Effect of organic acids on the determination of carbonate species in solutions

Accurate analysis of carbonate species in natural waters is important for diagenetic modelling and oilfield inorganic chemical scale precipitation estimation. The concentration of carbonate species is often measured by titration. Errors can occur when organic acids are present in the solution due to the titration end points not being clearly identified. In this paper a method is presented for the accurate analysis of bicarbonate when organic acids are present. An automatic titrator has been used. We find that the pH at the carbonic acid (H2CO3∗) titration end point varied with the total alkalinity, the organic acid concentration and solution ionic strength. In some cases the H2CO3∗ end point could not be identified due to masking but the bicarbonate and organic acid anion concentrations could be obtained by estimation using equations developed in this paper from the total alkalinity, titration end point pH and ionic strength. If not identified, significant errors in carbonate precipitation estimation will otherwise occur; examples are presented.