Chemical Forums
Chemistry Forums for Students => Analytical Chemistry Forum => Topic started by: Hunt on December 23, 2007, 12:16:00 PM
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In aqueous solution Barium chromate ppt has a yellow color. If a small amount of diluted acetic acid is added , the ppt should remain , but does its color change ? If yes , Why ?
Thanks for any help
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CrO42- exists in equlibrium with Cr2O72- in an aqueous solution.
2 CrO42− + 2 H3O+ ⇌ Cr2O72− + 3 H2O
By decreasing the pH, you shift the equilibrium to the right, Cr2O72− is orange in color. So the precipitate should become more orange. You can find the percent decrease in yellow ppt by using the Keq and Ksp to find the percent increase in Cr2O72− . In this situation knowing the pH before and after the addition of the acetic acid is the most important consideration. The change in the amount of Cr2O72− may be too negligible to detect a change outside of a photodetector.
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Cr2O72− is orange in color. So the precipitate should become more orange.
Do ypou know what the barium dichromate Kso is? Especially when compared to barium chromate Kso?
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Yah thanks for your input Rabn. I had found something similar here (http://www.carlton.srsd119.ca/chemical/equilibrium/dichromate/dichromate3.htm) . What perplexes me though is that the manual im reading in mentions something about adding few drops of acetic acid to the barium chromate ppt and observing how the color of the ppt changes.
Few drops of acetic acid wont cause any serious change and the ppt should not change in amount or color. But suppose that enough acetic acid is added so that the pH is sufficiently low and much of the ppt dissolves, it is the color of the solution that changes not the ppt! There's no formation of any new ppt ( barium dichromate is soluble in water ). Isn't that what you implied, borek ?
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I did a lot of searching for a value of the Ksp of the dichromate and could not find it. It is easy to calculate if you have the concentration CrO4(-) from using both the Keq and Ksp(chromate) and Ksp(dichromate); without any numbers it is a little difficult to get an idea of the magnitude of the Ksp(dichromate)(I'm assuming that is what you meant when you typed Kso). Keq(chromate/dichromate)=9.23*10^-2 Ksp(barium chromate)=1.17*10^-10. The key to solving for the Ksp of dichromate is using a mass balance equation for the amount of chromium available. That allows you to solve for [BaCr2O7] in terms of the other species whose concentrations can be calculated with Keq and Ksp of chromate. I still submit that the amount of insoluble barium dichromate will be relatively small so unless the [H(+)] is very high. So you would need to use a spectrophotometer to detect the change in color.
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Isn't that what you implied, borek ?
More or less. I have no idea what is barium dichromate solubility, but it is completely different salt from barium chromate. Chromic acid protonation in low pH (it is a weak one, especially when it comes to the 2nd dissociation step) and dichromate creation removes chromic ions from the solution, effectively dissolving chromate. Could be it is replaced by dichromate, honestly I don't know.