CHEMISTRY : Metal Cation Identification

C A L C I U M

INTRODUCTION:

This is an account on how to detect aluminum ions in solution by simple precipitation reactions. Aluminium is not very easy to distinguish since most of its compounds are soluble, and those which are not are white. With the following set of tests it will be easy to confirm Aluminium without requiring complex procedures or sophisticated equipment.

PRINCIPLE

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

Ca++ 2X- (aq) + 2 Na+ Y- (aq) ===> Ca++ 2Y- (s) + 2 Na+ X- (aq)
(s) Solid precipitate forming a colour change in soultion

One type of reaction is not enough, to confirm the presence of aluminium, 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 Aluminium cations. However the verification of 4 or 5 such test will be enough to confirm Aluminium in an unknown sample.

PROCEDURE

In 10ml testtubes, 4mls of Aluminium 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 aluminium salt of which only 16 produced a valuable result. These are marked with an Y in the React Column .

RESULTS

01: Sodium Hydroxide

a) This gave a MILKY WHITE PPT. insoluble in exess Hydroxide

b) No reaction on heating Unslouble (slightly soluble) white Calcium Hydroxide was formed.

02: Ammonium Hydroxide

a) This gave no immediate ppt.

b) On heating and standing for some hours, few cloudy white ppt was formed. Ammonia is unable to precipitate the sparingly soluble Calcium Hydroxide

03: Sodium Carbonate

a) A WHITE PPT was immediately formed

b) No reaction on heating or exess Insoluble white Calcium Carbonate was immediately formed.

04: Potassium Sulphate

a) A white precipitate was formed slowly.

b) Heating the mixture, made faster the reaction, and the white precipitate was quickly formed. Calcium Sulphate is an insoluble white substance.

05: Sodium Sulphite (or Sodium Metabisulphite)

a) No reation when mixture is room temp.

b) On boiling, the reaction formed a white ppt, and the liquid bumped violently on reaching the boiling point. Propably the white compound is Sodium sulphite, and it's reaction rate is slower from the sulphate one. Also it is possible that heating have in some way converted the Sulphite into Sulphate, and the ppt formed was due the sulphate.

06: Sodium Sulphide

a) After letting to stand for few minutes, the clear solution turns slightly dirty due the formation of a very faint white ppt.

b) On heating a light grey, faint white ppt was formed and fell to tha bottom of the tube. Slow formation of the insoluble light grey Calcium Sulphide took place. Heating increased the rate of reaction.

07: Sodium Fluoride

a) A faint white ppt was formed, which did not intensified on standing.

b) No further reaction on heating. An exact similar ppt took place with the Al and Mg solutions.

08: Potassium Iodate

a) No immediate precipitate or colour change. c) A WHITE PPT was formed only after standing for some quite time or heating. Precipitation of the insoluble Calcium iodate. Don't know why this was not spontanous (i.e formed as soon as the two solutions were mixed.)

09: Ammonium Phosphate

a) A WHITE PPT formed at once.

b) No further reaction on heating. Calcium also bears an insoluble phosphate.

10: Sodium TetraBorate

a) A dense WHITE PPT. was formed rapidly.

b) No further reaction on heating White insoluble Calcium Borate was formed.

11: Sodium Citrate

a) No reaction at all

b) On heating a WHITE precipitate was formed. This is an interesting test, since Citrate does not form a ppt in any way with Mg and Al. An excellent test to distinguish between Mg and Ca for example. White insoluble Calcium Borate was formed.

12: Sodium Silicate

a) WHITE ppt was formed rapidly.

b) No reaction on heating or standing. White Calcium Silicate was formed.

13: Potassium Ferro(II)Cyanide

a) No reaction or ppt detected

b) On heating a little white fine ppt was formed. Don't know if its an experimental error, but I would not rely on and perform this test.

14: Sodium Tungstate

a) A MILKY WHITE ppt was formed rapidly.

b) No further reaction, due to heating, xs tungstste or standing

15: Ammonium Molybdate

a) No initial precipitation or reaction

b) On heating, a FAINT WHITE PPt was formed.

CONCLUSION

From the results aboove, Calcium can be easily traced. The first test is the formation of a white precipitate with Sodium Hydroxide (undissolved in exess) and the lack of a ppt with Ammonium Hydroxide. Next comes the sulphate test, which eliminates most transition metals, Al and Mg.

The citrate ppt will confirm the presence of calcium. Of course further tests such as the borate and metabisulphite can be performed for further confirmation if desired. The Phosphate, Iodate, Fluoride and Carbonate are of no help.

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