Determining the titratable properties of aqueous and non-aqueous samples by 1H NMR Matthew Wallace, Kevin Lam, Agne Kuraite, Esther Bolarinwa, Nduchi Abiama, Josh Holroyd and Haider Hussain School of Pharmacy, University of East Anglia, UK. Titrations have a wide range of applications, from measuring the acidity of apple juice to studying the ion-binding properties of macromolecules. We have shown that 1 H NMR can have many advantages over conventional potentiometric, conductometric or colorimetric titrations. 1,2 In aqueous samples, the titratable acidity, basicity or carboxyl content can be determined from a single 1 H spectrum just by measuring the chemical shifts of ‘indicator’ molecules. 1 This approach avoids the tedious addition of titrant associated with conventional titrations. Furthermore, the detailed chemical composition of the sample can be determined simultaneously from the same spectrum. I will briefly discuss our published method for aqueous systems 1 and present our latest results on the measurement of the acidity and base content of oils. In situations where the properties of a system must be studied as a function of the sample conditions, for example the pH or ionic composition, our group is developing a new approach to titrations based on 1 H chemical shift imaging (CSI) NMR. 2-4 In our method, acid or salt is diffused into a sample to create a gradient of pH or ionic composition. 1 H spectra are recorded at different positions along the gradient using CSI to provide a comprehensive set of spectra as the conditions are varied. 2,3 Using pH buffer components, we can create programmable gradients in aqueous samples spanning five or more units. Our approach can be used to extract the p K a values of organic compounds with several closely spaced p K a values. These compounds are especially difficult to analyse using conventional titrimetric methods. Finally, I will demonstrate a method to study the affinity of macromolecules for Ca 2+ and Mg 2+ (M 2+ ). 4 In our approach, a solution of macromolecule is layered on top of crystals of M 2+ acetate salt. Dissolution and diffusion of the salt along the sample creates concentration gradients of acetate and M 2+ . The concentration of acetate and free (unbound) M 2+ are measured along the sample by CSI using our published method. 3 Binding of M 2+ to the macromolecule causes a discrepancy between the concentration of M 2+ and acetate, from which the binding affinity can be assessed. The stability of the macromolecule in the presence of M 2+ can be assessed simultaneously by 1 H NMR. Results will be presented for sodium polyacrylate, alginate, polystyrene sulfonate, nanocrystalline cellulose, polyethyleneimine, EDTA and maleate.
Scheme1. Method to assess the affinity of macromolecules for Ca 2+ or Mg 2+ (a) and the base content of lubricating oil (b) by 1 H NMR.
References 1. Wallace, K. Lam, A. Kuraite and Y. Z. Khimyak, Anal. Chem. 2020, 92 ,12789-12794. 2. Wallace, D. J. Adams and J. A. Iggo, Anal. Chem. 2018, 90 , 4160-4166. 3. Wallace, T. Hicks, Y. Z. Khimyak and J. Angulo, Anal. Chem. 2019, 91 , 14442-14450. 4. Wallace, J. Holroyd, A. Kuraite and H. Hussain, submitted.
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