XI, A, 4 Witt: Testing of Galvanized Iron 161 



is subjected to the hydrochloric acid test. It is evident that 

 the removal of the zinc is more uniform than with either copper 

 sulphate or lead acetate solutions, because there is no metallic 

 deposit to interfere. 



As long as only the outer layer of nearly pure zinc is exposed 

 to the action of the acid, the rate of solution should remain 

 almost constant.^** The surface exposed remains about uniform, 

 and since the acid is present in considerable excess, the change 

 in concentration is not great. The results show that this is 

 true. 



Manipulation. — A sheet of galvanized iron 7.6 by 30.6 centi- 

 meters was bent into the form of a cylinder and placed in a large 

 beaker. The edges were suitably protected. The apparatus 

 was fitted with a stirrer, and 1 liter of hydrochloric acid 

 (density, 1.052) was rapidly poured in. At intervals of one 

 minute 25 cubic centimeters of the acid were removed for anal- 

 ysis. Effervescence had apparently ceased by the end of the 

 sixth minute, but the reaction was allowed to run ten minutes. 

 After ten minutes the iron was removed and found by analysis 

 to be free from coating. 



Table Vtl shows the results. The iron was determined gravi- 

 metrically, and the zinc was titrated with potassium ferrocyanide. 

 The results were calculated to total weight of iron and zinc 

 dissolved in terms of ounces per square foot. 



" In a heterogeneous system, such as hydrochloric acid and zinc, the 

 velocity of the reaction may be expressed by the equation dx/dt=kOia—x). 

 Here O represents the area of the exposed surface 

 a, the original concentration of the acid 



X, the decrease in concentration of the acid caused by x equivalents 

 of metal going into solution in the time t. [See Nernst, W., 

 Theoretical Chemistry. Macmillan & Co., London (1911), 584.] 



In the present work a large excess of acid was used, so that the concentra- 

 tion remained practically constant. It can be assumed that the total surface 

 remains constant. Therefore, theoretically the right member of tho equation 

 becomes a constant, or dx/dt=K. This condition cannot be realized exactly, 

 however, because of other factors which affect the velocity. In the first 

 place no attempt was made to keep the temperature of the acid constant. 

 In actual practice there is a rise in temperature, and it was desired to 

 keep conditions as nearly similar as possible. Each zinc-iron alloy has its 

 velocity constant and each has its velocity equation, dx/dt=kO{a—x) . 

 Therefore, as the reaction proceeds and various alloys are exposed, it 

 becomes more and more complex. The surface, O, for some of the alloys 

 is not a constant because usually the entire surface is not exposed at the 

 same time. While the outer coating of zinc is being dissolved, the reaction 

 comes nearest to the theoretical. 



