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chromium compounds

sasan - 5-3-2014 at 07:57

Hi every one.recently I searched for making chromium salts from dichromate from sceincemadness but nothing
I Know how to make chrom alum,but this is a double salt.the separation of chromium sulfate from pottasium sulfate is the question.there is a way to adding ammonia to gain the Cr(OH)3.the trouble is this hydroxide contains the inert trioxide of chromium and would not dissolve in nitric or hydrochloric acid,ofcourse the hydroxide would dissolve but it is not economical at all in my country Iran and if I want to buy the chromium salts,the american and the germanian are just exist in iran and I should buy them by pay a high price!!
Recently I found the chromium sulfate with a low price,but it is fairly soluble in water.I think it is the anhydrous form of it according to the wikipedia but my sulfate was green powder and the wiki states that the anhydrous is reddish brown!
I want you guys to tell how to separate the chromium sulfate and potassium sulfate from the solution of reducted dichromate by sulfuric acid and ethanol and guess what is my mysterious green compound?
please post your suggestions.sorry for the bad english language

Metacelsus - 5-3-2014 at 08:07

Are you trying to make a chromium(iii) or chromium(vi) salt?

The green color is due to chromium(iii). Chromium(vi) is orange.

sasan - 5-3-2014 at 08:19

chromium 3 is the target.separate the chromium sulfate from the potassium sulfate solution and obtain the Cr2(SO4)3.n H2O
I know the green color is due to chromium 3 but it is not soluble in water just a little

sasan - 5-3-2014 at 08:29

I ask this question from an expert of chemistry lab in my college but he said I dont konw how to separate the chromium sulfate from pottasium sulfate solution!! fool!!I dont know how he is a professor and working in the lab!!

elementcollector1 - 5-3-2014 at 09:19

Seriously? Lighten up! Just because he's a professor doesn't mean he knows the answer to whatever question you might ask. This isn't a matter of general knowledge of chemistry, it's more a required specific knowledge of chromium and its salts.

Your 'mysterious green compound' is likely chromium sulfate hydrate, hexahydrate if I remember correctly.

I'm not sure I'm remembering this right, but can't double salts be 'broken' via acidification?

MrHomeScientist - 5-3-2014 at 10:32

I don't quite understand why precipitating chromium hydroxide, filtering, washing, and redissolving in sulfuric acid would not work for you. This should separate insoluble chromium hydroxide from soluble potassium hydroxide and return you to a solution of hydrated chromium sulfate alone. I haven't tried this myself so I can't say from experience.
Wikipedia states "heating chromium(III) sulfate leads to partial dehydration to give a hydrated green salt," and based on other information it seems to me that the full hydrate is reddish brown, partial hydrate is green, and anhydrous is purple.

sasan - 6-3-2014 at 00:34

last night I put some of that green powder in a glass,add a lot of water,didnt dissolve.I add some calcium chloride to the solution because I thought that if that powder was chromium sulfate,then it would react with CaCl2 in the solution:Cr2(SO4)3(s)+3CaCl2(aq)=2CrCl3(aq)+3CaSO4(s)
today I saw that some white powder(CaSO4) and a lot of unreacted green powder was in the bottom of glass and the solution was deep green maybe due to the CrCl3 solution.(probably the green powder is the chromium sulfate because of the creation of white powder at the bottom of glass,maybe calcium sulfate that is not soluble in water)
But...but why its not soluble in water if it is sulfate??

blogfast25 - 6-3-2014 at 06:29

Sasan:

I think you should really start all over again because something isn't quite right here.

Reduce your potassium dichromate with sulphuric acid and ethanol or methanol. Allow to cool. Add strong ammonia SLOWLY until pH is about 8 - 9. The precipitate formed is Cr(OH)3.nH2O. Filter it off and wash it with copious amounts of water.

This hydroxide, if you've done things properly, will dissolve in any strong, dilute acid to form a solution of the corresponding Cr (III) salt. A little gentle heat may be required.

All this is straightforward and should pose you no particular problems whatsoever.

