What is FC and what is being measured by those yellow drops.

[Cindy_H]

I am still trying to wrap my head around FC. What is FC and what isn't? Is FC the sum of all free chlorine ions in solution so FC = HOCl + OCl + Cl? And just to ask outright: what does the color indicator FC drop test measure exactly?

I take it there is no direct measure of HOCl (the sanitizing form of Cl) so we need to know our pH, temp and CYA levels to estimate the sanitizing % and lifetime of HOCl (i.e. from the graphs in the water chemistry section)?

Thanks!

 

[YippeeSkippy]

FC is the portion of chlorine available to kill the cooties.

That test block is basically an estimate... go for the FAS-DPD test if you need (and you do!) exact numbers.

Maddie

 

[mknauss]

Cindy -- you might be interested in the following -- Pool Water Chemistry

Take care.

 

[needsajet]

For pool water chemistry purposes, here's my simplification

"Free chlorine" (FC) includes hypochlorous acid (HOCl), hypochlorite ion (OCl^-) and chlorinated cyanurates (chlorine in reserve, tied up with CYA). It does not include chloride (Cl^-). FC is easily measured with the inexpensive and accurate FAS-DPD test (the pink to clear test).

A third step in the FAS-DPD test can also be used to test for combined chlorine (CC).

"Total chlorine" (TC) includes HOCl, OCl^- and chlorinated cyanurates, but adds in combined chlorine (CC). This is what the yellow test (OTO) measures. OTO is not needed for TFPC, but some members find it useful as a day-to-day yes/no indicator for presence of chlorine.

"Active chlorine" is just HOCl and OCl^-. The OCl^- is only a weak sanitizer, so some would say it's not very active in this sense. We don't test for active chlorine. You may also see this called "available chlorine", meaning 'available for immediate sanitizing'.

It's the HOCl that matters most in terms of sanitation. There is no inexpensive and reliable test for HOCl that works for TFPC. This is partly because in a stabilized pool, the amount of HOCl is so small. Calculators and tables are available to help determine the HOCl concentration, provided that FC, pH, and CYA are known.

Active chlorine + chlorinated cyanurates = free chlorine (FC)
Free chlorine + combined chlorine = total chlorine (TC)

 

[Cindy_H]

woo hoo! Thank you - that was exactly what I was after.

I guess Cl2 doesn't exist in water?

Then - the next question - what happens to all the chlorine that keeps getting added to the pool? Just accumulates or does it gas off??

 

[needsajet]

Excellent, I'm glad it was helpful

Again, from just the pool management practical perspective, chlorine gas does not remain in the water. Chlorine gas exists only very briefly in the water if added directly to the water by a commercial gas injection system, or, also very briefly, when added to the water passing through a salt water chlorinator.

The vast majority of the chlorine ends up as chloride ions, adding to the saltiness of the water. There are other lesser pathways which are more or less important depending how poorly or well the pool is operated. Post #3 in this link provides lots of explanation and description: Pool Water Chemistry

Cheers