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Vol. 28 No. 3
May-June 2006

Making an imPACt | Recent IUPAC technical reports and recommendations that affect the many fields of pure and applied chemistry.
See also www.iupac.org/publications/pac

Guidelines for NMR Measurements for Determination of High and Low pKa Values (IUPAC Technical Report)

Konstantin Popov, Hannu Rönkkömäki, and Lauri H. J. Lajunen
Pure and Applied Chemistry
Vol. 78, No. 3, pp. 663–675 (2006)
doi:10.1351/pac200678030663

Factors affecting the NMR titration procedures for the determination of pKa values in strongly basic and strongly acidic aqueous solutions (2 > pH > 0 and 14 > pH > 12) are analyzed. Guidelines for experimental procedure and publication protocols are presented in this report. These include:

  • calculation of the equilibrium H+ concentration in a sample
  • avoidance of measurement with glass electrode in highly acidic (basic) solutions
  • exclusion of D2O as a solvent
  • use of an individual sample isolated from air for each pH value
  • use of external reference and lock compounds
  • use of a medium of constant ionic strength with clear indication of the supporting electrolyte and of the way the contribution of any ligand to the ionic strength of the medium is accounted for
  • use of the NMR technique in a way that eliminates sample heating to facilitate better sample temperature control (e.g., 1H-coupled NMR for nuclei other than protons, GD-mode, CPD-mode, etc.)
  • use of Me4NCl/Me4NOH or KCl/KOH as a supporting electrolyte in basic solution rather than sodium salts in order to eliminate errors arising from NaOH association
  • verification of the independence of the NMR chemical shift from background electrolyte composition and concentration
  • use of extrapolation procedures
 

www.iupac.org/publications/pac/2006/7803/7803x0663.html


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