Colorimetric determination of low pH with Malachite Green

A spectrophotometric method was developed for determination of concentration-based pH values from 0 to 2 with Malachite Green indicator. A quadratic model equation was based on the ratio of the absorbances of the peak at 618 nm and the isosbestic point at 518 nm. Normalization to the isosbestic point was used to stabilize the response because the color faded; the useful time interval was within 5 min after indicator addition. Model and verification sets agreed within +/-0.02 pH units between pH 0.3 and 1.8. This excellent precision makes the colorimetric method useful for acid determinations with a relative precision of > +/-5%. The presence of salts at a salt/acid equivalent ratio >0.1 caused a low pH bias.