What does free chlorine ppm mean exactly?

[blac1175]

I'm a little confused as to what does exactly "1 ppm free chlorine" mean?

I know that "ppm" stands for parts per million, and I also learned that it's always a mass ratio, not volume or moles or anything else. So, 1 ppm = 1 lbs in 1,000,000 lbs. But 1 lbs of what exactly? Of NaOCl or of OCl-? The difference is meaningful, 1 ppm[OCl-] = 1.45 ppm[NaOCl]. Which one is the widely reported "free chlorine ppm", that is found in tables on TFP and also measured by the Taylor free chlorine test?

Extra question: when I do the combined chlorine part of the Taylor free and combined chlorine test, I assume I get combined chlorine concentration expressed as free chlorine. Is that correct? That is, 1 ppm combined chlorine in a Taylor measurement means "1 ppm[NaOCl] worth of chlorine atoms", which is 0.69 ppm[NH2Cl], if I understand things correctly. Am I right?

Thanks a lot,
Balint

 

[JoyfulNoise]

PPM, Parts Per Million, is NOT specifically a mass ratio. It can be a ratio of anything - mass, volume, grains of rice, number of widgets, whatever. In SI units and at standard temperature and pressure, 1ppm = 1 mg/L because 1 liter of water weighs exactly 1 kilogram. So 1 mg/kg = 0.000001 = 1ppm.

When you measure "free chlorine" with a Taylor kit, all of the oxidizers in the solution are reacting with the DPD indicator dye and the FAS drops. The assumption is that the only oxidizer in solution is chlorine. The test is standardized to measure the total amount of oxidizer in solution as "ppm Cl2", that is, part per million of chlorine gas equivalent. The CC test uses the same measure (ppm Cl2). In quantitative chemistry, titration tests are always standardized to a known chemical species in the reaction.

FC = [HOCl] + [OCl-] + [HCyCl]

Free chlorine = concentration of hypochlorous acid + concentration of hypochlorite anion + concentration of chlorinated cyanurate

 

[JamesW]

If you have 999,999 lbs of water and you add one lb of chlorine gas, the total weight is 1 million lbs.

The chlorine gas will mostly become hypochlorite and hypochlorous acid.

The test is not specific to any specific type of chlorine. It counts the moles and that's converted to weight of chlorine gas.

So, a million pound sample will contain the equivalent of 1 pound of chlorine gas.

It's the same principle as percentage. If you have a mixture of sand and rocks and you say that the mix is 5% by weight rocks, then you know the ratio.

Percentage just uses 100 as the reference sample size, whereas parts per million uses 1,000,000 as the reference sample size.

Different tests use different standard equivalents to express the test results.

Chlorine....................Chlorine gas.
Total alkalinity..........calcium carbonate.
Calcium hardness...calcium carbonate.
Cyanuric acid...........cyanuric acid.
Borate.......................boron.

So, when you say that you have 5 ppm chlorine, that means that you have the equivalent of 5 lbs of chlorine gas per 1 million pounds of water.

 

[blac1175]

You rock, both of you. Very precise answers. Thank you very much.

So to make sure I understand, if I pour 1 gal of "Pool Essentials Chlorinating Liquid" (10% NaOCl) into a 30,000 gal pool, then I raised free available chlorine by 3.715 ppm. That is because the density of the chlorinating liquid is 1.17 kg/l (from MSDS), the chlorinating liquid is 10% mass ratio of NaOCl, NaOCl is 74.44 g/mol, 1 mol NaOCl is equivalent to 1 mol of Cl2 and Cl2 is 70.91 g/mol:

(1 gal * 1.17 kg/l * 10% / 74.44 g/mol * 70.91 g/mol) / (30,000 gal * 1 kg/l) = 3.715 ppm

That pretty much matches what I have seen happening in my pool, and now I understand why

Balint

 

[JamesW]

10% is trade percent, which is grams chlorine gas per 100 milliliters. 1 gallon of 10% provides 3.33 ppm.

1 gallon of 10 trade contains the equivalent of 378.5 grams of chlorine gas (13.35 oz).