PHOSPHONATES

Abstract: The stability of aqueous solutions of diethylene triamine penta (methylene phosphonic) acid, comprising at lest 20% by weight of active material, is improved if the acid is partially neutralised. The major part e.g. 55 to 85% by weight of the acid is preferably diethylenetriaminepenta (methylenephos­ phonic) acid itself. The preferred products are derived by neutralising from 10 to 30% of the replaceable hydrogen with sodium potassium or ammonium ions. The salt solutions are less corrosive than known products to which mineral acid has been added.

Description: This invention relates to aqueous solutions of partially neutralised phosphonic acids which exhibit improved long term storage stability, to processes for their preparations and to processes for stabilising solutions of phosphonic acids. The phosphonic acids are useful as sequestrants for metal ions and as scale inhibitors when added in threshold quantities to aqueous media. The amino-methylene phosphonic acids are established articles of commerce. They may be manufactured by the reaction of the appropriate polyamine with phosphorous acid and formaldehyde. The composition of these acids can be expressed as that of acids having the general formula 1 EMI1.1 wherein n is zero or an integer having a value of from 1 to 5. The commercial products sold as amino-methylene phosphonic acid may comprise compounds having the above general formula or they may comprise those compounds in admixture with other less highly substituted phosphonic acids, i.e. those wherein at least one of the groups -CH2P03X2 is replaced by a hydrogen atom or a methyl group. The term amino methylene phosphonic acids as used in this specification encompasses these mixtures as well as the pure compounds. These acids may be prepared as crystalline solids or as solutions in an appropriate solvent which is usually water. The less sol-uble actds.may#be prepared as solids and dissolved if appropriate to a particular application. The more soluble acids are usually manufactured and sold in the form of concentrated aqueous solutions. These concentrated solutions suffer to varying degrees from a tendency of the acid to precipitate crystals of acid which detracts from their long tern storage stability and is thereby disadvantageous. This invention is particularly concerned with solutions comprising di ethyl enetri ami nepenta (methyl enephosphoni c acid) i.e. the phosphonic acid of the above formula 1, wherein n has a value of 2. This acid has been manufactured in the form of relatively concentrated aqueous solutions thereof. Such solutions areknown to suffer from a particular tendency to precipitate solid material or to solidify completely upon long term storage. European Patent 47150 discloses that this tendency is reduced by the addition of at least 10% by weight of the solution of a mineral acid which is preferably hydrochloric acid. This addition of acid material is disadvantageous in that the resulting product is highly corrosive and also in that when the product is neutralised prior to its use, an additional quantity of base is required to neutralism the mineral acid which has been added. The solutions comprise di ethyl enetri ami nepenta (methyl ene- phosphonic) acid alone or more usually a mixture of this acid with other less highly substituted phosphonic acids as mentioned above particularly diethyl enetri ami netri (methyl enephosphonic acid). The penta methylene phosphonic acid may provide a major or minor proportion of the phosphonic acids which are present. The solutions which are manufactured as commercial products will normally comprise a mixture of phosphonic acids. The preferred solutions according to the present invention are those wherein at least 25L by weight and more preferably at least 40 or 50L of the total weight of phosphorus containing species takes the iorm of diethylene triamine penta (methylene phosphonic acid). Most preferably from 55 to 85%, e.g. from 60 to 80% by weight of the phosphonic acid present is present as diethylenetriaminepenta (methylene phosphonic acid). In the case of products which comprise a mixture of phosphonic acids the composition of the acid may be assessed by titration for complexing power for metal ions. The result is conventionally expressed in terms of the amount of the pure acid which would give the identical titration result. This method of analysis produces a figure for the concentration of phosphonic acid which is commonly only 90-99E of the actual value. Alternatively, the phosphorus content of the composition may be determined and the amount of phosphonic acid calculated on the basis that all the phosphorus is present as the pure penta(methylenephosphonic acid). This latter convention will be adopted for the purpose of the present disclosure. All statements set out herein regarding eg the degree of neutralisation of the acid are put forward on the basis of this convention. The statements regarding the number of replaceable hydrogen atoms which are neutralised are based upon the assumption that the only phosphonate species present is the penta(methylenephosphonic acid). For the purpose of this diclosure the term diethylene triamine penta(methylenephosphonic acid) will be use to describe this acid and mixtures thereof with other phosphonic acids which are derived from diethylene triamine except where specifically indicated. We have now dicovered that certain salts of diethylene triamine penta(methylenephosphonic acid), wherein at least 10% and not more than 80% of the replaceable hydrogen atoms have been replaced by a water soluble