Application of Henderson equation for the determination of weakly acidic pharmaceutical compounds. Part 3: Determination of phenylbutazone.

: A new method based on the Henderson equation is proposed for the determination of sulfathiourea. The determination is achieved in mixed organic solvent/water media. To apply the Henderson equation, the dissociation constant (pKa) of sulfathiourea in such media should be known. Thus, the values of pKa as function of the composition of the media have been determination potentiometrically according to Albert and Serjeant. Equations have been derived to calculated the pKa of sulfathiourea in any medium. The pKa of sulfathiourea in water and the other solvents, methanol, ethanol, isopropanol and acetone, are reported. The proposed method is simple and rapid and gives reliable results. The mean percent recoveries of the powder in 50% solvent: methanol, ethanol, isopropanol and acetone are: 97.87 +/- 1,30, 100.85 +/- 1.35, 102.01 +/- 1.39 and 99.35 +/- 0.29 respectively. For the tablets, the mean percent recovery in 50% acetone is 97.95 +/- 0.60. The mean percent recovery of added standard sulfathiourea to the tablet powder is 101.55 +/- 0.83.