Spectrophotometric methods for the measurement of soil pH: A reappraisal

The pH of aqueous soil extracts is generally measured potentiometrically by glass electrode (GE). Spectrophotometric methods have also been used till around the '80s, and then they have apparently been abandoned. However, the use of microplates and spectrophotometers able to read absorbance values in them can significantly increase the speed of the data collection (saving analysis time), thus justifying their reappraisal. Three spectrophotometric methods are proposed in this work: a one-indicator (OISM), a separated three-indicator (STISM) and a mixed three-indicator (MTISM) spectrophotometric method. They are based on the addition of one or three colorimetric indicators (methyl red, bromocresol violet, and bromothymol blue) to the aqueous extract. The pH is measured through its absorption properties in the visible region. The analysis of 60 soil samples showed that STISM and MTISM results correlate well with the ones obtained by GE. The STISM method, being more simple and general than the other two methods, is proposed for quick routine analyses. The repeatability, reproducibility and accuracy of STISM (and of GE, for comparison) were evaluated by measuring several times the pH of a series of soil samples and of a certified reference soil. The standard deviations of STISM results were slightly worse than those of GE, whereas the accuracy was slightly better, indicating that STISM and GE have overall similar performances. STISM method is much faster than GE one: the analysis time saving is around 2 min per sample, and it becomes very high when hundreds of samples have to be analysed. It follows that STISM can represent an advantageous alternative to GE for rapid and accurate soil pH measurements.