THERMAL STABILITY AND CHEMICAL TRANSFORMATIONS OF OXALATES

A. V. Shkarin, G. M. Zhabrova, N. D. Topor, M. Ya. Kushnarev

(Presented by the scientific seminar of the Department of Radiation Chemistry)

Oxalates of various metals constitute a very interesting class of inorganic compounds. Recently they have been used increasingly often as catalysts and as starting substances for the preparation of other inorganic compounds: oxides and metals, which in turn are widely used in various areas of science and technology. Therefore, knowledge of the thermal stability of oxalates, clarification of the temperature range of their decomposition, the sign of the thermal effect, and the study of chemical and phase transformations are of great interest.

In the present investigation an attempt was made, in order to solve the questions posed, to apply a comprehensive method with simultaneous study of the curves of weight change (TG), differential weight change (DTG), and differential temperature change (DTA) during thermal decomposition at a uniformly increasing temperature (T). For this purpose a derivatograph of the OD-101 system, described in [1—2], was used. It should be noted that this method was applied to the study of oxalates for the first time, although many investigations had been carried out using either thermogravimetric methods [3, 4] or differential thermal analysis [5].

The oxalates of copper, cobalt, nickel, iron, whose decomposition, depending on the experimental conditions, leads to the formation of metals or oxides (or their mixtures), and the oxalates of magnesium, calcium, zirconium, and thorium, the final decomposition products of which are only oxides, were selected as objects of investigation. The decomposition of sodium oxalate was also studied.

The oxalates of nickel, copper, and thorium were prepared by precipitating 0.2 N solutions of the corresponding nitrate salts with a 0.4 N solution of ammonium oxalate. The precipitated oxalates were washed and dried at a temperature of 50—60° C. Zirconium oxalate was prepared by the method described in [6]. Cobalt oxalate was prepared and kindly provided to us by Prof. V. Danes from the Institute of Physical Chemistry of the Czechoslovak Academy of Sciences. The remaining oxalates used were of “chemically pure” grade. A weighed portion of 0.07—0.30 g was placed in a platinum crucible, into the bottom of which a platinum–platinum-rhodium thermocouple had been introduced. In a second identical crucible, aluminum oxide calcined at a temperature of 1200° C was placed as a reference. The crucibles were covered with quar-

...with a quartz cup, over which an electric furnace was placed. The programming device of the derivatograph made it possible to vary the heating rate from 1°/min to 20°/min. In our experiments the heating rate was 10°/min. The experiments were carried out in air and in a nitrogen atmosphere. In the latter case, preliminarily dried nitrogen was passed over the crucibles at a rate of 200 cm³/min throughout the entire heating process, and also during cooling of the resulting products to room temperature. The sensitivity of the thermoscales was 0.2 mg, which, with the weighed portions of starting substances used, made it possible to estimate the change in weight with an accuracy of up to 0.5%. After recording the thermograms (DTA) and derivatograms (DTG), repeated experiments were set up for X-ray phase analysis, with samples taken at the extreme points. X-ray patterns were recorded in standard Debye cameras using copper and chromium X-radiation filtered from β-radiation.

Experimental Data and Discussion of Results

Figure 1 presents the DTA and DTG curves for nickel oxalate dihydrate (NiC₂O₄·2H₂O), recorded in air (a) and in a nitrogen atmosphere (b). The first effect on the DTA curves, at \(T = 240–360^\circ\mathrm{C}\), is endothermic both during decomposition in air and in nitrogen. The inflection points (\(T_{\max}\)) on the DTA and DTG curves coincide and correspond to a temperature of 300–310°C. Calculation from the thermogravimetric curve (TG) shows that the weight loss by the end of this thermal process corresponds to 21%, which practically coincides with the calculated value of the weight loss due to removal of hydration water.

X-ray phase analysis records the formation of anhydrous nickel oxalate in this temperature region. The position and sign of the next effect depend on the composition of the gaseous medium in which decomposition takes place: an endothermic effect during decomposition in nitrogen (\(T_{\max} = 400^\circ\mathrm{C}\)), and an exothermic one during decomposition in air (\(T_{\max} = 480^\circ\mathrm{C}\)). In the latter case the position of the maximum on the DTA curve differs by 80°C from the position of the extremum on the DTG curve. As the X-ray phase analysis data show, in a nitrogen atmosphere at 400°C there occurs mainly the formation of metallic nickel with a small content of NiO, possibly due to the presence of an oxygen impurity in the nitrogen used by us.

