| Abstract
| - The dissociation constants (KI = [H+][I2-]/[HI-]) of twosulfonephthalein indicators (bromocresol purple and phenolred) were determined as function of temperature (10−30°C) at zero ionic strength. Freshwater pH, on the free hydrogenion concentration scale (molal units), can be preciselycalculated from measurements of indicator absorbanceratios (λ2A/λ1A) using the following equations: pH = pKI + log((R − e1)/(e2 − Re3)) and pKI = pKI° − AΔZ2(μ1/2/(1 +μ1/2) − 0.3 μ), where R = λ2A/λ1A, pKI = −log KI, μ is theionic strength, ΔZ2 = 4, and values of A for 283 ≤ T ≤ 303can be estimated from the equation: A = 0.5092 + (T −298.15) × 8.5 × 10-4. For bromocresol purple (λ1 = 432 nm,λ2 = 589 nm), pKI° = 5.226 + 378.1/T, e1 = 0.00387, e2 =2.858, and e3 = 0.0181. For phenol red (λ1 = 433 nm, λ2 = 558nm), pKI° = 5.798 + 666.7/T, e1 = 0.00244, e2 = 2.734,and e3 = 0.1075. These two indicators can be used to makeaccurate pH measurements of freshwaters (river water,lake water, groundwater, rainwater, etc) within the range4.5 ≤ pH ≤ 8.5. The precision of pH measurementsusing phenol red in well-buffered freshwaters is on theorder of ±0.001 or better.
|
