chromate sol.

[akcapr]

Adding h2o2 to a solution of ammonium dichromate causes it to turn black- no ppt I dont think. Adding HCl causes it to turn brownish (originally orange). Whats happening??

[woelen]

Adding h2o2 to a solution of ammonium dichromate causes it to turn black- no ppt I dont think. Adding HCl causes it to turn brownish (originally orange). Whats happening??

You get a peroxo-complex. I did some research after these complexes. Have a look at this page from my site. Especially the section on chromium complexes may be interesting for you:

 

http://woelen.scheikunde.net/science/chem/exps/peroxo/index.html

 

Your complex most likely is Cr(NH3)3(O2)2, which also is described on the page, mentioned above.

 

If you want to see a really beautiful complex, then dissolve just 1 or 2 small crystals of ammonium dichromate in 50 ml of water, such that the liquid is pale yellow, add 1 or 2 ml of dilute (10%) hydrochloric acid and also 1 or 2 ml of 3% H2O2. Do not use higher concentrations, then the result will be less interesting. This beautiful blue complex also is described on my website:

 

http://woelen.scheikunde.net/science/chem/solutions/cr.html

 

Look at the section on chromium in the +6 oxidation state.

 

If I were you, I would try to get some potassium dichromate instead of ammonium dichromate. K2Cr2O7 is more interesting, because the ammonium ion interferes with many aqueous experiments, while the potassium ion is inert. The potassium salt also allows you to do some very interesting syntheses of beautiful complexes of chromium.

 

PS1: If you experiment with dichromate salts, please be aware that dichromate is a suspect human carcinogen. So, to be on the safe side, avoid any contact with your skin, but also avoid inhalation of very fine crystalline dust.

 

PS2: On my site, the formation of an orange gaseous hexavalent chromium compound (CrO2Cl2) is mentioned. Do NOT attempt this with ammonium dichromate! Use either potassium dichromate or sodium dichromate for that experiment! This is a serious warning for your safety!