Limit tests are special tests for impurities allowed to be present in trace quantities in some cases or not allowed to be present at all in other cases according to pharmacopoeia.
The amount of impurities present in the sample can be determined by comparing either (1) Opalescence.
or (2) Turbidity.
or (3) Color.
Produced by reacting fixed amount of standard solutions of the impurities with the same reagents.
Notes:
1- Nesseler tubes must be (same glass, same diameter,same height)
2- Standard solution must be treated in the same way as sample (same reagent quantities with same arrangement)
3- For comparing white turbidity use Black background For comparing color use White backgroundLimit test for acid radicals
1) Limit test for chloride
Principle:
The test depends on the precipitation of Cl- impurity with silver nitrate in the presence of conc. nitric acid. conc. HNO3
Cl - + AgNO3 ———→ AgCl ↓
White ppt
Comparing the turbidity produced with standard turbidity of a specified amount of Cl - treated in the same manner. The turbidity in the solution is compared visually by examination in two identical Nessler tubes by viewing them transversely through the solution against black background.
Role of HNO3
a) To dissolve any Ag salts formed except Agcl.
b) HNO3 oxidizies any reducing substance which may reduce AgNO3 into Ago (black) which masks our test.
Procedure (E.P)
Test Standard
5 ml sample 1ml 0.01 N HCl
+ +
1 ml conc. HNO3 1 ml conc. HNO3
+ +
Distilled water to 50 ml Distilled water to 50 ml
+ +
1 ml AgNO3 T.S 1 ml AgNO3