Sciencemadness Discussion Board - colour of ferric nitrate

colour of ferric nitrate

CHRIS25 - 13-5-2012 at 01:13

You know, typing this into google is about as useful as an ashtray on a motorbike in a force 8 storm along the beach! Not only to you get everything unrelated to ferric nitrate but when you finally hit some images they are all the colours of the rainbow.

So, yes I would be grateful if someone could tell me please a) what colour should ferric nitrate in solution appear and b)what colour as a precipitate/solid. I thought blackish/brown but google's multi-coloured talent show put paid to that when I started seeing yellow to reds...

Thanks gentlemen.

Vargouille - 13-5-2012 at 06:26

The wikipedia says that it's a light violet crystal in nonahydrate form.

After checking out the CRC, it says that the anhydrous solid is colorless, the hexahydrate is violet, and the nonahydrate is violet-gray. In solution, the iron should make the solution a red color.

[Edited on 13-5-2012 by Vargouille]

barley81 - 13-5-2012 at 07:03

That is not true. Iron (III) ions are actually very pale violet in solution (very nearly colorless). When they hydrolyze (which is very easy), they turn a yellowish color. In acidic solution (i.e. dilute nitric acid) iron (III) is nearly colorless, while a solution of ferric nitrate in distilled water is yellow and turbid.

http://woelen.homescience.net/science/chem/solutions/fe.html

CHRIS25 - 13-5-2012 at 07:17

Yes I had already checked out woelens excellent site and after 30 hours the solution is very dark reddish brown and the iron has not really dissolved. I guess I must have Ferric Hydroxide at the moment. Do not know where to go with this experiment. Have read so much but really most of it is pure theory which does not help and apart from woelens photos I have no other guidelines for how to bring this to a successful conclusion.

barley81 - 13-5-2012 at 07:29

Did you use hydrogen peroxide/nitric acid to dissolve the iron?

CHRIS25 - 13-5-2012 at 07:38

No I decided not to use any H2O2, partly because I only have 6% and really do not want to add more water to the solution. It was just 6grams of cut up cast iron and 89.3 mils of 30% nitric which I then heated up - the beaker inside boiling water which was inside a stainless steel tea can.

Vargouille - 13-5-2012 at 10:02

Barley, I have a solution of FeCl3 that's a really dark red, and I used to have a solution of FeCl2 that was a light yellow color before it got oxidized. Looking on my little sheet of ion colors, it says that Fe+3 solutions range from orange to red depending on the anion present.

EDIT: After looking over Woelen's site, I see my mistake, with the caveat that in nearly every solution of Iron (III), it isn't clear. The way to get pure Fe(NO3)3 is probably to just leave the solution to percolate until it does turn clear.

[Edited on 13-5-2012 by Vargouille]

barley81 - 13-5-2012 at 10:31

FeCl3 is dark red because iron (III) forms a variety of chloro-complexes in solution. The iron (III)-aquo complex is very nearly colorless, and hydrolyzes to form yellowish species (which are not nearly as dark as the chloro-complexes of iron).

EDIT:
OK, sorry woelen, it's not red. It's yellow/brown.

[Edited on 13-5-2012 by barley81]

woelen - 13-5-2012 at 11:23

FeCl3 in solution is yellow/brown, even at very high concentration. If it is red, then it contains quite some Fe(OH)3 or mixed ferric chloride/hydroxide.

Solid FeCl3.6H2O has a mustard color and solid FeCl3 is black and somewhat shiny, like iodine crystals, but even darker.

CHRIS25 - 13-5-2012 at 13:07

Aaahh....I meant FERRIC NITRATE not chloride, must be because I am working on four things at once... Making corrections below..

I have then FeNO3 in solution, extremely dark but with reddish tint. Despite the fact that hardly any of the Iron has dissolved. I read some time ago that the safest way to ensure max FeNO3 was to keep it topped up with excess iron. Is this true?

[Edited on 13-5-2012 by CHRIS25]

Vargouille - 13-5-2012 at 18:57

I'm a bit confused with what you mean, Woelen. When I was making a 0.1 M solution of FeCl3 for the next year's AP students from the hexahydrate (only realizing afterwards that it would go bad), it was a distinctive orange color. This was reagent grade, mind you, so I can't imagine that there would be any significant amount of hydroxides. Do you mean that the iron hydroxide contaminant would be formed in situ to make the reddish color?

