# Chemistry of toilet bowl mineral stains on porcelain

I live in Massachusetts, my house has 3 bathrooms and I live alone. If I don't scrub my toilet bowls once a week they build up brownish-yellow mineral stains. These seem independent of whether I've actually used that toilet bowl since the previous cleaning (since I live alone some toilets are seldom used).

Looking on the web there's lots of advice from "folksy" sources like "frugal blonde" or "professional mom" about how to clean toilets - usually involving vinegar, Clorox, pumice-sponges, etc, and lots of elbow grease, rubber gloves, bathroom fans and time. One guy uses WD-40! But being "folksy" none of these involve any discussion of the chemistry.

I thought if I understood the science I could save work and maybe even prevent the stains but I can't find ANYTHING on the web about this! What are the minerals in my water and what form are they in? Are they ionized or just some sort of suspension? What are the principal forms - I assume calcium, magnesium (and silicate?) compounds - and what form do they precipitate out in on the porcelain? What IS porcelain (chemically-speaking) anyway, and what makes such tight bonds at the interface between the minerals from the water and the porcelain surface?

If I knew the answers to these maybe I could even prevent these porcelain-mineral bonds from forming

• Is the water drinkable, I.e. the same you could use as tap water? If so, it can only be calcium carbonate. Perhaps the water is pretty hard in your area. I would made a test using a small amount of a product against limestone in the toilet reservoir as well as directly in the toilet. Then I – Alchimista Dec 21 '17 at 23:40
• ...then I would use only one bathroom for a while. Finally compare the porcelain of the two spare :) bathroom, the treated and the untreated one. It may work. Congratulations for the number of bathrooms, by the way. – Alchimista Dec 21 '17 at 23:43
• Addendum limestone can accumulate every kind of dirt and therefore assume tints – Alchimista Dec 21 '17 at 23:47
• What do you mean " a product against limestone "? Limestone is CaCO3, so what is the chemical reaction we're trying to obtain with such a product? Say I use vinegar (acetic acid) - I get CaCO3 + 2CH3COOH --> Ca(CH3COO)2 +H2O +CO2. But what, in colloquial terms, is Ca(CH3COO)2 - is it something I want in my plumbing system? – user316117 Dec 22 '17 at 2:08
• I mean you buy a product (they are existing in liquid form too) normally use to prevent limestone (not the rock, the deposit, CaCO3 anyway) in washing machine. In your case you just poured it to the toilet not in use and you let there. It is prevention and it should work. The aftermath treatment you refer to require to solubilze the already formed trace and thus is more difficult. Obviously you have to replace the product after flushing the bowl. – Alchimista Dec 22 '17 at 9:45

Many places have water with dissolved iron and manganese, which precipitates out as a brown or black stain when exposed to air.

Massachusetts has areas of podzol, where the fine sandy glacial till had been covered by pine forests. This makes for acid soil, which in anaerobic conditions readily dissolves iron in the ferrous form, $\ce{Fe++}$ from the ground, which is a pale green color, almost unnoticeable. If you dig into a podzol, exposing it to air, the soil turns brownish or orange in a matter of minutes as the iron is oxidized to its ferric, $\ce{Fe+++}$, form. That's what is happening on your porcelain fixtures.

It's hard to dissolve the brown ferric iron stain, but there are two ways to do so:

• Reduce the iron (and manganese) to the lower-valence state. Sodium metabisulfite $\ce{Na2S2O5}$ is sold for this purpose under names such as Red-B-Gone. It works well on clothing and light stains on porcelain. Use this with caution, since it can generate noxious sulfur dioxide fumes if it comes in contact with an acid.

• Dissolve the oxides in a strong acid, such as $\ce{HCl}$, hydrochloric acid. Though more dangerous than reducing agents, 10%-30% $\ce{HCl}$ can remove thicker layers of stain. Use with great caution, including eye protection and rubber gloves. Rinse immediately with plenty of water if any gets on you or your clothing! $\ce{HCl}$ is available in hardware stores and in plumbing supply shops.

You can also treat the water to remove iron and manganese or to sequester it with a polyphosphate additive.

Do not use alkaline drain cleaners (e.g. $\ce{NaOH}$ and Drano®) on vitreous china, as it etches the glaze and allows stains to soak into the porcelain, making the stains difficult or impossible to remove.