in Ferric Salts and in Chromic Acid. 317 



predominates, thus tending to bring copper into agreement 

 with the other two metals. 



Rates of Solution in Chromic Acid. 



In the experiments with chromic acid as oxidizing agent the 

 solutions were initially about 0*015 molar with respect to 

 Cr0 3 , and contained also definite known amounts of free 

 sulphuric acid. The precaution, employed with ferric salts, of 

 keeping an atmosphere of carbon dioxide above the liquid 

 during the reaction, was, of course, unnecessary in working 

 with chromic acid, but in other respects the procedure remained 

 as before. As compared with the case of ferric salts, a higher 

 concentration of hydrogen ion is essential for the reaction to 

 proceed smoothly, and a further difference exists in the fact 

 that the acidity decreases during the reaction. But unless the 

 initial acidity is low this decrease is relatively small. In the 

 experiments accepted as trustworthy, the initial acidity was 

 always 0'5 normal (0*25 molar H 2 S0 4 ) or above, and the 

 decrease in acidity, even in the extreme cases, was only about 

 6 per cent, an amount too small to have any marked effect 

 upon the observed reaction velocity. 



The analyses of the solution samples for chromic acid were 

 carried out by one or the other of the two following methods : 

 (a) Treatment with an excess of potassium iodide and titration 

 of the liberated iodine with thiosulphate, (J) Treatment with 

 a known volume of standard ferrous sulphate, and titration of 

 the unoxidized ferrous sulphate with permanganate. 



Method (a), which was conducted according to the directions 

 of Seubert and Henke,* was applicable only in certain cases, 

 and seemed to have no advantage in accuracy over method (b). 

 The experiments in which method (a) was used are designated 

 in the tables by asterisks. Full experimental data are given 

 for Experiment 1, the values of c being volumes of 0*02 normal 

 thiosulphate used, which are evidently proportional to the con- 

 centrations of chromic acid at the corresponding times, and 

 can therefore be directly substituted in the velocity equation 

 in calculating k. 



Method (b) was applicable in all cases, and was on the whole 

 preferred, after experience had proved that the presence of 

 the green chromic salt in the solution did not seriously 

 diminish the sharpness of the permanganate end point. Data 

 for a typical experiment (No. 4 of Table IX) are given in 

 detail, the values of c being here cubic centimeters of - 02 

 normal permanganate used in titrating 20 cm 3 of the given 

 ferrous sulphate solution after the addition of the 20 cm 3 sample 



* Zeitschr. angew. Chemie, 1900, 1147. 



Am. Jouk. Sci. — Fourth Series, Vol. XLII, No. 250. — October, 1916. 

 22 



