AU4754700A - Method for producing mononatrium trihydrogen pyrophosphate - Google Patents

Abstract

The invention relates to a novel method for producing mononatrium trihydrogen pyrophosphate and to the use of products produced by said method as acidifiers, buffering agents and complexing agents. The novel method is characterised in that glassy, acidic natrium polyphosphate containing a Na/P ratio of 1: 2 in fine powder form is tempered at temperatures ranging between 100 and 200 DEG C in moist air with a steam pressure of between 10 and 200 mBar, or in a polar solvent, in which the phosphate is insoluble, in the presence of a quantity of water sufficient for the hydrolysis, until the mass has been predominantly converted into NaH3P2O7.

Description

This is an authenticated translation from the German language Process for the synthesis of monosodium trihydrogenpyrophosphate The subject of the present invention is a new process for the synthesis of monosodium trihydrogenpyrophosphate and the application of products so made as acidifiers, buff 5 ers and complexing agents. The utilisation of alkaline phosphates and especially sodium phosphates in the food, textile and paper industries as acidifiers, buffers and complexing agents is widespread. In order to achieve reproducible applications, homogeneous crystalline salts are pref 10 erably employed. A preferred group comprises the diphosphates or pyrophosphates with the formula MeH 4

..P

2 0 7 , which crystallise well and are highly soluble, relat ively speaking. Frequently, metaphosphates or polyphosphates are also utilised, such as Maddrell's salt, Kuroll's salt or Graham's salt (acid phosphate glass), which can be made by the dehydration of suitable solutions of sodium hydroxide or sodium carbon 15 ate and phosphoric acid. However, these frequently have reduced solubility and, espe cially due to the synthesising process, differing properties and compositions, so that the diphosphates are advantageous in many cases. Diphosphates in technical use include, principally, tetrasodium pyrophosphate 20 Na 4

P

2 0 7 (synthesised from disodium phosphate Na 2

HPO

4 by calcination at tempera tures of between 3000 C and 900* C) and the acid diphosphate Na 2

H

2

P

2 0 7 (obtainable from monosodium phosphate NaH 2

PO

4 by heating to 200-250* C). Relatively rarely, the phosphate Na 3

HP

2 0 7 (from a mixture of Na4P 2 0 7 and Na 2

H

2

P

2 0 7 sprayed with water) is used. NaH 3

P

2 07 has not previously been used technically, since a practicable 25 process for its synthesis has been lacking, although it should rable due to its greater acidity. U1a nau This is an authenticated translation from the German language 2 The following synthesis procedures are described in the literature, although they only produce impure mixtures. Giran (C.r. 134 [1902] 1500): A mixture of Na 2

H

2

P

2

O

7 and pyrophosphoric acid 5 H 4

P

2 0 7 (in excess) is heated for several hours at 1000 C. The paste is then spread be tween clay plates and heated in vacuo over P 2 0 5 to 110-120* C and then pressed 5-6 times between clay plates to remove completely the surplus acid. The product is a fri able, highly deliquescent substance, whose chemical composition is given by the for mula NaH 3

P

2 0 7 . Other properties, and especially a physical-chemical characterisation 10 of the product, are not given. Details of the conversion equation and the yield are also lacking. A disadvantageous factor in this process is the use of pyrophosphoric acid (H4P 2 0 7 ), since its production and handling is difficult and significant amounts of residue remain in the product. 15 A. Winkler, H. Hofsass, E. Thilo (Z. Anorg. Allgem. Chem. 306 [1960] 317/32, 329): on heating a mixture of Na 2

H

2

P

2

O

7 and H 3

PO

4 (molar ratio 90:10) at temperatures of 160-225* C and a H 2 0 vapour pressure of about 17 or alternatively 300 tor, mixtures of Na 2

H

2

P

2 0 7 , H 4

P

2 0 7 and NaH 3

P

2

O

7 form which on further heating are converted to Maddrell's salt - i.e. high molecular weight phosphate. 20 The following properties of the NaH 3

