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Chemical tests for inorganic anions: Part-I

Chemical tests for inorganic anions
 Chloride, Sulphate and Nitrate
Test for Chloride (Cl-)

i)          2M Nitric acid
ii)         Silver nitrate
iii)        Lead acetate.

Preparation of reagents:
Reagent A: Nitric acid (2M). Pour 128 ml conc.HNO3 into 500ml water, dilute to 1lit.

Reagent B: Silver nitrate (0.1M). Dissolve 16.99 g of AgNO3   in water and dilute to 1lit.

Reagent C: Lead acetate solution (0.25M). Dissolve 95 g lead acetate trihydrate in a mixture of 500 ml water and 10ml glacial acetic acid and dilute the solution with water to 1lit.

I.     Take 1ml of water extract in a clean test tube. Add 2-3 drops of reagent A to it. Then add 2-3 drops of reagent B. White precipitate indicate the presence of chloride ion, which is soluble in NH4 OH and insoluble in HNO3.

II.   Take 1ml of extract in a clean test tube. Add 2-3 drops of reagent C to it. White precipitate indicates the presence of Cl-    

III. The above tests must be carried out along with the reference standard as well as blank.

Additional tests
i)                    Potassium dichromate
ii)                   Conc. H2SO4
iii)                 Diphenylcarbazide

Preparation of reagents:
Reagent A:
1% Diphenylcarbazide prepared in ethanol
Take  5 ml aqueous extract in a clean test tube, evaporate to dryness,add a small quantity of solid potassium dichromate and 1 ml conc. H2SO4 . Heat the mixture gently. A red vapour of Chromyl chloride evolves which turns diphenylcarbizide spot on filter paper violet.


Test for Sulphate: (SO4 2-)

i)                    Dilute HCl   
ii)                   Barium Chloride
iii)                 Sodium rhodizonate

Preparation of reagents:
Reagent A: Dilute HCl

Reagent B: 0.25 M Barium chloride (Dissolve 61.1 g barium chloride dihydrate in water and dilute to 1 lit.)

Reagent C: 0.5 % aqueous solution of rhodizonate. Dissolve 0.5 g of sodium rhodizonate in 100ml of water. Reagent should be freshly prepared.

Procedure :

I.   Take 1ml of extract in a clean test tube. Add 2-3 drops of reagent A to it and then the test tube is kept in a water bath at 1000C for 5 minutes, then add 2-3 drops of reagent B to it. A white  crystalline precipitate indicates the presence of SO4.2-

II.   Place 1 drop of reagent B upon filter paper, followed by a drop of freshly prepared solution of reagent C. Treat the reddish-brown spot with a drop of acidified extract. The coloured spot disappears.

III.               The above test must be carried out along with the reference standard as well as blank.

Additional test

i) Barium chloride
ii) Potassium permanganate
iii) Hydrogen peroxide

Preparation of reagents:
ReagentA; 0.25M Barium chloride (Dissolve 61.1 g barium chloride dihydrate in water and dilute to 1 lit.)
Reagent B: 0.02 M Potassium permanganate solution in water. (Dissolve 3.16 gm Potassium permanganate in water and dilute to 1 liter)
Reagent c: 3% Hydrogen peroxide solution
            Take 3 drops of extract in a semi-micro test tube. Add 2 drops of reagent B and 1 drop of reagent A. a pink ppt appears. Add a few drops of reagent c and shake well. The coloured ppt becomes clearly visible.

Test for Nitrate ion (NO3-)

i)          Sulphanilic acid             ii)         a-naphthylamine                      
iii)         Acetic acid                   iv)        Zinc dust                     
v)         Sulphanilic acid             vi)        Ferrous sulphate  
vii)        Sulphuric acid

Preparation of Reagents:
Reagent A: 1% sulphanilic acid in 30% acetic acid. Dissolve 1 g of sulphanilic acid in 100ml of warm 30% acetic acid.
Reagent B: a – naphthylamine Boil 0.3 g of a-naphthylamine with 70ml of distilled water, filter or decant and mix  with 30ml of glacial acetic acid.

I.    Place 1 drop of neutral or acidic extract in a clean spot plate. Add 1 drop of the reagent A and then  1 drop of reagent B. Then add a pinch of zinc dust. Pink to red colour indicates the presence of Nitrate. (Griess test)

II.    Place a crystal of ferrous sulphate on a spot plate. Add a drop of extract and allow a drop of conc. sulphuric acid to run at the side of the drop. A brown ring forms round the crystal of ferrous sulphate indicating presence of nitrate. 

N.B: If nitrite is present in the extract, then at first it is removed from the solution by sulphamic acid or by sodium azide and then test for nitrate is performed.

The above tests must be carried out along with the reference standards as well   as blank.

Additional test

i)                    Brucine
ii)                   Conc. H2SO4
Place a few drops of aqueous solution on a clean spot plate. Add one drop of conc. sulphuric acid and small crystal of Brucine and stir the mixture with a glass rod . A blood red colour is produced.


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