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CHEM 213 Instrumental Analysis. Lab Lecture – Fe and Extraction. 3 + Fe 2+ Fe(bipy) 3 2+. red. Bipyridyl (bipy). Determination of Trace Iron (LM, p14-22). Chem 102 – used Crystal field theory to explain colour of coordination complexes

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Chem 213 instrumental analysis

CHEM 213Instrumental Analysis

Lab Lecture – Fe and Extraction


3 + Fe2+ Fe(bipy)32+

red

Bipyridyl

(bipy)

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

Spectrophotometry – any method using light to measure chemical composition


Fe: Concentration to Colour

Intensity of colour = Absorbance (A)

Measure absorbance of soln to determine the iron concentration.

P/P0 %T A

1 100 0

0.1 10 1

0.01 1 2

Transmitted

Transmittance (T) = P/Po

Absorbance A = -log(T) = abc

a = absorptivity

b = pathlength

c = concentration


Mode

Readout

Readout

On/Off

On/Off

Zero control

Spec 20D+

pg 21

Genesys 20

pg 21

Sample comp.

Spectrophotometer (Text p410, LM p19-21)


  • Rotate tube to get T=90%

  • Mark tube with line

  • Put on alignment line

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

  • Set wavelength to 510 nm

  • Insert test tube in sample compartment

  • Adjust trans/absorb control to 90%

  • Repeat until you have 8

  • matched tubes


3 + Fe2+ Fe(bipy)32+

Bipyridyl

(bipy)

2. xs + Fe2+ Fe(bipy)32+

Determination of Trace Iron (LM, p14-22)

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

red

+ xs NH2OH  Fe2+

hydroxylamine

reducing agent

1. Fe3+acidic

3. Adjust pH to ~ 5 by adding sodium acetate


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.


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


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


Prepare Calibration Curve

standards

Asample

Blank

Csample



Calibration curves
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


Drawing a Calibration Curve

Manually on graph paper



Manual Graphing of Calibration Curve

Useful for spotting: outliers


Manual Graphing of Calibration Curve

Useful for spotting: outliers

curvature


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


Solvent extraction

Determination of Dextromethorphan in Cough Syrup

Preservatives

Flavouring

Colouring

Expectorant

Glyceryl guaiacolate

Antihistamines

Chloropheniramine maleate

Diphenyhydramine HCl

Decongestants

Phenylephrine HCl

Analgesics

Acetaminophen

Dextromethorphan

cough suppressant

toluene

water


(DMH+ BTB-)org

yellow

toluene

water

+

(DMH+BTB-)

Dextromethorphan

DMH+

Bromothymol blue

BTB-

B. Solvent Extraction System

 [BTB-]org

= Absorbance @ 400 nm

Colour

 [DMH+]aq

Calibration curve:

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

Ion-association Extraction

ion pair

xs


First week solution preparation (pg 50)

Do NOT add BTB

Do NOT prep sample


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

b) Mohr pipet – use like a buret


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