[Edited on 6-3-2014 by blogfast25]

sasan - 7-3-2014 at 01:29

Ok adding ammonia is the best path to Cr salts reacting the hydroxide with acid
I put some aluminium powder in the reducted solution of dichromate,because I thought that it can make chromium and the Al goes to solution instead of Cr.but after a day all of Al was at the bottom of reducted solution.is it possible Al goes to the solution instead of Cr3+?
At the end I think adding ammonia is the best selection and obtain the hydroxide
Please post your suggestions.sorry for bad language

woelen - 7-3-2014 at 03:18

Chromium is very peculaliar and unfortunately things are not as easy as blogfast25 writes. His suggestions can be followed, but great care must be taken.

Heating a solution of a chromium(III) salt in many cases leads to problems. Chromium(III) in such cases forms complexes with anions which are very hard to break and may take weeks or even months to break again, even in the presence of ammonia or hydroxide.

Reduction of the dichromate with ethanol or methanol must be done very carefully, with cooling and good stirring. If the solution becomes warmer than appr. 40 C, then a green sulfato-complex is formed, which is amazingly hard to crystallize and amazingly hard to break down. If the reduction is done, such that the liquid remains cold, then you end up with a dark purple/grey solution and from that you can prepare quite pure Cr(OH)3 with ammonia. If the solution is green, then the sulfato-complex is formed.

blogfast25 - 7-3-2014 at 06:11

Woelen, I don't dispute what you say and maybe I've just been 'lucky' but I've never encountered these problems, having extracted Cr(III) quite a few times. Perhaps using acetic acid (instead of sulphuric) to effectuate the reduction of dichromate solves that problem for once and for all?

I do have a sample of Cr alum solution that I overheated and months later there's still no sign of it crystallising.

[Edited on 7-3-2014 by blogfast25]

sasan - 8-3-2014 at 07:51

seems that every body have problems with chromium,woelen is right,chromium make such a strong complexes that Al powder cant reduct the Cr(3+) to form elemental chromium
If I use nitric acid instead of sulfuric,can Al goes to the solution instead of Cr(3+)?maybe there would no any strong complexes of chromium with the sulfate anions
Another question is that the reaction of HF(aq) and the K2Cr2O7 can release flourine gas?like reacting with HCl(aq) that releases chlorine vapors.if no then what would be happen?or no reaction occurs???

woelen - 8-3-2014 at 07:59

HF and K2Cr2O7 do not react. Fluorine is one of the strongest chemical oxidizers and no common chemical is capable of oxidizing fluoride ion to fluorine gas. Not even the strongest available oxidizers like perxenates or peroxodisulfate or ozone can make fluorine from fluorides.

chornedsnorkack - 9-3-2014 at 00:00

Quote: Originally posted by sasan

Another question is that the reaction of HF(aq) and the K2Cr2O7 can release flourine gas?like reacting with HCl(aq) that releases chlorine vapors.if no then what would be happen?or no reaction occurs???

Something may happen depending on the HF/H2O ratio.
HF is notorious for being a weak acid when dilute yet very strong when concentrated. Whereas H2Cr2O7, while a strong acid is not very strong, and nowhere close to H2SO4 or concentrated HF.
First step:
K2Cr2O7+2HF<->2KF+H2O+2CrO3
Second step:
CrO3+2HF<->CrO2F2+H2O

What are the respective HF/H2O ratios where the first and the second reaction go to right?

Chemical synthesis of fluorine is notoriously not straightforward. It can be done, but in two steps not one. And the oxidant is manganese, not chromium.

Failed attempt at hexaamminechromium(III)

Brain&Force - 9-3-2014 at 15:49

I attempted to produce the compound hexaamminechromium(III) sulfate using chromium sulfate and household ammonia. Instead, I got a bluish-grey precipitate that doesn't redissolve on the addition of excess ammonia. From what I've read chromium complexes tend to be non-labile and substitution of the coordinate sphere occurs only slowly.

What other interesting coordination compounds of chromium can I make in a school lab?

DraconicAcid - 9-3-2014 at 16:34

Quote: Originally posted by Brain&Force

I attempted to produce the compound hexaamminechromium(III) sulfate using chromium sulfate and household ammonia. Instead, I got a bluish-grey precipitate that doesn't redissolve on the addition of excess ammonia. From what I've read chromium complexes tend to be non-labile and substitution of the coordinate sphere occurs only slowly.

What other interesting coordination compounds of chromium can I make in a school lab?

If you want it to work, you need to reflux the chromium(III) sulphate in ammonia with a catalytic amount of zinc. Chromium(III) complexes are very non-labile; chromium(II) complexes are labile and will substitute quickly.

