Abstract:
The invention concerns high purity phthalcin derivatives enabling their use for medical applications or in the field of biotechnology, as well as their preparation method whereby a phthatic anhydride derivative is condensed with a naphthol or phenol derivative in an organic acid ester and the crystals of the resulting condensate are converted by action of a strong acid or one of its precursors in anhydrous medium.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is the National Stage of International Application No. PCT/FR2003/003205, filed Oct. 28, 2003, which claims the benefit of France Patent Application Serial No. 02/13,528, filed on Oct. 29, 2002. The contents of both applications are hereby incorporated by reference in their entireties. 
     FIELD OF THE INVENTION 
     The present invention relates to high purity phthaleins and to the method for preparing them. It relates more particularly to high purity fluorescein. 
     In the present description, the term “high purity phthalein” is intended to mean a phthalein containing at most 1% by weight, preferably at most 0.5% by weight, of impurities. 
     BACKGROUND OF THE INVENTION 
     Phthaleins are molecules having the following xanthene unit: 
     
       
                 
         
             
             
         
      
     
     These products are useful as dyes in various industries, in particular the textile industry, the paper industry, printing, reprography, the food industry, the cosmetics industry and the pharmaceutical industry. Phthaleins are, currently, the subject of a considerable resurgence of interest in the health field, for their diagnostic use, in particular in the context of medical imaging and in the field of the biotechnology for labeling biological molecules (nucleic acids, proteins, lipoproteins, membrane lipids) and following the intracellular or extracellular biochemical activity of biological molecules. 
     For example, fluorescein is a phthalein commonly used in ophthalmology for performing retinal angiography by fluorescence. The diagnostic advantage of fluorescein angiography is currently increased by the appearance of new medicinal products for treating vascular pathologies of the retina and of the choroid and the availability of a new generation of retinographs that make it possible to perform digital fluorescence imaging that has higher performance levels and gives greater resolution than the former systems of acquisition on photographic emulsions. 
     In parallel, the quality and safety requirements of the new international pharmaceutical standards (ICH: International Commission of Harmonization, ICH Q Topic Q3A 1999) have considerably increased. The same is true with regard to the use of phthaleins in biotechnologies that require reagents of increasingly high quality. In order to satisfy the current requirements, the level of purity of the phthaleins used in the pharmaceutical field or that of biotechnology must necessarily be very high. By way of example, in the publication “Effective differences in the formulation of intravenous fluorescein and related side effects”, Am. J. Ophthalmol. 78, 2: 217-221, 1974, L. Yannuzzi showed a correlation between the purity of phthaleins, in particular that of fluorescein, and the tolerance of these substances when they are administered to humans by injection. 
     SUMMARY OF THE INVENTION 
     The high purity phthaleins according to the invention have the general formula (I): 
                                
in which R1, R2, R3, R4 and R5, which are identical to or different from one another, are chosen from the group comprising the following radicals or groups: hydrogen, hydroxyl, halogen, acetyl, amino, phosphate, nitro, sulfonate, carboxyl, alkylcarboxyl having from 2 to 30 carbon atoms, alkyl having from 1 to 30 carbon atoms, cycloalkyl having from 3 to 12 carbon atoms, alkyloxy having from 1 to 30 carbon atoms, haloalkyl having from 1 to 30 carbon atoms, hydroxyalkyl having from 1 to 30 carbon atoms, alkyl ester having from 2 to 40 carbon atoms, nitroalkyl having from 1 to 30 carbon atoms, carboxyalkyl having from 2 to 30 carbon atoms, aminoalkyl having from 1 to 30 carbon atoms, sulfoalkyl having from 1 to 30 carbon atoms, aryl, aryloxy, arylalkyl, haloaryl, aryl ester, succinimidyl ester, isothiocyanate, maleimide, iodoacetamide, haloacetamide, chlorosulfonic, purine or pyrimidine bases, monosaccharides, preferably hexoses or pentoses, oligosides and polyosides, polypeptides, proteins and phospholipids,
 
R1 and R5 are not hydrogen when R1 is a group —CH 2 —CH 2 —COOH, R2 is a hydroxyl group and R4 is a group —COOH,
 
these phthaleins containing no more than 1% by weight, preferably no more than 0.5% by weight, and even more preferably not more than 0.2% by weight, of residual impurities.
 
