Organic chemistry question

[TwoBitTxn]

I'll try to explain this

Hypochlorous Acid disassociates into hypochlorite ion and hydrogen ion in water. This reaction is reversable and the ratio is pH dependent.

HOCL--><--OCL^-+H⁺

Take a concentration of HOCL of 3ppm

If you add PO₄^-3 at a concentration of 5000ppb is the phosphate strong enough to pull the hydrogen away from the OCl^- and essentially force it to stay in that form due to a lack of free hydrogen?

 

[swmarc]

You kind of illistrated it wrong that H is actually bonded to the Cl^-

It depends on what your solution is in. If it is an aqueous solution then no it will never stay as the OCl but if you dissolved 3ppm of HClO into 5000ppm PO₃^- it would probably force the reaction to the right in the direction of OCl^- so instead of this
HClO--><--OCl+H

you get this

HClO---------><--OCl + H


Curious why are you using ppm? I am a chemist and I never use them.

 

[spoolin01]

Without dredging the memory banks and analytical chem books to do the exact calculation, the strongly basic pKa3 (12.4) of PO4-3 and the stoichiometric+ amount added should pull the hydrogen strongly away from OCl- and towards the phosphate.

I think your supposition is correct.

 

[spoolin01]

By the way, that's an inorganic chemistry question.

 

[BlueDreamDivers]

1 ppm means one part per 1,000,000 parts i.e., 1 Ž× 10^?.
1 ppb = 1 x 10^-9
5000 ppb = 5 x 10^-6, which is the concentration of phosphate ion.
Whereas the concentration of HOCl is just 3 ppm.
So if the dissociation constant of HOCl is higher than that of H_3PO_4 , then you can get free OCl- . That will also depend on the counter ion attached to phosphate ion.

 

[TwoBitTxn]

You kind of illistrated it wrong that H is actually bonded to the Cl^-

It depends on what your solution is in. If it is an aqueous solution then no it will never stay as the OCl but if you dissolved 3ppm of HClO into 5000ppm PO₃^- it would probably force the reaction to the right in the direction of OCl^- so instead of this
HClO--><--OCl+H

you get this

HClO---------><--OCl + H


Curious why are you using ppm? I am a chemist and I never use them.

I deal with commercial swimming pools. HOCl is the readily accepted written form of hypochlorous acid. I deal in ppm because those are the units I deal with on a day-to-day basis.

 

[TwoBitTxn]

So to put this in context:

HOCl is the main sanitizer for a swimming pool. As shown above it undergoes a disassociation reaction. The balance is pH dependent. As the pH drops the concentration of HOCl is higher vs OCl^-. Which is what we want.

However, I had a sales guy state that phosphate will scavenge the hydrogen from the OCl^- in the water and essentially force the equation to the right due to a now lack of hydrogen for the ion to recombine with.

The most common measure for chlorine in water is ppm. The most common measure for phosphate is ppb. 2000ppb is considered an action level for phosphate in a swimming pool. 2000ppb=2ppm. Or in laboratory chem form... 2000micrograms/liter=2milligrams/liter.

I wanted to check what I was being told.

Thanks,

 

[ianr33]

I just looked up the pKa of HOCl ,it is 7.5
pKa3 of H3PO4 (i.e.HPO4-- <--> PO4--- + H+) is 12.4

i.e. HOCL is a much stronger acid

So HOCl + PO4--- -----> OCL- + HPO4--

As the phosphate is in higher concentration there will be essentially no HOCl in the mixture.

 

[spoolin01]

So to put this in context:

HOCl is the main sanitizer for a swimming pool. As shown above it undergoes a disassociation reaction. The balance is pH dependent. As the pH drops the concentration of HOCl is higher vs OCl^-. Which is what we want.

However, I had a sales guy state that phosphate will scavenge the hydrogen from the OCl^- in the water and essentially force the equation to the right due to a now lack of hydrogen for the ion to recombine with.

The most common measure for chlorine in water is ppm. The most common measure for phosphate is ppb. 2000ppb is considered an action level for phosphate in a swimming pool. 2000ppb=2ppm. Or in laboratory chem form... 2000micrograms/liter=2milligrams/liter.

I wanted to check what I was being told.

Thanks,

The effect you're considering is due to effects on pH, not the particular acid-base components themselves. You could dump sodium hydroxide in and have the same effect. At some point the patrons will start screaming! I assume that is the reason for the phosphate action level. I don't know pool chemistry, but I recall they use HCl as well, must be to adjust pH. You can add all the tri-phosphate base you want but if you neutralize it with acid, you're back to the same HOCl species' distribution. Also, pretty sure OCl- is unstable so decomposition rate at higher pH (if that's what you're considering for your pool) could be a contrary result. The hypochlorous ion is reactive - that's why they call it bleach - so the fact that only a part of total HOCl is ionized at neutral pH probably isn't the whole story with respect to useful activity. As the ionized OCl- 'bleaches' something, the remaining HOCl dissociates to keep providing OCl- (law of mass action), esp. if pH is controlled by other intervention.

Whether overall there's a benefit to adding a small amount of phosphate, how much and which pH and counter-ions might be more stabilizing of the OCl- is beyond my basic chemistry . Maybe the rep knows what he's talking about.

 

[pdelannoy]

Phosphate is a much stronger base than hypochlorite (OCl-), so if you combine phosphate and HOCl, the HOCl will be completely deprotonated. And yea, in a swimming pool with a large assortment of other acids, bases, conjugate acids and bases, the story is more complicated.