Antraptseva, N. M.

Petrichenko, T. S.

BSU Digital Library

 42 
Thermal properties of manganese(II) dihydrophosphate tetrahydrate 
N. M. Antraptseva, T. S. Petrichenko   
National University of Life and Environmental Sciences of Ukraine, Kiev, Ukraine, 
 e-mail: aspirant_nubipu@ukr.net 
Heat treatment of crystal hydrates is one of the ways to obtain anhydrous salts. This pro-
cess in many respects is defined by an extent of hydration of crystalline hydrate. The thermal 
transformation of manganese(II) dihydrophosphate dihydrate Mn(H2PO4)2 · 2H2O have been de-
termined [1]. Thermal properties of  Mn(H2PO4)2 · 4H2O tetrahydrate are studied insufficiently. 
The aim of this work was to study thermal behaviour of Mn(H2PO4)2 · 4H2O to determine the 
sequence of thermal and structural transformations and how the extent of hydration affects these 
transformations.  
Thermal transformations were studied, as in [1], using Q-1500D derivatograph. According 
to the results of the differential and thermal analysis, Mn(H2PO4)2 · 4H2O is resistant when heat-
ing in air with a rate of 2.5 K/min to 50 oC. Further temperature increase is followed by a loss of 
weight. It is registered by four accurate steps on TG curve. 
Interpreting the results of the complex characterization of the products of partial and com-
plete dehydration of Mn(H2PO4)2 · 4H2O, we found the following. Heat treatment of 
Mn(H2PO4)2·4H2O in the range of 50–110 oC is followed by the removal of two molecules of 
water by the molecular mechanism and the formation of Mn(H2PO4)2 · 2H2O. At the same time 
partly the process of the anion disproportionation is  realized with the formation of MnHPO4 · 
3H2O and Н3РО4.  
The second stage of dehydration ends up in the range of 110–160 oC with the formation of 
two mixed crystalline phases which are Mn(H2PO4)2 and  Mn5(HPO4)2(PO4)2 · 4H2O. The quanti-
ty of free H3PO4 in the thermolysis products  of Mn(H2PO4)2 · 4H2O  increases  and  at 160 oC it 
makes 2.12 wt % (based on P2O5).  
The process of the anion condensation begins at the third stage of Mn(H2PO4)2 · 4H2O ther-
molysis. At 185 oC the losses of the weight correspond to a removal of 4.55 mol of H2O. The salt 
component includes the condensed phosphate with n = 2–5. The acid component includes poly-
phosphoric acids of the general formula Нn+2РnО3n+1 (n = 2–4).  
Most difficult it is to determine the composition of the products at heating of 
Mn(H2PO4)2·4H2O tо 275 oС. It is presented by a mixture of condensed phosphates 
and polyphosphoric acids with 2 ≤ n ≤ 7. The condensed phosphates with n = 3–8 are X-ray 
amorphous. 
At heating of Mn(H2PO4)2 · 4H2O to 300 oС the anion composition of thermolysis products 
is simplified. The finish products of dehydration appear. It is identified as cyclotetraphosphate 
with composition of Mn2P4O12. Its crystallization ends up at 335
0С.   
Thus, the sequence of thermal solid-phase transformations accompanying Mn(H2PO4)2 · 
4H2O  thermolysis was established. The composition of the products of its partial and full dehy-
dration was identified; the temperature intervals of their formation and thermal stability were 
concretized. It was shown that the final product of thermolysis is Mn2P4O12 (monoclinic system, 
sp. gr. С2/с, Z = 4; а = 1.2084, b = 0.8471, с = 1.0171 nm; β = 119.29о) is formed by two direc-
tions. The first direction provides thermal dehydration of the protonated condensed phosphates 
(up to 65 %). According to the second direction, up to 35 % of Mn2P4O12 is formed as a result of 
solid-phase interactions of thermolysis intermediate products. The general scheme of thermal 
solid-phase transformations of Mn(H2PO4)2 · 4H2O was proposed. 
References 
1. L.N. Shchegrov, N.M. Antraptseva, I.G. Ponomareva. Izv. Akad. Nauk SSSR, Neorg. Mater. (1989) 25 
(2): 308. 


Thermal properties of manganese(II) dihydrophosphate tetrahydrate

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Thermal properties of manganese(II) dihydrophosphate tetrahydrate

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