Vogelzang
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Potassium formate
Making potassium formate from 85% formic acid and anhydrous potassium carbonate
With the following information:
HCOOH 46 g/mole,
density = 1.22 g/cc (100%?; Merck Index)
density of 85% HCOOH should be between 1.15 and 1.22 g/cc based on the Merck Index and a formic acid chart in the Handbook of Chemistry and Physics
that only goes up to 70% formic acid and gives the density as about 1.15 g/cc for 70% formic acid.
K2CO3 138.21 g/mole
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I mixed 13.8 g K2CO3 (1/10 mole), some deionized water and gradually added a little over 10 ml of 85% formic acid (excess). After the fizzing went
away I checked the pH using pH paper which turned orange. I added a little more formic acid and there wasn't any more fizzing and then checked the pH
again and the pH paper was still orange. I figured it must have something to do with the fact that potassium is so electropositive and formic acid is
a relatively weak acid that this keeps the pH paper from turning red, which typically happens with strong acids. Does this look right?
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bbartlog
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Potassium formate in solution is alkaline, yes. I think a concentrated solution is over pH 10. You'd likely need to add a fair excess of formic acid
to actually render the solution acidic. I'd have to look up the equations if I actually wanted to calculate the proportions for some particular pH.
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Vogelzang
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The reason I used excess formic acid is to make sure all of the potassium carbonate has reacted. Boiling off the water and excess formic acid should
leave practically pure potassium formate. It says in the Merck Index the melting point is 167 degrees C and at higher temperatures gives off
hydrogen. I guess it would decompose into potassium carbonate which could be tested by adding formic acid to see if there's any fizzing.
I tried making anhydrous sodium acetate a couple times from acetic acid and sodium carbonate. According to one book I have, you can heat the
non-anhydrous salt until it dissolves in its water of hydration which then boils off and then the salt melts again. Its recommended not to heat it
too long because it would tend to decompose.
[Edited on 4-12-2011 by Vogelzang]
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