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The thermogravimetric analysis of some complex oxalates
Notes
739
Acknowledgement--The author wishes to thank Dr. J. S. MACKENZIEfor helpful discussions and Mr. H. FP.ANKEL,Miss R. JtrORIK and Mrs. N. BOLAS for their assistance. This work was supported by the Advanced Research Projects Agency (ARPA) under Contract No. DA-30-069-ORD-2638.
Research Laboratory, General Chemical Division Allied Chemical Corporation Morristown, New Jersey
G. FRANZ
The thermogravimetric analysis of some complex oxalates (Received 10 February 1963) IN the course of a study into the decomposition of a number of inorganic oxalates the thermogravimetric decomposition in air of chromium potassium oxalate Cr2(C204)8"3K2C204"5H~O, sodium chromium oxalate Cr~(C204)3"3Na2C204"5HzO and potassium titanyl oxalate, TiO'C=O4"K2C204"2HzO have been carried out. 80
70
f/
60
40
30
...........
I /"
50
W
.............................................. L / " :L._C
~//
_._./
/// /!/
-
JJ
20
_ _.-- -~t y
iO0
J
200
r
300
t
400
I
500
Ternperoture,
I
I
600
700
I
SO0
I
900
I000
*C
Fio. 1 .--Thermogravimetric decomposition of complex oxalates. I Chromium sodium oxalate. II Chromium potassium oxalate. III Titanyl potassium oxalate. IV Chromium oxalate. The apparatus used was an automatically recording decomposition balance in which the rate of heating was 6°/min. The recorded traces were analysed and plotted as percentage weight lost (w) against temperature (T°C) (Fig. 1). Each sample was first heated at 100°C until constant weight was attained, which accounted for the loss of some water in each case. The decomposition of the double salts of chromium can be compared with that of the corresponding simple oxalates. The decomposition of chromium oxalate, Cr=(C20~)3"6H20 proceeds directly
740
Notes
to Cr~O8 without any intervening plateau corresponding to the anhydrous salt (see figure) and is complete at 400°C. Sodium and potassium oxalates are very similar to each other in behaviour, the potassium salt being the less stable. The decomposition temperatures are: (a) M~C~O~ ~ M~COs; 500--600°C (M = K), 550-600°C (M = Na); (b) M~CO3---~ M~O; 800-900°C (M = K), above 950°C (M = Na). These temperatures agree with those reported by AKALANtX~for the first stage of this decomposition. The most plausible interpretation of curves I and II which can be undertaken at present rests on the assumption that water in excess of that in the formulae can be taken up, but is lost at 100°C. The extent of this deliquescence is quite considerable, 5"3 water molecules having been adsorbed by the potassium salt and 2.8 by the sodium salt. The slow part of each decomposition up to about 300°C can be accounted for by the loss of more strongly combined water. The theoretical values of W for the loss of the water of crystallization are 15.5 for the sodium salt and 19.0 for the potassium salt, so that the first plateau on curve I does not occur until the compound has decomposed beyond the dehydration stage. This is in contrast to the TABLE 1.-----STAGES I N THE DECOMPOSITION OF POTASSIUM TITANYL OXALATES
Experimental w
Temperature range (°C)
Possible product
Theoretical w
11"5 39-41 46"5-47"5 50"5 55-5
100 400--475 525-600 650-750 850-900
K2TiO (C~O~)2 KaCOs'TiO~ KIO2"TiO~ K~O'TiOs K~O'TiO
10'5 38"5 46'5 50"5 54.5
simpler oxalates. '~' The next stage in the decomposition which is complete at 450°C for the sodium salt but finishes rather more slowly in the case of the potassium salt, continues until the most stable oxides of each metal remain (3MzO, + Cr~Os). Theoretical values of w based on this assumption are W, 62.6 (M = Na), and w, 59.4 (M = K). Samples of the two double salts of chromium heated to between 900-950°C showed no trace of carbonate in the residue. Although a certain amount of instability is shown by both these products, this interpretation would be in accord with the notion of oxalate ions co-ordinated to a central chromium ion. The decomposition temperature is similar for each compound and higher than that of the simple chromium oxalate, and this is compatible with the expected stabilizing effect of complex formation. Such an explanation is also supported by the absence of carbonate in the solid residue, as indicated by the chemical tests, or by calculations based either on the stoichiometric formula or the formula used in the interpretation of the results. In the case of the potassium titanyl oxalate no information regarding a simple titanyl oxalate could be obtained. Attempts to prepare this salt by a method quoted in the literature ta~ were not successful. A comparison of the five experimental plateaux with possible products is made in Table 1. One of us (D. N.) wishes to thank the D.S.I.R. for a research grant.
The Chemistry Dept. The Royal College of Advanced Technology Salford, Lanes. {a~ S. ALKALAN, Rev. Fac. Sci., Univ. lstanbu121C, 184 (1956). {9~ j. ROBIN, Bull. Soc. Chim. France, 1078 (1953). ~*}P. P. BHATNAGERand T. BAN~tOEE, J. Sci. Ind. Res. 1511, 715 (1956).
