Ascorbic versus Oxalic Acid / CYA pre-dilution

The pool is gunite, holds about 9000 gals and is located in Hawaii. Last week I attempted to remove, using ascorbic acid, 10-year-old red dirt stains - oxidized iron/rust -these look yellow in the pool.

Red dirt stains comes from airborne volcanic dust, on the island of Kauai they sell red dirt T-Shirts.

The following method was used. The chlorine was reduced to zero via chlorine reducer. Algaecide 60 was used to stop a bloom. The ph was dropped to 6 and three times the normal amount of metal-out was introduced. Then 1 1/2 lbs of Ascorbic Acid was put in and left for 2 days. The pool was then drained and refilled and the water balanced.

I am now using liquid chlorine with a CYA of 25. The ascorbic acid did a great job of removing most of the yellow stains.

Interestingly after 24 hours it stopped removing stains leaving some areas of the copping tile with the stain and on the neighboring tiles it was completely gone! I removed these with 600 grade wet/dry sand paper, worked great. My theory is that the ascorbic acid reached some maximum absorption point, any theories on this?

There are still some light yellow stains around the pool, below the pool light there is a long narrow yellowish stain which could be iron so the question is should it be retreated with ascorbic acid or should oxalic acid be used? What is the difference between the two? Or is this a waste of time and money because the pool is 10 years old?

The plaster is still good and the pool looks 90% better than before. If I do this again I would like to pre-dilute the cyanuric acid in a bucket of water so I do not have to wait 3 to 5 days before the levels rise. Is this dangerous or even possible and is there any special procedure such as dropping the ph of the tap water?

Finally using Ben's CYA chart what would be the ideal minimum and maximum CYA level for 25 parts? The reason for asking this question is that below that too much chlorine is lost and above that chlorine needs to be maintained at a minimum of 3 which United Chemical states, on their website, can cause staining or is that sales talk?

Any help would be appreciated.

Follow up: It seems that after 5 days some of the iron is settling back into the pool, not bad but still noticeable.

Would it have been better to use the Cal Hypo method as described under "Wayyy too much copper--my fault!" to maintain the CL at 1.5 for a few weeks and then raising it to 5 - 10 rather than Clorox Regular , instead of 3 with clorox? Then not using everything else listed in the following except Calcium Hardness, Algaeside 60 (poly)?

Additional info follows; when I refilled the pool I adjusted it by adding Calcium Hardness to bring the water from 70 to 175, alkalinity increaser to raise it from 60 to 90, 3oz Algaeside 60 (poly), added CYA to bring it up to 25, one treatment of metal-out, then 2 days later one treatment of United Chemicals "Stain Remover," and kept the chlorine with clorox at 3. It seems that after I added the "Stain Remover" the iron started to settle out; this may only be a function of time.

The pool now looks 60% better than when I started.

Oh I would dearly like to solve this problem and I love this site.


Further Update:

After adding the "Stain Remover," which I think has 12.5% oxalic acid in it, my CL dropped to near zero, in the early morning. I then added a great deal of chlorine to bring it back to 3. Then thinking I might get an algae bloom I added Polyquat 60 to the maximum recommended. I did not realize that doing this would zero out my chlorine which is exactly what it did. I then added more chlorine and put pucks in my feeder and turned it up to max. This has brought the chlorine back to 2 and has stabilized there. Hopefully it will go above 4. I also added more CYA so that level might reach 30 from 25 as I read on this forum that below this number it is very hard to keep up with the loss. The water is glistening and it seems that the light stain that deposited itself is going away. Could it be the Polyquat 60? Any ideas as to why? Thank goodness I am working at home and able to keep an eye on it, or I might have a nice algae bloom.

