Spectrophotometric pH measurements in the presence of dissolved organic matter and hydrogen sulfide. (27th November 2017)

• Record Type: Journal Article
• Authors: Müller, Jens Daniel; Schneider, Bernd; Aßmann, Steffen; Rehder, Gregor
• Is Part Of: Limnology and oceanography, methods. Volume 16:Number 2(2018:Feb.)
• Page Start: 68
• Page End: 82
• Publication Date: 2017-11-27
• Subjects: Limnology -- Methodology -- Periodicals; Oceanography -- Methodology -- Periodicals; 551.48
• DOI: 10.1002/lom3.10227
• Languages: English
• ISSNs: 1541-5856

Abstract

Abstract: Spectrophotometric pH measurements were first introduced for oceanic environments and they facilitated the determination of the marine CO2 system, including the direct observation of Ocean Acidification. Extended characterizations of the indicator dye m‐Cresol purple over the past two decades enabled the application of this method to natural waters ranging from brines to freshwaters. However, the required determination of the dye's dissociation constants and absorbance properties were exclusively performed in buffer solutions prepared with artificial seawater. Potential perturbations by substances that occur in natural waters, but are not included in the buffer solutions, have never been tested. Therefore, we studied the impact of elevated amounts of dissolved organic matter (DOM) and hydrogen sulfide (H2 S) on spectrophotometric pH measurements. We did not observe an impact on spectrophotometric pH measurements by H2 S concentrations up to 400 μmol kg −1, which reflect high levels such as those reported from the Black Sea. Likewise, natural DOM did not interfere with the spectrophotometric measurements at concentrations typical for oceanic environments and large estuarine systems. However, strongly colored river waters can cause spectral disturbances resulting in calculated pH values that are up to tenths of pH units too low. To circumvent such disturbances, we recommend using intense light sources, a shorter cuvette length or spectrophotometrically calibrated glass electrodes when performing spectrophotometric measurements under critical conditions.