Note 410 — Chapter XI · THE COPPER AND SILVER GROUPS. PRECIPITATION WITH HYDROGEN SULPHIDE

When 10% of the zinc in a 0.1 molar solution has been precipitated, 0.01 molar sulphuric acid has been formed. For the sake of a rough approximation, the acid may be considered completely ionized and then [H+] = 0.02, which is 200 times the value of [H+] in a saturated H2S solution (p. 200); if the presence of a little sulphuric acid in the original zinc sulphate solution, resulting from a slight hydrolysis of the salt, is ignored, the concentration of the sulphide-ion is decreased roughly (200)2 or forty thousandfold, while the concentration of zinc-ion falls 10%. The corrections, that have been indicated, would change the quantities involved, but they would not modify the character of the result.