Spectrophotometric measurement of the pH of soil extracts using a multiple indicator dye mixture

Abstract

This paper describes the development of a spectrophotometric method with an expanded pH range of 3–9 that uses a mixed indicator solution (equimolar bromophenol blue, bromocresol purple, m‐cresol purple and thymol blue). The method uses measurements of absorbance of the dye mixture at two wavelengths (434 and 585 nm), chosen to represent the average acid and base peak maxima of the individual dyes within the mixture. The ratio of absorbance at these two wavelengths was used to calculate pH based on measured dye properties (pKa, molar absorptivity) and fundamental equations derived from Beer's law. The mixed dye spectrophotometric pH measurement was tested using certified pH buffers (pH (NBS/NIST) 4.00, 6.86, 9.18) and was found to be accurate to within ± 0–0.06 pH units. Measurements made with the mixed dye showed good correlation against conventional soil pH measurement using a glass electrode (r = 0.99), and also an alkalinity titration (r = 0.99) through the pH range 3–9. The average standard deviation was 0.07 for spectrophotometric soil pH measurement (n = 30) using the dye mixture. The mixed dye technique expands the working range of spectrophotometric pH measurement methods in soils and other applications.