# Diluting down solutions method

I'm designing an experiment and it involves testing various concentrations of solutions, and I will be testing 5 in total. I proposed to make up a stock solution, and distribute it evenly in 5 different beakers and add different amounts of water to each solution but my teacher doesn't like this method and says I should simply dilute down but I don't know what he means. So if I had a 1.00 mol/dm$^{-3}$ solution and wanted concentrations of 0.800, 0.600, 0.400 and 0.200 mol/dm$^{-3}$, how do I dilute it down? To be honest, the concentrations don't really matter, it's just the method of making them - would it be easier to dilute them by half each time? Each solution is going to be used up so I can't simply reuse them...

Ultimately I will need 200ml of each solution in the experiment design, and the precision of the concentrations is not very important. How can I dilute one stock solution to 4 other solutions?

It depends on what degree of accuracy you need and what order of magnitude the final concentrations will be.

If you need high accuracy and concentrations are in the range you are describing, I would put a class A volumetric flask on a balance, add the appropriate weight of solute, add water to not quite the full volume, dissolve the solute, and add water to the volumetric line, seperately for each solution.

If you need concentrations much lower than what you are saying, when weighing becomes your largest source of error, then it is more accurate to make a concentrated solution, and the dilute the concentrated solution. For example, you would use a class A TD (to deliver) volumetric pipette to measure the concentrated solution into a class A volumetric flask.

• Thanks @DavePhD . My degree of accuracy does not really matter at all, as this is just an experiment design. It's more about having 5 solutions of different known concentrations. How can I use a 1 mol/dm^3 solution to make 4 other solutions then? Ultimately, I will need 200ml of each solution. – Jim Mar 13 '15 at 15:34
• @Jim "known concentrations" and "accuracy does not really matter at all" are opposites. You could take a graduated cylinder, fill to 160ml with concentrate, add water to 200mL, then repeat for the next solution using 120mL concentrate, etc. – DavePhD Mar 13 '15 at 16:00
• +1, but first add some solvent, then the solute or stock solution (the concentrated solution), then solvent to the mark. – cbeleites Mar 14 '15 at 21:11

@DavePhD already gave a good description of how to prepare the solutions.

Here are possible good reasons why the teacher doesn't like your original proposed method:

distribute evenly

does not work in practice (unless you mean: prepare a bit more, and then pipette 5 equal amounts). But if you pipette, there is no reason not to pipette the amounts of stock solution that directly lead to the neeeded volume for the dilutions.

beaker

is no volumetric glassware, and in addition not the preferred glassware for mixing (though mixing with magnet stirrer is OK). Volumetric flasks are much better: you can close them and then thorough mixing is possible. If you don't have volumentric flasks, Erlenmeyers are better than beakers.

would it be easier to dilute them by half each time

That would lead to an exponential series of dilutions, like 1 mol/l, 0.1 mol/l, 0.01 mol/l etc. instead of the equispaced concentration series you describe. Also, serial dilution means that concentration errors build up: each dilution step adds error.

Ultimately I will need 200ml of each solution

With volumetric flasks, you can directly prepare 250 ml of each solution. Your proposed method would lead to varying volumes. It is unlikely that the proper volumes are available as volumetric flask.

Calculating the needed volumnes for the volumetric flask + pipetting stock solution or the graduated cylinder version is easy. A plausibility check of this can probably be done on the fly in the lab without calculator.
Calculating the needed additional volume for your method is somewhat more elaborate. This is of course a bad reason if it is only about laziness. But it is a good reason if it is about doing plausibility checks while doing lab work.

Last possible reason: working with volumetric flasks thorough mixing is easier and the volume is anyways more precise than with a graduated cylinder. (Far more precise if you are still learning the practical skills of lab work: with the graduated cylinder you neet to get both volumes right at the first attempt. With a pipette getting the volume right is much easier. So only the flask volume needs to be right at the first attempt - and the flask has a neck that helps with precise volumina.)