Evaluation of Zinc Dust

1 gram of Zn dust is dissolved in a solution of 4 grams of sodium bi­chromate in 1 liter water, and 20 cc. 20 per cent sulfuric acid added with thorough stirring. A 250-cc. portion of this solution is diluted with 900 cc. water, and 150 cc. 20 per cent sulfuric acid and 100 cc. 0.1 N potassium iodide solution are added. The mixture is allowed to stand in the dark for 30 minutes, and then the iodine is back-titrated with 0.1 N thiosulfate solution. To determine the strength of the bichromate, exactly 0.800 grams of it are treated in the same way.

Calculation: В = cc. thiosulfate for 0.800 g. bichromate Calculation: A = cc. thiosulfate for 4 g. bichromate + 1. zinc dust

Then, (В X 1-25—A) X 1.308 = per cent zinc metal

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