As for Chris' problem, perhaps you've run out of HNO3 in the system. Only thing I can think of, considering that a solid reagent shouldn't shift equilibrium and you still have a good deal of iron.

woelen - 13-5-2012 at 22:53

Quote: Originally posted by Vargouille

[...]Do you mean that the iron hydroxide contaminant would be formed in situ to make the reddish color?

Yes, if no acid is added, then iron(III) easily hydrolyses in aqueous solution and even more so if the solution is so dilute as you made it.

You get equilibria as follows (simplified equations, no presence of chloride taken into account):

[Fe(OH2)6](3+) <---> [Fe(OH2)5(OH)](2+) + H(+)
[Fe(OH2)5(OH)](2+) <---> [Fe(OH2)4(OH)2](+) + H(+)

Ions like the above can combine to larger ions with bridging O-atoms (through the lone pairs of electrons on these atoms) and large complicated ions may finally combine to bigh structures having net formula [Fe(OH2)3(OH)3]n, with n very large and this is hydrous ferric hydroxide.

Just as a test for you, make a solution of 0.1 M FeCl3 and then add a few drops of hydrochloric acid to it. The solution then will turn clear and beautifully yellow, no red at all. You only see the effect of chloride on the color of the solution.

Waffles SS - 14-5-2012 at 05:26

Pure Ferric Nitrate is White/Light violet but Homemade or self Laboratorymade is red/orange(due to oxidizing it to Iron(III)Oxide)

CHRIS25 - 14-5-2012 at 09:29

Quote: Originally posted by Waffles SS

Pure Ferric Nitrate is White/Light violet but Homemade or self Laboratorymade is red/orange(due to oxidizing it to Iron(III)Oxide)

Thanks, but "Homemade or self Laboratorymade is red/orange(due to oxidizing it to Iron(III)Oxide, then have I still ferric nitrate of what percent purity? After 2 hours of gentle heating I have 15ml of solution and 4.5grams of Iron still left. the Solution is a very deep brown,rusty red colour and flows now like watered down syrup. Is there anything I should do at this stage? I must admit I find this is the first reaction I have performed that seems overly complicated to understand, but I'm getting there and to know when it is finished? -this info I can not seem to work out through all the material that I have been reading.

[Edited on 14-5-2012 by CHRIS25]

barley81 - 14-5-2012 at 12:25

When ferric nitrate turns into hydrous ferric oxide, oxidation does not occur. Hydrolysis occurs.

Try pouring off the solution and adding a bit more nitric acid. This should redissolve the ferric hydroxide and reduce hydrolysis. Test it first: take a very small portion in a tube and add some nitric acid. If it turns a lot lighter, then you should add nitric acid to the solution until it also turns lighter. (The test would be a sign that the dark color is caused by presence of hydroxide).

CHRIS25 - 14-5-2012 at 14:34

OK, but all this really puts the theoretical balanced equation that I did at the start, with the stoichemetry, out the window. I mean to say that what then is the point of measuring everything, getting the equation in order when so many other things are happening at the same time? When or how on earth do I obtain ferric nitrate?

Instead of : Fe + 6HNO3 = Fe(HNO3)3 + 3H2O + 3NO2 which is not at all accurate

maybe something more akin to what is going on? like Fe + (x)HNO3 = Fe(HNO3)3.(x)H2O + 3NO2 + FeOH ?? I don't know I am still trying to learn, but what started out as a simple reaction between two ingredients turns out rather more complex than the theoretical balancing of an equation.

[Edited on 14-5-2012 by CHRIS25]

Waffles SS - 14-5-2012 at 20:48

Quote: Originally posted by barley81

When ferric nitrate turns into hydrous ferric oxide, oxidation does not occur. Hydrolysis occurs.

Try pouring off the solution and adding a bit more nitric acid. This should redissolve the ferric hydroxide and reduce hydrolysis. Test it first: take a very small portion in a tube and add some nitric acid. If it turns a lot lighter, then you should add nitric acid to the solution until it also turns lighter. (The test would be a sign that the dark color is caused by presence of hydroxide).

