How much solvent do you add for a 1:20 dilution, and why is it called 1 to 20?

solutions concentration solvents

3,069

Dilution ratio 1 : N has 2 different, conflicting meanings:

(1) The ratio of the 2 mixed volumes ( typical for 1 : 1 to 1 : 3, for everyday/household chemistry )
(2) The ratio of the final and initial concentrations (typical for 1 : 5 and more, for using calibrated volumes and exact concentrations ).

Consult the context of your question, but the quote obviously considers the latter case. 1 : 20 means the solute concentration is 20 times lower than before dilution ( the second row below).

1 : 20(1) means 1 : 21(2) ( approximately(*) )
1 : 20(2) means 1 : 19(1) ( approximately )

(*) - There is one trap: It is better to say you take 1 volume of the stock solution and while mixing, you fill up by solvent the final volume N. As volumes are not generally additive. By other words, by mixing $\pu{10 mL}$ of solution and $\pu{190 mL}$ of solvent, you may not get $\pu{200 mL}$.

3,069

CottonTheButton

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Updated on August 22, 2022

Comments

CottonTheButton 9 months

A 1:20 dilution implies that you take 1 part of stock solution and add 19 parts of water to get a total volume of diluted solution equal to 20 times that of the stock solution

So is 1:20 the way it is because the ratio is solute:solvent/diluent, right? So the solvent/diluent is just n-1 of the solvent?
- Alchimista over 1 year
  
  I would make a final solution with conc 1/20 of the initial one. See Poutnik point (2).
CottonTheButton over 1 year

I read more articles and I (think) finally get it. The ratio is simply just the solute ratio to the solvent ratio.
Poutnik over 1 year

Be sure about that ......