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Author: Subject: Purifying Copper Sulphate
Reflux-and-Chill
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[*] posted on 14-6-2016 at 08:15
Purifying Copper Sulphate


I am wondering what would be the most efficient (and not overly complicated) way to purify 200g of CuSO4·5H2O... I was thinking recrystallization, but maybe there is some better way. It would be great if you could write a quick guide. Thank you.
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aga
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[*] posted on 14-6-2016 at 08:32


Recrystallisation is THE way.

Maybe 2 or 3 times, depending on your skill, and how pure you need it to be.

Edit:

Both purity and skill increase after three recrystallisations.

[Edited on 14-6-2016 by aga]




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Reflux-and-Chill
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[*] posted on 14-6-2016 at 08:43


Would you be so kind as to teach me the best way to recrystallize it? My skill level is currently quite low (but it's not the worst) and I'm interested in leveling up.
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aga
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[*] posted on 14-6-2016 at 09:18


I'm by no means an expert, but here goes :-

Recrystallisation sounds quite easy, however the exact execution of the process can be tricky.

Fore-warned is fore-armed, so use a solubility table to see what is going to happen, e.g. : https://en.wikipedia.org/wiki/Solubility_table

That table says that CuSO4.5H2O (copper sulphate pentahydrate, which is probably what you have) will dissolve in water, and 27.5g will dissolve in 100 ml water @ 20C, yet 114g will dissolve in 100ml water at 100C.

Pick a vessel to do your recrystallisation in.

If it's a standard 250ml beaker, you can safely hold 200ml of water in it.

Now we have some data and can do a bit of maths : 200ml water.
The solubility table says 114g in 100 mls of water @100C, so you're going to need to weigh out exactly 228g of your impure crystals.

Wrong. They're impure so take a guess and add one or two grammes more.
(with this particular recrystallisation, you don't actually have to weigh it at all, but it is good practice to do so.)

Heat the 200ml of distilled water to boiling point - it will be at about 100C, so no need for a thermometer for this particular process.

Start adding your 230g of impure crystals, with stirring. Wait for the last crystals to fully dissolve before adding the next lot.

Repeat until you have added all the crystals, OR the last crystals simply will not dissolve. If you added them all and they all dissolved, you will need to add more, which is an indication that the estimate of impurities was wrong.

When your solution will not dissolve any more copper sulphate, it is certainly saturated, which is what we wanted in the first place.

While the liquid is hot, filter it. This removes any larger impurities, like dust, bugs, trees, elephants etc.

Now you will have a hot, clear, saturated copper sulphate solution.

Just leave it to cool - better to wait than to force it to cool in a fridge.

As it cools, the solvent (water in this case) cannot hold 114g per 100ml anymore, so the copper sulphate has nowhere to go - it must crystallise.

We started with crystals and we're getting purer crystals, hence the term 're-crystallisation'.

Slow cooling actually forms purer crystals : http://www.ce.gxnu.edu.cn/organic/net_course/content/Crystal...

Crystals can form instantly or take days to appear. With copper sulphate, you should see some after 24 hours at the most (if you did it right).

Now some Art : you have to Judge when you have enough crystals, or you think no more will form, then pour off the solvent that sits above the crystals.

The solvent contains the impurities, which is why you discard it.

This is why recrystallisation is a trade-off between how much product you get and how pure it is - you always lose some in this process.

You can chill your solvent to extract a 'second crop' of crystals or even a third, however you'll also be adding back in more impurities.

Give it a go, and please report back with photographs, weights, volumes etc.

I'll get some started as well : we can compare results when you post your photos.

[Edited on 14-6-2016 by aga]




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elementcollector1
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[*] posted on 14-6-2016 at 09:19


1) Add just a little water, not enough to dissolve the crystals. This can be tricky to judge, as CuSO4 is slow to dissolve, but use Wikipedia's solubility guidelines for this.
2) Heat solution to boiling. The rest of the crystals should dissolve.
3) Let cool. Crystals will begin precipitating, growing larger as the solution cools back to room temperature. Slower cooling means bigger and more pure crystals.

EDIT: I actually needed something to do on my few days off, so excuse me while I go try this.

[Edited on 6-14-2016 by elementcollector1]




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aga
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[*] posted on 14-6-2016 at 09:28


nice one elementcollector1 !

Please hold off posting results until Reflux-and-Chill gets a chance to try it out at least twice - first time it may all screw up, so have some patience.

Edit:

I screwed up the maths by forgetting the .5H2O already in the crystals.

It won't all fit in a 250ml beaker !

Start with 125ml water and recalculate accordingly.

