Asc. acid v. sodium bisulphate

[Carolina]

I have a problem with metals. There is 0.25 ppm (I think it's ppm) Cu and Fe in the water, and light brown staining on my gunnite pool. Ascorbic acid rubbed on the stain does nothing, nor does treating the entire pool with ascorbic acid. However, sodium bisulphate in a sock (placed on the stain) takes it away in a flash.

I am assuming that the ascorbic acid is most likely a weaker acid than the sodium bisulphate, and that is why one removed the stain rather than the other. If that is the case, I am also assuming that bringing the pH of my pool down to a certain point and proceeding from there would do the trick. However, I would like to know if anyone has any experience with this. I don't want to drop it too much and ruin my equipment. I can bypass my filter, but unfortunatly, not my heater.

Currently, I am using 4 quarts of Proteam Metal Magic in my ~12,000 gallon pool (on my 6th day), and it is lifting the stains *very* gradually. I'm also hoping it took care of the metals in the water.

[chem geek]

Carolina,

The reason that ascorbic acid works to remove stains is that it is what is known as a "reducing agent" which is the opposite of an "oxidizing agent" such as chlorine. It reduces metal, in particular iron, from a form called ferric (as in ferric oxide, or rust) to another form called ferrous that is more soluble in water and in particular can be kept soluble by another compound called a sequestrant.

The fact that ascorbic acid is weaker is not why it works to remove stains. To remove iron stains, a reducing agent combined with a sequestrant is what is required. And it is true that acidic conditions help dissolve the already formed rust as well as copper stains, but it does not take a very strong acid to do that. By having the pump off during application, the local pH near the stain can be lower and when combined with the reducing effect from the ascorbic acid and the overall sequestrant in the water, the metal is dissolved and then kept soluble in the water.

For older stains, it is true that more acidic conditions may be required since the stain may be deeper into the plaster, but again this only requires localized acidic conditions and at some point requires what is essentially an acid wash which removes both stain and some plaster at the same time, but this is a last resort and again only for the deepest (usually oldest) of stains.

Richard

[Carolina]

OK, that makes sense. But why does the bisulfate work so easily, whlie the ascorbic acid does not? Not only does the ascorbic acid not take the stain away, it makes the surrounding area darker.

And as such, do I simply use sodium bisulfate to treat the entire pool?

Thanks for your reply, Richard.

[chem geek]

The sodium bisulfate is a stronger acid than the ascorbic acid so for dissolving harder more embedded stains in plaster it is more effective, at least locally as you have applied it. Essentially, you are doing the "acid wash" locally removing the stain and probably a small amount (thin layer) of plaster as well. So it works faster, but is harsher. If that is what is needed for your particular stain, then that's fine, but usually you start by trying ascorbic acid first, especially for iron stains (since they are more readily kept away from re-staining by reducing and sequestering them).

[waterbear]

Actually, the light brown staining you see might be scale depost and not metal stains. .25 ppm is a very small amount of metal in the water...you said it was both copper AND iron. Copper usually produces brown stains while copper stains are either blue or grey black iron stains and blue copper stains are easy to remove. Black copper stains are difficult. Scale often produces a tan stain that is easily removed by acid since it dissolves the calcium carbonate, which is what a scale stain is. Since you said that the stains are easily removed by acid but not by a reducing agent I suspect they are scale. Dropping your pH to 7.0 for a period of time might remove them (and the reason the Metal Magic is removing them is because it does drop the pH of the water but you will have better results by either acid washing the pool or using a no drain acid wash kit such as the one by United Chemical.

I would be interested in a full set of test results. I suspect that your calcium levels are high, possibly your TA is high, and you have had periods of high pH which caused the staining in the first place.

[Carolina]

alk: 85
pH: 7.4
Calcium: 220

The older stains are caused by running my pool at a higher pH, and probably overchlorinating. This is something I've since corrected. However, recently (2 months ago) someone dropped a metal nut into the pool, which made half a dozen penny-like stains on the bottom.
Since then, light tan stains have been appearing all over the pool. An ascorbic acid treatment along with a sequesterant did nothing for them. I followed the protocol on this site exactly, hoping that it might work for me.

Would dropping the pH to 7.0 damage the heater? There is another site on the web that advocates dropping the pH to 6.0 for a day or so, but that seems to have the potential to cause too much damage.