Why Does the pH of My Pool Constantly Drop?

[Exchemist]

I hear most people need to constantly add acid to their pools as the pH rises. The pH of my pool always goes down. I have never added acid and am constantly adding washing soda. Typical test results:
FC: 3 to 6 ppm depending on the time of day
CC: 0 ppm
TA: 80 ppm
CYA: 40 ppm
pH: 7.4

I'll bring the pH up to 7.6 with washing soda and over about 10 days it drops again to 7.2. Repeat every 10 days. Pool is kept covered when it's cool out and uncovered when it's warm. I notice no difference in the rate of pH change with the cover on or off.

Any ideas?

 

[faby3003]

What type of chlorine do you use in your pool? If you use trichlor, that will lower your pH.

 

[Exchemist]

I use liquid chlorine -- the 182 oz bottles of Chlorox from Costco. I need to add between 1/3 and 1/5 bottle each day.

 

[faby3003]

Besides bleach and washing soda, is there anything else that you add to your pool on a regular basis?

With your TA at 80, it seems to me that your pH should be fairly stable.

 

[linen]

What is the history on your TA? If you are regularly adding washing soda and your not adding chemicals that are acid sources (MA, ph down, trichlor, dichlor, etc.), then your TA should also be going up.

How much make up water are you putting in to the pool (do you have an autofill)? This would be unusual, but if the makeup water is lower in TA and higher in ph, and you are using quite a lot of it, that could cause what you are seeing.

 

[Exchemist]

I haven't added anything other than bleach and washing soda since opening several weeks ago. TA does creep up over the season and typically is a bit over 100 by the time I close in the fall.

There is no autofill. The makeup water is mostly rain -- it's been raining regularly here. I can't believe the pH of a 1/2" of rain water would change the pool pH measurably. I did need to add a bit of tap water right after opening, but was also around 1/2".

 

[frogabog]

Rain lowers my pH too. Doesn't seem like it would, but sure seems to. Have you tested the pH of your rain? However, does this occur when there's no rain? Is there perhaps a left over trichlor puck in a feeder or is some well meaning person adding trichlor that you don't know about? (seriously, I know it sounds silly to ask but last year a forum member actually had this happen, a kid was trying to "help")

 

[JasonLion]

That doesn't make much sense. Still, raising the TA should take care of it (as long as you raise TA enough).