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Why is reference solution needed when determining the absorbance? Why is a blank solution usually used as the

reference?


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Experiment 4
Determination of Iron by Absorption Photometry -
Application of Spectrophotometric Technique
A copper solution is blue because it can absorb the complementary color of
yellow color from white light, and transmit the blue color. Table III.4.1 gives the
relationship between the color of a solution and the color of the light absorbed by the
solution. The stronger the concentration of a copper solution, the darker the blue color
of the solution. This is the principles of absorption photometry. Absorption
photometry is a method that uses the selective light absorptions of materials for
chemical analysis.
Table III.4.1 Relationship between the Color of a Solution and the Color of the
Light Absorbed by the Solution
Color of the
Light Absorbed
Color of the
Light Absorbed
Solution
Color
Wavelength
Solution
Color
Wavelength
(nm)
Yellowish
Purple
Purple
Yellowish
(nm)
400 - 450
Green
560 -580
Green
Yellow
Blue
450- 480
Blue
Yellow
580 - 600
Orange
Greenish
480 -490
Greenish
Orange
600 - 650
Blue
Blue
Red
Bluish
490 -500
Bluish
Red
650 - 750
Green
Green
Purplish
Green
500 - 560
Red
The molecules of each substance have a characteristic absorption spectrum.
Each substance absorbs light selectively at certain wavelengths. By measuring the
selective light absorption of a substance at different wavelength, a curve that shows
how the extant of light absorption varies with the wavelength can be drawn. This
curve is called the absorption spectrum or the absorption curve. The absorption
spectrum of a substance reflexes the properties of its molecules, which can be used to
identify the substance. This is to perform qualitative analysis. While quantitative
analysis can be performed by measuring the relationship between the extent of light
absorption by a substance (measured by absorbance A) at a specific wavelength and
the concentration of the substance. This relationship between absorbance and
concentration is represented by the Law of Light Absorption, i.e. the Beer's Law:
A = 1g (1/T) = 1g (lo/I) = Ebc
A is absorbance, T is transmittance, I, is the intensity of incident light, I is the
intensity of the transmitted light after light has been absorbed by a substance, & is the
absorptivity, b is the path length of light absorption (the width of the liquid that
transmits the light), c is the concentration of the measured substance in the solution.
When an experiment is performed under the same condition, the incident light, the
absorptivity, and the path length of the light absorption do not change, absorbance
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Reference solution is used as base line for measuring transmittance. The transmittance of reference solution will be taken as... View the full answer

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