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Lets say I have a precise solution, and would like to store it long term (up to two years) with as little as possible change in concentration. The volume is 10-100ml. What container should I choose? What temperature? Is there any other factors? I would prefer not to use sealed glass container. Is there a way to approximate a solvent loss? What if I stored some water in exactly the same conditions together with my sample, would I be able to approximate water loss in my sample from the water loss in this reference container, or does the difference in the concentration of the solute affect the rate of evaporation too much?

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    $\begingroup$ A sealed glass ampule would be the most certain way of ensuring your solution stayed the same concentration, removing the possibility of solvent loss. Even in that case, concentration is likely to change as the reagent in the solution would likely degrade to some extent (unless it was a very simple solution such as NaCl in water). Since a glass container isn't what you want to use, I'd suggest storing the solution in a tightly capped vial, sealing the cap with Parafilm,and then storing this vial in the freezer. This should minimise solvent loss and change in concentration due to degradation. $\endgroup$ – NotEvans. Nov 14 '15 at 23:39
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What if I stored some water in exactly the same conditions together with my sample, would I be able to approximate water loss in my sample from the water loss in this reference container, or does the difference in the concentration of the solute affect the rate of evaporation too much?

That would be a gross approximation, not suitable for a standard solution. Your solution and pure solvent do have different vapor pressures according to Raoult's law.

What temperature?

It depends on solvent, solute, and their interaction. I'd check two main things: i) solubility at the storage temeprature, which must be as close as possible to the solubility at the working temperature; ii) vapor pressure of the solvent, that gives you a rough indication of the storage temperature. You could take a look at the temperature dependence of the solvent vapor pressure and solute solubility - or, if seriously in doubt, even try simulating the thermodynamics of your solution using a software.

What container should I choose?

Volumetric flasks are usually the best choice. They minimize evaporation compared to other glassware (bottles, beaker, etc.) and, most important, they have the most accurately placed graduation mark. Use the appropriate rubber cap and close accurately with laboratory film. PTFE band between cap and flask cone might also help preventing losses.

At times, a slight overpressure above the liquid can be desirable. In this case, a gas valve (or a quality glass faucet) will be placed on top of the flask and nitrogen will be flushed inside. The flask can be stored capped by the sealed valve. Also consider the opportunity of degassing your solution by placing sieves on the bottom of the flask or by bubbling nitrogen through the liquid. Finally, consider the need for a dehydrating agent in the case of a moisture-sensitive solution.

But, most of all, don't keep standard solutions for two years but replace them every ~6 months.

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