Black spot after Vitamin C tablet (Complicated)

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MarkGLN

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Sep 27, 2016
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I dropped a Vitamin C tablet in the pool on top of a small green stain. It turned black. I may have added fuel to the fire by putting a Tri-chlor tablet on it afterward as Tri-chlor has always been my go-to stain remover. My immediate goal is to get rid of this stain and I bought some Dry Acid to put in a sock and rub over the black spot in the hopes that might help. Please disabuse me of this notion if I am wrong.

While I'm most interested in getting rid of this black spot, we do have a larger staining problem so any help there would be appreciated.

Background

Our pool has a lot of stains. 95% of them are green and some are organic as they came directly from leaves with dirt mixed in. In recent weeks, I superchlorinated the pool hoping to get rid of the stains. It didn't do much good, possibly because the staining occurred about one and half years ago. I also tried putting a Tri-chlor tab on the stains that used to respond to a Tri-chlor tab. They no longer respond, not even after sitting on the stain for five hours. The chemistry of the plaster/stain seems to have changed.

While some of these green stains had definite organic origins, the rest are probably copper related. Their slight blueish tinge is characteristic of oxidized copper. This is not too surprising as we had a copper pipe that corroded to the point of creating major leaks.

Overall, the pool surface is kind of a mess and yet the stains are only noticeable when there is no sunshine. Otherwise, with the sun sparking on the water, our pool looks like a turquoise Caribbean paradise. I remember reading that this type of disparity can be characteristic of scaling, and we do have a high calcium hardness. However, there is no white calcium line on the tile, which I understand is typical in scaling.

Short of an acid wash or replastering, we're trying to do spot removal of the more noticeable stains.

What we've tried

  • Trichlor tablets sitting on stain - used to work after minutes or an hour, now doesn't work even after five hours
  • Rubbing with Vitamin C - no effect
  • Vitamin C sitting on stain - Turned a green stain black (I had thought this was one of the green organic stains, but it may have been one of the green copper stains.)

What we haven't tried

  • Jack's Magic Stain ID - the reason we haven't tried is because some of the solutions (like for Copper and Scaling) probably won't work with our pool. We still have copper piping and a heater, and a simple off/on filtering system that means any concentrated Jack's chemicals in the pool will either go through all our equipment or the equipment will have to be turned off.
  • Dry acid - I bought some today as I've heard that dry acid placed in a sock can sometimes work with copper. I'm hoping this will be the answer for lightening some of the green stains as well as getting rid of the black spot. Again, please tell me if I'm wrong as this is the immediate problem I'd like to solve.

Current chemistry:

FC - 5.5 (coming down from my superchorinating level of 15 or so)
CYA - 40-60 (I'm still learning how to measure this)
pH - 7.0
CH - 650-750 (note that I don't have any white calcium line on the tile so I'm not sure whether I have scaling.)
TA - I think it was around 80 or so
 
Vitamin C will often turn copper stains black (a different oxidation state of copper). It is not useful for copper stain removal. Dry acid is sodium bisulfate and it's typically not used to remove copper because it's doesn't react strongly with copper and it will etch the plaster. That's why people report dry acid lifting stains because it actually removes plaster.

If your pH of 7.0 is common and low pH is an issue for you, then you need to boost your TA to keep your pH from being so low. Low pH causes copper pipe corrosion and so you are only adding more copper to your water when the pH goes below 7.2.

You should consult with the folks at Jacks Magic as they may have a copper stain removal process that can work in your situation.
 
JoyfulNoise said:
Vitamin C will often turn copper stains black (a different oxidation state of copper). It is not useful for copper stain removal. Dry acid is sodium bisulfate and it's typically not used to remove copper because it's doesn't react strongly with copper and it will etch the plaster. That's why people report dry acid lifting stains because it actually removes plaster.
Click to expand...

Thanks Matt. You're against dry acid for spot-treating then? We're avoiding an acid wash since the pool is very old. I was hoping some judicious spot removal of stains was an option. How much plaster removal are we talking about? The same as with an acid wash? And can the removal be lessened by shortening the contact time?

If your pH of 7.0 is common and low pH is an issue for you, then you need to boost your TA to keep your pH from being so low. Low pH causes copper pipe corrosion and so you are only adding more copper to your water when the pH goes below 7.2.
Click to expand...

