A REVISION OF THE ATOMIC WEIGHT OF ZINC. II. The Electrolytic Determination of Zinc in Zinc Chloride
Journal of the American Chemical Society
In a recent paper Baxter and Grose' have described the electrolytic determination of the zinc .in zinc bromide by deposition in a weighed mercury cathode. The atomic weight of zinc calculated from the results of this work, 65.38S, agrees very well with the outcome of Richards and Rogers's2 comparison of zinc bromide and silver, 65.376. Since zinc composes very nearly one half of zinc chloride, the latter substance is better suited than zinc bromide for the purpose in question. On the other hand
... the preparation of zinc chloride in an anhydrous condition and free from basic chloride is an unusually troublesome process. The difficulty was overcome, however, by first preparing anhydrous zinc bromide, and then converting this salt into the chloride by heating it in a current of chlorine. The result of the electrolytic analysis of the salt is in accord with the atomic weight of zinc as determined by the analysis of the bromide. Purification of Reagents. Water, nitric acid, sulfuric acid, alcohol and mercury were purified as described by Baxter and Hartmann.8 Bromine was freed from impurities by the processes used by Baxter and Grover,' and from the pure bromine, hydrogen bromide and its solution were prepared by synthesis with pure electrolytic hydrogen in the same way and with the identical apparatus employed by Baxter and Grover. The Preparation of Zinc Bromide. Since the zinc chloride was prepared from the bromide, the purification of the various samples of the latter substance will be described first. Sample A.-This specimen was prepared by the method used by Baxter and Grose.6 Crude stick zinc was electrolytically transported through a concentrated solution of zinc bromide. The resulting crystalline sponge was washed many times with water and then leached for a week in a very dilute solution of hydrobromic acid. After another thorough washing the metal was dissolved by covering it with dil. hydrobromic acid in a quartz flask and slowly adding pure bromine. During this operation the flask was cooled with running water to prevent undue rise of temperature. Since Baxter and Grose found electrolysis effective in removing iron, but not cadmium and lead, the solution was allowed to stand with an excess of zinc for 3 months with occasional shaking, in order to precipitate, in part a t least, the above metals as well ,as others of smaller solution tension than zinc.