in Ferric Salts and in Chromic Acid. 303 



which was initially 600 cm 1 in volume, were taken at convenient 

 intervals with a 20 cm 3 pipette, and the velocity constants h 

 calculated in the ordinary way from the ohserved concentra- 

 tions at the beginning and end of the five to ten-minute time 

 intervals. The rate of stirring was kept at 200 revolutions per 

 minute in all experiments, and although the variations were 

 usually very small they were systematically determined and 

 corrected for by the method described in the article just cited. 

 The temperature was 24 , 6° C. in all experiments with ferric 

 salts, and 25° C. in all those with chromic acid. 



Rates of Solution in Ferric Sulphate. 



Previous experiments on the rate of the reaction between 

 ferric sulphate and metals have been made by T. E. Thorpe* 

 (1882), and by C. G. Schleuderbergf (1908). Thorpe com- 

 pared the action of the ferric salt on zinc, magnesium, and 

 iron, but under poorly defined experimental conditions, further 

 complicated by the evolution of hydrogen. Schleuderberg 

 studied only the reaction between ferric sulphate and copper, 

 so that his work affords no comparison between the rates for 

 different metals. Though primarily interested in testing for a 

 possible effect of light on the reaction velocity, he was led to 

 the conclusion that diffusion was the determining factor. 



In the experiments of the writers, the ferric alum solutions 

 used were approximately - 05 molar with respect to RFe(S0 4 ) 2 , 

 and contained, besides, various known amounts of free sulphuric 

 acid together with a little ferrous sulphate to take up dissolved 

 oxygen. Several liters of solution were prepared at one time, 

 and the total iron concentration determined once for all by 

 reducing duplicate samples in a Jones Reductor and titrating 

 with - 02 normal permanganate. The ferrous iron concentra- 

 tion at various stages of the reaction was found by direct 

 titration with the same permanganate solution in the presence 

 of phosphoric acid, and the ferric iron obtained by difference. 

 In the tables C is the total iron, and c the (variable) amount of 

 ferrous iron contained in 20 cm 3 of solution, all expressed in 

 cubic centimeters of the - 02 normal permanganate. The 

 values of C-c were used in calculating Tt.. 



To prevent oxidation of the ferrous salt in the solution by 

 the stirring in contact with air, an atmosphere of carbon 

 dioxide was maintained above the liquid by passing a brisk 

 current of the gas into the reaction vessel throughout the ex- 

 periment. 



In most of the experiments the ferric alum used was the 

 potassium salt. Those, however, in which the sulphuric acid 



*Jour. Cheni. Soc. Loud., xli, 287. f Jour. Phys. Chem., xii, 574. 



