A colorimeter is an instrument that compares the amount of light passing through a sample solution to the amount passing through a pure solvent. It contains a light source, aperture, detector, set of filters, and holds sample cuvettes. Substances absorb light at specific wavelengths depending on their properties. The colorimeter measures transmittance (T) or absorbance (A) of solutions, which are related by a logarithmic function. Beer's law states absorbance is directly proportional to concentration, allowing determination of unknown concentrations from a standard curve. Wavelength selection depends on the color of light absorbed by the sample. Colorimetry is used to analyze metals in wet chemistry via titration with indicators.

1. DefinitionDefinition
 A colorimeter is an instrumentA colorimeter is an instrument
whichwhich comparescompares thethe amount ofamount of
lightlight getting through agetting through a solutionsolution
with the amount which can getwith the amount which can get
through athrough a sample of puresample of pure
solventsolvent..
 Substances absorb light for a variety ofSubstances absorb light for a variety of
reasons. Pigments absorb light atreasons. Pigments absorb light at
different wavelengths. A cloudydifferent wavelengths. A cloudy
solution will simply scatter/block thesolution will simply scatter/block the
passage of lightpassage of light
 The %The % transmissiontransmission or the %or the %

2. How the Colorimeter Works
 Light from a LED
light source passes
through a Cuvette
containing a solution
sample,
 Some of the incoming
light is absorbed by the
solution.
 As a result, light of a
lower intensity strikes a
photodiode.

3. ConstructionConstruction
 The essential parts of aThe essential parts of a
colorimeter are:colorimeter are:
 aa light sourcelight source, which is usually, which is usually
an ordinary filament lampan ordinary filament lamp
 anan apertureaperture which can bewhich can be
adjustedadjusted
 aa detectordetector which measures thewhich measures the
light which has passedlight which has passed
through the solutionthrough the solution
 a set ofa set of filtersfilters in differentin different
colorscolors
 filters are used to select thefilters are used to select the
wavelength of light which thewavelength of light which the
solutionsolution absorbsabsorbs the most.the most.
 Solutions are usuallySolutions are usually
placed in glass or plasticplaced in glass or plastic
cuvettes.cuvettes.
(1)(1) Wavelength selection,Wavelength selection,
(2)(2) Printer buttonPrinter button
(3)(3) Concentration factor adjustment,Concentration factor adjustment,
(4)(4) UV mode selector (DeuteriumUV mode selector (Deuterium
lamp)lamp)
(5)(5) ReadoutReadout
(6)(6) Sample compartmentSample compartment
(7)(7) Zero control (100% T),Zero control (100% T),
(8)(8) Sensitivity switch.Sensitivity switch.

4. TransmittanceTransmittance

5. Transmittance “T”
 The amount of light that passes through a solution is
known as transmittance T.
 Transmittance can be expressed as the ratio of the
intensity of the transmitted light It to the initial intensity
of the light beam Io
 The transmittance formula is:
T = It/Io
 The Colorimeter produces an output voltage which
varies in a linear way with transmittance, allowing a
computer, calculator, or handheld to monitor
transmittance data for a solution.

6. Absorbance “A”Absorbance “A”
 The reciprocal of transmittance of the sample varies
logarithmically (base ten) with the product of three factors:
ε, the molar absorptivity of the solution, bthe cell or cuvette width,
and C the molar concentration
A = log(1/T) = ε b C
 The relationship between these two variables is inverse and
logarithmic (base 10). It can be expressed as
A = log(1/T)

7. T & AT & A

8. T & AT & A

9. Relation between AbsorbanceRelation between Absorbance
and concentrationand concentration
 Beer’s lawBeer’s law

10. Beer’s lawBeer’s law
Mathematical statement of Beer’s law
 For a given solution contained in a cuvette with a
constant cell width, the Absorbance is proportional
to the concentration:
This equation shows absorbance to be related directly to
concentration and represents a mathematical statement of Beer’s
law.
b is cuvette path lengthb is cuvette path length
C is concentration of absorbing substanceC is concentration of absorbing substance
ε is absorptivity (~ substance)is absorptivity (~ substance)
A = ε C b = k C

11.  To obtain a Beer’s law curve,
several standards (solutions of
known concentration) are prepared
and their absorbance values are
determined using a Colorimeter.
 A graph of absorbance vs.
concentration is then plotted.
 A solution of unknown
concentration is placed in the
colorimeter and its absorbance
measured.
 When the absorbance of this
solution is interpolated on the
Beer’s law curve, its concentration
is determined on the horizontal
axis.
 Alternatively, its concentration may
be found using the slope of the
Beer’s law curve.
Beer’s Law Curve
Determination of C of an unknown substance

12. Concentration of unknown solutionConcentration of unknown solution
CCuu
CCuu= C= Css AAuu/A/Ass
CCuu is unknown concentrationis unknown concentration
AAuu is unknown absorbanceis unknown absorbance
CCss standard concentrationstandard concentration
AAss is standard absorbanceis standard absorbance

13. Ranges of A and T for the colorimeter
during calibration
 For best results our laboratory testing of the
colorimeter indicates that absorbance or
transmittance values should fall with these
ranges:
Transmittance (T) 0.28 - 0.90
Absorbance (A) 0.050 - 0.550

14. Determining of the wavelength
 You can select three LED light colors:
red (635 nm), green (565 nm) or blue (470 nm).
There are several ways you can decide which of three
wavelengths to use:
 Method 1.
 Look at the color of the solution. (Remember that the color of
solution is the color of light which is not absorbed).
 use a different color of light that will be absorbed
 For example: with a blue CuSO4 solution, use the red LED
(635 nm).

15.  Method 2.
 Directions for most colorimetry experiments express a
recommended wavelength. Use the closest of the three
wavelengths on the colorimeter. Even if the LED wavelength
is somewhat different, a Beer's law curve can usually be
obtained at almost any wavelength around the recommended
wavelength.

16.
Observed Color of
Compound
Color of Light
Absorbed
Approximate
Wavelength of Light
Absorbed
Green
700 nm
Blue-green
600 nm
Violet
550 nm
Red-violet
530 nm
Red
500 nm
Orange
450 nm
Yellow
400 nm

17.
Observed Color of
Compound
Color of Light
Absorbed
Approximate
Wavelength of Light
Absorbed
Green
Red 700 nm
Blue-green
Orange-red 600 nm
Violet
Yellow 550 nm
Red-violet
Yellow-green 530 nm
Red
Green 500 nm
Orange
Blue 450 nm
Yellow
Violet 400 nm

18. Basic Colorimeter SchematicBasic Colorimeter Schematic
Electronics

21. Wet analysis of metalsWet analysis of metals
 ZnZn
Titrated with EDTATitrated with EDTA
Indicator Eri Chrom Black TIndicator Eri Chrom Black T
ColorColor redred toto blueblue
By complexometric titrationBy complexometric titration
 FeFe
Titrated with KMnO4Titrated with KMnO4
ColoreColore permanent pinkpermanent pink
by redox titrationby redox titration

22.  Mg, Ca, Pb, Al, Mn, CrMg, Ca, Pb, Al, Mn, Cr
Titrated against EDTATitrated against EDTA
Indicator Eri chrom black TIndicator Eri chrom black T
ColorColor pinkpink redred toto blueblue
 NiNi
Titrated against EDTATitrated against EDTA
Indicator Eri chrom black TIndicator Eri chrom black T
ColorColor blue toblue to vine redvine red

23.  Cu , Co , PbCu , Co , Pb
Titrated against sodium thiosulfateTitrated against sodium thiosulfate
ColorColor blueblue toto pale pinkpale pink