Flyingfenix
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Problem with Potassium Dichromate solubility
I recently purchased about 1kg (970g to be more precise) of supposedly technical grade potassium dichromate, and am currently trying to purify it by
recrystallization from water. The material was a free flowing powder, grains about 0.1m-1.0mm in size, deep orange color, no noticeable smell.
AFAIK, the solubility of potassium dichromate ranges from about 102g/100ml @ 100°C, 13g/100ml @ 20°C to about 4.9g/100ml @ 0°C from Wikipedia
So, in theory, had I K2Cr2O7, I would be able to fully dissolve about 500g in 500ml of boiling water, but only 65g at
20°C.
Then, I proceeded to weigh about 500g of raw material and added to about 500ml of room temperature water, and started heating and stirring. Ambient
temperature was slightly above 25°C that day.
To my surprise, everything dissolved right away, much, much before reaching boling point.
Thinking, maybe my raw material was severely contaminated with potassium chromate, or even sodium chromate, compounds that have higher
solubilities at this temperature, I started adding small weighed portions of raw material to the solution while heating.
Now, to my astonishment, the full 970g of raw material dissolved!, I had to add all of my raw material, leaving nothing behind. And that was
before boiling (reached about 80°C).
I cannot understand what happened.
Had I pure potassium dichromate, I shouldn't be able to dissolve more than 510g in 500ml of boiling water.
Had I pure potassium chromate, I shouldn't be able to dissolve more than 396g in 500ml of boiling water.
Heck, had I pure sodium chromate, I shouldn't be able to dissolve more than about 630g in 500ml of boiling water! (impossible, it would be yellow and
not orange)
Any ideas/help?
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unionised
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Probably a stupid question but...
When you weighed out 500g initially, did it seem to be about half the material?
(Or, even better, did you weigh the whole lot before you started?)
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Ubya
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ammonium dichromate is also out of the list (156g/100ml 100°C).
are you sure this is really dichromate? can you do a test? just put some of your solution with some sulphuric acid and ethanol, under mild heat it if
it is really a dichromate salt the solution should change color to a green or blue
[Edited on 3-2-2019 by Ubya]
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clearly_not_atara
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Can someone please think of a good test for carbonates in the presence of chromates? I'm still sleepy this morning
[Edited on 04-20-1969 by clearly_not_atara]
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walruslover69
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Sodium Dichromate sounds like the most likely culprit. It has extremely high solubility 73g/100ml at 20C.
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unionised
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Add dilute acid + see if it fizzes
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Ubya
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on pubchem they say 187g/100g at 25°C and 236g/100ml at 20°C so there's a bit of confusion. sodium dichromate probably is the culprit
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woelen
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Must indeed be sodium dichromate. Too bad you dissolved all of it. Getting back solid sodium dichromate will be very difficult. The material is very
hygroscopic and it will be very hard to obtain a nice dry solid from your solution again.
May I give an advice to you? When you do experiments or try some procedure, first try at a test tube scale. In your case I would have taken 2 ml of
water and added the orange solid gram by gram and maybe I would have lost 3 or 4 grams. You lost the whole lot (at least, getting it back in solid
crystalline form will be very hard).
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Ubya
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If you have a very soluble potassium salt (potassium nitrate maybe) you could convert your sodium di chromate to potassium di chromate by adding it to
your solution, the potassium salt should precipitate
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