CHEM 213Instrumental Analysis

Lab Lecture – Fe and Extraction

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Chem 102 – used Crystal field theory to explain colour of coordination complexes

Chem 213 – use intensity of colour to determine concentration of metal ion

3 + Fe2+ Fe(bipy)32+

Bipyridyl(bipy)

red

Determination of Trace Iron (LM, p14-22)

Spectrophotometry – any method using light to measure chemical composition

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P/P0 %T A 1 100 0 0.1 10 10.01 1 2

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) = abc

a = absorptivity

b = pathlength

c = concentration

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Spectrophotometer (Text p410, LM p19-21)

Spec 20D+pg 21

On/Off Zero control

Readout Mode

Sample comp.

Readout On/Off

Genesys 20pg 21

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Spectrophotometer (Text p410, LM p19-21)

Sample cell matching LM p 19 (1 hour)- Need 8 test tubes with the same pathlength (b)- prepare dilute amaranth dye soln

- Set wavelength to 510 nm

- Insert test tube in sample compartment

- Adjust trans/absorb control to 90%

- Insert 2nd tube in spec

- Put on alignment line

- Rotate tube to get T=90%

- Repeat until you have 8 matched tubes

Record spectrometer number

- Mark tube with line

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• Reduce any Fe(III) to Fe(II) so all iron is in the Fe(II) state, because only Fe(II) forms coloured complex

Determination of Trace Iron (LM, p14-22)

3 + Fe2+ Fe(bipy)32+

Bipyridyl(bipy)

red

1. Fe3+acidic

2. xs + Fe2+ Fe(bipy)32+

3. Adjust pH to ~ 5 by adding sodium acetate

+ xs NH2OH Fe2+

hydroxylamine reducing agent

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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 = abc

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.

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Why is 100 %T (A = 0) reset at each wavelength?

Determination of Trace Iron (LM, p14-22)

Light intensity (Po) varies with wavelength)

A is maxBest sensitivity

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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

Prepare Calibration Curve

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Prepare Calibration Curve

Blank

standards

Asample

Csample

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Calibration Curves LM, Appendix A

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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

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Drawing a Calibration Curve

Manually on graph paper

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0 1 2 3 4 50

0.1

0.2

0.3

0.4

0.5

0.6

Concentration, ppm

Inst

Res

po

nse

Useful for spotting:

Manual Graphing of Calibration Curve

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0 1 2 3 4 50

0.1

0.2

0.3

0.4

0.5

0.6

Concentration, ppm

Inst

Res

po

nse

Useful for spotting: outliers

Manual Graphing of Calibration Curve

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0 1 2 3 4 50

0.1

0.2

0.3

0.4

0.5

0.6

Concentration, ppm

Inst

Res

po

nse

Manual Graphing of Calibration Curve

Useful for spotting: outliers curvature

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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

Manually on graph paper

- best for spotting outliers, curvature, etc.

- disgard outliers

Manual Graphing of Calibration Curve

Calculate least squares line

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Solvent extraction

Dextromethorphancough suppressant

Expectorant Glyceryl guaiacolate

Antihistamines Chloropheniramine maleate Diphenyhydramine HCl

Decongestants Phenylephrine HCl

Analgesics Acetaminophen

Preservatives

Flavouring

Colouring

Determination of Dextromethorphan in Cough Syrup

toluenewater

19Dextromethorphan

DMH+

Bromothymol blueBTB-

+ (DMH+BTB-)

ion pair

(DMH+ BTB-)org

yellow

B. Solvent Extraction System

Ion-association Extraction

water

toluene

Colour = Absorbance @ 400 nm [BTB-]org

xs

[DMH+]aq

Calibration curve: (absorbance-blank) vs. [DMH+]aq

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First week solution preparation (pg 50)

Do NOT add BTBDo NOT prep sample

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b) Mohr pipet – use like a buret

d) Fluted filter paper – allows liquid to pass through quickly