Molecular spectroscopy and structure: Quantitative spectroscopy Flashcards
What is the Beer-Lambert Law?
I/I0 = e^-ε c l
A = log(I0/I) = ε c l
What is transmittance?
the ratio I/I0, given the symbol T an often quoted as a percentage
What happens to a typical plot of data from a Beer-Lambert Law experiment at high concentrations?
it deviates from the predicted linear behaviour
Why do these deviations occur?
- the refractive index changes with concentration
- the absorbance saturates
- there may be association or dissociation of the solute
- the solute ma fluoresce, thus contributing to I by emission
Can absorption spectroscopy be used to find the concentrations of two distinct solute species in a solution?
yes (with some assumptions)
When is the measured absorbance at a given wavelength the simple sum of the contributions from each solute?
if all solutes present are non-reacting and separately obey the Beer-Lambert Law
How is this method involving simultaneous equations applied most effectively?
if the two wavelengths are chosen such that solute A absorbs more strongly than solute B at one wavelength and vice versa at the other wavelength
What happens if more than two wavelengths are chosen to get a larger data set?
The problem of finding the concentrations becomes one of least squares fitting rather than simply solving a pair of simultaneous equations.
What is the Henderson-Hasselbalch equation?
pH = pKa + log( [A-] / [HA] )
How is this type of analysis made as efficient as possible?
absorbance measurements are performed at a wavelength where the absorbance of HA is very different from that of A- and so there is a large variation in the absorbance with changes in pH
What feature of the plot of absorbance against wavelength gives the isobestic point?
the point at which all the absorbance curves converge
What feature of the plot of absorbance against wavelength gives the best wavelength?
the maximum point
What is the isobestic point?
The wavelength at which the absorbance of the solution is independent of pH.
What is indicated by the existence of an isobestic point?
a true equilibrium exits between two (and only two) species in the solution
What is signified by a more complex equilibrium, involving a significant concentration of an intermediate species?
it is unlikely that there will be a wavelength at which all three species have the same molar extinction coefficient
