THERMAL PROPERTIES OF Mn(H2PO4)2•4H2O AND PRODUCTS OF HIS DEHYDRATION

Анотація

Hydrated dihydrogenphosphates of manganese(ІІ) and products of their dehydration are widely used as the basis of various inorganic materials: catalysts of organic synthesis, thermophosphoric decorative coatings, corrosion inhibitors and others. Thermal properties of manganese(ІІ) dihydrogenphosphate tetrahydrate - Mn(H₂PO₄)₂·4H₂O was discussed in literature, superficially, in the context of research of its structure. The purpose of the present work - to investigate thermal properties of Mn(H₂PO₄)₂^.4H₂O, composition, temperature intervals of formation and thermal stability of products of its partial and complete dehydration, to set the sequence of thermal solidphase transformations.

In accordance with the results of differential thermal analysis, Mn(H₂PO₄)₂^.4H₂O proof at heating on air with speed a 2.5 °/min to 50°С. The further increase of temperature is accompanied by the loss of mass, that on the thermogravymetric curve (ТG) registers four quite distinct stages in the intervals of 50-110°С (3 maximums at 60, 80 and 100°С), 110-160°С (maximum at 120°С), 160-220°С (maximum at 180°С) and 275-335°С (2 maximums at 295 and 310°С).

The results of complex analysis of products of partial dehydration showed that practically simultaneously with beginning of dehydration except the solid phase, registers the liquid - free phosphate acid amount of which during the removal of 1.66 mole Н₂О at 105°С is 1.21% (in a count on Р₂О₅). The main component of multiphase product of dehydration of Mn(H₂PO₄)₂·4H₂O on the first stage is dihydrate - Mn(H₂PO₄)₂·2H₂O, identified according to X-ray and IR spectroscopic analysis. The second product is less protonated phosphate of composition MnHPO₄·3H₂O. Interpretation of the obtained data shows that the heat treatment of Mn(H₂PO₄)₂·4H₂O in the range of 50 - 110°С is accompanied by removal after the molecular mechanism of two least strongly adhesive in its structure crystallizational water molecules. However, accordingly to the TG curve, their complete removing is not happening (the losses of mass is 1.7 mole Н₂О only). This indicates that the free phosphoric acid, which is formed, dissolved in water, preventing its removal.

The same for the chemistry process proceeds on the second stage of dehydration of initial tetrahydrate. Its ends with formation in an interval 110 - 160°С mixture of two crystalline phases, the main of which is anhydrous dihydrogenphosphate Mn(H₂PO₄)₂. The second phase is a phosphate of composition Mn₅(HPO₄)₂(PO₄)₂·4H₂O - product of disproportionation of hydrogenphosphate on a scheme: 5MnHPO₄·3H₂O → Mn₅(HPO₄)₂(PO₄)₂·4H₂O  + H₃PO₄ + 11H₂O.

As a result the amount of free Н₃РО₄ in partial thermolysis products of Mn(H₂PO₄)₂·4H₂O increases and at 160°С is 2.12 % P₂О₅. One of the sources of its formation on the second stage of dehydration of initial tetrahydrate, in addition, there is a process of intramolecular hydrolysis of Mn(H₂PO₄)₂·2H₂O.

On the third stage of thermolysis of Mn(H₂PO₄)₂·4H₂O the process of anionic condensation begins actively. At 185°С, when the losses of mass answer removing of 4.55 mole Н₂О, in composition of salt component the condensed phosphates formed with n = 2 ÷ 5, in composition of acid - polyphosphate acids of general formula Н_n+2Р_nО_3n+1, where n = 2 ÷ 4. At heating of Mn(H₂PO₄)₂·4H₂O to 275°С it is presented by mixture of X-ray amorphous high molecular oligohosphates, the amount of phosphorus atoms in the chain of which varies from 3 to 8 and more, and polyphosphate acids with 2 ≤ n ≤ 7. The total content of Р₂О₅ in a salt component increases to 53,4 mas.%, in acid - reduced to 11 mas.%, characterizing cooperations that will be realized between them. The degree of transformation of monophosphatic anion in a salt component arrives at 94,6 %, in acidic - 84,5 %.

Further heating of Mn(H₂PO₄)₂·4H₂O to 300°С is accompanied by simplification of anionic composition of thermolysis products and reduction to the amount of polyphosphate acids, specifying on their direct participating in formation of new phosphate with the circular structure of anion. About beginning of crystallization of finish thermolysis products, identified as cyclo-tetraphosphate of composition Mn₂P₄O₁₂, shows exothermic effect with a maximum at 295°С. Forming of crystalline structure of cyclo-tetraphosphate is completed at 335°С - to the temperature practically complete dehydration of initial tetrahydrate. Mn₂P₄O₁₂ is crystallizable in monoclinic system (sp. gr. С2/с, Z = 4) with unit cell parameters, nm; а = 1.2084, b = 0.8471, с = 1.0171; β = 119.29^о.

Thus, dehydration of Mn(H₂PO₄)₂·4H₂O occurs simultaneously in two directions with formation of cyclo-tetraphosphate of Mn₂P₄O₁₂. First - envisages thermal dehydration of the protonated condensed phosphates (to 65 %). Accordance with the second direction, to 35% Mn₂P₄O₁₂ formed as a result of solidphase cooperation of partial dehydration products (polyphosphate acids and middle oligohosphates).