Recommendations for NMR Measurements of High pK Values and Equilibrium Constants in Strongly Basic Solutions
IUPAC Projects
Recommendations for NMR Measurements of High pK Values and Equilibrium Constants in Strongly Basic Solutions
Nuclear Magnetic Resonance (NMR) is a well-established, powerful method for monitoring the dissociation of acidic groups. Chemical shift-pH titration is widely used with potentiometric measurements within 2<pH<12 and also as an alternative to the glass electrode at high (pH>12) and low (pH<1) pH-range. In the former case an excellent agreement between potentiometric and NMR equilibrium data obtained in the same background medium could be observed. For extremely high and low hydrogen ion concentrations it is believed that NMR provides more accurate data.
Unlike "normal" procedure, the chemical shift-pH titration of highly basic solutions normally requires a complete change of the background electrolyte composition (e.g., 1 M NaCl to 1 M NaOH). Moreover, frequently a titration under variable ionic strength has to be used in order to measure anomalously high (low) pK values. At the same time, little is known about the NMR chemical shift sensitivity of different nuclei to the effects that are not associated with particular chemical protonation/deprotonation equilibrium (e.g., to the "indifferent" supporting electrolyte concentration and a drastic variation of its composition as well as to the presence of internal reference and uncontrolled D2O/H2O ratio).
At high pH such background electrolytes as sodium and potassium salts form undissociated NaOH and KOH species. Usually this process is not accounted for in acidity constants calculations. Another problem with these salts is a complex formation with an acid under the study. This normally leads to a significant decrease in pK values. At the same time, the comparison of pK values for different acids requires the data obtained under similar conditions (e.g., I=0.1 M or 1.0 M). This raises the problem of high pK value measurement at a common ionic strength with reasonable accuracy.
Everyday practice reveals a large diversity of experimental approaches to the chemical shift-pH titration procedure: internal (external) references; D2O, H2O or D2O/H2O solvents; titration at a constant (variable) ionic strength; use of different nuclear. As a result, a large disagreement for high (low) pK data could be observed.
A recent IUPAC project is intended to indicate some real and possible sources of errors in chemical shift-pH titration at high (low) pH range and to formulate some recommendations for this procedure.
For more information, contact the Task Group Chairman K. Popov <ki-popov@mtu-net.ru>. Additional task group members are H. Rönkkömäki and L. H. J. Lajunen.
© 2014 by Walter de Gruyter GmbH & Co.
Artikel in diesem Heft
- Contents
- What Lies Ahead for the Union
- PEDECIBA–Programa de DEsarrollo de CIencias BAsicas
- Scientific and Technical Information
- Macromolecular Nomenclature and Terminolo
- Aligning the Revised Red and Blue Books and Preparing for IUPAC Preferred Names
- The Address is www.iupac.org
- Dana Knox Receives the Franzosini Award
- New SIT Software Makes Speciation Calculations Easier
- Recommendations for NMR Measurements of High pK Values and Equilibrium Constants in Strongly Basic Solutions
- Performance Evaluation Criteria for Preparation and Measurement of Macroand Microfabricated Ion-Selective Electrodes
- New Solubility Data Projects
- Concepts and Structure for Requests in Clinical Laboratories
- Properties and Units for Transfusion Medicine and Immunohaematology
- Global Use of the C-NPU Concept System for Properties in Toxicology
- Definitions Relating to Stereochemically Asymmetric Polymerizations (IUPAC Recommendations 2001)
- Potentiometric Selectivity Coefficients of Ion-Selective Electrodes
- Provisional Recommendations
- The Science of Sweeteners
- Ionic Poylmerization
- Green Chemistry in Africa
- Vermeer's Camera: Uncovering the Truth Behind the Masterpieces
- Bioinformatics 2002: North-South Networking
- CHEMRAWN XIV Follows Up With Green Chemistry Activities
- Functional π-Electron Systems
- Organic Synthesis
- Solubility Phenomena–Application for Environmental Improvement
- Physical Organic Chemistry
- 11th International Conference on Polymer Characterization (POLYCHAR-11) 6-10 January 2003, Denton, Texas, USA
- 6th Annual UNESCO School & IUPAC Conference on Polymer Properties 14-17 April 2003, Mpumalanga, South Africa
- International Symposium on Ionic Polymerization 30 June-4 July 2003, Boston, USA
- 22nd Discussion Conference of P.M.M. on Spectroscopy of Partially Ordered Macromolecular Systems 21–24 July 2003, Prague, Czech Republic
- Calendar of IUPAC Sponsored Conferences
Artikel in diesem Heft
- Contents
- What Lies Ahead for the Union
- PEDECIBA–Programa de DEsarrollo de CIencias BAsicas
- Scientific and Technical Information
- Macromolecular Nomenclature and Terminolo
- Aligning the Revised Red and Blue Books and Preparing for IUPAC Preferred Names
- The Address is www.iupac.org
- Dana Knox Receives the Franzosini Award
- New SIT Software Makes Speciation Calculations Easier
- Recommendations for NMR Measurements of High pK Values and Equilibrium Constants in Strongly Basic Solutions
- Performance Evaluation Criteria for Preparation and Measurement of Macroand Microfabricated Ion-Selective Electrodes
- New Solubility Data Projects
- Concepts and Structure for Requests in Clinical Laboratories
- Properties and Units for Transfusion Medicine and Immunohaematology
- Global Use of the C-NPU Concept System for Properties in Toxicology
- Definitions Relating to Stereochemically Asymmetric Polymerizations (IUPAC Recommendations 2001)
- Potentiometric Selectivity Coefficients of Ion-Selective Electrodes
- Provisional Recommendations
- The Science of Sweeteners
- Ionic Poylmerization
- Green Chemistry in Africa
- Vermeer's Camera: Uncovering the Truth Behind the Masterpieces
- Bioinformatics 2002: North-South Networking
- CHEMRAWN XIV Follows Up With Green Chemistry Activities
- Functional π-Electron Systems
- Organic Synthesis
- Solubility Phenomena–Application for Environmental Improvement
- Physical Organic Chemistry
- 11th International Conference on Polymer Characterization (POLYCHAR-11) 6-10 January 2003, Denton, Texas, USA
- 6th Annual UNESCO School & IUPAC Conference on Polymer Properties 14-17 April 2003, Mpumalanga, South Africa
- International Symposium on Ionic Polymerization 30 June-4 July 2003, Boston, USA
- 22nd Discussion Conference of P.M.M. on Spectroscopy of Partially Ordered Macromolecular Systems 21–24 July 2003, Prague, Czech Republic
- Calendar of IUPAC Sponsored Conferences
