#### CHEMICAL FORUM

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Jr. Member

Joined: 02 Apr 2012
Posts: 8
 Posted: Mon Apr 02, 2012 7:28 am    Post subject: Neutralizing Phosphoric Acid 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.

Sr. Member

Joined: 22 Mar 2012
Posts: 26
Location: Olomouc - Czech Republic
 Posted: Mon Apr 02, 2012 9:43 am    Post subject: 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

Jr. Member

Joined: 02 Apr 2012
Posts: 8
Posted: Mon Apr 02, 2012 8:14 pm    Post subject:

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

Sr. Member

Joined: 22 Mar 2012
Posts: 26
Location: Olomouc - Czech Republic
 Posted: Tue Apr 03, 2012 1:10 am    Post subject: 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

Jr. Member

Joined: 02 Apr 2012
Posts: 8
 Posted: Tue Apr 03, 2012 4:02 am    Post subject: 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

Jr. Member

Joined: 02 Apr 2012
Posts: 8
 Posted: Tue Aug 07, 2012 10:19 pm    Post subject: Clarification re Phosphoric Acid 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.

Distinguished Member

Joined: 19 Mar 2012
Posts: 396
 Posted: Wed Aug 08, 2012 1:30 am    Post subject: Phosphoric or Phosphorous? 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.

Jr. Member

Joined: 02 Apr 2012
Posts: 8
 Posted: Thu Aug 09, 2012 3:13 am    Post subject: 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,
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