Color i Meters

7/22/2019 Color i Meters

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Colorimeter


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A = abcA is the absorbance

Beer-Lambert Law(a.k.a. Beer's law) - the linear relationship

between absorbance and concentration of an absorbing species.

Absorbance

Main use of Beers Law is to determine the concentration

of various solutions.

c is the concentration of the sample in (mol/L)

a is molar absorptivity in L/[(mole)(cm)]

Also called extinction coefficient or ;it is dependent on the material being studied.

b is the path length in cm

The diameter of the cuvette or sample holder which is the distance

the light travels through the absorbing sample. b is a constant

when the same size cuvette is used for all samples.


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Visual ObservationsBecause colorimetry is based on

inspection of materials with the human eye, it is

necessary to review aspects of visible light.Visible light is the narrow range of electromagnetic

waves with the wavelength of400-700 nm.

= the mnemonic used to remember the colors of the visible spectrum.

Colorimetry

ROYG. BIV


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Definition

A colorimeter is an instrument which compares theamount of light getting through a solution with the

amount which can get through a sample of pure

solvent.

Substances absorb light for a variety of reasons. Pigments

absorb light at different wavelengths. A cloudy solution will simply

scatter/block the passage of light (sometimes a colorimeter is

used to monitor the growth of a bacterial or yeast culture).

The % transmission or the % absorbance is recorded (you can

use either).

It is possible to change the color of the light that is used by using

filters in the simplest equipment or an "optical wedge" .


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How the Colorimeter Works

Light from a LEDlight source passesthrough a Cuvettecontaining a solution

sample,

Some of the incominglight is absorbed by the

solution.

As a result, light of alower intensity strikes

a photodiode.


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Construction The essential parts of a

colorimeter are:

a light source, which is usuallyan ordinary filament lamp

an aperture which can beadjusted

a detector which measures the

light which has passedthrough the solution

a set offilters in differentcolors filters are used to select the

wavelength of light which thesolution absorbs the most.

Solutions are usuallyplaced in glass or plasticcuvettes.

(1) Wavelength selection,

(2) Printer button

(3) Concentration factor adjustment,(4) UV mode selector (Deuterium

lamp)

(5) Readout

(6) Sample compartment

(7) Zero control (100% T),

(8) Sensitivity switch.
http://en.wikipedia.org/wiki/File:Spectrophoto-controls.svg


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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


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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


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Intensity: For light shining through a colored solution,the

observed intensity of the color is found to be dependent on both

the thickness of the absorbing layer (pathlength) and the

concentration of the colored species.

For One Color: A series of solutions of a single color

demonstrates the effect of either concentration or pathlength,

depending on how it is viewed.

Side view

Top view

(a.k.a. Birds eye view)

Visual Colorimetry


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For more than one color: the ratio of an unknown mixture

can also be determined by matching the shade of the color to

those produced from known ratios.In this example, the ratio of a mixture ofred and blue can be

determined visibly by comparing the mixture to purples

produced from known ratios of red and blue.

Ratio used

Purple produced

Visual Colorimetry


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TRANSMITTANCE


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Transmittance is given by the equation:

T = I/Io

where I is the intensity of the light after it has gone

through the sample & Io is the initial light intensity.

Absorbance is related to the %T:A = -logT = -log(I/ Io)

Transmittance is Related to Absorbance


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To obtain a Beers law curve,several standards (solutions ofknown concentration) are preparedand their absorbance values aredetermined using a Colorimeter.

A graph of absorbance vs.

concentration is then plotted. A solution of unknown

concentration is placed in thecolorimeter and its absorbancemeasured.

When the absorbance of thissolution is interpolated on theBeers law curve, its concentrationis determined on the horizontalaxis.

Alternatively, its concentration may

be found using the slope of theBeers law curve.

Beers Law CurveDetermination of C of an unknown substance


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Concentration of unknown solution

Cu

Cu= Cs Au/As

Cu is unknown concentration

Au is unknown absorbance

Cs standard concentration

As is standard absorbance


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Maintenance

Calibration

Adjustment

Replacement of burned-out lamp and photodetector

Electronic problems are rare

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