Risk of using ph down (dry acid) to remove brown waterline stain?

[2WeeksNotice]

The attached photo is a brownish stain at the waterline and 3-4" below. Not sure exactly when it started, but believe it was after adding make up water (well, with known high iron, but run thru a metals pre-filter), or possibly after shocking with CalHypo. This was all before finding TFP earlier this year. I'm skeptical of the true efficacy of the pre-filter, but it may help a little. The CalHypo...well, now I know what to do to shock.

Also attached are recent tests at a local pool store for metals of the hard well water, softened well water, and current pool water all taken the same day.

Today, I tried AA test and it does a very very little bit to change the stain. CalHypo in a sock did nothing. But ph down instantly erased it! From everything I've read, this means copper. Pls comment if it could still be iron (or something else), as that is what I'd expect from my well water. Regarding copper: I have never used algecides (unless the pool co put some in to open up?), have a CuLater put in by the PB. Only other possible thing was the phosphate remover and clarifier the PB used (Phosphate 9000 and Pool Blue; spec sheets here if needed Spec Chem - MSDS). Not sure if those contain copper or not, or would create staining? I do have copper pipe in the house (20 yrs old).

The questions are:
1. If I would put the ph down in a sock and use 3-5 lbs (big guess) to do the all the waterline, what's the risk? Sulfates & corrosion are what I've read. Is this amount in a 20,000 gal SWCG pool w/heater damaging?
2. If I do that, what's the recommendation for bringing ph back up?
3. Is it copper for sure?
4. Will the stains come back if I use the softened water as make up water?...well, I think I know that answer, or at least a way to find out.

I don't really want to go down the Jack's process for 2 days to 2 weeks. Seems a bit involved for a cosmetic stain like this.

Thanks for any comments.
Allen

 

[Texas Splash]

Even though the test results don't show it, the impact of pH Down points to copper. To your questions:

1. If I would put the ph down in a sock and use 3-5 lbs (big guess) to do the all the waterline, what's the risk? Sulfates & corrosion are what I've read. Is this amount in a 20,000 gal SWCG pool w/heater damaging?

I would do a portion of the waterline at a time then re-test the pH. It should be okay, but best to be cautious. Your TA appears to be high enough to keep the pH fairly elevated.

2. If I do that, what's the recommendation for bringing ph back up?

You could aerate, but the TA should allow the pH to rise fairly soon.

3. Is it copper for sure?

The pH Down points to it, yes. Perhaps from the old plumbing.

4. Will the stains come back if I use the softened water as make up water?...well, I think I know that answer, or at least a way to find out.

Depends on the copper level in the water and your current chemistry. This is where your TF-Pro is critical. After you've completed the stain removal, be sure to keep the pH under control. An elevated FC and pH tend to aggravate metals. Winter water exchanges may help.

Hope that helps.

 

[2WeeksNotice]

So the resulting sulfate/corrosion from the ph down is not a big deal? You only mentioned making sure the ph doesn't get too low. Or is the low ph what leads to potential corrosion?
Thanks again!

 

[Texas Splash]

So the resulting sulfate/corrosion from the ph down is not a big deal?

If you used dry acid all the time yes. But since this is basically a one-time effort, there should be no lasting side effects.

 

[2WeeksNotice]

Just updating the results in case anyone else needs to do this. It works very well, just have to take it in sections.

I did half the perimeter one day and half the next. Took about 6 lbs of dry acid to do the whole pool (37'x16'). I put a tennis ball size amount of it in the sock at a time. It would dissolve pretty quickly as I rubbed, so if there was something else to put the dry acid in that wouldn't dissolve so fast, it might help. pH did drop ~0.6 each day, but then used homemade pvc fountain (thanks for the idea @duraleigh on an old post) to aerate. Also needed make up water today, and my fill pH is pretty high, so this helped too. It may be a good idea to do this when the water level is lower (or even remove some water) so the acid doesn't dissolve so fast.

There is some very light staining on the steps and walls still, but barely noticeable. Going to let that go. I think all this waterline staining started when I did my first SLAMs. Looked at my PB invoice from opening and there is an algaecide on there, so that could be the copper source. Also they installed a Nature2 express...which will be removed very soon.

And darn it, be careful not to drip any water from the sock onto aluminum or concrete...It will leave a mark!

