Colorimetry

1. Colorimetry In colorimetry, the concentration of a coloured solution can be found, by comparing the solution to a set of standard solutions of known concentrations. This is achieved by comparing the absorbance of coloured (visible) light of the unknown solution with the absorbance of light by the standard solutions.

2. Electromagnetic Spectrum Col

3. In colorimetry, the concentration of a coloured solution can be found, by comparing the solution to a set of standard solutions of known concentrations. This is achieved by comparing the absorbance of coloured (visible) light of the unknown solution with the absorbance of light by the standard solutions.

4. Colorimetry measures the intensity of light passing through a coloured solution. A detector (eg a photocell) calculates the % transmittance by comparing the intensity of light (I) passing through the coloured solution with the intensity of light entering the solution (Io) . (Io) is measured by finding the intensity of light which passes through a solution of the pure solvent. When (Io) is measured, this “standardises” the colorimeter so coloured solutions can be compared with it. The colorimeter is “zeroed” so that the (Io) is 100% ie all light entering the solvent also passes through it.

5. Colorimetry measures the intensity of light passing through a coloured solution. A detector (eg a photocell) calculates the % transmittance by comparing the intensity of light (I) passing through the coloured solution with the intensity of light entering the solution (Io) . (Io) is measured by finding the intensity of light which passes through a solution of the pure solvent. When (Io) is measured, this “standardises” the colorimeter so coloured solutions can be compared with it. The colorimeter is “zeroed” so that the (Io) is 100% ie all light entering the solvent also passes through it. % Transmittance (T) = I / (Io) Absorbance (A)  = -log(I/Io) = -log(T) Absorbance (A) is directly proportional to the concentration of the coloured solution (provided the path length for the light is constant).

6. Colorimetry The measurement of the intensity of electromagnetic radiation in the visible spectrum transmitted through a solution or transparent solid. It is used to identify and determine the concentrations of substances that absorb light of a specific wavelength or colour according to Lambert's law, which relates the amount of light absorbed to the distance travelled through the absorbing medium, and Beer's law, relating it to the concentration of absorbing substance in the coloured solution. A photocell is often used to measure the amount of light transmitted through a glass tube containing the solution to be analyzed; the result is compared with results from a similar tube containing solvent alone. Most elements and many compounds, in appropriately treated samples, may be identified by colorimetry.

9. The blue color of a solution of copper sulphate is due to the fact that the copper ions absorb a portion of the visible spectrum.  That part of the spectrum which is not absorbed is transmitted and gives the solution its blue colour. As shown, the copper ions absorb orange light ( wave length of about 630 nm). of Copper sulphate

10. The concentration of the copper ions can be determined by measuring the amount of light absorbed.  The measurement of light absorption will be accomplished with a colorimeter.

11. The blue colour of a solution of copper sulphate is due to the fact that the copper ions absorb a portion of the visible spectrum.  That part of the spectrum which is not absorbed is transmitted and gives the solution its blue colour. As shown, the copper ions absorb orange light ( wave length of about 630 nm). So we use an orange or yellow filter in the colorimeter – as it is only orange light that is affected by the copper solution Using this filter in the colorimeter, orange light  from the light source passes through the sample solution and reaches the light detector.  The light detector measures the intensity of the light reaching it.  The only light absorbing substance is the copper sulphate in the cuvette so the amount of light absorbed by the solution is a measure of the copper sulphate concentration. The higher the concentration of copper ions in the solution the lower the intensity of the light striking the detector. 

12. The higher the concentration of copper ions in the solution the lower the intensity of the light striking the detector.  The amount of light absorbed by the solution is measured in terms of absorbance, A.  The absorbance is defined as A  = -log(I/Io) where Io is the intensity of the light reaching the detector when the cuvette (cell) is filled with pure solvent (water), and I is the intensity observed when the cell contains the reaction mixture • The absorbance of the solution is directly proportional to the • concentration of the copper sulphate.  • That is A = Constant x copper sulphate concentration • So plotting a graph of Absorbance against concentration should give • a straight line, which can be used as a “calibration curve”

13. A colorimeter works by using a photocell to detect the intensity of light shining through a coloured solution and comparing this with the intensity of light shining onto the solution. The detector thus calculates the transmittance (T) (% of light passing through) of the solution. The transmittance is related to the absorbance (A = -log10T) and the absorbance is directly proportional to the concentration (Beer-Lambert law).

14. An Excellent Colorimetry Report should contain: • • Scientific principles of colorimetry and relation of absorbance to • concentration; an outline of how a colorimeter works • • Standard procedures and risk assessments. • How samples are prepared for use • • Preparation, via serial dilution, of a range of standards and • measurement of absorbance leading to a calibration curve. • Excellent graphical work • • All raw data tabulated, good precision, units, etc • • Measurement of an unknown of suitable dilution • • Conclusions based on the concentration of the unknown found from • the graph • Comparison with the result “by eye” • • Evaluations, including reference to the graph, line of best fit, scatter, • anomalies, difficulties • Suggested improvements