TY - JOUR
T1 - Spectrophotometric Determination of Freshwater PH Using Bromocresol Purple and Phenol Red
AU - Yao, Wensheng
AU - Byrne, Robert H.
PY - 2001/1/1
Y1 - 2001/1/1
N2 - 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.
AB - 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.
UR - https://digitalcommons.usf.edu/msc_facpub/1624
UR - https://doi.org/https://doi.org/10.1021/es001573e
U2 - 10.1021/es001573e
DO - 10.1021/es001573e
M3 - Article
VL - 35
JO - Environmental Science Technology
JF - Environmental Science Technology
ER -