Before I learned about BBB, the pool guy was adding acid every week and when I finally tested the pH, I found that it was 6.8. After some long discussions, PG is coming around to my point of view that the pH needs to be higher. The reading before today's 7.0 was 7.8 and I told PG it was okay to lower it. The reason I'm not complaining about 7.0 (vs 7.2, for example), is that the staining seems to look worse at the higher pHs. (I'm hoping someone will address this appearance issue as I remember somebody else making the same comment.) Some others on this board seem to believe getting into the 6's is dangerous for copper leaching, but anything in the 7's range is okay. It sounds like you disagree on that, so maybe we should try harder to shoot for 7.2. Do you think 7.0 is low enough that I should actually add a little Borax?

You should consult with the folks at Jacks Magic as they may have a copper stain removal process that can work in your situation.
Click to expand...

Good idea, thanks.

BTW, do you have any ideas about that small black spot?
 
I'm wondering if I should try citric acid on the black spot. I believe it's postulated to be the test material for copper in Jack's Magic Stain ID.

Any thoughts on this? Or on using a little bit of dry acid for a short period of time?
 
pabeader said:
How many days have you had the black spot? I've read where someone had a similar reaction and it cleared after a few days and then spot was stain free.
Click to expand...

The stain happened yesterday.

I read something similar except that it was minutes not days after which the spot was stain free. That's why I waited several minutes and when nothing happened, I briefly put a chlorine tab on it. Probably not the wisest thing to do but it was sort of a panic re-action. At any rate, the stain is still there.

I'm hoping to get some answers on the dry acid, since that's an official recommendation of this site for copper:

From: Pool School - Metals in the Water and Metal Stains

Copper stains can be removed by putting dry acid in a sock and rubbing it on the stain.
Click to expand...
 
Sorry I missed all these posts, Tapatalk has been acting very weird for me lately.

Yes, dry acid will remove metal stains. So will trichlor tabs, or muriatic acid, or just about any strongly concentrated acid near the stain/plaster surface. There's lots of complicated chemical interactions I can write about regarding the etching and removal of copper stain as copper is a fairly complex ion in aqueous solutions with lots of interactions between it and chlorides and sulfates, etc. But let's just simplify it by saying that the copper metal ion (Cu2+) is not stable in aqueous solution with a pH above 7.0 and it will always want to come out of solution and form stains. So, you can temporarily remove a copper stain using an acid, but it won't fix the underlying problem which is you have an over-saturation of copper metal ions in the water and the chemical driving force wants those ions out of solution and into a solid form (typically starts as copper hydroxides and copper carbonates which then convert slowly into copper oxides that are very hard to remove).

So go ahead and use the dry acid to remove the stain...but don't be surprised if it reforms or you get larger stains elsewhere. Your water has too much copper in it and the ONLY solution is to drain the pool and very carefully manage your pH as you have copper pipes. I think the only long term solution for your current stains is to drain and acid wash the plaster BUT if you plaster is old and damaged, then it's just simply time for a replaster job. Then, once you have a fresh new surface, you MUST carefully manage your water chemistry as those copper pipes represent a huge (YUUUUGE!) source of metal contamination.

You might want to consider a resurfacing product called aquaBright by ecoFinish - Plaster and Aquabright Comparison . It is a flame-sprayed thermoplastic polymer coating that is perfect for plaster remodeling as it gives you all of the rigidity and texture of plaster but with the chemical inertness of vinyl or fiberglass. Metals would have a much harder time staining this type of surface and would be more easily removed without having to worry about damaging the plaster surface beneath the stain. I don't know what your willing to spend but replumbing a pool by removing the copper pipes and installing all new PVC would seem like a very costly option to me and not very practical. Keeping the existing plumbing in place but using a different surface material and properly managing your pool water chemistry would be the less expensive/less complicated approach.
 
Just want to weigh in for the interim re: advice.

It is true that you ultimately need to lower your copper level and a partial water change right now would stand you in better stead. Eg. if you were to drain and replace a third, that would buy you some time against the ongoing corrosion and reduce the frequency of staining.

With that said and given that staining is/will be an ongoing problem since the source is pipe corrosion, you can manage this staining by acquiring an "underwater acid wash" tool that can be used very sparingly (compared with a real acid wash) and address the spots, (see this: Amazon.com: Purity Pool UAW Underwater Acid Wash Set: Patio, Lawn Garden) then SEQUESTER the metals with a Jack's or Metal Magic sequestrant to forestall new staining. (Note: Yes, you could use dry acid. In the article, dry acid is referred to because it is an easy way to test. But over time, dry acid adds unwanted sulphates to your water. Ergo, the use of Muriatic Acid is preferable for ongoing "stain clearing" work with copper.)

