How much Phosporic acid to reach Ph 4?

[Vularin]

Here's a bit of Trivia!

I need to make 5L of acid water with a Ph of 4,
now i can use 75% or 85% Phosporic Acid.

how much ml of Phosporic acid will i need?

Cheers,
V

 

[lorentz5]

Hey Vularin, I tried calculating it for you.

For a pH of 4, you need a [H3O+] of 0.0001M (10^-4) since H3PO4+H2O<=>H3O+H2PO4
0.0001M H3PO4 = 0.00196g H3PO4 / 5L (this is what you want)
For 75%, 75g/100ml = 0.00196g/x, x=0.0026ml
So to make a 5L solution of pH 4 using 75% H3PO4, you'd need to pour 0.0026ml of the 75% H3PO4 into 4999.9974ml of H2O.
That way, you'll have 0.00196g (like 2mg) of H3PO4 per 5 L and this will give you a pH of 4.
Sounds highly impractical unless you have a mg scale. Someone should check my chem.

P.S. Don't forget: never add water to acid. Do it the other way around.

 

[psilyguy]

Do yourself a favor and get a digital ph tester, or at the very least some test strips. It's not going to be much...a few drops.

 

[Vularin]

Cool! thx! thats some crazy math skills!

So whats the pH sweet spot? cause i hear some people have bad experience with pH 2 (less yield).
Am i right to presume the sweet spot is between pH 4-3?

Hey Vularin, I tried calculating it for you.

For a pH of 4, you need a [H3O+] of 0.0001M (10^-4) since H3PO4+H2O<=>H3O+H2PO4
0.0001M H3PO4 = 0.00196g H3PO4 / 5L (this is what you want)
For 75%, 75g/100ml = 0.00196g/x, x=0.0026ml
So to make a 5L solution of pH 4 using 75% H3PO4, you'd need to pour 0.0026ml of the 75% H3PO4 into 4999.9974ml of H2O.
That way, you'll have 0.00196g (like 2mg) of H3PO4 per 5 L and this will give you a pH of 4.
Sounds highly impractical unless you have a mg scale. Someone should check my chem.

P.S. Don't forget: never add water to acid. Do it the other way around.