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 degrees 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 (lambda2A/lambda1A) using the following equations: pH = pKI + log((R - e1)/(e2 - Re3)) and pKI = pKI(degrees) - AdeltaZ2(mu1/2 /(1 + mu1/2) - 0.3 mu), where R = lambda2A/lambda1A, pKI = -log KI, mu is the ionic strength, deltaZ2 = 4, and values of A for 283 < or = T < or = 303 can be estimated from the equation: A = 0.5092 + (T-298.15) x 8.5 x 10(-4). For bromocresol purple (lambda1 = 432 nm, lambda2 = 589 nm), pKI(degrees) = 5.226 + 378.1/T, e1 = 0.00387, e2 = 2.858, and e3 = 0.0181. For phenol red (lambda1 = 433 nm, lambda2 = 558 nm), pKI(degrees) = 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 < or =pH < or =8.5. The precision of pH measurements using phenol red in well-buffered freshwaters is on the order of +/-0.001 or better.