Analytical Sciences

Abstract − Analytical Sciences, 26(6), 709 (2010).

Open-cell Titration of Seawater for Alkalinity Measurements by Colorimetry Using Bromophenol Blue Combined with a Non-linear Least-squares Method
Kei OKAMURA,* Hideshi KIMOTO,** and Takashi KIMOTO**
*Center for Advanced Marine Core Research, Kochi University, B200 Monobe, Nankoku, Kochi 783-8502, Japan
**Kimoto Electric Co. Ltd., 3-1 Funahashi, Ten-noji, Osaka 543-0024, Japan
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 e3/e2) and a tentative acid dissociation constant value (pKa2). 2) The perturbation of hydrogen ion was evaluated from the change in titration mass (Δm). The total hydrogen ion concentration at m + Δm, pHT(at m+Δm), was calculated using pHF(ind) for a mass m and constants for sulfate (ST) and fluoride (FT). 3) The alkalinity (AT) was computed from the titrant mass (m + Δm) and the corresponding pHT(at m+Δm) through a non-linear least-squares approach using the pKa2 value as a variable parameter. Seawater sample at 2000 m depth from the West Pacific was analyzed. The resulting AT (2420.92 ± 3.35 μmol kg−1) was in good agreement with the AT measured by potentiometric electric force (2420.46 ± 1.54 μmol kg−1). The resulting pKa2 was 3.7037, in close proximity to that reported by King et al. (pKa2 = 3.695).