     A phthalein that is particularly advantageous, in particular for ophthalmic applications, is fluorescein having such a purity. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     It is known practice to prepare the phthaleins of formula (I) by the condensation of a phthalic anhydride derivative with a phenol derivative having a free ortho-position with respect to a hydroxyl group. 
     This condensation is carried out by heating at the melting temperature of a mixture of the phthalic anhydride and of the phenol derivative, in the desired proportions. 
     This condensation can optionally be carried out in a dilution solvent. It can also be carried out in the presence of a catalyst. 
     In the absence of solvent, during the heating, the reaction medium rapidly thickens and has a tendency to harden. Within the reaction medium, areas where the temperature is too high and areas where the temperature is not high enough then form. 
     In the areas where the temperature is too high, the reactants and/or the reaction product are degraded and, in the areas where the temperature is not high enough, the reaction is not complete. The product obtained is of mediocre quality since it contains by-products that are very difficult to remove. 
     In order to improve this known method, the addition of an inert solvent or the use of a catalyst has been considered. 
     Thus, U.S. Pat. No. 1,931,049 describes the addition to the reaction medium of an inert solvent consisting of a benzene-based or aliphatic hydrocarbon, more particularly ortho-dichlorobenzene. However, the condensation reaction is then incomplete, and generate intermediate products that are subsequently difficult to remove. Therefore the method described in U.S. Pat. No. 1,931,049 does not make it possible to obtain a high purity phthalein according to the definition of the present application. Moreover, higher aliphatic hydrocarbons are not miscible with the reaction medium; consequently, they introduce no improvement in terms of thermal transfer and impair the elimination of the water formed, thereby slowing down the reaction, which takes place exclusively in a hydrophobic medium. 
     Regarding the catalysts that are used to improve the reaction yield, these are concentrated sulfuric acid, anhydrous zinc chloride and tin chloride. 
     The German patent DE 360691, describes the use of an aromatic sulfonic acid as a catalyst, in particular benzenesulfonic acid, alone or combined with one of the abovementioned three catalysts. 
     It so happens, however, that the addition of these catalysts leads to a reaction product that sets and hardens with entrapment of impurities, which can no longer be eliminated from the desired product. 
     In order to eliminate both the generated by-products and the impurities, isolation and purification methods have been developed, but none have made it possible to substantially improve the quality of phthaleins. 
     A conventional method consists in basifying the phthalein in an aqueous medium so as to dissolve it, and then in acidifying it so as to allow it to precipitate. These two steps are repeated successively in an attempt to eliminate the impurities. However, this method does not bring about a noticeable improvement in the purity of the product since, while the impurities dissolve with the phthalein during the basifying step, they precipitate again with the phthalein during the acidification step. In addition, this purification method has the drawback of producing a considerable amount of salts which are difficult and expensive to subsequently eliminate. 
     U.S. Pat. No. 1,965,842 describes the purification of phthaleins, derived from hydroxybenzene, by direct extraction of the by-products with dichlorobenzene alone or in a mixture with other solvents. This direct extraction of the crude product with the solvent in question does not, however, make it possible to obtain extensive elimination of impurities, which remain partly trapped in the phthalein crystals. 
     None of the purification methods described in the prior art makes it possible to reach a level of purity that is sufficient to allow the pharmaceutical use of phthaleins. 
     SUMMARY OF THE INVENTION 
     Given the advantage of these molecules for medical diagnosis, the production of high purity phthaleins is a real medical need and would satisfy a high demand, in particular for pharmaceutical applications in ophthalmology, for diagnosis, in particular in medical imaging, or in the field of biotechnological applications (for example, as dye to label molecules). 
     The inventors have, to their credit, at the end of extensive research, found that it is possible to obtain high purity products by the condensation of a phthalic anhydride with a phenol or naphthol derivative in a specific solvent which consists of an organic acid ester. 
     They have also found that the use of organic acid esters as solvents makes it possible to carry out this condensation with an excellent yield, greater than 75%. 
     Specifically, these solvents have the particularity
         firstly, of leading to specific crystallization of the phthalein that derives from the condensation and of excluding, from the generated crystals, all the impurities that remain dissolved in the reaction medium, and   secondly, of allowing a complete condensation that is predominant, to detrimental and unwanted side reactions, which results in the total consumption of the reactants and minimizes the formation of by-products.       

     The method in accordance with the invention thus makes it possible to obtain high purity phthaleins with a yield that is satisfactory from an industrial point of view. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a radiocrystallography spectrum of crude fluorescein. 
         FIG. 2  is a radiocrystallography spectrum of the fluorescein after conversion with acid (yellow). 
         FIG. 3  is a radiocrystallography spectrum of the crude 4′,5′-dimethyifluorescein. 
         FIG. 4  is a radiocrystallography of the 4′,5′-dimethyifluorescein after conversion with acid. 
         FIG. 5  is a radiocrystallography spectrum of the crude 4′,5′-dihydroxyfluorescein. 
         FIG. 6  is a radiocrystallography spectrum of the 4′,5′, dihydroxyfluorescein after conversion with acid. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     More particularly, the invention is related to a method for preparing phthaleins, from which the residual impurities have been removed, having general formula (I): 
                                
in which R1, R2, R3, R4 and R5, which are identical to or different from one another, are chosen from the group comprising the following radicals or groups: hydrogen, hydroxyl, halogen, acetyl, amino, phosphate, nitro, sulfonate, carboxyl, alkylcarboxyl having from 2 to 30 carbon atoms, alkyl having from 1 to 30 carbon atoms, cycloalkyl having from 3 to 12 carbon atoms, alkyloxy having from 1 to 30 carbon atoms, haloalkyl having from 1 to 30 carbon atoms, hydroxyalkyl having from 1 to 30 carbon atoms, alkyl ester having from 2 to 40 carbon atoms, nitroalkyl having from 1 to 30 carbon atoms, carboxyalkyl having from 2 to 30 carbon atoms, aminoalkyl having from 1 to 30 carbon atoms, sulfoalkyl having from 1 to 30 carbon atoms, aryl, aryloxy, arylalkyl, haloaryl, aryl ester, succinimidyl ester, isothiocyanate, maleimide, iodoacetamide, haloacetamide, chlorosulfonic, purine or pyrimidine bases, monosaccharides, preferably hexoses or pentoses, oligosides and polyosides, polypeptides, proteins and phospholipids,
 
R1 and R5 are not representing hydrogen when R1 is a group —CH 2 —CH 2 —COOH, R2 is a hydroxyl group and R4 is a group —COOH,
 
by condensation of a phthalic anhydride derivative of formula (II) with a phenol or naphthol compound of formula (III)
 
                                
in which R1, R2, R3, R4 and R5 have the same meanings as above,
 
in a solvent consisting of an organic acid ester.
 
     Advantageously, the starting compound (III), which is condensed with the phthalic anhydride (II), is chosen from the group comprising in particular resorcinol, orcinol, naphthol, pyrogallol, alkylaminophenol and arylaminophenol. 
     When resorcinol is used as starting product, the method in accordance with the invention makes it possible to prepare fluorescein. 
     Advantageously, the solvent used in the method in accordance with the invention is an organic acid ester of formula (IV)
 
R 6 —COOR 7   (IV)
 
in which R 6  is chosen from the group comprising the following radicals or groups: hydrogen, alkyl having from 1 to 30 carbon atoms, cycloalkyl having from 3 to 12 carbon atoms, haloalkyl having from 1 to 30 carbon atoms, hydroxyalkyl having from 1 to 30 carbon atoms, nitroalkyl having from 1 to 30 carbon atoms, aryl, aryloxy, alkylaryl, arylalkyl, substituted arylalkyl, haloaryl, aryl ester, alkyl ester having from 2 to 40 carbon atoms, and alkyloxy having from 1 to 30 carbon atoms, R 7  representing one of the following groups: alkyl having from 1 to 30 carbon atoms, cycloalkyl having from 3 to 12 carbon atoms, haloalkyl having from 1 to 30 carbon atoms, hydroxyalkyl having from 1 to 30 carbon atoms, nitroalkyl having from 1 to 30 carbon atoms, aryl, aryloxy, alkylaryl, arylalkyl, substituted arylalkyl, haloaryl, aryl ester, alkyl ester having from 2 to 40 carbon atoms, or alkyloxy having from 1 to 30 carbon atoms.
 
