Principle Involved in the Limit Test for Chloride, Sulphate, Iron, Arsenic, Lead, and Heavy Metals

A limit test is defined as a quantitative or semiquantitative analysis that identifies and controls very small amounts of impurity that might exist in the substance being examined. Generally, limit tests are used to determine whether an ingredient contains any inorganic impurities. The purpose of limit testing is simply to identify the contaminants present in a substance and to compare them to the standard.

Limit tests have the following importance:

• To determine how much pollution is harmful
• In order to find out the number of impurities that can be avoided or cannot be avoided

Limit test for chloride

Principle: The precipitation reaction is the limit test for chloride. In the presence of dilute nitric acid, chlorides precipitate from soluble chloride when silver nitrate reacts with soluble chloride to produce silver chloride, the form of which appears as solid particles in the solution (opalescence). Based on how much chloride is present in the test substance, the intensity of turbidity is affected.

NaCl + AgNO3 (HNO3) → AgCl + NaNO3

Limit test for sulphate

Principle: For the sulphates limit test, precipitation method is applied to the test solution, followed by comparison with a standard solution containing sulphates known in proportion. As a result of hydrochloric acid reaction with barium chloride, barium sulphates are precipitated as barium. sulphate.

SO2− 4 + BaCl2 (HCl) → BaSO4 + 2Cl−

Precipitates form in solutions, causing turbidity. The degree of turbidity depends on the amount of sulphate in the solution. Test results that are less turbid than the standard signifies a sample that contains sulphates in accordance with the prescribed limits of the test.

Limit test for iron

Principle: By removing iron from thioglycollic acid and reacting with ammonium citrate, purple dye results are produced by forming ferrous thioglycollate.



By comparing the colour intensity of test solution to standard iron solution containing 0.04 mg iron, we can determine the concentration of iron in the test solution.

Limit test for arsenic

Principle: An Arsenic Limit Test is based on the formation of yellow stains on mercuric chloride paper when arsenic gas reacts with hydrogen ion in the presence of a reducing agent such as potassium iodide. A Gutzeit test requires special equipment and is also known as the Gutzeit test. A sample of arsenic-containing arsenic acid is reduced to arsenious acid in a reaction involving reducing agents such as zinc, stannous acid, potassium iodide, and hydrochloric acid. Further reduction of arsenious acid to arsine (gas) occurs in the presence of hydrogen, causing a yellow stain in mercuric chloride paper

H3AsO4 + H2SnO2 → H3AsO3 + H2SnO3
Arsenic acid                   Arsenious acid

H3AsO3 + 3H2 → AsH3 + 3H2O
Arsenious acid        Arsine

Assuming that there is arsenic in the sample, how much yellow stain will appear on mercuric chloride paper depends on the quality of the arsenic present.

Limit test for lead

Principle: This limit test of lead is conducted by reacting lead in alkaline solution with diphenyl thiocarbazone (dithizone) to form a lead dithizone complex that is detected in colour.

The chloroform dithizone compound is green in colour, while the lead-dithizone complex is violet in colour, making the result a red colour after the reaction is complete.

Limit test for heavy metals

Principle: An acidic medium is used in heavy metal limit testing, where metallic impurities react with hydrogen sulphide to produce brownish-coloured solution. The results of this test show that lead, mercury, bismuth, arsenic, antimony, tin, cadmium, silver, copper, and molybdenum exhibit corrosion. A metal impurity in a substance is expressed as a number of parts of lead per million parts of the substance. According to the Indian Pharmacopoeia, the maximum concentration is 20 ppm.

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