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Phenolphathalein 2019.04.10

Kinetic of degradation of phenolphthalein in basic media works well as shown in the previous experiment.

Goal

We want to measure the order of the reaction. In the literature, it is told that you are expected to use constant ionic force. But is this necessary ?

Let's check if we replace NaCl 0.3M by water to see if it yields to the same result.

Solutions

  • solution A: NaOH 0.3M in water
  • solution B: water
  • solution C: phenolphthalein 3.02 g/L in MeOH / EtOH
ExperimentABC
14 mL0 mL40 µL
23 mL1 mL20 µL
32 mL2 mL20 µL
41 mL3 mL20 µL

Results

Raw data for the 4 kinetics are attached in the corresponding documents:

The data were processed online on the following page.

An exponential regression has been applied for the 4 experiments for all the points having an absorbance over 0.3 and the following results are observed:

Experiment[NaOH]kr2
10.300 M0.28780.99997
20.225 M0.19840.99998
30.150 M0.12170.99997
40.075 M0.04600.99959

What is r2 ???

NaOH regression

NaOHregression.svg

  • y = −0.03705 + 1.069 x
  • r2 = 0.9983

Even without constant ionic strength the results are still quite good. The slope is close to 1 despite the fact that it is far from passing through 0,0.