Acid-free continuous alkalinity measurement by equilibration with CO2 gas across a silicone membrane

In an effort to develop a portable instrument to comprehensively characterize all four CO2 system parameters, we have designed a continuous-flow acid-free spectrophotometric alkalinity device for natural water measurements. Samples are equilibrated with a 30% CO2 gas mixture along a 2-meter length of thin wall silicone tubing, followed by spectrophotometric measurements with a pH optical cell. As a sample enters the silicone tubing, it gradually equilibrates with the CO2 gas and attains full equilibration as it exits the tubing. The equilibrated sample is mixed with a solution containing the pH indicator bromo cresol purple (BCP) and then passes into an optical cell for measurement of BCP absorbance ratios. Total alkalinity (A_T) is directly quantified using this relationship

log(A_T+[H⁺]) = log( (K₀)_i(pCO₂)/K_I) + E + log((R(25)-e₁)/(e₂-R(25)e₃))

where the pCO2 is partial pressure of CO2 gas. E is a system calibration constant. R is ratio of absorbances at wavelengths 589 and 432. K0 and KI are dissociation constant of CO2 and BCP.

This measurenent system combined with a channel for pH measurements, constitutes a robust system for characterizing CO2 system chemistry, providing either in situ, underway, or discrete measurements. It is especially useful for applications in which sample volumes are limited such as pore waters extracted from sediments.