Question

How to anslyse Zn^2+ cation

Answer 1

This is an account on how to detect zinc ions in solution by simple precipitation reactions. Zinc is quite easy to distinguish since many of its compounds are insoluble, however many are white in color, end so it is not so easy to distinguish between other amphoteric metal cations such as Barium, Aluminium, Lead, etc. With The following set of tests it will be easy to confirm zinc without requiring complex procedures or sophisticated equipment.

PRINCIPLE

As mentioned, the tests are simple precipitation reactions. A solution of a zinc salt (Zinc Chloride sol.) was mixed with an equal ammount of another solution, which give a physical change, usually a colour change due to a precipitation of the zinc insoluble compound.

Zn X- (aq) + Na+ Y- (aq) ===> Zn+ Y- (s) + Na+ X- (aq)
(s) Solid precipitate forming a colour change in soultion



One type of reaction is not enough, to confirm the presence of zinc, since other metal salts give the same results. For Example with NaOH, many metals give a white ppt., and hence one can't say that the formation of a white ppt of an unknown sample with NaOH is 100% due to zinc cations. However the verification of 4 or 5 such test will be enough to confirm zinc in an unknown sample.

PROCEDURE

In 10ml testtubes, 4mls of zinc solution was placed. To this, about 2mls of solution of the following compounds all having different anions (-ve) was added. If desired, the mixture was heated gently to increase rate of reaction or added in exess to detect further complex reactions, usually the dissolving of the ppt just formed.

The following compounds was mixed with the zinc salt of which 16 produced a valuable result. These are marked with an Y in the React Column .

No.REAGENT NAMEREACTION01Sodium HydroxideY02Ammonium HydroxideY03Sodium CarbonateY04Potassium Sulphate05Sodium ThioSulphate06Sodium (Metabi)SulphiteY07Sodium SulphideY08Sodium Fluoride(y)09Sodium Chloride10Potassium Bromide11Ammomium Iodide12Potassium Iodate(y)13Ammonium PhosphateY14Sodium TetraBorate (Borate)Y15Sodium Salicylate16Sodium Benzoate17Tannic AcidY18Sodium Malate sol.19Sodium Methanoate sol.20Sodium Ethanoate21Sodium Citrate22Sodium Tartarate23Sodium SilicateY24Potassium Ferro(II)CyanideY25Potassium Ferri(III)CyanideY26Sodium VanadateY27Potassium Permanganate28Potassium Dichromate29Sodium TungstateY30Ammonium Molybdate31Sodium BiSeleniteY32Potassium Thiocyanate