The application of a spectrophotometric method to determine pH in acidic (pH<5) soils

Sima Bargrizan; Ronald J Smernik; Rob W Fitzpatrick; Luke M Mosley

doi:10.1016/j.talanta.2018.04.074

The application of a spectrophotometric method to determine pH in acidic (pH<5) soils

Talanta. 2018 Aug 15:186:421-426. doi: 10.1016/j.talanta.2018.04.074. Epub 2018 Apr 24.

Authors

Sima Bargrizan¹, Ronald J Smernik¹, Rob W Fitzpatrick², Luke M Mosley³

Affiliations

¹ The School of Agriculture, Food and Wine, The University of Adelaide, South Australia, Australia.
² Acid Sulfate Soils Centre, School of Biological Sciences, The University of Adelaide, South Australia, Australia.
³ Acid Sulfate Soils Centre, School of Biological Sciences, The University of Adelaide, South Australia, Australia. Electronic address: luke.mosley@adelaide.edu.au.

PMID: 29784382
DOI: 10.1016/j.talanta.2018.04.074

Abstract

pH is a "master variable" controlling many biogeochemical processes in soils. Acid sulfate soils undergo rapid and large pH changes from circumneutral pH under anaerobic soil conditions to sulfuric soils with ultra (pH < 3.5) and extremely (pH 3.5-4.4) acidic properties following oxidation. Measuring soil pH using a glass electrode has several potential drawbacks including liquid junction errors, drift, suspension effects and clogging. Spectrophotometric pH measurement, involving addition of an indicator dye to the sample, is widely used in seawater and has recently been developed for soil extracts at circumneutral pH ranges. The aim of this study was to extend the spectrophotometric method for application in ultra and extremely acidic soils. The acid dissociation constant (pK_a = 5.02) and molar absorptivities of the indicator dye bromocresol green were determined and shown to enable spectrophotometric pH measurement between pH 3 - 5.3. To demonstrate the performance and application of the method, pH and metal availability (Fe, Al, Zn) were measured during the incubation of two acid sulfate soils, which both classified as hypersulfidic soils (pH > 4) and transformed to sulfuric soils (pH < 4) after incubation for 12 weeks. The method compared well (r² > 0.99) to glass electrode measurements under acidic conditions with high metal availability. The method has potential to improve understanding of biogeochemical processes in ultra and extremely acidic soils.

Keywords: Acid sulfate soils; Bromocresol green; Indicator dye; Soil pH.