Keywords and Phrases
In situ NMR; Micro-sensor compound; PH measurement; Quantitative NMR; Temperature
"The pH of an NMR sample can be measured directly by NMR experiments of signal intensities, chemical shifts, or relaxation time constants that depend on the pH. The 1H NMR peak intensities of the pH indicator phenolphthalein change as it changes from the OH-depleted form to the OH-rich form in the range of pH = 11.1 to 12.7. Because this range is rather small, another NMR technique was utilized based on 19F chemical shifts. The shift of F- in an aqueous solution of NaF changes in the alkaline range between pH = 11.0 and 14.0 and even more so in the acidic range between pH = 1.0 and 4.0. A new device made it possible for NMR samples to accurately determine pH values. The device consists of three parts: (1) an external reference (trifluoroacetic acid), (2) a temperature-sensing compound based on the chemical shifts of ethylene glycol or methanol, and (3) the pH micro-sensor compound NaF. Because pH micro-sensor compounds added to an aqueous solution have an influence on the pH, only a minimum amount of an NMR micro-sensor compound should be added to the sample. Quantitative NMR experiments in different setups (spherical NMR tubes, tubes with susceptibility plugs, Shigemi tubes) were conducted to determine the minimum amount of micro-sensor compound necessary for NMR measurements. A minimum number of 4.02 x 1016 nuclei was found to be sufficient for NMR signal observation using a 400-MHz spectrometer. The chemical shifts of pH-sensing NMR signals generally depend on temperature. Temperature-dependent NMR experiments were conducted to establish calibration curves through which the influence of temperature on the chemical shift can be corrected. The 19F signal of trifluoroacetic acid was found to have the least temperature-dependent chemical-shift variation and is suggested as independent standard for temperature-correction curve"--Abstract, page iii.
Nam, Paul Ki-souk
M.S. in Chemistry
Missouri University of Science and Technology
ix, 58 pages
© 2017 Ming Huang, All rights reserved.
Thesis - Open Access
Electronic OCLC #
Huang, Ming, "In situ pH determination based on the NMR analysis of ¹H-NMR signal intensities and ¹⁹F-NMR chemical shifts" (2017). Masters Theses. 7867.