Definition of limit test:
Limit test is defined as a quantitative or semi-quantitative test designed to identify and control small quantities of impurities which are likely to be present in the substance.
Apparatus Required
Nessler cylinders
Glass rod
Stand
Chemicals Required
Dilute hydrochloric acid
0.5 M Barium chloride: 122.1 g of BaCl2. 2H2O dissolved in distilled water.
Barium sulphate reagent containing 0.5 M barium chloride in 1000 ml of water. (This is prepared as follows: Mix 15 ml of 0.5 M BaC/2, 55 ml of water and 20 ml of sulphate free alcohol. Add 5 ml of 0.0181% w/v potassium sulphate. Dilute to 10 ml with water and mix.)
Reaction
Principle
It is based upon the chemical reaction between barium chloride and soluble sulphate in the presence of dilute hydrochloric acid. The turbidity produced is compared with the standard solution. Barium chloride reagent contains barium chloride, sulphate – free alcohol and small quantity of potassium sulphate. The inclusion of the small quantity of potassium sulphate in the reagent increases the sensitivity of the test. Alcohol prevents super saturation and a more uniform turbidity develops. If the turbidity produced in the test is more intense than the standard turbidity, then the drug fails the test, otherwise, it passes the test.
Procedure
Take two 50 ml Nessler cylinders. Label one as “Test” and the other as “Standard”.
| Standard | Test |
| 1. Place 1 ml of 0.1089% w/v solution of K2SO4 in a Nessler cylinder. | 1. Dissolve the specified quantity of the substance in distilled water and transfer to Nessler cylinder. |
| 2. Add 2 ml of dil. HCl. | 2. Add 2 ml of dil. HCI. |
| 3. Dilute to 45 ml with water and add 5 ml of barium sulphate reagent. | 3. Dilute to 45 ml with water and add 5 ml of barium sulphate reagent. |
| 4. Stir immediately with a glass rod and allow to stand for 5 minutes. | 4. Stir immediately with a glass rod and allow to stand for 5 minutes. |
| 5. Observe the turbidity developed and compare with that of the sample. | 5. Observe the turbidity developed and compare with that of the standard. |
The turbidity in test is seen less than that of the STD; thus, the sample pass the limit test of sulphate
