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Modified Limit Test for Chloride and Sulphate

The term limit in Chemistry refers to a value or amount likely to be present in a substance, while test refers to examining or studying it.
The term limit in Chemistry refers to a value or amount likely to be present in a substance, while test refers to examining or studying it. As such, a limit test is nothing more than identifying the impurities in a sense and comparing them to a standard. In general, limit tests are qualitative or semi-quantitative tests designed to identify and control small amounts of impurities in a substance that is likely to exist. For example, the limit test is generally done to determine whether an alloy contains inorganic contaminants.

Modified limit test for chloride

Aim: Test the Modified Chloride Limit of the specific test substance.

Glassware: measuring cylinder, Nessler’s cylinder, and glass rod.
Chemicals: 0.1M silver chloride, distilled water, conc. HCl, dil. Nitric acid.

Principle: In chloride limit tests, precipitation is used to measure the concentration. 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.

Procedure: According to the 6th Edition of the International Pharmacopoeia, the limit test for chloride has been modified within the context of the preparation of standard solutions. In the past, the chlorine solution was prepared through the dissolution of sodium chloride (NaCl, known as Cl- impurity), but now it has been modified by substituting sodium chloride (NaCl) for hydrochloric acid (HCl).

HCl               + AgNO3 → AgCl + HNO3
Hydrochloric   Silver         Silver    Nitric acid
acid                Nitrate       Chloride

Conclusion: When the opalescence produced in the sample solution is lower than that produced in the standard solution, the sample will pass the limitation test for chloride and vice versa.

Modified limit test for sulfate

Aim: Test the sulfate level in the given sample using the modified limit test.

Glassware: glass rod, measuring cylinder, and Nessler’s cylinder.
Chemicals: test substance, distilled water, barium sulfate, hydrochloric acid, potassium sulfate.

Principle: The limit test for sulfates uses the precipitation method as its basic principle. As a result of reacting with barium chloride in the presence of hydrochloric acid, the sulfates precipitate as barium sulfate. In the presence of hydrochloric acid, only sulfate precipitates as other acid radicals do not react with barium chloride as hydrochloric acid prevents the reaction of different acid radicals with barium chloride.

SO4(2−) + BaCl2 (HCl) → BaSO4 + 2Cl−

Precipitates form in the solution, leaving it turbid, with the degree of turbidity being dependent on the amount of sulfate present. In order for the test results to indicate that the sample contains sulfates within prescribed limits, the sample's turbidity must be less than that of the standard.

Reagent preparations:
Barium sulfate reagent - Making a 0.05M barium chloride solution requires dissolving 12 grams of barium chloride (BaCl2.2H2O) in 1000 ml of water. Add 55 ml water, 20 ml alcohol, and 5 ml of 0.0181% w/v potassium sulfate (K2SO4) solution to 15 ml of the prepared solution. Makeup the volume up to 100 ml.

Standard potassium sulfate solution - The volume of K2SO4 and water was made up to 100 ml using 0.1089g of K2SO4 accurately weighed.

Test solution:
Sodium chloride - 20 ml of water should contain 2 grams of sodium chloride.
Sodium bicarbonate - In small quantities of water, dissolve 2 grams of sodium bicarbonate.

Modified sulfate limit test -
From I.P.1996 on, limit tests for sulfate have undergone an extensive modification. By doing so, it eliminated the need for barium sulfate reagents. While turbidity is comparable through the use of alcohol and barium chloride, the method still uses alcohol.

Conclusion: According to the I.P.1996 standard, the sample has passed the limit test if the opalescence produced by the standard exceeds the opalescence of the test.
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