Experiment 3
Spectrophotometric determination of an indicator constant
Aim
To find pKln by a spectrophotometric method in which the absorbance of a solution containing the indicator is measured as a function of pH.
Experimental requirement
Quantity Apparatus Uncertainty
1 Lambda 25 scanning uv/visible spectrophotometer -
2 50 cm3 burettes ±0.1
2 50 cm3 volumetric flasks ±0.1
1 5 cm3 pipette ±0.1
1 pH meter and electrode assembly ±0.01
Table 1: Experimental requirements
List of substance
List of substance Quantity/ cm3 Purpose
Methyl red, code number = 10 55.0 Indicator solution
0.02M Acetic acid 550.0 Used to make the 11 solutions
0.02M Sodium acetate 550.0
Buffer pH 7.0 20.0 Used to calibrate pH meter
Buffer pH 4.0 20.0
Table 2: List of Substances
Experimental Method
Eleven solutions were prepared so that each consisted of a total volume of 50.0 cm3 and comprised y cm3 of acetic acid (y = 50.0 …0 cm3) and (50.0 – y) cm3 of sodium acetate. Methyl red indicator solution (5.0 cm3) were added to each of the prepared buffer solution and mixed well. pH meter was calibrated each at pH 4.0 and pH 7.0 at 20°C. pH of the eleven solutions was measured. The spectra of each solution were obtained by using the Lambda 25 scanning uv/visible spectrophotometer where method P3 had the following parameters as pre-sets:
Start wavelength 750 nm
End wavelength 340 nm
Data interval 1.0 sec
Scan speed 960 nn/min
Number of samples 11
Display Overlay
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