Lessons · Pharmacy · how much diluent to add
How much diluent to add
The diluent is the final volume less the volume of stock you started with, and it is never the whole final volume.
Hone is a place to practise a career, one idea a day. This is one of its lessons, written out in full and free to read without an account.
What it is for
The compounding log records what went in, and what goes in is stock plus diluent. Write the final volume in the diluent column and the batch is made at the wrong strength and the record says it is right.
How to think about it
Work the stock volume first with C1V1 = C2V2. Then subtract it from the final volume. Then check by counting the drug: the grams in the stock you took must equal the grams in the batch you made.
Worked example
Make 250 mL of a 4% from a 20% stockThe question behind most dilutions on the bench.
V1 = (4 x 250) / 20 = 50 mL of stockC1V1 = C2V2, rearranged for the stock volume.
Diluent = 250 - 50 = 200 mLAdd two hundred millilitres to the fifty.
Check: 50 mL of 20% holds 10 g, and 250 mL of 4% holds 10 gThe drug did not change. That is what makes it right.
Your turn
You need 30 mL of stock to make 180 mL of the finished product. Write the diluent to add.
180 - 30 = mL
Solve one, graded on the server
The trap
Adding the final volume of diluent to the stock. The final volume already includes the stock, so doing that makes a batch that is too big and too weak.