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I have a 2% solution of Zinc sulfate in water (in heptahydrate form) that I want to turn into 4% solution instead of having to buy 4% solution separately.

Would heating it to 100 degrees Celsius and waiting for extra water to vaporize be the right way to do it?

Thanks.

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2 Answers 2

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The best approach would be to crystallize $\ce{ZnSO4.7H2O}$ and then weigh the required amount and prepare a fresh solution.

How:

  1. Slowly evaporate the solution until it is about one-fifth of its original volume. Caution: Do not boil the solution as it may spit.

  2. Allow the concentrated solution to cool until crystals form.

  3. Filter off the crystals and put the filter paper and crystals on a watch glass and dab dry with another piece of filter paper.

  4. Allow it to dry thoroughly (if you're keen on very accurate concentrations)

I assume by $x\%$ solution you mean $x\%$ weight($\pu{g}$)/volume($\pu{mL}$)

You may proceed to prepare the solution by dissolving $4$ grams in 100 $\pu{mL}$ water. In case you need a different volume, you may use unitary method.

Edit:

Crystallization is certinly more tedious than simple evaporation and volume restoration, but is bound to be very accurate. There might be loss of salt in trace amounts during evaporation, adheration to vessel walls et cetera. But if much precision is not your requirement, you may follow the latter, by all means.

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    $\begingroup$ Even though boiling the original solution down to half the volume might create technical issues, once you boil it down to 1/5 the volume couldn't you let it cool and add distilled water until the volume is back to half of the original amount without having to isolate crystals? $\endgroup$
    – Joseph Hirsch
    May 15, 2019 at 15:13
  • $\begingroup$ @JosephHirsch I suppose that'd work. A lot easier too! $\endgroup$
    – William R. Ebenezer
    May 15, 2019 at 15:23
  • $\begingroup$ Or might we lose some small trace Zinc Sulfate during evaporation? I think not if it's slow enough. $\endgroup$
    – Joseph Hirsch
    May 15, 2019 at 15:23
  • $\begingroup$ @JosephHirsch zinc sulfate isn't that volatile, I believe $\endgroup$
    – William R. Ebenezer
    May 15, 2019 at 15:29
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I believe that evaporation would work, but for technical reasons, I would evaporate it to below half the original volume, cool it, and then add distilled water until the volume is half of the original level.

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    $\begingroup$ Thanks, this is what I will do. I wasn't sure which answer to pick as correct here. I picked William's because it also contains the warning not to boil the solution, which might be useful to anyone who finds this question later. $\endgroup$
    – Milan Babuškov
    May 15, 2019 at 16:02

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