[AllylicAzideUMN]

I'm trying to incorporate oxygen-18 into a substrate via a hydrolysis reaction using sulfuric acid and want to replace the oxygen-16 water in sulfuric acid with oxygen-18 labelled water. To do this, I need to completely dry 95% sulfuric acid of its water but I have been struggling to find procedures in the literature. Would it be feasible to use drying salts (e.g. CaCl2 or MgSO4) or molecular sieves with 95% sulfuric acid? I feel that drying salts may be more effective. Does anyone have further insight?

[wildfyr]

Buy it anhydrous. You don't want to dry it yourself.

[rolnor]

Is the concentrated acid not 98,3%?

[AllylicAzideUMN]

I believe 18M sulfuric acid (concentrated sulfuric) is 95% H2SO4, 5% water. It probably depends a bit on the bottle too.

[rolnor]

https://en.m.wikipedia.org/wiki/Sulfuric_acid

[Enthalpy]

The attempt needs more accurate figures where you write "completely", because 99%, 99.99% and 99.9999% are not the same effort.

And for instance, is at least the heavy water more pure than the sought sulphuric acid?

I wonder - maybe a silly interrogation, but: can H₂0, H₃O⁺, SO₄^2-, HSO₄^- swap their oxygen atoms? If this happens, even at a slow pace, then ¹⁸O won't stay pure in the water, as some ¹⁶O will leak from the SO₄^2- part. This gag happens with ²H.

For the sought solution, several steps may be necessary, depending on the desired ¹⁸O purity and the achievable dehydration. Like:
100% ¹⁶O, dehydrate as possible, add ¹⁸O to obtain 90% ¹⁸O,
again dehydrate as possible, add ¹⁸O to obtain 99% ¹⁸O,
again and again

During production and storage, air moisture may be a difficulty.

[wildfyr]

Just to add, I very much doubt that sieves or typical drying salts will work here. They stand a good chance of reacting with sulfuric acid. Even if not, it's a very hygroscopic material.

You could try to dry it over P2O5 if you're familiar with that procedure, since sulfuric acid has a very low vapor pressure.... But I still would only use a pump you are willing to possibly destroy.

[OrganicDan96]

are you trying to hydrolyse an ester? if you could give us more detail we could help you to find an alternative procedure

[AllylicAzideUMN]

Thank you for the suggestions so far. In response to Enthalpy, I would like as close to 100% as possible but anything over 99% would likely work too. In response to OrganicDan96, I'm attempting the following reaction, the hydrolysis of two adjacent aromatic nitrile groups into an anhydride. The diacid is unstable relative to the more entropically-favored anhydride and water molecule. 

[rolnor]

If you add P2O5 to sulphuric acid you will get SO3.

[wildfyr]

That's not how P2O5 drying works. The P2O5 never comes into contact with the thing being dried

[rolnor]

Sorry, I misunderstood you wildfyr. You could make 100% by mixing conc. acid with oleum, or?

[OrganicDan96]

could you hydrolyse using 18O NaOH then dehydrate the resulting carboxylic (probably thermally)