Chlorine Rise After Non-chlorine Shock

[sl3238]

Anyone know why my FC and TC would rise after a non-chlorine ( Leslie Pools Fresh N Clear) shock ( tested about 2 hours after the shock)...My FC went from 2 to 4...My TC went from 3 to 4...

[chem geek]

Non-chlorine shock (MPS) will interfere with the CC test and register as CC. It might also bleed through to the FC test, though that is more rare. There are interference removers you can get, but they are for FAS-DPD chlorine tests (such as Taylor K-2006) and it sounds like you are using plain DPD.

[quantumchromodynamics]

You started with 2 ppm FC and 1 ppm CC for a total of 3 ppm TC.

The non-chlorine shock could have released the 1 ppm of combined chlorine to make it free chlorine. That would raise your FC from 2 to 3 ppm and would make your TC = 3 ppm.

Since your new FC (4 ppm) = TC (4 ppm), you shouldn't have any CC.

The MPS could be registering as 1 ppm CC, which would make a TC of 4 ppm.

If you're not using FAS-DPD, then you could have enough test error to account for the difference.

Also, MPS will oxidize some chloride to chlorine, although I don't think it would be enough to register on a test.

HSO5^- + Cl^- --> SO4^2- + HOCl

The standard electrode potential E⁰ of KHSO₅ = 1.85V

The standard electrode potential E⁰ of Cl₂ = 1.36 V

[Edit] I should have given the E⁰ for HOCl, not Cl₂, as Richard shows below. [End edit]

[chem geek]

HOCl + H⁺ + 2e^- ---> Cl^- + H[/sub]2[/sub]O ..... E₀ = +1.482V

Generally, the MPS doesn't oxidize chloride to chlorine, or not very quickly though thermodynamically it is favored. That is how it persists to oxidize other chemicals before chlorine does. Adjusting for typical concentrations, E = 1.15V for the chlorine reaction.

[quantumchromodynamics]

This reference has this in the "Blending Oxone" section:

Sodium chloride can be used if some conversion to active chlorine is desired, provided the blend is pH buffered to avoid evolution of chlorine gas,

Of course, this is not the same as what happens in a hot tub, but I thought it indicated that some chloride would be converted into chlorine. I don't think it would be much in any case.

Also, note that there are some "Multifunctional" shock products that contain MPS and dichlor, such as "DuPont Pool Care Multifunctional Shock", which is 15 % Sodium dichloro-s-triazinetrione.

Dupont

MSDS