Spectrophotometric determination of pH in seawater off Taiaroa Head, Otago, New Zealand: Full-spectrum modelling and prediction of pCO 2 levels

Abstract

Accurate measurement of seawater pH has long been sought by marine chemists (for example: [Dickson, A.G. 1993a. The measurement of sea water pH. Marine Chemistry, 44, 131–142, Dickson, A.G. 1993b. pH buffers for sea water media based on the total hydrogen ion concentration scale. Deep-Sea Research, 40, 107–118; Zhang, 1996; Tapp, M., Hunter, K.A., Currie, K. and Macaskill, B. 2000. Apparatus of continuous-flow underway spectrophotometric measurement of surface water pH. Marine Chemistry 72(2–4), 193–202; Friis, K., Koetzinger, A., Wallace, D.W.R. 2004. Spectrophotometric pH measurement in the ocean: Requirements, design and testing of an autonomous charge-coupled device detector system. Limnology and Oceanography: Methods 2, 126–136]. Recently, such attempts have taken on greater significance as anthropogenic carbon dioxide emissions may create rapidly changing oceanic pH. Spectrophotometric techniques have been accepted generally as the best for determination of seawater pH. Here we report a new technique using thymol blue as the indicator dye and fitting the entire spectrum from 400 to 900 nm rather than measuring the absorbance values at only two or three points in the spectrum. This full-spectrum modelling enables a reduction in signal to noise over other techniques. In the laboratory, we find with seawater samples a pH precision increase of five-fold “within” a sample and seven-fold “between” samples when comparing the full spectrum to the three-point method of analysis [Zhang, H., Byrne, R.H. 1996. Spectrophotometric pH measurements of surface seawater at in-situ conditions: absorbance and protonation behaviour of thymol blue. Marine Chemistry 52, 17–25]. We then use this analysis technique for a series of transects between July 2005 and March 2006 east of Taiaroa Head, New Zealand. The pH values determined at sea using the full-spectrum modelling technique along with independently measured alkalinity ( A T) are used to determine the total inorganic carbon concentration ( C T). C T in turn is used with A T to calculate pCO 2 levels in surface seawater. Comparing pCO 2 levels from independently measured pCO 2 acquired in parallel with pH, we show an average internal consistency agreement of 1.15% ( σ = 0.23%) when using the Mehrbach et al. [Mehrbach, C., Culberson, C.H., Hawley, J.E., Pykowicz, R.M. 1973. Measurements of the apparent dissociation constants of carbonic acid in seawater at atmospheric pressure. Limnology and Oceanography 18, 897–907] (as refit by Dickson and Millero[Dickson, A.G., Millero, F.J. 1987. A comparison of the equilibrium constants for the dissociation of carbonic acid in seawater media. Deep-Sea Research 34, 1733–1743] carbonate dissociation constants. The pCO 2 values predicted from pH are very close to the measured values of pCO 2. We indicate that while both pH and pCO 2 are of great interest in surface seawater, it may be possible to measure only pH and alkalinity to obtain an excellent predictor of pCO 2 levels. The instrument we use to measure pH combined with the full-spectrum modelling show that regular, real-time analysis of the carbonate system in surface seawater on a small boat (14 m) can be precise and consistent.