Atomic Weight of Zinc, History
On the International List of Atomic Weights zinc is given as 65.38. It has apparently four isotopes, with atomic weights of 64, 66, 68, and 70. They can be partially separated by distillation in a high vacuum. The atomic number is 30. The following determinations have been made of the atomic weight of zinc:   In 1814 Wollaston gave the ratio Zn:O = 100:24.41. Somewhat higher values than the atomic weight for zinc of 64.73, which this ratio gives, had been previously obtained.
Jacquelaine obtained the ratio Zn:O = 100:24.169 by two conversions of the metal into the nitrate and ignition to oxide. He found no occluded gases in his oxides, Zn = 66.201.
Erdmann, by the same method, obtained the ratio Zn:O = 100:24.6. The atomic weight of zinc is 65.04.
Morse and Burton made fifteen determinations by this method, and tested in all cases their zinc oxide for nitrogen oxides. According to their ratio Zn:O = 100:24.5139, the atomic weight of zinc is 65.269.
Morse and Arbuckle corrected this investigation by eight determinations, in which allowance was made for occluded gases. Their corrected value for the atomic weight of zinc was 65.457.  Several experimenters ignited the sulphate into the oxide. The oxalate and the lactate have also been ignited.
Jacquelaine obtained the ratio Zn:O = 100:24.147 by converting a known weight of metal into sulphate and igniting to oxide. From this ratio, which is the sum of two determinations, zinc is 66.26.
Erdmann gave the ratio ZnSO_{4}:ZnO = 100:50.26. The sum of two ignitions of the sulphate by Baubigny gave the ratio ZnSO_{4}:ZnO = 100:50.414. The atomic weight of zinc is 65.40 from the latter ratio.  Some of the earlier investigators determined the hydrogen evolved by dissolving known quantities of zinc in dilute acid. Van der Plaats, from three determinations, obtained Zn = 65.68.
Reynolds and Ramsay, as the mean of five determinations, obtained Zn = 65.4787 ± 0.016135. This figure, which was calculated on the assumption that H = 1.0, makes Zn = 65.9776 if H = 1.00762.  Gladstone and Hibbert obtained the ratio 2Ag:Zn = 3.2980:1 by the simultaneous electrolytic deposition of silver and zinc. Their result, obtained from six experiments, makes the atomic weight of zinc 65.421.
 Marignac analysed the double salt ZnCl_{2}.2KCl. Baxter and Hodges determined the percentage of zinc in the chloride by estimating the metal electrolytically. Their ratio Zn:Cl_{2} = 0.92195:1, obtained as an average of seven determinations, gave Zn = 65.379.
Baxter and Grose operated similarly on the bromide and obtained the ratio Zn: Br_{2} = 0.409103:1. They made eight determinations, and calculated an atomic weight of 65388 for zinc. Richards and Rogers determined the ratios ZnBr_{2}:2 Ag and ZnBr_{2}:2 AgBr by analysing the bromide. Their paper also contained the following determinations by Richards alone. The ratio ZnBr_{2}:2 Ag = 104.379:100 was obtained from three experiments. This makes Zn = 65.376. From his ratio ZnBr_{2}:2 AgBr = 0.599611:1, obtained from three determinations, Zn = 65.377. The International Committee on Atomic Weights for 1925 have adopted the value Zn = 65.38.