Yes.I Did mistake,Hydrolysis Occur(Sorry)
But i have to say according to my Experiments:even hot con.Nitric acid cant dissolve Iron(III)Oxide(its really stable)

Just possible route is reacting Iron Metal with con.Nitric acid (by water jet system)I know Chinese company that make Ferric Nitrate and sodium Nitrate(by dissolving NO2 in NaOH solution) by this method

http://www.sciencemadness.org/talk/viewthread.php?tid=17161#...

[Edited on 15-5-2012 by Waffles SS]

Neil - 15-5-2012 at 05:14

You are going to have all the carbon from the cast iron floating around in that goo as well.

watson.fawkes - 15-5-2012 at 05:34

Quote: Originally posted by CHRIS25

Instead of : Fe + 6HNO3 = Fe(HNO3)3 + 3H2O + 3NO2 which is not at all accurate

Not at all. Ferric nitrate is Fe(NO₃)₃; no hydrogen. The Wikipedia page on iron(III) nitrate has a reaction, though without citation.

Dissolving iron in nitric acid isn't as simple as it might seem on first glance. It's easier to understand if you consider that the ferric nitrate is ionic, dissociates in solution, and you can consider the product is a combination of Fe³⁺ and NO₃^-. To get to the ferric ion, you have to oxidize the iron: Fe⁰ --> Fe³⁺ + 3 e^-. Oxidizing the iron means reducing some nitrate: 4 H⁺ + NO₃^- + 3 e^- --> NO + 2 H₂O. You'll get more NO than NO₂ because your nitric acid isn't that concentrated. Add these two equations together and add three (spectator) nitrate ions to both sides and you get a balanced and neutral reaction.

The other problem I'd guess you're having is that you've got a lump of iron with relatively low surface area. Low surface area means low reaction rate, because the reaction can only occur at the solid-liquid interaction surface. If the solution is motionless, you get depletion of nitrate at the surface and it has to diffuse in, which is slow. Agitation and stirring will help. It's going to be slower than using steel wool, in any case, just because of the surface area.

I would have to guess that much of the red you're seeing is dissolved nitrogen oxides. If it's gotten syrupy, it's going to react even more slowly, because diffusion will be that much slower. Add more acid or dilute with some distilled water.

arsphenamine - 15-5-2012 at 07:38

Inspect your solution at different dilutions ranging, oh say, 10%, 20% 40% 80%.

Aqueous Fe(NO3

3 will appear in varying shades of violet, not red.

Solid Fe(NO3

3 is the nonahydrate, and also has a violet color.

If it don't, it ain't.

CHRIS25 - 15-5-2012 at 12:07

Ok Watsonfawkes and arsphenamine. Thanks. Today I filtered the solution and heated it for two hours over a meth burner. I added 20ml 30% HNO3 first, the whole solution was now 40ml. After two hours I have a colour that can be best described as deep golden brown yellow, or to put it another way the same colour as Hennesey Cognac but much darker, I compared the two side by side. I am going to be honest and say that I actually have no idea what I have other than what WatsonFawkes mentioned. The explanation you gave is very helpful, though I will need to digest it slowly.

I don't think I can take this Ferric Nitrate attempt any further, I never knew how complex a reaction it was, and Wikpedias Blundering over simplicity fooled me when it said - quote: ... the compound is prepared by treating iron metal powder with nitric acid, and in another place they said: adding nitric acid to scrap Iron" Obviously misleading.

Anyway I have learned quite a lot and unexpectedly gained some insight, but I guess I will have to buy the darn stuff.....
Kind Regards
Chris

Vargouille - 15-5-2012 at 13:57

Chris, the solution is simple: Start over with steel wool. Watson's right, lumps of the metal is no good for quick reactions. Also, for wiki's comment about adding HNO3 to scrap, I think it means concentrated HNO3 in that scenario.

watson.fawkes - 15-5-2012 at 14:37

Quote: Originally posted by CHRIS25

I don't think I can take this Ferric Nitrate attempt any further

I can think of two things you can do with this mixture, other than just tossing it out.

The first is to wait. Put the mixture in a jar, ensuring it has a vent to let evolving nitrogen oxides out, leave it outside (because you don't want those nitrogen oxides inside a building), and forget about it for a while. Your reaction is slow, not non-existent. It will eventually react to completion, though it may take a while (months?). If you want an art project out of this, make a time-lapse movie of the reaction.