During decomposition in air, oxidation of nickel evidently occurs at the moment of its formation, and on the DTA curves (Fig. 1a), beginning at a temperature of 420°C, an exothermic effect appears, whose maximum corresponds to \(T = 480^\circ\mathrm{C}\). The character of the thermogravimetric curve also changes. In a nitrogen atmosphere, beginning at a temperature of 400°C, it runs almost parallel to the abscissa axis (Fig. 1b), which indicates completion of the decomposition of nickel oxalate and the absence of appreciable transformations of the nickel formed. In an air atmosphere (Fig. 1a), after 400°C an increase in the weight of the sample is observed, caused by oxidation of nickel.

Figure 2 shows derivatograms of the thermal decomposition of trivalent iron oxalate in an atmosphere of air (a) and nitrogen (b). During decomposition in air, on the DTA curves, after an endoeffect in the temperature range 180–260°C, there follows a complex exothermic “hump” extending over the temperature range 300–400°C. In a nitrogen atmosphere the DTA curves are characterized by three clearly expressed endoeffects (Fig. 2b). The decomposition of hydrated iron oxalate...

Fig. 1. Derivatogram of \( \mathrm{NiC_2O_4 \cdot 2H_2O} \):
a) in air. Sample weight 0.28 g;
b) in nitrogen. Sample weight 0.28 g.

Fig. 2. Derivatogram of \( \mathrm{Fe_2(C_2O_4)_3 \cdot 5H_2O} \):
a) in air. Sample weight 0.27 g;
b) in nitrogen. Sample weight 0.07 g.

\( \mathrm{Fe_2(C_2O_4)_3 \cdot 5H_2O} \) in a nitrogen atmosphere proceeds by a distinctive pathway. At a temperature of \(150\text{–}220^\circ\mathrm{C}\), processes of dehydration and decomposition evidently occur simultaneously, with the formation of ferrous oxalate, as indicated by the data of X-ray phase analysis.

Table 1

Summary data on the temperature characteristics of reactions of decomposition of oxalates according to chemical composition

and the Mössbauer effect. In the next stage of decomposition, in the temperature range 220–290°C, according to the data of weight and X-ray phase analysis, as well as the Mössbauer-effect data, \(Fe_3O_4\) and Fe are formed [10].

Upon further heating in the temperature range 300–520°C, another endothermic effect appears, caused by the decomposition of \(FeC_2O_4\). X-ray phase analysis indicates the formation of \(Fe_3O_4\) and Fe in this temperature range. To verify this supposition, the decomposition of ferrous oxalate \(FeC_2O_4 \cdot 2H_2O\) was carried out. It turned out that after the dehydration process in the range 160–240°C, decomposition of the dehydrated iron oxalate begins in the temperature range 330–440°C. According to X-ray phase analysis, the final products formed during the decomposition of ferrous oxalate are identical to the decomposition products of ferric oxalate.

Without dwelling on a detailed consideration of all the derivatograms obtained, some of which are presented in Figs. 3 and 4, let us turn to the summary of results given in Table 1.

Fig. 3. Derivatogram of \(Zz(C_2O_4)_2\): a) in air. Sample 0.15 g;
b) in nitrogen. Sample 0.15 g

As follows from Table 1, for the oxalates studied in hydrated form the beginning of the chemical transformation is dehydration. Dehydration is characterized by the independence of the temperature interval and of the sign of the effect from the composition of the gaseous medium. These processes are endothermic in all cases, although, depending on the chemical nature of the oxalate, they may proceed in one stage (nickel, magnesium, calcium oxalates) or in two (thorium oxalate). In some cases the next stage—decomposition—is superimposed on the dehydration process (iron oxalate).

