Problem need some help from pharmacists and pharmacy students?

Question

I'm provided with levothyroxine tablets, each weighing 75mg and contains 25mg of active compound. I need to make 12 powders from these tablets, each to contain 16mg. What should I do in terms of trituration and weighing to obtain these powders and how many tablets should I use? The conditions are that I have to use a class B balance, can't weigh any value lower than 100mg at any one weighing and of course a powder should be a minimum weight of 120mg.

Answer 1

Once more into the breach, this time with (hopefully) all the starting data.  Sorry I proposed 70 mg weights -- forgot the minimum weighment of a Class B balance is 100 mg.

(14)(16) = 224 mg

(9)(25) = 225 mg

(5)(16) = 80 mg -- under balance minimum

(10)(16) = 160 mg, smallest "weighable" packet

(10)(14)(16) = 2240 mg gross

(9)(75) = 750 - 75 = 675 mg gross

(10)(9)(25) = 2250 mg

2250 - 675 = 1575 mg inert

Crush 9 tablets.  Set up the balance for 2250 mg.

Put the power on the balance and add inert ingredient to reach 2250 mg.  Mix resulting power thoroughly, clean the scale, and weigh out (14) 160 mg packets.  The 14th packet may be light because of product lost in transfers and sticking to your mixing vessel, but that was your purpose in choosing 14 in the first place.  In any case you will have waste of 10 mg gross powder (1 mg active ingredient).  Technically, the 13th & 14th packets are also waste, but that's unavoidable, and represents a minimum.

(13)(16) = 208 mg, but you still need 9 tablets, so the wast in active ingredient would be the same, although you would waste less inert ingredient.