6.4. Solubility of Copper FormateBinary Systems (1) The Cu(CHO2)2-HCHO2 system. There are two early publications giving data for the solubility of copper formate in anhydrous formic acid.1,2 Davidson and Holm1 measured the solubility at 308, 315, 322 and 325 K while Kendall and Adler2 measured the solubility at 413 K. The purity of the materials appears to have been the same for both investigations but nothing is stated about the precision of the analytical results. No direct comparison can be made between these two sets of values because the temperatures selected are different. There are no other published results with which these two sets of data can be compared. Consequently, both sets of values are classified as tentative.
Ternary Systems (1) The Cu(CHO2)2-NH4CHO2-HCHO2 system. Davidson and Holm1 studied this system over the 302 to 350 K temperature range. One feature of this system is the large increase of solubility of Cu(CHO2)2 as the concentration of NH4CHO2 increases. There is more than a tenfold increase in the solubility of Cu(CHO2)2 when the concentration of NH4CHO2 in the solvent increases from 30 to 35 mol %. The data are classified as tentative because there is no other report of solubility data in this system.
(2) The Cu(CHO2)2-HCHO2-H2O system. Kalalova published the results of a study of the solubility of Cu(CHO2)2 in a 2 % aqueous solution of formic acid.3 The solid phases identified are: Cu(CHO2)2·4H2O, Cu(CHO2)2·2H2O and Cu(CHO2)2. The transition points between the phases was determined to be:
tetrahydrate to dihydrate: 317.7 ± 0.5 Kdihydrate to anhydrous salt: 332 ± 1 KOstanii et al.4,5 studied this system at 298.2 and 308.2 K and at 323.2 and 343.2 K . However, the data are presented only in graphical form—four solubility diagrams at each of the temperatures selected. Cu(CHO2)2·4H2O and Cu(CHO2)2·2H2O were identified as equilibrium solid phases with the following transition points:
tetrahydrate to dihydrate—316.0 ± 0.8 K
dihydrate to anhydrous salt—335.2 ± 1.0 KA least squares analysis gave the following equations:
for tetrahydrate solubility—log S = -1766.0(1/T) + 5.720
for dihydrate solubility—log S = -1125.9(1/T) + 3.702.From these equations a value of 316.7 ± 0.5 K is obtained for the transition temperature of tetrahydrate todihydrate. This is consistent with the experimentally value.3 The published solubility data diagrams indicate that, depending on the temperature, there are three equilibrium solid phases in this system: Cu(CHO2)2·4H2O, Cu(CHO2)2·2H2O and Cu(CHO2)2 with the following transition points:
at 298.2 K: tetrahydrate to dihydrate—28.8 ± 0.5 mass % HCHO2
dihydrate to anhydrous salt—61.1 ± 0.5 mass % HCHO2;
at 308.2 K: tetrahydrate to dihydrate—23.8 ± 0.5 mass % HCHO2
dihydrate to anhydrous salt—56.2 ± 0.5 mass % HCHO2;
at 323.2 K: the only stable solid phases are the dihydrate and the anhydrous salt
the solution composition at the transition point is 28.5 ± 0.5 mass % HCHO2;
at 343.2 K: the only stable solid equilibrium phase is the anhydrous Cu(CHO2)2.All these values are classified as tentative because they are the only published values available.