cation, exhibit improved storage stability as compared to the free acid. Accordingly from one aspect of our invention provides an aqueous solution comprising at least 20% by weight of a salt of diethyl-enetriaminepenta(methyl enephosphonic acid). wherein at least 10% and not more than 80% of the replaceable hydrogen atons have been replaced by a water soluble cation. The adoption of the convention referred to above provides an alternative method of defining the quantity of base which is present in the solutions of the invention. The molar quantity of a monovalent cation which need be included in a particular phosphonic acid solution in order to produce a solution according to the invention will be at least 1/5 the molar quantity of phosphorus present in the solution. Where a multi valent cation is present the proportion of cations will be reduced in proportion to the valency of the cation e.g will be halved in the case of a di-valent cation. The salts of the present invention are most conveniently formed by adding the appropriate quantity of a base to a solution of the phosphonic acid in water. Bases which are appropriate for this purpose are basic derivatives of the alkali metals, e.g. sodium potassium and ammonium, the alkaline earth metals, e.g. magnesium and calcium, ammonium and organic amines, e.g. ethanolamines, propanol amines and isopropylamine. Mixtures of these bases may be empl oyed. The preferred bases for addition to the acid are sodium, potassium and ammonium hydroxide or mixtures of these bases. The most preferred base is sodium hydroxide. The quantities of base needed to produce a product having the desired degree of storage stability varies with the concentration of the phosphonic acid solution, the composition of the phosphonic acid solution and the nature of the base which is added. In general, we prefer to add sufficient base to provide a solution wherein from 10 to 30% preferably from 15 to 25%, e.g. from 18 to 22% of the replaceable hydrogen atoms represent water soluble cations. The precise preferred quantities which will provide acceptable stabilities may be determined empirically for each acid solution and each base additive. In general ,where an amnonium compound is used as the base a quantity of base sufficient to provide that at least 15 and preferably at least 20E of the replaceable hydrogen atoms represent ammonium cations pffll be added. Where the base used is a derivative of potassium, the quantity of base added is preferably such thatat least 10% but no more than 25% and more preferably no more than 20% of the replaceable hydrogen atoms are replaced by potassium cations. Where the base used is a derivative of sodium, the quantity of base added is preferably such that at least 10% and preferrably from 15 to 25% of the replaceable hydrogen atoms are replaced by sodium ions. The more highly neutralised ammonium salts, i.e. the:tetra ammonium salt wherein 40% of the replaceable hydrogen atoms are replaced by ammonium cations and more highly neutralised salts are less preferred for most purposes. Nevertheless the stability of the more completely neutralised ammonium salts, i.e. those wherein the molar ratio of ammonium ions to phosphorus atoms in the acid solution is at least 4:5 and in the range of 4:5 to 8:5 may be advantageous in some circumstances and solutions of this type form a further aspect of our invention. Generally the preferred solutions are those where the degree of neutralisation is lower. In the case of ammonium and especially sodium and potassium the preferred ratio of cations to phosphorus atoms is in the range of 1:5 to 3:5. In the case of the sodium and potasssium ions in particular these salts exhibit improved stability as compared to salts having a higher or a lower degree of neutralisation. In the case of potassium salts the more highly neutralised salts of the more concentrated acids are less storage stable and solid material may be deposited upon long term storage. Thus the preferred ratio of potassium ions to phosphorus atoms in the novel solutions is from 2:5 to 3:5 this proportion preferably being present in a solution containing from 30 to 45% preferably from 35 to 40E by weight of active material. In the preferred case of the sodium salts the preferred ratio of sodium ions to phosphorus atoms is also from 2:5 to 3:5. However the sodium salts are generally more stable than the corresponding potassium salts and more concentrated phosphonic acid solutions e.g those comprising from 40 to 50% by weight of phosphonate will preferably be stabilised by the addition of sodium hydroxide. Equally if it is desired to add a minimum quantity of base to produce a stable solution sodium hydroxide will generally be preferred a proportion of from 1:5 to 2:5 (as molar ratios of sodium ions to phosphorus atoms) being sufficent to give improve stability especially at acid concentrations of from 30 to 45% by weight of phosphonate. The base can be added to a concentrated solution of the acid, i.e. those solutions which are available as commercial products which comprise, e.g. from 25 to 35 or 45X by weight of active material. More conveniently the base can be added to the highly concentrated phosphonic acids Which are produced in the conventional manufacturing processes as hereinafter described. In this embodiment the selection of an aqueous solution of the base of the necessary concentration enables the phosphonic acid to be neutralised and diluted in a single operation. The process of stabilising a diethylenetriaminepenta (methylenephosphonic acid) which comprises the addition of an appropriate quantity