Content deleted Content added
No edit summary |
m Added dtata sources |
||
Line 11:
:<math>K=\frac{{[S]} ^\sigma {[T]}^\tau ... } {{[A]}^\alpha {[B]}^\beta ...}
</math>
== Types of equilibrium constants ==
=== Association and dissociation constants ===
In organic chemistry and biochemistry it is customary to use pK<sub>a</sub> values for acid dissociation equilibria.
Line 28 ⟶ 30:
:<math>M+L+H \rightleftharpoons MLH:\lg \beta_{111} =\lg \left(\frac{[MLH]}{[M][L][H]} \right)</math>
Note how the subscripts define the stoichiometry of the equilibrium product.
=== Stepwise formation constants ===
The stepwise constant for protonation of ML can be easily derived as follows.
Line 54 ⟶ 57:
In biochemistry equilibrium constants are often measured at a pH fixed by means of a buffer. Such constants are, by definition, conditional.
== Experimental methods ==
A general equilibrium expression for an equilibrium constant
:<math>K=\frac{{[S]} ^\sigma {[T]}^\tau ... } {{[A]}^\alpha {[B]}^\beta ...}
</math>
shows that it is a function of the concentrations [A], [B] etc. of the chemical species in equilibrium. The equilibrium constant value can be determined if any one of these concentrations can be measured. The general procedure is that the concentration in question is measured for a series of solutions with different compositions. The data are then treated by a more or less complicated mathematical procedure to get the values. There are four main experimental methods.
=== Potentiometric measurements ===
A free concentration [A] or activity {A} is measured by means of an [[ion selective electrode]] such as the [[glass electrode]]. If the electrode is calibrated using activity standards it is assumed that the [[Nernst equation]] applies in the form
Line 66 ⟶ 72:
:E=E<sup>0</sup>+s lg[A]
''s'' an empirical slope factor
=== [[Absorbance]] measurements ===
It is assumed that the [[Beer-Lambert law]] applies.
:<math>A=l\sum {\epsilon c}</math>
where l is the optical path length, <math>\epsilon</math> is a molar absorbance and ''c'' is a concentration. More than one of the species may contribute to the absorbance. In principle absorbance may be measured at one wavelength only, but in present-day practice it is common to record complete spectra.
=== [[Fluorescence]] (luminescence) measurements ===
It is assumed that the scattered light intensity is a linear function of species’ concentrations.
:<math>I=\sum{ \phi c}</math>
where <math>\phi</math> is a proportionality constant.
=== NMR [[chemical shift]] measurements ===
Chemical exchange is assumed to be rapid on the NMR time-scale. An individual chemical shift is the mol-fraction weighted average of the shift of contributing species.
:<math>\Delta=\frac{\sum c_i \delta_i}{\sum c_i}</math>
=== Calorimetric measurements ===
Simultaneous measurement of K and <math>\Delta</math>H for 1:1 adducts is routinely carried out using [[Isothermal Titration Calorimetry]]. Extension to more complex systems is limited by the availability of suitable software.
=== Range and limitations ===
#Potentiometry. The most widely used electrode is the glass electrode which is selective for the hydrogen ion. This is suitable for all acid-base equilibria. Lg values between about 2 and 11 can be measured by potentiometric titration using a glass electrode. This enormous range is possible because of the logarithmic response of the electrode. The limitations arise because the Nernst equation breaks down at very low or very high pH.
Line 84 ⟶ 95:
#NMR. Limited precision of chemical shift measurements also puts an upper limit of about 4 on lg <math>\beta</math>. Limited to diamagnetic systems.
#Calorimetry. Insufficient evidence is currently available.
== Data sources ==
[[http://www.acadsoft.co.uk/scdbase/scdbase.htm IUPAC SC-Database]] A comprehensive database of published data on equilibrium constants of metal complexes and ligands
[[http://www.nist.gov:80/srd/nist46.htm NIST Standard Reference Database 46]] Critically Selected Stability Constants of Metal Complexes
[[http://daecr1.harvard.edu/pKa/pKa.html Inorganic and organic acids and bases]] pKa Data in water and [[DMSO]]
[[category: Analytical chemistry]]
| |||