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In one of my projects (which is about alpha-voltaic cells, don't worry, I know that thorium isn't a strong alpha source, it's only for measurements) I would need to get water soluble thorium compound from a small sample of thorium dioxide for purification with selective reactions and further measurements. Would any mineral acid work? I found some sources and publications on the internet by google searching but the biggest amount of information I could get is that

"...Several hot acids can solve it..."

Which is not nearly as accurate as I would need it because I don't want to spend the budget of the project on expensive acids which doesn't even work instead of better equipments. (Their price is from their necessary purity)

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Thorium dioxide can be dissolved in the mixture of hydrofluoric acid and hydrochloric acid, as described by Kluger and Lieser in 1981.1

Proceed with caution since hot condensed acid are known to be highly corrosive and hydrofluoric acid is also highly toxic and penetrating.

  1. Kluge, E.; Lieser, K. H. Dissolution of ThO2 in HCl/HF-mixtures. Radiochim. Acta 1981, 29 (2–3), 167–168. DOI: 10.1524/ract.1981.29.23.167.
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    $\begingroup$ Hot hydrofluoric acid. That gives me the willies just thinking about it. $\endgroup$
    – zeta-band
    Commented Jan 29, 2018 at 23:47
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    $\begingroup$ @zeta-band Actually it's drops of $\ce{NH4F}$ solution added to hot $\ce{HCl}$ ... but I agree with you. $\endgroup$
    – Weijun Zhou
    Commented Jan 29, 2018 at 23:59
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$\color{red}{WARNING:DO \ NOT \ ATTEMPT \ TO \ REPEAT \ THE \\ FOLLOWING \ EXPERIMENTS \ WITHOUT \ EXPERIENCE \\ IN \ WORKING \ WITH \ HF \ OR \ WITHOUT \ THE \ PROPER \\ WORK \ ENVIRONMENT \ AND \ PERMISSIONS\ !}$

In the procedure of refining used nuclear fuel pellets which are typically consisting of $(Th,U)O_{2}$ with 1-4% $UO_{2}$ content The most common acid mixtures which are used are $HCl/HF,HNO_{3}/HF,H_{2}SO_{4}/HF.$ If the Thorium oxide was previously heated above $1000°C$ It makes the process much more slower. The least dangerous way to obtain a water soluble thorium compound is to dissolve the Thorium oxide in a solution of $8-13 \ mol\cdot l^{-1} \ \ HNO_{3}$ and $0.02-0.05 \ mol\cdot l^{-1} \ \ F^{-}$ at temperatures of $50-70°C$ (The fluoride ion concentration can be achieved by adding any fluoride salts which will produce it or by adding hydrofluoric acid to it). This method does not needs the acids to be at the temperature of boiling.The produced Thorium nitrate is much more soluble in water than it's halides so this is a good procedure from this side too.

Reference: DISSOLUTION OF $(Th,U)O_{2}$ IN NITRIC ACID - HYDROFLUORIC ACID SOLUTIONS

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