Open-cell Titration of Seawater for Alkalinity Measurements by Colorimetry Using Bromophenol Blue Combined with a Non-linear Least-squares Method

Abstract

The open-cell titration of seawater was studied for alkalinity measurements by colorimetry. 1) The colorimetric pH of free hydrogen ion concentration, pHF(ind), was calculated from the ratio of the absorbances at 436 and 590 nm (R = _590nmA/_436nmA), along with the molar absorption coefficient ratios (e₁, e₂ and e₃le₂) and a tentative acid dissociation constant value (pK_a2). 2) The perturbation of hydrogen ion was evaluated from the change in titration mass (Δm). The total hydrogen ion concentration at m + Δm, pH_{T(at m+Δm)}, was calculated using pHF(ind) for a mass m and constants for sulfate (S_T) and fluoride (F_t). 3) The alkalinity (A_t) was computed from the titrant mass (m + Δm) and the corresponding pH_{T(at m+Δm)} through a non-linear least-squares approach using the pK_a2 value as a variable parameter. Seawater sample at 2000 m depth from the West Pacific was analyzed. The resulting A_t (2420.92 ± 3.35 µmol kg^-1) was in good agreement with the A_t measured by potentiometric electric force (2420.46 ± 1.54 µmol kg^-1). The resulting pK_a2 was 3.7037, in close proximity to that reported by King et al. (pK_a2 = 3.695).