what's the "ideal" ph?

[gonefishin]

Hi all,

I was wondering what the ideal" ph would be for chlorine to work most effectively (yet still in the acceptable range) and what the "ideal" is ph for pool water and the effects on your skin/hair?


Hope this doesn't seem like an off the wall question

thanks,
dan

[medvampire]

Dan
Chlorine is more active at lower at a lower pH. The main catch 22 is like you mentioned is the body aspect. The ph of the eye is slightly basic. Like has been mentioned here each pool has its own personality and finding an ideal pH is kinda like getting a pair of goggles, find something that’s fits and wear them out. I can only answer your question from my own humble ideals and I am sure the pros here will have more to say.
I like to run my pH around 7.2 to 7.4 due to a high CH. I have swam in a pH of 7.0 with out any real effects to my skin and hair but it felt a little harsh to my eyes. I think the water feels better on my eyes at around 7.8 to 8.0 but the water gets a little hazy due to a high ALK and CH. I have just salted my pool (any one have some popcorn? ) so I will have to see how this effects the water feel at different at pH levels.
Steve

[waterbear]

From what I understand the 'best' pH is 7.4-7.5 but anything in the range of 7.2-7.6 is fine. some people run pools at higher pH with no problems.

EDIT: just caught my typo and fixed it. I typed 7.-7.5 when I meant to type 7.4-7.5!

[chem geek]

At some point with lower pH it is harder to keep alkalinity at a certain level since the pH will tend to drift up. This is dependent on physical factors such as aeration, but with those factors constant, a lower pH outgasses carbon dioxide at a much faster rate. The following link shows a chart with this info.

CO2.png

The horizontal black "limit" line I drew at 15 is somewhat arbitrary but I have heard from others and have my own personal experience that trying to hold a pH and alkalinity combination above this line seems to be more difficult.

So, a lower pH may require lower alkalinity to prevent an unacceptable rate of upward pH drift, but the lower alkalinity will also make the pH swing more rapidly to changes in pool acid/base equilibrium. A bit of a catch-22 that is mostly avoided at a pH of 7.4 (7.3 or higher).

Richard

[nater]

Chem Geek,
What are the units on the CO2 outgasing rate?

By the way, I find the information you and Waterbear share on this board extremely useful, keep up the good work!

[chem geek]

Chem Geek,
What are the units on the CO2 outgasing rate?

By the way, I find the information you and Waterbear share on this board extremely useful, keep up the good work!

Because the actual outgassing of CO2 is partly a chemical process (CO3(2-), HCO3-, H2CO3 and CO2(aq) all in equilibrium) and partly a physical process (dissolved CO2 physically moving from water out into the air [EDIT]that are not in equilibrium[END-EDIT]) I can't show an actual rate. However, for a constant physical process (that is, constant aeration conditions) the rate of outgassing is proportional to the concentration of dissolved CO2 (or combined H2CO3 and dissolved CO2 since that is the usual convention). So the numbers on the Y-axis of the chart are the unitless quantity "(actual - equil.)/equil." where "actual" is the actual concentration of H2CO3 & CO2(aq) and "equil." is the equilibrium amount of this same species based on the normal concentration of CO2(g) in the atmosphere.

So a Y-axis value of 0 would mean that the pool and the air are in equilibrium with no outgassing at all. A value of 1 would mean that there is twice as much dissolved CO2 in the water as there is during equilibrium. So a value of 15 means that there is 16 times as much CO2 in the water as there is when there is equilibrium. Since we don't know the actual rate of outgassing, the numbers are only useful as a relative scale. A value of 30 will outgas twice as fast as a value of 15. The reason for the "actual - equil." in the numerator is that there is both a forward outgassing and a reverse ingassing going on so "actual - equil." is proportional to the NET outgassing. The division by "equil." is arbitrary since any scaling factor could be used, but I chose this so that the numbers had some reasonable meaning with respect to how far things are out of equilibrium.

I'm sure that's more than you wanted to know!

Richard

[waterbear]

I'm sure that's more than you wanted to know!

Richard

I appreciate the technical explanations! They help me better understand what is occuring.

[chem geek]

I appreciate the technical explanations! They help me better understand what is occuring.

No problem. As you can tell, I enjoy this stuff.

The thread A Cheap Pool Cooler shows that two people with seemingly similar conditions can still get different levels of upward pH drift (and alkalinity lowering when acid is added to compensate for this pH drift). There is no question that all else equal, lower pH, higher alkalinity, and more aeration will cause more outgassing of CO2 which is why Ben's approach to lowering alkalinity tries to maximize two of these effects (obviously, making alkalinity higher so that you can lower it faster is, well, not the brightest thing to do). However, trying to predict the actual amount is really hard. There are formulas to approximate it, but they have so many parameters to put in that are really hard to measure that it's really not worth it -- it's much better to just get to "know your pool" and then you can use my graph as a relative guide using your own pool's behavior for a fixed set of conditions.

[gonefishin]

Thanks all

My ph has drifted up a little. Consequently my pool has seemed to lose that super soft feeling that it has previously had. I didn't know if this could have been related to my ph drifting up or perhaps I was just getting used to the water.

I thought I'd get some opinions before bringing the ph down. I'll see what happens.


thanks,
dan