Ascorbic Acid Treatment - Further Reading

WARNING: If you intend to use AA to lift stains, you have to use a sequestering agent or else the iron will simply come back out of solution once the FC neutralizes all the AA and the FC begins to hold. Unless the intention is to drain and refill the pool with iron-free or nearly iron-free fresh water, not using a sequestering agent is simply going to cause one of two outcomes - either the stains will come right back, OR the water will turn green/brown. Only when minimal stains are caused by an errant roofing nail in the water can one use a Vitamin C spot treatment and then forget about it. But if a pool has stains, you need a sequestrant.

How to Lift Iron Metal Stains Using Ascorbic Acid

Ascorbic acid is ineffective against copper stains. They will go from brown to black (ascorbic acid catalyzes the further oxidation of copper from its initial red/brown color to black). Ascorbic acid is only effective against iron stains.

Here are the directions to use Ascorbic Acid treatment to rid your pool of iron metal stains:

Before AA Treatment

1. Take your chlorine down to 0 and PH down to 7.2; if there is chlorine in the water, it won't hurt; it will just eat up the ascorbic acid, so you will need more to get rid of the stains. Let it decline naturally. Remove tablets, stop adding liquid chlorine, or turn off the SWG.
2. You can add Polyquat 60 per directions on the bottle to avoid getting algae while the chlorine is low.
3. You will need about a half to one pound of ascorbic acid per 10,000 gal. Go lighter on it and see if all the stains lift off before adding more.
4. Put the filter on RECIRCULATE if you have a Multi-Purpose Valve(MPV) on your filter. If you have a cartridge filter, then remove the cartridges.
5. Use a cup, go around the pool's perimeter, and drop it down the sides.
6. Let the ascorbic acid circulate for 1/2 hour. You will be amazed how the stain disappears before your eyes.
7. If the stain is not all gone, leave the filter in CIRCULATE and add more ascorbic acid close to where you still see the stain.
8. Leave it in CIRCULATE until all of the stain is gone. (add more ascorbic acid if it circulates for 1/2 hour and there are still stains)
9. When the stain is gone, add enough sequestering agent for the volume of your pool - more is better than not enough.
10. Put the filter back on FILTER if you changed it on your MPV, or replace your filter cartridges and leave it on 24/7.

The ascorbic acid will bring your ph down, and sometimes the alkalinity.

After 24 hours, you can start to rebalance the water.

After AA Treatment

• Bring up your pH and alkalinity - use Arm and Hammer Washing Soda first; it will raise both pH and alkalinity.
• Make sure you test in between because you don't want your pH to exceed 7.2.
• If your alkalinity is in range, and you still need to raise your pH, use Borax to take it up to no higher than 7.2.
• You can start to bring up your chlorine. You want to do this slowly. It will take a lot of chlorine - preferably using liquid chlorine only at this time, and try to take it up to your minimum chlorine for your cya according to the Chlorine/CYA Chart.
• As you slowly raise the chlorine, watch for stains starting to form.
• If you see stains, ensure the pH is no higher than 7.2, and add more sequestering agent. You should see the stain lift in a couple of hours.
• Do not raise to SLAM FC levels! Do not do a SLAM for at least 2 weeks!
• Make sure you keep your pH low (7.2) for a week or two - it won't be hard because the ascorbic acid and sequestering agent will help it stay low.
• Once your chlorine starts to hold, it means that you have used up the ascorbic acid in the water, and it will be easy to rebalance the pool back to recommended levels.

Here are two important notes:

• High pH and chlorine will precipitate metals not sequestered out of the water and onto the pool's surface again. If this happens, reduce the pH to 7 or 7.2 and add more sequestering agent. This should lift the stain off.
• You can add Polyquat 60 to the water before starting the stain treatment to avoid getting algae while the chlorine is low.

Where to buy ascorbic acid:

• Guardian Supplements (Reasonable shipping cost)
• chemistrystore.com (High shipping cost)

Where to buy Citric acid:

• chemistrystore.com (High shipping cost)

Citric Acid is cheaper than Ascorbic Acid, but you will have to use more. It takes a little longer, but it works.

Should You Use Ascorbic Acid or Citric Acid or Oxalic Acid?

People have settled on ascorbic acid because it seems to work more effectively than citric acid, though it is more expensive.

Also, chlorine reacts with citric acid, forming chloroform and other nastier chlorinated organics. They’ll offgas and go away, but it could cause eye/skin irritation until it does. Ascorbic acid is much safer.

One can compare the different reducing agents by comparing their molecular weights:

• Ascorbic Acid: 176.12 g/mole
• Citric Acid (monohydrate): 210.14 g/mole
• Oxalic Acid: 90.03 g/mole

So, one pound of ascorbic acid equals 1.2 pounds of citric acid and 0.5 pounds of oxalic acid.

As described in this paper, chlorine oxidizes citric acid to produce significant amounts of chloroform (a trihalomethane, THM), especially at lower pH, so it is better to use ascorbic acid instead even though it is more expensive.

