Ethanedioic Acid Titrations

It’s that time of year where if you are in IB1 you may well be starting to think about your IA. What will you do?!

I hope today’s post gives you some ideas.

Have you thought about working with Ethanedioic acid (HOOCCOOH) or oxalic acid to give you its more common name? Ethanedioic acid occurs as a free acid in beetroot leaves and rhubarb. Could you extract it and see how much you have? You could titrate it with a base, such as NaOH (what would you use as an indicator?) but things get interesting if you titrate it with potassium manganate (VII) or KMnO₄ (why don’t you need an indicator for this reaction?)

Source: BriannaWalther (CC BY-SA 3.0), via Wikimedia Commons

The reaction is interesting as occurs at a variable rate. In solution there are MnO4- and -OOCCOO- ions. The reaction is slow to start as the MnO₄^– and ^–OOCCOO^– are both negatively charged and therefore repel each other. They have a high activation energy. However, once they do start to react the reaction proceeds quickly and one of the products, Mn²⁺ catalyzes the reaction. This referred to as autocatalysis. The reaction then slows down towards the end as the end point is reached.

The reaction is usually warmed to begin with to allow the initial reaction to proceed at a faster rate.

A few things to think about with regards to the chemical reaction occurring – can you write the half equation fo the reaction of MnO4^–? What about oxalic acid? As a clue, it is the ^–OOCCOO^– (the ethanediotate ion) that reacts and it is turned into CO₂. And can you also deduce (or find) the autocatalysis reaction? Can you think of a use for this reaction?

Oxalic acid is also used for textile finishing and cleaning and also, due to its bleaching effects for whitening leather. It can also be used to clean metals and to remove rust or blood stains. Could you investigate one of these things? Maybe looking at how changing the concentration of the acid alters the rate of reaction?

Be careful though as it is a poisonous chemical. Before you do any work with it you should ensure that you have thoroughly risk assessed the practical and discussed what you are intending to do with your teacher.

So, food for thought? Has this blog post given you any ideas? If so, I’d love to hear what they are – please feel free to post them below.