Total Alkalinity

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Thanks for that, I will process it. :)

That last part- if there is twice as much base floating about in the water as the acid, why isn't the hypochorous acid neutralised and thus rendered useless? How can it last long enough to sanitise in the form of free chlorine?

Answered in your link no doubt but probably too complex for me to understand....I am an IT guy not chemist.
 
The hypochlorous acid is not neutralized because the water is pH buffered. So the base mostly goes towards moving carbon dioxide (or carbonic acid) to bicarbonate ion and bicarbonate ion to carbonate ion in that buffer system -- so you can think of the base as neutralizing carbonic acid. In order to convert hypochlorous acid to hypochlorite ion (to "neutralize it" as you put it), the pH would need to rise significantly. This is especially true when there is CYA in the water since it is a hypochlorous acid buffer so resists changes to its concentration when pH changes. It basically releases chlorine bound to CYA in order to form more hypochlorite ion thereby keeping the hypochlorous acid concentration from changing as much.
 
Cool. Thanks.

I'm going to be brave and try to summarise what I have learnt in a "for dummies" style...

1. Salty water passes through the chlorinator cell;
2. Electrolysis breaks this water up into Chlorine gas, Hydrogen gas and Sodium Hydroxide;
3. The Chlorine gas dissolves in the water rushing through the cell forming hypochlorous acid:hypochlorite ions (the ratio dependant on pH);
4. The Hydrogen gas dissolves in the water rushing through the cell basically linking with dissolved oxygen somewhere and forming water;
5. Some of the Sodium Hydroxide, a strong base, is buffered by the TA. The rest of the Sodium Hydroxide pushes the pH up slightly (?) as does the degassing of CO2;
6. The hypochlorous acid:hypochlorite ions, now out in the body of the pool, do their job of sanitising, eventually turning back into salt.
7. The cycle repeats.

Notes on point 3: The higher the pH of the water, the more the ratio of hypochlorous acid:hypochlorite ions swings towards the hypochlorite ions. Hypochlorite ions are nowhere near as effective at sanitising than hypochlorous acid therefore keeping the pH around 7.4 (the "sweet spot" is best).

Notes on point 5: Shifts the carbon dioxide (or carbonic acid) to bicarbonate ion and bicarbonate ion to carbonate ion.

Can you remind me why the TA doesn't absorb ALL the sodium hydroxide?

I tried reading your detailed thread Chem Geek but it was a bit too much for me. :)

Cheers.
 
In point 4, much of the hydrogen gas tends to escape to the atmosphere, often as tiny bubbles that are barely visible coming out of the return jets. If it doesn't escape right away, nearly all of it will eventually outgas to the atmosphere in other ways.

In point 6, as that happens the PH goes back down. In practice you are unlikely to be able to measure any PH change as both the creation of hypochlorous acid (basic) and break down of hypochlorous acid (acidic) are more or less continuous.

Your note on point 3, ratio of the different forms of chlorine, is only true when there isn't any CYA present. CYA changes the balance so the ratio is close to constant at the PH range we care about and it mostly stops mattering what the PH is (within reason).
 
Cheers Jason.

That detail about CYA - so the fact that the CYA makes the ratio close to constant is why you need to have more Free Cholorine (i.e. to get the hypochlorous acid levels up)?

The CYA 'overrides' the pH somewhat in terms of this ratio?
 
CYA has several effects. The need for more chlorine at higher CYA levels is a separate effect. CYA binds to some percentage of the chlorine, the percentage bound is higher at higher CYA levels. The chlorine bound to CYA is a far less effective sanitizer while it is bound to CYA, so you need to use higher FC levels to get the same sanitizing power. Separately, CYA changes the ratio of hypochlorous vs hypochlorite among the unbound chlorine so that the dependance on PH is almost eliminated and the sanitizing power remains strong though the whole range of reasonable PH values (7.2-8.2).
 
1. Salty water passes through the chlorinator cell;
Yes, but note that only the chloride ions are relevant to the reaction in the salt cell that produces chlorine.

2. Electrolysis breaks this water up into Chlorine gas, Hydrogen gas and Sodium Hydroxide;
The chloride (salt) ions in the water are what become chlorine gas. The water becomes hydrogen gas and hydroxide ions.