Citric acid (CA) is a tricarboxylic acid with a bent/linear structure that will fully deprotonate, giving up three hydrogen ions. In comparison, ascorbic acid is a furan molecule (a ring structure related to glucose) with two -OH radical groups that will deprotonate fully in water, giving up two hydrogen atoms. So, on a simple molar basis, citric acid is more acidic than ascorbic acid. But that isn't all that relevant.

Ascorbic acid is also a powerful reducing agent, whereas citric acid is not as strongly reducing. So, in terms of iron chemistry (which is where most pool stains come in), ascorbic acid is very good at reducing the oxidation state of iron -

Fe3+ (rust) + e- --> Fe2+ (highly soluble)

So not only does ascorbic acid locally reduce the pH, which is good for dissolving metals, but it also converts most iron from its insoluble rust form to a highly soluble 2+ oxidation state. Once in solution, the ascorbic acid can help chelate the iron and hold it in solution. The only problem is when you add chlorine back into the water, the ascorbate molecule that chelates the iron gets oxidized into the dehydroxyascorbic acid form, which has no chelation effect (this is why chlorine and AA neutralize one another).

There is also another downside to citric acid. One might be tempted to think it would work better because it is more acidic. But that is not how it works. When citric acid loses all of its protons to the solution, it has three negatively charged sites that can chelate a positively charged metal ion. However, what tends to happen is the complex that forms between the metal ion and the citric acid molecule, which involves multiple numbers of citrate molecules, making the ring structure crowded and less stable. Citric acid works better at a lower pH to chelate because it releases fewer protons and thus forms a smaller, more stable complex. That would typically occur when the solution has a pH below 6, but in a pool, low pH can be dangerous to equipment and surfaces. So, in the end, citric acid is less desirable as a metal stain remover, even though it is cheaper.

If you only use citric acid for a single treatment and keep your water exposed to air, that's probably OK, but it is not recommended to use citric acid for repeated multiple treatments.

Another alternative is oxalic acid, but one has to be careful not to overdose or use it in high CH conditions due to the precipitation of calcium oxalate (i.e., it's OK as a local spot treatment, but not so good for a broad amount -- probably no more than 1 ounce per 10,000 gallons). Oxalic acid has a higher toxicity profile (so just be careful not to get it on your hands or inhale dust), though it's still less hazardous than some other chemicals we use for the pool.

What is the Role of the Sequestrant?

Ascorbic acid converts iron staining into dissolved iron in the water. Phosphonic acid (and other sequestrants) bind to the iron dissolved in the water, preventing it from redepositing as stains. However, the phosphonic acid breaks down over time, so you must add more to keep the iron in the water. To some extent, phosphonic acid can remove stains, but it isn't nearly as good as ascorbic acid.

When there is iron in the water, low pH (say around 7.0 to 7.3) tends to prevent staining, while high pH (say around 7.8 to 8.2) tends to cause staining. Phosphonic acid helps control this so high pH levels can occur without staining. High FC levels also tend to cause staining. The chances of staining increase as the pH and/or the FC level go up. Likewise, as the phosphonic acid level decreases, the staining chance increases.

The top sequestrants are:

• ProTeam's Metal Magic
• Jack's Magic the Pink Stuff (regular)
• The Blue Stuff (fresh plaster)
• The Purple Stuff (SWG)

You can also find many other brands with similar products, some less expensive. Sequestrants based on HEDP, phosphonic acid, or phosphonic acid derivatives are the most effective.

A saltwater pool is considered a high TDS water body. The major component of that TDS are sodium ions (Na+). Because sequestering agents are anionic compounds (they have negative charges to them), you want to use a sequestering agent formulated to work when there are higher concentrations of metallic species in the water. The Jacks Magic products formulated for saltwater pools will give you the performance you seek.

Some clouding of the water after adding sequestrant is entirely normal.

How to Keep Stains in a Pool from Returning

Ascorbic acid converts iron staining into dissolved iron in the water. To remove this iron, bind it to a sequestrant or replace it with metal-free water.

Replacing the water with metal-free water is the best long-term solution by far. That is assuming you can get some metal-free water. The metals in the pool must have come from someplace. Usually, they come from the fill water. "Draining" the pool is one of several ways to replace the water, not usually the best choice, however.

Some people have had success using the CuLator egg in their skimmer.

Tips When Doing an Ascorbic Acid Treatment for Pool Stains

• Putting the filter on RECIRCULATE or removing the filter cartridges helps avoid the slight chance of coating your filter media with metals, which might force you to replace your filter media. This doesn't usually happen, even if the filter isn't on recirculate, but best to avoid even the slight chance of a problem by using recirculate or removing cartridges.
• Ascorbic acid is way milder than an acid wash. There won't be any plaster etching, though the metals coming off can change the texture of the surface.
• A copper heat exchanger is safe from damage with a PH of about 7.0 and won't even notice a week or two at 6.6 or 6.8, though a couple of years at 6.6 or a few weeks at 4.0 would be a problem.
• If you can't wait to order ascorbic acid, go to your supermarket or hardware store canning section (you know, mason jars and stuff). You will find a product called Fruit-Fresh Produce Protector by Ball. Fruit-Fresh is Ascorbic Acid and Citric Acid and comes in a 5 oz. Container for about $6. It is quick and easy to get and works like a charm.

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