A rapid and accurate spectroscopic method for alkalinity measurements in sea water samples

The authors propose a new spectrophotometric method for the determination of sea water alkalinity. The method consists to neutralize all the basic species taken into account in the alkalinity expression by a weak acid (formic acid) mixed with a pH sensitive dye, the Bromo-Phenol Blue, which has a dissociation constant close to those of formic acid. The neutralization reaction leads to a final mixture having an absorbance at 590 nm which is a function of the original alkalinity of the sample. Two alternative calibrations procedures are proposed and alkalinities measurements on sea water from different origins are compared with the results obtained by the classical Granʹs potentiometric titration. The agreement between the two methods is usually better than 1%. The main advantages of the method are the possibility to run very small samples (1–2 mL) and its rapidity.