136 The Philippine Journal of Science i9i6 



the coating, especially since the lack of uniformity in galvanized 

 plate and the difficulty often encountered in getting sample 

 pieces cut to exactly the same size make very concordant results 

 out of the question. Therefore the determination of the amount 

 of zinc and zinc-iron alloy per unit area is often relied upon 

 for practical tests. Many quick methods have been developed to 

 determine this factor. With these methods the zinc is removed 

 by immersing the plated iron in a solution of the salt of some 

 metal which 'will be replaced in solution by zinc, but not by 

 iron, or else advantage is taken of the difference in solubility 

 of zinc and iron in acids and alkalies. 



The Preece test ^ employs a solution of copper sulphate into 

 which a measured piece of zinc plate is dipped. Copper forms 

 a spongy black deposit on zinc and a bright, hard deposit on 

 iron ; the number of one-minute immersions necessary to remove 

 the zinc, shown by the first appearance of a bright red copper 

 deposit, serves as a basis for calculating the thickness of the 

 coating. The limitations of this method have been thoroughly 

 discussed elsewhere.* Patrick and Walker ^ have suggested the 

 use of basic lead acetate to replace the copper sulphate of the 

 Preece test ; * in this case zinc is dissolved and spongy lead 

 is precipitated, while the iron remains unattacked. 



Both acids and alkalies can be used to remove zinc from iron 

 without appreciable attack on the latter. In this case, owing 

 to the difference in potential of the two metals, the zinc in 

 contact with the iron inhibits the solution of the latter, while 

 the rapidity with which the former dissolves is increased. A 

 simple method consists in dissolving the zinc from the iron 

 by immersion in sulphuric acid, ^ determining the amount of iron 

 in solution, and adding it to the weight of the remaining plate. 

 The weight of the coating per unit area is calculated from the 

 difference between the weight of iron and the original weight 

 of plate. For certain concentrations of (dilute) sulphuric acid 



10 



' Walker, loc. cit. 



' Walker, loc. cit. ; Patrick, W. A., and Walker, W. H., Joum. Ind. & Eng. 

 Chem. (1911), 3, 238. 



' Loc. cit, 



' It is interesting to note that had an ammoniacal, instead of a neutral, 

 solution of copper sulphate been used, the Preece test would have lost many 

 of its limitations. Although perhaps not quite so convenient as the lead 

 acetate method, the use of ammoniacal copper sulphate solution (see 

 experiment 25) gives reliable and concordant results. 



'Burgess, C. F., Electrochem. & Met. Eng. (1905), 3, 19; Dudley, C. B., 

 Trans. Am. Soc. Test. Materials (1909), 9, 441. 



"Burgess, loc. cit., recommends the use of % N sulphuric acid. 