P

2

O

7 fraction are described: * the substance cannot be extracted with dioxan * it contains strongly acidic OH groups e it is only stable at a certain minimum water vapour pressure, which prevents its 25 further condensation to higher molecular weight compounds * yield: the NaH 3

P

2

O

7 quantity corresponds to about 10 % of total P content "W M1#f Und be. "91otr Urkurnd-.nubeo Spift 8?3sn _a7 & / 6' 1mbr This is an authenticated translation from the German language 3 0 since pure NaH 3

P

2 0 7 was not obtained, details of further chemical properties and analytical data for the substance are likewise absent here. A. Norbert, C. Dautel, C.R. Acad. Sc. Paris, 1, 262 (23.5.1966), p. 1534-1536: an 5 aqueous solution of diphosphoric acid H 4

P

2 0 7 and Na 2

H

2

P

2 0 7 in equimolar proport ions is slowly dried in a vacuum in the presence of phosphoric acid anhydride. The reaction temperature should be maintained at about 0* C in order to avoid hydrolysis of P 2 0 7 2 - to P043~. Following a reaction time of several days, a hard, white, but readily pulverised product is obtained, which is weakly hygroscopic (melting point 185-200* 10 C). The product, which can be characterised by X-ray diffraction analysis (the Debye Scherrer process), differs from mixtures of orthophosphate and polyphosphate of equivalent composition and can therefore be regarded as mainly pure NaH 3

P

2 0 7 . The NaH 3

P

2 0 7 decomposes in organic solvents such as alcohol, acetone and ether. A dis 15 advantageous factor, apart from the use of pyrophosphoric acid (H 4

P

2 0 7 ) is, above all, the costly drying over P 2 0 5 , which makes the product impracticable for technical uses. A. Norbert et al., C.R. Acad. Sc. Paris, 1. 267 (04/11/1968), p. 1237 - 1239 also at tempted to obtain the compound from these components at higher temperatures (1000 20 C - 125* C), but obtained a mixture of orthophosphate and pyrophosphate with the formula Na 2

H

8

(PO

4

)

2

P

2 0 5 , which could also be characterised by its diffraction pattern. The aim is therefore to find a process for the synthesis of NaH 3

P

2 0 7 , which permits a relatively high yield while also utilising relatively easily accessible starting materials. 25 Surprisingly, the acidic polyphosphate glass (NaxHy(PO 3 )), which is obtained through the fusing of a mixture of sodium phosphate and phospho lar ratio of 1:2 at ,*j~te n be if ~Ba~~ , ni ' This is an authenticated translation from the German language 4 a temperature of over 300* C, represents such a starting material, since with a purely thermal process in which the acidic polyphosphate glass is tempered under water vap our pressure at temperatures of 1000 C - 200* C, it is converted by hydrolysis to NaH 3

P

2 0 7 . 5 The tempering is preferably carried out at 110 - 180* C and especially at 110 - 130* C, whereby the water vapour pressure over the finely powdered starting material is 30 to 100 mbar, and especially 35 to 50 mbar. 10 The reaction is continued until the reaction product consists mainly of NaH 3

P

2

O

7 - i.e. it contains less than 20 %, and preferably less than 10 % polyphosphate, and mono phosphate formed by excessive hydrolysis. Depending on the reaction temperature, reaction times of 5 to 100 hours are necessary. On tempering in a solvent such as diglyme or dioxan, with temperatures of 50 to 150* 15 C, and preferably 90 to 1200 C, and reaction times of 0.5 to 10 hours, good yield re sults are obtained. The acidic pyrophosphates NaH 3

P

2 07 (I) according to the invention represent to a certain extent a solid form of the diphosphoric acid H 4