Thanks again @Texas Splash for the advice!

 

[JamesW]

Your rope guides are aluminum, so you are very close to the tracks with acid.

 

[2WeeksNotice]

Yes… first day I was in the pool scrubbing, so I didn’t drip anything or get near the guides as they are well above. Second day I was on the deck leaning over the edge, so that’s when I noticed a couple spots. Next time ( hopefully not necessary) I will be in the water.

Also will be curious to see if this copper stain comes back at next SLAM or not.

 

[2WeeksNotice]

Slightly frustrating and embarrassing ...nonetheless a learning experience...I just realized I used Sodium Ascorbate (a mineral salt of Vitamin C) and not Ascorbic Acid for the iron stain test! Chemists, I'm sure this will be no surprise to you that Sodium Ascorbate had no effect on iron stains? But wow, tonight a few AA tablets sure worked wonders.

The reason I went back with AA is that it appeared like the waterline stains were coming back a bit (not unexpected since I didn't put any sequestrant in). Was planning to, just didn't think it would come back that fast. But also knew I had other light staining on steps and some walls that dry acid didn't seem to clean and wanted to try AA on those areas.

Question is if it's expected that the dry acid would take off most of the waterline iron stains last week...or is it possible to have both iron and copper stains at the waterline? From my well water, I'd really expect iron to be the main contributor.

...And FYI on the Sodium Ascorbate was on my kitchen shelf, I assumed to be "Vitamin C". I use it in the winter when I lap swim indoors at the local recreation center. Mix AA (or Sodium Ascorbate) with water in a spray bottle to remove the heavy residual chlorine smell on my skin. I'll be sure to tell the rec plex about TFP!

 

[mknauss]

heavy residual chlorine smell

That is not chlorine. That is chloramines. The stuff left over after the chlorine has oxidized urine and fecal matter ------------

 

[2WeeksNotice]

thanks for that....i like my pool more every day!

 

[JamesW]

Chemists, I'm sure this will be no surprise to you that Sodium Ascorbate had no effect on iron stains? But wow, tonight a few AA tablets sure worked wonders.

Ascorbic acid and sodium ascorbate are basically the same thing except for the extra hydrogen ion.

A pKa of 4.70 indicates L-ascorbic acid will exist almost entirely in the anion form at pH values of 5 to 9.

So, once you put ascorbic acid into water, it becomes ascorbate.

So, it should do the same as ascorbic acid.

The main reaction is an oxidation reduction reaction where the ascorbic acid or ascorbate gets oxidized (loses electrons) and the iron gets reduced (gains electrons).

Note: Gaining electrons causes the oxidation state to be lower because electrons have a negative charge.

The acidity of the ascorbic acid might make a difference in the results, but I would think that they would work about the same.

Ascorbic Acid: The chemical formula of ascorbic acid is C₆H₈O₆.

Sodium Ascorbate: The chemical formula of sodium ascorbate is C₆H₇O₆Na.

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[JamesW]

When ascorbic acid is added to the pool, it becomes the ascorbate ion by losing a hydrogen ion.

pK1 = 4.17 (or 4.7); pK2 = 11.57.

Dehydroascorbic acid (DHA) is the oxidized form of ascorbic acid (vitamin C)/Ascorbate.

Ascorbic acid (vitamin C)/Ascorbate is oxidized with the loss of two electrons to form dehydroascorbic acid.

It is an electron donor to the iron, which reduces the iron and makes it soluble again.

As you can see, it is the carbon atoms that lose electrons to go to a higher oxidation state.

 

[2WeeksNotice]

@JamesW ...wow, thank you. But I really shouldn't be hanging out in the deep end! I'm in the shallow end...by the steps. Figuratively and literally! Tonight I put a Vit C tablet and some Sodium Ascorbate powder on the stained step next to each other and Vit C won visibly and easily. My SA is a couple years old, but not sure it would have a shelf life.

Regardless of all that, the question remains: is it expected that the dry acid would take off most of the waterline iron stains last week? Or was it really copper?...or is it possible to have both iron and copper stains at the waterline? From my well water, I'd really expect iron to be the main contributor. And if I have both, does one sequestrant take care of both? Thanks!!

 

[Darin]

i just use iron-out in a sock to get all of my stains. It works very good and it is not harsh.