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Chemistry International
Vol. 24, No. 6
November 2002
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 <[email protected]>. Additional task group members are H. Rönkkömäki and L. H. J. Lajunen.
www.iupac.org/projects/2001/2001-038-2-500.html
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