Sciencemadness Discussion Board

Theoretical photocatalyzed oxidation of halides

plante1999 - 4-1-2013 at 07:31

I got an idea of a silver catalyzed oxidation of halide using various oxidizer. It is known that silver halides (excluding fluoride) decompose to there composing element in light. It is also known that silver surface is oxidized easily. I thought that if a bromide solution, a suitable oxidizer a small amount of sulphuric acid was in solution, and a silver piece was added in presence of light, it could be possible that silver oxidize in surface, the acid increase the solubility and the bromide do metathesis with the silver sulphate. Then light would turn the bromide to bromine and silver metal.

2Ag + 'O' -) Ag2O
Ag2O + H2SO4 -) H2O + Ag2SO4
Ag2SO4+ 2NaBr -) 2AgBr + Na2SO4
2AgBr -) 2Ag + Br2
Br2 + H2O -) HOBr + HBr
HOBr + HBr + Na2SO4 -) NaBr + NaOBr + H2SO4
3NaOBr -) NaBrO3 + 2NaBr

Overall:
NaBr + 3'O' -) NaBrO3

One could then filter to get most of his silver back, since silver halide are mostly insoluble. Take note that it would be better to use the corresponding 'Hydrohalic' acid.

Adas - 4-1-2013 at 10:03

Quote: Originally posted by plante1999  

HOBr + HBr + Na2SO4 -) NaBr + NaOBr + H2SO4


I don't think that this reaction can proceed.

AndersHoveland - 4-1-2013 at 10:50

Chlorine oxidizes bromide to bromate in the presence of sunlight. This is actually a significant problem with some water reservoirs. Los Angeles Ivanhoe Reservoir is using a creative strategy to try to solve this problem, covering their reservoir with floating black plastic balls to try to block sunlight, and thus prevent carcinogenic bromates from forming.

plante1999 - 4-1-2013 at 13:01

Quote: Originally posted by Adas  
Quote: Originally posted by plante1999  

HOBr + HBr + Na2SO4 -) NaBr + NaOBr + H2SO4


I don't think that this reaction can proceed.


It is equilibrum, and there is no need for sulphuric acid, the need is for an acid.

Hexavalent - 4-1-2013 at 13:06

Quote: Originally posted by AndersHoveland  
Chlorine oxidizes bromide to bromate in the presence of sunlight.


What favours this as opposed to chlorine oxidizing bromide to bromine? Theoretically, as chlorine is a more powerful oxidizing agent, it would capture an electron from the bromide anion, oxidzing it to a neutral bromine atom and producing a chloride anion.

elementcollector1 - 4-1-2013 at 13:33

Quote: Originally posted by Hexavalent  
Quote: Originally posted by AndersHoveland  
Chlorine oxidizes bromide to bromate in the presence of sunlight.


What favours this as opposed to chlorine oxidizing bromide to bromine? Theoretically, as chlorine is a more powerful oxidizing agent, it would capture an electron from the bromide anion, oxidzing it to a neutral bromine atom and producing a chloride anion.

Entirely true. I don't think chlorine and bromide can make bromate.

Hexavalent - 4-1-2013 at 13:44

Actually, thinking about it, it may be possible....would the photolytically-induced oxidation be caused by a radical mechanism? If so, just how does sunlight act as a "catalyst"?

AJKOER - 4-1-2013 at 14:06

Quote: Originally posted by AndersHoveland  
Chlorine oxidizes bromide to bromate in the presence of sunlight. This is actually a significant problem with some water reservoirs. Los Angeles Ivanhoe Reservoir is using a creative strategy to try to solve this problem, covering their reservoir with floating black plastic balls to try to block sunlight, and thus prevent carcinogenic bromates from forming.


Some discussion of reaction paths may aid some clarity. First,

2 NaBr + Cl2 --> 2 NaCl + Br2

Cl2 + H2O <--> HOCl + HCl

Now, per Watts' Dictionary of Chemistry, HOCl can oxidize I2 (also S and P and I would believe Br2) all the way to Iodate. For Bromine, I would similarly expect:

Br2 + H2O <--> HBr + HOBr

HOCl + HBr <--> BrCl + H2O ( see http://homepage.uibk.ac.at/~c724117/publications/voegele02-j... equation 2)

HOCl + BrCl --> Cl2 + HOBr (see http://infoscience.epfl.ch/record/33222/files/EPFL_TH2746.pd... equation (4.6.12) )

3 HOBr --uv-> 2 HBr + HBrO3

and the bromate is formed. The references provided will also provide some insights on process.

Note also:

HBr + BrCl --> HCl + Br2 ( see http://www.deepdyve.com/lp/american-institute-of-physics/a-q... equation R2 )

As previously noted: HOCl + HBr <--> BrCl + H2O, so

HOCl + 2 HBr --> HCl + Br2 + H2O


[Edited on 5-1-2013 by AJKOER]