PAPERS ON CHEMISTRY AND PHYSICS 123 



A XOTE ON THE EFFECT OF TEMPERATURE 



ON THE TRANSITION OF CALCITE TO 



ARAGONITE 



R. Edman Greenfield, Chemist, State Water Survey, 



Urbana. 



In a study of the solubility of the different forms of 

 calcium carbonate in carbon dioxide solutions and its 

 bearing on corrosion problems, some things were noted 

 which led to a little study on the condition of change of 

 one form to another. The studies on solubility were car- 

 ried out by shaking carbon dioxide solutions with solid 

 calcium carbonate in a constant temperature bath until 

 equilibrium was reached. Over a week was always al- 

 lowed for this reaction. 



Bicarbonate alkalinity, which represented the calcium 

 carbonate dissolved, and free carbon dioxide were de- 

 termined on the resultant solution. The results at any 

 one temperature when plotted on ordinary coordinate 

 paper gave a smooth graft of an exponential form. It 

 was found that if they were plotted on logarithm paper 

 they gave straight lines within the limit of error. Re- 

 sults for 0°, 20° and 65°C are shown in Fig. 1. It will 

 be seen that the solubilities of both forms increase with 

 temperature from 0° to 20° and that aragonite is ap- 

 preciably more soluble than 'calcite at all times. At 25° 

 and 35° results were obtained not much different from 

 those at 20° ; if any thing they were a little lower in 

 both cases than were the 20° results. These results are 

 not shown in Fig. 1. At 65° the two forms were about 

 the same solubility and were lower than even the a 

 calcite. At first these results were considered to indi- 

 cate a transition temperature between the two forms of 

 calcium carbonate, but later experiments on mixtures 

 and a study of Johnson's 1 article shows this to be im- 

 possible. They may well indicate that the solubilities of 

 the two forms become quite close at this temperature. 

 This is supported by certain conductivity data and solu- 



1 J. Johnson, H. E. Merwith & EM. Williamson, Am. J. Science, 4th ser. 

 \U 473 (101fi>. 



Studies conditions of formation of different forms of calcium car- 

 bonate under different conditions. 



