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j_sum1
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You could go for ammonium dichromate.
I have just re-read this whole thread and I didn't notice anyoine mentioning it.
Acidify the calcium chromate solution to form dichromate. Precipitate by adding a solution of ammonium sulfate. Let it settle and decant the ammonium
dichromate solution. Recrystallise. This is what Tdep did and it seems to me to be a good route.
I have an assortment of Cr waste to process some time soon and I am thinking of this method.
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Fulmen
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Not a bad idea. The problem is purity, without a way to filter&wash the CaCrO4 I will have a ton of sodium chloride in it. Can it be separated
properly?
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Fulmen
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Before I discard the calcium route completely, any suggestions on how to produce a better precipitate?
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JJay
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I didn't have extremely serious trouble filtering after simply boiling off water to produce crystals. The slower the water evaporates, the larger the
crystals will be. I do remember having serious problems with bumping towards the end, but I think they were caused mainly by other calcium salts after
almost all of the calcium chromate precipitated. The filtrations with high concentrations of calcium chlorate took a while to filter and required
switching filter paper regularly because the filtrate was syrupy and degraded the filter paper. I used a 1L borosilicate Buchner with a vacuum pump
for most of the filtrations.
[Edited on 30-3-2018 by JJay]
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Fulmen
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That's pretty much the brute force approach. Sadly I haven't finished the buchner funnel yet, so I'm stuck with gravity filtering. But a thorough boil
did improve things a bit, and if I prioritize purity over yield I should be able to make this work. It would be fun to try reacting the solid calcium
chromate with sulfuric acid to produce pure chromic acid, but I fear the calcium sulfate would mess up everything.
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JJay
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I don't think mine is extremely high purity just yet... I recrystallized it 3x, but I still don't think it's quite there.
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Fulmen
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Without an actual analysis it's just guesswork, might as well guess that it's fairly pure ;-)
I did have to add a shitload of NaOH to the solution to get things to precipitate. I started by adding NaOH until the solution turned yellow, that
should indicate CrO4-2 right? Still, without a large excess I get no precipitate when adding Ca(NO3)2. And the precipitate is whiter than I
expected, shouldn't it be yellow? I wonder if I'm getting a co-precipitate of some sort...
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JJay
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Calcium chromate is pretty soluble... calcium hydroxide has low but significant solubility... barium chromate is insoluble IIRC. Iron chromate and
iron hydroxide are insoluble.
[Edited on 30-3-2018 by JJay]
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Fulmen
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F*ck, I forgot about Ca(OH)2. I need to rethink this a bit, I assumed calcium chromate would be the least soluble salt. Damn.
---
I feel like such an idiot. My data indicated a low solubility for calcium chromate in hot solutions, so I figured I could precipitate it out. Instead
I've spent a lot of time making and filtering off calcium hydroxide. I did wonder why the filtrate kept coming out yellow though. Jeeeez...
Anyways, it's all good. I hadn't tossed out anything so I just need to acidify and boil down a lot of liquid.
[Edited on 30-3-18 by Fulmen]
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S.C. Wack
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Potassium sulfate would not be unwise then, to get something nicely crystalline. When K, Na, Ca chromates and dichromates are made from roasting
chromite ore red hot in air, potassium or sodium sulfate and CaO are useful things.
The best although high energy route I suspect is dissolving SS in acid, precipitating the hydroxide sludge, and roasting it as for chromite ore. The
sludge retains some of the alkali; if someone wants K dichromate it's best not to use NaOH to precipitate.
I might have tried that and failed, but I would have done it like the KOH fusion with Cr2O3 in air, which works but sucks, at least when using pottery
grade Cr2O3.
Earlier I mentioned how chromite doesn't form with excess base in the presence of the Fe, but maybe it will form a soluble and H2O2-friendly chromite
when the oven-dried hydroxide sludge is heated with KOH?
