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Author: Subject: Recrystallisation question - help me design an experiment for junior students
j_sum1
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[*] posted on 3-1-2024 at 16:06
Recrystallisation question - help me design an experiment for junior students


I am hoping someone can help me with a puzzle.

As part of a suite of experiments for 11 year old students, I would like a simple-to-conduct crystallisation as a means for separating mixtures.
I need a good, reliable recrystallisation with easily identifiable product.

Ideally, students would start with two powders (or concentrated solutions), mix them together, add water and stir until dissolved and then chill in an ice bath to selectively crystallise one of the species. I have had a play with a few chemicals but have not hit upon anything that works reliably.

A few constraints
  • Reagents must be non-hazardous. To give a feel for the standard, I could do copper sulfate, but I could not do sodium hydroxide.
  • I do not want a displacement reaction. That is not the idea.
  • There must be a means to distinguish the two species when done so there is some visible evidence of separation. Copper sulfate is attractive in this respect. A pH test would also be suitable.
  • The experiment must not be reliant on accurate measuring of masses or volumes. I would prefer to work with "spoonfuls" than digital scales.
  • Crystallisation time in an ice bath needs to be on the order of minutes. A result in less than 15 minutes would allow students time to filter and recover the product.

    Best one I have found so far is copper sulfate and sodium sulfate dissolved in warm water. Dissolving was quite slow and of course Na2SO4 has a negative solibility curve above 32°C -- both of which complicated things. The assumption most students would have cone to was that the sodium salt never actually dissolved. Saturated solutions seemed to take too long to crystallise in an ice bath.

    Any ideas?
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    [*] posted on 3-1-2024 at 17:27


    What about copper sulfate and sodium chloride? Sodium chloride has a pretty flat solubility curve.



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    [*] posted on 3-1-2024 at 19:09


    I have always had good experience with potassium nitrate when it comes to recrystallization. It has such a nice solubility curve.
    Mix copper sulfate and potassium nitrate, dissolve it in boiling water, chill it, and you will get clean potassium nitrate crystals, leaving a blue solution.
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    [*] posted on 3-1-2024 at 19:15


    Quote: Originally posted by Sir_Gawain  
    What about copper sulfate and sodium chloride? Sodium chloride has a pretty flat solubility curve.

    Yes. But the most likrly species to precipitate will be sodium sulfate which has the lowest solubility at 0C.
    I would rather avoid substitution reactions. The unit of work is on physical changes and ohysical properties and separating mixtures. It seemed a shame to leave out somethjng as fundamental as crystallisation, but I just could not think of a good example to demonstrate. Even food colouring and a suitable salt would suffice. But afaik, most of the common candidates absorb food colouring quite well.
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    [*] posted on 4-1-2024 at 05:24


    Quote: Originally posted by Sir_Gawain  
    What about copper sulfate and sodium chloride? Sodium chloride has a pretty flat solubility curve.

    If you add salt to a copper sulphate solution it goes green and then yellow.


    The "classic" example of a recrystallisation used to be naphthalene from alcohol.
    The solubility at room temperature is fairly low (about 10% I think), and the solubility in boiling alcohol is almost infinite (because it melts and the two liquids mix).
    So it's almost impossible to get it to fail.

    Unfortunately the toxicity of the solid raises more concerns than it used to and people get twitchy about flammable solvents.
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    [*] posted on 4-1-2024 at 06:28


    What about chrome alum and potassium nitrate? There isn't any dissplacement reaction, potassium nitrate is very soluble while chrome alum has very low solubility at low temperatures, you get nice dark purple crystals.
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    [*] posted on 4-1-2024 at 14:50


    potassium chlorate and potassium chloride maybe?

    Potassium chlorate crystalizes easily and quickly with cooling, while the temperarure/solubility curve of potassium chloride is much flatter.

    Both salts are white, but there are lots of simple and experiments you can do to distinguish between the two. Spectacular options (mix with sugar and ignite) add excitement but may perhaps be too dangerous and/or illegal. There are also less spectacular but simple and clear reactions based on color- or precipitations that could be used.

    [Edited on 5-1-2024 by phlogiston]




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    [*] posted on 4-1-2024 at 17:18


    Chlorates would be off the list.
    Chrome alum is a good idea but I do not have any on hand to experiment with.
    KNO3 (or NaNO3) with CuSO4 sounds good. I will have to check whether it is allowed for that age group under the school's risk assessment matrix.

    (I know. Ridiculous. But that is how it works. In another experiment in the suite I am seeking exemption for students to handle 100mg of paradichlorobenzene which is restricted to 17 year olds. Don"t anyone mention that dozens of urinal cakes are left just lying around for anyone to pick up.)

