Asc. acid v. sodium bisulphate

[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