[Edited on 14-6-2016 by aga]




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Reflux-and-Chill
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[*] posted on 14-6-2016 at 10:00


THANK YOU for such great replies! I have a little bit more work to do and then I'm all over this project. I'll post the results when it's done.
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[*] posted on 14-6-2016 at 11:16
DO NOT DISTURB


From experience I can tell you that IMHO the most important thing here is not to disturb the copper sulfate solution during cooling in any way, shape, or form! Put it somewhere up high, in an isolated area where there is very little opportunity for vibration or air current, and of course make sure not to poke at it!



“Imagination is more important than knowledge. For knowledge is limited to all we now know and understand, while imagination embraces the entire world, and all there ever will be to know and understand.” -Albert Einstein
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aga
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[*] posted on 14-6-2016 at 13:02


Quote: Originally posted by Reflux-and-Chill  
then I'm all over this project. I'll post the results when it's done.

Be sure that you do !

I've already got mine weighed, photographed, dissolved, filtered and is recrystallising as i type.

You got a week before i auto-post my results, photos, yields etc. Hopefully elementcollector1 will give you the same grace period.

The 'active' part is at most 3 hours. The rest is basically waiting.




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[*] posted on 14-6-2016 at 13:51


There is a sentence in a book that I've read that has stuck with me to this day, and it goes a little something like this:
Quote:
There is no point in trying to purify a material if you cannot assess the improvement in purity that the process has afforded


What this means is that without some means to show that the purity has been improved, there is no point in trying to purify. Back in the old days this may have been "recrystallise to constant melting point". Whilst I do not wish to discourage any efforts to recrystallise the copper sulfate, what makes you so sure it needs purifying in the first place? Unfortunately with inorganic compounds a melting point (range) is rarely indicative, but perhaps you may attempt some gravimetric or titrometric analysis of the "impure" material and after a single recrystallisation in order to determine what improvement (if any) has been acheived.

[Edited on 14-6-2016 by DJF90]
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[*] posted on 14-6-2016 at 14:46


My last attempt at this...
:)




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[*] posted on 14-6-2016 at 15:08


This is basically a very simple DOING project.

Please, oh please, let someone Do something for a change.




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CharlieA
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[*] posted on 14-6-2016 at 17:11


Quote: Originally posted by DJF90  
There is a sentence in a book that I've read that has stuck with me to this day, and it goes a little something like this:
Quote:
There is no point in trying to purify a material if you cannot assess the improvement in purity that the process has afforded


What this means is that without some means to show that the purity has been improved, there is no point in trying to purify. Back in the old days this may have been "recrystallise to constant melting point". Whilst I do not wish to discourage any efforts to recrystallise the copper sulfate, what makes you so sure it needs purifying in the first place? Unfortunately with inorganic compounds a melting point (range) is rarely indicative, but perhaps you may attempt some gravimetric or titrometric analysis of the "impure" material and after a single recrystallisation in order to determine what improvement (if any) has been acheived.

[Edited on 14-6-2016 by DJF90]


Excellent, professional advice! I would suggest determining Cu by titration with a standard carbonate solution. Alternatively, sulfate could be determined gravimetrically as barium sulfate.
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[*] posted on 14-6-2016 at 17:27


Quote: Originally posted by DJF90  
There is a sentence in a book that I've read that has stuck with me to this day, and it goes a little something like this:
Quote:
There is no point in trying to purify a material if you cannot assess the improvement in purity that the process has afforded


What this means is that without some means to show that the purity has been improved, there is no point in trying to purify. Back in the old days this may have been "recrystallise to constant melting point". Whilst I do not wish to discourage any efforts to recrystallise the copper sulfate, what makes you so sure it needs purifying in the first place? Unfortunately with inorganic compounds a melting point (range) is rarely indicative, but perhaps you may attempt some gravimetric or titrometric analysis of the "impure" material and after a single recrystallisation in order to determine what improvement (if any) has been acheived.

[Edited on 14-6-2016 by DJF90]

I agree with this sentiment entirely which is why I do not feel the need to recrystallise what I have. (Besides, the lab is out of action.) I have pottery grade copper sulfate that I have no reason to suspect is impure.

That said, there is a "just in case" mentality that one might apply. If there was some impurity present then recrystallisation would be a way of getting rid of most of it.
Also one way to assess purity is to compare the purified crystals visually with the discarded crud. If there is any sign of the crud being discoloured or some notable difference in the crystals then you can be reasonably certain that some impurities have accumulated there.

But I think it is possible to overstate the importance of purity in an amateur lab setting. The last chemplayer video I watched had a bug crawling inside his beaker. Maybe some bug parts made their way to the product but I am guessing that they filtered out at a later step.




