How much Calcium Hydroxide to manufacture calcium formate?

First, I have no experience with stoichiometry.


I currently manufacture calcium formate for a specific use because here in my country, it is very difficult to get calcium formate in small quantities. If you do, its very expensive.


The easiest and cheapest formic acid I can get is CP grade, 85%, which I have a lot of. I use CP grade anhydrous calcium hydroxide.


I would like to know what the correct amount by weight of calcium hydroxide I should dissolve (Too much of calcium hydroxide makes calcium carbonate. I think that from its reaction with $\ce{CO2}$ in the air, which makes the filtration step a nightmare).


**My current recipe:** To $\pu{4 L}$ of distilled water, I add $\pu{390 mL}$ of 85% formic acid. I turn on the stirring and start adding calcium hydroxide until the $\mathrm{pH}$ is somewhere between 7 and 8. I then filter at room temperature. Is this optimal given the chemicals I mentioned?




One of the interesting observations I have noted in neutralizations with calcium hydroxide on a large scale is that the reaction of a not-very strong acid (like organic acids) with Ca(OH)2 is quite rapid initially, but slows down as the neutralization becomes more complete, say around 98-99%. At this point, the pH is on the acid side, maybe around 5-6, and it just seems to take forever (several minutes, anyway). It's not nearly as rapid as with NaOH or KOH.


Stirring manually, checking every few minutes with pH paper, then stirring the sludge up again will certainly take a lot of time and labor. With a large quantity, like 4 L, mechanical stirring would seem to be necessary so you can let the reaction go to completion while doing something else.


I suspect you have come across this slowing down, after correctly calculating the amount of Ca(OH)2, and concluded that CaCO3 is produced. It is also possible that CaCO3 is present in your supposedly CP Anhydrous Ca(OH)2, and that would dissolve much slower in the nearly exhausted formic acid. Perhaps that is why you need to filter.


Filtration of 4 L is a pain. I propose another plan: Add 90% of the calculated Ca(OH)2 and stir until clear. The pH will be acidic, ~4-5. Any CaCO3 in your Ca(OH)2 should dissolve moderately rapidly; when the liquid goes as clear as it's going to get, add the rest of the Ca(OH)2. This last addition should dissolve completely, except if it actually does contain CaCO3, that CaCO3 may dissolve so slowly, especially if your pH has gone up to 7-8 (because perhaps your formic acid is only 84.6% - rounded up to 85%). But if your pH stays high, the neutralization is done, and your filtration should be 10 times easier.


BTW, I shouldn't do any homework problems for you, but a final concentration of 10.66% calcium formate solution seems quite nice, considering that it is soluble to the extent of 16.2% at 0ºC. And if you need any other numbers for reference, consider 266.4.


 4