Spectrophotometric measurement of total inorganic carbon in aqueous solutions using a liquid core waveguide

Abstract

This work describes spectrophotometric procedures for direct measurements of total inorganic carbon in aqueous solutions. The procedures used for these measurements involve CO 2 equilibration across the permeable wall of a Teflon AF-2400 liquid core waveguide. The waveguide acts as both an equilibration membrane and an optical cell in which spectrophotometric pH measurements, obtained via measurements of absorbance ratios, are used to determine the CO 2 fugacity and total carbon in acidified solutions surrounding the waveguide. Total carbon concentrations ( C T) a in the acidified external medium are calculated using the equation: log(C T ) a = log (K 0) a (K 0) i +B(t)− log R−e 1 1−Re 3/e 2 where B( t) is an empirically derived constant, ( K 0) a and ( K 0) i are Henry’s law constants appropriate to the acidified external solution (a) and a solution inside the waveguide (i) whose total alkalinity is known, R is an absorbance ratio of bromocresol purple (BCP) at 589 and 432 nm, and both e 1 and e 3/ e 2 are molar absorptivity ratios appropriate to bromocresol purple. Spectrophotometric ( C T) a measurements of natural riverine samples obtained using the procedures described in this work are in good agreement with C T measurements obtained via coulometry.