felis
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Washing crystals
Gentlemen,
This is my first post on this nice board and I meant to post it in the beginner's section - sorry! I am refining a compound by crystallization.
Fertilizer grade compound gets dissolved, shaken with activated carbon, filtered, evaporated to supersaturation, moved to crystallization beaker and
cooled. Crystals grow very well. The compound is monopotassium phosphate.
The next step is going to be washing crystals with alcohol, most likely IPA. I see two possibilities as how to proceed - one, crush the crystals and
then wash and dry, and two, wash them right where they grew, then dry and take out (crush or dissolve and evaporate). Does it make much difference as
far as purity (I don't need analytical grade)? Or is there even better way to do the wash?
Thank you,
[Edited on 18-8-2009 by felis]
/felis
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chemoleo
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Thread Moved
17-8-2009 at 17:53 |
ammonium isocyanate
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Welcome to the forum!
Your recrystalization procedure sounds very good, although if you start with a solution that is saturated at r.t. then I imagine that the losses are
high. Depending on how expensive KH2PO4 is to you, it may be worthwhile to reconcentrate the solution and do a second crystalization to improve your
yield, or to start with a higher temperature solution (of course, I'm assuming that the solubility of KH2PO4 increases with temperature, which may not
be the case).
If you don't need really pure reagents, I think you should just stop where you are. However, if you really feel like washing them with isopropanol,
It will only be really effective if you cruch the crystals, so any impurities present will be exposed to the IPA and won't be trapped in the crystal.
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DJF90
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Crushing the crystals may not be necessary, especially if they are grown very slowly. If this is the case then the crystals will be very pure and
there will be very little impuries present (they will be concentrated in the mother liquor). Alcohol could be added to a saturated solution to force
the crystals to crash out (this compound is supposedly insoluble in alcohol).
I would suggest recrystallising twice from water, and then washing the crystals with methanol, ethanol or IPA. After drying, you should have fairly
pure crystals; I'd expect LR grade, if not better.
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felis
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Thanks for the reply. Solubility of KH2PO4 in water does increase with temperature. I start at about 90C and about 2x saturation. Crystallization
happens in the refrigerator at 4C. I don't know how long it would take to grow, it's been a full day so far and growth continues.
Of course, solution left after crystallization goes back to the cycle; at the end I'm planning to lose 50ml of solution or 15g of KH2PO4. I have a
kilo to process.
/felis
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DJF90
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You may want to use fresh solvent after each crystallisation, as the mother liquor from the previous run will contain a higher concentration of
impurities and so your crystals will be less pure. The slower it crystallises, the purer the crystals are (generally).
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not_important
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How many stages of recystallisation are needed depends on the levels and types of impurities, and the intended use of the product. About the only
rule of thumb would be that if the mother liquor is off color or gives noticeable different coloured crystals when a bit of it is evaporated, then
another recrystallisation is likely a good idea.
While recycling the mother liquor is generally a good thing, there is a limit to how much it can be done, depending on the type and level of
impurities involved. Increasing concentrations of other compounds can lead to modification of the crystal form, to increased numbers of inclusions,
to in some cases double compounds crystallising out.
Immediately washing with alcohol generally will lead to an increase in retention of impurities, as that will tend to force inorganics and thus
impurities out of solution onto the surfaces of the product crystals. Better is a wash with ice water, only a small amount if the product has
noticeable solubility in water at 0 C, or with a saturated solution of the product using a bit of it from a previous batch to make the solution.
Using vacuum filtration or a centrifuge helps removing mother liquor and wash liquid from the product, the vacuum filter is the easiest for the
amateur.
If the solubility in ice water is low enough, actually covering the product with it and stirring it up s even better, as it opens up pockets that may
have formed between crystals that are adhering to each other; however this is typically only possible for solubility low enough to be considered
precipitates. Again, depending on impurity amounts, the wash filtrate might be used to dissolve the next batch of raw compound, possibly with some of
the mother liquor from crystallisation.
Alcohol washes of inorganic and ionic compounds are most useful when
1) you know some of the impurities are soluble enough in the alcohol or water-alcohol mix to be dissolved.
2) you intend to dry the product, rather than to redissolve it, and you know the product cakes badly on slow drying, or that it is fairly hygroscopic.
3) you want to quickly dry the product, in which case a second wash of acetone or ether is generally used, assuming the product is not soluble in
them.
For compounds that are hydrated, such as Na2CO3.10H2O, an alcohol wash can remove water and cause the crystals to break up.
Some compounds, if crystallised slowly to form larger crystals, will have more inclusions that trap the mother liquor and thus impurities, than if
crystallised more rapidly so as to form smaller crystals. In general large crystals are best avoided if there is much impurities, as that seems to
favour the formation of inclusions over crystallising when there are only traces of impurities.
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felis
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Quote: Originally posted by not_important  | Better is a wash with ice water, only a small amount if the product has noticeable solubility in water at 0 C, or with a saturated solution of the
product using a bit of it from a previous batch to make the solution.
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That's probably the best way. KH2PO4 has good solubility in water, but it dissolves slowly so I won't lose much.
Thanks!
/felis
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zed
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Generally speaking, the faster crystals are formed, the smaller they are, and the more internal purity they possess.
Sounds good, until you consider, that collectively, the mass of small crystals formed, has a tremendous surface area. This expanded surface area,
can be damned hard to efficiently rinse, and the small size of the crystals, makes them vulnerable to completely redissolving during the rinsing
process.
Very large crystals are usually less "internally" pure than small crystals. But, they contain a massive amount of material in relation to their
small surface area, and rinsing is easily accomplished, without huge losses.
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