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  1. 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. 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. 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. 4. TransmittanceTransmittance
  5. 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. 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. 7. T & AT & A
  8. 8. T & AT & A
  9. 9. Relation between AbsorbanceRelation between Absorbance and concentrationand concentration  Beer’s lawBeer’s law
  10. 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. 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. 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. 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. 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. 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. 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. 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. 18. Basic Colorimeter SchematicBasic Colorimeter Schematic Electronics
  19. 19. 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
  20. 20.  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
  21. 21.  Cu , Co , PbCu , Co , Pb Titrated against sodium thiosulfateTitrated against sodium thiosulfate ColorColor blueblue toto pale pinkpale pink