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Fulmen
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I think I know where it all started going wrong. I used the data from the Wikipedia Solubility Table which lists data for the anhydrous salt not the dihydrate:
anhydrous
4.5 g/100 mL (0 °C)
2.25 g/100 mL (20 °C)
1.83 g/100 mL (30 °C)
1.49 g/100 mL (40 °C)
0.83 g/100 mL (60 °C)
dihydrate
16.3 g/100mL (20 °C)
18.2 g/100mL (40 °C)
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JJay
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I have observed that calcium chromate that is heated to dryness on a hotplate is hard to dissolve without a large quantity of water. Come to think of
it, I wonder if that is a good way to purify it... by first heating to dryness and dehydrating, grinding, washing with water, dissolving with a large
quantity of water, concentrating, and crystallizing....
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Fulmen
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It's worth a try I guess.
I'm about done cleaning up this giant cluster f*ck. I ran out of HCl, so a large portion of the Ca(OH)2 had to be filtered. Oh well, at least I have
another project cleaning that up as a bonus. Might come in handy. Even though I used recrystallized calcium nitrate I got a lot of dark brown turbid
matter in the solution that really clogged up the filters, but now that's gone and I'm left with about a liter of chromate solution that I can work
with again.
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JJay
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I just tested some of my calcium chromate with peroxide... I currently have four fractions, but they are not the same fractions as before. Fraction 1
turns brown, almost opaque with .8 grams of yellow, crunchy crystals in 10 mL of water with 10 mL of 29% H2O2. It's not extremely dark brown and is
certainly less dark than Jones reagent with hydrogen peroxide, but it's nearly opaque in a 20 mL vial. It has a greenish cast but when held up to the
light looks more reddish.
Fraction 2 is a bit damp and does not produce a solution that is as strongly yellow. It turns brown with peroxide but not as brown. Fraction 3 is also
a bit damp and is even less brightly colored and produces less brown.
Fraction 4, which is dry and was actually the first fraction taken, produces an even brighter yellow and darker brown.
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JJay
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I tried the same treatment with a mL of Jones reagent, and the addition of peroxide produced considerable fizzing and a persistent dark color that
appeared blue-green when diluted.
I put .8g of fraction 1 into a vial and added 10 mL of water, followed by a few drops of sulfuric acid. Addition of peroxide had the same effect as
with Jones reagent except the solution was not quite as dark and did not fizz as long.
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Fulmen
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What should this test indicate?
Personally I'm loosing my frikkin' mind here. I keep getting this dark brown, muddy precipitate that instantly clogs the filters. At first I thought
it was some sort of clay/powder coating residue from the fertilizer grade calcium nitrate, but it keeps appearing every time I boil down the solution
further. Any idea what it can be?
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JJay
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It indicates chromate. A while back I had noticed that the yellow substance didn't react much with peroxide; it appears that acid is required for a
strong reaction. I could try extracting the chromium (VI) oxide peroxide with ether or ethyl acetate; it should be blue in solution, and it forms
stable complexes with some organic molecules, but it's said to be very dangerous to isolate it in pure form.
I do remember seeing something like the mud that you describe that but only when I boiled the solution down the first time and before seeing any
yellow or white crystals... I did not try filtering it but decanted from it instead. Eventually, it stopped forming. I don't know what it was or if it
was the same as what you are seeing... I assumed it was nickel or iron oxides and hydroxides that made it through the early decantations and
filtrations somehow.
Filtering through a bed of cotton might work for removing it, but it's awfully hard to beat vacuum filtration for that sort of thing.
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Fulmen
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It doesn't look like a simple suspended solid, settling even for a day doesn't do anything to it. Besides, a solid would most likely act like a seed
and get trapped in the salt as it precipitates, right? It's not something that has passed through previous filterings either, the solution has always
been crystal clear before boiling. After boiling it down it has a brown turbid look.
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Fulmen
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Re: your rest, I don't really see the point. The color and pH-response should be enough to confirm the presence of chromates, considering the
reactants there's really nothing else it could be.
