Sciencemadness Discussion Board

How can i convert HPO4 salt to PO4 salt

idrbur - 18-2-2017 at 21:11

Hello friends, i want to make some elemental phosphorous and i found some text about making it from Ca3(PO4)2 but i dont have any phosphate salt available near me from which i can prepare it , the only salt of phosphorous i have is diammonium phosphate. So plz tell me how can i convert HPO4 salt to PO4 salt or how can i prepare elemental phosphorous from CaHPO4 or other HPO4 salt.

Meltonium - 19-2-2017 at 06:49

Can you get Calcium hydroxide? I believe adding 3 equivalents of Ca(OH)2 to a solution of 2 equivalents (NH4)2HPO4 would yield calcium phosphate.

3Ca(OH)2 + 2(NH4)2HPO4 -> Ca3(PO4)2 + 4NH4OH + 2H2O

Then the ammonium hydroxide would decompose into water and ammonia gas. The calcium phosphate would precipitate out of solution and you would essentially have only calcium phosphate and water left.

BromicAcid - 19-2-2017 at 09:50

Of note regarding the calcium phosphate method is that you are not actually going to be looking for calcium phosphate as the starting material. The 'wet method' starts from bone ash which can be found at some stores for use in gardening. The bone ash is treated with sulfuric acid, filtered, then concentrated. I made the mistake during my attempt of using bone meal which made for a stinky, floaty precipitate along with very low yields of my target phosphate salt. You may know all of this already but I thought it might be worth repeating since bone ash is not what would be regarded as a standard starting material.

At one time I also tried diammonium phosphate. Looking into the MSDS sheets and such I found that the material 'decomposed' at relatively low temperatures. I was hoping that the decomposition route would results in loss of ammonia leaving behind phosphoric acid. Instead it left a sold strangely colored mass.

[Edited on 2/19/2017 by BromicAcid]

WGTR - 20-2-2017 at 08:28

If you're starting with (NH4)2HPO4, there's an easier way than the calcium phosphate method.

Refer to patent 3,619,133:
Attachment: US3619133.pdf (617kB)
This file has been downloaded 219 times

Different sodium salts can be used; here I show sodium carbonate, but the patent refers to sodium chloride as well. The resulting ammonium carbonate is volatile, and easily driven from the mixture with heating:

2(NH4)2HPO4 + Na2CO3 --> 2Na(NH4)HPO4 + (NH4)2CO3

Upon further heating ammonia is driven off, leaving sodium hexametaphosphate behind in the molten state.

Na(NH4)HPO4 --> NaPO3 + (NH4)OH

The cooled glassy NaPO3 is then smashed to a powder (quickly, keeping moisture away). At this point, refer to page one of this awesome thread:

There's a method outlined there that uses aluminum powder and SiO2 with the NaPO3; that information should get you started.

I've made a small amount of phosphorus in a borosilicate test tube this way. I didn't isolate it, it was more just to say that I did it. However, I performed the reaction under a vacuum. The reaction seemed to proceed more easily than under an inert gas. Perhaps this is because of the lack of convective heat loss from the reaction mixture. The phosphorus condensed in the tube right above the reaction mixture.

Take care because of the implosion risk when working with a vacuum. I never had a problem with the stopper getting pulled into the tube, but that risk is there if the stopper isn't sized correctly (or regardless, for that matter). Also, borosilicate was needed rather than sodalime glass. The boro could barely hold up to the heat provided by the propane torch while under a vacuum. In fact, it was caving in at places. A sodalime tube would be too soft under the same conditions. A better way would be to place the evacuated tube itself into a larger vacuum chamber, that way the pressure is more or less equalized on both sides of the hot tube. Of course in that case, one would probably need electric heating.

Another idea is to place the reaction mixture in a steel thimble, and place this inside some firebrick sitting inside a glass tube, and then heat this evacuated tube underwater inductively. The water would keep the glass tube and the inductor cool, while the reaction zone would remain very hot. It's the same idea as running a light bulb underwater, the filament would be very hot, while the glass would stay cool. If the tube imploded, the water would prevent glass shards from flying, and also protect the phosphorous from air exposure.

[Edited on 2-20-2017 by WGTR]

idrbur - 21-2-2017 at 17:52

Thanks guys that was very helpful. Now i'm gonna try that making phosphorous from NaPO3.