DefinitionDefinition

A colorimeter is an instrument which A colorimeter is an instrument which comparescompares the the amount of lightamount of light getting through a getting through a solutionsolution with the with the amount which can get through a amount which can get through a sample of pure solventsample of pure solvent.. Substances absorb light for a variety of Substances absorb light for a variety of

reasons. Pigments absorb light at different reasons. Pigments absorb light at different wavelengths. A cloudy solution will simply wavelengths. A cloudy solution will simply scatter/block the passage of lightscatter/block the passage of light

The % The % transmissiontransmission or the % or the % absorbanceabsorbance is is recorded (you can use either).recorded (you can use either).

It is possible to change the color of the light It is possible to change the color of the light that is used by using filters in the simplest that is used by using filters in the simplest equipment or an "optical wedge" .equipment or an "optical wedge" .

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.

ConstructionConstruction The essential parts of a The essential parts of a

colorimeter are:colorimeter are: a a light sourcelight source, which is , which is

usually an ordinary usually an ordinary filament lamp filament lamp

an an apertureaperture which can which can be adjusted be adjusted

a a detectordetector which which measures the light measures the light which has passed which has passed through the solution through the solution

a set of a set of filtersfilters in in different colors different colors

filters are used to select filters are used to select the wavelength of light the wavelength of light which the solution which the solution absorbsabsorbs the most. the most.

Solutions are usually Solutions are usually placed in glass or placed in glass or plastic cuvettes. plastic cuvettes.

(1)(1) Wavelength selection, Wavelength selection, (2)(2) Printer buttonPrinter button(3)(3) Concentration factor Concentration factor

adjustment, adjustment, (4)(4) UV mode selector UV mode selector

(Deuterium lamp)(Deuterium 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.

TransmittanceTransmittance

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.

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, b the 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)

T & AT & A

                                  

T & AT & A

Relation between Relation between Absorbance and Absorbance and concentration concentration Beer’s lawBeer’s law

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 lengthC is concentration of absorbing substanceC is concentration of absorbing substanceε is absorptivity (~ substance)is absorptivity (~ substance)

A = ε C b = k C

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 CurveDetermination of C of an unknown substance

Concentration of unknown Concentration of unknown solution Csolution Cuu

CCuu= C= Css A Auu/A/Ass

CCu u is unknown concentrationis unknown concentration

AAu u is unknown absorbanceis unknown absorbance

CCs s standard concentrationstandard concentration

AAs s is standard absorbanceis standard absorbance

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 - 0.90Absorbance (A) 0.050 - 0.550

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).

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.

 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

 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

Basic Colorimeter Basic Colorimeter Schematic Schematic

Electronics

Wet analysis of metalsWet analysis of metals ZnZnTitrated with EDTATitrated with EDTAIndicator Eri Chrom Black TIndicator Eri Chrom Black TColor Color redred to to blueblueBy complexometric titrationBy complexometric titration FeFeTitrated with KMnO4Titrated with KMnO4Colore Colore permanent pinkpermanent pink by redox titrationby redox titration

Mg, Ca, Pb, Al, Mn, Cr Mg, Ca, Pb, Al, Mn, Cr Titrated against EDTATitrated against EDTAIndicator Eri chrom black TIndicator Eri chrom black TColor Color pink pink redred to to blueblue NiNiTitrated against EDTATitrated against EDTAIndicator Eri chrom black TIndicator Eri chrom black TColorColor blue to blue to vine redvine red

Cu , Co , PbCu , Co , Pb

Titrated against sodium thiosulfateTitrated against sodium thiosulfate

Color Color blueblue to to pale pink pale pink