Sciencemadness Discussion Board

Easy preperation of dichromate

theAngryLittleBunny - 2-11-2017 at 08:54

So, I needed some dichromate to oxidize an alcohol to a carboxylic acid, so I decided to make some today, and I found a suprisingly easy way to make it.

I decided to make sodium dichromate by reacting chromic oxide with sodium nitrate:
Cr2O3 + 2NaNO3 -> Na2Cr2O7 + 2NO
I think it's worth noting that the chromic oxide I used was kinda impure, since I got it from recycled chromium waste, and probably had a lot of sodium carbonate. Anyway, I used 30g od the chromic oxide and assuming that it was pure (which it wasn't), I used a stocheometric amount (34g) of sodium nitrate. I melted the sodium nitrate in a steel can, and started to slowly add the chromic oxid, which fizzed like if you put baking soda in vinegar, and then dissapeared completely, while the melt became an orange colour. The first half of the addition was pretty quick, and the melt remained liquid and clear. Above the can, I saw some slight brown fumes, which suggests that NO is formed, which oxidized in air to NO2. Towards the end, the reaction slowed down, and the reaction mix became really dark orangy black and kinda syrup like (looked pretty scary actually O.o). The syrup like consistency might be because the sodium carbonate contermination made some sodium chromate, which has a really high melting point of about 600 to 700°C, but that's just my wild guess.

Right now, I'am letting the melt cool down, in theory, this should be pure sodium dichromate (which it obviously isn't), but let's see how high the yield is. The theoretical yield as potassium dichromate is about 58g.

Anyway, this seems like a really nice reacton, because it doesn't use any alkali hydroxide, and you eliminate the danger of foaming over, since you add the chromic oxide in small portions.

Velzee - 2-11-2017 at 09:50

I believe the reaction produces sodium chromate, not sodium dichromate:

8 NaNO3 + 2 Cr2O3 + O2 ---> 4 Na2CrO4 + 8 NO2

You'd have to add a strong acid to the resulting product in order to produce dichromate:

2CrO4(2-) + 2H(+) ----> Cr2O7(2-) + H2O


[Edited on 11/2/2017 by Velzee]

[Edited on 11/2/2017 by Velzee]

Velzee - 2-11-2017 at 10:04

Therefore your theoretical of NaCrO4 yield is about 32.4g, and 21g of Na2Cr2O7.

theAngryLittleBunny - 2-11-2017 at 15:21

Quote: Originally posted by Velzee  
I believe the reaction produces sodium chromate, not sodium dichromate:

8 NaNO3 + 2 Cr2O3 + O2 ---> 4 Na2CrO4 + 8 NO2

You'd have to add a strong acid to the resulting product in order to produce dichromate:

2CrO4(2-) + 2H(+) ----> Cr2O7(2-) + H2O


[Edited on 11/2/2017 by Velzee]

[Edited on 11/2/2017 by Velzee]

If this would be the case, there would have been A LOT of brown fumes, but the brown fumes were barely visible. And at the end, the mix was still somewhat molten, if it would have been sodium chromate, it would be a yellow solid, but it was a dark orange liquid, which solidified to an orange solid when cooling. In your reaction equasion, you added oxygen from the air, but the reaction was way too fast as that air would have played a role in that. I know, dichromate is acidic, and it might seem confusing that you can make something acidid from neutral starting chemicals. But acidic compounds can also arise from oxidation, like sulfur turns to acidic sulfur dioxide when burned, phosphorus to acidic diphosphorus pentoxide, and so on.

Boffis - 3-11-2017 at 06:56

Its better to add sodium hydroxide too. This not only helps reduce the melting point of the mixture, it also reduces the amount of sodium nitrate used and the gas evolved is then only nitrogen. Use sulphuric acid to neutralise the excess alkali and acidify to precipitate the dichromate.