Development of a Spectrophotometric Method for Determining pH of Soil Extracts and Comparison with Glass Electrode Measurements

Soil pH measurement using conventional glass electrodes has several limitations. A spectrophotometric method was developed for determining soil pH involving indicator dye (bromocresol purple or phenol red) addition to soil extracts. Results were compared against values determined using a glass electrode for a range of soils (n = 13) with varying properties using different extraction conditions (1:1 w/v soil/water, 1:1 soil/0.01 mol L^–1 CaCl₂, 1:5 soil/water and 1:5 soil/0.01 mol L^–1 CaCl₂) and high and low ionic strength buffer calibrations of the electrode. For all extraction conditions, there was a strong relationship (r² > 0.95, slope ≈ 1) between values of the spectrophotometric (pH_spec) and glass electrode (pH_elec) methods. The precision of pH_spec was similar to pH_elec measurements across the different extraction conditions (±0.02–0.08 average standard deviation of triplicate measurements, n = 39). Large and variable differences were observed between pH_elec measured following calibration with high (µ = 0.1 mol L^–1) and low (µ = 0.005 mol L^–1) ionic strength buffers. In contrast, ionic strength effects on the indicator dye and resulting pH_spec calculation are implicitly accounted for. A spectrophotometric reflectance probe in situ method was also successfully trialed. The spectrophotometric pH method circumvents many of the problems associated with the use of glass electrodes in soil solutions.