This document describes the limit test for iron. The test involves reacting a sample solution containing iron in ammoniacal solution with citric acid and thioglycollic acid. This produces a pale pink to deep reddish purple color due to ferrous compounds. The color produced from the sample solution is compared to a standard ferric ammonium sulfate solution. If the sample solution color is less dark than the standard, then the sample passes the limit test for iron. Citric acid forms complexes that prevent interference from other metal cations.

1. LIMIT TEST FOR
IRON

2. PRINCIPLE
 The limit test for iron is based on the reaction of iron in
ammoniacal solution in presence of citric acid, with
thioglycollic acid when a pale pink to deep reddish purple
color is formed due to ferrous compounds.
 Thioglycollic (HS)CH2COOH, is useful analogue of glycollic
acid, CH2(OH)COOH which is prepared by the action of
potassium hydrogen sulphide on monochloroacetic acid.

3. PRINCIPLE
 The colour produced from a specified amount of
substance from the test is compared by viewing
vertically with a standard(ferric ammonium sulphate).
 If the colour from test solution is less dark than the
standard, then the sample passes the test.
 Citric acid does not allow the precipitation of iron by
ammonia by forming a complex with it. The colour due
to ferrous compound gets destroyed by oxidising agents
and alkalis. So interference of other metal cations is
eliminated by making use of 20% citric acid, which forms
a complex with other metal cations.

4.  PREPARATION OF STANDARD SOLUTION OF IRON: It is prepared by
adding 0.173g of ferric ammonium sulphate [NH4Fe(SO4)2.12H2O]
to 1.5 ml of HCl and adding sufficient water to produce 1000ml.
 METHOD: A solution is first of all obtained from a specified amount
of substance. It is then taken in a Nessler cylinder and to it 2ml of
20% citric acid solution and 2 drops of thioglycollic acid are
added.
 The solution are mixed, and made alkaline with a solution of iron
free ammonia (Ferrous thioglycolate is colourless in acid or
neutral solutions.In acidic media Fe2+ is oxidised to Fe3+ in
presence of thioglycollic acid hence solution becomes colourless.
The purple colour is developed only in the presence of alkali) and
diluted to 50ml mark with water.
 The colour obtained is compared with standard prepared from
2ml of standard solutions of iron, with 50ml of water in Nessler
cylinder following the same procedure as for the test solution.

5.  If the colour produced by test solution has been less than that
of standard, the sample passes the limit test for iron.