Talanta 77 (2008) 533–540 Contents lists available at ScienceDirect Talanta journal homepage: www.elsevier.com/locate/talanta Underway determination of alkalinity in estuarine waters by reagent-injection gas-diffusion flow analysis Sarah M. Gray 1 , Peter S. Ellis, Michael R. Grace, Ian D. McKelvie ∗ Water Studies Centre, School of Chemistry, P.O. Box 23, Monash University, Clayton, Victoria 3800, Australia article info Article history: Received 29 October 2007 Received in revised form 11 March 2008 Accepted 17 March 2008 Available online 25 March 2008 Keywords: Gas diffusion Reagent-injection Alkalinity Spectrophotometry abstract The development and application of a portable, hybrid reagent-injection gas-diffusion flow analysis tech- nique is described for the underway measurement of total alkalinity in estuarine waters. Injection of pH 4.5 buffer into a continuously flowing sample stream produced gaseous CO 2 that diffused across a microporous PTFE membrane into a weakly buffered acceptor stream containing bromothymol blue indi- cator. The resultant change in acceptor stream pH was detected photometrically using a super-bright LED with a multi-reflection flow cell and charge coupled device detector. This method gave a detection limit of 0.5 mg CaCO 3 L -1 , with reproducibility of 1.0% R.S.D. at 160 mg CaCO 3 L -1 , and a measurement rate of 71 injections h -1 . The portable FIA system was used for underway analysis of estuarine waters with salin- ities ranging from that of freshwater to seawater, and there was close agreement between the results obtained by underway analysis and from a reference titration method. © 2008 Elsevier B.V. All rights reserved. 1. Introduction The total alkalinity (TA) of natural waters is practically defined as the amount of base (HCO 3 - , CO 3 2- , OH - ) that must be titrated with acid in order to reach a pH of 4.5, the point at which hydroxyl ions and dissolved carbonate species are converted to carbonic acid [1]. In marine waters, the total alkalinity will also include other basic or weakly basic species with pK a values of ≥4.5 that are present at detectable concentrations, such as borate and silicate [2]. Total alkalinity is historically expressed in terms of the mass equivalent of calcium carbonate, and in freshwaters total alkalin- ity can range from 0.05 to 500 mg CaCO 3 L -1 (1–10,000 M) [3], while in marine systems the concentration usually lies within a narrow range of 115–130 mg CaCO 3 L -1 (2300–2600 M) [4]. Total alkalinity is an important water quality parameter because it pro- vides a measure of the buffering capacity of a waterbody. Acidic species from atmospheric or catchment sources, or from internal biogeochemical processes, may exceed the buffer capacity of a nat- ural waterbody, causing measurable pH change. There have, for example, been widespread reports of the acidification of freshwa- ter lakes in response to acid rain [5]. It is estimated that 30–40% of the anthropogenic carbon dioxide added to the atmosphere due to the burning of fossil fuels is absorbed by the oceans [6,7], and there ∗ Corresponding author. Tel.: +61 3 99054558; fax: +61 3 99054196. E-mail address: ian.mckelvie@sci.monash.edu.au (I.D. McKelvie). 1 Current address: Nanotechnology Victoria Ltd., PO Box 229, Dingley 3127, Victoria, Australia. is now concern that this will decrease the pH of the oceans, by as much as 0.4–0.5 pH units by 2100 [8,9]. In estuaries, total alkalinity may behave conservatively, i.e. the observed concentration change is due only to dilution by mixing of marine and freshwaters, or it may behave non-conservatively in response to the effects of processes such as calcite dissolution or deposition [10], rapid, high river flow events [11], or respiration and primary production. In the anoxic bottom waters and/or sedi- ments of an estuary, alkalinity increases in response to microbially mediated processes such as denitrification, and sulfate, iron and manganese reduction that consume H 3 O + [12]. Because of the transient nature of dispersion and mixing pro- cesses in estuaries, which are highly dependent on river flows and tidal fluxes, high frequency or spatially intense sampling and analy- sis is desirable in order that these processes be properly studied and understood. Ideally rapid, on-line or in situ analytical techniques should be employed either onboard or from a sampling vessel to enable underway collection of chemical information. Total alkalinity in marine and estuarine systems has histori- cally been determined by potentiometric acid–base titration [13], and this approach has subsequently been adapted for shipboard use [14]. However shipboard measurements of total alkalinity are technically demanding, and involve painstaking measurements of combined titrant-acid and sea water volumes, as well as careful standardisation and storage of acids [15]. Thus, the development of simpler, more robust methods for the determination of total alkalinity in a variety of aquatic systems, that do not require the constant supervision of an experienced analyst, and which can be operated autonomously are highly desirable. While automated lab- 0039-9140/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.talanta.2008.03.020