Achieving accurate spectrophotometric pH measurements using unpurified meta-cresol purple

Abstract

Abstract For best accuracy, spectrophotometric characterizations of seawater pH are obtained using a purified pH-sensitive dye—usually meta-cresol purple (mCP) for typical ranges of seawater pH. In recognition of practical limitations, though, a straightforward method is here proposed to improve measurements made using unpurified mCP. The user first determines, for a particular lot of unpurified mCP, the absorbance contribution of indicator impurities at 434 nm (434Aimp). Correction for this contribution is then mathematically applied to the measurements of seawater pH. We tested this approach using six unpurified lots of mCP and, for comparison, purified mCP in a synthetic experimental solution over the pH range 7.25–8.25. The 434Aimp correction yielded substantial improvements in pH accuracy: on the order of 0.005 at low pH (~ 7.25) and 0.01 or more at higher pH (~ 8.25). The pH accuracy achieved by the corrective model was also examined relative to the Global Ocean Acidification Observing Network (GOA-ON) “weather” and “climate” goals for pH measurements (uncertainties of ± 0.02 and ± 0.003, respectively). When previously published algorithms (appropriate for purified mCP) were used, none of the unpurified dyes met the more stringent “climate” goal in waters of pH > 7.6. With the algorithms proposed here (i.e., incorporating the lot-specific 434Aimp correction), three of the six lots came into “climate” compliance over the full experimental pHT range and two additional lots achieved “climate” compliance up to pH ~ 8.0. This protocol offers a simple, user-determined correction to significantly improve the accuracy of pH measurements made with unpurified mCP.