Brain&Force - 9-3-2014 at 18:25

I had never thought of making a chromium(II) complex - I'll try that instead. I'll also see if I can plate chromium metal using a chromium(III) solution. I know most industrial processes use chromates instead, but I just want to figure out if it's possible.
And I'll also try to make a Cr(OH)₆^3- complex with NaOH.

sasan - 10-3-2014 at 07:45

chornedsnorkack states that: First step:
K2Cr2O7+2HF<->2KF+H2O+2CrO3
Second step:
CrO3+2HF<->CrO2F2+H2O
the reaction of chromyl chloride with water at first makes the blue complex of chromium oxyperoxide then decomposes to Cr(3+) with bluish green tint
Chromyl flouride like chromyl chloride decomposes in the water to Cr(3+) maybe chromium flouride

Brain&force,see handbook of "Handbook of preparative inorganic chemistry"VOLUME 2-second edition edited by Gerog barauer,there you will see a lot of chromium complexes in its all states specially in the (3+).It will be very helpful for you to make complexes of chromium,search it in google and download it

blogfast25 - 10-3-2014 at 09:51

I'm running a few experiments to check whether complexed Cr3+ can be reduced with Al or not. Quite surprising results so far. But more tests are needed.

sasan - 11-3-2014 at 02:33

I think Al (like Zn) can reduce Cr3+ into Cr2+.not to the elemental chromium,but Im not sure

blogfast25 - 11-3-2014 at 05:50

Quote: Originally posted by sasan

I think Al (like Zn) can reduce Cr3+ into Cr2+.not to the elemental chromium,but Im not sure

Check the Electrochemical Series: 'naked' Cr3+ is reduced by Al to Cr (0) with volts to spare (so to speak). But so far I've not seen any reduction. More tests to follow 'soon'.

[Edited on 11-3-2014 by blogfast25]

blogfast25 - 15-3-2014 at 09:53

Here are some results from attempts to reduce Cr(III) with aluminium.

Firstly a bit of chromium alum solution was prepared at room temperature (RT) and to half of it was added some ammonium sulphate and some heat: the solution turned dark green. On cooling it didn’t revert back to the original colour.

Some of that solution, as well as some of the cold Cr alum solution, was loaded into separate test tubes and a piece of aluminium sheet, lightly sanded, added to each. This is what that looked like but bear in mind strong colour distortion due to flash (left is green solution, right is blue solution):

After over a week (second photo), no change was noted in EITHER of the tubes, clearly no plating of Cr(0) had taken place (more about the third tube below). I have to conclude tentatively that in BOTH cases the chromium is complexed too strongly to be reduced by Al.

I then went on to prepare a solution of Cr(NO3)3 as follows. The required amount of KCr(SO4)2.12H2O was dissolved in about 400 ml of water, at RT and the Cr precipitated as Cr(OH)3.nH2O with the equivalent amount of 33 % NH3. The precipitate was filtered and the filter cake washed several times on a Buchner and sucked dry.

Quantitatively transferred into a large beaker, it was dissolved in the required amount of 70 % HNO3, yielding a deep blue solution which was diluted to about 250 ml. The molarity of the Cr(NO3)3 solution was about 0.35 mol/L. The deep blue is of course caused by the [Cr(H2O)6]3+ complex ion.

The third tube in the second photo is that solution (slightly diluted), with a piece of Al added, to see if reduction take place. Nitrate ions aren’t suitable ligands for Cr(III). So far (a few hours) no change has been observed.

A couple of simple tests were carried out too. Two tubes loaded with the solution, slightly diluted, were heated on steam bath. No change in colour was observed. To the first (left in the picture below) was then added a good pinch of ammonium sulphate, to the second (centre in the picture below) ammonium chloride. Third tube is Cr(NO3)3 + aluminium.

Left (sulphate) and centre (chloride) tubes changed colour to deep green quite quickly:

This suggests ligand exchange.

Another interesting observation was made by adding an excess 33 % NH3 to the two green tubes. The one with the suspected sulphate complex turned mauve immediately (but not all precipitate has dissolved yet). The one with the suspected chloride complex first precipitated a green precipitate, somewhat later this then turned mauve too. The mauve is due to the Cr(III) ammonia complex and these results seem to suggest the strength of complexation is NH3 > Chloride > Sulphate > H2O.

