Spectrophotometric Titration of Weak Acids or Bases in Aqueous Solution Using the Type II Plot

The Type II plot of Higuchi, Rehm, and Barnsfein for the spectrophotometric titration of weak acids or bases has been tested in aqueous solutions. Accurate results are obtained provided the ionic strength is held constant during titration, A theoretical analysis of the range of application of the Type II plot is considered for solutions of weak acids or bases. Experimentally, it is found that acids (or bases) with pK ≦ 9.0 can be determined with an accuracy of 0.2% in 0.1M concentration, and 0.07M sodium carbonate can be titrated to bicarbonate with 0.2% accuracy. The Type II plot can be used to determine the end point of weaker acids (or bases) than is possible using more conventional methods of end point detection.