Aloha,


Paul

How is your pool doing now? What was your ph when you refilled your pool? I have been trying to figure out stains for a while now - and am interested in results from those of us who do battle! This is my opinion on what works. I have only used ascorbic acid, so I can't give you any info on the Oxalic. First let me say that if you use pucks in your feeder, your cya will go up from the stabilizer in the pucks. The reason your chlorine was being used up is that the stain treatment uses chlorine - that is why they tell you to lower your chlorine before doing the treatment - otherwise you are only using up the treatment with the chlorine that is already there. When trying to get rid of the stains, you should always have a sequestering agent in the water too, something like metal free or sequasol. The stain treatment is lifting the stain from the surface of the pool and puting the metal back into solution. You need the sequestering agent to bind onto the metal so that the metal will be filtered out. I always keep my filter running 24/7 when doing a stain treatment. I use polyquat as a preventative till I can get my chlorine back up. Your chlorine will be used up while it is filtering out the metals - you may even get cloudy water if there is a lot of metal in solution that has to be filtered out. Keep your ph no higher then 7.2. You will notice that doing the treatment will also bring down the ph. It isn't necessary to drain your water. You just want to make sure that there is enough sequestering agent to keep the metals suspended until they are filtered out. It is a good idea to put a chlorine puck in the skimmer so that the stain will come off on the filter. Raise the chlorine in the pool slowly - when you notice that the chlorine is lasting, then you can start to bring it up to (with a cya of 25 - 30, you only need your chlorine at 2 to 3. Do not shock for at least 2 weeks, keeping the filter running 24/7. If you see stain starting to come back, lower your ph, back to 6.8, add sequestering agent and see if they lighten up. It means that there is still some metal in the pool. Keep the filter running and keep a chlorine puck in the skimmer. Low cya means you don't need high chlorine. There is no need to shock if there is no algae. Keeping up with the water is the best bet. Keep your ph at about 7.4 - higher ph combined with high chlorine will make any metals stain the pool. I hope this helps you, if you have any questions feel free to ask. You can look in the metals forum for more discussions on stains. Welcome to the forum!

Thank you for your reply.

The pool is much better although there are some stains reappearing. The pool water was 7.8 when I refilled it and I am now maintaining it at 7.5, which seems to just stay there as long as the alkalinity is maintained at 100 not allowing for the CYA adjustment. The CYA is now 33 is this too high?

The cl is between 4 and 6 is this too high?

I did add metal out and a polyquat when I refilled.

Next time I'll try holding it 7.2 for two weeks and putting pucks in the skimmer

Your cya is good. Your chlorine level is fine - you can keep it closer to 4 - here is the best guess chart:


Stabilizer . . . . . . Min. FC . . . . Max FC . . . 'Shock' FC
=> 0 ppm . . . . . . . 1 ppm . . . . . 3 ppm . . . . 10 ppm
=> 10 - 20 ppm . . . . 2 ppm . . . . . 5 ppm . . . . 12 ppm
=> 30 - 50 ppm . . . . 3 ppm . . . . . 6 ppm . . . . 15 ppm
=> 60 - 90 ppm . . . . 5 ppm . . . . . 10 ppm . . .. 20 ppm
=> 100 - 200 ppm . . . 8 ppm . . . . . 15 ppm . . .. 25 ppm

As you can see with a cya of around 30 you can keep it on the low side around 3-4. I would run my ph around 7.2 to 7.4. You may want to add some more metal out, if the stains are coming back you don't have the metal in the water staying in solution. - and run your pump 24/7 for a couple of more days. Don't shock the pool for at least 2 weeks, I only shock if I have combined chlorine. Remember the pucks will add cya, and also bring down your ph because they are acidic, so I would keep the puck in the skimmer for a couple of days, if you have either a DE or sand filter.

Hi, Paul,

I can't comment on the stain issue...wish I knew more.

Your CYA of 33 (is that a pool store reading?) and Cl of 4-6ppm are just fine. Your CYA will slowly rise if you continue using pucks. You might want to think about an alternate source of Cl.

Dave S.

Once again thank you to both of you.

I will drop the ph to 7.2 and put a puck in the basket. I will also add more metal out and use liquid chlorine to adjust cl in the future.