Sciencemadness Discussion Board

Calcium and chromate

TmNhRhMgBrSe - 2-8-2019 at 06:13

I put calcium carbonate into k2cr2o7 to make cacro4. caco3 don't dissolve. I put na2co3 into k2cr2o7 to make k2cro4. na2co3 dissolve and solution become yellow. Why don't the caco3 dissolve like na2co3?

walruslover69 - 2-8-2019 at 08:39

If you don't have an understanding of basic solubility rules, you definitely shouldn't be working with chromates/dichromates

TmNhRhMgBrSe - 2-8-2019 at 08:39

I put some calcium nitrate into slightly acidified k2cro4 to make cacro4. some precipitate formed but solution become very orange. Why?

TmNhRhMgBrSe - 2-8-2019 at 08:44

Quote: Originally posted by walruslover69  
If you don't have an understanding of basic solubility rules, you definitely shouldn't be working with chromates/dichromates
Fuck, wrong word. I should say "why don't caco3 react with k2cr2o7 to form co2 gas?"

vmelkon - 2-8-2019 at 12:53

Quote: Originally posted by CELamCheng  
I put some calcium nitrate into slightly acidified k2cro4 to make cacro4. some precipitate formed but solution become very orange. Why?


It depends on the amount of acid, but when acidity is increased, the dichromate converts partially to trichromate which has a deep red color.
I think this is the reaction

Cr2O72- + CrO42- + 2 H+ ->Cr3O102- + H2O

TmNhRhMgBrSe - 3-8-2019 at 07:38

I added a few drops of acid into the chromate so it should have a few % dichromate before adding excess calcium nitrate. All chromate should precipitate as cacro4. The solution should contain some remaining dichromate. Is that acidic enough to form trichromate?

vmelkon - 3-8-2019 at 18:38

Probably not. According to this video, you need a very acidic solution to make trichromate
https://www.youtube.com/watch?v=Db1PsI_YXQM

TmNhRhMgBrSe - 21-8-2019 at 05:53

Quote: Originally posted by CELamCheng  
I put some calcium nitrate into slightly acidified k2cro4 to make cacro4. some precipitate formed but solution become very orange. Why?
I think I know why now. The calcium nitrate has some remaining nitric acid. I found out this by adding this solution to some new calcium carbonate.

MrHomeScientist - 21-8-2019 at 06:51

Good job sticking with it and performing your own testing! It's very rewarding to figure out problems like this.