Reconstitution math for lab handling — a worked example

#reconstitution #concentration #math #how-to

ReconRosa 1mo ago

The single most-asked handling question, worked end to end. This is bench math for documenting a stock concentration — nothing about consumption.

Say a vial contains 10 mg of compound and you add 2 mL of bacteriostatic water. Concentration = mass ÷ volume = 10 mg ÷ 2 mL = 5 mg/mL. That's it. That number is what you write on the label and in the notebook.

Change the water volume and the concentration changes inversely: same 10 mg in 1 mL is 10 mg/mL; in 5 mL it's 2 mg/mL. Pick a volume that makes your concentration a round, easy-to-record number and your future notes will thank you.

4 replies

research-notes New-U Staff ▲ 1 · New 1mo ago

Clean worked example. The only thing we'd underline: pick the diluent volume so the resulting concentration is a round number you can record without rounding errors. The math is trivial; the documentation discipline is the actual skill.

MicrogramMick ▲ 1 · New 1mo ago

Units pedantry, lovingly: keep mg and mL consistent throughout and your mg/mL falls out clean. Most "my math is wrong" posts are actually "I mixed up my units" posts.

NewBenchNadia 1mo ago

The inverse relationship (more water = lower concentration) is the bit that finally clicked reading this. Wrote the 10 mg / 1 / 2 / 5 mL examples straight into my notebook.

ReconRosa 1mo ago

That's exactly the goal — make future-you's records effortless. If your concentration is an ugly decimal, change the diluent volume, not your handwriting.

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