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

Area of Science:

• Environmental Science
• Analytical Chemistry
• Oceanography

Background:

• Total alkalinity (A_T) is crucial for understanding oceanic carbon dioxide (CO₂) cycling.
• Traditional A_T measurements involve labor-intensive titrations requiring precise sample volumes.
• Existing methods necessitate expertise and can be time-consuming.

Purpose of the Study:

• To develop simplified, precise, and accurate spectrophotometric methods for determining total alkalinity (A_T).
• To reduce sample volume requirements for A_T analysis.
• To introduce novel instrumentation for carbonate chemistry assessment.

Main Methods:

• Spectrophotometric A_T determination using CO₂ gas equilibration (stopped-flow, continuous, direct bubbling).
• Utilizing a linear relationship between A_T, pCO₂, and pH, with a temperature-dependent calibration constant.
• Development and application of the Minimal Volume Multiparameter Inorganic Carbon Analyzer (MVMICA).

Main Results:

• Equilibration-based A_T methods achieved high precision (±1.0–2.0 μmol kg^-1).
• Results showed strong agreement with standard titration methods (±2.0 μmol kg^-1).
• MVMICA enabled precise pH_T (±0.002) and A_T measurements with minimal sample volumes (∼1.0 mL).

Conclusions:

• Presented methods offer flexible configurations for various applications, including automated and in situ A_T analysis.
• Membrane equilibrations are suitable for continuous and automated water analysis.
• Direct equilibration allows for rapid A_T measurements of small sample volumes (as low as 0.50 mL).