     Particularly advantageously, the abovementioned solvent is chosen from the group comprising methyl, ethyl, propyl or butyl benzoates, methyl, ethyl, propyl or butyl heptanoates, methyl, ethyl, propyl or butyl octanoates, methyl, ethyl, propyl or butyl laurates, methyl, ethyl, propyl or butyl myristates and methyl, ethyl, propyl or butyl palmitates, and mixtures thereof. 
     The solvent is chosen according to its boiling point, so as to make it possible to carry out the condensation reaction at a temperature of between 150° C. and 250° C. 
     The condensation reaction can be carried out at atmospheric pressure or under a pressure that is adjusted according to the difference that exists between the temperature corresponding to the boiling point of the solvent and the temperature necessary to carry out the reaction, in particular under a pressure greater than atmospheric pressure when the boiling point of the compound (IV) is lower than the reaction temperature. 
     The condensation reaction can be carried out in the presence of a catalyst chosen from the group comprising in particular Lewis acids, such as ZnCl 2  or AlCl 3 , Brönsted acids such as H 2 SO 4 , polyphosphoric acid, or salts thereof. 
     Advantageously, the catalyst used is an alkali metal salt of hydrogen sulfate. Particularly advantageously, the catalyst used is potassium hydrogen sulfate (KHSO 4 ) or sodium hydrogen sulfate. The use of hydrogen sulfate as catalyst makes it possible to obtain an excellent yield for the condensation reaction and has the advantage, unlike other catalysts, of making it possible to obtain complete condensation of the reactants, of being able to be completely eliminated from the phthalein obtained, and of not inducing the formation of tars in the reaction medium. 
     At the end of the condensation reaction, a crude product is obtained, the organic purity of which is already much higher than that of the products obtained with the methods of the prior art, since it is equal to or greater than 95%. 
     However, this purity is not yet sufficient to allow pharmaceutical use, in particular by injection. Furthermore, as shown above, the purification methods described in the prior art do not result in a process that makes it possible to notably improve this purity. 
     At the end of extensive research, surprisingly and unexpectedly, the inventors have found a method that considerably increases the purity of crude phthaleins resulting from the condensation reaction, by treating them with a strong acid or one of its precursors in an anhydrous organic medium. 
     According to an advantageous embodiment, the method in accordance with the invention consequently comprises, after the condensation reaction, a step consisting in acidifying the product resulting from the condensation, in an anhydrous organic medium, by addition of a strong acid or one of its precursors, chosen from the group comprising in particular sulfuric acid, hydrochloric acid, hydrobromic acid, hydrofluoric acid, hydriodic acid, polyphosphoric acid, pyrophosphate (P 2 O 5 ), and mixtures thereof. This acidification is carried out until the phthalein crystals resulting from the condensation are converted to phthalein crystals having a different structure. 
     The effect of this acidification step is to convert the crude phthalein that is insoluble and dispersed in the organic medium into a form that has a different crystalline structure and is slightly soluble in this same medium. This conversion is very rapid, and takes place via an intermediate solubilization phase during which the impurities are released and completely eliminated from the phthalein crystals. This new purification method is very advantageous since it requires very little solvent and yields to a very high purity in a very short time. 
     According to the method of the present invention, this purification step is carried out by dispersing the crude phthalein resulting from the condensation, in an anhydrous solvent, preferably in an alcohol, a ketone, an ether or a halogenated solvent, that is either used alone or as a mixture, even more preferably in absolute ethanol or acetone, alone or as a mixture. 
     The dispersion of crude phthalein thus obtained is acidified by the addition of a strong acid or one of its precursors, chosen from the group comprising in particular sulfuric acid, hydrochloric acid, hydrobromic acid, hydrofluoric acid, hydriodic acid, polyphosphoric acid, pyrophosphate (P 2 O 5 ), and mixtures thereof. 
     According to a particularly advantageous embodiment, the acidification is carried out either by sparging gaseous hydrochloric acid into the phthalein dispersion, or by adding hydrochloric acid dissolved beforehand in an organic solvent. 
     According to another advantageous embodiment of the method in accordance with the invention, the product obtained after acidification is washed with a washing solution chosen from the group comprising water, polar solvents such as alcohols or ketones, or slightly polar solvents such as ethers and halogenated solvents, used pure or as a mixture. 
     At the end of the purification step, the phthaleins thus treated have a purity of greater than 98%, preferably greater than 99%, more preferably greater than 99.5%, and even more preferably of 99.8%. 
     The phthaleins thus obtained have the suitable quality to prepare other phthaleins of formula (I) by chemical modification of the groups R1, R2, R3, R4 and R5 according to the conventional methods of the art, in particular to obtain phthaleins that can be used for labeling biological molecules (nucleic acids, proteins, lipoproteins, membrane lipids) and in the field of biotechnological applications (for example, for the labeling of molecules and of their intracellular or extracellular biochemical activity). 
     The invention is particularly suitable for preparing very high purity fluorescein, i.e. such that its content of each of the by-products of the reaction is less than or equal to 0.2%, and preferably less than or equal to 0.1%, the sum of the contents of each of these by-products being less than or equal to 0.5%. 
     The method for preparing a fluorescein having the abovementioned purity comprises the following successive steps:
         condensing phthalic anhydride with resorcinol, in a solvent which is an ester of an aliphatic or aromatic organic acid, preferably methyl benzoate, in the presence of a catalyst chosen from the group of Brönsted acids,   suspending the red-colored crystals obtained in the preceding step in an anhydrous solvent chosen from the group comprising alcohols such as absolute ethanol, ketones such as acetone, ethers, halogenated solvents, or mixtures thereof,   acidifying the suspension obtained, by the addition of a strong acid or one of its precursors, chosen from the group comprising sulfuric acid, hydrochloric acid, hydrobromic acid, hydrofluoric acid, hydriodic acid, polyphosphoric acid, pyrophosphate (P 2 O 5 ), or mixtures thereof, until yellow-colored crystals are obtained,   washing the crystals thus obtained with a washing solution chosen from the group comprising water, alcohols, ketones, ethers and halogenated solvents, pure or as a mixture, until red-colored crystals are obtained.       