D. DOLLIMORE D. NICHOI.,soN
739
Acknowledgement--The author wishes to thank Dr. J. S. MACKENZIEfor helpful discussions and Mr. H. FP.ANKEL,Miss R. JtrORIK and Mrs. N. BOLAS for their assistance. This work was supported by the Advanced Research Projects Agency (ARPA) under Contract No. DA-30-069-ORD-2638.
Research Laboratory, General Chemical Division Allied Chemical Corporation Morristown, New Jersey
G. FRANZ
The thermogravimetric analysis of some complex oxalates (Received 10 February 1963) IN the course of a study into the decomposition of a number of inorganic oxalates the thermogravimetric decomposition in air of chromium potassium oxalate Cr2(C204)8"3K2C204"5H~O, sodium chromium oxalate Cr~(C204)3"3Na2C204"5HzO and potassium titanyl oxalate, TiO'C=O4"K2C204"2HzO have been carried out. 80
70
f/
60
40
30
...........
I /"
50
W
.............................................. L / " :L._C
~//
_._./
/// /!/
-
JJ
20
_ _.-- -~t y
iO0
J
200
r
300
t
400
I
500
Ternperoture,
I
I
600
700
I
SO0
I
900
I000
*C
Fio. 1 .--Thermogravimetric decomposition of complex oxalates. I Chromium sodium oxalate. II Chromium potassium oxalate. III Titanyl potassium oxalate. IV Chromium oxalate. The apparatus used was an automatically recording decomposition balance in which the rate of heating was 6°/min. The recorded traces were analysed and plotted as percentage weight lost (w) against temperature (T°C) (Fig. 1). Each sample was first heated at 100°C until constant weight was attained, which accounted for the loss of some water in each case. The decomposition of the double salts of chromium can be compared with that of the corresponding simple oxalates. The decomposition of chromium oxalate, Cr=(C20~)3"6H20 proceeds directly
740
Notes
to Cr~O8 without any intervening plateau corresponding to the anhydrous salt (see figure) and is complete at 400°C. Sodium and potassium oxalates are very similar to each other in behaviour, the potassium salt being the less stable. The decomposition temperatures are: (a) M~C~O~ ~ M~COs; 500--600°C (M = K), 550-600°C (M = Na); (b) M~CO3---~ M~O; 800-900°C (M = K), above 950°C (M = Na). These temperatures agree with those reported by AKALANtX~for the first stage of this decomposition. The most plausible interpretation of curves I and II which can be undertaken at present rests on the assumption that water in excess of that in the formulae can be taken up, but is lost at 100°C. The extent of this deliquescence is quite considerable, 5"3 water molecules having been adsorbed by the potassium salt and 2.8 by the sodium salt. The slow part of each decomposition up to about 300°C can be accounted for by the loss of more strongly combined water. The theoretical values of W for the loss of the water of crystallization are 15.5 for the sodium salt and 19.0 for the potassium salt, so that the first plateau on curve I does not occur until the compound has decomposed beyond the dehydration stage. This is in contrast to the TABLE 1.-----STAGES I N THE DECOMPOSITION OF POTASSIUM TITANYL OXALATES
Experimental w
Temperature range (°C)
Possible product
Theoretical w
11"5 39-41 46"5-47"5 50"5 55-5
100 400--475 525-600 650-750 850-900
K2TiO (C~O~)2 KaCOs'TiO~ KIO2"TiO~ K~O'TiOs K~O'TiO
10'5 38"5 46'5 50"5 54.5
simpler oxalates. '~' The next stage in the decomposition which is complete at 450°C for the sodium salt but finishes rather more slowly in the case of the potassium salt, continues until the most stable oxides of each metal remain (3MzO, + Cr~Os). Theoretical values of w based on this assumption are W, 62.6 (M = Na), and w, 59.4 (M = K). Samples of the two double salts of chromium heated to between 900-950°C showed no trace of carbonate in the residue. Although a certain amount of instability is shown by both these products, this interpretation would be in accord with the notion of oxalate ions co-ordinated to a central chromium ion. The decomposition temperature is similar for each compound and higher than that of the simple chromium oxalate, and this is compatible with the expected stabilizing effect of complex formation. Such an explanation is also supported by the absence of carbonate in the solid residue, as indicated by the chemical tests, or by calculations based either on the stoichiometric formula or the formula used in the interpretation of the results. In the case of the potassium titanyl oxalate no information regarding a simple titanyl oxalate could be obtained. Attempts to prepare this salt by a method quoted in the literature ta~ were not successful. A comparison of the five experimental plateaux with possible products is made in Table 1. One of us (D. N.) wishes to thank the D.S.I.R. for a research grant.
The Chemistry Dept. The Royal College of Advanced Technology Salford, Lanes. {a~ S. ALKALAN, Rev. Fac. Sci., Univ. lstanbu121C, 184 (1956). {9~ j. ROBIN, Bull. Soc. Chim. France, 1078 (1953). ~*}P. P. BHATNAGERand T. BAN~tOEE, J. Sci. Ind. Res. 1511, 715 (1956).
D. DOLLIMORE D. NICHOI.,soN