На главную

Научные подразделения Центра
Научная библиотека
Научные советы
Издательская деятельность
История ИК СО РАН
Версия для печати | Главная > Центр > Научные советы > Научный совет по катализу > ... > 2003 год > № 26

№ 26

Обложка номера


Важные научные результаты в области катализа,
полученные в России за 2000 -2002 гг.

За рубежом

Важные научные результаты области катализа, полученные в России за 2000 -2002 гг.


За рубежом

IUPAC Projects

New SIT Software Makes Speciation Calculations Easier

Metal ion speciation in environmental systems is a hot topic because speciation is strongly linked to bioavailability. For labile systems, the speciation can be determined by equilibrium modelling calculations to give the well known "speciation diagram," a plot of composition (species concentration) vs. pH or composition vs. metal/ligand ratio.

Speciation calculations require values for the stability constants of all the species that may form from the system components. These values must be valid at the ionic strength of the medium concerned. Here is a major problem: the vast majority of equilibrium constants have been determined at a fixed ionic strength that is typically much higher (ca. 0.1-1.0 M) than applies in freshwater environmental systems (<0.02 M).

To correct the stability constants to the required ionic strength one must be able to calculate the activity coefficients of all species in the equilibrium reaction. These corrections may be quite large (e.g., log KI for the reaction Fe3+ + nta3- is 16.26 for I=0.10 M NaClO4 and the value calculated for I=0.001 M is 17.89).

The second problem is that not all equations for activity coefficient calculations (e.g., Debye-Huckel and Davies [eqtn. 1]) are valid to such high ionic strengths:

log γ1= -AZj2{|½/(l+|½)-0.3|} (1)

Specific Interaction Theory, or SIT (and Pitzer parameters), provides a valid tool for calculating stability constants over a wide range of ionic strengths. The calculations are not trivial; they require a database of ionic coefficients, ε M, X, for each ion-ion interaction:

log γm = -AZi2{|½/(l+l.5|½)+ε(M, X)mx} (2)

Now for the easy and exciting part! An IUPAC project, undertaken by L. D. Pettit of Academic Software, has produced the SIT program that will calculate:

  • ionic activity coefficients in a user-specified medium, up to
    5.0 molal
  • stability constants in the range 0-5.0 m, based on published values at a single ionic strength
  • molarity from molality, and vice versa

This is an enormous asset for those involved in speciation calculations. It is a user-friendly program that is now in the public domain at <www.iupac.org/projects/ 2000/2000-003-l-500.html>. Interested persons are invited to test the SIT program and send comments to the author. The program includes a comprehensive, but not exhaustive, file of SIT parameters. The file can be edited, but if users have access to reliable SIT parameters that are not included, please send these to the author at <www.acadsoft.co.uk> so that the release version of the program can be updated for all to benefit.

The SIT program can be readily applied to industrial processes for which stability constants may be required at very high ionic strengths.

Two other IUPAC products will assist in the understanding and application of stability constants. The IUPAC Stability Constant database, SC-database1 should be a starting point for stability constant data. It links data directly to the program SPECIES that will calculate speciation curves. The HELP files in the database provide definitions and information about temperature, ionic strength, and solvent effects. For a much wider discourse, the program Sol-Eq2 explains the principles of solution equilibrium and explores many applications in environmental, biological, and industrial systems. It includes a primer on how to do your own speciation calculations.

1SC-database, Stability Constants Database; IUPAC, Academic Software. 2001.

2Sol-Eq. Solution equilibria: Principles and Applications. Academic Software. 2001.

Both of these programs are available to order via <www.acasoft.co.uk>.
Reviewed by Kip Powell, vice president, Analytical Chemistry Division.

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<l) 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., 1=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.

Performance Evaluation Criteria for Preparation and Measurement of Macro-and Microfabricated Ion-Selective Electrodes

During the past several years, the application of micro-fabrication technologies of practical microsensor devices has entered the field of biology and medicine and is beginning to serve as the driving force for discoveries in cell biology, neurobiology, pharmacology, and tissue engineering. In parallel, the methodology of ionophore-based liquid membrane ion-selective electrodes (ISEs) has now entered the arena of trace analysis and precision science due to the latest updated and upgraded research and development of this method. This circumstance has made it necessary to also upgrade and update the evaluation criteria for the preparation and measurements of ISEs, primarily microfabricated ISEs and potentiometric cells. This is in fact the purpose of this project.

There are more or less generally accepted evaluation criteria for ISEs. Some of those are summarized in Pure and Applied Chemistry documents (PAC 1994, 66, 2527; PAC 1995, 67, 507).

In this new document, we will deal with the specialities of macro- and microfabricated ISE sensors; standard procedures for the microfabricated devices are much more critical compared to macroelectrodes. In addition, some evaluation criteria for the micro reference electrodes are of the same importance. Items we will discuss include the following:

  • conditioning of the multilayered sensor system (sensing membrane, underlying hydrogel; and eventually biocompatible coating) before use to reach high stability readings, due to water transport through the membrane
  • conditioning overnight or storage in humid atmosphere

Chemistry International, 2002, Vol 24, N.6

Copyright © catalysis.ru 2005–2024
Политика конфиденциальности в отношении обработки персональных данных