Sciencemadness Discussion Board - Conversion of potassium dichromate to ammonium dichromate

Conversion of potassium dichromate to ammonium dichromate

blogfast25 - 1-8-2013 at 09:27

In the UK, ammonium dichromate is now all but outlawed from what I understand. Potassium dichromate is likely to follow shortly but at least that compound is not beyond the synthesis capability of many a home chemist.

Ideas on how to convert K2Cr2O7 into (NH4

2Cr2O7 would be welcome.

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[Edited on 2.8.13 by bfesser]

AJKOER - 1-8-2013 at 11:44

Blogfast:

Here is a suggested preparation route based on my research. First prepare CrO3, the anyhydride of Chromic acid, whose preparation per this source (http://chromium.atomistry.com/chromium_trioxide.html ) is described as follows:

"Chromium Trioxide, Chromic Anhydride, CrO3, was first obtained by Unverdorben by the interaction of chromyl fluoride and water. It is usually prepared by the action of concentrated sulphuric acid on solutions of a chromate, usually of potassium dichromate. Zettnow recommends that 300 grams of potassium dichromate be warmed until dissolved with 500 c.c. of water after the addition of 420 c.c. of concentrated sulphuric acid; after standing for ten to twelve hours, the liquid is poured away from the potassium hydrogen sulphate crystals that have separated, heated to 80° to 90° C., and 150 c.c. of concentrated sulphuric acid added, then water, a few drops at a time, until the chromium trioxide crystals which separate at first are just redissolved. Crystals are allowed to deposit during twelve hours, and subsequently, after concentration, they are collected upon a platinum, asbestos, or pumice-stone filter, and washed with pure nitric acid of density 1.46. The crystals are then mixed with a little concentrated nitric acid, and dried upon a porous plate, the process being repeated until the product is quite free from potassium sulphate and sulphuric acid. The potassium sulphate may also be removed as potassium alum by addition of aluminium sulphate, and the sulphuric acid separated by fusion. The crystals are freed from nitric acid by warming, preferably in a current of dry air in a tube at 60° to 80° C. Other methods of formation are by the action of concentrated sulphuric acid upon lead chromate; from barium and strontium chromates by the action of sulphuric acid or nitric acid; by the action of chlorine, or of hydrochloric acid upon silver chromate."

Next, per this link http://chromium.atomistry.com/ammonium_chromate.html , to quote:

"Ammonium Chromate, (NH4

2CrO4, is formed when a solution of chromic acid is neutralised by ammonia, and the mixture evaporated. It yields golden-yellow needles (monoclinic prisms):

a:b:c = 1.9603:1:1.2226; β = 115° 13',

of density 1.886, and soluble in water. On heating or on exposure to air they decompose, evolving ammonia, and form the dichromate:

2(NH4

2CrO4 = (NH4

2Cr2O7 + 2NH3 + H2O"

Hope this helps.

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[Edited on 2.8.13 by bfesser]

unionised - 1-8-2013 at 11:51

That would be one approach.
A less suicidally hazardous way would be to make Na2CrO7 instead of the K salt, dissolve it in water, add ammonium chloride, let the ammonium salt crystallise and then clean it up by recrystalisation.

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[Edited on 2.8.13 by bfesser]

blogfast25 - 1-8-2013 at 12:32

Yes, NaCl is much more soluble at 0 C than (NH4

2Cr2O7. A few recrystallisations would do it.

AJ: dichromate synth from Cr(III) and a real base like NaOH or KOH is much simpler than via CrO3.

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[Edited on 2.8.13 by bfesser]

AJKOER - 1-8-2013 at 14:04

I agree with the comments as CrO3 is properly described as highly toxic, corrosive, and carcinogenic although remains more popular in older textbooks being a powerful oxidizer (readers be wary).

However, assuming one has to start with the Potassium salt due to availability, my concern (perhaps unfounded, just based on experience working with NH4 salts) centers around the tendency of ammonia to readily form double salts (Potassium ammonium...), hence the more direct (and unsafe) approach employing Chromic acid. Having access to a Sodium salt would obviate this argument.
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My personal manner to prepare (still unsafe) is to add aqueous H2C2O4 to nearly any available dichromate salt, filter out the insoluble oxalate or, more safe, just pour out the clear solution into an excess of aqueous ammonia. Let evaporate and expose to air and/or gently heat to form ammonium dichromate.

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[Edited on 2.8.13 by bfesser]

S.C. Wack - 1-8-2013 at 17:18

CrO3 is relatively benign IME. It's no benzidine.