     An X-ray diffraction analysis of the red-colored fluorescein crystals is represented in  FIG. 1 ; an X-ray diffraction analysis of the yellow-colored fluorescein crystals is represented in  FIG. 2 ; these X-ray-diffraction analysis being obtained on the following equipment: Philips 1729 generator, Philips 1050 goniometer, Cu Kα radiation, Gonio acquisition software and Rayon processing software, under the following operating conditions: 
     
       
         
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 Voltage 
                 40 mV 
               
               
                   
                 Intensity 
                 40 mA 
               
               
                   
                 Number of points 
                 4000 
               
               
                   
                 Number of passages 
                 10 
               
               
                   
                 Acquisition period 
                 250 ms 
               
               
                   
                 Start angle (°θ) 
                 3.000 
               
               
                   
                 End angle (°θ) 
                 23.000 
               
               
                   
                 Standard 
                 silicon 
               
               
                   
                   
               
             
          
         
       
     
     According to an advantageous embodiment of the method for preparing fluorescein, the catalyst used for the condensation reaction is the hydrogen sulfate of an alkali metal, preferably potassium hydrogen sulfate or sodium hydrogen sulfate. 
     According to another advantageous embodiment of the method for preparing fluorescein, the acidification is carried out by sparging gaseous hydrochloric acid into the fluorescein suspension or by the action, on this fluorescein, of hydrochloric acid in solution in an anhydrous organic solvent, preferably chosen from the group comprising alcohols, ketones, ethers and halogenated solvents, used alone or as a mixture, even more preferably isopropanol, absolute ethanol or acetone, pure or as a mixture. 
     Advantageously, the washing step is carried out with a mixture of water, of ethanol and of acetone. 
     By means of this method in accordance with the invention, it is possible to prepare fluorescein having a purity of greater than or equal to 99.7%, which provides unquestionable advantages, in particular for pharmaceutical uses in diagnosis, especially in medical imaging or else in the field of biotechnological applications. 
     The inventors have demonstrated a novel crystallographic form of fluorescein being yellow-colored crystals. This novel crystallographic form is identified by the X-ray diffraction analysis given in  FIG. 2 , which was determined under the conditions mentioned above. 
     They have also demonstrated a novel crystallographic form of the following compounds: 4′,5′-dihydroxyfluorescein and 4′,5′-dimethylfluorescein. 
     Thus, the invention relates to the yellow-colored fluorescein crystals for which the X-ray diffraction analysis is given in  FIG. 2 . 
     The invention also relates to the yellow-colored 4′,5′-dihydroxyfluorescein crystals for which the X-ray diffraction analysis is given in  FIG. 4 . 
     It also relates to the dark red-colored 4′,5′-dimethylfluorescein crystals for which the X-ray diffraction analysis is given in  FIG. 6 . 
     All these spectra were determined with the equipment and under the conditions mentioned above. 
     The invention will be described in more detail with the aid of the following examples which are not limiting but relate to advantageous embodiments. 
     EXAMPLES 
     Example 1 
     Preparation of High Purity Fluorescein 
     Synthesis of Fluorescein 
     A mixture comprising 55 g of resorcinol, 30 g of phthalic anhydride, 2 g of potassium hydrogen sulfate and 500 ml of methyl benzoate is brought to 200° C. for 6 hours. After cooling, the red crystals of crude fluorescein are washed with acetone and dried. 
     Mass obtained=51.8 g (78%). 
     These crystals are analyzed by X-ray crystallography under the following operating conditions: 
     Equipment 
     Philips 1729 generator 
     Philips 1050 goniometer 
     Cu Kα radiation 
     Gonio acquisition software 
     Rayon processing software 
     
       
         
               
             
               
               
               
             
           
               
                   
               
               
                 Conditions 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Voltage 
                 40 mV 
               
               
                   
                 Intensity 
                 40 mA 
               
               
                   
                 Number of points 
                 4000 
               
               
                   
                 Number of passages 
                 10 
               
               
                   
                 Acquisition period 
                 250 ms 
               
               
                   
                 Start angle (°θ) 
                 3.000 
               
               
                   
                 End angle (°θ) 
                 23.000 
               
               
                   
                 Standard 
                 silicon 
               
               
                   
                   
               
             
          
         
       
     
     The radiocrystallogram of  FIG. 1  is obtained, for which the peaks are identified below: 
     
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 Theta 
                   
                 % 
                 Number 
               
               
                   
                 (degrees) 
                 Distance (Å) 
                 Intensities 
                 of counts 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 5.3600 
                 8.2457 
                 18.09 
                 647 
               
               
                   
                 5.9500 
                 7.4306 
                 31.10 
                 1112 
               
               
                   
                 6.7000 
                 6.6020 
                 36.30 
                 1298 
               
               
                   
                 8.3750 
                 5.2883 
                 45.95 
                 1643 
               
               
                   
                 9.1050 
                 4.8675 
                 10.79 
                 386 
               
               
                   
                 9.4300 
                 4.7012 
                 14.09 
                 504 
               
               
                   
                 9.9950 
                 4.4379 
                 8.03 
                 287 
               
               
                   
                 10.9150 
                 4.0678 
                 9.73 
                 348 
               
               
                   
                 11.6200 
                 3.8241 
                 16.41 
                 587 
               
               
                   
                 11.7300 
                 3.7888 
                 5.40 
                 193 
               
               
                   
                 12.4200 
                 3.5813 
                 2.82 
                 101 
               
               
                   
                 13.2150 
                 3.3694 
                 100.00 
                 3576 
               
               
                   
                 13.5050 
                 3.2983 
                 7.47 
                 267 
               
               
                   
                 13.8850 
                 3.2097 
                 79.03 
                 2826 
               
               
                   
                 14.9650 
                 2.9828 
                 5.62 
                 201 
               
               
                   
                 15.3100 
                 2.9172 
                 3.91 
                 140 
               
               
                   
                 15.7600 
                 2.8359 
                 11.69 
                 418 
               
               
                   
                 16.0000 
                 2.7945 
                 8.67 
                 310 
               
               
                   
                 16.9100 
                 2.6481 
                 4.28 
                 153 
               
               
                   
