Ceric Ammonium Sulphate: attempted preparation

I’ve attempted to prepare ceric ammonium sulphate (CAS) - (NH4)4Ce(SO4)4.2H2O (CAS 10378-47-9) by means of a brute force fusion, with stoichiometry:

CeO2 + 2 H2SO4 + 2 (NH4)2SO4 = (NH4)4Ce(SO4)4.2H2O

For that purpose 10.0 g of reagent grade CeO2 (a light yellow, very fine powder), 7.7 g 2 (NH4)2SO4 (the stoichiometric amount) and 7 ml of 95 % H2SO4 (approx. twice the stoichiometric amount) were mixed in a 250 ml RBF and an air cooled refluxer was fitted to it. The reagent mix was a viscous slurry and had already heated up significantly during mixing.

The RBF was heated on an oil bath for about 20 minutes, starting at RT and ending at about 250 C (bath temperature). The run was ended because a striking colour change from a light yellow paste to a reddish brown cake (appearing quite dry and immobile) had taken place.

This was then allowed to cool, then force cooled on ice bath. Slowly about 35 ml of cold DIW was added and much of the cake dissolved to form an amber looking liquid, presumed a solution of CAS with excess H2SO4 and (NH4)2SO4. It was clear though that much of the initial CeO2 was still present as unreacted oxide. This was separated from what was presumably ceric ammonium sulphate solution by filtering. The unreacted amount of CeO2 will be determined tomorrow.

The last bits of this reddish cake/crust was dissolved in a bit of hot water and added to the filter. This is the perfectly transparent solution obtained (still during filtering), about 40 ml at the end of filtering:



The same solution during boiling: it appears to be slightly thermochromic, it is darker coloured at BP than RT:



At about 25 ml during boiling in, the first turbidity was observed and boiling was stopped (beaker was taken off the heat). After a bit of cooling, the beaker was ice cooled. A crop of rather non-descriptive small red/amber crystals of presumably CAS was obtained.

Of the original solution (pre boiling) about 1 ml has also been transferred into a test tube. Three drops of 35 % H2O2 were needed to reduce all the Ce(IV) to Ce(III) and thus a colourless solution resulted. To that some K2SO4 solution was added and the white and poorly soluble potassium cerous sulphate double salt precipitated, this typical is of REs (III).

To follow:

• Yield determination
• Second attempt with larger amount of H2SO4 and longer time/higher temperature.



[Edited on 15-6-2013 by blogfast25]

Hey there, blogfast!
Cool idea!, if those are the crystals to pick doesn't it comes as a homogenous goopy mass of the material??
anyway I think you should try higher temperatures with less solvent, and czochrawski it off, I put my pets it won't work out pretty good fella

In the mean time, there’s been a development. When I went back to check on the ice bath and crystals later on last night, the crystals had redissolved completely. On heating they reappeared. So this material has an inverted solubility-temperature dependence (like many RE sulphates) and this can be used to isolate/recrystallise the substance.

I can also confirm its thermochromic nature: solutions darken noticably when heated and I believe this might explain the inverted solubility-temperature dependence, by means of structural changes at higher temperature, affecting both colour and solubility.

This is the first crop of crude CAS (still wet) obtained exploiting the poor solubility at BP (bottom of a 250 ml beaker):



Lovely vibrant colour!

Quote: Originally posted by platedish29

[…], and czochrawski it off, I put my pets it won't work out pretty good fella

Quote: Originally posted by elementcollector1

I'd like to see how much ceria went unreacted

Quote: Originally posted by smaerd

Neat blogfast, did you create this procedure yourself or was it based on any literature?

The unreacted CeO2 of the first run weighed in at about 6.7 g, so barely 33 % actually dissolved. Shockingly bad, really…

The second run was as follows: 14 ml of conc. H2SO4 and 2 hours of runtime, starting at RT and ending at 266 C.

This time I actually got a bit of bubbling from start to finish. Here’s the RBF after cooling:



After chilling, 50 ml of DIW was added on ice bath. Again unreacted CeO2 was prominently present. But is does look less. Actual numbers tomorrow.

After filtering - solution strength is in the order of about 0.5 M Ce⁴⁺:



The same solution when boiling:



I believe the colour change must be due to two different structures being in equilibrium:

CAS, yellow/amber ↔ CAS, amber/red

If ΔH = Hamber/red - Hyellow/amber > 0 (endothermic), heating would then push it to the right. ‘Amber/red’ could then also be less soluble than ‘yellow/amber’. The first crystals crop had also lightened considerably overnight.

The boiling CAS solution started bumping badly after the first turbidity showed, even with boiling pebbles, so it’s now evaporating on steam bath.

Czocrawski? I’ll have to look it up.

But getting decent crystals may prove difficult…

I’m also a bit baffled at the colour of Ce (IV). The electron configuration of Ce (0) is:

[Xe] 4f² 6s² (others claim [Xe] 4f¹ 5d¹ 6s² but that doesn't really change anything here)

That would indicate that Ce (III) would have one lone 4f electron (and thus be coloured) and Ce (IV) none, which would make it colourless. But reality is the other way around...



[Edited on 15-6-2013 by blogfast25]

Quote: Originally posted by blogfast25

Czocrawski? I’ll have to look it up.