An abstract in JCS 88ii, 707 (1905) kind of suggests that either salt is OK:
Potassium or sodium dichromate (1 mol.) is heated in aqueous solution with ammonium chloride (2 mols.) until the liquid assumes a deep garnet red colour. After evaporating to half its bulk, the solution deposits in one or two days deep garnet-red needle shaped crystals of ammonium dichromate,

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[Edited on 2.8.13 by bfesser]

12AX7 - 1-8-2013 at 19:55

Outside chance the K2Cr2O7 - (NH3

2SO4 system might work. K2SO4 is lower solubility than the other three possibilities, but it's not much less than K2Cr2O7, so it may be a challenge to crystallize (for example, it may exhibit a eutectic sort of behavior as the solvent evaporates).

Doing it at high pH (chromate) should be successful, as K2CrO4 is quite soluble.

Tim

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[Edited on 2.8.13 by bfesser]

No K2Cr2O7 crystallising from solution

Bezaleel - 2-8-2013 at 02:09

Maybe this is a good moment to share an experience in my attempt to synthesize CrO3.

CrCl3 soluiton was treated with NaOH solution, and the precipitate vacuum filtered, washed once and vacuum filtered again (coarse fritte, as huge size as available, to reduce filtering time).

Cr(OH)3 lumps were put in a beaker and bleach added through a filter (to keep as much residue out as possible, bleach was at least 1 year old). Soda lye solution was added, and the mixture stirred on a magnetic stirrer untill all the Cr(OH)3 had dissolved. Liquid temperature was about 60 C. A strongly coloured yellow solution of sodium chromate was obtained.

The solution was left to evaporate on the hotplate (liquid temperature was about 40 C) until half the volume had been obtained (bleach used was around 5% solution). Next the pH was lowered with HCl solution to around 3. A colour shift towards orange was observed near pH = 6. Take care here, to put some cottonwool in the beaker while allowing the HCl solution to drip in drip by drip under stirring (don't heat!!!) Excess chlorine and CO2 absorbed from the air in the previous steps were released; cottonwool is imperative to absorb aerosols containing hexavalent chromium. (Keep your environment clean, and the air in particular.) Note that there is a considerable heat-up in this step (40 to 50 C temperature raise observed).

Finally a concentrated solution of KCl was added to the solution, expecting K2Cr2O7 to crystallise out. This did NOT happen. Instead, after continuing evaporation at 40 C liquid temperature, NaCl and/or KCl crystallised out in square crystals.

If you look up the respective solubilities of Na2Cr2O7, K2Cr2O7, NaCl, KCl, NaClO3, KClO3, Na2CO3, and K2CO3, the (molar) solubility of K2Cr2O7 is by far the lowest of all these (liquid temperature is around 25 C). Why the K2Cr2O7 doesn’t crystallise out is a mystery to me.

The final step I wanted to do was to add H2SO4 to the crystallised K2Cr2O7.

Further steps
This solution has been standing around for half a week now, but nothing has changed. I think I will add NaOH until the yellow colour reappears, and then add BaCl2 solution to it. BaCrO4 is as insoluble as BaSO4, so this should work. In order to get conversion of Ba(OH)2 to the chromate, stirring the heated solution may be a good idea.

I wonder whether I may obtain a solution of CrO3 when adding a stoichiometrc amount of H2SO4 to the BaCrO4. A solution of CrO3 will do in my case.

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[Edited on 2.8.13 by bfesser]

plante1999 - 2-8-2013 at 04:04

Finally something still very easily available in Canada that is outlawed in other country, don't get me wrong, gennerally stuff you can't get easily in Canada are available throughout the world. Anyone with 150 CAD can buy 2.5 kg of one or other dichromate, and be shipped. When I say everyone, it include minors...

I can only think to one way from K dichromate to NH4 dichromate. Make K chromate with KOH and add a soluble solution of strontium salt. Filter the strontium chromate. Boil a solution of ammonium bisulphate with the strontium chromate and filter while hot to get the dichromate crystals . However I can think fast to a way to turn chromite ore/Cr2O3 into NH4 dichromate.

Melt calcium/barium nitrate/sodium (barium works better) at high heat and add the chromium source througly melt for an hour and dissolve the solubles, decant and add ammonium salt of your choice (not sulphate) and boil till crystals form (ammonia gas will escape) . Put it the cold to crystalise the dichromate.

Reference: My personal work.

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[Edited on 2.8.13 by bfesser]

blogfast25 - 2-8-2013 at 04:48

@Tim: at high pH that's gonna stink!

@Bezaleel: highly unusual, indeed! But I think adding a concentrated solution of KCl was a mistake. Just acidifying with HCl would yield the dichromate, without an ocean of KCl... Your bleach also turns to chloride, you just seem to have far too much Cl^- in there!

