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