of a base so as to replace at least 10% and not more than 80E of the replaceable hydrogen ions by a water soluble cation is believed to be novel and constitutes a further aspect of the invention. These processes are carried out by adding a sufficent quantity of a base to ensure that the molar ratio of the cation of that base to phosphorus atoms in the solution is at least 1:5, and not more than 4:5. The process of stabilising the acid solution by the neutralisation thereof may conveniently be carried out by adding a solution of a base thereto until the pH of the solution rises to a pre-determined level. The optimum level will vary with the composition and concentration of the acid and to a lesser degree with the nature of the base, the temperature of the solution and the instrument used to monitor the pH. Consequently the optimum pH for a particular product must be determined by experiment and the neutralisation then carried out until that pre-determined pH is reached. The present invention is most useful when applied to acid solutions which would otherwise tend to be unstable upon prolonged storage at ambient temperatures in order to improve the stability thereof. In the case of diethylenetriaminepenta(methylene- phosphonic acid) the phosphonic acid solution may contain at least 30 and preferably at least 40, 45, or 50% of active material. The phosphonic acid solutions may be produced using the conventional synthetic techniques. For example they can be produced by the reaction of the appropriate polyamine with phosphorous acid and formaldehyde in an aqueous reaction medium in the presence of a strong acid catalyst, e.g. hydrochloric acid. This reaction is carried out at elevated temperatures usually at least 900C. The reaction medium may conveniently be maintained under gentle reflux for the course of-the reaction. At the end of the reaction the product will comprise a significant quantity (usually about 12 to 13% by weight) of hydrochloric acid. Some of this hydrochloric acid may be removed, e.g. by distillation of the reaction product although removal of all the hydrochloric acid is impractical and usually unnecessary. The removal of this hydrogen chloride also removes water from the phosphonic acid product. If the chloride content is reduced to less than 5% which is the preferred level the concentrated acid product will contain relatively small quantities of water and will normally be diluted to form a solution containing the preferred amounts of active material, e.g. 20 or 30 to 55% by weight of active material. As previously mentioned, this dilution and the partial neutralisation may be carried out simultaneously by diluting the concentrated acid with the appropriate quantity of an aqueous solution of a base of appropriate concentration. The product will comprise a mixture of phosphonic acids derived from the polyamine. Usually at least 60%, more usually 70 or 85% of those phosphonic acids will take the form of the fully substituted phosphonic acid, i.e diethylenetriaminepenta(methylenephosphonic acid) itself. The commercial phosphonic acid products also typically comprise a number of impurities. In particular they commonly contain significant quantities of Chloride ion. This can be stripped out as hydrogen chloride and we prefer to utilise acids which contain less than 5% by weight of chloride ion. Solutions containing a greater proportion of chloride ion can be used (e.g. up to 10% by weight) although the quantity of base needed to achieve optimum stabilisation will increase in proportion to the amount of chloride ion which is present, because of the need to neutralise the hydrogen chloride present in the solution. The invention is illustrated by the following Examples: Example 1 a) Preparation of diethyl enetriaminepenta(methyl enephosphonic acid) (AMP acid ] ) 808 parts by weight of a 36% aqueous solution of formaldehyde was added to a refluxing mixture comprising 195 parts of diethylene triamine, 780 parts of phosphorous acid and 966 parts of a 36% aqueous solution of hydrochloric acid. The temperature of the mixture was maintained within the range 1050C to 1150C throughout this addition. Thereafter the reaction mixture was maintained under gentle reflux for a period of one hour. The temperature of the reaction mixture was then raised to 1250C and aqueous hydrochloric acid was distilled off. The product was diluted with 730 parts of water to give 2007 parts of an AMP phosphonic acid solution containing 54% by weight of active material (estimated by phosphorus analysis. This solution contained approximately 5% by weight of chloride ion. b) Neutralisation of AMP acid 28.0 parts of 47% sodium hydroxide solution was added to 100 parts of the AMP acid prepared above with thorough mixing. The resulting solution was heated so as to evaporate water therefrom until 120.0 parts of a product remained. This - product contained 45% w/w of AMP acid (estimated on the assumption that all phosphorus is present as D5A or salts thereof). This product had a pH of 1.2 and was stable upon prolonged storage. Example 2 Preparation of disodium salt of diethylenetriamine pentamethylene phosphonic acid 808 parts by weight of a 36% aqueous solution of formaldehyde was added to a reflexing mixture comprising 195 parts of diethylene triamine, 780 parts of phosphorus acid and 966 parts of a 36X aqueous solution of hydrochloric acid. The temperature of the mixture was maintained within the range 1050C to 1150C throughout this addition. Thereafter the reaction mixture was maintained under gentle reflux for a period of two hours and distilled up to a temperature of 128or, Much of the water and hydrochloric acid was therefore removed. 