Chemistry Forum

Hi,

I’m using phosphoric acid (H₃PO₃) to clean cement off some brick pavers and I’d like to neutralize it with either bicarbonate of soda (NaHCO₃) or quicklime (CaO) before washing it into the soil.

I read on the net that because phosphoric acid has 3 hydrogen molecules and bicarbonate of soda has 1, I therefore need to use three times as much bicarb to neutralize the acid.

Is this correct?

Either way, can anyone tell me how many grams of bicarb or lime I’ll need per mm of acid to neutralize it?

Thanks in advance.

H₃PO₃ + 3NaHCO₃ = Na₃PO₃ + 3H₂CO₃
per mm of acid? dont you mean ml? It depends how concentrated is that acid
so for example:
for 81.9 g of 100% H₃PO₃ (1 mol) you need 3 * 84 = 252 g of NaHCO₃
so if you tell me how much of the acid and how concentrated it is, I (or someone else) can tell you how much of bicarb or quicklime you need

Hi,

KriggY wrote: per mm of acid? dont you mean ml?

Oops, yes, I meant ml.

The product is called Phosphoric Acid Cleaner made by Spirit, and "contains 22% phosphoric acid solution". I'm using it undiluted.

As for how much acid, I can only estimate that the entire job will require 1 litre.

So, 1 litre of 22% acid.

Cheers

ok:
1 litre of 22% acid means 220 ml of 100% acid and 780 ml of water.
the mass of the acid is 220*1.885 = 414.7 g
414.7g / 98g/mole = 4.23 moles of acid.

H₃PO₃ + 3NaHCO₃ = Na₃PO₃ + 3H₂CO₃

we need 3 moles of NaHCO₃ per 1 mole of acid i.e 12.7 moles

12.7*84.01g/mole = 1066.5 g of NaHCO₃

Thanks KriggY,

Now I have no more excuses for not doing this job... darn!!

If you're interested, here's the link for my information on neutralizing phosphoric acid.

http://www.kchemistry.com/molarity.htm

Cheers

Hi,

Can somebody please clarify whether the formula for phosphoric acid is H₃PO₄ or H₃PO₃?

KriggY went to some trouble to assist me with calculating the amount of bicarb needed to neutralize phosphoric acid, but I've since found a discrepency with the formula I gave.

Also, I'm not sure where the 84.01g/mole comes from. I need to know this as I'm using an 85% solution now instead of 22% and will need to make an adjustment.

Thanks a million.

Sodium bicarbonate is NaHCO3, with a molar mass of 84.007 g/mol.
___________________________________________________________

Phosphoric acid is H3PO4, with a molar mass of 97.994 g/mol. 85% solution has a density of 1.685 g/mL.

H3PO4 + 3 NaHCO3 = Na3PO4 + 3 H2CO3

w[base]
= r[base:acid] * m[base] * c[acid] * d[acid] / m[acid]
= 3 * (84.007 g/mol) * 0.85 * (1.685 g/0.001L) / (97.994 g/mol)
= 3683.(5) g sodium bicarbonate per L of phosphoric acid.
___________________________________________________________

H3PO3 is Phosphorous acid with a molar mass of 82.00 g/mol. Make sure you know which one you have. Among other things, phosphorous acid is dibasic, not tribasic like phosphoric acid. (It will only donate two hydrogen atoms in an acid-base reaction; the third hydrogen is not acidic.)

Also, it's usually stocked as a powder, not a liquid.

H3PO3 + 2 NaHCO3 = Na2HPO3 + 2 H2CO3

w[base]
= r[base:acid] * m[base] * w[acid] / m[acid]
= 2 * (84.007 g/mol) * 1 g / (82.00 g/mol)
= 2.049 g sodium bicarbonate per 1 g of phosphorous acid.

Thanks Graham for the effort.

Yes, it's definitely phosphoric acid (ortho) 85% and it's classed as S₅. Looks like I'm going to have to suit up for this stuff!

Cheers,