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Author: Subject: Particle size of chromium III hydroxide precitate
vinothan23
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[*] posted on 16-2-2012 at 02:02
Particle size of chromium III hydroxide precitate


What is particle size of Cr(OH)3 precipitates, what size(in microns) filter cloth/paper we can use to filter the precipitates without loss?

[Edited on 16-2-2012 by ScienceSquirrel]
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[*] posted on 16-2-2012 at 04:26


Can the name of this thread be any more vague?

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[*] posted on 16-2-2012 at 04:57


I recommend a 42000 micron sized piece of filter paper, which will fit nicely in a 56000 micron sized buchner funnel.

[Edited on 16-2-2012 by phlogiston]




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vinothan23
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[*] posted on 17-2-2012 at 01:04


Quote: Originally posted by phlogiston  
I recommend a 42000 micron sized piece of filter paper, which will fit nicely in a 56000 micron sized buchner funnel.

[Edited on 16-2-2012 by phlogiston]


42000 micron seems very big size holed filter paper, how are you justifying your recommendation?
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[*] posted on 17-2-2012 at 05:49


Ok, because this is in beginners I will give you a break.

You are not going to get any serious answers to your question because it is very vague. There are a lot of factors that affect particle size, and you have not given any details regarding the precipitate and how you obtain it.

It would also be far easier to simply try a few different types of filter paper than to predict the size of your particles, even if you do give a few more details.




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[*] posted on 20-2-2012 at 00:55


One way to increase particle size is boiling a precipitate for some time. This tends to make a precipitate more coarse, such that it settles quicker and is easier to filter.

Chromium(III) hydroxide is a notorious one. It is fine and hard to filter. One way to filter it is using the same filter paper over and over again. Filter with a nice fine paper and collect the liquid, which goes through the filter. If this is still very turbid, then again filter it through the SAME piece of filter paper. You'll see that the second time already much better results are obtained. You could try even a third time. Keep in mind though that filtering will become painfully slow when the filter paper is nearly clogged with the fine particles of the precipitate. Vacuum pumping helps a lot, otherwise be patient!




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[*] posted on 20-6-2012 at 19:45


How do you even get chromium hydroxide to precipitate?



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[*] posted on 21-6-2012 at 10:45


IIRC, Cr(OH)3 will precipitate on its own from solution due to low solubility, like a lot of other transition metal hydroxides.



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[*] posted on 21-6-2012 at 12:22


Chromium hydroxide can be precipitated from a chromium(III) salt by adding ammonia. Using NaOH is not really good, because even a small excess amount of NaOH causes the chromium hydroxide to redissolve, giving chromite solutions. Chromium(III) is amphoteric. Ammonia, however, is a sure method. Its pH is just good to precipitate the chromium(III) as hydroxide, while it does not redissolve in a strong excess amount of ammonia.
You must not allow a precipitate of chromium(III) hydroxide to stand for a long time in ammonia (hours or days). Slowly, over the course of hours, the chromium(III) hydroxide dissolves, due to formation of an ammonium complex of chromium(III). This complex has a nice purple/red color.




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[*] posted on 21-6-2012 at 14:12


Is filtration really necessary?



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[*] posted on 22-6-2012 at 07:08


Quote: Originally posted by vinothan23  
What is particle size of Cr(OH)3 precipitates, what size(in microns) filter cloth/paper we can use to filter the precipitates without loss?

[Edited on 16-2-2012 by ScienceSquirrel]


Just try it out on the small scale and run it through some different porosities.

Virtually no precipitate I've seen can get through 1-10 micron elements in any significant quantity, unless you're about to HPLC something or it's extremely expensive / rare. Most of them are caught around 50 microns.

It's always going to be a case of loss versus material worth as you won't get perfectly uniform particle sizes from most precipitations.

People like Mettler actually make weigh adapters specifically for taking filter papers for this kind of thing if it's critical.

A coffee filter paper or two will probably get it well enough if it's just at home work - put one inside the other, as the seams tend to be a bit leaky.
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[*] posted on 3-9-2012 at 09:15


Oh noes! Get rid of it! Cr2O3.x H2O is one of the worst stuff to be filtered ever having one of the smallest diameter in powder form it is also the hardest
Decant and evaporate
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