[Edited on 15-3-2014 by blogfast25]

sasan - 16-3-2014 at 01:42

I told you that it would not reduce with Al,just to the Cr2+ that is blue,for certainty if you add sodium acetate then red chromous acetate will be form but if no precipitate of acetate occurs,so Al cant reduce Cr3+ even to the 2+
blogfast try reducing with Mg,but I think maybe it would react with water to from hydroxide in precence of other ions.
I should purchase Chromium powder,treating with hydrochloric to form chromous chloride,add Al to see what will be form
I`m not professional in electrochemistry but I think electrolysis the solution of Cr3+ with Al/Cr(s) electrodes can be useful.sorry for bad english

blogfast25 - 16-3-2014 at 05:47

Sasan:

If complexation is the cause then reduction to Cr(II) won't work either. I'll check my tubes with acetate later on. Both green tubes didn't go blue though.

Last night I also tried Mg (with an even greater oxidation potential compared to Al) but it didn't seem to work: after chewing up the acid reserve in the solution, Cr(OH)3 started precipitating.

sasan - 17-3-2014 at 08:10

Chromium like his uncle(!!)manganese is somewhat complicated

I think Mg would not react with water only in the presence of other ions in water to form Mg(OH)2 and because of this Cr(OH)3 precipitated instead of Mg(s) + Cr(3+) = Cr(s) + Mg(2+),otherwise magnesium is much more electropositive than chromium
wikipedia:An aqueous solution of a Cr(III) compound is first reduced to the chromous state using zinc.[5] The resulting blue solution is treated with sodium acetate, which results in the rapid precipitation of chromous acetate as a bright red powder.

2 Cr3+ + Zn → 2 Cr2+ + Zn2+
2 Cr2+ + 4 OAc- + 2 H2O → Cr2(OAc)4(H2O)2

The synthesis of Cr2(OAc)4(H2O)2 has been traditionally used to test the synthetic skills and patience of inorganic laboratory students in universities because the accidental introduction of a small amount of air into the apparatus is readily indicated by the discoloration of the otherwise bright red product
Producing chromous acetate wont be easy because of air oxidation,watch here:www.tulane.edu/~inorg/.../Experiment%203.pdf‎

blogfast25 - 17-3-2014 at 13:48

There's a thread here somewhere on chromous acetate, from potassium dichromate reduced by Zn in the presence of acid.

It seems strange then that Zn, by far the weaker of the three reducing agents (Mg > Al > Zn, acc. electrochemical series), can at least reduce Cr(VI) to Cr(II).

Tomorrow I'll test my tubes for Cr(II) but I'm not hopeful: even the blue tube of Cr(NO3)2 + Al strip hasn't lost any colour intensity but it has turned dark green! I suspect this may be a pH effect: the Al will have reacted with any acid reserve in there, thereby increasing the OH- concentration, so that there may now be some OH ligands on the Cr3+.

I'll also try and reduce the Cr(NO3)2 with Zn powder and some HCl. The HCl should complex the Cr to the green complex, so if Zn does reduce the Cr(III) chloro complex, I should see a colour change to blue. Then I can test with acetate...

[Edited on 17-3-2014 by blogfast25]

woelen - 17-3-2014 at 23:55

Do you have anything else than the nitrate? Nitrate ion is oxidizing and zinc will reduce the nitrate in acidic solution. If you have chromium chloride or sulfate (or chrome alum), then I would try with that for a better chance of success.

I once made a solution of Cr(2+) (it's on my website) but this is amazingly air-sensitive. Even in strongly acidic solution, it quickly turns dark green when exposed to air.

blogfast25 - 18-3-2014 at 02:04

Quote: Originally posted by woelen

Do you have anything else than the nitrate? Nitrate ion is oxidizing and zinc will reduce the nitrate in acidic solution. If you have chromium chloride or sulfate (or chrome alum), then I would try with that for a better chance of success.

Ooopsie. Hadn't thought of that. But I've got the alum. Will test Zn, Mg and Al in comparable circumstances.

Thanks!

Brain&Force - 18-3-2014 at 16:24

I'll be trying the reduction of chromium(III) sulfate with magnesium. (I had already tried it, but I left it unmonitored and all I got was a clear solution with a bluish-gray powder on the bottom.)

