UV Vis

1. COLOURIMETRY/ UV-VISIBLE SPECTROSCOPY
Know your syllabus: Module 8: Applying Chemical Ideas
Topic: Analysis of Inorganic Substances
Inquiry question: How are the ions present in the environment identified and
measured?
● conduct investigations and/or process data to determine the concentration of coloured species
and/or metal ions in aqueous solution, including but not limited to, the use of:
– colourimetry
– ultraviolet-visible spectrophotometry
– atomic absorption spectroscopy

2. Background
• Light is a form of energy that can interact with chemical substances.
• By examining the wavelength of light (its color) that is absorbed by a chemical
substance, the chemicals identity and quantity can be determined.

3. Light reflected
Light absorbed
...Background
• How a chemical appears to our eyes depends on how
light interacts with the substance.
• Light can be transmitted (passed through), reflected
(scattered), or absorbed by a substance.
Light transmitted
Light absorbed

4. Self-Check
1. The green light in the diagram is being
____________.
a) reflected
b) absorbed
c) transmitted
d) transmogrified

5. • the chemical nature of the substance.
NiCl2
Co(NO3)2
K2Cr2O7 K2CrO4 CuSO4 KMnO4
...Background
The color observed depends on:
• the thickness of the sample the light
passes through.
• the concentration of the
substance in solution.
Concentrated
Dilute
20 mm path length
55 mm path length

6. Components of UV- Visible Spectrophotometer
Basic Design
 An instrument used to make absorbance or transmittance measurements is known as a
spectrophotometer

8. 40V ElectricArc
Electrode
Filament
D or H Gas
2 2
S
ealed QuartzT
ube
Fundamentals of Spectrophotometry
Basic Design
 Light Source: provides the light to be passed through the sample
- Tungsten Lamp: visible light (320-2500 nm)
- Deuterium Lamp: ultraviolet Light (160-375 nm)
In presence of arc, some of the electrical energy is
absorbed by D2 (or H2) which results in the
disassociation of the gas and release of light
D2 + Eelect  D*
2  D’ + D’’ + hn (light produced)
Excited state
Low pressure (vacuum)
Tungsten Filament
- based on black body radiation:
heat solid filament to glowing, light emitted
will be characteristic of temperature more
than nature of solid filament

9. Monochromator
 Wavelength Selector (monochromator): used to select a given
wavelength of light from the light source
- Prism:
- Filter:
Crude colorimeter
UV-Vis
Spectrophotometer

10. Beer’s Law
 The relative amount of a certain wavelength of light absorbed (A) that passes through a sample is
dependent on:
- distance the light must pass through the sample (cell path length - b)
- amount of absorbing chemicals in the sample (analyte concentration – c)
- ability of the sample to absorb light (molar absorptivity - e)
Increasing [Fe2+]
Absorbance is directly proportional to concentration of Fe+2

11.  Sample Cell: sample container of fixed length (b).
- Usually round or square cuvet
- Made of material that does not absorb light in
the wavelength range of interest
1. Glass – visible region
2. Quartz – ultraviolet
3. NaCl, KBr – Infrared region

12. ...Background
In this experiment, a colorimeter will be used to find the mathematical
relationship between the absorbance of orange light (λ=610 nm) and the
concentration of copper(II) sulfate.
Light Source
Concentration of a Solution (Beer’s Law)

13. • Since the colorimeter has a fixed path length b, and the experiment only uses
copper(II) sulfate (ε) Beer's law (A = ε × b × c ) can be simplified to:
• The direct relationship between absorbance and
concentration is mathematically represented by a straight
line: Y = mX + b.
• The mathematical relationship (equation) will be
determined by graphing measured absorbencies at known
concentrations of copper(II) sulfate and fitting the data to a
line (calibration curve).
• The equation of the linear fit will be used to determine the
concentration of an "unknown" copper(II) sulfate solution.
Y = mX + b
A = constant × c
Concentration of a Solution (Beer’s Law)
...Background

14. Sequencing Challenge
A. Determine the
linear equation for
the calibration
curve's line of best
fit.
B. Create a
calibration curve
by graphing
absorbance versus
concentration.
C. Measure the
absorbance of the
unknown and
determine its
concentration using
the equation.
The steps to the left are part
of the procedure for this lab
activity. They are not in the
right order. Determine the
correct sequence of the
steps, then take a snapshot
of this page.
D. Create five
copper(II) sulfate
solutions of known
concentration
(standards)and
measure the
absorbance of each.
Concentration of a Solution (Beer’s Law)

15. Analysis
1. State Beer - Lambert's law.
Concentration of a Solution (Beer’s Law)

16. Analysis
2. Explain the difference between “absorbance” and
“transmittance” of light.
Concentration of a Solution (Beer’s Law)
The relative amount of a certain wavelength of light absorbed (A) that passes through a sample.
The relative amount of light making it through the sample (P/Po) is known as the transmittance (T)

17. Analysis
3. Why is CuSO4 a bluish color? Are colors of light
other than blue transmitted?
Concentration of a Solution (Beer’s Law)

18. Analysis
4. Why was the absorbance of orange
(610 nm) light used instead of another
color?
Concentration of a Solution (Beer’s Law)

19. Synthesis
1a. A solution of sodium sulfate is clear and colorless, and yet a solution of copper(II)
sulfate is blue. Which ions are causing the blue color?
1b. Could you use Beer's law to find the concentration of a sodium sulfate solution?
Concentration of a Solution (Beer’s Law)

20. Multiple Choice
1. Which of the following variables affects the
absorbance of light in a solution?
a) The distance the light has to travel through
the solution (path length.
b) The amount of solute in each volume
(concentration).
c) The wavelength of the light that is interacting
with the solution.
d) All of the above variables affect the
absorbance of light in a solution.
Concentration of a Solution (Beer’s Law)

21. Multiple Choice
2. A sample 0.10 M nickel (II) chloride is placed into
a cuvette with a 1.00 cm path length. The solution
has a measured absorbance of 2.0. What would
you expect the absorbance of a 0.05 M nickel (II)
chloride solution to be?
a) 1.0
b) 2.0
c) 4.0
d) not enough information
NiCl2 solution
Concentration of a Solution (Beer’s Law)

22. Multiple Choice
3. What should be in the ‘blank’ cuvette when the
colorimeter is calibrated?
a) nothing
b) the solution with the greatest concentration
of solute
c) the solvent
d) a 1.0 M sample of the solution
Concentration of a Solution (Beer’s Law)
??

23. Multiple Choice
4. What color of light is transmitted through a
copper(II) sulfate solution?
a) red
b) orange
c) green
d) blue
Concentration of a Solution (Beer’s Law)

24. Multiple Choice
5. How many colors make up white light?
a) 1
b) 3
c) 4
d) more than five
e) none
Concentration of a Solution (Beer’s Law)