METHOD FOR DETERMINING CHLORIDE

Abstract: A method for determining chloride by adding an excess amount of a water-soluble silver salt to a chloride-containing sample, filtering off a precipitate of produced silver chloride, titrating silver in the filtrate with a thiocyanate aqueous solution in the presence of a titration indicator is disclosed, which comprises using a cobalt compound and a coloration sensitizer as titrating indicators. When a cobalt compound and a coloration sensitizer are used as indicators, color of the sample solution changes from pink to red upon completion of titration, which makes it possible to accurately know the end point of titration even with samples of yellowish brown to red color. Therefore, this method is particularly suited for inspection of plasma, infusion, urine, etc.

Description: Technical Field This invention relates to a method for the quantitative determination of chlorides. Particularly, it is concerned with a method for detecting and quantitatively determining chlorides in samples including specimens such as plasma, transfusions and urine. Background Art One of the known methods for the quantitative determination of chlorides or chloride ions is the Volhard method, according to which a predetermined excess amount of silver nitrate is added to a sample solution containing chloride ions to form white precipitates of silver chloride, which is separated by filtration, and the excess silver ions remaining in the filtrate is titrated with a thiocyanate solution using Fe³⁺ as an indicator. Concentration of the chlorides or chloride ions is calculated from the amount of the silver nitrate and that of the thiocyanate consumed, wherein the point at which ferric thiocyanate (ferric rhodanate) formed after the consumption of the silver develops a color (yellow - red) is set as the end point of the titration. In this method, if a specimen is in a color range of yellowish brown to red, the above-mentioned change to a red color was not clear enough to detect the end point of titration. Disclosure of the Invention It is an object of the present invention to provide a method for quantitatively determining chlorides in a sample with accuracy even if the test material is in a color range of yellowish brown to red, wherein the end point of titration is made distinct, thus solving the prior art problems. According to the invention there is provided a method for the quantitative determination of chlorides as set forth below. 1) A method for the quantitative determination of chlorides which comprises adding an excess amount of a water-soluble silver salt to a chloride-containing specimen, filtering off precipitates of the silver chloride thus formed and titrating the silver in the filtrate with an aqueous solution of a thiocyanate in the presence of a titration indicator, characterized by using a cobalt compound as a titration indicator and a color sensitizer. 2) A method according to item 1 wherein the cobalt compound is cobalt(II) nitrate. 3) A method according to item 1 wherein said water-soluble silver salt is a strong electrolyte. 4) A method according to item 3 wherein said strongly electrolytic water-soluble silver salt is silver nitrate. 5) A method according to item 1 or 2 wherein the color sensitizer is a quaternary ammonium salt, methanol, ethanol, acetone, polyvinyl pyrrolidone or a phosphate compound. 6) A method according to item 3 wherein the quaternary ammonium salt is tetrabutylammonium nitrate. 7) A method according to item 5 wherein the phosphate compound is tributyl phosphate. As a chloride-containing specimen used in the invention are mentioned, for example, plasma, plasma substitutions, amino acid and/or vitamin-containing transfusions, beverages such as fruit juice, wine and beer, solutions of a solid food, urine and other test materials. The method of the present invention is effective especially for specimens such as vitamin B₁ or like substance-containing transfusions and urine with which the color reaction was hardly detectable by prior-art methods such as the Volhard method. As examples of the chloride contained in these liquid specimens are mentioned sodium chloride, potassium chloride, magnesium chloride, calcium chloride and the like. Measurement of these chlorides is usually carried out for the assay of components of various pharmaceutical preparations. In carrying out the present invention, first, a predetermined excess amount of a water-soluble silver salt is added to a sample solution to form white precipitates of silver chloride, which are then filtered off. To the filtrate are added Co²⁺ as a color indicator and a color sensitizer, and then, a thiocyanate solution is added thereto until reaching the end point of titration when color of the solution is changed from pink to blue. The water-soluble silver salt includes silver nitrate, silver acetate, silver malonate, silver perchlorate and the like, silver nitrate being most preferable. Cobalt(II) nitrate is a suitable Co²⁺-providing cobalt compound, which is used in the form of an aqueous solution in a concentration of usually 0.1-1.0%, and preferably 0.2-0.6%. The present invention is characterized by using a color sensitizer together with a cobalt compound as a titration indicator. As the color sensitizer are mentioned quaternary ammonium salts, methanol, ethanol, acetone, polyvinyl pyrrolidone and phosphate compounds. The quaternary ammonium salts include aliphatic quaternary ammonium salts such as tetraalkyl ammonium salts containing four or more carbon atoms, for example, tetrabutylammonium nitrate, tetrabutylammonium perchlorate and tetrabutylammonium borofluoride. Aromatic quaternary ammonium salts and heterocyclic quaternary ammonium salts may also be used. The phosphate compounds include triethyl phosphate, tributyl phosphate, trioctyl phosphate, tributoxyethyl phosphate and the like. Such color sensitizer is used in the form of an aqueous solution desirably in an amount so as to provide a molar concentration equal to that of Co²⁺ ion. The thiocyanate used for the titration of silver in the present invention may be, for example, ammonium thiocyanate, potassium thiocyanate, sodium thiocyanate and the like. Silver ions remaining in the filtrate after filtration of the silver-chloride precipitates are precipitated with an aqueous solution of the above-mentioned thiocyanate to form white precipitates of silver thiocyanate. After the silver ions have entirely been consumed, excess SCN⁻ and Co²⁺ of the color indicator are reacted to form cobalt thiocyanate, which in turn forms a complex with the color sensitizer and develops a blue color. Concentration of the chlorides can be calculated from respective amounts of the aqueous solutions of silver nitrate and thiocyanate dropped. The invention will be described below in more detail with reference to an example and a comparative example. Example 1 To 10 ml of a specimen was added 0.4 ml of concentrated nitric acid. To the mixture was dropwise added an aqueous solution (0.1 N) of silver nitrate to precipitate all of the chloride ions as silver chloride. The silver chloride precipitates were filtered off. To a 5-ml aliquote of the filtrate were added 150 mg of tetrabutylammonium nitrate and 10 mg of cobalt(II) nitrate (hexahydrate). Excess silver ions were back titrated with a standard solution of ammonium thiocyanate (0.05 N). The area to which the solution has been dropwise added turns blue, but the color desappears when stirred. It is set as the end point when the blue-colored precipitates no longer disappear as it is stirred. NaCl concentration (%) in the specimens was calculated from the dropped amount of the aqueous solution of silver nitrate a (ml) and the dropped amount of the standard solution of ammonium thiocyanate b (ml) using the following equation. [10a - b(a + 10.4)] x 5.85 x 10⁻³ (%) The results were as follows. Comparative Example 1 A similar experiment was made using a wine as specimen and ferric ions (Fe³⁺). As color of the wine and the color changed at the end point of titration were similar, determination of the end point was difficult. When Co²⁺ and a sensitizer were used together, the color change area was differentiated to allow the easy determination of the end point. Industrial Applicability According to the present invention, there is provided a method for the quantitative determination of chlorides or chloride ions in chloride-containing specimens. In the method of the invention, use of Co²⁺ as a titration indicator in place of the Fe³⁺ in the prior Volhard method together with a color sensitizer allows to give a blue coloration. Consequently, when the the quantitative determination is conducted with a specimen having a color range of yellowish brown to red, the end point of titration can more easily be determined than in the prior art methods wherein change from a yellow color to a red color is judged. The quantitative assay of the present invention is used for the quantitative determination of chlorides in plasma, plasma substitutions, amino acid- and/or vitamin-containing transfusions, beverages such as fruit juice, wine and beer, solutions of a solid food, urine and other test materials. Therefore, the invention is utilizable in the fields of medical industry and food industry.

