Determination of the Equilibrium Constant Using Colorimetric Method

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Experiment 19 - Determination of the equilibrium constant for the reaction Fe3+ (aq) + SCN( (aq) = FeSCN2+ (aq)

Object: To determine the equilibrium constant by a colorimetric method

Theory:
Colorimetric methods of analysis are usually applied to the determination of small concentrations of either inorganic or organic materials in a solution. The constituent sought must be coloured or must be capable of reacting with a reagent to produce a substance having a suitable colour. Beers Law, which relates the degree of absorption of monochromatic light to the concentration of an absorbing substance in a solution; is combined with Lambert's and Bouguer's relationships of absorption to length of the light path, to form the following exponential expression.

[pic]
where [pic]= intensity of light transmitted by the absorbing layer
[pic] = intensity of light incident on the absorbing layer
C = concentration of substance in absorbing layer T = thickness of absorbing layer
k = constant of proportionality
This combined expression is frequently called "Beers Law" and may be re-written in the form
[pic]
The log ratio of [pic] is known as absorbance and has no unit. This term is also called optical density and is the quantity that the instrument measures and displays on the meter. As the thickness of the absorbing layer is maintained constant by examining the solution in standard test tubes, the value of absorbance A is proportional to the concentration of the solution.

[pic]
[pic] ((((((((((((((((((((((((((1)
If Beers Law holds for a range of concentrations of a solution, as A is directly proportional to the concentration.
[pic]
Beer’s Law

Hence [pic] ((((((((((((((((((((((((((2)
where C1 = concentration of unknown
C2 = concentration of standard
A1 = absorbance of unknown
A2 = absorbance of standard
It is usually to construct a...
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