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The application of a spectrophotometric method to determine pH in acidic (pH<5) soils
Ist Teil von
Talanta (Oxford), 2018-08, Vol.186, p.421-426
Ort / Verlag
Netherlands: Elsevier B.V
Erscheinungsjahr
2018
Link zum Volltext
Quelle
Elsevier ScienceDirect Journals Complete
Beschreibungen/Notizen
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 (pKa = 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.
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•A spectrophotometric method was developed for pH measurement in acidic soils.•Bromocresol green dye properties were characterised at low ionic strength.•An acid sulfate soil incubation was used to validate the method between pH 3 − 5.3.•Results compared well to glass electrode pH measurements in the acidic range.•High metal availability in acidic soils did not significantly affect dye function.