Calcium monohydrate oxalate, effects

In a previous paper (11)/ we described the effects of small ionic species on the surface charge of calcium oxalate monohydrate (COM). The effects were detected by measuring the electrophoretic mobility of the particles in the aqueous phase. The influences of the activity of calcium and oxalate ions monovalent electrolytes and sulfate/ phosphate/ pyrophosphate/ and citrate ions on the electrophoretic mobility were studied. It was found that the results could be accounted for hy certain established theories for the electrical double layer/ which is also useful for analyzing the results of the present work. 

Calcium oxalate monohydrate has a solubility of 0.0067 g and 0.0140 g L 1 at 25° and 95 °C respectively. The solubility is less in neutral solutions containing moderate concentrations of ammonium oxalate owing to the common-ion effect (Section 2.7) hence a dilute solution of ammonium oxalate is employed as the wash liquid in the gravimetric determination. 

Fig. 37. Effect of sample packing on the dehydration course of calcium oxalate monohydrate shown by mass spectrometry...

In practice AT vs furnace temperature is plotted givinga thermogram of the type illustrated in figure 11.8. Figure 11.9 shows a curve for the DTA examination of calcium oxalate monohydrate exemplifying also the effect of changing the atmosphere from nitrogen to air. 

When a hot solution of a calcium salt in HCl is treated with ammonium oxalate, calcium oxalate monohydrate is precipitated. The small solubility of the precipitate in water is reduced in ammonium oxalate solutions (common ion effect). The precipitated calcium oxalate is washed, dried and weighed. However, because of its hygroscopic nature and the possibility of coprecipitation, it is preferable to convert it to the carbonate in a muffle furnace. 

Figure 3.3 DTA curves of calcium oxalate monohydrate measured in air (upper curve) and in nitrogen (lower curve). The second effect at 500°C corresponds to the elimination of carbon monoxide, a process that is endothermic in an inert atmosphere. If oxygen is present, the CO immediately burns to CO2, which produces an exothermic effect (above).

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