TGWS - 23-4-2020 at 09:32
What did I make?
So I was following the synthesis of Potassium Chromate according to:
http://www.sciencemadness.org/talk/viewthread.php?tid=84436
wherein - and as to why I made a separate thread - instead of adding the usual H2SO4 to acidify to dichromate (I use
NaHSO4 as a substitute for Sulfuric Acid, as the latter is very hard to come by in my country), I added oxalic acid.
I have a 1 molar solution of oxalic acid, which I added dropwise to the solution of K2CrO4. Perhaps I should note that the
solution also contains { Cr2O3, KOH, Na2S2O8, and other products formed in the reaction}
I added the oxalic acid solution until the solution turned orange and stopped bubbling, but as I left it, it slowly turned red and began slowly
releasing a gas, which I believe was CO2. I'll admit I didn't test it properly, as the setup I had for this was very crude. I wasn't
expecting much to come of this reaction, just that it would turn to dichromate as expected. I do not have any stoppers with which to allow me to
bubble the gas into lime water, so I just pipetted a few drops into a solution of lime water, whereupon it began to turn cloudy. I understand this is
probably the worst way of attempting to ascertain the presence of CO2, next to breathing in copious quantities of the gas and seeing if I
asphyxiate.
I suspended the test tube that I was performing the reaction in in an ice water bath, whereupon some grey-white crystals began to form.
I did some research online, and found several papers that suggest a 'red chromoxalate', or 'Croft's Salt', with the formula
K2H2Cr2(C2O4)4(OH)2. What the papers I found did, was also add some
potassium oxalate/potassium hydrogen oxalate, as well as warming the reaction. Would the leftover KOH from the reaction be sufficient to react
with the oxalic acid to form enough potassium oxalate to facilitate the formation of this red chromoxalate?
I found an answer on Yahoo! answers that says the reaction between oxalic acid and potassium dichromate should go as follows:
7C2H2O4.2H2O + K2H2Cr2O7 =>
K2H2Cr2(C2O4)4(OH)2 + 6CO2 + 19H2O. This answer
also states the product is called 'potassium dichromoxalate', or 'Croft's Salt'.
So my main question is this: What did I make?
While the solution itself turned red, the precipitate formed upon cooling was grey-white. The primary product in the equations that I've found is
identical in formula, yet the description states it is red. If I am to accept that the solution is this 'Croft's Salt', what is the precipitate?
Was this simply a redox reaction, and while the oxalic acid was oxidised to carbon dioxide, the chromate was reduced to a deep-red complex? I'm not
aware of a chemical that matches this - but I may be very wrong
I suppose another follow-up question could be: did I actually just mess up, get some impurities in there that messed everything up, and form a
completely random compound? Am I an idiot and I misread the papers? Most likely, yes. But if I'm not, please help me.
Sorry for the ridiculously long post for what amounts to a very basic question, but I wasn't sure just how much information I should provide, and I
didn't want to ask here before doing some research at least.
woelen - 23-4-2020 at 10:58
Oxalic acid is capable of reducing chromium(VI) to chromium(III). While doing so, an oxalato-complex of chromium(III) is formed. Different such
complexes exist, and which one is formed strongly depends on pH. Colors can be nearly anything. I made green complexes, but also deep purple
complexes. At the pH's in your situation you most likely make a basic oxalato-complex. The deep red color most likely is due to a mix of orange
dichromate and deep purple oxalato-complex.
wg48temp9 - 23-4-2020 at 11:36
I was curious about Croft's salt. I found Croft's paper on it perhaps it will help you. From a quick read of the paper woelen is right there are
various colourful complexes.
Crofts paper Attachment: croft-salt-MP8410100089.pdf (284kB)
This file has been downloaded 522 times
The colour also depends on the trans cis of the oxalate. see Attachment: croft2-1691103a0.pdf (386kB)
This file has been downloaded 447 times
TGWS - 23-4-2020 at 23:42
Thank you for your replies - that makes a lot of sense.
How easy is it to obtain the complexes in crystalline form - do they decompose as solids, or is it possible to crystallise them out of solution in
such a way that does not alter the structure?
In the first paper, they mention how in their attempt to create the salt via Turner's method, they created a red granular precipitate. Of course this
was performed with the pure salt in a concentrated solution, whereas mine was very dilute. Afterwards they note that 9 times out of 10, when
attempting to recrystallise the precipitate, it formed a granular bluish-grey powder.
Are these the same compound?
To be fair, I did crystallise it out via. placing in an ice-water bath, rather than evaporating it - maybe I should try that and see which I obtain.
Amos - 24-4-2020 at 10:45
You might want to look at the experiments conducted in this paper:
Werner, E. A. (1888). XLIV.—Oxidation of oxalic acid by potassium dichromate. J. Chem. Soc., Trans., 53(0), 602–609. doi:10.1039/ct8885300602
I can't remember exact ratios for the preparation I did (I don't think it came from Werner) but years ago I intimately mixed powdered oxalic acid
dihydrate and potassium dichromate in a mortar and pestle and quickly turned the contents out into a watch glass, and the two spontaneously reacted
quite strongly and exothermically, spitting small droplets of water and eventually melting together. After cooling I dissolved the mass in a minimal
amount of hot water and after cooling I got a crop of dark red-black crystals. I seem to recall a similar experiment with hydrated sodium dichromate
gave instead a dark green syrup that was hard to form decent-sized crystals of. Sorry I don't have any notes for you.
[Edited on 4-24-2020 by Amos]
TGWS - 25-4-2020 at 07:20
That's wonderful, thank you so much. 
From what I've read, many suggest a ratio of 1 parts potassium dichromate will react with 6 or 7 parts oxalic acid. I had no idea the powders
themselves would react spontaneously - this is interesting indeed. I had glanced at that paper before, and noted how they mixed the two powders
intimately, then heated them to the point of interaction, then in an air oven at ~115°C. Would you say the reaction is regularly spontaneous, or is
the Gibbs Free Energy close enough to 0 that it could swing either way based on temperature/pressure conditions?
Boffis - 1-5-2020 at 02:38
The greyish white colour of your ppt suggests a potassium oxalate ppt. I believe the so called potassim tetraoxalate is sparingly soluble.
The so called croft's red salt is really dark red-purple and doesn't sound like your ppt. I produced this salt as a minor contaminant of my potassium
trioxalatochromate III when I didn't use enough oxalic acid. The contamination was only apparent when I examined my product under the microscope. This
was one of the problems I encountered when following Brauer's method that I mentioned in my thread on these complexes http://www.sciencemadness.org/talk/viewthread.php?tid=150143...