Effect of chelating agents on the stability of injectable isoniazid solutions.

The chelating agents NTA, EDTA, HEDTA and DTPA afford a stabilizing action towards isoniazid solutions. The stabilizing effect of the chelating agents is highly dependent on their concentration. The stability of isoniazid solutions increases markedly with increasing NTA or EDTA concentration up to 1 mmol/l. Beyond this concentration, the stabilizing effect of NTA is more or less independent of its concentration and the dependency of the degradation rate of isoniazid on the concentration of EDTA becomes less pronounced. There is an optimum concentration for HEDTA and DTPA at which their stabilizing effect is maximum; below or above this concentration, the stability of isoniazid decreases. The stabilizing efficiency of the investigated chelating agents decreases in the order DTPA greater than HEDTA greater than EDTA greater than NTA, which runs parallel with the stability constants of their metal chelates.