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Colorimetric pH Determination with Computer Modeling

A colorimetric method with azoviolet indicator was developed to determine pH of concentrated waste solutions with pH >11. Experimental data with calibration solutions in 4 M NaNO3 were modeled two ways. A single-wavelength quadratic model was made with the peak wavelength at 554 nm. A more elaborate model with spectral data from 400 to 650 nm used a polynomial fit to the ratio of the PLS eigenvalues. For both models, the data were normalized and made independent of indicator concentration through division by the sample absorbance at the isosbestic point. Both models showed a precision of fit of ±0.1 pH unit in solutions that contained 3 to 4 M NaNO3. However, the full-spectrum model was able to flag inappropriate samples through goodness of fit calculations.