Purifying Hypo

[SHORTY]

Purification

Hypophosphorous acid from almost any source will serve as a starting material, so long as it is about 10 formal.  The commercial grade 50% acid (Merck), which usually contains from 2 to 3 mole percent phosphorous acid, meets this requirement and was used in all the preparations.

About 600ml. of concentrated hypophosphorous acid is placed in a one liter filtering flask which is connected to a water aspirator.  Through a two hole stopper in the top of the flask run a thermometer and a coarse gas dispersion tube, the tube being connected to a source of dry nitrogen.  The flask rests on a hot plate.  After all of the air has been thoroughly flushed out of the system with nitrogen, the aspirator and hotplate are turned on, the nitrogen flow is cut down, and the temperature regulated so that it remains around 40C during the evaporation, which is continued until the volume of solution is about 300ml.  the hot plate is then turned off and the solution allowed to cool.  The liquid in the flask is then poured into a wide mouthed Erlenmeyer flask and placed in a dry ice – acetone bath.  After a few hours in the bath, the contents of the flask will freeze. The flask is then removed from the bath allowed to stand in a cold room for about 12 hours, after which time the flask contents should be from 30 to 40% liquid.

The following operations are carried out in the cold room.  The mixture is filtered by suction through Whatman #44 filter paper.  The filtrate is discarded, the solid pressed dry on the paper, transferred to a crystallizing dish and allowed to stand until 20 to 30% of it liquefies.  It is then refiltered, and the resulting white solid stored over Mg(ClO₄)₂ in an evacuated desiccator in a cold room. The yield in this preparation is about 15%.  Large crystals of this product can be prepared  by recrystallizing the white solid from n-butanol.  (The high solubility of hypophosphorous acid in the lower alcohols renders them impractical for recrystallization purposes.)  Analysis showed that the purity of the product was not increased by recrystallization.

Analysis

Aqueous solutions of the white solid were analyzed for hypophosphorous acid and phosphorous acid by a combination acidimetric-iodometric method in which total acid was determined by titration with standard NaOH to the phenolphthalein and point, and phosphorus acid was determined by the method of Jones and Swift.  The results of these analyses showed that the phosphate impurity had been reduced from 3% in the crude acid to about 0.2% in the purified material.  This purification method was subsequently extended by Mr. R. T. Jones , who found that three filtration-liquefaction cycles reduced the phosphate impurity to less than 0.1 mole percent.

Qualitative test for phosphate were run on aqueous solutions of some batches of H₃PO₂ prepared by this method, using the ammonium molybdate procedure recommended by Swift.  At no time was any yellow coloration or yellow precipitate noted.  This test is sensitive to about 0.1 mole percent phosphate in the presence of hypophosphorous acid.

Would this bee possible to do in a clandestine setting?  Where could one obtain dry nitrogen? 

Any one interested in viewing the rest of this document on hypo can download it here

http://etd.caltech.edu/etd/available/etd-04242003-090233/unrestricted/Jenkins_wa_1953.pdf

but be forewarned it is 10mb (152) scanned pages.

[Rhodium]

Would this bee possible to do in a clandestine setting? 

I see no reason why it shouldn't. Do you really want to use this particular procedure though? In the third paragraph it is sadly enough stated that "The yield in this preparation is about 15%". 

This information was interesting though: "Large crystals of this product can be prepared  by recrystallizing the white solid from n-butanol (The high solubility of hypophosphorous acid in the lower alcohols renders them impractical for recrystallization purposes).  Analysis showed that the purity of the product was not increased by recrystallization".

Where could one obtain dry nitrogen?

It should probably be available in gas tubes from any welding supply or similar.

[SHORTY]

I just assumed that it would not bee suitable for a lab. Actually, if 15% yeild was obtained by a real chemist then my chances of getting any yeild at all are probably not even worth the effort. 

Rhodium, did you happen to look at the rest of the document?  Just curious as most of it was above my understanding of chemistry so it may not bee of any signifigance to someone who understands it.

[Rhodium]

I don't think that there were anything else of intrerest other than that they say the following refs contain disccusions about the reaction of iodine with (hypo)phosphorous acid(s).

Mitchell, J. Chem. Soc. 117, 1322 (1920)
Mitchell, J. Chem. Soc. 119, 1266 (1921)
Mitchell, J. Chem. Soc. 121, 1624 (1922)
Mitchell, J. Chem. Soc. 123, 629 (1923)
Griffith and McKeown, Trans. Faraday Soc. 30, 530 (1934)
Griffith and McKeown, Trans. Faraday Soc. 36, 767 (1940)

[8ball]

Why purify what you already have that works. If that makes any sense. Would'nt purifying only get rid of some existing salts maybe giving the hypo a longer shelflife. Unless this is what you are after for the purpose of a saleable product.

[SHORTY]

If it wasn't too hard i would do it just to see the difference between using pure hypo as opposed to what i use now.  However, as Rhodium pointed out, 15% yeild was achieved by a chemist in a real lab so i doubt that i would get even 1% yeild.  Especially since i don't have a coldroom and its about 90-100F in my lab this time of year.  My fridge is a little to small for me to fit in too.