SPECTROPHOTOMETRIC PROCEDURES FOR DETERMINATION OF SEA-WATER ALKALINITY USING BROMOCRESOL GREEN

Spectrophotometric procedures are presented for the determination of sea water total alkalinity over a range of temperatures and salinities. One-step volumetric acid addition with prompt spectrophotometric measurement of excess acid provides total alkalinities precise to +/- 2 micromolal. Excess acid concentrations are quantified using the sulfonephthalein indicator, bromocresol green. Measurements of bromocresol green absorbance ratios [R(t)=616A/444A] at temperature t(degrees-C) and salinity S provide molal scale total hydrogen ion concentrations ([H+]T = [H+] + [HSO4-] + [HF]) through the equation pH(T) = -log [H+]T = log (I)beta(IT) + log {[R(25) - 0.00131]/[2.3148 - 0.1299R(25)]}, where R(25) = R(t) [1 + 0.00907(25 - t)] and log (I)beta(IT) = 4.2699 + (2.578 x 10-3) (35 - S).