Question about non-chlorine shock (MPS)

[Gibson00]

For a chlorine tub, no ozone, no silver ion product....is there really any need to use MPS?  If I boost the chlorine levels to about 4 or 5ppm after each soak, and then maybe shock it once ever 7-10 days by raising the chlorine to 10-15ppm, does adding MPS really benefit anything?  The chlorine (bleach) will take care of the chloramines and any other contaminants, correct?
Thanks!

[Vinny]

Yes, the actual number for CC is 10x their amount - so if you measured 0.4 PPM CC then 4 should burn it off. The only real benefit is that you can use the tub sooner (20 minutes) than if you have a lot of chlorine in the tub. But there's something you can add to the tub to neutralize the chlorine as well (I think peroxide).

[chem geek]

You shouldn't even need to shock if you are properly maintaining the chlorine level between soaks and properly adding enough chlorine after a soak to handle the bather waste.

The main benefit to MPS is that it oxidizes some bather waste without producing chlorinated disinfection by-products.  It does not oxidize all the same chemicals that chlorine does, however.  It doesn't work well against ammonia, for example.

As for CC and the 10x rule, that rule is incorrect.  The rule came about because of the following chemical relationship for how much chlorine it takes to oxidize ammonia:

3HOCl + 2NH₃ ---> N₂(g) + 3H₂O + 3H⁺ + 3Cl^-
Hypochlorous Acid + Ammonia ---> Nitrogen Gas + Water + Hydrogen Ion + Chloride Ion

You can see that there is a 3:2 or 1.5 ratio of chlorine to ammonia in MOLAR units.  Chlorine is measured in parts-per-million (ppm) of chlorine gas (Cl₂) equivalent where the molecular weight is 70.906 g/mole.  Ammonia is measured in ppm of nitrogen (N) equivalent where the molecular weight is 14.0067 g/mole.  So the ratio of chlorine to ammonia in their respective ppm units is 1.5*(70.906/14.0067) = 7.59.

Because of some side reactions and inefficiencies the rough rule-of-thumb is that it takes an amount of chlorine that is 8-10x the amount of ammonia.  HOWEVER, Combined Chlorine (CC) is NOT measured in ammonia nitrogen units but rather is measured in chlorine units so there is no factor of 5 difference from molecular weight derived ppm so the ratio would be closer to 1.5.  Furthermore, CC from ammonia is mostly monochloramine so already has one of the chlorine attached to it (NH₂Cl) so the ratio is actually closer to only 0.5.  Because one must get over the "hump" and because of side reactions the rough rule-of-thumb for CC that is monochloramine is 1x or roughly an equal amount of chlorine.  The reaction of chlorine with monochloramine is shown below:

HOCl + 2NH₂Cl ---> N₂(g) + 3H₂O + 3H⁺ + 3Cl^-
Hypochlorous Acid + Monochloramine ---> Nitrogen Gas + Water + Hydrogen Ion + Chloride Ion

where you can see the 1:2 or 0.5 ratio.  Note that both hypochlorous acid and monochloramine are both measured in the same Cl₂ ppm units.

It is true that using a higher concentration of chlorine will make the reaction go faster, but in terms of how much chlorine will get used up it will be closer to the 1x rule-of-thumb.  Of course, this is when the CC is monochloramine.  If it is instead monochlorourea then more chlorine is needed, but it's still only around 3x so still not the incorrectly applied 10x.

[Vinny]

I stand corrected! I'll quote 3x next time ... Funny thing is that the Taylor book still has 10x for breakpoint chlorination for getting rid of CC.

I wonder why they haven't updated their info.

[chem geek]

No one has updated their info, at least not completely.  The closest is that they finally DID figure out that there was one chlorine already attached to monochloramine as described in this Taylor article.  But NO ONE has admitted to the bonehead mistake of using the wrong units of measurement -- not recognizing that CC is in chlorine units while the 10x rule was based on chlorine vs. ammonia nitrogen units!  You really have to stop blindly trusting what the industry says.  Yes, much is fine, but you cannot assume that all of it is accurate just because it's from the industry.