Spectroscopic /Colorimetric
Analysis
Emily Scheerer
AP Chemistry Pd. 2
Concept
Colorimetric/Spectroscopic Analysis – variation of intensity of
color of a solution with changes in concentration.
How a Spectrometer works:
There are five parts: a light source, a monochromator, the cell, a
detector and a meter
The monochromator selects a wavelength from the light source to
send to the cell. Light shots through the cell and the detector
measures the intensity of light from the cell. The meter indicates
the intensity in %T (percent transmittance) or A (absorbance)
Concept cont’d

There are 2 ways to express absorbance:
–
–
–
Percent transmittance T= I/I0*100
Absorbance A = log I0/I
A = 2 – log10%T
Beer’s Law

A = abc
A – absorbance
a – molar absorbity
b – solution path length
c – concentration in moles per liter
Since Beer’s law makes Absorbance proportional to concentration we
can use it to determine concentration.

Purpose
To determine the concentration of an unknown iron substance based
on Spectroscopic Analysis.
Determine this by comparing the unknown measurement to the scale
you create with samples of known concentration.
For a given substance, the amount of light absorbed depends on:
~ Concentration
~ cell/path length
~ wavelength of light
~ solvent
Spectrophotometer diagram
Picture Credit: http://www.boomer.org/c/p3/c03/Fig04.gif
Spectrophotometer (inside diagram)
Picture credit:
http://www.wellesley.edu/Chemistry/Chem105manual/Lab04/lab04_spectrophotometer.gif
These are cuvettes 
Materials
Standard iron solution Fe(NO3)3 + HNO3 (1 mL = 0.05 mg Fe)
1M
NH4C2H3O2
10% hydroxylamine hydrochloride
Cuvettes
0.30% o – phenanthroline
6M
50 mL volumetric flask
1,2, and 5 mL pipets
Spectrophotometer
125 mL Erlenmeyer flask
H2SO4
Unknown iron sample
Picture credit - http://www.globescientific.com/product-type-spectrophotometer-cuvettes-c-21_630_89.html
Procedure
~ Weigh out 0.1g iron unknown into a 50-mL volumetric flask
~ add 5 drops of 6M sulfuric acid and dilute to 50 mL
~ mix and transfer to 125 mL Erlenmeyer flask
~ pipet exactly 1 mL of this solution into a 50-mL volumetric flask
~ add 1 mL of 1M ammonium acetate & 1mL of 10% hydroxylamine
hydrochloride & 10 mL of 0.30% o-phenanthroline solution
~ dilute to 50 mL
~ mix to develop orange red iron color (45 min)
~ Fill a clean and dry cuvette with colored solution
~ put cuvette in spectrophotometer and determine absorbance at 510 nm
~ Repeat with 1mL, 2mL, 3mL, 4mL, 5mL standard iron solution
Potential Data (to explain calculations,
DO NOT use this as a factual data
table)
Wavelength (nm)
%T of standard solution
540
36.4
560
15.5
580
7.5
600
32.6
620
8.1
640
16.3
Unknown: 22.4 %T at 600 nm
Potential Data (cont’d)



Mass of standard solution used = 7 g
Mass of unknown = 5.5 g
Concentration of standard Fe = 0.0228 M
Calculations
Calculate the absorbance of the standard and unknown solutions from their
measured transmittance.
Remember: A = 2 – log10%T
A (standard) = 2.00 – log(32.6) = 0.487
A (unknown) = 2.00 – log(22.4) = 0.650
Then calculate concentration of unknown given that:
As/ Cs = ab = Au/Cu
As = absorbance of standard solution
Cs = Concentration of standard
Au = absorbance of unknown
Cu = Concentration of unknown
Rearrange to find the concentration of the unknown:
Cu = Aux Cs x 1/As= .650 x .0228 x 1/0.487 = 0.0304 M
Alternate Websites
Beer’s Law explanation:
http://teaching.shu.ac.uk/hwb/chemistry/tutorials/molspec/beers1.htm
Alternate Lab – Crystal Violet solution
http://www.smes.org/classes/chemistry/ap/apCrystalVioletRateLab.htm
Book Citation:
Nelson, John H. Laboratory experiments for Brown and LeMay
Prentice-Hall Inc, New Jersey, 1977