Influence of m-cresol purple indicator additions on the pH of seawater samples:: correction factors evaluated from a chemical speciation model

The perturbation of the indicator m-cresol purple on the pH in seawater is illustrated in diagrams, representing measurements in l-cm and 5-cm cells. The diagrams apply to a measured pH interval of 7.4-8.4 using a 2-mM stock solution of m-cresol purple sodium salt dissolved in seawater. The magnitude of the perturbation is described as correction values, i.e., the change in seawater pH caused by the indicator. The diagrams are based on calculations made by using the equilibrium speciation programme, MARINHALT. From these calculations, and least squares fitting methods, pH correction values are described in terms of the pH difference between each seawater sample and the pH of an indicator stock solution. Calculations are performed for a typical high latitude water and a north Pacific deep water. Diagrams are presented for a salinity of 35 and a temperature of 15 degrees C. Responses to salinities between 32 and 36 and temperatures 15-25 degrees C are illustrated as well. A +/- 0.05 pH difference between a seawater sample and an indicator stock solution gives a correction of less than 0.001 pH unit for a 1-cm cell. For a 5-cm cell, pH differences between the indicator stock solution and a seawater sample as large as +/- 0.3 cause corrections smaller than +/- 0.001 pH unit. Calculations demonstrate that the five-fold lower indicator concentration used with 5-cm cells decreases the perturbation effect by approximately a factor of five relative to l-cm cells. (C) 1999 Elsevier Science B.V. All rights reserved.