Titration of alkali solutions containing aluminum

Application of differentiation circuits to potentiometric titration. I

Abstract

In acidimetry of sodium hydroxide solution containing aluminum, colloidal precipitation appears during titration and a complicated potentiometric titration curve is obtained. A rapid method was investigated to titrate such alkali solutions with good accuracy.
(1) It was ascertained that an inflection point corresponding to total sodium hydroxide in the solution always appears on the titration curves of the solutions containing different amount of aluminum. (2) In order to detect the equivalence point of the titration, two stages of differentiation circuits were constructed and a change in the second derivative of the output from a pH indicator electrode was shown on an ammeter. (3) The time interval was measured from the start of titration to the indication of the end point on the ammeter, and the amount of total alkali was obtained from a calibration curve. The titration was finished within one or two minutes, and the determination was made with a relative error less than about 3%.