CS266780B1 - A method for determining the content of dibasic diphosphates in calcinates in their preparation by thermal dehydration - Google Patents

Abstract

The solution is based on the determination of the reaction heats during the dissolution of calcinate, which contains two basic components - diphosphate and hydrogen phosphate anhydride - while the anion of the first component is hydrolytically cleaved during dissolution. From the predetermined reaction heats corresponding to the dissolution under these conditions of pure diphosphate and pure hydrogen phosphate anhydride and from the determined reaction heat during the dissolution of the calcinate, the content of diphosphates in the calcinate is determined by calculation.

Description

The invention relates to a method for determining divalent metal diphosphates in calcinates during their preparation by thermal dehydration.

Diphosphates are classified as condensed phosphate compounds. They have anions in the form of two tetrahedra (kO^) connected by a P-O-P bond, which has a relatively high bond energy (compared to the energies of other bonds in the diphosphate molecule). Technologically, the most important methods for preparing diphosphates are those based on dehydration by calcination of a suitable raw material, which can be either hydrogen phosphates or a corresponding mixture of a phosphorus compound with a volatile cation and a compound of the relevant divalent metal cation with a volatile anion. Determination of the diphosphate content in calcinates of the aforementioned raw materials has not been practically possible so far. The methods that have been used for this purpose have not yielded satisfactory results. In general, the most widely used method for the qualitative and sometimes quantitative evaluation of condensed phosphates is chromatography. However, this is unsuitable for the given purpose, as it necessarily requires transferring the samples into a solution. However, during dissolution, the hydrolytic cleavage of diphosphate anions and their disintegration into orthophosphate-type anions cannot be quantitatively prevented. Even if the hydrolytic cleavage of diphosphates during dissolution could be prevented (which is not possible), the quantitative evaluation of chromatographically separated components is very difficult, lengthy and inaccurate. The same is true for nuclear magnetic resonance (NMR) methods, which currently require the transfer of samples into solution in most cases. Non-destructive analytical methods, such as X-ray diffraction analysis, IR spectroscopy, Raman spectroscopy or luminescence spectroscopy, cannot be used for the given purpose either. X-ray analysis is a method that is demanding in terms of equipment, energy and operation, and is also burdened with a significant error when used for the given purpose, resulting from the necessity of performing a number of extensive calibration measurements before the determination, with standards that are also difficult to prepare. Using this method for the given purpose moreover, in addition to the above, the existence of multiple structural modifications of individual diphosphates greatly complicates the process. Although it is possible to qualitatively distinguish diphosphates from other types of condensed phosphates by IR spectroscopy and Reman spectroscopy, their absorption bands are not so characteristic that they can be used for the quantitative determination of diphosphates. The use of luminescence methods for determining the content of diphosphates has also been proposed and verified. However, these determinations again require a large number of calibration measurements, moreover with substances that should have a very precisely characterized purity, which is practically impossible based on what was stated above. Moreover, according to the experience of the authors of this invention, their cations also significantly contribute to the luminescent properties of condensed phosphates, which would further degrade the accuracy of the measurement. ,

The above knowledge is eliminated by a method for determining the content of divalent metal diphosphates in calcinates during their preparation by thermal dehydration, according to the invention, characterized in that for a sample of calcinate, preferably previously quenched with water, the reaction heat corresponding to its dissolution is determined, which occurred in such a way that the diphosphate anions contained in the calcinate were hydrolytically cleaved into simple phosphates, while the determined reaction heat is the sum of the heats corresponding to the individual components contained in the calcinate, i.e. hydrogen phosphate anhydride and diphosphate, when the heat corresponding to the dissolution of diphosphate

- 1 2óo7b^ determined previously is true of the heat associated with β its hydrolytic cleavage significantly higher than the heat corresponding to the dissolution of hydrogen phosphate anhydride also determined previously; from the values of these three reaction heats, since there are no other components in the calcinate, the content of diphosphate in the calcinate can be calculated.