The second is to sparge. Sparging is just bubbling air through a solution. Sparging can remove many dissolved gases by giving the dissolved gas somewhere to go other than the solution. At the boundary between the bubble and the solution, some of the dissolved gas will come out of solution and then be carried out with the bubble. Sparging also provides agitation. Use a long, tall vessel; introduce air at the bottom with a glass tube; a small aquarium pump should suffice. Leave it alone for a few days and see what color may change.

You can combine these two, if that's feasible. Neither, however, will give you ferric nitrate quickly.

CHRIS25 - 15-5-2012 at 15:11

Quote: Originally posted by Vargouille

Chris, the solution is simple: Start over with steel wool. Watson's right, lumps of the metal is no good for quick reactions. Also, for wiki's comment about adding HNO3 to scrap, I think it means concentrated HNO3 in that scenario.

Hi there, that is why I broke the 6 grams of Iron up into 5 chunks, but yes I suppose not even tis is really effective. However about the steel wool, I did not use this because steel contains at least 10% chromium, plus some other additives, but Cast iron is well.. Iron. EDIT, whoops, stainless steel contains chromium my mistake, but steel still is not the same as Iron, it is an alloy and that was the only reason I never used it. But if people have success with this then I will certainly try that one as well, I'll do both experiments.

Watson.Fawkes, yes I already put the solution into a jar, I hate wasting stuff thinking that one day it will have a purpose. But thanks, I have put the jar outside and loosened the top. I am glad you wrote back, I am renewed now determined to see this through. I will leave it a few days and then try and get hold of a pump and follow your advice.

By the way, as an offshoot, while you are here, I said I would keep you up to date with that Potassium Polysulphide mixture (LOS) I measured out 2.4 grams of sulphur and 1.6 of KOH, heated until melted and places in 100ml hot water. I did this three times and reduced the Sulphur to 1.5 and KOH to 0.4 grams twice, I got a good solution and was able to perfect the colours for a couple of projects, but on the chemistry side, there was always 0.3 to 0.5 grams of undissolved sulphur in the solution, the same range of amount no matter what I did. At least I can always factor this into my equations when I make a solution every week. But Nope, I can never get it to fully dissolve. Kind Regards.

[Edited on 15-5-2012 by CHRIS25]

Vargouille - 15-5-2012 at 15:48

It works for FeCl2 and FeCl3, with some amount of contamination, so I assume that it would work for Fe(NO3)3. Just a little extra work to isolate the nitrate salt from the contaminants. IIRC, steel wool is a good deal purer than structural steel, and is a low-carbon steel, so isolation shouldn't be too difficult.

Neil - 15-5-2012 at 18:27

Iron metal is soft, almost as soft as pure copper. It has a disgustingly high melting point and very little chemical resistance.

Cast Iron is a eutectic mixture of carbon and iron, it has much more carbon then steel. It may or may not have a multitude of other elements mixed up with it, Mn, P, Cr and so on.

Further more the carbon present in cast iron may be present as carbide formed by rapid cooling of the hot metal, aka white cast iron or it may be in the format of flaked graphite which gives Grey cast iron it's appearance.

The difference in carbides and flake graphite modify the reactivity of the cast iron and the corrosion resistance.

Soft iron bailing wire might be a better source, steel wool is at the very least a potentially known alloy.

CHRIS25 - 16-5-2012 at 01:56

Ok Neil. Thankyou for your extra information. I will try this one out as well and see the difference between the two reactions. Always good to learn.

barley81 - 16-5-2012 at 03:45

If you use steel wool, you will probably have to filter the solution because lots of gunk is produced when you dissolve it. At least this is the case when you use sulfuric acid. The ferrous sulfate made in that way is actually pretty pure, so I think you can get a purer product if you use steel wool.

CHRIS25 - 16-5-2012 at 06:06

Ok thankyou Barley I will be doing this one in the next couple of days.

Ferric Nitrate...well not quite

CHRIS25 - 28-5-2012 at 07:18

I apologise to the purists out there for the un-scientific methods and explanations here, it is just that I had given up on making ferric nitrate proper because of the difficulties in this reaction but not wanting to waste what I already had attempted and always willing to try new things out, so have no precise calculations for this one.