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JJay
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Quote: Originally posted by Fulmen  | Re: your rest, I don't really see the point. The color and pH-response should be enough to confirm the presence of chromates, considering the
reactants there's really nothing else it could be.
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That is kind of what I thought, but it was suggested that it might be an iron compound.
One of these will make your life a lot easier: https://www.ebay.com/itm/ISO-Standard-Borosilicate-1000ml-24...
It's a Yee Chen special, but it's a useful piece of equipment.
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Fulmen
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I did use vacuum, I managed to McGyver the broken funnel good enough for a couple of runs. Besides, I now have the acrylic funnel up and running: http://www.sciencemadness.org/talk/viewthread.php?tid=81589&...
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Fulmen
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The new funnel and a very coarse filter has helped a lot. Or at least postponed the problem with the unfiltrable precipitate a bit. I'm down to a real
concentrate now, but that's also where things get hard because I'm a greedy motherf. by nature. I want good yields, and that means recrystallizing the
unwanted salts several times. There is a lot of it and it keeps coming out yellow.
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Fantasma4500
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for fine particulate you may attempt decantation, im however ignorant of the relative density of calcium chromate, particle size also means something
in terms of how long it takes to settle, for instance silver is quite heavy, but nanoparticulate silver wont settle in a whole year.
how about lead chromate? PbO*PbCrO4 regardless of how fine the ppt would be should settle quite fast.
another thing.. for calcium chromate you could try to ppt most of the remaining in solution adding some ethanol shifting polarity
a little trick for calcium chemistry is to turn the calcium hydroxide into carbonate using CO2, from here you can then apply more concentrated CO2,
dry ice. this forms CaHCO3 which is actually soluble, i managed to produce this using 5% H2CO3 a year ago or so, think i did it at room temperature
even. CaHCO3 appeared somewhat stable to temperature, but once it dries out and gets some heat it should all convert into CaCO3 which is then
perfectly insoluble
now as the calcium route uses pool chemicals... what about trichloroisocyanuric acid? with NaOH this forms NaClO, this would mean NaCrO4 and NaCl
which would be easy to seperate, filter hot and let NaCrO4 crystallize out, problem with NaClO usually is its poor concentration and cost, you would
also have some NaClO3 in solution which could be a problem when acidifying to get dichromate.
NaClO3 and NaCrO4 is not very soluble in ethanol, NaClO3 seems soluble in methanol however, where NaCrO4 is not.
maybe ppt with lead wouldnt be a really great idea supposing you got a lot of chloride in solution, many years ago i attempted to ppt copper chromate
from stainless steel chlorides, the ppt was sorta gellish, and copper chromate is supposedly somewhat unstable, but it could work to avoid ppting
chlorides at least, if its not unstable after all then it would be doable to boil the whole thing dry where the gelly ppt of copper chromate would
lose its gel like state of precipitate and you could easily leech out the watersoluble chemicals with decantation
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Fulmen
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Decanting sort of works, but it's awfully slow and inefficient. But it's not the calcium chromate causing problems, it's something else that keeps
precipitating out as I reduce the volume. Using a coarser filter does help, but it still clogs up after a while. And of course a lot of this ppt
passes through, so I will have to deal with it eventually.
As for lead chromate, how would you turn that into a more soluble salt afterwards?
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Fulmen
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I feel I've come as far as I can with the calcium route. Either I'm getting chromate precipitation or the liquid is simply so concentrated the
precipitated white salts look yellow. Either way it's getting hard to work with the solution now. I also realize the high pH was a mistake, it caused
carbonates to precipitate onto the bottom of the beaker.
So I decided to precipitate out all the Ca with an excess H2SO4. But now I'm faced with another dilemma. My initial plan was to precipitate potassium
dichromate as it is less soluble than the various sodium salts that would be remaining. By precipitation out the colored dichromates it should be easy
to see when it's all out, right? But this probably won't work with sulfates as both the sodium and potassium salt has comparable solubility when cold.
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