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
| Experiment | A | B | C |
|---|---|---|---|
| 1 | 4 mL | 0 mL | 40 µL |
| 2 | 3 mL | 1 mL | 20 µL |
| 3 | 2 mL | 2 mL | 20 µL |
| 4 | 1 mL | 3 mL | 20 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] | k | r2 |
|---|---|---|---|
| 1 | 0.300 M | 0.2878 | 0.99997 |
| 2 | 0.225 M | 0.1984 | 0.99998 |
| 3 | 0.150 M | 0.1217 | 0.99997 |
| 4 | 0.075 M | 0.0460 | 0.99959 |
What is r2 ???
NaOH regression
- 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.