Getting inpatient after a week I decided to add steel wool to my solution of filtered dissolved cast Iron in nitric acid. As previously posted on this subject I had a clear solution of ferric nitrate in rather a lot of nitric acid still and was leaving outside indefinitely. Anyway, getting impatient because I desperately needed some ferric nitrate no matter how polluted so to speak - I decided to add steel wool to this until no more dissolved. As expected I got a very very thick Red brown solution. Was on the point of heating this up when I decided to experiment with the as is solution on copper and various heat treatments etc (metal work purposes - not chemistry), I got some very unexpected artistic possibilities from the various stages of painting the solution on the copper drying and heating with blow torch. Due to this I would like to know what could basically be in this solution besides an acidic Iron hydroxide and iron nitrate mixed, (has an exact PH of 2). And can I realistically do anything more with this "Gunge" that I now call it, in order perhaps to purify things a little further.

[Edited on 28-5-2012 by CHRIS25]

Nicodem - 29-5-2012 at 01:12

Quote: Originally posted by CHRIS25

Getting inpatient after a week I decided to add steel wool to my solution of filtered dissolved cast Iron in nitric acid.

As far as I would expect, ferric nitrate in aqueous solution should react with iron, so adding steel wool was not a very bright idea. Iron is a reducing reagent and will reduce most Fe(III) species, especially in aqueous solutions.
You could try to reoxidize the iron in the solution by using nitric acid, but the only way to do this without contaminating too much with excess HNO3 would be to titrate it at a higher temperature until full conversion is achieved.

weiming1998 - 29-5-2012 at 01:20

Quote: Originally posted by CHRIS25

I apologise to the purists out there for the un-scientific methods and explanations here, it is just that I had given up on making ferric nitrate proper because of the difficulties in this reaction but not wanting to waste what I already had attempted and always willing to try new things out, so have no precise calculations for this one.

Getting inpatient after a week I decided to add steel wool to my solution of filtered dissolved cast Iron in nitric acid. As previously posted on this subject I had a clear solution of ferric nitrate in rather a lot of nitric acid still and was leaving outside indefinitely. Anyway, getting impatient because I desperately needed some ferric nitrate no matter how polluted so to speak - I decided to add steel wool to this until no more dissolved. As expected I got a very very thick Red brown solution. Was on the point of heating this up when I decided to experiment with the as is solution on copper and various heat treatments etc (metal work purposes - not chemistry), I got some very unexpected artistic possibilities from the various stages of painting the solution on the copper drying and heating with blow torch. Due to this I would like to know what could basically be in this solution besides an acidic Iron hydroxide and iron nitrate mixed, (has an exact PH of 2). And can I realistically do anything more with this "Gunge" that I now call it, in order perhaps to purify things a little further.

[Edited on 28-5-2012 by CHRIS25]

Try adding some water to your iron nitrate. It might not all have dissolved. If you can still see chunks of Fe(OH)3 in your solution (actual chunks, not just opaque, as an aqueous Fe3+ solution is partially opaque from hydrolysis), then filter with fine filtering paper. This should get rid of most insolubles. This mix probably contains some Fe(II) nitrate, some Fe (III) nitrate, and some oxide/hydroxide.

As your purpose is metalworking, whether the compound is Fe(II) nitrate or Fe(III) nitrate or a mix of both shouldn't really matter, as they all decompose to Fe2O3 when heated (the Fe(II) oxide gets aerobically oxidized), which gives it the artistic effect (I think, correct me if I'm wrong).

CHRIS25 - 29-5-2012 at 05:00

Hi Nicodem and Welming, I don't think I will titrate, I might actually start all over and get what I originally had just to see what possibilities there are. Welming, No I have no insolubles I filtered this earlier on before working with it and there were no insolubles then. I added water and wee bit of H2O2 to make sure all was oxidized to complete the reaction that I saw, so yes you are right I probably have all those ingredients mixed in.

[Edited on 29-5-2012 by CHRIS25]

Just an after thought: Would there be an advantage to using Rusted barbed wire (or rusty nails but barbed wire has more surface area) in place of Iron, or Iron Filings? The rust is Iron Oxide and in the reaction with nitric acid I am sure that I read somewhere that rusty is preferable to clean, (I think it was another science site dealing with making chemicals for patination) Anyway I collected some rusty barbed wire, I have some iron filings, I have cast Iron (which did not really go well last time). Thanks guys for your tips.

In either case the stoichemetry would be 5.6g Iron and 126g acid(due to the fact that my acid is @ 30%) This would give theoretically 24.2g Fe(HNO3)3.

[Edited on 29-5-2012 by CHRIS25]