Spectrophotometric procedures allow rapid and precise measurements of the pH of natural waters. However, impurities in the acid-base indicators used in these analyses can significantly affect measurement accuracy. This work describes HPLC procedures for purifying one such indicator, meta-cresol purple (mCP), and reports mCP physical-chemical characteristics (thermodynamic equilibrium constants and visible-light absorbances) over a range of temperature (T) and salinity (S). Using pure mCP, seawater pH on the total hydrogen ion concentration scale (pHT) can be expressed in terms of measured mCP absorbance ratios (R = λ2A/(λ1)A) as follows: [formula in text] where -log(K(2)Te2) = a + (b/T) + c ln T – dT; a = -246.64209 + 0.315971S + 2.8855 × 10(-4)S2; b = 7229.23864 – 7.098137S – 0.057034S2; c = 44.493382 – 0.052711S; d = 0.0781344; and mCP molar absorbance ratios (ei) are expressed as e1 = -0.007762 + 4.5174 × 10(-5)T and e3/e2 = -0.020813 + 2.60262 × 10(-4)T + 1.0436 × 10(-4) (S – 35). The mCP absorbances, λ1A and λ2A, used to calculate R are measured at wavelengths (λ) of 434 and 578 nm. This characterization is appropriate for 278.15 ≤ T ≤ 308.15 and 20 ≤ S ≤ 40.