Quote: Originally posted by 12AX7

Also, is the solid salt a single form, or does it have different allotropes? Tim

Quote: Originally posted by watson.fawkes

I have to wonder if all the ammonia you added is still in the reaction mixture. You may have significant amounts of cerium (IV) sulfate. It's decomposition point is 350 °C, per Wikipedia. A melting point test would give you some idea of what you've got.

Quote: Originally posted by blogfast25

You don’t have an explanation for the Ce (III)/Ce(IV) colour conundrum I presented above? Today I'm running a test WITHOUT ammonium sulphate, so that might shed some light on it too.

It could be informative to slowly recrystallize the substance at low and high temperatures, in order to grow crystals of an observable size (and also because large crystals of anything are cool ). The habit should be different if the crystals are different, and thermochromism of the crystal will be more easily observed than in a polycrystalline mass. Well, maybe you have (or can make) a microscope with a heated stage -- in any case, recrystallization is of course a good idea for purity if nothing else.

I would be very surprised if ammonia were present as a complex: boiling sulfuric acid was used! It would have to be a very strong complex indeed (which I suppose isn't impossible), but still, it would be obvious from the formula if ammonium is not present. An ammonium complex is impossible, as far as I know.

How does ammonium sulfate behave? Does it ultimately decompose to N2, H2O and SO2 at, what, like 400C+? Most ammonium salts sublimate, which is a reversible reaction (perhaps with NH3 + H2SO4 in the gaseous phase for this one). I would expect similar behavior here, leaving CeO2 behind in addition.

Tim

Quote: Originally posted by 12AX7

I would be very surprised if ammonia were present as a complex: boiling sulfuric acid was used! It would have to be a very strong complex indeed (which I suppose isn't impossible), but still, it would be obvious from the formula if ammonium is not present. An ammonium complex is impossible, as far as I know.

The unreacted CeO2 from run #2 was only 0.75 g, so about 90 % dissolution had been achieved.

The products obtained from both runs are substantially different in colour (left: #1; right: #2):



The third run was as follows: 10.0 g CeO2, no ammonium sulphate, 22 ml of conc. H2SO4, 1 hour runtime.

The amount of non-digested CeO2 has yet to be determined but seems negligible going by filter cake. After filtering:




The same solution near BP:



This was then boiled in for what felt like an eternity, when bumping started and a large crop of material dropped out over the course of about 1 minute:



This lightened a lot on cooling.

A small sample of this product was dissolved in iced water, it dissolved completely.

Tests using Ce(OH)4:

The crops of ‘CAS’ products of run #1 and #2 were combined and dissolved in cold water and neutralised with 0.89 SG NH3, ceric hydroxide precipitated. This was then copiously washed with hot water:



Dissolution tests with this ceric hydroxide proved it dissolves easily in hot 50 % H2SO4, hot HNO3 70 % and hot HCl 37%. In all cases similar looking solutions were obtained and all were thermochromic.

Here is a Ce(NO3)4 solution at RT:



In the case of hot HCl, the dark red slowly fades to colourless, and chlorine could be detected:

2 Ce(IV) + 2 Cl(-I) == > 2 Ce(III) +Cl2(0)

This reaction was fairly slow. It's a fairly wasteful way of preparing CeCl3 solutions.


So, ceric sulphate, ceric ammonium sulphate, ceric nitrate and ceric chloride solutions all show the same colour and thermochromism. This strongly suggests the thermochromism is more or less independent of sulphate ions, nitrate ions, chloride ions or ammonium ions.

One final observation: the wet Ce(OH)4 responds slightly to UV light… like Nd(OH)3



[Edited on 16-6-2013 by blogfast25]

Quote: Originally posted by woelen

Nice results. Your observation confirms the colors of the chemicals I have. Without ammonium ions you have a bright yellow compound, with ammonium ions you get the orange/red stuff.

But run #1 AND run #2 both were prepared with ammonium sulphate present!

Also, the colour of the solutions (and their thermochromism) seem totally indifferent to the type of anion. Some more experiments tomorrow.

I think this CeO2 may contain small amounts of another RE, not sure which though. Now it turns out the Ce (III) chloride has a yellow/greenish tinge, but not like ferric yellow though. And the Ce(OH)4 really does respond to saver bulb light differently than incandescent light, no question about it...

Hot glacial acetic acid didn't manage to dissolve the Ce(OH)4, so it probably isn't worth trying NH4HF2 either... I wish I had me some perchloric acid.


[Edited on 17-6-2013 by blogfast25]

Quote: Originally posted by blogfast25

I think this CeO2 may contain small amounts of another RE. [Edited on 17-6-2013 by blogfast25]

Quote: Originally posted by blogfast25

Quote: Originally posted by woelen   Nice results. Your observation confirms the colors of the chemicals I have. Without ammonium ions you have a bright yellow compound, with ammonium ions you get the orange/red stuff. But run #1 AND run #2 both were prepared with ammonium sulphate present!

Quote: Originally posted by woelen

Sorry for the misread, I understood you did one run with ammonium sulfate and the other without. It still may be that the orange stuff contains a lot of ammonia while the yellow stuff does not. You can easily test that by adding some of the solid to wet solid crunched NaOH and carefully smell the mix.

Quote: Originally posted by elementcollector1

Lanthanum, maybe?