A SENSOR FOR IN SITU SPECTROPHOTOMETRIC MEASUREMENTS OF SEAWATER pH

Matthew Paul Seidel, The University of Montana

Abstract

The design of small autonomous sensors for measuring parameters in the ocean has been an important area of research. The development of a seawater pH sensor which is highly accurate and precise has unlimited potential for oceanographic studies. A seawater instrument for pH measurements was built based upon the design of the SAMI-pH for freshwater. The performance of the freshwater design in a seawater setting was unknown. During a cruise in the Labrador Sea the first design of the SAMI-pH for seawater based upon the previous instrument design was tested. The results showed promise but indicated that improvements needed to be made to the stability and reproducibility of the instrument to achieve the desired accuracy of 0.002 pH units and precision of 0.001 pH units. Since a full scale optimization of the SAMI-pH had never been undertaken, multiple parameters were tested. The old Delrin flow cell was replaced with a custom made Z-cell, which improved the absorbance precision. A commercially available static mixer was used to produce reproducible mixing of the indicator and sample. The indicator concentration and indicator pH were optimized to maximize the linearity of the [mCP]T vs. pH plots and to reduce the flushing volume which increased the deployment length. A new method of calculating molar absorptivity ratios was evaluated and the SAMI-pH accuracy was found to be 0.0017 when compared to the Cary UV/Vis. The precision showed excellent improvement from 0.004 to 0.0007 with the new SAMI-pH design. In situ testing of two SAMI-pH instruments was completed at Scripps Institution of Oceanography. Agreement of -0.0047 pH units between the two SAMI-pH instruments during a 22-day period with 883 measurements illustrates the highly accurate and stable performance of the SAMI-pH. An alternative method for pH determination based on theoretical calculations was also tested. The average offset between pH values calculated from the linear extrapolation technique and the theoretical model was -0.0008 pH units illustrating that the linear extrapolation is a rigorous method for determining the pH of the solution.