The limit test of iron depends on the reaction of iron in ammonical solution in the presence of citric acid , with thioglycolic acid to obtain a pink to deep purple color . Citric acid forms a complex with iron which is not precipitated by ammonia . The color is obtained due to formation of ferrous salt i.e ferrous thioglycolate [Fe ( HSCH2COO)2 ] which disappear in air due to oxidation.
Thioglycolic acid (H3CH2COOH) is a derivative of glycolic acid [CH2(OH)COOH] . It is a colorless liquid with independent odour . The color product from a known amount of the substance is compared with the standard color obtain from a known amount of iron under the same experimental condition.
standard iron solution - Accurately 0.1726 g of ferric ammonium sulfate is weighed and dissolved in 10 ml of 0.1 N H2SO4 and sufficient water to produce 1000 ml . Each ml of this solution contains 0.02 mg of iron.
* Procedure : (A) Preparation of test solution :
0.5 gm of the substance be examined is dissolved in 40 ml of distilled water and transferred to a 'nesseler cylinder' . 2 ml of a 20% w/v solution of iron free citric acid and 0.1 ml of 'thioglycollic acid' are added , mixed and made alkaline with iron free ammonia solution. Dilute it to 50ml with distilled water and allow to stand for 5 minutes. Any color is produced is not more instant than the standard solution.
(B)Preparation of the standard solution :
Standard iron solution is distilled with 40ml of water in a ' Nesseler cylinder' 2 ml of a 20% of a 20% w/v solution of iron . Free citric acid are added, mixed and made alkaline with iron free ammonia solution. Dilute it to 50 ml with distilled water and allowed to stand for 5 minute.
In the limit test of iron citric acid is added to prevent the precipitate of iron a with ammonia