Sodium sulphate anhydrite, 4.33

Solution. We investigate the transformation of sodium sulphate decahydrate Na2S04-IOH2O into sodium sulphate anhydrite Na2S04 by the reaction... 

C.R.Wilding, B. Mile, AT. Vincent, Effect of potassium and sodium sulphates on the reaction of synthetic anhydrite with water. Journal of Chemical Technology and Biotechnology, 1984,34A 80-90... 

Figure 4.43. Calculated relative humidity for the salt pair sodium sulphate decahydrate and anhydrite, respectively, at 15°C ( ) for comparison, the test data ( ) published by Timmermanns are shown.

Figure 5.33. Solubility of sodium sulphate into water. At temperatures below 32.4°C, decahydrate is a solid deposit. In this range, the solubility is strongly increased with rising temperatures (A-B). At temperatures above 32.4 °C, decahydrate is transformed into anhydrite. Above this temperature, the solubility is slightly reduced with rising temperatures (B-C).

In the equilibrium system, solid sodium sulphate can thus occur as an anhydrite Na2S04(s) and as a decahydrate Na2S04 10H2O(s). The decahydrate Na2S04 10H2O(s), in particular, cannot exist at temperatures above 32.4°C, because the hydrate is then transformed into the anhydrite Na2S04(s). 

Figure 5.34. Relative humidity RH over a saturated solution of sodium sulphate. At temperatures below 32.4°C, the solid deposit is decahydrate. In this range, RH decreases with rising temperature, because the solubility of decahydrate increases (A-B). Above 32.4°C the decahydrate is transformed into anhydrite, and RH over the saturated solution increases slightly with the temperature, because the solubility of anhydrite decreases with temperature (B-C).

Figure 5.35. If RH over a saturated solution of sodium sulphate decahy-drate (A-B) is reduced, the solution is dehydrated and solid decahydrate is formed. When below a certain RH value, decahydrate emits the chemically bound hydrate water and is transformed into anhydrite (B-D). The boundary curve denotes RH over a decabydrate-anbydrite salt pair in thermodynamic equilibrium. The curve B D corresponds to this expression (b) measurement values denote the experimental data, cf. Timmermanns.

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