A tracer monitored titration for seawater total alkalinity.

摘要

We introduce a new titration methodology named Tracer Monitored Titration (TMT), in which analyses are free of volumetric and gravimetric measurements, and insensitive to pump precision and reproducibility. Conventional seawater total alkalinity (TA) measurements require calibrated volumetric flasks, pipets and titration cells to quantify sample and calibrated burets or piston pumps to dispense titrant. Consequently, of the very few fully automated systems that exist, none are sufficiently simple to be readily adapted to an autonomous sensor. Spectrophotometric monitoring of titrant dilution factor rather than volume increment lays the burden of performance solely on the spectrophotometer. Titrant is prepared by quantitative addition of a sulfonephthalein indicator to a standardized solution of HCl. Dilution of a pulse of titrant in a titration vessel is tracked using the total indicator concentration, measured spectrophotometrically. The concentrations of protonated and unprotonated indicator species, derived from Beer's Law, are used to calculate titrant and sample dilution factors in addition to the equilibrium position of the titration mixture. We present data from 286 titrations on three types of total alkalinity standards: Na 2CO3 in 0.7 mol kg-soln-1 NaCl, NaOH in 0.7 mol kg-soln-1 NaCl and a seawater certified reference material (CRM). The accuracy and recision are +/-0.2% and +/-0.1% (4 and 2 mumol kg-soln-1 for TA ∼2100-2500 mumol kg-soln -1, n = 242), using low precision solenoid pumps (reproducible to no better than 10%). Excellent accuracy and stability are achieved without the need for standard calibrations as a result of optimizing the performance of the spectrophotometric detection system and relying upon basic chemical thermodynamics for calculating the equivalence point. The TMT system built for this study is fully automated, with maximum sample throughput of 90 day -1 (∼16 minutes per titration). We observed no drift over a 40-day period without calibration. The system is currently capable of ∼3000 continuous titrations without an operator (e.g. hourly samples for 4.5 months). Although applied to acid-base titrations in this project, the approach should be generally applicable to other types of titrations.