                 17.4750 
                 2.5650 
                 3.97 
                 142 
               
               
                   
                 17.8550 
                 2.5122 
                 3.80 
                 136 
               
               
                   
                 18.2300 
                 2.4622 
                 2.52 
                 90 
               
               
                   
                 18.5150 
                 2.4256 
                 2.96 
                 106 
               
               
                   
                 18.8250 
                 2.3871 
                 5.03 
                 180 
               
               
                   
                 19.0800 
                 2.3563 
                 3.27 
                 117 
               
               
                   
                 20.4300 
                 2.2066 
                 3.05 
                 109 
               
               
                   
                 20.6800 
                 2.1811 
                 4.53 
                 162 
               
               
                   
                 20.9300 
                 2.1562 
                 3.08 
                 110 
               
               
                   
                 21.1800 
                 2.1319 
                 2.38 
                 85 
               
               
                   
                 22.9400 
                 1.9762 
                 4.53 
                 162 
               
               
                   
                   
               
             
          
         
       
     
     Wavelength: 1.54051 Å 
     Purification of Crude Fluorescein 
     50 g of crude fluorescein obtained in the previous step are stirred into 1000 ml of ethanol/acetone mixture. Concentrated sulfuric acid is added to this mixture, until complete conversion of the red crystals to yellow crystals is obtained. 
     The yellow crystals obtained are analyzed by X-ray crystallography as above. The X-ray diffraction analysis of  FIG. 2  is obtained, for which the peaks are identified below: 
     
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 Theta 
                 Distances 
                 % 
                 Number of 
               
               
                   
                 (degrees) 
                 (Å) 
                 Intensities 
                 counts 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 3.3500 
                 13.1813 
                 8.55 
                 56 
               
               
                   
                 4.0050 
                 11.0283 
                 47.18 
                 309 
               
               
                   
                 4.8550 
                 9.1010 
                 12.98 
                 85 
               
               
                   
                 5.2650 
                 8.3940 
                 9.01 
                 59 
               
               
                   
                 5.4500 
                 8.1099 
                 11.45 
                 75 
               
               
                   
                 5.7900 
                 7.6352 
                 52.98 
                 347 
               
               
                   
                 6.3000 
                 7.0193 
                 16.95 
                 111 
               
               
                   
                 6.7700 
                 6.5340 
                 17.86 
                 117 
               
               
                   
                 7.4900 
                 5.9090 
                 17.56 
                 115 
               
               
                   
                 8.0300 
                 5.5140 
                 69.77 
                 457 
               
               
                   
                 8.4250 
                 5.2572 
                 33.13 
                 217 
               
               
                   
                 8.7800 
                 5.0462 
                 8.40 
                 55 
               
               
                   
                 9.1550 
                 4.8411 
                 15.88 
                 104 
               
               
                   
                 9.3400 
                 4.7461 
                 42.44 
                 278 
               
               
                   
                 9.6550 
                 4.5926 
                 34.66 
                 227 
               
               
                   
                 9.8800 
                 4.4891 
                 24.43 
                 160 
               
               
                   
                 10.1950 
                 4.3518 
                 57.71 
                 378 
               
               
                   
                 10.5800 
                 4.1951 
                 58.02 
                 380 
               
               
                   
                 10.7350 
                 4.1352 
                 38.17 
                 250 
               
               
                   
                 11.1650 
                 3.9779 
                 28.85 
                 189 
               
               
                   
                 11.4800 
                 3.8701 
                 30.23 
                 198 
               
               
                   
                 11.7200 
                 3.7919 
                 59.85 
                 392 
               
               
                   
                 12.1100 
                 3.6716 
                 26.41 
                 173 
               
               
                   
                 12.3950 
                 3.5884 
                 58.47 
                 383 
               
               
                   
                 12.6400 
                 3.5200 
                 100.00 
                 655 
               
               
                   
                 12.9450 
                 3.4384 
                 30.84 
                 202 
               
               
                   
                 13.1550 
                 3.3845 
                 56.64 
                 371 
               
               
                   
                 13.8350 
                 3.2211 
                 14.66 
                 96 
               
               
                   
                 14.0550 
                 3.1717 
                 24.12 
                 158 
               
               
                   
                 14.3550 
                 3.1068 
                 36.34 
                 238 
               
               
                   
                 14.6700 
                 3.0415 
                 42.90 
                 281 
               
               
                   
                 15.1300 
                 2.9511 
                 42.90 
                 281 
               
               
                   
                 15.7500 
                 2.8377 
                 18.47 
                 121 
               
               
                   
                 15.9400 
                 2.8047 
                 18.17 
                 119 
               
               
                   
                 16.2500 
                 2.7526 
                 15.42 
                 101 
               
               
                   
                 16.7550 
                 2.6719 
                 17.71 
                 116 
               
               
                   
                 16.8500 
                 2.6573 
                 19.39 
                 127 
               
               
                   
                 17.2250 
                 2.6011 
                 18.17 
                 119 
               
               
                   
                 17.7300 
                 2.5293 
                 15.73 
                 103 
               
               
                   
                 18.4200 
                 2.4377 
                 10.23 
                 67 
               
               
                   
                 18.5750 
                 2.4180 
                 10.23 
                 67 
               
               
                   
                 19.2650 
                 2.3345 
                 15.88 
                 104 
               
               
                   
                 19.5800 
                 2.2984 
                 10.84 
                 71 
               
               
                   
                 19.8000 
                 2.2739 
                 11.91 
                 78 
               
               
                   
                 20.1150 
                 2.2397 
                 13.28 
                 87 
               
               
                   
                 20.6500 
                 2.1841 
                 14.81 
                 97 
               
               
                   
                 21.1500 
                 2.1348 
                 11.30 
                 74 
               
               
                   
                 21.2750 
                 2.1228 
                 10.08 
                 66 
               
               
                   
                 21.8100 
                 2.0732 
                 7.94 
                 52 
               
               
                   
                 22.2500 
                 2.0342 
                 9.16 
                 60 
               
               
                   
                 22.6250 
                 2.0022 
                 9.62 
                 63 
               
               
                   
                 22.9100 
                 1.9786 
                 9.01 
                 59 
               
               
                   
                   
               
             
          
         
       
     
     Wavelength: 1.54051 Å 
     These crystals are filtered off and then washed with an ethanol/acetone/water (40/40/20) mixture. The washing turns the yellow fluorescein crystals to red fluorescein crystals, which have a higher purity. 
     Purity by HPLC: 99.8% 
     Example 2 
     Preparation of high purity 4′,5′-dimethylfluorescein 
     Synthesis of 4′,5′-dimethylfluorescein 
     A mixture comprising 62 g of 2-methylresorcinol, 30 g of phthalic anhydride, 2 g of potassium hydrogen sulfate and 500 ml of ethyl laurate is brought to 200° C. for 3 hours. After cooling, the crude product is filtered and washed with acetone and then dried. The product obtained is a dark orange powder. 
     Mass obtained=49.7 g (69%) 
     These crystals are analyzed by X-ray crystallography in the same way as in example 1. The X-ray diffraction analysis of  FIG. 3  is obtained, for which the peaks are identified below. 
     
