Spectrophotometric Determination of Freshwater PH Using Bromocresol Purple and Phenol Red

The dissociation constants (KI = [H+][I2-]/[HI-]) of two sulfonephthalein indicators (bromocresol purple and phenol red) were determined as function of temperature (10−30 °C) at zero ionic strength. Freshwater pH, on the free hydrogen ion concentration scale (molal units), can be precisely calculated from measurements of indicator absorbance ratios (λ2A/λ1A) using the following equations: pH = pKI + log((R − e1)/(e2 − Re3)) and pKI = pKI° − AΔZ2(μ1/2/(1 + μ1/2) − 0.3 μ), where R = λ2A/λ1A, pKI = −log KI, μ is the ionic strength, ΔZ2 = 4, and values of A for 283 ≤ T ≤ 303 can be estimated from the equation: A = 0.5092 + (T − 298.15) × 8.5 × 10-4. For bromocresol purple (λ1 = 432 nm, λ2 = 589 nm), pKI° = 5.226 + 378.1/T, e1 = 0.00387, e2 = 2.858, and e3 = 0.0181. For phenol red (λ1 = 433 nm, λ2 = 558 nm), pKI° = 5.798 + 666.7/T, e1 = 0.00244, e2 = 2.734, and e3 = 0.1075. These two indicators can be used to make accurate pH measurements of freshwaters (river water, lake water, groundwater, rainwater, etc) within the range 4.5 ≤ pH ≤ 8.5. The precision of pH measurements using phenol red in well-buffered freshwaters is on the order of ±0.001 or better