Talanta 56 (2002) 61–69 A high precision spectrophotometric method for on-line shipboard seawater pH measurements: the automated marine pH sensor (AMpS) Richard G.J. Bellerby a,b, *, Are Olsen a , Truls Johannessen a , Peter Croot c a Bjerknes Centre for Climate Research, Geophysical Institute, Uniersity of Bergen, Alle ´gaten 70, 5007 Bergen, Norway b Plymouth Marine Laboratory, Prospect Place, West Hoe, Plymouth, Deon, PL13DH, UK c Marine Chemistry and Geology Department, Netherlands Institute for Sea Research (NIOZ), Postbus 59, 1790 AB Den Burg, Texel, The Netherlands Received 3 November 2000; received in revised form 10 July 2001; accepted 8 August 2001 Abstract Measurement strategies for understanding the oceanic CO 2 (carbon dioxide) system are moving towards in situ and ship of opportunity sampling techniques. Automated instrumentation with high accuracy and sampling frequencies will enable a greater understanding of the fluxes of marine carbon and lead to a more reliable constrain on the calculated uptake of anthropogenic CO 2 by the oceans. This paper describes the automated marine pH sensor (AMpS); new instrumentation and methodology for the determination of seawater pH using dual spectrophotometric measurements of sulfonephthalein indicator in a semi-continuous seawater stream. The pH values measured during a recent study in the Weddell Sea are used to illustrate the excellent properties of the AMpS. The method has an on-line precision of better than 0.001 pH units and an estimated accuracy of better than 0.004 pH units. The instrument is compact, portable and has a measurement frequency of 20 samples per hour. The instrument is ideally suitable for operation on ships of opportunity. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Seawater pH; Marine chemistry; Spectrophotometry; Instrumentation; Underway measurements www.elsevier.com/locate/talanta 1. Introduction Notable advancements in instrument technol- ogy have occurred over the last decade in chemi- cal oceanography, no more so than with instruments concerned with the study of the marine inorganic carbon system. It is now possi- ble to measure very small changes in dissolved carbon dioxide (CO 2 ) concordant with the very small increases in total marine inorganic carbon due to the oceanic uptake of anthropogenically produced carbon dioxide. Methods to understand the net uptake of carbon by the oceans are varied and include measurements of the surface CO 2 fugacity ( f CO 2 ) field throughout the world oceans * Corresponding author. Tel.: +47-55-582565; fax: +47- 55-584330. E-mail address: richard.bellerby@gfi.uib.no (R.G.J. Bellerby). 0039-9140/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved. PII:S0039-9140(01)00541-0