Overcoming Limitations for Ultrasensitive In Situ pH Measurements in Marine Waters while Maintaining Traceability to Primary Standards

Abstract

Historically, pH was defined by Sørensen as the negative logarithm of the hydrogen ion activity. Today, primary pH standards are prepared and characterized by electrochemical measurements performed in metrological institutes. As such standards must have an ionic strength below 0.1 mol·kg^-1, uncertainties arise when studying seawater pH. For this and other practical reasons, oceanographers prefer to use the total hydrogen ion concentration scale and measure pH optically with pH indicator dyes rather than using electrochemical methods. Yet only the latter directly measure ion activity and are directly traceable to primary standards. To address this challenge our group focuses on improving the precision of electrochemical measurements using pH glass electrodes to about 0.001 pH units. This work highlights recent advances in enhancing electrode sensitivity using a novel coulometric method, reducing pH measurement uncertainty while maintaining traceability, and developing a submersible pH probe for in situ measurements in marine waters.