DOM and H2S Robustness of spectrophotometric pH measurements

The impacts of H₂S and organic substances on the spectrophotometric pH-determination were comprehensively studied. The overall concept to test for both potential perturbations was based on the assumption that potentiometric measurements are not affected by the presence of DOC and H₂S. Therefore, comparison measurements of potentiometric and spectrophotometric measurements can be used to identify potential biases associated with the spectrophotometric method. We set up a so-called “mini-ocean” experiment, where both measurements can be performed from a well-controlled water sample. Further, the set-up allowed for manipulation of the investigated water sample, e.g. by spiking with DOC and H₂S containing stock solutions. The results of the experiments formed the foundation for a manuscript entitled “Spectrophotometric pH measurements in the presence of dissolved organic matter (DOM) and hydrogen sulfide (H₂S)”, which was submitted to the peer-reviewed journal “Limnology and Oceanography: Methods”.

The main outcome of the study is:

• No impact on spectrophotometric pH measurements by H₂S up to concentrations of 400 μmol kg^-1, which reflects high levels as reported from e.g. the Black Sea.
• Natural DOM did not interfere with the spectrophotometric measurements at concentrations typical for oceanic environments and large estuarine systems.
• Strongly coloured river waters can cause spectral disturbance resulting in calculated pH values that are up to tens of pH units too low (when using an Agilent 8453 with 10-cm cuvette). This result is most likely related to instrument-specific properties at high absorbances rather than to interaction between DOM and the dye solution.
• Perturbations in strongly coloured samples can be reduced by using intense light sources, a shorter cuvette length (as applied in the instrument under development) or spectrophotometrically calibrated glass electrodes