PRACTICAL PHARMACEUTICAL INORGANIC CHEMISTRY – LIMIT TEST FOR CHLORIDES

Object – To perform the limit test for Chlorides.

Reference – Chatwal G.R., Pharmaceutical Chemistry-Inorganic, Volume I, Himalaya Publishing House, Mumbai, II Edition, 1996, 45-46

Materials required –

Chemicals – Nitric acid, Silver nitrate solution, Sodium Chloride, Distill water

Glasswares & Apparatus – Nessler cylinder, Measuring cylinder, Pipette, Glass rod, Spatula, Weighing balance

Theory – Limit tests are quantitative or semi quantitative tests designed to identify and control small quantities of impurity which are likely to be present in the substance. The limit tests for chlorides is based on the reaction between silver nitrate and soluble chlorides to obtain silver chloride which is insoluble in dilute nitric acid. The silver chloride produced in the presence of dilute nitric acid makes the test solution turbid, the extent of turbidity depends on the amount of chloride present in the substance is compared with the standard opalescence produced by addition of silver nitrate to a standard solution having a known amount of chloride and the same volume of dilute nitric acid as used in the test solution. If the turbidity from the sample is less than the standard turbidity, the sample will pass the limit test and vice versa.

Reactions involved in the test –

  1. Soluble chlorides in substance + silver nitrate in presence of dilute nitric acid gives silver chloride and nitrate ions
  2. Sodium chloride + silver nitrate (in presence of dilute nitric acid) gives silver chloride and sodium nitrate

Procedure –

Test solution –

  1. First of all, cleaned and dried Nessler cylinder was taken.
  2. In the Nessler cylinder, 1 gram of sample was taken.
  3. Then, 10 ml of water and 1 ml of nitric acid was added to it.
  4. Now, the volume was made upto 50 ml with water.
  5. Then, 1 ml of silver nitrate solution was added.
  6. It was stirred well.
  7. Finally it was kept aside for 5 minutes and then turbidity or opalescence was observed.

Standard solution –

  1. First of all, cleaned and dried Nessler cylinder was taken.
  2. In the Nessler cylinder, 1 ml of 0.058455% w/v solution of sodium chloride was taken.
  3. Then, 1 ml of nitric acid was added to it.
  4. Now, the volume was made upto 50 ml with water.
  5. Then, 1 ml of silver nitrate solution was added.
  6. It was stirred well.
  7. Finally it was kept aside for 5 minutes and then turbidity or opalescence was observed.

Inference –

  1. Turbidity of test solution > turbidity of standard solution – test failed
  2. Turbidity of test solution < turbidity of standard solution – test passed
  3. Turbidity of test solution = turbidity of standard solution – test passed

Result – On comparing the turbidity / Opalescence of test solution and standard solution, it was found that the turbidity of test solution is…………………………….. (more than/ less tan / equal to) the standard solution. Hence the sample ……………………………..(passed / failed) the limit test for chlorides.

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