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 Theta 
                 Distances 
                 % 
                 Number of 
               
               
                   
                 (degrees) 
                 (Å) 
                 Intensities 
                 counts 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 3.5700 
                 12.3700 
                 7.38 
                 81 
               
               
                   
                 5.0290 
                 8.7938 
                 45.49 
                 499 
               
               
                   
                 6.6650 
                 6.6365 
                 62.26 
                 683 
               
               
                   
                 6.7400 
                 6.5630 
                 60.07 
                 659 
               
               
                   
                 7.1300 
                 6.2057 
                 87.24 
                 957 
               
               
                   
                 7.4550 
                 5.9366 
                 20.97 
                 230 
               
               
                   
                 8.3700 
                 5.2915 
                 6.56 
                 72 
               
               
                   
                 8.9300 
                 4.9621 
                 30.63 
                 336 
               
               
                   
                 9.0950 
                 4.8728 
                 10.12 
                 111 
               
               
                   
                 9.3300 
                 4.7511 
                 21.24 
                 233 
               
               
                   
                 9.4550 
                 4.6889 
                 19.05 
                 209 
               
               
                   
                 10.0350 
                 4.4204 
                 12.12 
                 133 
               
               
                   
                 10.3250 
                 4.2975 
                 18.87 
                 207 
               
               
                   
                 10.6000 
                 4.1873 
                 11.49 
                 126 
               
               
                   
                 10.8300 
                 4.0994 
                 39.65 
                 435 
               
               
                   
                 11.2900 
                 3.9344 
                 12.49 
                 137 
               
               
                   
                 11.4500 
                 3.8801 
                 10.85 
                 119 
               
               
                   
                 11.7550 
                 3.7808 
                 28.17 
                 309 
               
               
                   
                 11.9050 
                 3.7339 
                 52.42 
                 575 
               
               
                   
                 12.2550 
                 3.6288 
                 79.95 
                 877 
               
               
                   
                 12.7050 
                 3.5023 
                 8.11 
                 89 
               
               
                   
                 13.2400 
                 3.3631 
                 21.15 
                 232 
               
               
                   
                 13.4200 
                 3.3188 
                 100.00 
                 1097 
               
               
                   
                 13.7100 
                 3.2499 
                 10.85 
                 119 
               
               
                   
                 14.5250 
                 3.0712 
                 8.84 
                 97 
               
               
                   
                 14.8350 
                 3.0084 
                 23.25 
                 255 
               
               
                   
                 15.0550 
                 2.9654 
                 17.59 
                 193 
               
               
                   
                 15.1550 
                 2.9463 
                 15.13 
                 166 
               
               
                   
                 15.3650 
                 2.9070 
                 25.80 
                 283 
               
               
                   
                 15.7050 
                 2.8456 
                 20.51 
                 225 
               
               
                   
                 16.1900 
                 2.7625 
                 14.13 
                 155 
               
               
                   
                 16.7550 
                 2.6719 
                 9.02 
                 99 
               
               
                   
                 17.5400 
                 2.5558 
                 7.47 
                 82 
               
               
                   
                 17.8850 
                 2.5081 
                 8.75 
                 96 
               
               
                   
                 18.1050 
                 2.4786 
                 6.75 
                 74 
               
               
                   
                 18.4900 
                 2.4288 
                 19.51 
                 214 
               
               
                   
                 18.7950 
                 2.3907 
                 6.11 
                 67 
               
               
                   
                 18.8900 
                 2.3791 
                 6.84 
                 75 
               
               
                   
                 19.6800 
                 2.2872 
                 15.50 
                 170 
               
               
                   
                 20.2100 
                 2.2296 
                 7.38 
                 81 
               
               
                   
                 20.3950 
                 2.2103 
                 9.66 
                 106 
               
               
                   
                 20.6800 
                 2.1811 
                 6.84 
                 75 
               
               
                   
                 20.9950 
                 2.1498 
                 5.38 
                 59 
               
               
                   
                 21.4650 
                 2.1049 
                 7.11 
                 78 
               
               
                   
                 21.6500 
                 2.0878 
                 8.84 
                 97 
               
               
                   
                 21.9350 
                 2.0620 
                 9.21 
                 101 
               
               
                   
                 22.1250 
                 2.0451 
                 9.02 
                 99 
               
               
                   
                 22.5300 
                 2.0102 
                 5.47 
                 60 
               
               
                   
                 22.7850 
                 1.9889 
                 5.93 
                 65 
               
               
                   
                   
               
             
          
         
       
     
     Wavelength: 1.54051 Å 
     Purification of 4′,5′-dimethylfluorescein 
     40 g of crude 4′,5′-dimethylfluorescein are added to 800 ml of ethanol/acetone mixture. Concentrated sulfuric acid is added to this mixture, until complete conversion of dark orange-colored crystals to yellow-colored crystals is obtained. 
     The yellow crystals obtained are analyzed by X-ray crystallography as above. The X-ray diffraction analysis of  FIG. 4  is obtained, for which the peaks are identified below. 
     
