chornedsnorkack - 22-6-2015 at 22:51
How could a substance be heated with a volatile acid for some time so that the said volatile acid does not escape until it is deliberately removed?
Say you´d like to produce Fe(OH)3, or FeOOH, or Fe2O3. How?
If you add strong alkali like NaOH rapidly to solution of e. g. FeCl3 then you get a mess:
iron hydroxides precipitate rapidly as fine nanoparticles
they have nonstoichiometric composition, not specifically Fe(OH)3 but somewhere between Fe(OH)3 and FeOOH, or Fe(OH)3
with some water
they trap some cations and anions which being involatile are hard to remove.
But how about forming iron hydroxide precipitate slowly, in weakly acidic environment, and then heating it under still acidic condition that dissolves
a lot of iron, and allows it to migrate to bigger, more perfect iron hydroxide crystals?
For example, dilute carbonic acid at 0,04 % in air has pH 5,5. How about concentrated carbonic acid? Of course 100 % carbonic acid would boil because
the vapour pressure of water would go in top of the vapour pressure of carbon dioxide. But suppose that you manage to heat water to 80 degrees and
then the headspace is 50 % steam and 50 % carbon dioxide, but no air. What would be the pH of this solution? Around 4?
How would you practically handle heating a precipitate with such a volatile weak acid, and make sure your 50 % carbon dioxide is not replaced by air?
In the end, the goal might be to gradually remove the carbon dioxide until the temperature is 100 degrees and headspace 100 % steam, 0,00 % carbon
dioxide, with pH 7, so that the dissolved Fe(HCO3)3 is disintegrated and added to the growth of iron hydroxide crystals.
Also, which other volatile acids would be suitable for such reflux?
deltaH - 22-6-2015 at 23:48
In heterogeneous catalyst preparation, this is a standard challenge, with iron in particular! Normally the 'trick' we used to use is to start from
nitrate salts, for example iron nitrate. This can be decomposed easily into iron oxides by heating, forming volatile NO2 and water etc. If you want to
precipitate it from solution, consider using ammonia or 'ammonium carbonate', that way the co-product formed is ammonium nitrate which can be
decomposed cleanly to gases in a calcination (heat treatment) but do be aware of the safety implications, ammonium nitrate is potentially explosive,
albeit rather insensitive, so washing as much of it as possible out is prudent (small amounts left will be destroyed by heating and does not pose a
large danger from explosive risk).
Organic counter-ions can also be used and then the co-formed salt burned off in a calcination, for example working with metal acetates and
precipitating with ammonia (the co-formed ammonium acetate easily forms acetamide on strong heating and is so driven off). Any carbonisation occurring
can be burnt off by heating to at least 500C in air.
There are lots of strategies, I hope this gives some ideas, but please be safe and think of the hazards if attempting any of this and take appropriate
precautions and work in small amounts: NO2 and acetamide are toxic and carcinogenic! Ammonium nitrate can be explosive. etc...
[Edited on 23-6-2015 by deltaH]