Spectrophotometric Determination of Freshwater PH Using Bromocresol Purple and Phenol Red

The dissociation constants ( K _I = [H ⁺ ][I ^{2 -} ]/[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 ( _{λ 2} A / _{λ 1} A ) using the following equations:  pH = p K _I + log(( R − e ₁ )/( e ₂ − Re ₃ )) and p K _I = p K _I ° − A Δ Z ² (μ ^1/2 /(1 + μ ^1/2 ) − 0.3 μ), where R = _{λ 2} A / _{λ 1} A , p K _I = −log K _I , μ is the ionic strength, Δ Z ² = 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 (λ ₁ = 432 nm, λ ₂ = 589 nm), p K _I ° = 5.226 + 378.1/ T , e ₁ = 0.00387, e ₂ = 2.858, and e ₃ = 0.0181. For phenol red (λ ₁ = 433 nm, λ ₂ = 558 nm), p K _I ° = 5.798 + 666.7/ T , e ₁ = 0.00244, e ₂ = 2.734, and e ₃ = 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.