600 parts of water and *475 parts of 47% caustic soda were then added to the cooled solution to yield 2370 parts of product after filtration. The product contained 45% parts w/w of AMP and (estimated by phosphorus analysis) the product had a pH of 1.2 and was stable upon prolonged storage. * This caustic soda solution is sufficient to form the sodium salt of diethlyene triamine penta (methylene phosponic) acid itself and neturalise residual hydrochloric acid from the preparation. Example 3 The stability of the phosphonic acid solution formed as the product of Example 1 (a) and a series of partially neutralised salts obtained by the addition of various bases to it was compared. The basic derivatives were prepared by adding the calculated quantity of base required to neutralise the free hydrochloric acid in the phosphonic acid solution and then enough base to from solutions of the mono, di, tri and tetra sodium and potassiumsalts containing 20, 30, 40 and 50% by weight of phosphonate. The instability of- the phosphonic acid was demonstrated by seeding the solution with solid acid and allowing the solution to stand at ambient temperature. The solution slowly solidified over a period of a few days. The storage stability of the partially neutralised salt solutions was assessed by allowing them to stand at ambient temperature and by storing them at 00C for a period of six months. The results of these tests are present in the following Tables: (a) Tests at ambient temperatures (i) Sodium Salts EMI11.1 (if) Potassium Salts Acid Concentration Mono Di Tri Tetra 20 < --------- Soluble ---------- > 30 f Soluble 40 < ------ Soluble ) 50 Soluble Crystals Crystals formed formed in 4 days immediately on cooling to ambient (b) Tests at 00C (i) Sodium Salts Acid Concentration Mono ¯ Di Tri Tetra 20 C Soluble ) 30 < --------- Soluble ---------- > 40 <RTI ID=12.8> < --------- Soluble ---------- > 50 Crystals < -- Soluble -- > Crystals after 3 in -12 days Months (ii) Potassium salts Acid Concentration Mono Di Tri Tetra 20 < --------- Soluble ) 30 < --------- Soluble ---------- > 40 < -----Soluble----- > Crystals al ready present EMI13.1 50 Crystals Crystal Crystals formed formed al ready after 3 in 4 days* present Months * these crystals dissolve when the solution was allowed to warm to ambient temperatures.

Claims: Claims 1. An aqueous solution comprising at least 20% by weight of a salt of di ethyl enetrimainepenta(methyl enephosphonic) acid wherein at least 10% and not more than 80% of the replaceable hydrogen atoms have been replaced by a water soluble cation which is stable upon storage at ambient temperature. 2. A solution according to claim 1, characterised in that the water soluble-cation is an alkali metal, an alkaline earth metal, an organic amine or an ammonium ion. 3. A solution according to claim 2, characterised in that the cation is either a sodium or a potassium ion or a mixture of these ions. 4. A solution according to claim 2, characterised in that the cation is an ammonium ion 5, A solution according to claim 4, characterised in that at least 15% of the replaceable hydrogen atoms have been replaced by ammonium ions. 6. A solution according to claim 5, characterised in that no more than 40S of the replaceable hydrogen atoms have been replaced by ammonium ions. 7. A solution according to any of claims 1, 2 and 3 characterised in that from 10 to 30E of replaceable hydrogen atoms have been replaced by water soluble cations. 8. A solution according to claim 7, characterised in that from 15 to 25% of the replaceable hydrogen atoms have been replaced by water soluble cations. 9. A solution according to any of the preceeding claims, characterised in that it comprises at least 30% by weight of phosphonic acid. 10. A solution according to claim 9, characterised in that it comprises at least 40% by weight of phosphonic acid. 11. A solution according to any of claims 1 to 10, characterised in that at least 25% by weight of the total weight of phosphorus containing species in solution is diethylene- triaminepenta(methylenephosphonic acid). 12. A solution according to claim 11, characterised in that at least 50X by weight of the total weight of phosphorus containing species is diethylenetriaminepenta(methylene phosphonic acid). 13. A solution acording to claim 12, characterised in that from 55 to 85% of the total weight of phosphorus containing species is diethylenetriaminepenta(methylenephosphonic acid). 14. A solution according to any of claims 1 to 13, characterised in that the phosphonic acid solution is produced by the reaction of diethylene triamine with formaldehyde and phosphorous acid in the presence of hydrochloric acid. 15. A solution according to claim 14, characterised in that hydrogen chloride and water are removed from the reaction product. 16. A solution according to claim 15, characterised in that the chloride content of the phosphonic acid solution is less than 5% by weight 17. A method for stabilsing a solution of dlethylenetrlamlnepenta- (methylenephosphonic acid) -which comprises adding a quantity of a base thereto which is sufficient to replace at least 10X of the replaceable hydrogen ions by water soluble cations. 18. A method according to claim 17 characterised in-that the product is a solution according to any of claims 1 to 16.

Inventors: HARGREAVES ROGER GEOFFREY, JONES TREVOR ERIC, HARGREAVES, ROGER GEOFFREY, JONES, TREVOR ERIC
Applicants: ALBRIGHT & WILSON LIMITED
Application year: 1984
Main classification: C07F9
IPC codes: C02F5, C07F9