1968
JOURNAL OF CHEMICAL EDUCATION
OCTOBER, 1931
A NOTE ON THE PRECIPITATION OF NICKEL AND COBALT AS SULFIDES IN QUALITATIVE ANALYSIS In the usual scheme of qualitative analysis, nickel, cobalt, manganese, and zinc are precipitated with H2S or (NH&S from ammoniacal solutions. This procedure often results in the formation of colloidal CoS and NiS, which is very undesirable. Haring, with Westfall and Leatherman [J.Am. Chem. Soc., 52, 513541 (1930)], has found that a t a pH of 4.4 or slightly lower in an ammonium acetate solution, both of these elements will give perfectly compact and easily filterable precipitates. I n an effort to arrive a t approximately this pH and get a good precipitate of these two elements we have made the following change in the usual procedure: After precipitating iron, aluminum, and chromium with NHIOH, the solution is made just acid with acetic acid, and 5y0(NH&S is slowly added until precipitation is complete. All of the above elements are completely precipitated in a good granular form. This was tried in our class of 400 students and in only four cases were colloids formed. We suspect that in these cases the students failed to acidify with acetic acid. In the procedure followed, the iron, aluminum, and chromium hydroxides were filtered off before testing for the other ions. But even if these are all mixed, as in the A. A. Noyes procedure, the method outlined may still be used. In this case the hydroxides will dissolve in the acetic acid but will reprecipitate with the (NH&S.
Language is only the instrument of science, and words are but the signs of ideas.-
JORNSON
