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Measuring Equilibrium Constant of pH Indicators Using Spectrophotometry and pH Meter

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8 diapositives · 236 Views · Jul 22, 2014

This experiment aims to calculate the equilibrium constant (Ka) of various pH indicators including phenolphthalein, bromthymol blue, and methyl red using a spectrophotometer (T60U) and pH meter. First, the concentrations of acidic indicator forms ([HIn]) and their basic counterparts ([In-]) will be measured. Then, pH values will be recorded to find [H+]. The results will be compared to literature values to determine deviations and their causes. The experiment will also explore the color change ranges of indicators through titrations.

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Measuring Equilibrium Constant of pH Indicators Using Spectrophotometry and pH Meter
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  1. Calculation of the equilibrium constant of indicator

  2. Purposes • 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 .

  3. Materials and Apparatus • Spectrophotometer (T60U) • pH meter • Phenolphthalein • bromthymol blue • methyl red • methyl orange • lacmus(litmus) • hydrochloric acid • sodium hydroxide

  4. 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 為什麼並列 Compare the result of the experiment and the Ka from literature to calculate the deviation and find the reason of the deviation .

  5. Acid-based Titration Figure out the color changing range of each indicator by Ka thymol blue methyl orange lacmus phenolphthalein fluorescein Figure out the appropriate color changing range of the indicator Find out the deviation of the indicator in the inappropriate range during the titration conclusion

  6. Step(One)-Preparation • 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 add NaOH into the 7th volumetric flask which turns it into the form of “ In- .” • (Not finished?)

  7. Step(Two)-Experiment 1 • 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.

  8. Step(One)-Experiment 2 • Use pH meter to measure the pH value of all the samples and infer their【H+】. • Calculate its equilibrium constant.