Tracer monitored titrations: Measurement of total alkalinity

Todd R. Martz, Andrew G. Dickson, Michael D. DeGrandpre

Research output: Contribution to journalArticlepeer-review

Abstract

We introduce a new titration methodology, tracer monitored titration (TMT), in which analyses are free of volumetric and gravimetric measurements and insensitive to pump precision and reproducibility. Spectrophotometric monitoring of titrant dilution, rather than volume increment, lays the burden of analytical performance solely on the spectrophotometer. In the method described here, the titrant is a standardized mixture of acid-base indicator and strong acid. Dilution of a pulse of titrant in a titration vessel is tracked using the total indicator concentration measured spectrophotometrically. The concentrations of reacted and unreacted indicator species, derived from Beer's law, are used to calculate the relative proportions of titrant and sample in addition to the equilibrium position (pH) of the titration mixture. Because the method does not require volumetric or gravimetric additions of titrant, simple low-precision pumps can be used. Here, we demonstrate application of TMT for analysis of total alkalinity (AT). High-precision, high-accuracy seawater AT measurements are crucial for understanding, for example, the marine CaCO3 budget and saturation state, anthropogenic CO 2 penetration into the oceans, calcareous phytoplankton blooms, and coral reef dynamics. We present data from 286 titrations on three types of total alkalinity standards: Na2CO3 in 0.7 mol kg-soln -1 NaCl, NaOH in 0.7 mol kg·soln-1 NaCl, and a seawater Certified Reference Material (CRM). Based on Na2CO 3 standards, the accuracy and precision are ±0.2 and ±0.1% (4 and 2 μmol kg-soln-1 for AT ≃2100-2500 μmol kg·soln-1, n = 242), using low-precision solenoid pumps to introduce sample and titrant. Similar accuracy and precision were found for analyses run 42 days after the initial experiments. Excellent performance is achieved by optimizing the spectrophotometric detection system and relying upon basic chemical thermodynamics for calculating the equivalence point. Although applied to acid-base titrations in this paper, the approach should be generally applicable to other types of titrations.

Original languageEnglish
Pages (from-to)1817-1826
Number of pages10
JournalAnalytical Chemistry
Volume78
Issue number6
DOIs
StatePublished - Mar 15 2006

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