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Vol.
28 No. 3
May-June 2006
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
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