Physical–chemical Characterization of Purified Cresol Red for Spectrophotometric pH Measurements in Seawater

The use of impure cresol red in spectrophotometric seawater pH measurements can introduce systematic inaccuracies greater than 0.1. Cresol red has been purified on a bulk scale to address this problem, but a characterization of the dye's physical–chemical properties has not been provided to date. This work reports the physical–chemical characteristics of purified cresol red for use in spectrophotometric seawater pH measurements over a range of temperatures and salinities. Seawater pH is expressed on the total hydrogen ion concentration scale (pHT) in terms of the ratio (R) of cresol red absorbances (A) at 433 and 573 nm (RCR = 573A/433A): pHT=−logK2Te2+logRCR−e11−RCRe3e2 where − log(K2Te2) = a + b/T + c ln T − dT a=−859.326051+0.14616S+7.81164×10−4S2b=22969.9366+8.04468S−0.20512S2c=152.209523−0.0317821Sd=0.259915 and cresol red molar absorptivity ratios are expressed as: e1=−0.00413+1.814×10−5Te3/e2=−0.021683+1.8107×10−4T+3.163×10−5S−35 for 278.15 ≤ T ≤ 308.15 K and 20 ≤ S ≤ 40.

We recommend using cresol red to measure the acidity of seawater that has (at 298.15 K) a pHT of 6.8–7.8. This range might be encountered in ocean areas such as oxygen minimum zones or, hydrothermal vent fields, or it might be imposed in controlled laboratory studies. Ocean acidification will make cresol red an increasingly important indicator in coming decades as waters within ever larger ocean areas shift into its optimal indicating range.