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 Theta 
                 Distances 
                 % 
                 Number of 
               
               
                   
                 (degrees) 
                 (Å) 
                 Intensities 
                 counts 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 3.7050 
                 11.9199 
                 22.22 
                 172 
               
               
                   
                 4.6150 
                 9.5732 
                 45.61 
                 353 
               
               
                   
                 4.9300 
                 8.9629 
                 46.77 
                 362 
               
               
                   
                 6.3300 
                 6.9861 
                 8.53 
                 66 
               
               
                   
                 6.6150 
                 6.6864 
                 6.20 
                 48 
               
               
                   
                 7.3000 
                 6.0619 
                 19.51 
                 151 
               
               
                   
                 7.4500 
                 5.9405 
                 32.56 
                 252 
               
               
                   
                 7.6500 
                 5.7861 
                 26.61 
                 206 
               
               
                   
                 8.1250 
                 5.4499 
                 64.21 
                 497 
               
               
                   
                 8.6300 
                 5.1332 
                 22.48 
                 174 
               
               
                   
                 9.5950 
                 4.6211 
                 6.59 
                 51 
               
               
                   
                 10.4450 
                 4.2487 
                 41.34 
                 320 
               
               
                   
                 10.8500 
                 4.0919 
                 10.21 
                 79 
               
               
                   
                 11.3550 
                 3.9122 
                 14.60 
                 113 
               
               
                   
                 11.7000 
                 3.7983 
                 9.17 
                 71 
               
               
                   
                 11.7950 
                 3.7682 
                 9.30 
                 72 
               
               
                   
                 12.2250 
                 3.6375 
                 23.13 
                 179 
               
               
                   
                 12.4500 
                 3.5728 
                 58.79 
                 455 
               
               
                   
                 12.7650 
                 3.4861 
                 19.90 
                 154 
               
               
                   
                 12.8950 
                 3.4515 
                 18.60 
                 144 
               
               
                   
                 13.1000 
                 3.3984 
                 100.00 
                 774 
               
               
                   
                 13.6350 
                 3.2675 
                 67.44 
                 522 
               
               
                   
                 14.1400 
                 3.1530 
                 30.75 
                 238 
               
               
                   
                 14.5250 
                 3.0712 
                 14.08 
                 109 
               
               
                   
                 14.8400 
                 3.0074 
                 22.74 
                 176 
               
               
                   
                 15.1750 
                 2.9425 
                 19.38 
                 150 
               
               
                   
                 15.3500 
                 2.9098 
                 30.23 
                 234 
               
               
                   
                 15.6950 
                 2.8474 
                 20.93 
                 162 
               
               
                   
                 16.0950 
                 2.7784 
                 7.24 
                 56 
               
               
                   
                 16.2850 
                 2.7468 
                 7.49 
                 58 
               
               
                   
                 16.5950 
                 2.6969 
                 8.01 
                 62 
               
               
                   
                 16.9100 
                 2.6481 
                 14.08 
                 109 
               
               
                   
                 17.3200 
                 2.5873 
                 8.66 
                 67 
               
               
                   
                 17.5400 
                 2.5558 
                 7.88 
                 61 
               
               
                   
                 17.9150 
                 2.5040 
                 13.31 
                 103 
               
               
                   
                 18.3550 
                 2.4460 
                 9.69 
                 75 
               
               
                   
                 18.4800 
                 2.4300 
                 10.59 
                 82 
               
               
                   
                 18.8600 
                 2.3828 
                 15.37 
                 119 
               
               
                   
                 19.2350 
                 2.3380 
                 9.43 
                 73 
               
               
                   
                 19.4850 
                 2.3092 
                 7.75 
                 60 
               
               
                   
                 20.2100 
                 2.2296 
                 14.99 
                 116 
               
               
                   
                 20.7100 
                 2.1781 
                 7.11 
                 55 
               
               
                   
                 20.8350 
                 2.1656 
                 6.59 
                 51 
               
               
                   
                 21.0850 
                 2.1411 
                 6.33 
                 49 
               
               
                   
                 21.5250 
                 2.0993 
                 12.92 
                 100 
               
               
                   
                 21.6500 
                 2.0878 
                 8.79 
                 68 
               
               
                   
                 21.8400 
                 2.0705 
                 6.98 
                 54 
               
               
                   
                 22.0900 
                 2.0482 
                 10.34 
                 80 
               
               
                   
                 226550 
                 1.9997 
                 7.11 
                 55 
               
               
                   
                   
               
             
          
         
       
     
     Wavelength: 1.54051 Å 
     After filtration and recrystallization from an acetone/water mixture or washing in ethanol/acetone/water, the yellow-colored crystals turn to dark orange-colored crystals. 
     Example 3 
     Preparation of 4′,5′-dihydroxyfluorescein 
     Synthesis of 4′,5′-dihydroxyfluorescein 
     A mixture comprising 63 g of pyrogallol, 30 g of phthalic anhydride, 2 g of potassium hydrogen sulfate and 500 ml of ethyl myristate is brought to 200° C. for 3 hours. After cooling, the crude product is filtered and washed with acetone and then dried. The product obtained is a grayish-brown- or anthracite-colored powder. 
     Mass obtained=43.5 g (59.7%) 
     The crystals obtained are analyzed by X-ray crystallography as above. The X-ray diffraction analysis of  FIG. 5  is obtained, the peaks of which are identified below. 
     
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 Theta 
                 Distances 
                 % 
                 Number 
               
               
                   
                 (degrees) 
                 (Å) 
                 Intensities 
                 of counts 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 3.6450 
                 12.1158 
                 73.08 
                 980 
               
               
                   
                 5.9200 
                 7.4681 
                 8.13 
                 109 
               
               
                   
                 6.7200 
                 6.5824 
                 13.35 
                 179 
               
               
                   
                 7.3350 
                 6.0331 
                 10.29 
                 138 
               
               
                   
                 8.5050 
                 5.2081 
                 11.78 
                 158 
               
               
                   
                 9.2300 
                 4.8021 
                 15.59 
                 209 
               
               
                   
                 9.7400 
                 4.5529 
                 18.20 
                 244 
               
               
                   
                 10.3400 
                 4.2914 
                 21.40 
                 287 
               
               
                   
                 10.9150 
                 4.0678 
                 6.71 
                 90 
               
               
                   
                 11.2600 
                 3.9447 
                 8.80 
                 118 
               
               
                   
                 12.2550 
                 3.6288 
                 18.64 
                 250 
               
               
                   
                 12.5550 
                 3.5434 
                 12.75 
                 171 
               
               
                   