@Plante: hmmm... not sure I'd want to get barium involved here: it'll be a job and a half to fully recover it. I think Cr source + NaOH + Na(ClO3,NO3

+ heat, leaching + HCl + NH4Cl might stand a good chance though, because acc. wiki's solubility table Na2Cr2O7 is highly soluble even cold.

[Edited on 2-8-2013 by blogfast25]

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[Edited on 2.8.13 by bfesser]

plante1999 - 2-8-2013 at 05:03

No need for NaOH. Only bare nitrate digest the chromium oxide. It directly make dichromate when done this way.

Antiswat - 2-8-2013 at 07:42

i know a guy in the UK who sells ''lead chromate'' i then realised that it wasnt chromate, but DICHROMATE
i think it was 6£ for 200g
looks dangerously alot like bacon powder (=
i believe converting it into the lead salt could perhaps be a good idea as most lead salts arent very soluble, PbCl2 (if considering NH4Cl) is 0.67/100 mL @ 20°C

(NH4

2CO3 could be used for this then..

Pb(Cr2O7

2 + (NH4

2CO3 > PbCO3 (s) + NH4Cr2O7

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[Edited on 2.8.13 by bfesser]

blogfast25 - 2-8-2013 at 08:59

Quote: Originally posted by Antiswat

i know a guy in the UK who sells ''lead chromate'' i then realised that it wasnt chromate, but DICHROMATE

No, it's very likely to be lead chromate (PbCrO₄), which is highly insoluble and a slightly surplus-to-requirements yellow pigment.

Dichromates exist in equilibrium with chromates:

Cr₂O₇^2- + H₂O < === > 2 CrO₄^2- + 2 H⁺

This equilibrium shifts to the right in alkaline conditions.

But because PbCrO₄ is so insoluble, adding Pb²⁺ to a dichromate also pulls the equilibrium to the right by precipitation of PbCrO₄.

[Edited on 2-8-2013 by blogfast25]

Quote: Originally posted by plante1999

No need for NaOH. Only bare nitrate digest the chromium oxide. It directly make dichromate when done this way.

Without any alkali?

Here's quite a decent page about chromates from Cr(III)/alkali/nitrate or chlorate fusion:

http://webpages.charter.net/dawill/tmoranwms/Chem_Chromate.h...

... although I don't see the need for KCl if you use KOH instead of NaOH.

Would 'bare nitrate' not be quite wasteful in nitrate? Any textbook mentions alkali + oxidiser although at high temp/forcing conditions air seems to work too.

[Edited on 2-8-2013 by blogfast25]

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[Edited on 2.8.13 by bfesser]

woelen - 2-8-2013 at 12:34

Heavy metal dichromates usually are much more soluble than heavy metal chromates.

Using BaCrO4 one can indeed make solutions of CrO3. I once tried adding H2SO4 to a suspension of BaCrO4 in water. This produces an orange solution and a white suspension of BaSO4.

You have an equilibrium:

BaCrO4 <--> Ba(2+) + CrO4(2-)
2CrO4(2-) + 2H(+) <-->>> Cr2O7(2-) + H2O (mostly to the right)

This equilibrium is driven to the right by means of the following equlibrium:

Ba(2+) + SO4(2-) ---> BaSO4

I never tried with PbCrO4. I am afraid that PbCrO4 is so insoluble that this does not convert to dichromate on addition of H2SO4, but I never tried personally.

plante1999 - 2-8-2013 at 17:27

It depend if you value nitrate. Dichromate can be more valuable depending on the case.

2 KNO3 + Cr2O3 --> K2Cr2O7 + 2 NO

Dirrecly dichromate, and my personnal work prove this too.

[<a href="u2u.php?action=send&username=bfesser">bfesser</a>: rxn. eqn. cleanup]

[Edited on 3.8.13 by bfesser]

12AX7 - 2-8-2013 at 21:14

As for synthesis, I find simply cooking chrome oxide with a chlorate suffices. Base is unnecessary, and if you use a slight excess of KClO3, a source of potassium is not required (of course). Some HCl, Cl2 and O2 is released in the reaction; perform it outdoors. Obviously, if you use NaClO3, you get the soluble sodium salt, which is more amenable to further reactions, like precipitating the ammonium salt.

My experience is with chlorates, but I'm sure nitrates work just as well. Anyone with Cr2O3 and NaNO3 could give it a shot and see if it works.