On a side note, I have some chromium sulfate, but it's green, not purple, and dissolves to give a green solution (but not very easily). It was purchased in 1984 if that helps (yeah, the chemicals are that old at my school).

sasan - 19-3-2014 at 09:21

Brain and force I have chromium sulfate that solve in water just a little and make the solution light green,if your sulfate is like mine(it is light green not deep green such as chromium chloride ),then it would not reduce with Mg or either Al,but test it I'm not sure
I search this insoluble sulfate in handbook of chemicals,maybe it is chrome cake because it stated that it is insoluble form of chromium sulfate and made by fusing sodium sulfate and chromium oxide(calcined form that is insoluble in acids and alkalies) followed by dissolving sodium hydroxide in water and collecting insoluble chromium sulfate,again I'm not sure
chromium chemistry is really complicated,because of this chromium(3+)salts are more expensive comparing to cooper and nickel and just sold by big famous chemical suppliers like sigma aldrich

blogfast25 - 19-3-2014 at 11:17

I prepared a 100 ml of 0.35 M chromium alum stock solution and tried to reduce it in various ways.

20 ml was mixed with 10 ml HCl 37 w% and chips of Al and Mg added. In both cases the blue solution turned green and stayed green. No reduction was observed.

I then tried zinc. I tried clean battery zinc, pellets of good purity zinc and even very pure fine powder. I never obtained proof positive of any reduction. Adding some of the solution to some glacial acetic acid or the other way around never precipitated any red Cr (II) acetate dimer.

I also reduced some potassium dichromate with a pellet of zinc and some HCl 37 w% and it reduced to a green solution very quickly but I never observed any blue.

I’m somewhat baffled that Al and Mg can’t reduce the complexed Cr(III): the complexation constants must be very large or something else is in play.

B&F:

My (now spent) Cr(III) sulphate was also green and of a very strange texture: like green wool.

[Edited on 19-3-2014 by blogfast25]

S.C. Wack - 19-3-2014 at 12:13

Quote: Originally posted by blogfast25

Reduce your potassium dichromate with sulphuric acid and ethanol or methanol.

Or perhaps hydrogen peroxide.

Quote: Originally posted by blogfast25

I then tried zinc. I tried clean battery zinc, pellets of good purity zinc and even very pure fine powder.

Perhaps contamination with a little mercury would be helpful.

blogfast25 - 19-3-2014 at 13:33

Quote: Originally posted by S.C. Wack

Perhaps contamination with a little mercury would be helpful.

There's a published procedure (by Science Squirrel, IIRW) somewhere on this board for preparing Cr(II) acetate with mossy zinc and HCl. It doesn't call for any mercury. But I'd try it if I had any.

Here:

http://www.sciencemadness.org/talk/viewthread.php?tid=18216#...

Re. reduction of dichromate with H2O2: works very well but it does have the disadvantage that any excess, remaining H2O2 will start oxidising the Cr(III) back to chromate, in alkaline conditions (for instance to precipitate Cr(OH)3).

[Edited on 19-3-2014 by blogfast25]

S.C. Wack - 19-3-2014 at 14:47

There is chromous literature using zinc both with and without mercury or acid. An interesting quote which the authors go on to demonstrate in Anal Chem 8, 188 (1936) (thx Russians. here, have Crimea.): "A chrome alum solution that has been freshly prepared, that is made up in dilute sulfuric acid solution, and that is held at room temperature or below, is violet in color and rapidly reduced by the zinc amalgam in a Jones reductor. If the solution is allowed to stand, if acid is not added, or if the solution is heated, there is a shift to the green color and the rate of reduction is less."

blogfast25 - 20-3-2014 at 05:53

Quote: Originally posted by S.C. Wack

There is chromous literature using zinc both with and without mercury or acid. An interesting quote which the authors go on to demonstrate in Anal Chem 8, 188 (1936) (thx Russians. here, have Crimea.): "A chrome alum solution that has been freshly prepared, that is made up in dilute sulfuric acid solution, and that is held at room temperature or below, is violet in color and rapidly reduced by the zinc amalgam in a Jones reductor. If the solution is allowed to stand, if acid is not added, or if the solution is heated, there is a shift to the green color and the rate of reduction is less."

S.C.:

That confirms the role of complexation. Strange how the electrochemical series (values for CrIII to Cr0 or CrIII to CrII) doesn't seem to reflect that... CrIII seems to be complexed almost always, in some way or another.

Crimea to the Crimeans, as far as I'm concerned.