## 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 } (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.

Original language | American English |
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Journal | Environmental Science Technology |

Volume | 35 |

DOIs | |

State | Published - Jan 1 2001 |

## Disciplines

- Life Sciences