Tim

plastics - 3-8-2013 at 01:45

Fairly straightforward synthesis of CrO₃ ('Preparation 62'). Tried it myself. Remember not to use standard filter paper to isolate the crystals!

http://www.sciencemadness.org/talk/viewthread.php?tid=6116

blogfast25 - 3-8-2013 at 06:42

<del>@bfesser: what does 'split thread' mean? Split into what and where?</del>

@plante and 12AX7: that's really interesting. The only snag is that chlorates and nitrates will become increasingly difficult to obtain in the EU, due to more chemophobic legislation being brought on.

@12AX7: the link I brought up higher up is one of your pages, is it not? Yet it prescribes alkali?

[Edited on 3-8-2013 by blogfast25]

[<a href="u2u.php?action=send&username=bfesser">bfesser</a>: del, off-topic question answered <a href="viewthread.php?tid=25472">elsewhere</a>]

[Edited on 3.8.13 by bfesser]

plante1999 - 3-8-2013 at 07:56

Blogfast, I have tried a 1/1 mix of sodium/calcium hydroxide, chromium source and air, it take a while, but eventually all dissolve.

blogfast25 - 3-8-2013 at 08:31

Quote: Originally posted by plante1999

Blogfast, I have tried a 1/1 mix of sodium/calcium hydroxide, chromium source and air, it take a while, but eventually all dissolve.

What's the idea with the slaked lime? Higher melt temperature?

1/1 molar or 1/1 w/w?

[Edited on 3-8-2013 by blogfast25]

plante1999 - 3-8-2013 at 09:07

Quote: Originally posted by blogfast25

Quote: Originally posted by plante1999

Blogfast, I have tried a 1/1 mix of sodium/calcium hydroxide, chromium source and air, it take a while, but eventually all dissolve.

What's the idea with the slaked lime? Higher melt temperature?

1/1 molar or 1/1 w/w?

[Edited on 3-8-2013 by blogfast25]

1/1 molar, to cheapen the cost and increase Oxygen absorption, in industry I believe pure calcium oxide is used because they have very high temp furnace.

blogfast25 - 3-8-2013 at 11:46

Any idea how the lime increases oxygen absorption? Or any references that confirm that?

[Edited on 3-8-2013 by blogfast25]

plante1999 - 3-8-2013 at 11:54

Peroxide formation maybe?

I will try to find a reference.

blogfast25 - 3-8-2013 at 13:28

These roastings of chromite/chrome sources with alkali in air are all done in rotary kilns: that points to very long residence times of the reagent mixture in the furnace.

One industrial process I found uses Na2CO3 (ash), Chromite (Fe/Cr oxidic ore) and Dolomite (presumably a carrier - increases specific surface area of the mix) in a cement kiln. After roasting the 'klinker' is then ground and water leached for NaCr2O4. Acidification with CO2 under pressure and cooling drops out bicar and leaves a Na2Cr2O7 solution.

[Edited on 3-8-2013 by blogfast25]

Antiswat - 3-8-2013 at 13:46

Quote: Originally posted by blogfast25

Quote: Originally posted by Antiswat

i know a guy in the UK who sells ''lead chromate'' i then realised that it wasnt chromate, but DICHROMATE

mmm well i cant say anything else but that its pure orange, in which shocked me abit when i looked up pictures of lead chromate, in which is yellow alike any other chromate
but what i have 200g of is orange
i dont see how you can miscolour lead dichromate, or well lead chromate, or why
im quite sure it is lead dichromate, he doesnt always really give many fucks about the titles he smacks on the bags he sells with chemicals and heatsealing

12AX7 - 3-8-2013 at 21:30

Quote: Originally posted by blogfast25

@12AX7: the link I brought up higher up is one of your pages, is it not? Yet it prescribes alkali?

'Tis; but only the first run got pictures.

Going straight to dichromate is pretty neat, the melt is thermochromic and looks something like blood lava, cooling to orange-red as it freezes. I suppose it's about as healthy as that sounds, too.

Tim

blogfast25 - 4-8-2013 at 04:53

@Antiswat:

Colour is a poor guide when it comes to chemical powders. How we perceive it depends on granulometry, purity, lighting and other factors. Ferric oxide ranges from ochre to almost black! Individuals also perceive colour slightly differently. Young people's vision often stretches a bit deeper into NIR than older people's.

No references to PbCr2O7 can be found by chem powerhouses like Merck, Sigma etc etc. One source mentions a complicated route to it and describes the product as red crystals. See http://chromium.atomistry.com/lead_dichromate.html . Assuming it really does exist then it would hydrolyse to PbCrO₄ immediately on contact with water, as per what was explained above.

