Nit-picking dilutions

[dae]

This is not important for most biological applications but...
I know that for making most simple solutions you dilute things up to a desired volume with solvent. E.g. a 20% w/v solution of glycine is 20g glycine with diH2O added until the final volume is 100mL. Same story with molarities. But is it the same when diluting concentrated stocks or do you add a specific volume of solvent. For instance if I have a 50X solution of TAE, do I add diH2O to 20mL of 50X stock until the final volume is 1L or do I combine 20mL 50X stock with 980mL diH2O and not worry about the final volume? For concentrated stock solutions the latter makes more sense.

Is there ever a case (besides %w/w) where you add a specific volume of solvent and just take whatever final volume emerges?

Thanks!

[JackBean]

Sure it is. E.g. when preparing PCR, the buffers are usually from 2x to 10x, but I guess you are not able to fill water till 50 ul So you have to add calculated volume and hope, it's alright

(but for large volumes like 1l or so, I guess it's usuall to fill to 1l)

[dae]

Thanks, I guess I should clarify: is there a type of dilution that is defined by the addition of specific amounts rather than final volume?

[canalon]

In fact this depend on what you are doing.
- As jackBean said, it is not always possible to have a total measured amount, and in some case you must rely on calculated amounts (ie PCR, but plenty other small volumes)
- Sometimes the exact volume is not really an issue, TAE would be typical (by the way your recipe woud be correct fo a 50X concentrate, but not that good if you have a 5X stock...). If your TAE is 1.1X or 0.9X so what... So in this case, nobody bothers.
- But it is not always desirable, and in this case you use volumetric flasks to make sure that the final volume is correct.

But As far as I know, there is no specific name that would stress the difference between the 2 way to prepare your dilutions. Just a level of accuracy.

[dae]

Thanks, I corrected the 5X above I realize that for a 1X buffer of TAE this is silly nuance but what I'm asking is if, for some reason, accuracy is paramount, what is the definitional way to make a working solution from a 50X stock?

Thanks for your patience

[canalon]

As I said, when accuracy is paramount, use a volumetric flask and accurate measuring implements.