JJay
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Shortcut Lab Preparation of 2 2/3 M Chromic Acid Reagent from Sodium Dichromate
199 grams of technical sodium dichromate dihydrate was added to a media bottle. The weighing dish was rinsed 3x with a small amount of distilled water
(~30 mL total) and the washings were added to the bottle. An additional 100 mL of distilled water was added to the media bottle and it was shaken
until nearly all of the sodium dichromate dissolved. The dissolution of sodium dichromate is endothermic, and the outside of the bottle became cold.
289 grams of 90.5% sulfuric acid were then slowly added, generating considerable heat. Towards the end of the addition, a voluminous precipitate
formed, probably chromic anhydride. The funnel and flask used to add the sulfuric acid were rinsed and the rinsings added. The solution was diluted to
just under 500 mL. Two layers were visible in the bottle, a lower layer that contained the undissolved chromic anhydride, and an upper layer
consisting of more dilute solution. An egg-shaped stirbar was added, but though stirring was evident, it could not effectively agitate the mixture.
The bottle was swirled, causing the upper layer to gradually grow until it filled most of the liquid portion of the bottle, and then the last of the
precipitate was dissolved under magnetic stirring, resulting in a homogeneous, cherry-red solution. The solution was then diluted to just under 500 mL
and allowed to cool.
![IMG_20170624_203313[1].jpg - 84kB](http://www.sciencemadness.org/talk/files.php?pid=486340&aid=60005)
Notes:
Next I plan to remove and rinse the stirbar before diluting as close as possible to 500 mL and labeling the bottle. I have no plans to use the
solution for volumetric work, and an excess of Jones reagent is typically used, so high precision is not necessary. I can easily get two significant
figures by simply diluting to the bottle markings. If more precision is required, carefully dried and measured reagents should be mixed in volumetric
glassware, and I would consider more dilute solutions of potassium dichromate.
I read Jones' original paper on his reagent, discussed by S.C. Wack here, before writing up this procedure: http://www.sciencemadness.org/talk/viewthread.php?tid=24744 The solution prepared here is clearly not exactly the same thing the reagent that
Jones specified in his paper, but it's very similar. Is there anywhere this solution cannot be substituted for Jones' reagent?
It looks like Jones used 2.67 moles of chromic anhydride (which he referred to as "chromic acid") in 3 moles of sulfuric acid, diluted to one liter.
It's possible to make chromic anhydride from sodium dichromate, but it's definitely easier to use sodium dichromate and additional sulfuric acid.
I have noticed that often, the exact concentration of Jones reagent is not specified in procedures in the literature. When concentration is specified,
it appears that the most popular is 3 M, with some procedures calling for 2.75 M.
It might be technically more accurate in some contexts to state that the solution contains NaHSO4, but most of the time, it's reasonable to consider
this formulation as a ~2.67 M Jones reagent with sodium and sulfate spectator ions. Right?
[Edited on 25-6-2017 by JJay]
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AvBaeyer
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JJay,
Keep in mind that Jones reagent is usually used in conjunction with acetone as a cosolvent. I have no reason to suspect that your preparation will not
work but addition of acetone will likely precipitate the sodium sulfate species perhaps complicating work up. Best thing is to just give it a try.
Nice write up,
AvB
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JJay
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Sounds like a plan.
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JJay
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I used some to oxidize some benzyl alcohol to benzoic acid at 25 C. I have read that doing this in the cold with insufficient oxidizer can create
benzaldehyde in moderate yield, but the reaction is said to be finicky, and it's very unlikely that anyone would inadvertently make a lot of
benzaldehyde that way if trying to make benzoic acid. I haven't finished working it up yet, but it smells like benzoic acid (the smell is a little
hard to describe... it's mild and kind of like gummy bears). This did make a huge mess of gummy green salts that smell mildly of almonds and probably
entrap a trace of benzaldehyde, which will go in my trivalent chromium salt waste container....
At this point, I have three chromium waste containers, one for chromic acid (for making potassium dichromate or perhaps chromium peroxides, especially
if I can find some cheap bipy), one for trivalent chromium salts (for making chromium hydroxides), and one for chromium hydroxide (for making
chromates).
[Edited on 30-6-2017 by JJay]
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JJay
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I was thinking about Jones oxidation again today, and I don't see how I came up with the numbers I used in my earlier writeup (perhaps I was just
being lazy), but they are in the ballpark... I think the correct ratio should be:
373.78 grams sodium dichromate dihydrate (2 2/3 moles)
555.78 grams sulfuric acid (5 2/3 moles)
dilute to 1 L to prepare 2.67 M reagent
I have found that this solution is almost utterly ineffective at cleaning glassware. It seems to function reasonably well in Jones oxidations, but it
also creates a large volume of gummy precipitate. I'm pretty sure most of that is chromium sulfate 18-hydrate, but there is undoubtedly a significant
amount of sodium sulfate in it as well.
I tried making a 3M version and had an extremely difficult time getting everything to dissolve. I had to heat the volumetric flask in a water bath
then dump it into a larger bottle then rinse it out and add slightly more water, defeating the entire purpose of using a volumetric flask. While it
wasn't a complete disaster, trying to make the 3M reagent with sodium dichromate wasn't as much fun as I had been expecting, so I will probably stick
with 2.67 M next time.
One of these days I am going to have to recycle all of my chromium waste, hopefully back into sodium dichromate....
[Edited on 9-7-2018 by JJay]
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