APPLICATIONS OF AMPEROMETRIC TITRATIONS IN PHARMACEUTICAL SUBSTANCES
Some pharmaceutical substances are assayed by amperometric titrations, namely : procainamide hydrochloride ;
Materials Required : Procainamide hydrochloride : 0.25 g ; 2M hydrochloric acid : 100 ml ; potassium bromide : 3 g ; 0.1 M sodium nitrite Vs (dissolve sodium nitrite in sufficient water to produce 1000 ml) Standardization of 0.1 M Sodium Nitrite Vs : Dissolve 0.3 g of sulphanillic acid in 50 ml of 2M hydrochloric acid, add 3 g of KBr, cool in ice and titrate with 0.1 M sodium nitrite Vs determining the end-point amperometrically. Each ml of 0.1 N sodium nitrite Vs is equivalent to 17.32 mg of C6H7NO3S.
Procedure : Dissolve 0.25 g of procainamide hydrochloride in 50 ml of 2 M hydrochloric acid, add 3 g of potassium bromide, cool in ice and titrate slowly with 0.1 M sodium nitrite Vs, stirring constantly and determining the end-point amperometrically. Each ml of 0.1 M sodium nitrite Vs is equivalent of 27.18 mg of C13H21N3O . HCl.
Procaine hydrochloride can be assayed exactly in a similar manner by using 0.4 g of the substance.
Each ml of 0.1 M sodium nitrite Vs is equivalent to 27.28 mg of C13H20N2O2, HCl.
Materials Required : 0.001 M Nickel solution ; supporting electrolyte [a mixture of NH4OH (1.0 M) and NH4Cl (0.2 M)] ; gelatin solution (0.2%) : 2 ml ;
Procedure : The following steps may be followed in a sequential manner :
· Weigh accurately a sample of Ni-salt to yield a 0.001 M Ni-solution. To 25 ml of this solution placed in a titration cell add an equal volume (25.0 ml) of a supporting electrolyte and 2 ml of gelatin solution,
· The solution must be deoxygenated. Set the applied e.m.f. to 01.85 V Vs SCE (standard-calomel electrode),
· The diffusion current is measured, and
· Finally, titrate with dimethylglyoxime solution (0.02 M) using the standard general method and obtain a V-shaped graph.
· Each ml of dimethylglyoxime solution is equivalent to 0.5869 mg of Nickel.
Materials Required :
(i) Buffered supporting electrolyte : Dissolve 10 g of KNO3 and 8.2 g of sodium acetate in 500 ml of DW. Add glacial acetic acid carefully until a pH of 4.2 is achieved (pH Meter) (approximately 10 ml of the acid will be required),
(ii) Standard 0.01 M K2Cr2O7 Solution : Weigh accurately ‘ANALAR’-grade 1.47 g K2Cr2O7 into a 500-ml volumetric flask. Dissolve in DW and make up the volume upto the mark, and
(iii) 0.1% w/v Gelatin Solution : Dissolve 0.1 g gelatin in 100 ml of boiling DW.
Procedure : The amperometric titration may be carried out in a 100 ml beaker. A saturated KNO3 salt bridge is employed to provide contact between the saturated calomel electrode and the analyte solution. The various steps involved are as follows :
1. Weigh accurately a sample of Pb-salt to give a 0.01 to 0.02 M lead solution,
2. Transfer 10.0 ml aliquot to the titration vessel,
3. Add to it 25 ml of the buffered supporting electrolyte, and 5 ml of the gelatin solution,
4. Determine the current at zero applied potential,
5. Add K2Cr2O7 (0.01 M) solution, in 1 ml increments, and measuring the resulting current after each addition,
6. Continue the addition to at least 5.0 ml beyond the equivalence point,
7. Correct the currents for the volume change, and plot the graph. Determine the end-point and calculate the number of milligrams of Pb in the given sample, and
8. Repeat the titrations at – 1.0 V. It is essential to bubble N2 through the solution for 10–15 minutes before the titration and while addition of reagents are made. However, the flow of N2 must be stopped at the time of measuring the current. Again, correct the currents for dilution, plot the graph, determine the end-point, and report the number of milligrams of Pb present in the given sample.