Accurate spectrophotometric pH measurements made directly in the sample bottle using an aggregated dye perturbation approach

Abstract

Spectrophotometric pH measurements of seawater (pHspec) are routinely made by the oceanographic community for a wide variety of studies. However, obtaining consistent measurements between laboratories that meet stringent thresholds such as the “weather” (± 0.02) and “climate” (± 0.003) standards, has been a challenge. One of the main sources of error for pHspec measurements is gas exchange of carbon dioxide during sample handling. Here, we present a simple method where pHspec measurements on the total scale are made directly in the sampling bottle, which minimizes sample handling errors because the solution is not transferred during analysis. We compared the performance of this method to a standard automated benchtop system on a hydrography cruise, and the two methods were consistent to 0.003 ± 0.0033 (1σ). This demonstrates that this simple method can produce pHspec that approaches climate quality, and comfortably meets weather quality standards. Additional benefits include high sample throughput, and the ability to rapidly quantify dye perturbation effects for each sample. The latter should be particularly useful for low salinity samples such as those taken from estuaries, insofar as modifications specific to the pHspec measurements of estuarine waters are employed.