3. The Chlorine gas dissolves in the water rushing through the cell forming hypochlorous acid:hypochlorite ions (the ratio dependant on pH);
Yes and the chlorine gas dissolving in water also produces hydrochloric acid which is a strong acid. Basically, one chlorine from chlorine gas goes to hypochlorous acid (a weak acid) and the other goes to hydrochloric acid (a strong acid). The hydrogen ions from the hydrochloric acid react with the hydroxide ions from step 2 above to form water where half of such hydroxide ions get neutralized in this way.

4. The Hydrogen gas dissolves in the water rushing through the cell basically linking with dissolved oxygen somewhere and forming water;
The hydrogen gas does not react and as Jason noted it outgases from the pool.

5. Some of the Sodium Hydroxide, a strong base, is buffered by the TA. The rest of the Sodium Hydroxide pushes the pH up slightly (?) as does the degassing of CO2;
Yes this is correct.

6. The hypochlorous acid:hypochlorite ions, now out in the body of the pool, do their job of sanitising, eventually turning back into salt.
Yes, but this process is an acidic one so the pH drops thus countering the pH rise from

7. The cycle repeats.
Yes.

Notes on point 3: The higher the pH of the water, the more the ratio of hypochlorous acid:hypochlorite ions swings towards the hypochlorite ions. Hypochlorite ions are nowhere near as effective at sanitising than hypochlorous acid therefore keeping the pH around 7.4 (the "sweet spot" is best).
Yes but as Jason noted when CYA is in the water the amount of hypochlorous acid does not change very much with pH because CYA is a hypochlorous acid buffer. Whereas with no CYA going from a pH of 7.5 to 8.0 drops the hypochlorous acid concentration by around 50%, when CYA is in the water the drop is only around 15% so can generally be ignored.

Notes on point 5: Shifts the carbon dioxide (or carbonic acid) to bicarbonate ion and bicarbonate ion to carbonate ion.
Yes this is true.

Can you remind me why the TA doesn't absorb ALL the sodium hydroxide?
pH buffers do not absorb all of the pH changing hydrogen ions. The reason has to do with chemical equilibrium. In the case of carbonic acid (including carbon dioxide) and bicarbonate ion this can be written as:

[Bicarbonate Ion] * [Hydrogen Ion] / [Carbonic Acid + Carbon Dioxide] = Constant

As an example using concentrations at a pH of 7.5 and a TA of 80 ppm would be 1591.7 * 0.034404 / 98.138 = 0.5580 and if you add an amount of acid "add" then it will have an amount that is shifted converting bicarbonate ion into carbonic acid so we have (1591.7 - shift) * (0.034404 + add - shift) / (98.138 + shift) = 0.5580 where you can see that to make this balance "add - shift" must be very small but not zero while "shift" will nearly equal "add". If the TA absorbed ALL of the sodium hydroxide, then "shift" would equal "add", but that equilibrium ratio would not balance. Basically, the ratio of bicarbonate ion to carbonic acid depends on the pH so you can't shift from one to the other without changing the pH at least a little.
 
Cheers guys really appreciate the help.

The final thing Im taking away from this - because I am using CYA - is that it's still a good idea to set the pH to mid 7's for swimmer comfort? And to help keep it here a TA around 60 is best...?

Any idea of a good CYA test kit available in Australia? The test strips I use seem ok for most other parameters but the CYA one is too hard to read.
 
Test strips are generally unreliable but the worst test does seem to be the CYA test. See if you can get the Palintest SP 315C. They even make a FAS-DPD test for chlorine though that may not be available in Australia. I think Dave is allowed to ship the TFTestkits TF-100 to you in Australia though that will of course be quite expensive due to the shipping costs.
 

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JasonLion said:
The CO2 outgassing rate depending on the surface to volume ration. More surface area means more outgassing, more volume means less. Swimming pools have drastically higher surface to volume ratios than fish tanks, so drastically slower outgassing rates.
Click to expand...




Did you mean swimming pools have lower surface to volume ratios?

Surface area to volume = Surface area/VOL

= Surface area/(Surface area x depth)

= 1/avg depth


If we are talking a home fish tank avg depth might be 1-2 ft and avg pool,is 5 ft

SA/VOL pool = 1/5 = 0.2

SA/VOL tank = 1/1 = 1.0
 
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