Sequential Injection Acid–Base Titration—A Useful Application for Process Analytical Science

Abstract

A sequential injection (SI) titration system is proposed for the titration of a strong acid with a strong base. The concept is based on the sequential injection of a base titrant, acid analyte and a base titrant zone into a distilled water carrier stream, giving a stack of well-defined zones adjacent to each other in a holding coil, which is swept by reversal through a reaction coil to the detector. The base zones contain Bromothymol Blue as indicator and the endpoint is monitored spectrophotometrically at 620 nm. A linear relationship between peak width and logarithm of acid concentration is obtained in the range 0.01 to 0.1 mol l ^{- 1} of HCl with 1 × 10 ^{- 3} mol l ^{- 1} of NaOH as titrant. Other linear ranges are possible by changing the titrant concentrations. The results obtained for the sequential injection titration of samples is in good agreement with a standard potentiometric method with an RSD <0.30%. The system is fully computerised with a sample frequency of 30 samples h ^{- 1} .