The effect of pressure on meta-Cresol Purple protonation and absorbance characteristics for spectrophotometric pH measurements in seawater

The sulfonephthalein indicator meta-Cresol Purple (mCP) is well suited to direct spectrophotometric pH measurements throughout the oceanic water column. In prior work the molar absorbance and proton exchange characteristics of purified mCP were characterized over a wide range of salinities and temperatures, allowing use of the indicator over a broad range of oceanic conditions in the surface ocean. Modern spectrophotometric instrumentation allows direct in situ measurements of seawater pH at depths that create substantial pressure-dependent changes in the physical chemical behavior of mCP. In order to allow use of purified mCP for quantitative pH measurements over essentially the full range of ocean depths, prior calibrations of mCP for measurements of seawater pH were extended to include pressures up to 827 bar. Using purified mCP, seawater pH is measured using the equation pH T = − log K 2 e 2 + log R − e 1 1 − R ⋅ e 3 e 2 where e 1 = − 0.007762 + 4.5174 × 1 0 – 5 T + 1.7 × 1 0 – 6 P e 3 / e 2 = − 0.020813 + 2.60262 × 1 0 – 4 T + 1.0436 × 1 0 – 4 S – 35 + 4.6 × 1 0 – 6 P – log K 2 e 2 = a + b / T + c ln T − d T − 0.05645 / T × P anda 64209 + 0.315971S + 2.8855 × 10− 4S2 .23864 − 7.098137S − 0.057034S2 93382 − 0.052711S 81344. T are gauge pressure (bar) and Kelvin temperature, and the coefficients a, b and c are salinity dependent coefficients determined in previous work. The influence of pressure on the properties of mCP is sufficient to cause easily measureable effects at depths less than 100 m.