Make a saturated solution, add water to the crystals so that some remain undissolve at bottom of jar ,use cold water.Its not a very agressive acid. My chemist sold me it instantly once I explained what was doing
[/ QUOTE ] Probably as one of my former colleagues would have said "a slack handfull in a jug of water".
It is not what you would call very soluble so saturated, or nearly so, would probably be about right. It is slow to disslove in cold water so warm water would be sensible.
It is what is termed a weak acid but it is corrosive, it is harmful and it is rather toxic so goggles and gloves would be sensible precautions. Keep any unused labelled and out of sight and reach of children.
I cannot find a reliable figure for the solubility, possibly because it is available in both the anhydrous and hydrated forms. (Before anybody quotes it back at me I have read 90g/litre from the above safety sheet If correct that's for the anhydrous material I would suppose and would make it 126g/litre for the dihydrate)
As you are in France don't get it from chemist/Pharmacy, it will cost an arm and a leg. Quincailler........(hardware shops) and decorating supply shops sell it at reasonable prices. €4 for 1kg IIRC.