Making a solution weaker
The page for when the main calculator gives you a draw of two units. A short draw carries a large reading error, and the only real fix is to spread the amount through more liquid.
Educational arithmetic only. Not medical advice and not a dosing protocol.
Before you act on that number
This is educational arithmetic on figures entered by the reader. It is not medical advice, not a dosing protocol, and not a recommendation that anything be prepared, obtained or administered.
Nothing here states what quantity is appropriate for anyone, and there is no function in it that produces an amount to administer. A question about anyone’s health belongs with a licensed clinician who can examine the person asking.
Compounds of the kind this arithmetic is used on are largely research chemicals that are not approved by the FDA for human use. An exact calculation is not an accurate measurement: what is actually in a syringe depends on the preparation and the device, neither of which this page can see.
No qualified clinical reviewer is recorded against this site, and no credential is claimed anywhere on it. what this is not, at full length.
See the arithmetic
C1 × V1 = C2 × V2. Adding liquid does not change how much compound is present, only the volume it is spread through.- Rearranged for the stock to take:
V1 = C2 × V2 ÷ C1. Five millilitres at 2 mg/mL from a 10 mg/mL stock needs2 × 5 ÷ 10 = 1 mLof stock. - The diluent is the remainder:
5 - 1 = 4 mL. - A target stronger than the stock is refused rather than returned as a negative diluent volume. Adding liquid cannot concentrate a solution.
What this buys, with numbers on it
Half a milligram out of a 10 mg/mL solution is 0.05 mL, which is five units on a U-100 barrel. Half a graduation on that barrel is five microlitres, so the smallest reading error possible is 10% of the measurement. The same half milligram out of a 2 mg/mL solution is 0.25 mL, which is twenty-five units, and the same five microlitres is now 2% of the measurement.
Nothing about the amount changed. The draw got five times longer and the error got five times smaller as a share of it, because the absolute uncertainty is a property of the barrel rather than of the draw. That is the entire argument for diluting, and it is arithmetic rather than preference.
What it costs
A second preparation step, and with it a second opportunity for the strength written on the container to stop matching the strength inside it. The identity also assumes two things it cannot check: that volumes add without change, and that the stock really is at the strength entered.
The first is close enough to true for watery dilutions at these strengths. The second is usually where the error is, and especially when the stock strength was itself derived from a mass over a diluent volume - in which case it was a ceiling, and that page explains what follows.