Total alkalinity measurements in small samples: methods based on CO₂ equilibration and spectrophotometric pH

Background: Total alkalinity (A_T) is a fundamental parameter in understanding the oceanic cycling of carbon dioxide (CO₂). Measurements of the A_T of natural waters are typically obtained through single- or multi-step titrations using a strong acid, with the endpoint pH determined via potentiometry or spectrophotometry. Conventional A_T determinations are labor-intensive and require precise knowledge of the sample's weight or volume. Equilibration with CO₂ gas, with or without a membrane, can simplify the procedure and reduce the required sample volume while maintaining high precision.

Results: Several spectrophotometric A_T methodologies involving CO₂ gas as a titrant are presented: stopped-flow equilibration across a liquid core waveguide (LCW), continuous equilibration using gas-permeable silicone tubing, and direct bubbling with CO₂ gas for measurements of small samples. Alkalinity determinations from CO₂ equilibration are based on a simple linear relationship between A_T, pCO₂, and spectrophotometric pH. Incorporating an empirically derived, temperature-dependent calibration constant, E(T), eliminates the need for precise CO₂ concentrations. Equilibration-based A_T measurements demonstrated high precision (±1.0–2.0 μmol kg⁻¹) and were in strong agreement with standard titration methods (±2.0 μmol kg⁻¹). Novel spectrophotometric instrumentation is introduced, named the Minimal Volume Multiparameter Inorganic Carbon Analyzer (MVMICA), capable of precise pH_T (±0.002) and A_T measurements with volumes ∼1.0 mL. The accuracy of MVMICA over a wide range of conditions makes it invaluable for assessing carbonate chemistry in aquatic systems using limited available sample volumes.

Significance: The three methods presented in this paper offer flexible configurations, each adaptable for specific applications. Membrane equilibrations using Teflon AF 2400 LCW or silicone tubing are appropriate for automated analysis of waters, with potential for in situ A_T determinations. Equilibration of samples across a silicone membrane facilitates rapid, continuous measurements. Alternatively, direct equilibration without a membrane enables analyses of samples as small as 0.50 mL.