P

2 0 7 (II), since they have an 20 acid hydrogen atom with an acidity comparable with (II). This results in applications as an acidifier, for instance in the food, textile and paper industries. At a pH value of about 2, (I) can both accept and release protons, making possible its use as a buffer. Generally speaking, this phosphate can be used in the production of other phosphates, 25 as a medium for stabilising emulsions and suspensions, as a catalyst for polymerisat ion and hydration, as an etching reagent for semiconductors, for the manufacture of fireproof materials, flameproofing materials and piezoelectric beatenter und be "Jqtgaf urkundenabef sezrder engIischen~ g Sprach te Baden &~WuIQbI This is an authenticated translation from the German language 5 Like all diphosphates, (I) complexes divalent and trivalent metals, although because of its higher phosphate content it has better bonding properties than these. The pyrophosphate (I) possesses preservative and bacteriostatic properties due to its acidifying effect. 5 It has stabilising properties with respect to vitamin C, debitterises the flavour of foods with potassium chloride added and prevents discoloration through enzymatic or non enzymatic browning (e.g. in fruit and vegetable juices, in soya and fish sauces, in po tatoes and other food products), so that it represents a favourable food additive. (I) decomposes in the presence of water to monophosphates and is therefore not toxic. 10 The following examples serve to illustrate its manufacture. Example 1: A laboratory drying oven from the firm of Heraeus was used. The samples of finely 15 pulverised acid polyphosphate were tempered for varying periods at 1100 C at normal pressure and under an appreciable water vapour partial pressure of about 45 mbar. The water vapour pressure was provided with well dampened silica gel (mass 10 times relative to acid polyphosphate), also in the drying oven under the same conditions. To achieve an even distribution, the acid polyphosphate powder was sprinkled onto flat 20 porcelain dishes with a sieve. The following tempering times were used: 2 hr, 1 hr, 3 hr and 5 hr, and 1, 3 and 5 days Even after short tempering times, distinct changes can be observed. Due to H 2 0 absorption from the atmosphere, the grain surfaces "deliquesce" and the 25 powder agglomerates. On cooling the samples in a desiccator, the lump hardens, al though it can be readily ground again to powder. Analytical results from the various samples are given in the following table. The distrib e various polymerisat ion stages is shown in Fig. 1. lObnaue This is an authenticated translation from the German language 6 Table 1: Results of analytical investigation of the tempered samples Sample Tempering pH 1% Solubility Hygroscopicity at 65% NMR polym time at 1100 C solution 1% solution relative hu idity after erisation level (TE/F) after 5 hr 24 hr n 30 min wt. (%) wt. (%) Wu 2455- untreated test 1.76 1.28 1.65 5.38 30.5 2456 sample ASPP1/5 /2 hr 1.69 1.23 0.52 3.31 22.4 ASPP1/6 1 hr 1.69 1.46 0.76 3.39 23 ASPP1/7 3 hr 1.71 1.08 1.78 4.97 10.9 ASPP1/8 5 hr 1.71 1.48 1.88 5.08 6.5 ASPP1/1 1 day 1.65 0.80 0.75 2.79 4.7* ASPP1/3 3 days 1.65 0.48 0.42 1.06 3* ASPP1/2 7 days 1.68 0.41 0.29 0.44 2.6* * These polymerisation levels are largely determined by the high concentration of 5 diphosphate. The results of the 3 P-NMR measurements show that even after short tempering times, significant break-down of the polyphosphate chains takes place, and this continues with increasing tempering of the samples. It can be seen from the spectrum that, apart 10 from a slightly raised proportion of monophosphate, the main constituent is diphos phate. The signal for the medium phosphate groups has reduced markedly, while a strong signal appears in the range of -6 to -12 ppm, providing evidence of a large diphos phate content. The singlet for monophosphate (0+2 ppm) remains largely unchanged. 15 By calculating the integration values, it can be found that the diphosphate content has increased by the same amount by which the polyphosphate,,onte as fallen. C:~ tif Bat"- CD &&r0eni .'r- This is an authenticated translation from the German language 7 The solubility has remained largely the same or somewhat improved with increasing sample tempering time. The pH value has also remained almost unchanged and is still significantly below 2 5 for all samples. Also of interest is the fact that the hygroscopicity falls with increasing diphosphate content. It is known from the literature that the diphosphate NaH 3

