Vol. 24, No. 6
for NMR Measurements of High pK Values and Equilibrium Constants in
Strongly Basic Solutions
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.
"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
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.
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.
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.
more information, contact the Task Group Chairman K.
Additional task group members are H. Rönkkömäki and L.
H. J. Lajunen.