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Type: Thesis
Title: Spectrophotometric determination of pH and its influence in soils
Author: Bargrizan, Sima
Issue Date: 2019
School/Discipline: School of Agriculture, Food and Wine
Abstract: It would be beneficial to develop an alternative pH measurement technique for soils, since the standard glass electrode method suffers deficiencies with unpredictability in liquid junction potential, high drift and the necessity for electrode calibration with variable ionic strength solutions if high accuracy is required. Other problems with the electrode method for use in soils include clogging of the porous fiber of electrodes, and the “suspension effect”, which can result in a negative bias when there is substantial H+ present on exchange sites. Spectrophotometric methods using indicators offer an alternative to eliminate several inherent issues with potentiometric pH measurement. This approach has been widely used in for pH determination of marine waters due to the high reproducibility achievable (≥ 0.001 pH units) but has not previously been developed for soils. The aims of this thesis were i) to develop a spectrophotometric method for measuring soil pH in the circumneutral (5-8) and acidic (< 5) pH range; ii) to develop a mixed dye spectrophotometric method that can be used for any the soil in the pH range 3-9; and iii) to use these techniques for evaluating the consistency of the thermodynamics of the soil carbonate system. In the first experiment, spectrophotometric determination of the concentrations of the acid and base forms of phenol red and bromocresol purple were used for soil pH measurement in the pH range of 5.0-8.5. This spectrophotometric method showed a strong relationship (r2 > 0.95) with values determined using a glass electrode in both water and CaCl₂ soil extracts. Similar precision of ±0.02-0.08 pH unit was obtained for measurement on replicate soil extracts for both spectrophotometric and glass electrode methods. The application of the spectrophotometric method was then extended to use with acidic soils by employing an indicator, bromocresol green, with a lower pKa; again, a strong correlation (r2 > 0.99) was achieved between spectrophotometric and glass electrode pH measurements. In the next experiment, a mixed dye (bromophenol blue, bromocresol purple, m-cresol purple, and thymol blue) method was developed that has a much wider working pH range of 3-9; in comparison the working pH range of single dye methods is approx. ±1 pH unit from their pKa. In the mixed-dye method, pH was calculated based on ratio of absorbance at selected two wavelengths and individual dye properties using fundamental equations derived from Beer’s law. The accuracy of the method was found to be within ±0.00-0.06 pH units against certified pH buffers. In the last experiment, measurements and modelling was conducted to evaluate the consistency of the thermodynamics of the soil carbonate system. pH was calculated from a known concentration of pCO₂ applied for soil solution equilibration and alkalinity titration and then comparing the results with pH measured using spectrophotometer and glass electrode. The internal consistency of the soil carbonate system was shown with a precision of ±0.03 pH units. Difference of calculated pH from measured pH was within 0.00-0.1 pH units in soil solutions with alkalinity > 0.5 meq L⁻¹. In conclusion, the application of novel spectrophotometric pH measurement methods for soils has been developed. The indicators which have been calibrated allow wider soil pH measurement between 3-9 which is also useful for other application such as oceanic pH. This study will provide a better understanding of the role of pH in illuminating acid-base reactions in soils, especially including the geochemically significant carbon dioxide system.
Advisor: Mosley, Luke
Smernik, Ron
Dissertation Note: Thesis (Ph.D.) -- University of Adelaide, School of Agriculture, Food and Wine, 2019
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