Introduction
Limit tests are the category of impurity testing in which solution of an unknown sample is compared to a standard solution. Limit test is defined as the quantitative or semi quantitative test designed to identify and control small quantities of impurity which is likely to be present in the substance. In these tests the turbidity/ opalescence/ colour intensity produced by test sample is compared with the turbidity/ opalescence/ colour intensity produced by standard solution. In this article we will see the limit tests for chlorides, sulphates, and iron.
Limit test for Chlorides
Limit test for chlorides is based upon the reaction between soluble chloride ions and silver nitrate reagent in nitric acid medium. The insoluble silver chloride formed by reaction makes the solution turbid. This turbidity is compared with the standard turbidity produced by addition of silver nitrate to the known amount of chloride ion solution. If the test solution shows less turbidity as compared to standard solution, the sample passes limit test.
Preparation of solution for test
A specific amount of substance is dissolved in distilled water and the volume made to 50ml in Nessler’s cylinder. Depending upon the nature of substance the solution for test is prepared, as follows;
- Alkaline substances like carbonates, hydroxides, etc. are dissolved in sufficient quantity of acid so that effervescence ends and free acid is present.
- For insoluble substances like kaolin, water extract is prepared, filtered and then filtrate is used as solution for test.
- Salts of organic acids like sodium benzoate, sodium salicylate, etc. liberate free water insoluble organic acids. During acidification it is filtered of and filtrate is used as solution for test.
- Coloured substances like crystal violate, malachite green, etc. are carbonised and ash produced is extracted in water.
- Substances like potassium permanganate are reduced by boiling with alcohol and the filtrate is used.
Principle
Limit test of chloride is based on the reaction of soluble chloride with silver nitrate in presence of dilute nitric acid to form silver chloride, which appears as solid particles (Opalescence) in the solution.
NaCl + AgNO3 = AgNO3 + NaNO3
Procedure
| Test sample | Standard compound |
| Specific weight of compound is dissolved in water or solution is prepared as directed in the pharmacopoeia and transferred in Nessler cylinder | Take 1ml of 0.05845 % W/V solution of sodium chloride in Nessler cylinder |
| Add 1ml of nitric acid | Add 1ml of nitric acid |
| Dilute to 50ml in Nessler cylinder | Dilute to 50ml in Nessler cylinder |
| Add 1ml of AgNO3 solution | Add 1ml of AgNO3 solution |
| Keep aside for 5 min | Keep aside for 5 min |
| Observe the Opalescence/Turbidity | Observe the Opalescence/Turbidity |
Observation
The opalescence produced in sample solution should not be greater than standard solution. If opalescence produced in sample solution is less than the standard solution, the sample will pass the limit test of chloride and vice versa.
Reason
Nitric acid is added in the limit test of chloride to make solution acidic and helps silver chloride precipitate to make solution turbid at the end of process.
Limit test for Sulphates
Principle
Limit test of sulphate is based on the reaction of soluble sulphate with barium chloride in presence of dilute hydrochloric acid to form barium sulphate which appears as solid particles (turbidity) in the solution.
SO42- + BaCl2 = BaSO4 + Cl–
Procedure
| Test sample | Standard compound |
| Specific weight of compound is dissolved in water or solution is prepared as directed in the pharmacopoeia and transferred in Nessler cylinder | Take 1ml of 0.1089 % W/V solution of potassium sulphate in Nessler cylinder |
| Add 2ml of dilute hydrochloric acid | Add 2ml of dilute hydrochloric acid |
| Dilute to 45 ml in Nessler cylinder | Dilute to 45 ml in Nessler cylinder |
| Add 5ml of barium sulphate reagent | Add 5ml of barium sulphate reagent |
| Keep aside for 5 min | Keep aside for 5 min |
| Observe the Turbidity | Observe the Turbidity |
Barium sulphate reagent contains barium chloride, sulphate free alcohol and small amount of potassium sulphate.
Observation
The turbidity produce in sample solution should not be greater than standard solution. If turbidity produces in sample solution is less than the standard solution, the sample will pass the limit test of sulphate and vice versa.
Reason
Hydrochloric acid helps to make solution acidic. Potassium sulphate is used to increase the sensitivity of the test by giving ionic concentration in the reagent. Alcohol helps to prevent super saturation.
Limit test for Iron
Principle
Limit test of Iron is based on the reaction of iron in ammoniacal solution with thioglycolic acid in presence of citric acid to form iron thioglycolate which is pale pink to deep reddish purple in color.
Procedure
| Test sample | Standard compound |
| Sample is dissolved in specific amount of water and then volume is made up to 40 ml | 2 ml of standard solution of iron diluted with water up to 40ml |
| Add 2 ml of 20 % w/v iron free solution of citric acid | Add 2 ml of 20 % w/v iron free solution of citric acid |
| Add 2 drops of thioglycolic acid | Add 2 drops of thioglycolic acid |
| Add ammonia to make the solution alkaline and adjust the volume to 50 ml | Add ammonia to make the solution alkaline and adjust the volume to 50 ml |
| Keep aside for 5 min | Keep aside for 5 min |
| Color developed is viewed vertically and compared with standard solution | Color developed is viewed vertically and compared with standard solution |
Earlier ammonium thiocyanate reagent was used for the limit test of iron. Since thioglycolic acid is more sensitive reagent, it has replaced ammonium thiocyanate in the test.
Observation
The purple color produce in sample solution should not be greater than standard solution. If purple color produces in sample solution is less than the standard solution, the sample will pass the limit test of iron and vice versa.
Reason
Citric acid helps precipitation of iron by ammonia by forming a complex with it. Thioglycolic acid helps to oxidize iron (II) to iron (III). Ammonia to make solution alkaline.
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