You also need to watch and control your ph more closely -- 7.8 is too high for a pool with metals, but 7.0 is a bit risky and a bit too low for someone with copper pipes. Your sweet spot is going to be in the 7.4 range. This may mean weekly measurement from a pool guy won't cut it, so if you don't already have one, please consider getting either the TFP100 or Taylor K2006 Test kits (avail at TFTestkits.net )

- Hope that helps you get a management plan together.
 
pabeader said:
How many days have you had the black spot? I've read where someone had a similar reaction and it cleared after a few days and then spot was stain free.
Click to expand...

I said earlier the black spot was still there. Now it is gone. I think it was replaced by a white spot, but can't tell just yet as there's too much sun glare. This apparently is one of those multi-state reactions that copper is famous for.
 

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JoyfulNoise said:
There's lots of complicated chemical interactions I can write about regarding the etching and removal of copper stain as copper is a fairly complex ion in aqueous solutions with lots of interactions between it and chlorides and sulfates, etc. But let's just simplify it by saying that the copper metal ion (Cu2+) is not stable in aqueous solution with a pH above 7.0 and it will always want to come out of solution and form stains. So, you can temporarily remove a copper stain using an acid, but it won't fix the underlying problem which is you have an over-saturation of copper metal ions in the water and the chemical driving force wants those ions out of solution and into a solid form (typically starts as copper hydroxides and copper carbonates which then convert slowly into copper oxides that are very hard to remove).
Click to expand...

You know, I don't really mind the color of at least some of the copper stains. Especially the ones that are a light minty green. It's more the splotchy pattern of the stains. I actually wouldn't mind if I could remove some of the splotchy stains and have them redeposit as a uniform light color over the surface. I'm wondering if after a stain removal process, by brushing the pool everyday, I could encourage a uniform deposit. I know this is kind of a weird trade-off, but I think a uniform light mint stain would be a lot more attractive than a mottled/splotched pattern of darker green. Is there any merit in the idea of brushing the plaster to encourage a more uniform resettling of the stains?

You might want to consider a resurfacing product called aquaBright by ecoFinish - Plaster and Aquabright Comparison . It is a flame-sprayed thermoplastic polymer coating that is perfect for plaster remodeling as it gives you all of the rigidity and texture of plaster but with the chemical inertness of vinyl or fiberglass. Metals would have a much harder time staining this type of surface and would be more easily removed without having to worry about damaging the plaster surface beneath the stain. I don't know what your willing to spend but replumbing a pool by removing the copper pipes and installing all new PVC would seem like a very costly option to me and not very practical. Keeping the existing plumbing in place but using a different surface material and properly managing your pool water chemistry would be the less expensive/less complicated approach.
Click to expand...

Thanks, I'll look into that.
 
Swampwoman said:
It is true that you ultimately need to lower your copper level and a partial water change right now would stand you in better stead. Eg. if you were to drain and replace a third, that would buy you some time against the ongoing corrosion and reduce the frequency of staining.
Click to expand...

I'm actually part of the way there. One of our copper pipes was capped after it corroded too much, and a PVC one installed in it's place. During the installation of the new pipe, about a fourth of the pool water was drained. Since that time, and after the refilling, I've been careful about the chemistry as I've been learning about the BBB method. So I think most of the damage has already been done and some of the copper taken out of the water during the partial drain/refill. We do have other copper piping, but armed with the knowledge of BBB, I'm in a better position to "baby" it and keep it from corroding.

With that said and given that staining is/will be an ongoing problem since the source is pipe corrosion, you can manage this staining by acquiring an "underwater acid wash" tool that can be used very sparingly (compared with a real acid wash) and address the spots, (see this: Amazon.com: Purity Pool UAW Underwater Acid Wash Set: Patio, Lawn Garden) then SEQUESTER the metals with a Jack's or Metal Magic sequestrant to forestall new staining. (Note: Yes, you could use dry acid. In the article, dry acid is referred to because it is an easy way to test. But over time, dry acid adds unwanted sulphates to your water. Ergo, the use of Muriatic Acid is preferable for ongoing "stain clearing" work with copper.)
Click to expand...

I'll look into this kind of process again. My wife doesn't want us to keep muriatic acid around the house, but it turns out the pool guy is familiar with this process.

