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Acidity of seawater

The anthropogenic increase in the CO2 concentration in the atmosphere is leading to an increased uptake of this gas into the oceans and thus to a decrease in the pH value. This has a significant influence especially on marine biology as, for example, calcifying organisms such as corals and slugs are decomposing. The effect of a decreasing pH value is currently being investigated in many projects. A reliably measured pH value therefore is an important parameter for such studies and for the monitoring of the oceans.

It is a difficult task in oceanography to measure the pH value by means of glass electrodes that are widely applied in other areas and use a potentiometric method of measurement. The complicated composition of seawater and its varying salinity have a disturbing influence on the voltages measured which can hardly be compensated for. Therefore, another measurement procedure has established itself in oceanography.

An established alternative to the measurement by means of glass electrodes is the measurement of the pH value by means of spectrophotometers. This method uses a pH-sensitive dye (e.g. m-cresol purple) whose protonated and deprotonated species have different percentages depending on the pH value. The absorption of light by the two species can be quantified by measuring a spectrum. The pH value can be calculated from the determined ratio and the dissociation constant of the dye used. The spectroscopic determination of the pH value provides very precise results, but at this time, there is no traceability concept that would be valid over the entire relevant range of salinity. This is especially important for pH value measurements in the Baltic Sea, in which the salinity underlies extremely heavy salinity variations.


  Dissociation equilibrium and absorption spectrum of the dye m-cresol purple

Cooperation projectse
In a cooperation project with the Leibniz-Institut für Ostseeforschung Warnemünde (IOW), a metrological basis for the precise and traceable measurement of the pH value in the Baltic Sea is planned to be established. The task of our Working Group is the measurement of the pH value of suitable buffer solutions by means of a primary potentiometrical procedure. The varying salinity of the solutions is adapted to seawater. The IOW measures the same solutions spectrophotometrically while using the dye m-cresol purple. The measurement results of both laboratories serve to determine the dissociation constant of m-cresol purple at different salinities that finally enable the traceability of the spectrophotometrically measured pH value at the respective salinity. This enables routinely conducted measurements of the pH values in the Baltic Sea by means of this optical method that is traced to the pH value of potentiometrically characterized reference solutions.