# How to make solution of specific concentration with non-stoichiometric hydrate?

I am currently working with a drug molecule known as Amiloride $\ce{HCl}$ and it seems to be a non-stoichiometric hydrate.

How would I go about making a solution with a specific concentration of Amiloride $\ce{HCl}$? I know its anhydrous molecular weight but if I don't know how much water is in the solid crystals how can I get a specific concentration?

• (1) You'll need to make a stock solution and then analyze that for concentration somehow. (2) It might be possible to dry the drug by heating in a oven (maybe vacuum oven?). (3) Analyze the sample for water content. (4) purify the sample somehow – MaxW Feb 9 '17 at 21:12
• @MaxW, I swear I'm not just copying your comments into answers today, we're just on the same wavelength or something ;) – airhuff Feb 9 '17 at 23:48

Anhydrous Amiloride HCl is commercially available and is stable in air. This suggests that you should be able to remove the waters of hydration, however many that may be, by repeatedly heating in a drying oven until a constant weight is reached. If you know the weight before drying you could of course calculate the original degree of hydration after the final drying, if that is of interest.

If you need to further verify dryness, you could have the dried product analyzed by Karl Fischer water anaysis (~\$50). Just be sure to seal the sample under a dry atmosphere and label "HYGROSCOPIC" all over the sample and submittal paperwork!

Once you have the constant-weight product, stored under a good desiccator, just use the formula weight of the hydrochloride salt, 266.1, to calculate the concentration of your Amiloride HCl solutions.

The simplest was would be to dissolve a fixed amount in water and then titrate the content of amiloride HCl. A number of different methods would jump to mind:

• precipitation of chloride with silver nitrate and gravimetric analysis
• potentiometric and/or pH titration with sodium hydroxide to determine the basic nitrogen content
• full deprotonation with a strong base and acidic back-titration.

If you choose your titration solution in such a way that the concentration is more than what you need afterwards, you can even dilute it to the desired stock solution concentration afterwards.