Considering that "he doesn't always really give many fucks about the titles he smacks on the bags he sells with chemicals", ergo on balance I think your product is lead chromate, not lead dichromate. Only elemental analysis for Cr can give the ultimate answer of course.

@12AX7: so you've kept the best to yourself!

[Edited on 4-8-2013 by blogfast25]

[Edited on 4-8-2013 by blogfast25]

Antiswat - 28-8-2013 at 10:31

hmh hmh..
i must draw pictures now!

http://people.bu.edu/straub/courses/demomaster/images/bloodp...

i can read PbCr2 clearly, then assuming that the rest would be O7, as in PbCr2O7, this has the EXACT same colour as my product does

this however is stated to be PbCrO4

http://upload.wikimedia.org/wikipedia/commons/thumb/8/84/C.I...

http://upload.wikimedia.org/wikipedia/commons/thumb/d/da/Lea...

http://en.wikipedia.org/wiki/Lead(II)_chromate

colours sometimes amaze me, i downright collect colourful liquids in bottles and store them away from UV to pull out sometimes and amaze myself of what i have seen through time of chemical colours (:
something that really blew my mind was something as simple as copper citrate being deep blue solution, i had it in a yellow plastic container, and this turned it visually DEEP GREEN!
when i poured out the copper citrate solution with crystals to boil to a solid i did get confused for a minute

i would like to test it, but im just so not glad about handling Cr2O7 in such fine powders, and especially when its lead dichromate..

blogfast25 - 28-8-2013 at 17:06

Photo 2 and 3 are clearly, at least going by colour, PbCrO4 ('Chrome Yellow')

Photo 1 is a darker product but that and a half legible label really doesn't constitute evidence of anything at all. It could be PbCr2O7 but I wouldn't bank on it... Only analysis can be the arbiter here.

What could be worth trying is to take a bit of the top material and suspend it in a bit of water. Then add some alkali to it: it should turn yellow, according to the principle explained higher up.

[Edited on 29-8-2013 by blogfast25]

Natures Natrium - 22-10-2013 at 09:26

I've been messing around with oxides of chromium. I used the high temp NaOH/NaNO3 method of oxidation. The reaction was done about 6 months ago, but just recently worked up, so I am largely working from memory.

Source of chromium was chromium dioxide obtained as chromium green for pottery purposes. Source of NaOH and NaNO3 was Drano Professional Strength Crystals, as recommended by a member (I believe it was plante).

A cast iron pot for lead smelting was used. Propane fueled bunsen burner was used as the heat source.

The melt itself was actually quite fascinating to watch. Slowly churning bright green and very hot caustic mud is the easiest description. Occasionally it would form a crust on top that had to be broken (easily) with an iron file; the file was also used as a stir stick at intervals. I would have left the crusts to contain the oxygen, but as the melt frothed under the crust it would create a foam that lifted the crust up to the edge of the pot (and over a bit, in one instance). Whenever the churning slowed down, more of the Drano was added, and the oxygen evolution would pick up again. This was continued for several hours.

A couple of interesting points:
1. The aluminum turnings contained in the Drano mixture sat at the bottom, and largely went through the entire process unreacted. That is some passivization, I would have thought that molten hydroxides would have annihilated the aluminum, given the effect of aqueous solutions of hydroxides on the metal.

2. Just adding more nitrate instead of the Drano mixture also would have been better, but the only nitrate I have available is potassium nitrate, and the only hydroxide is the sodium salt. This makes the Drano mixture convenient.

3. Towards the end, the melt was even more interesting to watch. As the mud churned, occasionally it would briefly bring to the surface nearly perfectly translucent crystals of bright yellow sodium chromate, sometimes half a centimeter long. I would imagine that someone with deft hands, iron tweezers, and a lot of patience could obtain very pure chromate salts this way.

4. I had originally intended to produce the sodium and potassium versions of both chromate and dichromate. However, in the interest of time and ease of crystallization, I decided to go with only sodium chromate and potassium dichromate.

5. At all times, gloves and eye protection. Fume hood was on.

Now on to a question: What do you guys use to clean up and reduce excess higher oxides of chromium? What is the industry standard?

I have been using lemon juice with vitamin C tabs dissolved in it. It appears to do a good job of reducing the chromium compounds, but I'm aware that color is not exactly the best indicator of a compounds oxidation state. Thoughts?

The orange mud is the potassium dichromate. It was recrystallized twice from water, but is still very wet. May hang a 'teabag' of dessicant inside the vial. Yellow is crude sodium chromate, and represents about 1/3 of the chromate recovered from this reaction. The green is more chromium dioxide than I will probably ever have a use for.