Abstract
The method depends on the addition of excess of Nessler reagent to the solution of isoniazide, and the back-titration of the excess of Hg with sodium diethyldithiocarbamate standard solution using CuSO4 as an indicator. The recommended procedure is as follows: transfer an accurately weighed portion of the sample powder, corresponding to about 20 mg of isoniazide, to a centrifuge tube. Add 10 ml of methyl alcohol (aldehyde-free), and shake the mixture for 10 minutes mechanically, centrifuge, pipet 10 ml of the alcoholic solution into a 25 ml Erlenmeyer flask, and evaporate to dryness on a steam bath. The residue is dissolved in 10 ml water, and transferred with washings into a 50 ml measuring flask. Add 5 ml of Nessler reagent and warm on a steam bath for 10 minutes.Cool to room temperature and dilute with water to the mark. Filter, discard the first 10 ml and pipet 10 ml from the subsequent filtrate into a 100 ml conical flask. Add 20 ml of water, 10 ml of pyridine, 3 ml of 0.1M EDTA, 1 ml of 20% tartaric acid and 1 drop of 0.1M CuSO4. Titrate with 0.02M sodium diethyldithiocarbamate until a stable yellow-brown coloration appears. Carry out a blank determination.
p-Aminosalicylate compounds do not interfere with the titration, while the presence of lactose, dextrose, and sugar etc. should be avoided.
