Need chemistry education

[JimInFL]

From my extensive education and working background in chemistry (about an hour long topic, in middle school 40 years ago) I remember one thing. An explanation of pH, and this sliding scale on the chalkboard (yes - chalkboard) that ranged between Acid and Alkaline. For simplicity, I think of Alkaline as bleach, and acid as, well, acid. If you had something more or less balanced in both, and you add one or the other, the scale shifted that direction.

Here's my dilemma.

We've had our pool almost 4 years. It's a salt pool so I don't have to mess so much with chlorine (alkaline - right ?). A little acid here and there, an adjustment on the generator, a bag of shock - no big deal. All performed under the careful direction of world renowned chemical experts at Pinch-A-Penny (very nice folks). How can a pool be low on chlorine, and also need acid ? Or chlorine fine, and need acid ? It's one or the other - right ? This just never made sense to me. Is there a simple explanation for what I'm missing here ?

jim in FL

[D.Hayden]

My understanding.. very limited.. was chlorine was just there to kill things in the water..

Don't know if this is true.. but on Wiki:

Chlorine for swimming pool usage is made to be Chlorine neutral (7,0) as to not raise or lower the p<br style="padding-top: 0px; padding-right: 0px; padding-bottom: 0px; padding-left: 0px; "><br style="padding-top: 0px; padding-right: 0px; padding-bottom: 0px; padding-left: 0px; ">Read more: http://wiki.answers.com/Q/Is_chlorine_an_acid_or_base#ixzz1KGiJZnnz

[Eager]

Your assumption that chlorine = alkali is wrong. Chlorine does not mean acid or base, it just means chlorine. "Chlorine" is easier to dissolve in alkali (base), as in "bleach" solution (sodium hypochlorite solution), and this is the most common form of "chlorine" for use, so it's common to think that "chlorine" and alkali are associated, but this is not the case. If you get too acidic, much of the dissolved "chlorine" will escape from the solution as chlorine gas, so it's hard to get very acidic and have a chlorine concentration, so we're usually talking about pH changes near neutral.

Edited April 22, 2011 by Eager

[Pigdawg]

I do not know anything about pool chemistry or the salt systems in pools but in water treatment chlorine is used as a disinfectant not for pH adjustment. Depending on the type of chlorine used it can have different effects on pH. My plant use to use pure chlorine gas and that would cause the water pH to go down due to the formation of hydrochloric acid and Hypochlorous acid. We found that when we switched over to sodium hypochlorite(house hold bleach just 5X stronger) it slightly adjusted our pH up. Chlorine will decay from exposure to sunlight and warm weather. Our storage room has its own ac system and the windows are blacked out to help with this. Chlorine residues will drop from a chlorine demand.

[Erik S.]

Your assumption that chlorine = alkali is wrong. Chlorine does not mean acid or base, it just means chlorine. "Chlorine" is easier to dissolve in alkali (base), as in "bleach" solution (sodium hypochlorite solution), and this is the most common form of "chlorine" for use, so it's common to think that "chlorine" and alkali are associated, but this is not the case. If you get too acidic, much of the dissolved "chlorine" will escape from the solution as chlorine gas, so it's hard to get very acidic and have a chlorine concentration, so we're usually talking about pH changes near neutral.

EXACTLY right! That's why it's important to buy supplies that are meant for pools and trust the salesmen. Controlling the pH of your (non salt) pool DOES have a bit to do with sanitation, but there are many other reasons for keeping your water solution balanced. A few are: To keep the mechanical parts of your pool (pumps etc) from corroding/dissolving due to incorrect pH; prevent pool from "naturally" trying to balance itself by absorbing surface materials, thus decreasing the total dissolved solids in the water; and finally, reducing the risk of skin rashes and/or eye irritation from bacteria etc.

[Kyle O]

The pH is simply the negative log of the hydrogen ion concentration, so a larger concentration equals a lower number.

[mpeltier]

Bieng this question is based on Salt water pool chlorine generation, I will give my take on it from that perspective. The true chlorine we use to disinfect is actually Hypochlorous Acid. This is realeased thru electrolysis of salt water thru the chlorine generator (probably should be called a Hypochlorous Acid generator, but who knows what that means, right?). There are no chloromines produced or released in this process like those contained in chemical additives of chlorine from the pool store. These chloramines are actually more acidic than the Hypochlorous Acid produced by generation and are missing from the water in acidity tests conducted on your (saltwater) pool. This may be why your saltwater pool tests (like mine does) less acidic when its chlorine tests fine. If everything else tests in balance but the acid is slightly off, I generally ignore it.

[Jimmy76]

http://www.troublefreepool.com/index.html

Try out this site. I've been folowing the advise on there for two years and my pool has been the best its ever been, and cheaper to maintain. There's a tab for "Pool School" and it has some very good informational reading. If you need any help, the people on that forum are very helpful.

Good luck,

Jim