# What is the rate constant k for the reaction between potassium manganate and oxalic acid?

For my school's chemistry classes, I am investigating the kinetics of the redox reaction between $$\ce{K2MnO4}$$ and $$\ce{C2H2O4}$$. I analyzed the data using a colorimeter set to $$\pu{565nm}$$. underneath are my results:

This picture shows the different results I obtained at different temperatures. Since absorption is directly related to concentration, I tried to take $$\mathrm{ln(a)}$$ over time and $$\frac{1}{a}$$ over time to determine the order of reaction and thus the rate constant k (by using the gradient of the straight line. however, none of the above resulted in a straight line as shown below.

As you can see, neither of these lines are straight and therefore I cannot find rate constant k. Is it because the reaction is auto-catalytic? If so, can someone please explain how to find the rate constant?

• Are you sure that you have used potassium manganate $\ce{K2MnO4}$ and oxalic acid ? Because potassium manganate is a green substance that is stable in basic solution, and oxalic acid does not exist in basic solution : it is transformed in oxalate ion. So I suspect you have worked in acidic solution with potassium permanganate $\ce{KMnO4}$ which is purple in solution. Jun 16 '21 at 15:50
• What is series $1$, series $2$, and series $3$? What do you report in figure $2$ and $3$ ? Versus what ? Jun 16 '21 at 15:54
• @Maurice I used manganate, a green substance which I then dissolved in water for a 0.001M purple solution. then I added 0.1M oxalic acid and placed it in a colorimeter.
– Clau
Jun 16 '21 at 16:25
• @Maurice the series 1, 2 and 3 can be ignored, it's just excel placing it in. and for the second chart it is ln(absorbance) vs. time and the third is 1/absorbance vs. time.
– Clau
Jun 16 '21 at 16:26
• Why did you choose potassium manganate ? it is not a good choice. Because, even without oxalic acid, potassium manganate gets slowly decomposed in water, due to the acidity of atmospheric $\ce{CO2}$ producing $\ce{MnO2 + KMnO4}$ $$\ce{3 K2MnO4 + 2 CO2 -> 2 KMnO4 + MnO2 + 2 K2CO3}$$ Jun 16 '21 at 17:09