Is it possible to weigh out less than 10mg on a class B balance?

Question

Not weighing something like 2mg on it's own but rather 102mg?

Answer 1

It is possible, but the accuracy will be a matter of luck, not precision.  According to the book, Pharmaceutical Practice (p. 79), the class B balance has a minimum of 100 mg with a minimum increment of 10 mg.  This implies that the precision of the balance itself is ± 5 mg.  In addition to this limitation, setting up the differential weighing risks contamination of the weights.  If you don't use calibrated tare weights you find yourself in the position of "weighing out" the "tare" product, and more than doubling your chance for error. You should use a class A balance to obtain ± 0.5 mg weights.

Edit:

given the additional information,

With 75 mg tablets you can get an "accurate" weight on a class B scale with any multiple of 2 tablets. (It seems to me that the only reason for using the balance at this point is to verify that 10 tablets indeed weigh 750 mg).  At this point you have 2.50 mg active ingredient (10*.25).

Discarding 1 tablet before you crush them leaves you with 2.25 mg active ingredientand a total mass of 675 mg. Now adding 325 mg lactose makes a total mass of 1,000 mg.  After mixing thoroughly you can weigh out (10) 100 mg batches of finished product, each of which has 0.225 mg active ingredient.  The extra 0.001 mg represents a 0.45% deviation from the desired dosage.

If you want to get closer to the 0224 mg active ingredient per dose, you can weigh out 250 tablets for a total mass of 18.750 g and a total mass of active ingredient of 62.5 mg.  Ratioing 62.5 mg with 0.224/0.250 gives 56 mg active ingredient which, divided by 0.25 mg/tablet yields 224 tablets, massing 16,800 mg.  This means discarding 26 tablets. 56 mg/0.224 mg = 250 doses.  16,800/250 = 67.2 mg/dose, which you cannot weigh out, but you CAN weigh out 70 mg doses, so 70*250 = 17,500 mg.  Adding 700 mg lactose to the 224 tablets (16,800 mg) gives 17,500 mg.  After mixing thoroughly you can weigh out 250 doses of 70 mg, each having 0.224 mg active ingredient.

I hope that last didn't get too convoluted for you.

Reiterating, you CAN use finer weights on a class B balance, but it won't necessarily increase your precision.