Sciencemadness Discussion Board

Strange colors during hydrohalic acid syntheses

ave369 - 11-12-2015 at 02:45

When I make hydrobromic acid and distill it out of solution, it is colored brownish-red, but no free bromine is observed on the bottom of the flask. I think it's the tribromide ion in action.

Now when I do the same with hydrochloric acid, the boiling solution turns yellow! What is this, trichloride?

And if we had glassware coated with diamond and could distill HF the same way, what the color would be? Pale yellow?

[Edited on 11-12-2015 by ave369]

shivam - 11-12-2015 at 06:49

While making Hydrobromic acid (or Hydrochloric acid as well), If the sulfuric acid is too concentrated, it will further oxidize the Hydrobromic acid to bromine, hence the brownish red solution.
A few of these side reactions can happen through a few different ways:

2HBr + H2SO4 --> Br2 + SO2 + 2H2O

6HBr + H2SO4 --> 3Br2 + S + 4H2O

8HBr + H2SO4 --> 4Br2 + H2S + 4H2O

The amount of free bromine formed is not usually so much that it can form separate bromine layer at bottom. A little amount of bromine is readily soluble in aqueous media.
During distillation, bromine comes over at much lower temperature than Hydrobromic acid, so contamination of bromine in distillate is inevitable (unless you do a precise fractional distillation)
The key is to avoid using too much concentrated sulfuric acid and keeping temperature under control..
Use sulfuric acid that is a bit more diluted down. Having to remove more water afterwards through distillation is always more welcomed than poor quality product.

chemrox - 11-12-2015 at 18:50

Try some less hazardous operations until you understand basics and are familiar with techniques. All the shit about LAH comes from workers not trained in chemistry.

ave369 - 12-12-2015 at 00:21

I do not use concentrated sulfuric acid for this. Standard battery acid, 36%. And my hydrobromic acid is clear as water, not even slightly yellow. Try to presume less when you want to sound pedantic. We in Russia have a good saying: "All telepaths are on a leave", said to someone who behaves like they are a telepath.



[Edited on 12-12-2015 by ave369]

woelen - 12-12-2015 at 05:19

Quote: Originally posted by chemrox  
Try some less hazardous operations until you understand basics and are familiar with techniques. All the shit about LAH comes from workers not trained in chemistry.
Why this answer? Please be helpful, or do not answer at all. Your answer is the best way to put off people and kill their interest in chemistry (or science in general) :mad:

There is nothing wrong with the question of ave369, he just wonders what causes these colors he observes and does not show any evidence of doing stupid or dangerous things without any understanding.

woelen - 12-12-2015 at 05:25

In the case of HBr, I think that the color is due to formation of some free bromine, which indeed remains in solution as Br3(-) when the concentration of bromide is high. I myself once made some 40% HBr and this also turned yellow/brown. I believe this is due to oxidation by oxygen from air. Only a small amount of free bromine can give a noticeable color.

With HCl I also noticed the formation of a pale yellow/green color, but on distillation of 22% HCl I obtained a perfectly colorless solution. I did this by distilling hardware store 30% HCl. I diluted this to 22% or so with boiled tap water and then I distilled over appr. 90% of the liquid. The liquid in the receiver was perfectly colorless and I use that sample of HCl for experiments where I want to be sure that no metal ions are in solution. The remaining 10% of liquid was deep yellow and I discarded that.

ave369 - 12-12-2015 at 05:40

At last, Woelen, the true scientist, not a snarker. I've got the same results as you. Maybe you answer the question where does the yellow-green come when I distill HCl (az.) from 36% sulfuric and table salt? Is it truly trichloride, or just a solution of chlorine?


And I'm a SHE, the last time! I wonder why Polverone ignores my request for the custom title "Eastern European Lady of Mad Science" that would stop gender confusion once and for all. Maybe you, woelen, can do that?

[Edited on 12-12-2015 by ave369]

shivam - 12-12-2015 at 06:27

Okay ave369. Sorry for trying to help..
Didn't mean to come off as pedantic....
(please understand that people learn things with time, they grow into it...which is what i am doing --- and enjoying!)

SIGH...Good day! again Sorry for wasting your time..

[Edited on 12-12-2015 by shivam]

Amos - 12-12-2015 at 09:50

Quote: Originally posted by shivam  
Okay ave369. Sorry for trying to help..
Didn't mean to come off as pedantic....
(please understand that people learn things with time, they grow into it...which is what i am doing --- and enjoying!)

SIGH...Good day! again Sorry for wasting your time..

[Edited on 12-12-2015 by shivam]


I don't think she was talking to you about being pedantic; you were nothing but informative. I think the quote was directed at chemrox.

Amos - 12-12-2015 at 09:58

Quote: Originally posted by ave369  
where does the yellow-green come when I distill HCl (az.) from 36% sulfuric and table salt?

[Edited on 12-12-2015 by ave369]


I can't speak about Russian table salt necessarily, but a common additive to table salt over here is a small amount of potassium or sodium ferrocyanide, which will react with acids to give iron(II) chloride(pale green, yellowing as oxidation takes effect) as well as a small amount of cyanide, which would be harmless in such a low concentration so long as your distillation receiver isn't your mouth.

ave369 - 13-12-2015 at 03:12

I was specially buying pure table salt from Iletsk. It surely did not contain any anti-blobbing agents, because it was in huge blobs.