Claims: A method for the quantitative determination of chlorides which comprises adding an excess amount of a water-soluble silver salt to a chloride-containing specimen, filtering off precipitates of the silver chloride thus formed and titrating the silver in the filtrate with an aqueous solution of a thiocyanate in the presence of a titration indicator, characterized by using a cobalt compound as a titration indicator and a color sensitizer. A method according to Claim 1 wherein the cobalt compound is cobalt(II) nitrate. A method according to Claim 1 wherein said water-soluble silver salt is a strong electrolyte. A method according to Claim 3 wherein said strongly electrolytic water-soluble silver salt is silver nitrate. A method according to Claim 1 or 2 wherein the color sensitizer is a quaternary ammonium salt, methanol, ethanol, acetone, polyvinyl pyrrolidone or a phosphate compound. A method according to Claim 3 wherein the quaternary ammonium salt is tetrabutylammonium nitrate. A method according to Claim 3 wherein the phosphate compound is tributyl phosphate.

Inventors: MORIKAWA NAOKI TERUMO KABUSHIK, SUZUKI HIROSHI TERUMO KABUSHIK, MORIKAWA, NAOKI TERUMO KABUSHIKI KAISHA, SUZUKI, HIROSHI TERUMO KABUSHIKI KAISHA
Applicants: TERUMO CORP, TERUMO KABUSHIKI KAISHA
Application year: 1988
Main classification: G01N31
IPC codes: G01N31, G01N33