                 13.2000 
                 3.3731 
                 11.86 
                 159 
               
               
                   
                 13.7200 
                 3.2476 
                 100.00 
                 1341 
               
               
                   
                 14.7150 
                 3.0324 
                 17.75 
                 238 
               
               
                   
                 14.8600 
                 3.0034 
                 13.57 
                 182 
               
               
                   
                 16.1600 
                 2.7675 
                 6.79 
                 91 
               
               
                   
                 17.2550 
                 2.5967 
                 7.83 
                 105 
               
               
                   
                 18.3550 
                 2.4460 
                 6.86 
                 92 
               
               
                   
                   
               
             
          
         
       
     
     Wavelenth: 1.54051 Å 
     Purification of 4′,5′-dihydroxyfluorescein 
     40 g of crude 4′,5′-dihydroxyfluorescein are added to 800 ml of ethanol/acetone mixture. Concentrated sulfuric acid is added to this mixture, until complete conversion of grayish-brown- or anthracite-colored crystals to reddish-brown- or mahogany-colored crystals is obtained. 
     The crystals obtained are analyzed by X-ray crystallography as above. The X-ray diffraction analysis of  FIG. 6  is obtained, for which the peaks are identified below: 
     
       
         
               
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 Theta 
                 Distances 
                 % 
                 Number of 
               
               
                   
                 (degrees) 
                 (Å) 
                 Intensities 
                 counts 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 3.2500 
                 13.5865 
                 90.53 
                 526 
               
               
                   
                 3.5350 
                 12.4923 
                 18.76 
                 109 
               
               
                   
                 3.7850 
                 11.6683 
                 11.70 
                 68 
               
               
                   
                 3.9450 
                 11.1958 
                 13.25 
                 77 
               
               
                   
                 4.1650 
                 10.6053 
                 15.15 
                 88 
               
               
                   
                 4.5400 
                 9.7310 
                 10.33 
                 60 
               
               
                   
                 4.6350 
                 9.5319 
                 10.15 
                 59 
               
               
                   
                 6.3950 
                 6.9154 
                 9.29 
                 54 
               
               
                   
                 6.5500 
                 6.7525 
                 11.53 
                 67 
               
               
                   
                 6.8200 
                 6.4863 
                 27.71 
                 161 
               
               
                   
                 7.1150 
                 6.2187 
                 10.50 
                 61 
               
               
                   
                 7.4800 
                 5.9168 
                 21.51 
                 125 
               
               
                   
                 7.8700 
                 5.6253 
                 12.91 
                 75 
               
               
                   
                 8.2200 
                 5.3874 
                 27.88 
                 162 
               
               
                   
                 8.7800 
                 5.0462 
                 41.48 
                 241 
               
               
                   
                 9.2200 
                 4.8073 
                 10.15 
                 59 
               
               
                   
                 9.5350 
                 4.6499 
                 11.70 
                 68 
               
               
                   
                 9.7950 
                 4.5276 
                 42.17 
                 245 
               
               
                   
                 10.1000 
                 4.3923 
                 17.21 
                 100 
               
               
                   
                 10.5400 
                 4.2108 
                 16.70 
                 97 
               
               
                   
                 10.8850 
                 4.0789 
                 16.52 
                 96 
               
               
                   
                 11.6500 
                 3.8144 
                 25.13 
                 146 
               
               
                   
                 11.8900 
                 3.7385 
                 13.60 
                 79 
               
               
                   
                 12.0450 
                 3.6911 
                 13.08 
                 76 
               
               
                   
                 12.3900 
                 3.5898 
                 16.52 
                 96 
               
               
                   
                 12.5800 
                 3.5365 
                 14.80 
                 86 
               
               
                   
                 12.8750 
                 3.4568 
                 27.37 
                 159 
               
               
                   
                 12.9950 
                 3.4254 
                 25.82 
                 150 
               
               
                   
                 13.2900 
                 3.3507 
                 67.64 
                 393 
               
               
                   
                 13.5750 
                 3.2816 
                 27.02 
                 157 
               
               
                   
                 13.7150 
                 3.2488 
                 27.02 
                 157 
               
               
                   
                 14.1100 
                 3.1596 
                 100.00 
                 581 
               
               
                   
                 14.6200 
                 3.0516 
                 15.66 
                 91 
               
               
                   
                 14.7750 
                 3.0203 
                 18.24 
                 106 
               
               
                   
                 15.0400 
                 2.9683 
                 25.13 
                 146 
               
               
                   
                 15.3050 
                 2.9181 
                 24.78 
                 144 
               
               
                   
                 15.6850 
                 2.8491 
                 16.35 
                 95 
               
               
                   
                 15.9400 
                 2.8047 
                 11.02 
                 64 
               
               
                   
                 16.2500 
                 2.7526 
                 10.67 
                 62 
               
               
                   
                 16.5950 
                 2.6969 
                 12.22 
                 71 
               
               
                   
                 17.1650 
                 2.6099 
                 9.81 
                 57 
               
               
                   
                 17.5700 
                 2.5516 
                 8.78 
                 51 
               
               
                   
                 17.7600 
                 2.5252 
                 9.81 
                 57 
               
               
                   
                 18.0750 
                 2.4826 
                 11.36 
                 66 
               
               
                   
                 18.5150 
                 2.4256 
                 8.43 
                 49 
               
               
                   
                 18.7650 
                 2.3944 
                 13.08 
                 76 
               
               
                   
                 19.0450 
                 2.3605 
                 11.36 
                 66 
               
               
                   
                 20.1950 
                 2.2312 
                 50.09 
                 291 
               
               
                   
                 20.6150 
                 2.1877 
                 9.12 
                 53 
               
               
                   
                 21.0550 
                 2.1440 
                 12.56 
                 73 
               
               
                   
                 21.4650 
                 2.1049 
                 9.29 
                 54 
               
               
                   
                 21.6850 
                 2.0846 
                 9.98 
                 58 
               
               
                   
                 22.0600 
                 2.0509 
                 9.81 
                 57 
               
               
                   
                   
               
             
          
         
       
     
     Wavelength: 1.54051 Å 
     After filtration and washing in water, the reddish-brown- or mahogany-colored crystals turn to grayish-brown- or anthracite-colored crystals.