Effect of Dissolved CO2 on Gran Plots

This paper discusses the determination of the p K a of a weak acid by the use of a Gran plot and the effect of CO 2 dissolved in the titrant, a strong base solution. Experimental examples are shown for the titration of benzoic acid with "CO 2 -free" and CO 2 -containing KOH solutions. Dissolved CO 2 does not cause a large error in the determination of p K a by the use of Gran plots, even when the amount of CO 2 dissolved in the titrant is so high that the simulation of the titration curve gives a serious error in the p K a value. Owing to this advantage of Gran plots, the preparation and storage of strong base solutions used for Gran plots are simple, and hence the determination of the p K a value of a weak acid can be completed in a single 3-hour undergraduate laboratory.