P

2 0 7 has a minimum value of hygroscopicity (A. Norbert et al., 1968, p. 1238). The X-ray diffraction analysis of sample ASPP1/3 (tempered 3 days) revealed that the 10 compound is the already suspected pyrophosphate. The line distribution corresponds to that given by A. Norbert, C.R. Acad. Sc. Paris, 1966, p. 1535. Example 2 As example 1, but drying oven temperature 1500 C. After 3 days there is an 80 % conversion to NaH 3

P

2 0 7 (determined by "P-NMR). 15 Example 3 As example 2, but in a mixing apparatus (L6dige mixer), whereby a saturated water vapour-air mixture at 250 C with 35 mbar water vapour is fed into the mixing cham ber. After 8 hrs, a yield of 90 % NaH 3

P

2 0 7 is obtained. Example 4 20 25 g acid polyphosphate (ground) is added to 50 ml diglyme, to which 2ml water is added, and hydrolysed for 0.5 hr under intensive stirring at 120* C. Immediately fol lowing addition, the acid polyphosphate agglomerates. The agglomerate gradually crystallises out, whereby stirring encourages the disintegration of the glass-like aggre gate while further crystallisation takes place. Subsequently, the solid material is re 25 moved by suction with exclusion of air moisture, washed in acetone and dried under vacuum. Yield of NaH 3

P

2 0 7 75 % (determined with 31 P-NMR). ,,,Oks un e- ' zo, ~ o 4 keaen-

Claims (2)

1. Process for synthesis of NaH 3 P 2 0 7 , characterised in that 5 glass-like acid sodium polyphosphate with a Na/P ratio of 1:2 is tempered as a fine powder at temperatures of 100-200* C in damp air with a water vapour pressure of

10-200 mbar or in a polar solvent, in which the phosphate is insoluble, in the pres ence of a quantity of water sufficient for the hydrolysis, until the mass has over whelmingly been converted into NaH 3 P 2 0 7 . 10 2. Process according to Claim 1, characterised in that the temperature is 1 10-180* C, preferably 110- 135* C, the water vapour pressure is 30-100 mbar and the reaction time is 5-100 hours. 15 3. Process according to Claim 1 or 2, characterised in that the glass-like acid sodium phosphate is obtained by evaporation of a mixture of sodium phosphate salts and phosphoric acid in the ratio of Na/P of 1:2, melting of 20 the mixture at over 300* C and grinding of the cooled product. 4. Process according to Claims 1 to 3, characterised in that the NaH 3 P 2 0 7 contains less than 25 %, preferably less than 10 %, polyphosphate 25 and monophosphate. Translator's note: there is at least one word missing in the German original here. It has assumed that the intended word is Mischung ( mixture). This has been used in the translation. a ,. -4Q~B~ w au' This is an authenticated translation from the German language 9 5. Process according to Claim 1, characterised in that the reaction is carried out in an ether, particularly diglyme or dioxan, at tempera 5 tures of 50-150* C, preferably 90-120* C over a period of 0.5-10 hours. 6. Application of NaH 3 P 2 0 7 according to Claims 1 to 5 as an acidifier, buffer or complexing agent for the food, textile or paper industry. 10 Fig. 1 P 2 0 5 distribution in the individual phosphate fractions P 2 0 5 , % 1/2 hr tempering 5 hr tempering 3 day tempering 15 Fraction Fig. 2 cps 2-theta scale 20 This translation of the document presented to me in the German language is correct and complete. Donau- WOW eschingen, 12 January 2001. James Walker, publicly - fsche appointed and court-sworn translator of the English "l s***~ language for the State of Baden-Wurttemberg. bol-