You also need to watch and control your ph more closely -- 7.8 is too high for a pool with metals, but 7.0 is a bit risky and a bit too low for someone with copper pipes. Your sweet spot is going to be in the 7.4 range. This may mean weekly measurement from a pool guy won't cut it, so if you don't already have one, please consider getting either the TFP100 or Taylor K2006 Test kits (avail at TFTestkits.net )
Click to expand...

I do have the test kits, but I'm having a hard time convincing my wife to keep muriatic acid on the premises. The compromise has been to let the pool guy add the acid, but he comes only once a week. I notice that the pH can go up one number per day, so I don't think a once-a-week acid add will do it.

Hope that helps you get a management plan together.
Click to expand...

Thanks.
 
One bit of knowledge I learned today while looking into the Jack's Copper and Scaling product, is that it requires stringent chemistry parameters. I don't think with my 700-ish CH, that I'll be able to make the cut.

The fact that the black spot created by the Vitamin C tablet apparently turned to white seems to bode well for an ascorbic acid treatment. Does that treatment have similar chemistry requirements with respect to Calcium hardness? I took a quick scan of one of the main ascorbic acid threads and didn't note a CH requirement.
 
Typically we advise that an AA treatment be done with the heater bypassed to protect against heater core corrosion. I think your copper pipes preclude AA as a treatment option.
 
Hi Mark. It is "possible" the AA would work theoretically if you allowed it to circulate long enough but if you can't use Jack's Stain 2 then as Matt said, you probably shouldn't do a full, long sitting AA treatment either. In this case, the Jack's copper would be the superior treatment that would be more bulletproof. Perhaps its worth considering installing a bypass to your heater.

But even with the $100 on the reusable tool, the MA treatment would likely be as effective as Jacks and ultimately the cheaper date.

So let me ask this...is your wife's ban on MA based on any particular trauma or is there a chance that if you took extra precautions, such as storing the MA outdoors in an isolated locked cabinet, she would compromise. Because frankly, you are really hamstrung here on the key component of metal management...not just the spots, but the finer control of the pool on a day to day basis.

While I don't love MA myself, I've also come to understand my initial paranoia about having it around was out of scale...I've since used it (diluted) to scrub/correct over-grout on 300 SF of new slate on my hands and knees...so i think its fair to say I've come a long way from early days when I didn't even want to use it for the pool ;) I wouldn't leave it lying about for small children to get into and I still respect its power, but it is possible to overcome MA-anxiety, just so you know ;)
 
JoyfulNoise said:
Typically we advise that an AA treatment be done with the heater bypassed to protect against heater core corrosion. I think your copper pipes preclude AA as a treatment option.
Click to expand...

I found a way around that -- using a submersible pump.

Edit: To clarify, the pool equipment would be shut off and a submersible pump used to circulate the water. Once the treatment is over, the pH could be brought back up and the equipment turned back on.

That said, I don't think I have the guts to use AA. I was kind of freaked out by the black spot. It eventually went away but what if I do the whole pool and the stains stay black? I read that citric acid is actually better for copper than AA, but I think I also read that if there's any trace of chlorine, a reaction will occur that produces chloroform.
 
Swampwoman said:
H

So let me ask this...is your wife's ban on MA based on any particular trauma or is there a chance that if you took extra precautions, such as storing the MA outdoors in an isolated locked cabinet, she would compromise. Because frankly, you are really hamstrung here on the key component of metal management...not just the spots, but the finer control of the pool on a day to day basis.
Click to expand...

My wife is just not a big believer in "Better living through chemistry." I don't think there's been any trauma, she just is very protective of the house and doesn't want any kind of accident. She's also concerned about the health hazards of the fumes. She's the type that uses a face mask even when using Clorox in the house for cleaning. :p

This is something we'll have to work out. It's becoming more apparent.
 
Before taking any major chemical action, I decided to try one of those mechanical stain removal products. -- one made of a patented non-abrasive material that claims to remove stains and/or scaling. In the back of my mind, I was thinking scaling was involved in my pool's plaster stains, whether or not the actual green material was organic, copper or both.

So..... I have been spending my afternoons mechanically rubbing away stains from my pool and it's working amazingly well. The stains are lightening considerably to the point that the plaster actually looks pretty good. There's a lot of "dust" in the pool now and I presume that's calcium scaling.

The question now is how much of this particulate matter, if any, can be expected to be returned to the pool after it goes through the filter. If there's copper embedded in the scaling, I presume it will return. But what about the calcium/scaling itself?
 

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