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, pH_F(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 e₃/e₂) 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 pH_F(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).