Traceability of pH measurements in brackish waters to a primary pH standard

Spectrophotometric pH measurements need to be traced back to a primary pH standards, i.e., buffer solutions to which a pHT value has been assigned by Harned cell measurements. For seawater applications the most widespread buffer substance is TRIS, which has previously been characterized for salinities of 20-40 by DelValls and Dickson (1998).

To enable traceable pH measurements in brackish waters TRIS buffer solutions, which cover the salinity range from 5 to 20 and additionally 35, were prepared in cooperation of PTB and IOW and under advice of Prof. A. Dickson. The pH of these buffers was analysed by Harned cell measurements, within the temperature range 5-45°C.

Subsequently, these buffer solutions were used to determine the dissociation constant, pK, of the pH-indicator dye mCP by spectrophotometric measurements for the same S- and T-range. The figure below shows preliminary result of the obtained TRIS pH and mCP pK values. Additional work up and quality and consistency checks of the data is required before the results can finally be summarized with the aim to be published in scientific journals. The aim of this work is to close a currently existing gap of fundamental knowledge for the application of spectrophotometric pH measurements, which results from a lack of well characterized buffer solutions in the salinity range 5-20. This also affects the degree of knowledge of the pK of mCP in this salinity range. Moreover, the dye has not been characterized over the range of temperature in natural environments. Closing this gap, which is only possible in cooperation with the PTB and considerably extending the work originally planned within WP2, will enhance accuracy of the measurement in the salinity range 0-20, and also allow for design of spectrophotometric pH-measurement systems running over a range of (in situ) temperatures. This work will continue beyond the duration of the project.

Figure 1: (a) Extended characterization of the pH of TRIS buffered artificial seawater solution for the salinity range 5-20 allow for (b) the accurate determination of the dissociation constant of mCP in brackish waters, covering a wide range of temperatures.

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