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[*] posted on 14-6-2016 at 19:44


With really pure copper sulfate you can get huge perfect crystals. Impurities disrupt the crystal formation resulting in smaller crystals with less regular structure. Pottery grade is seldom the best purity but is usually sufficient for a home lab. Maybe we could have a crystal growing contest.
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[*] posted on 14-6-2016 at 21:07


For those wondering why I am interested in purifying it; it's because it was cheap to buy, and the person told me that it's not the purest sample.
And come on, crystallization is cool and you know it.

[Edited on 15-6-2016 by Reflux-and-Chill]
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[*] posted on 14-6-2016 at 21:44


Quote: Originally posted by Reflux-and-Chill  
For those wondering why I am interested in purifying it; it's because it was cheap to buy, and the person told me that it's not the purest sample.
And come on, crystallization is cool and you know it.

[Edited on 15-6-2016 by Reflux-and-Chill]

And that's a good enough reason to attempt it.


After you have had a go, maybe you could organise a competition as per mackone's suggestion.




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Reflux-and-Chill
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[*] posted on 14-6-2016 at 23:00


That sounds good.
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[*] posted on 15-6-2016 at 05:29


Ok count me in! Discard everything i said about copper sulphate crystals in other threads :D. I am making my copper sulphate as i want to use up some conc nitric acid. If nothing else i am bound to have impure stuff if i am making it :D.

Great idea for a competition to improve skills.
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[*] posted on 15-6-2016 at 09:47


I made a nice inch long Cu(II)SO4 crystal as described above, then rinsing the residue with ethanol (it's insoluble in EtOH), repeating again. Then forgot about the solution in the freezer. Some crystals were larger but couldn't be recovered since the mass had to be broken.

I'll try to post a pic when I get home.

Actually recrystallization is an important technique, partly if you're trying to salt out a product how made, though eve then there are other ways of approaching fast.

However, without an expensive machine or if you have a special technique to detect impurities, you won't know how "pure" it is.
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[*] posted on 15-6-2016 at 09:49


One more point if you have a pure sample crystal, it can be used as a seed
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Reflux-and-Chill
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[*] posted on 15-6-2016 at 10:42


I would like to learn using pure sample crystals as seeds.
P.S. UPDATE: They are growing nicely.
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[*] posted on 16-6-2016 at 07:34


Looks like i can start tomorrow evening, so for various reasons i would like to go a slightly different route! I am just checking i am on the right track.

I have some conc nitric acid (more than i am happy having), the bottle is clear and says 68% but dosnt fume much when opened. Its a Amazon buy so not too sure if the 68% is true or not, what i am think is murder a few birds with one stone.

So take Nitric and add same volume of water (distilled), i will decide and measure the amount tomorrow but i am thinking roughly 200ml Acid and 200ml water.

I have some copper wire left over from a loft extension, so i intend to weigh portions of copper and add to the acid until no more dissolves. maybe while heating to around 30-40C?

the idea is too get some copper nitrate plus from the left over copper nitrate i can add sodium carbonate? This should give me basic copper carbonate? The sodium carbonate is fairly pure according to the container but it dosnt seem anhydrous, so i will just add until no more reacts.

take some of the copper carbonate and add sulphuric acid until no more reaction to give me copper sulphate? Then purify the copper sulphate by recapitalization.

Does this sound ok? The reason for going this route is to add some more reagents to my collection and to use up some conc nitric Acid. As i am not 100% sure of the concentrations and water contents of what i have, i will weigh the amounts of what add as i go. Does this sound ok? Or am i missing something that is going to make this fail?

I also need to do this in the shed so when the copper is dissolving in the nitric Acid is sodium carbonate in a wash bottle ok to neutralize any NOx gas given off? i will blow the NOx through the set up with a small air pump. Comments welcome.

edit i might actually start with more acid or use some Magnesium carbonate as well, i dont have any magnesium nitrate so that is another i could make.
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[*] posted on 16-6-2016 at 07:46


Too complicated.

Better to just make some copper sulphate from your sulphuric acid with electrolysis.

Nitric acid is less of a worry than conc sulphuric !

Edit:

Also, Reflux-and-Chill has had plenty of time to boil 125ml of water, add/dissolve crystals, filter, leave to cool ...

... so will probably post his results pretty soon ... ?

[Edited on 16-6-2016 by aga]




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[*] posted on 16-6-2016 at 08:30


I see that you're subtly and politely telling me that it's time to post the results, with which I agree. I think the crystals are looking pretty good at this point. Let me just dry them and snap a few pics, and I'll be back in no time.
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