UV-spectrometry is frequently used to determine the concentration of a substance in solution by meas — Physical Chemistry — Electrochemistry Chemistry Question
UV-spectroscopy as an Analytical Tool
UV-spectrometry is frequently used to determine the concentration of a substance in solution by measuring the UV absorbance at a certain wavelength or either visible or ultraviolet light. The law of Lambert and Beer states that the absorbance is directly proportional to the concentration in moles per dm3 at a given wavelength: A = ε c l (ε is a molar absorptivity or extinction coefficient in dm3 mol-1 cm-1, the path length in cm. A = log Io / I.
Here the maximal and minimal concentration that can be measured for the redox concentration Fe(II) fenathroline (ferroin) will be considered: (λmax = 512 nm, ε = 10500 dm3 mol–1 cm–1).
Calculate the lowest concentration of ferroin that can be measured in a 1 cm cuvet at 512 nm. If a 2 % difference of light intensity still can be measured.
Model Answer
2% decrease in light intensity implies: I / I0 = 98/100 and A = – log98/100 = 0.01
This absorption corresponds with 0.01 = 10500 × cmin × l cmin = 0.95×10–6 mol dm–3
Calculate the highest concentration of ferroin that can be measured in a 1 cm cuvet at 512 nm if at least 2 % of the incident light must reach the detector.
Model Answer
2 % light throughput means that: I / I0 = 2/100 and A = log100/2 = 2 – 0.3010 = 1.6990
For ferroin this absorption at 512 nm corresponds with:
1.6990 = 10500 × cmax × l cmax = 1.618×10–4 mol dm–3
The composition of a complex between a metal M and a ligand L can also be determined spectrometrically using the method of Continuous Variations, also known as Job’ s method whereby the sum of molar concentrations of M and L is kept constant as their ration is varied. The following graph of absorbance vs. mol fraction for a complex is given, whereby the mol fraction xM = cM / (cM + cL) is varied (measurement at 552 nm).
Determine the composition of the complex and show your calculation.
Model Answer
Minimum in curve at xM = 0.33: cM = 0.33 cM + 0.33 cL or 0.67 cM = 0.33 cL
Which compounds absorb at xM = 0?
Which compounds absorb at xM = 1?
Show how you derive your answer.
Model Answer
For xM = 0: cM / cM + cL = 0, cM = 0 and cL = 1
For xM = 1: cM / cM + cL = 1, cM = 1 and cL = 0
M and L both absorb and have an absorption of AM = 1.0 and AL = 0.5, respectively.
Calculate the ratio of the extinction coefficients of M and L.
Model Answer
xM = 0, cL = 1, AL = L cL l
xM = 1, cM = 1, AM = M cM l
I has the same value in booth formulas, just as cL and cM, so that
L / M = AL / AM = 0.5 / 1, thus cM = 2 L
Calculate the percentage of the incident light that has been transmitted through the solutions belonging to xM = 0 and xM = 1, respectively.
Model Answer
For xM = 0 holds 0.5 = – log I / I0 I / I0 = 0.32 thus 32 % has been transmitted
For xM = 1 holds 1 = – log I / I0 I / I0 = 0.1 thus 10 % has been transmitted