For cobalt oxalate, after dehydration in the range 180—

250°C, its partial decomposition to \(\mathrm{Co_3O_4}\) was observed. Upon further heating in the temperature range 370–430°C, very rapid decomposition of dehydrated cobalt oxalate occurred. According to X-ray phase-analysis data, in this temperature region the formation of \(\mathrm{Co_3O_4}\) and Co was detected, which agrees with the data of [4].

In the decomposition of sodium oxalate and dehydrated calcium and zirconium oxalates, the formation of carbonates of the corresponding metals was observed. In the last two cases the carbonates were intermediate compounds, decomposing upon further increase in temperature with the formation of calcium oxide and zirconium dioxide. In the case of calcium oxalate, the carbonate formed was recorded not only from the weight loss but also by the X-ray phase method. In the case of zirconium oxalate, the carbonate apparently formed, \(\mathrm{Zr(CO_3)_2}\), was amorphous. Gravimetric analysis showed a weight loss corresponding to the calculated value.

To confirm the formation of zirconium carbonates, special experiments were carried out on the decomposition of zirconium oxalate at \(T = 150\text{–}250^\circ\mathrm{C}\) in a vacuum apparatus. The evolution of CO was recorded in amounts corresponding to the calculated value, according to the change in weight on the TG curve of the derivatogram of zirconium oxalate. With a further increase in temperature (400–450°C), in air and in nitrogen, the next exoeffect appears, independent of the gaseous medium and associated with the transformation of the tetragonal structure of \(\mathrm{ZrO_2}\) into the monoclinic one. Such an exothermic process also took place in the decomposition of zirconium hydroxide [7].

Analysis of the derivatogram of thorium oxalate (Fig. 6) shows that hydrated thorium oxalate \(\mathrm{Th(C_2O_4)_2 \cdot 6H_2O}\) decomposes stepwise—

Fig. 4. Derivatogram of \(\mathrm{Th(C_2O_4)\cdot 6H_2O}\): a) in air. Sample 0.14 g; b) in nitrogen. Sample 0.15 g

is completed. In the temperature interval (120–200°C) four molecules of water are split off and an X-ray-amorphous dihydrate of thorium oxalate is formed. With a further increase in temperature (240–300°C), according to gravimetric-analysis data, thorium oxalate dihydrate is converted into the monohydrate. The latter decomposes at 350–450°C with formation of the final product—thorium dioxide. It should be noted that in works [8, 9] the assumption was made that thorium carbonate might be formed as an intermediate product decomposing at 390°C. As follows from the derivatogram (Fig. 4 b), on the DTG curve there is a small “step” in the temperature region 380–390°C, which may be associated with the formation of some unstable intermediate product. Clarification of this question requires additional investigations using other, more sensitive methods.

In conclusion it may be noted that the decomposition of the oxalates studied by us was characterized mainly by endothermic effects in an inert atmosphere (nitrogen). A weak exoeffect was observed only in the decomposition of dehydrated copper oxalate.

The exoeffects observed during the decomposition of dehydrated oxalates of nickel, copper, cobalt, and iron in air are due to oxidation of the metal formed. The exoeffects appearing during the decomposition of dehydrated oxalates of calcium, zirconium, magnesium, and thorium are evidently due either to catalytic oxidation of the carbon monoxide evolved during the reaction, or to burning-out of carbonaceous compounds formed during decomposition of oxalates [8].

Conclusions

1. The decomposition of oxalates of nickel, copper, cobalt, iron, magnesium, calcium, sodium, zirconium, and thorium was investigated on a “derivatograph” instrument, with analysis of the decomposition products by the X-ray phase method.

2. The beginning of the chemical transformation of oxalates in hydrated form is dehydration. In the case of trivalent iron oxalate, the next stage of decomposition—the formation of divalent iron oxalate—is superimposed on the dehydration process.

3. Dehydrated oxalates of nickel and copper in a nitrogen atmosphere decompose mainly to metals. Decomposition of trivalent iron oxalate leads to the formation of a mixture of oxides and metal.

During decomposition of dehydrated oxalates of magnesium, calcium, zirconium, and thorium in nitrogen and in air, oxides are formed. Decomposition of zirconium and calcium oxalates proceeds through the formation of intermediate products—carbonates. Sodium oxalate decomposes to sodium carbonate.

Literature

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