Limit test for Iron

 

Aim: To carry out the limit test for iron for given sample

Apparatus: Nessler's cylinder, glass rod , pipette, 20% w/v citric acid. Thioglycolic acid, Dil. Ammonia Solution /Ammonium Hydroxide solution, Litmus paper, distilled water and standard ferric ammonium sulphate solution( 0.0173gm in 1000ml of water).

Principle:

The limit test for Iron is based upon the reaction between ferrous ion and Thioglycolic acid to produce pale pink color to reddish violet color, due to formation of ferrous thioglycolate complex in ammonia solution.

The oxidation state of Iron is not important because the thioglycolic acid is a strong reducing agent which reduce the ferrous ion.

The color produce by the sample is compared with that of the Standard color produced by specific amount of ferric ammoinum sulphate solution under the same reaction condition.

For the sample to pass the limit test for iron, the sample color should not be more than that of standard colour.

Reaction: 

Reasons:

• Citric acid solution is added  because it forms a soluble complex with iron and avoids precipitation as its hydroxide when Dil. ammonia solution is added.
• Dil. Ammonia solution is added to make the solution alkaline because ferrous thioglycolate gives pink color only in basic media but remain color less in acidic or neutral media.

Procedure:

Standard Color	Sample color
Pipette out 2 ml of standard ferric ammonium sulphate solution in to a clean Nessler"s cylinder and dilute to 25ml with distilled water. Add 2ml of 20% w/v citric acid and 0.1 ml thioglycolic acid. Make up the solution alkaline by adding dil. ammonia solution and check with  Litmus paper. Dilute to 20 ml with distilled water, mixed and allowed to stand for 5 min.	Dissolve the specified amount of sample with 25 ml distilled water in a clean Nessler's cylinder. Add 2ml of 20%w/v  citric acid and 0.1ml thioglycolic acid. Make up the solution alkaline by adding dil. Ammonia solution and check with Litmus paper. Dilute to 50 ml with distilled water, mixed and allowed to stand for 5 min.

Preparation of standard ferric ammonium sulphate solution:

Dissolve 0.173g of ferric ammonium sulphate[ NHFe (SO4)2], 2H2O to 1.5 ml HCl. Add sufficient water to make up the volume 1000ml.

Comparison:

compare the sample color with standard color by viewing the Nessler's cylinder against white back ground.

Obsevation & Report : When viewed transversely against a white background, the color produced in the sample is less/more than the standard color. Hence the given sample passes/ fails the limit test for Iron.