Capsules containing 500 mg of hydroxizine and 250 mg of secobarbital are hard after 1 week.

I suspect we have to do with a base-acid reaction with salt formation here. The hydroxizine salt of the secobarbital is formed. The first being a strong base; The second a weak acid. As base, Hydroxizine will not be properly soluble in water; The same applies to secobarbital. The formed salt, on the other hand, probably does. And this is obviously positive. In the stomach, HCl will expel secobarbital from its salt and we reach a situation completely equal to this without interaction between the two components. I can assume that the operation is not compromised. Perhaps the dissolving speed is influenced in a positive sense. The only thing that strikes me is the word "rock hard". One would have to lay the obtained rod in HCl 0.02 M and see if it solves fast or slow.

now maybe it is desirable that no stone hard rod is formed. The following improvements can be tried out by the trainee:

1. Dry all powders in a hot air oven e.g. 1 hour at 60 to 70 ° C; work quickly and sheltered the finished capsules against humidity. I'll read correctly that Nasecobarbital is hygroscopic; Presumably hydroxyzinesecobarbital too?
2. add 1% coll. SiO2 (Aerosil); Mix half the amount with hydroxyzine and the other half with secobarbital
3. add 1% mgstearate and mix according to 2