Powder Calculations

[pfdunlop]

Hi can someone please give me an explanation on how to do powder calculations. I have an exam in 4 hours and last minute revision I'm still struggling.

Calculate for 12 x 120mg powders each containing 0.2mg digoxin starting with a weight of 100mg. Individual powders should weigh 120mg.

I know I need to dilute the powder to get it to weigh, but not sure after that.

Thank you

[johnep]

"weight of 100mg" of what? digoxin, lactose? a dilution containing ? of digoxin?

Ok. key to all these is to start at the end:
12 x 120mgm = 1,440mg ie 1.44grm
12 x 0.2mg = 2.4mg

that is theoretical. In practice would need considerable overage to compensate for slight weighing errors, spillage etc.
I'll return to this after lunch.
johnep

[bolam]

Ive done this quickly so im not too sure if its right...
right so you want 0.2mg in 120mg / 2.4 mg in 1440mg for 12 powders. if you look this is a 1in 600 dilution so from the original 100mg you are given you need to dilute/ titriate
i started with a 1 in 6 so 100mg + 500= 600mg
takin 100mg of this and adding 900mg =1000mg. at this point here you have completed a 1 in 60 dilution... check for yourself.
you want a final total of 1440. I made this 1500mg for a nice number.
taking 150mg of your 1 in 60 and making upto 1500mg is your final dilution and you now have your 1 in 600.
divide these 1500mg into 12 times 120mg powders with 60mg to spare.

not sure if its right/ the best way
but thats how i would approach it.