I recently read some articles on PoolForum about pH increasing over time as you add bleach each day so you have to lower it using muriatic acid. For years I have just accepted this as a fact from the bleach having a high pH. Decreasing the pH creates the conditions favorable for Alkalinity to decrease and so mine does. But recently, this post raised some questions about whether or not the bulk of the pH increase is actually due to the bleach. I used to be a lab chemist years ago, so I decided to run some tests to see if I could find anything unusual. I don't have a lab, labware or highly accurate measuring instruments like I once did, so my results are "ball park".

I used 3/4 gallon bleach jugs that were cleaned, rinsed multiple times and allowed to sit dry for 3 days. In bottle #1 I collected straight tap water. My water is provided by the city and the source is a local lake. So it is chlorinated and soft (tastes good too!). In bottle #2 I collected straight pool water. Bottle #3 was half pool water and half tap water.

For the water in each bottle I took initial readings. Then I added 3 drops of 6% bleach, mixed and took readings again after 10 minutes. Each drop of bleach should add approximately 1ppm chlorine. For those interested, there are 20 drops in 1ml and there are 3785 ml in a gallon. So 3785 X 0.75 X 20 = 56,775 drops. 6% bleach is 60,000 ppm. So if you do the concentration conversion C1V1=C2V2, you come up with each drop adding 1.06 ppm chlorine (basic emergency plan training).

After the 10 minute readings, I cut the top off of each bottle so that the water depth was about 5 inches and set them outside on my deck table. The air temperature was about 95 and there was a good 7 hours of direct sunlight, so the water in each bottle got to about 110F. I mixed and analyzed again after 4 hours and again after 24 hours. I basically wanted to see what the increase in pH was at various alkalinity.

To minimize this post length, here's a link to my results in an Excel spreadsheet.

My conclusion is that pH in my pool increased more at higher alkalinity and it takes more acid to reduce the pH. In the first test (3 drops/ppm), the pH of Tap Water (Alk 20) increased from 7.5 to 7.6 after 24 hours. However pool water with 70ppm Alkalinity increased from 7.4 to 7.7. In the second test (20 drops/ppm) a few days later, the pH of Tap Water (Alk 20) increased from 7.5 to 7.6 after 24 hours. However pool water with a 60 ppm Alkalinity went from 7.4 to 7.8.

This surprised me as I thought higher alkalinity would keep the pH from increasing so fast. I have also noticed that as my alkalinity has been decreasing over the past month that I have in fact not had to adjust the pH as often. At an alkalinity of 120, I was having to adjust the pH twice a week. Now that my alkalinity is down to 60, I am only adjusting once a week. And it takes half the acid to bring the pH down.

So, am I dreaming here or what? Is this possible? I thought higher alkalinity slowed pH change but my tests seem to indicate that it increases it. I intend to let my alkalinity decrease over time to see if I notice any problems.

Any feedback, comments or things I have not considered are welcome. I am just trying to learn more.

Thanks,
Jim