Supersolubility Curves for Copper Sulfate Pentahydrate Solutions

Abstract

The authors carried out experiments similar to the work of Tin and McCabe. Tin and McCabe found two groups of supersolubility curves for MgSO4·E7H20 in continuously cooled, stirred, and seeded solutions. The location of these curves wa shown as dependent on several factors, but no attempt was made to derive any quantitative relations among them.In our experiments, unseeded and seeded solutions of copper sulfate were allowed to cool in a batch agitating vessel with cooling jacket at constantstirring speed. The flow rate of cooling water was kept constant, without contrriling the cooling rate.The ranges of experimental variables were as follows:Stirring speed in r. p. m. 140-780Rate of cooling water (w) in g/sec 10-80g. of seed crystals (CuSO4·5H20) per 100g. soln.(S) 0-10Initial sat'd. concn. of anhydrate in wt. per cent 22-32Corresponding initial sat'd. temperature in °C 38-70Observations were made on the point where the first nucleus appeared and the point where sudden crystallization took place, while simultaneously, the relations of time with temperature and concentration were determined.The point where temperature and concentration producedthe first appearance of crystals was named“first supersaturation point” and the point wheresudden crystallization occurred was called“second supersaturation point.”The temperature and concentration values at these points were determined from an analysis of the curves and the results of observation obtained under experimental conditions. The first supersaturation point thus obtained was plotted in Fig. 4, and the second supersaturation point in Figs. 2 to 3e.Fig. 4 shows an unique supersolubility curve which is obviously independent of the variables tested here. This fact differs from the results of Tin and McCabe. On the contrary, Figs. 2 to 3e indicate clearly the effects of stirring speed, theamount of seed crytals and the flow rate of cooling water. This shows a coincidence with their work qualitatively.The authors, therefore, attempted to correlate the data of the second supersaturation point, and obtained an experimental but dimensionless relation, as follows:CII′/CSII′=1.18(wcw/UAk)0.1·exp.(-0.028S)where CII′ and CsII′ are the second supersaturated and the saturated concentrations of CuSO4 at the same temperature, U is the over-all heat transfer coefficient, Ah, is the heat transfer area of agitated vessel, and cw, , is the heat capacity of cooling water, respectively.It is shown in Fig. 7 that the above relation coincides with the results of experiments within an accuracy of 10 per cent.