Sciencemadness Discussion Board

Phosphoric Acid from TSP

DFliyerz - 1-6-2015 at 07:03

I'm giving myself a pretty dumb project which is synthesizing phosphoric acid from trisodium phosphate, and I've narrowed the process down to two main equations:

2 Na3PO4 + 3 CaCl2 --> Ca3(PO4)2 + 6 NaCl
Ca3(PO4)2 + 3 H2SO4 --> 2 H3PO4 + 3 CaSO4

However, I've run into two problems with my process: the trisodium phosphate I have is 20-25% sodium sesquicarbonate, which will cause some calcium carbonate to precipitate along with the tricalcium phosphate, so I need to separate the sodium sesquicarbonate from the trisodium phosphate beforehand. Next, I don't have any clean sulfuric acid, but just have sodium bisulfate. That means that theoretically I'll have soluble sodium sulfate in my phosphoric acid unless I can find a way to remove it beforehand. I know that this is a pretty dumb project, but I'd like to know if there are any ways to fix these problems, even if it makes it extremely time/money inefficient.

j_sum1 - 1-6-2015 at 07:24

Well, if you really have time on your hands, I have a similar project on the go.
I have no H2SO4 except that which I make myself and so I have refrained from using that. I do have plenty of HCl that I can use.

I place a layer of Ca3(PO4)2 on the bottom of a plastic container with a sealable lid. Inside I place an open container of concentrated HCl. I put the lid on and wait a few months.
After that time the HCl vapour has reacted with the calcium phosphate to form CaCl2 and phosphoric acid. The mixture is a sludge that oozes with the acid. You can get some to decant off but you probably want a better method with higher yield. It would probably respond reasonably well to vacuum filtration. The method I used was to dissolve the phosphoric acid in acetone, leaving the calcium chloride behind. The acetone was then distilled off.

Unfortunately I picked up quite a bit of contamination in my first batch -- It was left sitting for a week with the acetone in an unsuitable container. Still, it was enough for a proof of concept. I expect that my next batch will come out quite clean.

As a process, it is not real elegant, but it will fix both your problems.

DFliyerz - 1-6-2015 at 07:45

What about the sodium sesquicarbonate in the TSP?

j_sum1 - 1-6-2015 at 07:58

So, you end up with some calcium carbonate in with your calcium phosphate.
When exposed to the HCl, the carbonate will convert to CaCl2 giving off some carbon dioxide. The phosphate also converts to CaCl2 so there is no real difference.

Texium - 1-6-2015 at 08:08

How can you be so sure that the HCl will react in an appreciable way with the calcium phosphate though? It seems to me like that reaction would tend to favor the other side and be very inefficient for phosphoric acid production. Now I'm curious about this, and when I get home I'm going to try adding some phosphoric acid to a calcium chloride solution so see if a precipitate will form. If so, it would seem unlikely that the reverse reaction would work for producing phosphoric acid.

DFliyerz - 1-6-2015 at 08:10

I'd kinda prefer not to wait a few months, are you certain there's no way to do it with sodium bisulfate or some other homemade sulfuric acid?

j_sum1 - 1-6-2015 at 08:24

Quote: Originally posted by DFliyerz  
I'd kinda prefer not to wait a few months, are you certain there's no way to do it with sodium bisulfate or some other homemade sulfuric acid?

You could probably work out a way. I'm just letting you know what I did. It was mostly a way of doing something useful with some calcium phosphate before I had any equipment or much else n the way of reagents. It worked. (I think it helped that temperatures were quite high over summer.)

@zts16
You tell me. I ended up with a thick acid with a slight yellow tinge. It was soluble in acetone and had a high boiling temperature. The powder that accompanied it was not soluble in acetone but was soluble in water.
I'll admit, I did not attempt to obtain a precipitate from the acid and calcium chloride. But, as you say, that is the opposite of what I was attempting to achieve.

Texium - 1-6-2015 at 18:42

Quote: Originally posted by j_sum1  

@zts16
You tell me. I ended up with a thick acid with a slight yellow tinge. It was soluble in acetone and had a high boiling temperature. The powder that accompanied it was not soluble in acetone but was soluble in water.
I'll admit, I did not attempt to obtain a precipitate from the acid and calcium chloride. But, as you say, that is the opposite of what I was attempting to achieve.
Well, I stand corrected after testing it. I added about a mL of concentrated phosphoric acid to a few mL of calcium chloride solution and it remained perfectly clear with absolutely no precipitation. I guess the greater strength of HCl overrides the low solubility of the calcium phosphate in this scenario. It seems like a plausible method as long as the acid is distilled afterwards.

j_sum1 - 1-6-2015 at 18:50

Plausible if you have a lot of time at your disposal.
Hey, I have a bucket with a lid and a spare spot on my bench. It isn't costing me anything.

macckone - 1-6-2015 at 20:26

Quote: Originally posted by zts16  
It seems like a plausible method as long as the acid is distilled afterwards.

You don't distill phosphoric acid. The acetone technique seems like a good separation method.

j_sum1 - 1-6-2015 at 22:14

If you are impatient you could always speed up the rate using a fluidised bed of your phosphate and pass HCl gas through.

Repuroposed buchner for H3PO4 production.jpg - 259kB

Texium - 2-6-2015 at 16:54

Quote: Originally posted by macckone  
Quote: Originally posted by zts16  
It seems like a plausible method as long as the acid is distilled afterwards.

You don't distill phosphoric acid. The acetone technique seems like a good separation method.
My apologies, I forgot about the polymerization/glass etching that comes with hot concentrated phosphoric acid.

Why not just TSP and excess HCl

chloric1 - 2-7-2023 at 11:12

I’d like to make the point that sodium chloride is virtually insoluble in concentrated hydrochloric acid. So the sedimentation of NaCl will drive the equilibrium to the right:
Na3PO4 + 3HCl >>>3NaCl + H3PO4.

I’m thinking the yield would be atrocious unless a later acetone separation was used. Personally, I’d prefer to separate disodium phosphate from a TSP solution only partially neutralized to pH 8-9 by HCl since it has such low solubility in cold water. Also dosodium phosphate dodecahydrate is so easy to melt and you could just add the remainder of the hydrochloric acid to that and just get a viscous slurry on heating. When excess HCl is removed, add acetone

unionised - 2-7-2023 at 13:32

Quote: Originally posted by DFliyerz  


Next, I don't have any clean sulfuric acid, but just have sodium bisulfate.

I'm sure I read somewhere that adding ethanol to sodium bisulphate gives a precipitate of sodium sulphate and a solution of sulphuric acid in alcohol.

chloric1 - 2-7-2023 at 19:53

Quote: Originally posted by unionised  
Quote: Originally posted by DFliyerz  


Next, I don't have any clean sulfuric acid, but just have sodium bisulfate.

I'm sure I read somewhere that adding ethanol to sodium bisulphate gives a precipitate of sodium sulphate and a solution of sulphuric acid in alcohol.


I’ve read the same thing numerous times. I feel similar about trisodium phosphate. Seems like TSP is effectively dibasic sodium phosphate and sodium hydroxide. Sodium hydroxide is soluble in ethanol and especially methanol. Maybe you could remove the alkali from TSP leaving the dibasic salt? Not sure.