Spectrophotometric Determination of Freshwater PH Using Bromocresol Purple and Phenol Red

Wensheng Yao, Robert H. Byrne

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Abstract

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 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.

Original languageAmerican English
JournalEnvironmental Science Technology
Volume35
DOIs
StatePublished - Jan 1 2001

Disciplines

  • Life Sciences

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