1. CHEM 213 Instrumental Analysis
Lab Lecture – Fe and Extraction

2. Determination of Trace Iron (LM, p14-22)
Chem 102 – used Crystal field theory to explain colour of coordination complexes
Chem 213 – use intensity of colour to determine concentration of metal ion
Fe Fe(bipy)32+
Bipyridyl
(bipy)
red
Spectrophotometry – any method using light to measure chemical composition

3. Fe: Concentration to Colour
Intensity of colour = Absorbance (A)
Measure absorbance of soln to determine the iron concentration.
Transmitted
Transmittance (T) = P/Po
Absorbance A = -log(T) = abc
a = absorptivity
b = pathlength
c = concentration
P/P %T A

4. Spectrophotometer (Text p410, LM p19-21)
Spec 20D+
pg 21
On/Off
Zero control
Readout
Mode
Sample comp.
Readout
On/Off
Genesys 20
pg 21

5. Spectrophotometer (Text p410, LM p19-21)
Record spectrometer number
Sample cell matching LM p 19 (1 hour)
Need 8 test tubes with the same pathlength (b)
prepare dilute amaranth dye soln
Rotate tube to get T=90%
Set wavelength to 510 nm
Insert 2nd tube in spec
Mark tube with line
Put on alignment line
Insert test tube in sample compartment
Adjust trans/absorb control to 90%
Repeat until you have 8
matched tubes

6. Determination of Trace Iron (LM, p14-22)
Fe Fe(bipy)32+
Bipyridyl
(bipy)
red
Reduce any Fe(III) to Fe(II) so all iron is in the Fe(II) state, because only Fe(II) forms coloured complex
1. Fe3+acidic
+ xs NH2OH  Fe2+
hydroxylamine
reducing agent
2. xs Fe2+  Fe(bipy)32+
3. Adjust pH to ~ 5 by adding sodium acetate

7. Determination of Trace Iron(LM, p14-22)
Measure absorption spectrum of the complex to determine the wavelength of max. absorbance.
Absorbance spectrum – graph of absorbance of light vs. wavelength
A = abc
Measure A for most conc standard from 640 to 400 nm at 20 nm intervals.
The 0 %T (20D Only) and the 100 %T (A = 0) settings must be reset for each new wavelength.

8. Determination of Trace Iron (LM, p14-22)
Why is 100 %T (A = 0) reset at each wavelength?
Light intensity (Po) varies with wavelength)
A is max
Best sensitivity

9. Prepare Calibration Curve
Standards = pipet 5, 10, 15 & 20 mL Fe3+ into flasks
Blank = soln with all of the reagents but no analyte
[Fe3+]=0
Zero (set A =0) instrument using blank
Measure absorbance of all standards and samples
Re-zero
Take second absorbance reading for all stds and samples

10. Prepare Calibration Curve
standards
Asample
Blank
Csample

11. Calibration Curves LM, Appendix A

12. Calibration Curves
Calibration Curve – graph of instrument response vs. conc of analyte
when response for sample is measured, its conc is determined by interpolating from the graph
- blank value should be part of the calibration curve, i.e. signal for [C] = 0.0
-standards must cover the complete range of expected unknowns
- plot signal on the Y axis and conc on the X axis.
Normally response is linear
or with math transformation can be made linear
Signal = m(conc) + b
m = slope b = intercept

13. Drawing a Calibration Curve
Manually on graph paper

14. Manual Graphing of Calibration Curve
Useful for spotting:

15. Manual Graphing of Calibration Curve
Useful for spotting: outliers

16. Manual Graphing of Calibration Curve
Useful for spotting: outliers
curvature

17. Manual Graphing of Calibration Curve
Manually on graph paper
- best for spotting outliers, curvature, etc.
- disgard outliers
Calculate least squares line
Mathematically calculate line using least squares
fits data to y = mx + b ?
determines m and b
determines error in m and b?
allows accurate calculation of sample conc

18. Determination of Dextromethorphan in Cough Syrup
Preservatives
Flavouring
Colouring
Expectorant
Glyceryl guaiacolate
Antihistamines
Chloropheniramine maleate
Diphenyhydramine HCl
Decongestants
Phenylephrine HCl
Analgesics
Acetaminophen
Solvent extraction
Dextromethorphan
cough suppressant
toluene
water

19. + B. Solvent Extraction System Colour = Absorbance @ 400 nm
 [BTB-]org
 [DMH+]aq
Calibration curve:
(absorbance-blank) vs. [DMH+]aq
(DMH+ BTB-)org
yellow
water
toluene
Ion-association Extraction
Dextromethorphan
DMH+
Bromothymol blue
BTB- +
(DMH+BTB-)
ion pair xs

20. First week solution preparation (pg 50)
Do NOT add BTB
Do NOT prep sample

21. b) Mohr pipet – use like a buret
d) Fluted filter paper – allows liquid to pass through quickly