Salt Stain removed by Ascorbic Acid

[NotYou]

Hi All,

I've been using ascorbic acid to remove salt stains for a while, but it always amazes me that it's immediate - I've attached photo's of before and after - ~3 minutes apart.

What I'd like to know is why? What happens?

Thanks in advance,

Buddy.

[exchemist]

1 hour ago, NotYou said:

Hi All,

I've been using ascorbic acid to remove salt stains for a while, but it always amazes me that it's immediate - I've attached photo's of before and after - ~3 minutes apart.

What I'd like to know is why? What happens?

Thanks in advance,

Buddy.

Those stains look like iron salts to me. You'll have to explain to me why you associate them with salt, as I'm a Brit and we don't have many outdoor swimming pools here. Do you use salt to treat the water in some way? Could it have iron as a contaminant, like the rock salt we put on the road in winter, which always looks a bit pink or brown?

Chemically, I would expect ascorbic acid to form a "chelate" with iron Fe³⁺ ions, which is a sort of cage molecule enclosing it. This could serve to dissolve the iron salts off the sides of the pool, if that it what it is.  

[NotYou]

I, too, suspect that it's iron stains. The stains sometimes appear at the spot where I pour the salt into the pools. The salt I use is highly refined pool salt - ultra-fine pure white crystalline salt. However I don't think they remove all the contaminants, too costly.

Here's what Fishel Pools says about pool salt:

Quote

 

Why Is Salt Used in Pools?

You probably already know that chlorine is used to keep pools safe by disinfecting the water and killing disease-causing bacteria, algae, and other organic matter. Chlorine is necessary for a safe swimming environment, but in its liquid form, it can sometimes cause damaging side effects like skin irritation, stinging eyes, and brittle hair.

These days, many pools are equipped with a salt chlorine generator system (also known as a salt water chlorinator) instead. With this system, pool salt (NaCl, or sodium chloride) is dissolved into the water, and the mechanical system electrolyzes the salt and transforms the chloride portion of the salt into chlorine. This chlorine functions in the same way as traditional liquid chlorine, making it a great replacement for a vital piece of pool maintenance and sanitation. However, unlike liquid chlorine, chlorine electrolyzed from salt lacks a strong odor and it won’t lead to irritated eyes, dry skin, or green hair. Plus, it can decrease pool maintenance costs by as much as 80%! (source)

WHAT YOU NEED TO KNOW ABOUT SALT CHLORINE SYSTEMS

The salt that you add to your pool should be as pure as possible—you can purchase pool-grade sodium chloride specifically for this purpose. Also, you’ll be glad to know that the amount of salt added to a pool produces a concentration far less than the salt concentration of seawater (so when you exit the pool, your skin won’t have a salty residue). It is important, however, that you maintain the appropriate salt concentration to decrease the possibility of corrosion while still maintaining the pool’s cleanliness and sanitization.

 

I think you're right about the chelation, here's what I found at PubMed:

Quote

Ascorbic acid and its metabolites, including the ascorbate anion and oxalate, have metal binding capacity and bind iron, copper and other metals. The biological roles of ascorbate as a vitamin are affected by metal complexation, in particular following binding with iron and copper. Ascorbate forms a complex with Fe3+ followed by reduction to Fe2+.

Amazing stuff, hey?

[exchemist]

7 hours ago, NotYou said:

I, too, suspect that it's iron stains. The stains sometimes appear at the spot where I pour the salt into the pools. The salt I use is highly refined pool salt - ultra-fine pure white crystalline salt. However I don't think they remove all the contaminants, too costly.

Here's what Fishel Pools says about pool salt:

I think you're right about the chelation, here's what I found at PubMed:

Amazing stuff, hey?

Ah, so the salt is used to generate chlorine by electrolysis - which I can see makes sense, if you don't mind swimming in salt water and the associated potential for corrosion, I suppose. I didn't know that. Thanks.    

Edited April 30, 2021 by exchemist