Oxalic Acid degeneration

[keith pierce]

Does oxalic acid break down over time. I bought a quantity about 5 years ago and am still using it but i have a feeling that its not as effective.

 

[Little John]

In what form - liquid or dihydrate crystals ? If the latter, then it won't have 'gone off'. Not sure about the liquid - depends what it's been mixed with (if anything).
LJ

 

[Nige.Coll]

The liquid has a short shelf life.

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[keith pierce]

thanks. they are in dihydrate crystals

 

[coatesg]

The liquid has a short shelf life

Pure oxalic solution, I would have thought to be quite stable? If sugar mixed in it's different (formation of HMF, etc)

 

[mbc]

thanks. they are in dihydrate crystals

Then if kept dry they shouldn't degrade in years and years.

 

[beebopper]

Sorry Coat esg but you would have thought wrong then. Solution would degrade certainly over years.
Solid is fine for many years without worry.
A pure solution of oxalic acid is contradiction in terms ie add to water and it is not pure.

 

[beebopper]

Then if kept dry they shouldn't degrade in years and years.

Yes that is correct.

Editted

 

[Little John]

Sorry Coat esg but you would have thought wrong then. Solution would degrade certainly over years.
Solid is fine for many years without worry.
A pure solution of oxalic acid is contradiction in terms ie add to water and it is not pure.

Degrade into what, exactly ?

Of course it isn't a contradiction in terms - and to be uber-pedantic - Oxalic Acid crystals, as usually supplied, already has water added to it - that's why it's called a dihydrate !
LJ

 

[coatesg]

Degrade into what, exactly ?

That was my thought as well. HOOC-COOH can't easily be oxidised further (without anything like an MnO4- ion or H2O2) so I was struggling to see exactly what might happen to it in aqueous solution. If you have other impurities then yes, the oxalates will react (eg iron -> complexes, calcium -> calcium oxalate (beerstone!)).

Thermal decomposition of oxalates is very slow at room temp.

 

[coatesg]

(And "degradation over years", if the case, I would not consider to be a short shelf life)

 

[mbc]

If its in solution then it's usually a sugar solution and hmf rises over time in acidic sugar solutions and hmf is toxic to bees in high concentration.

 

[coatesg]

The OP was talking (I presume) about oxalic alone - esp given then have dihydrate crystals.

 

[Little John]

Thermal decomposition of oxalates is very slow at room temp.

Exactly - the oxalate cation is pretty stable. Perhaps if the solution were to be kept in a clear glass container and subjected to light for a very long time it 'might' undergo some UV degradation - however, that's only conjecture. But I fully agree - add some impurity or other - such as sucrose, and it's a whole different ball-game.

BTW - as I've just mentioned 'impurity', for the benefit of Beebopper's scant knowledge of organic chemistry - and anyone else who's a bit short in this area: if you were to take a quantity of Oxalic Acid (dihydrate or otherwise) which has been assayed as being 99.7% 'pure', with the remaining 0.3% consisting of (typically) heavy metals such as iron and it's oxides, sulphates, carbonates, and so on, and add a quantity of water to it ... it's 'purity' will remain the same, at 99.7%.

What changes is it's Molar Concentration (an organic chemist's usual method of describing the degree of dilution of a solute within a solvent) - but NOT it's purity. It's purity stays exactly the same. So if you were then to remove the solvent, you'd then be back to where you started. Simples.
LJ

 

[oliver90owner]

Oxalic acid was used as a volumetric standard solution years ago (when wet chemistry analyses were the norm).

There are two distinct chemical forms of oxalic acid. Anhydrous and the dihydrate. The normal one, referred to these days, is the dihydrate which does not have 'water added' - it is a chemical combination which is stable until the temperature is sufficiently high to disassociate the water from the molecule, or possibly be slowly removed by a very strong dessicant. Only chemists would look at the formula (or perhaps the relative molecular mass) to confirm the form they might be using.

Anhydrous oxalic acid could be used for sublimating, but the dihyrate is easier to handle. Dissolving anhydrous oxalic in water is not as easy as using the dihydrate form (likely for two reasons, at least).

 

[Beefriendly]

There are two distinct chemical forms of oxalic acid. Anhydrous and the dihydrate. The normal one, referred to these days, is the dihydrate which does not have 'water added' -

I thought that a dihydrate had 2 molecules of water attached, shirley it's the anhydrous that has 'no water added'.

 

[Ely]

I thought that a dihydrate had 2 molecules of water attached, shirley it's the anhydrous that has 'no water added'.

Yes. Olly has written it correctly. It isn't 'added'. The water is part of the molecular bond

 

[Beefriendly]

Yes. Olly has written it correctly. It isn't 'added'. The water is part of the molecular bond

Yes you are correct. I misread the 'added bit' as suggesting it contained no water molecules.