Zinc may be precipitated by 8-hydroxyquinaldine (2-methyloxine) in acetic acid-acetate solution: it can thus be separated from aluminium and magnesium [see Section ll.ll(G)]. It can be weighed as Zn(C10H8ON)2 after drying at 130-140 °C. The co-precipitated reagent is volatile at 130 °C.
The solution may contain up to 0.05 g Zn in 200 mL. Add dilute aqueous ammonia solution until a white precipitate of zinc hydroxide just appears. Re-dissolve the zinc hydroxide with a drop of acetic acid. Add a slight excess of the reagent (see Note) (2 mL for each 10 mg of Zn present) and then 2-3 drops of concentrated ammonia solution; the pH should be at least 5.5. Digest the precipitate at 60-80 °C for 15 minutes, allow to stand for 10-20 minutes, and filter through a sintered-glass or porcelain filtering crucible. Dry to constant weight at 130-140 °C. Weigh as Zn(C10H8ON)2.
If aluminium is present, add 1 g of ammonium tartrate to the clear, slightly
acid solution. Introduce the reagent (2 mL for each 10 mg of Zn present), dilute the solution to 200 mL, and heat to 60-80 °C. Neutralise the excess of acid by adding dilute ammonia solution (1:5) dropwise until the complex salt which forms on the addition of each drop just re-dissolves on stirring. Add, with stirring, 45 mL of 2M ammonium acetate solution. The pH should be at least 5.5. Allow the solution to stand for 10-20 minutes, and complete the determination as above.
Note. The reagent is prepared by dissolving 5 g 8-hydroxyquinaldine in 12 g glacial acetic (ethanoic) acid and diluting to 100 mL with water.