Recrystallization of salts

  • Thread starter cnidocyte
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In summary, if you have a mixture of NaOH and NaNO3 and want to separate them via crystallization, you would add ethanol to the mixture and stir it up while filtering the result.
  • #1
cnidocyte
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If I have a mixture of NaOH and NaNO3 and want to separate them via crystallization how would I go about it? Let's say my solvent is ethanol. 1g NaOH dissolves in 7.2ml ethanol whereas it takes 125ml to dissolve a gram of NaNO3. I'm not well versed in solutions, if I added a 72ml of ethanol to a 1:1 mixture weighing 20g would the ethanol dissolve mainly the NaOH since it has a far greater solubility?
 
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  • #2
Well, yes, but there's technique involved, too. That depends mostly on what your starting material is and what you want to accomplish.

Let's say that for some bizarre reason you took two 10g samples of the pure reagents and mixed them together - but the individual crystals were still pure. You could just add ethanol at room temperature and stir it up a while, and filter the result, and you'd have mostly NaNO3 crystals left on the filter. But frankly, both reagents are pretty cheap, and that's a lot of bother to get impure sodium nitrate, and you'd be better off tossing the mix down the drain and buying some more reagents.

On the other hand, let's say you're after the evil bad guy (played by Gary Oldman) because he's going to do terrible things to your girlfriend (played by Angelina Jolie), but you can save her with some sodium nitrate (who knows why, it's Hollywood). Of course, you can't call Sargent-Welch for a delivery because their next secret evil lair delivery is next Tuesday, so you have to wing it. So you go into the secret evil bad guy lab, and you get his inexplicable mixture of sodium hydroxide and sodium nitrate. You put it in an erlenmeyer flask, and heat it up, and add boiling ethanol until it all dissolves. Then you take it off the heat and let it cool to room temperature. The sodium nitrate will drop out of solution as pretty crystals, and you filter off the liquid, and save the day. Gary Oldman gnashes his teeth, Angelina Jolie kisses you really hard, and then Brad Pitt shows up and punches you in the snoot for canoodling with his wife. So there you are. :-)
 

1. What is the purpose of recrystallization in the purification of salts?

Recrystallization is a commonly used technique in chemistry to purify solid compounds, such as salts. It involves dissolving the impure salt in a solvent and then allowing it to slowly cool and form crystals. This process helps to remove impurities and yield a purer form of the salt.

2. How do you choose the appropriate solvent for recrystallization?

The choice of solvent for recrystallization depends on the solubility of the salt. It is important to choose a solvent in which the salt is highly soluble at high temperatures, but less soluble at lower temperatures. This allows the impurities to remain dissolved while the pure salt crystals form upon cooling.

3. How do you prepare a saturated solution for recrystallization?

To prepare a saturated solution, add the desired amount of salt to the solvent in a beaker and heat it until the salt dissolves completely. Then, continue adding small amounts of salt until no more will dissolve in the solution. This indicates that the solution is saturated and ready for recrystallization.

4. How can you improve the efficiency of recrystallization?

To improve the efficiency of recrystallization, it is important to minimize the loss of product during the process. This can be achieved by using the minimum amount of solvent necessary to dissolve the salt, avoiding overcooling the solution, and filtering the crystals quickly to prevent them from redissolving.

5. What are some common techniques used to dry the recrystallized salt?

After filtering the crystals, they must be dried to remove any remaining solvent. This can be achieved by using a drying oven, vacuum oven, or desiccator. Another common technique is to air dry the crystals on a filter paper or using a hairdryer on a low heat setting.

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