Time Series Measurement of pH at Station ALOHA

Dan Sadler

Abstract

Measurements of pH have been made for over 2 years at Station ALOHA using both electrode and spectrophotometric techniques. A comparison shows the analytical precision of both methods is better than 0.002 pH units. However, an intercruise comparison of the deep water pH record indicates that the spectrophotometric technique is better suited for time series analysis. We feel this is due to the spectrophotometric method's freedom from uncertainties inherent in the electrode determination such as drifting electrodes, changing junction potentials, and difficulties in preparing sea water buffers.

The pH of surface waters at Station ALOHA ranges between 8.075 and 8.105 on the sea water scale (pHsws) at 25C. This range is consistent with the pH calculated using DIC and total alkalinity measured at ALOHA and commonly accepted apparent equilibrium constants. Temporal variability of pH decreases with depth. While variation in surface water is about 0.03 pH units, waters near the oxygen minimum (800-1000 m) vary as much as 0.01 pH units. Waters deeper than 2000 m show the least intercruise difference with variations less than 0.005 pH units.