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I'm planning on preparing a water solution using the following salts of reagent grade with approximately these concentrations: \begin{array}{lr} \ce{CaCl2} & \pu{10 mg/L}\\ \ce{Na2SO4} & \pu{1 mg/L}\\ \ce{NaHCO3} & \pu{20 mg/L}\\ \ce{KHCO3} & \pu{5 mg/L}\\ \ce{NaF} & \pu{0.25 mg/L}\\ \ce{NaNO3} & \pu{1 mg/L}\\ \end{array} I suspect some of these salts are hygroscopic. Do any of them need to be oven dried prior to weighing? What is an appropriate procedure for this (i.e. what temperature, for how long, etc.). Are there any other requirements in order to correctly produce a solution of the above composition?
It's for an experiment where I'm needing to replicate a natural water source so I'm needing to be as accurate as possible.

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    $\begingroup$ @airhuff, I'd disagree with your statement. Chemicals like calcium chloride are almost certainly not anhydrous unless explicitly stated, and even then once opened they'd absorb any moisture in contact. $\endgroup$ – NotEvans. May 20 '17 at 23:30
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    $\begingroup$ @NotBaran , somehow I read into this statement that the "artificial water" was for an aquarium, though that's clearly not stated...don't know why I assumed that, and I will delete the comment. But, with that in mind I figured worrying about drying, or other special handling, of the chemicals was overkill. Regarding the labeling of the degree of hydration, we are still in disagreement. If I buy a chemical reagent, I expect the bottle to be accurately labeled with it's contents, including whether a salt is anhydrous or not. I guess an exception would be a bucket of CaCl2 ice melt or something. $\endgroup$ – airhuff May 20 '17 at 23:50
  • $\begingroup$ Often labs have a sealed nitrogen/argon environment, with gloves on the glass so you can manipulate what's inside. There's usually an airlock to move things in and out. Put a balance inside one (Or ask to use someone who's already set one up), weigh your material in the safe environment, and take it out, or set up your reaction also inside the safe area. You of course need anhydrous material, or it'll kind of defeat the purpose. $\endgroup$ – MadisonCooper May 22 '17 at 18:08
  • $\begingroup$ @airhuff The best attempt I have ever seen from a vendor to acknowledge that a hygroscopic salt was not anhydrous was the following: $\ce{CaCl2 --X H2O}$ I don't know if this meets your standard for "clearly labeled" or not. $\endgroup$ – J. Ari May 23 '17 at 19:07
  • $\begingroup$ @J.Ari , this is how I expect the bottle to be labled. Sigma-Aldrich has another CaCl2 dihydrate product labled "Calcium Chloride", sans the dihydrate part, but it is clearly shown as a dihydrate in the chemical formula on the label. $\endgroup$ – airhuff May 23 '17 at 19:30
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You are dealing with very low concentrations and for those you will need very accurate equipment and a very clean lab. I am not sure that reagent purity is enough. In some parts of the world ordinary tap water might already have higher concentrations.
Unfortunately you have not given enough context for what you are doing, so I will outline some procedures which should guarantee quite good accuracy.

First thing you will need is bidistilled water probably lots of it.

For very accurate (and reproducible) results I would recommend purchasing stock solutions and the doing serial dilutions.

If the above is no option you will have to do some more research by yourself, because it depends on the quality and variation of the chemicals you buy. (And preparing a list of all that for your list of chemicals is beyond the scope of this summary.)
For example calcium chloride comes on merck in at least four variations: $\ce{CaCl2}$, $\ce{CaCl2.xH2O}$, $\ce{CaCl2.2H2O}$, $\ce{CaCl2.6H2O}$, and solutions. Depending on your starting material you will need to look up the decomposition temperatures. For calcium chloride you may find them on Wikipedia.

You first need to dry your container with which you will measure the mass of your reagent. Make sure you use gloves as fats and skin can change the result at these low concentrations. Measure it multiple times until you have a constant value (within the limitations of your scale). You need to dry the container at the same temperature which is appropriate for drying your chemical.
Then you follow the same procedure with the chemical. Obviously you will not hit the concentration exactly, but in science that hardly matters, as long as you know the correct value. Once you started the drying process you should keep the container closed until you are ready to transfer it into the solution.
You shouldn't store your prepared stock solutions for a long time and at below room temperature in appropriate flasks.

The other way would be checking the concentration against a primary standard. For calcium chloride you can probably use silver nitrate, which you have to dry like explained above. (It doesn't have the nasty crystal water, which is a plus as $\pu{50 - 90 ^\circ C}$ should be sufficient.)

For setting up the solutions you will need a good assortment of volumetric flasks, pipettes, micro pipettes, thermometers, and probably a bit more I just can't think of right now.

You might want to check with this open access publisher what is common in the field and adjust appropriately. It very much depends on your goal; approximately can cover a pretty large range of desired accuracy.

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