pH perturbation by the addition of the dye to the sample

The problem of perturbation of a sample’s pH by addition of the indicator dye is by far more complex for measurements in brackish waters than in the open ocean, where variations in salinity and alkalinity are relatively small, and the buffer capacity is high due to the high alkalinity. Therefore, parameterization and extrapolation to zero indicator pH was tackled experimentally, theoretically, and considered in the instrumental design. For three contrasting waters of the Baltic sampled in Norrbyn, Askö, and, Kristineberg, pH measurements were performed using a series of well defined indicator volume additions. A speciation model was then developed which accurately reproduced the observed change in the measured pH (Fig. 1). Additional model runs were performed to specifically address the processes within the developed instrument prototype, which uses an injection flow through principle. The theoretical approach thus allowed to scrutinize and optimize data fitting options such as the best fitting function and the fitting window used for the extrapolation to zero mCP concentration.

Figure 1: Measured (dots) and modelled pH as a function of indicator addition for the three natural waters of the Baltic
Sea covering a large salinity range, reported on the free pH scale.