One for Chem Geek

[goneaway]

Rather than me hijacking a thread, i'll start a new one, maybe Chem Geek could answer this.
In laymans terms though please 

At school we were taught that acids and alkali's were measured on a PH scale.

If water at 7.2 is neutral and then by adding chlorine, we turn it acidic.
Then what the heck is TA (i assumed Total Alkalinity ??), and how can you affect the PH without affecting the TA?

I'm confused.

To compound my confusion, i was also under the impression that Chlorine was a bleach, and that bleach was alkaline.
How come the stuff we add in the hot tub is acidic.

I really should have been more attentive in chemistry.

[Dr. Spa™ Ret.]

It's the other way round. Bleach is a form of chlorine, and has a pH of 13 (very alkaline). Pure chlorine gas has a pH below 1 (very acidic). Other forms of chlorine;

Sodium Dichlor - 6.9
Tri Chlor - 2.9
Calcium Hypochlorite - 11.8
Lithium Hypochlorite - 10.7

[Tman122]

Rather than me hijacking a thread, i'll start a new one, maybe Chem Geek could answer this.
In laymans terms though please 

At school we were taught that acids and alkali's were measured on a PH scale.

If water at 7.2 is neutral and then by adding chlorine, we turn it acidic.
Then what the heck is TA (i assumed Total Alkalinity ??), and how can you affect the PH without affecting the TA?

I'm confused.

To compound my confusion, i was also under the impression that Chlorine was a bleach, and that bleach was alkaline.
How come the stuff we add in the hot tub is acidic.

I really should have been more attentive in chemistry.

It's difficult to adjust TA (total Alkalinity) without affecting the PH. You should try and get a TA of 80-120 first. This gives you a buffer for your PH. Then try and make adjustments to your PH. Soda ash raises PH and TA but effects PH more than TA. Baking soda raises TA more with less effect on PH so depending on where your water starts.........You may have to live with one (TA, PH) slightly off. No biggy.

Keat, your turning into a water Nazi like I was....LOL 

[chem geek]

The different chlorine sources have different pH as indicated by Dr. Spa.  However, when any of the sources of chlorine is used/consumed, this is an acidic process so the net effect of using bleach, chlorinating liquid, Cal-Hypo or lithium hypochlorite is that it is fairly pH neutral.  The rise in pH when using these sources of chlorine comes from carbon dioxide outgassing because pools and spas are intentionally over-carbonated.  Total Alkalinity (TA) is a measure of this over-carbonation so a higher TA will outgas more carbon dioxide and therefore rise in pH more, all else equal.

So one usually uses a higher TA when using a net acidic source of chlorine.  For example, Trichlor pucks in pools with a TA of 120 ppm or more.  In spas, using Dichlor is net acidic so a TA of 80 ppm or so is usually OK, but the actual amount that works well depends on the amount of aeration in the spa.  Using bleach or other hypochlorite sources of chlorine needs a lower TA, as low as 50 ppm, and you usually need additional pH buffering from borates (say, from boric acid).

So TA is mostly a measure of the carbonates in the water and they are good for buffering pH, BUT are a SOURCE of rising pH.  So the general rule is that if your pH tends to rise over time, then target a lower TA level; if your pH tends to drop over time, raise your TA level.

[goneaway]

Wished i'd never asked 
My head hurts now 

I'm currently using Bromine, as the smell isn't so bad.