### Calculation of the equilibrium constant of indicator

```CALCULATION OF THE
EQUILIBRIUM CONSTANT OF
INDICATOR
Purposes
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Use the Spectrophotometer (T60U) and pH meter to
measure【HIn】,【In-】 and 【H+】in the solution ,
and calculate the equilibrium constant (Ka) of the
equation .
Compare the result with the literature to calculate
the deviation .
Observe the effect of the indicator through the
titration .
Materials and Apparatus
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Spectrophotometer (T60U)
pH meter
Phenolphthalein
bromthymol blue
methyl red
methyl orange
lacmus(litmus)
hydrochloric acid
sodium hydroxide
Procedures
Prepare the solution of indicator
Experiment One：
Use the Spectrophotometer
(T60U) to measure【HIn】
and【In-】
Experiment Two：
Use pH meter to measure 【H+】
Compare experiment one
and two , and calculate the
Ka
calculate the Ka of the equation

Compare the result of the experiment and the
Ka from literature to calculate the deviation
and find the reason of the deviation .
Acid-based Titration
Figure out the color changing range
of each indicator by Ka
thymol blue
methyl orange
Figure out the appropriate
color changing range of the
indicator
lacmus
phenolphthalein
fluorescein
Find out the deviation of the indicator
in the inappropriate range during the
titration
conclusion
Step(One)-Preparation
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Prepare the solution of the indicator：mix the
indicator with 20% of C2H5OH、0.1M NaOH
solution、0.1M HNO3.
Take seven volumetric flasks( 50ml ) , formulate
seven kinds of solutions ( from acid to basic ) , and
into the form of “ In- .”
(Not finished?)
Step(Two)-Experiment 1
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Set the Spectrophotometer’s wavelength at the
range of 400nm~700nm , scan the sample in the
acid (1th volumetric flask )and the basic(7th
volumetric flask ) situation , and find the maximum
wavelength(λHIn 、 λIn- ).
Set the wavelength at λHIn and measure the
absorbance of all the samples.
Set the wavelength at λIn- and measure the
absorbance of all the samples.
Calculate its equilibrium constant.
Step(One)-Experiment 2
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Use pH meter to measure the pH value of all the
samples and infer their【H+】.
Calculate its equilibrium constant.
```