Simplified seawater alkalinity analysis: Use of linear array spectrometers

Modified spectrophotometric procedures are presented for the determination of seawater total alkalinity using rapid scan linear array spectrometers. Continuous monitoring of solution pH allows titrations to be terminated at relatively high pH, whereby excess acid terms are very small. Excess acid concentrations are quantified using the sulfonephthalein indicators, bromocresol green and bromocresol purple. The outlined spectrophotometric procedures require no thermal equilibration of samples. Using bromocresol green, solution pHT ([H+]T in moles per kg of solution) is given as:pHT=4.2699+0.002578(35−S)+log((R(25)−0.00131)/(2.3148−0.1299R(25)))−log(1−0.001005S)and R(25)=R(t){1+0.00909(25−t)}, where 29⩽S⩽37, 13°C⩽t⩽32°C, and R(t) is the absorbance ratio (A616/A444) at temperature t and salinity S. Using bromocresol purple, the solution pHT is given as pHT=5.8182+0.00129(35−S)+log((R(25)−0.00381)/(2.8729−0.05104R(25))) and R(25)=R(t){1+0.01869(25−t)}, where 29⩽S⩽37, 13°C⩽t⩽32°C, and R(t)=A589/A432. Alkalinity measurements using bromocresol purple had a precision on the order of 0.3 μmol kg-1 and were within 0.3–0.9 μmol kg-1 of the alkalinities of certified seawater reference materials.