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Limit test

Introduction

In Chemistry, Limit means a value or amount that is likely to be present in a substance and test means to examine or to investigate. Thus, limit test is nothing but to identify the impurities in the substance and compare it with standard. In general, limit test is defined as quantitative or semiquantitative test designed to identify and control small quantities of impurity which is likely to be present in the substance. Limit test is generally carried out to determine the inorganic impurities present in compound.

 

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.

 

Limit test of sulphate is based on the reaction of soluble sulphate with barium chloride in presence of alcohol and potassium sulphate to form barium sulphate, which appears as solid particles (turbidity) in the solution. Here alcohol is added to prevent super saturation.

 

Limit test of heavy metals is based on the reaction of metallic impurities with hydrogen sulfide in acidic medium to form colored solution. Metals that response to this test are lead, mercury, bismuth, arsenic, antimony, tin, cadmium, silver, copper, and molybdenum.

Limit test of lead is based on the reaction of lead and diphenyl thiocabazone (dithizone) in alkaline solution to form lead dithizone complex which is read in color.

 

Limit test of Iron is based on the reaction of iron in ammonical solution with thioglycollic acid to form iron thioglycolate which is pink-reddish purple in color.

 

Limit test of Arsenic is based on the reaction of arsenic gas with hydrogen ion to form yellow stain on mercuric chloride paper in presence of reducing agents like potassium iodide. It is also called as Gutzeit test and requires special apparatus.

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