potassium phosphite solutions --> phosphorous acid

loki · guinea pig

I was looking at some of those K/P ferts/fungicides which are used to stop phytophthora (sp?) etc, and just so happened to be a proper msds on one i was looking at, and i found it came out to a 4:3 K Razz

ratio (molar), i dunno how other ones are set up, but that's why they call it a mixture of mono and di-potassium salts of phosphorous acid. So, i am not sure if all preparations have the same proportions, but i would think that any chem company worth its salt tells you the g/l of each ion in the solution on the msds... which makes separation much simpler if you do a bit of math.

now, i see, from reading some old hive archives about the subject, that hydrochloric acid is usually used. I personally don't understand why a much more obvious way to clean it up isn't used - namely, the fact that tartaric acid forms an insoluble precipitate with potassium ions, potassium tartrate, aka cream of tartar. I don't forsee any troublesome reactions with tartaric acid and phosphorous acid, both of them are reducing agents (tartaric acid much much weaker and more specific tho)...

So, wouldn't it work, if one gets the raw solution, mix up a concentrated solution of tartaric acid of known mole/volume, carefully titrates, (i am inclined to think taking a progressive halving strategy is the easiest way to ensure as close to perfect titration) and out of the solution falls potassium tartrate, which can then be filtered or decanted separated from the liquid, and then one would have phosphorous acid solution.

I'm not quite sure how one goes from there, in this situation you end up with probably about 1M/L solution (about 10%) - is it ok to gently boil this off until it reaches the desired 50%ish? or would it be better to maybe concentrate it to say 30% and then refrigerate it and precipitate and and repeat (since it can form phosphine gas if it gets too hot)

loki · guinea pig

https://www.synthetikal.com/hiveboard/crystal/000456171.html

heh worth reading a bit closer, i'd think once the K-tartrate is precipitated just boil it down carefully...

or maybe it is actually a better idea to start by concentrating the whole solution, as the K-H3PO3 salt solution until it starts to precipitate, then do the tartaric acid titration, would save a lot of risking making a mess with phosphorous acid. the idea of using the oven to dry it is probably the most sensible way, at a gentre 80 degrees temp.

Would it work to bring the solution quite a bit further down than just starting to precip, maybe bake the whole thing down overnight at 70`C in the oven after boiling to precipitation point. it might be good to intentionally make a concentration of tartaric acid, if one is adding it directly to the solution, that is designed so that the phosphorous acid is goodly concentrated after the conversion, measure accurately, and then just throw in the tartaric acid solution with the dryed k.phosphite salts, give it a bit of heat to ensure full ion transferring happens, decant and then reduce the solution, should be able to then start the reduction process with better than 50% concentrated phosphorous acid, meaning much less time needed when reducing this potentially dangerous acid to a usable concentration.

knowing the molarities of everything of course would mean you would know by volume at what point you reach the concentration of acid you want for reactions.