I came into some fairly cheap (surplus stock) 99 % phthalic acid anhydride (phthalic anhydride) and wanted to synthesise some potassium hydrogen
phthalate (KHP) from it. KHP is a useful material as primary standard material for acidometry and for calibrating pH meters. This labnotes *.pdf:
…provides some interesting data on the temperature dependence of the water solubility of KHP. Using a bit of extrapolating the estimated solubility
at 80 C is about 26 g/100 g and about 5.5 g/100 g at about 0 C. The dependence is almost linear.
As far as anhydrides go, phthalic anhydride is a bit unusual because of the furan-like structure which causes it to rehydrate quite sluggishly. Only
hot water eventually rehydrates it to phthalic acid.
About 0.25 mol of the anhydride was weighed off in a clean beaker and 0.25 mol of technical KOH was dissolved 200 ml of water. The KOH solution was
added to the phthalic anhydride and not much happened. The quantities correspond stoichiometrically to about 26 g of KHP for 100 g of solvent.
This mixture was then heated to boiling point, stirring continuously, and the phthalic anhydride remarkably slowly dissolved in the KOH solution. It
took well over an hour to obtain a perfectly clear, hot solution of mainly KHP. Seed crystals were obtained with the ‘glass rod trick’ and added
to the solution. After cooling for a bit this was then fridged. A couple of hours later the first crop of crude KHP crystals was obtained, going by
the link’s data about 40 g of KHP should have been obtained:
These will be harvested, washed with iced water and recrystallised from hot solution.
[Edited on 19-7-2012 by blogfast25]Magpie - 19-7-2012 at 14:14
Nice work. I have a small bottle of KH phthalate reagent that was excessed where I once worked. I have used it several times to standardize NaOH
titrating solutions. As I recall it is fairly expensive so you did well to make it yourself.blogfast25 - 20-7-2012 at 05:05
Nice work. I have a small bottle of KH phthalate reagent that was excessed where I once worked. I have used it several times to standardize NaOH
titrating solutions. As I recall it is fairly expensive so you did well to make it yourself.
Yes, it's pricy enough to try and make it yourself.
Do you dry it again prior to use (I'll be using it as an acidometric standard too) and if so in what conditions?Endimion17 - 20-7-2012 at 06:10
This is nice. Do inform us on the yield after the purification.Magpie - 20-7-2012 at 06:54