Calcinates in the thermal preparation of diphosphates can be relatively easily dissolved in aqueous solutions of mineral acids due to the components they contain. In addition, those calcinates for which the determination of the content of diphosphates is considered were prepared at such temperatures that they may contain primarily an intermediate product of the hydrogen phosphate type in the form of anhydride or the final product diphosphate. If non-stoichiometric amounts of phosphorus compounds and divalent metal compounds were used as starting materials for the preparation of diphosphate, they may also contain by-products of the simple phosphate (orthophosphate) type, as well as by-products of the dihydrogen diphosphate type and by-products of the cyclic tetraphosphate type. The influence of the first two by-products is removed during the determination by advantageously pre-leaching the calcinate with water, when, unlike the other components, they dissolve. Third by-products, if present in small quantities in the calcine, due to their insolubility even in a strongly acidic environment, do not affect the heat of reaction during the dissolution of the calcine and can therefore only introduce an error in the final calculation of the determination result. However, this can be easily eliminated if the content of cyclotetraphosphates in the calcine is suspected by determining the content of cyclotetraphosphates (Czech author's certificate 232 090). To convert the calcine into a solution for determining the heat of reaction corresponding to its dissolution, it is necessary to use a solvent in which both components of the calcine - diphosphate and hydrogen phosphate anhydride - are soluble, and in the case of diphosphate, the anion is hydrolytically split during its dissolution. For example, diluted mineral acids or, possibly, diluted solutions of alkali hydroxides are suitable. A suitable device for determining the heat of reaction of dissolution is an isoperibolic reaction calorimeter, preferably a double one, which works on the principle of measuring the temperature difference before dissolution and after dissolution, with the possibility of dosing the solid phase; the heat of reaction is calculated from the temperature difference based on the calorimeter calibration. The combined calorimeter suitably compensates for the thermal effects of a mechanical nature that may occur during the determination (the influence of stirring, sorption of the liquid phase on the surface of the solid phase, evaporation of the liquid phase, etc.). Under the same conditions (using the same device, solvent and its concentration, etc.), the heat of reaction of dissolution of the pure diphosphate of the respective divalent metal and the pure hydrogen phosphate anhydride of the same divalent metal are determined in advance.

To calculate the content of divalent metal diphosphate in calcinate (x), a simple relationship can be used: (y ₃ ^X ~ ^2^2 ' ^Q 3 ^{) +} ^1^3 ^Q l ^}

2M _l (Q ₃ - QJ _{χ r} ___. 100 ^(!4) ' M ₂ (Q ₂ - Q ₃ ) + 2M _L (Q ₃ - Q _l )

- 2 266780

- reaction heat of dissolution of calcinate (J/g)

- heat of reaction of dissolution of diphosphate (J/g)

- heat of reaction of anhydride dissolution (J/g)

- molecular weight of hydrogen phosphate (anhydride)

- molecular weight of diphosphate $3 ^Q 2 ^1 ^M 1 ^M 2

Advantages of the method according to the invention:

- the method according to the invention represents a solution for determining the content of diphosphate, for which no method existed until now,

- the method of determination is fast and undemanding in terms of energy, material and operation, - a suitable device usable for the determination (isoperibolic double reaction calorimeter) can be assembled from commonly available materials and components.

Example 1

In the calcinate prepared from the starting cobalt hydrogen phosphate at a temperature of 200 °C, the content of dicobalt diphosphate was determined by the method according to the invention. The measurement of the reaction heats of dissolution was carried out using an isoperibolic reaction calorimeter. A 2M HCl solution was used as a solvent. After seven minutes, complete dissolution of the calcinate was achieved. The determined reaction heat was 244.2 J/g. The previously determined reaction heat of pure Οθ2Ρ ₂ °^ was 282.1 J/g (Qg) and of pure C _o HPO^ 114.7 J/g (Q^). The calculated content of dicobalt diphosphate was 78.4%.

Example 2

In the calcinate prepared from the starting manganese hydrogen phosphate at a temperature of 200 °C, the content of dimanganese diphosphate was determined by the method according to the invention. The dissolution of the calcinate was carried out in a solution of 1 M HCl in an isoperibolic double reaction calorimeter. After six minutes, complete dissolution of the calcinate was achieved and the determined heat of reaction Qj is 320.0 J/g. The previously determined heats of reaction of pure Μην^Ογ (Q ₂ ) and MnHPO^ (Qp under these conditions were 355.5 J/g (Q ₂ ) and 126.7 J/g (Op. The content of dimanganese diphosphate in the calcinate was 85.3%·

Claims (1)

• FIELD OF THE INVENTION Method for determining the divalent metal diphosphate content of calcinates in their preparation by thermal dehydration, characterized in that a sample of calcinate, preferably pre-cured with water, is determined by the heat of reaction corresponding to its dissolution, which occurs by hydrolytic cleavage of diphosphate contained in the calcine. anions to simple phosphate anions, the heat of reaction determined being the sum of the heat corresponding to the dissolution of the individual components contained in the calcine, i.e. hydrogen phosphate anhydride and diphosphate, when the heat corresponding to the dissolution of diphosphate hydrogen phosphate also determined in advance, the diphosphate content of the calcine being calculated from the values of these three heats.