150 The Philippine Journal of Science i9i6 



The copper sulphate, or Preece, test is the one still in most 

 general use for all galvanized articles except sheets. An outline 

 of the method is as follows : 



A strong solution of copper sulphate is shaken with copper 

 oxide to neutralize any acid present and is then diluted to 

 density 1.186 at 18.5 °C. A galvanized article is placed in this 

 solution for one minute and is then removed and cleaned. This 

 operation is then repeated until a bright deposit of copper is 

 formed on the iron base. In order to pass specifications, an ob- 

 ject must withstand a given number of immersions. Many criti- 

 cisms may be offered to this method as pointed out by Walker * 

 and by Patrick and Walker, '° who have studied it in detail. They 

 state that since the zinc coating is not homogeneous and the 

 zinc-iron alloys are less electropositive to the iron than is 

 the zinc, the zinc does not go into solution at a constant rate 

 of speed : 



Since the rapidity with which the zinc or zinc alloy will pass into solution, 

 and an equivalent weight of copper be precipitated in its place, is a function 

 of the difference of potential between the two metals, it will be seen that 

 the rate of solution must of necessity change as we pass from the zinc to 

 the alloy. The coating in the so-called sherardized iron and some kinds 

 of electro-galvanized iron consists so largely of an iron-zinc alloy that any 

 test or measurement based upon an assumed uniformity in rate of solution 

 is liable to grave error and untrustworthy. 



THE LEAD ACETATE METHOD 



Patrick and Walker ^^ devised the lead acetate method as a 

 substitute for the Preece test. They recommended that it be 

 used to determine the weight of zinc coating per unit length or 

 area, but stated also that, even if it is used in the same way 

 as the Preece test, it is more satisfactory. A solution is made 

 by dissolving 400 grams of crystallized lead acetate in one liter 

 of water. To this are added 4 grams of litharge to neutralize 

 any acid present and to give the solution a slightly basic re- 

 action. After filtering, the solution is ready for use. A piece 

 of wire 7.6 to 15.2 centimeters (3 to 6 inches) long or a gal- 

 vanized plate 5.08 by 5.08 centimeters (2 by 2 inches) is weighed 

 and placed in a quantity of the solution for three minutes. The 

 zinc goes into solution arid a deposit of metallic lead forms on 

 the plate or wire. The object is taken from the liquid, and 

 the lead is removed. The operation is repeated until no more 

 lead coating forms. The object is first washed in water and 



" Electrochem. & Met. Ind. (1909), 7, 440. 

 "Joum. Ind. & Eng. Chem. (1911), 3, 239. 

 " Loc cit. 