    I will do some experimentation today and see if any of these ideas work satisfactorily. In the meantime, keep the suggestions coming.
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    [*] posted on 5-1-2024 at 00:55


    I am little bit worried about double dissplacement with NaNO3/KNO3/CuSO4 mix.

    Solubility of CuSO4.5H2O at 0 °C is 0,097 mol/100 ml H2O.

    Solubility of K2SO4 at 0 °C is 0,041 mol/100 ml H2O.

    Solubility of Na2SO4.10H2O at 0° is 0,033 mol/100 ml H2O.

    So sodium/potassium sulfate should crystallize first. With potassium salt it's little bit complicated because K2SO4 and CuSO4 form together Tutton's salt K2Cu(SO4)2.6H2O, so this salt should actually crystallize first with potassium. So I propose mixture of K2SO4 + KNO3 + CuSO4.5H2O, so you get pure crystals of K2Cu(SO4)2.6H2O.
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    [*] posted on 5-1-2024 at 03:48


    Thanks Bedalsky. That was my concern too.
    I played around with it a bit this afternoon and indeed, if the sulfate concentration was too high, the mixture became cloudy. I also had copper coloration entrapped in the crystals that formed.

    I got a good result with a small amount of copper sulfate and more KNO3 (approx 4x the volume). When dossolved in a minimum amount of warm water the mixture crystallised easily and quickly and the separation was good.

    I think this is the way forward.

    Next week's task will to make 8 short procedural videos. Then the suite will be ready to go.
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    [*] posted on 5-1-2024 at 11:34


    Is it not possible to use any salt, like potassium nitrate ? The idea is to dissolve it in hot water, and add one or two drops ink. The dark blue solution will crystallize by cooling. The saved crystals are colorless. This is impressive for young pupils. It is better than separate a blue powder from a blue mixture of CuSO4 + other sulfate.s
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    [*] posted on 5-1-2024 at 18:47


    maybe Manganese II sulfate and Iron II sulfate. or any other sulfate (copper sulfate)

    different colors/different solubility

    MnSO4 Solubility in water
    52 g/100 mL (5 °C)
    70 g/100 mL (7 °C) (from sm wiki)
    52 g/100 mL (5 °C) 70 g/100 mL (70 °C) from wikipedia

    (probably health hazard)

    FeSO4
    Heptahydrate
    15.65 g/100 ml (0 °C)
    20.5 g/100 ml (10 °C)
    29.51 g/100 ml (25 °C)
    39.89 g/100 ml (40.1 °C)
    51.35 g/100 ml (54 °C)

    CuSO4
    It has a solubility of 31.6 g/100 mL at 0˚C and 203.3 g/100 mL at 100˚C



    [Edited on 6-1-2024 by RU_KLO]




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    [*] posted on 6-1-2024 at 10:57


    Mn2+, Fe2+ and Cu2+ have similar ionic radii, so they can replace each other in a crystalline structure. So you end up with mixed crystalls.
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    [*] posted on 6-1-2024 at 15:55


    @Bedlasky

    Solubility of K2SO4 at 0 °C is 0,041 mol/100 ml H2O.

    Solubility of Na2SO4.10H2O at 0° is 0,033 mol/100 ml H2O.

    So sodium/potassium sulfate should crystallize first. With potassium salt it's little bit complicated because K2SO4 and CuSO4 form together Tutton's salt K2Cu(SO4)2.6H2O, so this salt should actually crystallize first with potassium. So I propose mixture of K2SO4 + KNO3 + CuSO4.5H2O, so you get pure crystals of K2Cu(SO4)2.6H2O.[/rquote]

    K2SO4 and Na2SO4+H2O together form a double salt compound. So it wouldn't be a good choice...



    [Edited on 7-1-2024 by Admagistr]
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    [*] posted on 6-1-2024 at 16:13


    Recrystallisations are most commonly performed to purify compounds (ie reduce the amount of one or several impurities present as minor amounts to start with), not to separate a mixture compounds that are present in comparable concentrations.
    That is also why crystallisation is so universally useful (while you are having difficulty finding a suitable combination of salts): the impurities are much less likely to crystallise due to their low concentration.

    So, wouldn't it be a more realistic lesson to task them with removing a small amount of an intensely colored impurity from a much larger quantity of a non-colored salt?




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    [*] posted on 6-1-2024 at 16:41


    +1 for Phlogiston's reasoning



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    [*] posted on 6-1-2024 at 22:52


    Thanks Phlogiston. That is effectively what I ended up with: a relatively dmall amount of copper sulfate in potassium nitrate. There is enough to impart some colour so all is well.
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