Why does pH drop only in the winter

[jereece]

When I close my pool I do raise the chlorine concentration to about 8ppm. However I use bleach as my only source of chlorine. During the season, my pH never needs adjusting. Well that is once I get it right at opening. But my point is that bleach does not affect my pH over the course of the swimming season so I would not think it would affect the pH over the winter. The only thing I can think of is the water on top of my cover. I need to check into that.

Thanks for the information.

Jim

[jereece]

Here a new wrinkle in this problem. I tested the water on top of my winter cover and the pH is 7.9! So the water in my pool (under the winter cover) was 6.4 and the water on top of my winter cover is 7.9. The water on top of my cover is fairly clear even thought there are leaves under the water at the bottom of the cover.

After the discussions below, I was thinking the pH of the water on top of my cover would be low and hence leached into the pool water underneath. Obviously this is not the case. This problem began only a couple years ago. Prior to that, I never had this problem. I am still treating my pool water the same (bleach for chlorination), it's the same cover. I'm still perplexed as to why my pH would drop so low over the winter but during the summer be steady as a rock.

Could algae cause this? It seems like last year I had what appeared to be black algae on the bottom of my pool when I opened last year. It was a ring a couple feet wide around the circumference of the pool. Wiping with a Miracle Eraser easily removed it and I did not think about it again. I have not looked this year as I have not removed the cover yet. But can black algae cause the pH to drop? That's the only change I have noted over the past couple years.

I appreciate your thoughts.

Jim

[chem geek]

I am still perplexed. Even bacterial conversion of CYA to ammonia would tend to make the pH rise, not fall. The only thing that has the pH fall is when chlorine gets used up or consumed -- oxidation of ammonia or other organics is an acidic process. However, if you add a hypochlorite source of chlorine, then that exactly counteracts that and in fact has a small net pH rise due to the excess lye in bleach and chlorinating liquid. If Trichlor or Dichlor were being used, then a pH drop would be expected.

How about your TA level? Has that dropped some as well? An acid would drop both pH and TA.

[jereece]

Actually since my pool is AG I learned not to worry about alkalinity. Testing showed that as alkalinity increases the greater my pH would fluxuate. With alkalinity low, pH is stable as a rock. See this string of posts - http://www.poolforum.com/pf2/showthread.php?t=7870. I have not checked it in years but am confident my alkalinity is very low.

I added 2 boxes of Borax and pH has come up to normal.

This has me perplexed as well.

Jim