Notes on the new NurdRage video: making sulphuric acid from sulphates and oxalic acid

The newest NurdRage's video shows an interesting way to produce sulphuric acid, namely by reacting a sulphate salt with oxalic acid. Since oxalic acid is way weaker than sulphuric acid, one expects only a minute amount of sulphuric acid to be made, but the fact that most metal oxalates are insoluble displaces the equilibrium towards the right and, in fine, leads to an almost quantitative reaction (IMHO).

NurdRage tested the reaction with iron sulphate, which might not have been the best idea: someone on the Patreon thread remarked that iron (II) sulphate is easily oxidised into iron (III) sulphate, whose oxalate is much more soluble, and that impacts the yield negatively. I therefore suggested to use copper (II) sulphate instead, which is widely available, does not risk over-oxidation (This reaction is even quoted verbatim on Wikipedia). Furthermore, copper (II) oxalate is given as ‘insoluble’ in water, even more so than iron (II) oxalate.

NurdRage agreed, and, as you can see in his video, while the reaction did happen as expected, he was unable to filter out the resulting copper (II) oxalate which seems to crystallise in very fine powder, and went all through his fritted funnel.

So I decided to retry his experiment, changing a few things.

To 20 mL of water in a beaker is added 6 g (24 mmol) of copper (II) sulphate. This is well under the solubility at 25 °C, but I wanted to keep leeway. When the copper (II) sulphate had dissolved, I directly added 3.8 g of oxalic acid dihydrate (30 mmol, a slight excess). The solution immediately turned from a medium blue to a light blue – copper (II) oxalate has a somewhat fascinating blue colour, both light and saturated, almost fluorescent. (See picture.)
[NurdRage dissolves the oxalic acid in water and mix both solutions, but that leads to pointless dilution of the result.]

After five minutes of stirring at room temperature I set up the vacuum filtration by using a fritted funnel into which I put a small filtering disk made of glass fibres, and atop of it a not-too-thick layer of diatomaceous earth (Celite). (See picture). I then cautiously tipped the contents of the beaker into the funnel so as to avoid to disturb the layer of Celite, and turned the vacuum pump on. To my delight, all the copper oxalate remained stuck over the Celite, while I was able to collect 20 mL of a limpid, transparent liquid in my filtering flask. (See pictures). I will have to titrate the resulting solution tomorrow. If the reaction is quantitative, it should result in 24 mmol of sulphuric acid into 20 mL of water, or a 1.2 M solution.

In any case, this process seems to work fine, provided extra mesures are taken to filter out the resulting mixture.



EDIT: PS. Since boiling water can dissolve around 2 kg of copper (II) sulphate per litre, it is theoretically possible to produce a 8 M solution of sulphuric acid, which is slightly under 50% w/w.

But thinking about it, this process could even produce much more concentrated sulphuric acid. Since the reactants are consumed, water is liberated to dissolve more, so, in theory, there could be more copper (II) sulphate and oxalic acid put in than a given amount of water can dissolve. Assuming that:
1. Copper oxalate is insoluble in sulphuric acid, whatever its concentration.
2. There is no reverse reaction.
But that seems somehow highly suspicious.

P.PS: If some copper oxalate makes it to your fritted disc, you can remove it using 23% HCl. Copper oxalate will react to some extent with concentrated HCl to form yellowish soluble CuCl₂ that you can then wash out.

[Edited on 23-6-2024 by Keras]

Quote: Originally posted by Sulaiman

Nice experiment. Worth trying myself. Would a coagulant/flocculent such as potassium alum help?

Quote: Originally posted by Alkoholvergiftung

[…] filter paper. Maybe the same goes for Copper sulfate and oxalic acid.

Quote: Originally posted by Keras

Quote: Originally posted by Alkoholvergiftung   […] filter paper. Maybe the same goes for Copper sulfate and oxalic acid. The problem with filter paper is that it will get destroyed by the acid if it is too concentrated.

Quote: Originally posted by clearly_not_atara

What was wrong with magnesium sulfate? Shouldn't that be massively cheaper than copper, and with no disposal issues?