Can iron stains be avoided if pH is 7.7

[smallpooldad]

I am looking after my neighbours SWG 20,000 gal plaster pool. As he is recovering from a major operation, it might take a year.

He wants to spend as little as possible on pool maintenance as funds are presently tight. I did explain that although the pH settles at 7.7 it would be better to add acid to keep it at about 7.3 to avoid iron stains which do develop over time in our pools. I did do a no drain AA treatment which worked well, using Jack's Magic Purple Stuff as the sequestrant. He does not wish to purchase additional Purple Stuff nor add acid to lower the pH from 7.7.

The current test results are as follows:

Cl 5
pH 7.7 (stays there - does not seem to go higher)
TA 90 (does not budge)
CH 400
CYA 400
Salt 3300
Borates 0 (2 dogs)
Temp 82-84
CSI + 0.04

Would moving the CH down to 250 from 400 help with the iron staining issue and perhaps calcium scale, this would give a CSI 0f - 0.16? Or is the pH at 7.3 or below the primary factor affecting staining, I believe that a lower CH also helps or is this assumption incorrect if the pH is not adjusted to 7.3 or below?

Any help would be appreciated, thank you.

Please note the specs below are for my pool not his.

 

[JasonLion]

He does not wish to purchase additional Purple Stuff nor add acid to lower the pH from 7.7.

Then there will be stains.

 

[Richard320]

If I remember my college chemistry, pH is the deciding factor whether the iron stays in solution or not.

 

[smallpooldad]

JasonLion,

Thank you for the quick reply. That is sad news indeed.

Aloha

 

[smallpooldad]

Richard,

That is what I thought as well but would a lower CH of 250 help with calcium staining?

Thank you.

 

[Richard320]

Richard,

That is what I thought as well but would a lower CH of 250 help with calcium staining?

Thank you.

Lowering CH will lower CSI, so of course there's less chance of Calcium Scale. Just change numbers on pool calculator and see how much. Temperature has a lot more effect than I thought it would, but pH and TA seem to carry more weight than the actual Calcium level.

 

[JasonLion]

Calcium has hardly anything to do with the appearance of iron stains. High CH levels do reduce the effectiveness of sequestrant at binding to iron, but that effect isn't significant unless CH levels are very high (say over 600).

 

[smallpooldad]

Apologies for the late thank you reply.

His salt cell was starting to get some calcium build-up not a lot but enough for me to explain to him that it would be best to keep the pH at 7.3, he agreed so at least we can keep the iron in solution hopefully. "Hopefully" is the the correct word as he does not wish to buy anymore sequestrant.

His alkalinity was 110, like mine it is high, mine is presently 90 -100 and I keep my pH at 7.3 -sometimes 7.2. For the life of me I cannot figure out why it stays so stubbornly high, a year ago we use to fight to keep at 80, if not looked after with baking soda additions, it could get down to 50, now the reverse seems to be the case. The well water must be of a different composition. Years back when I tested the municipal well water it used to have a pH of 8.2 now it comes in at 7.8, maybe that has something to do with it.

I will also drop the Calcium Hardness slowly to a level of 300 from the present level of 400 by not adding calcium after back washing as he does not wish to drain. Based on what you both wrote I think that seems to be a minor issue.

Anyways thank you both for your quick replies and kind help.

 

[smallpooldad]

What is even more odd is that the municipal water has an alkalinity of 80.

Both of our salt generators generate about 3 ppm of chlorine per day to maintain a level of FC of 5 ppm, about 2 ppm per day in the winter October through March. Both pools have a CC of zero. Our UV is high here at our latitude. Could that be raising the alkalinity?

Or could it be that both our plaster pools are quite old, his 20+ years and mine 16+ years old, and the surface is releasing something into the pool?

A mystery indeed, unless either of you can figure it out.

 

[chem geek]

Evaporation and refill will add whatever is in the fill water to your pool water and that includes both CH and TA. So TA can certainly rise over time from evaporation and refill though at some point the carbon dioxide outgassing makes up for that gain (assuming you are adding acid to then lower the pH back down which of course also lowers TA) and you reach a steady state.

 

[smallpooldad]

Chem Geek,

That seems to be case as after a few backwashes we do need to add calcium as our fill water has less than 60 ppm calcium.

And yes I do add acid at about a rate of 2 oz per day to maintain 7.2 - 7.3 ppm in my pool (10,000 gals), which seems to hold my alkalinity in the 80 to 90 range.

Hopefully this will be the case in his pool (20,000 gals) as we have just started adding acid yesterday to lower it to 7.3 from 7.7. Maintaining it at 7.3 - 7.2 should over time drop his alkalinity back down to 80 -90 ppm, presently we need to add 16 oz of acid as it bounces up to 7.7 overnight, so driving it down should, I think, cure the excessive need back into the 4 oz of acid daily area. This will make him much happier as regards cost.

Thank you for your informed reply.