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

Abstract

Core Ideas Soil pH is a critical parameter in soils as it influences many biogeochemical processes. There are problems in pH measurements in soils using glass electrodes. A new spectrophotometric method was developed to measure pH in soils. The method compared well to measurements using glass electrodes in a range of soils. A reflectance probe technique was also successfully trialed for in situ measurement. 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 (pHspec) and glass electrode (pHelec) methods. The precision of pHspec was similar to pHelec 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 pHelec 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 pHspec 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.