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I use rainwater in my house for my water supply. I collect the water in plastic-lined tanks. Some of my piping at home is copper and I have blue stains in my shower because I think the copper from my pipes is being dissolved by the carbonic acid in the rainwater.

I'm planning to hang magnesium rods into my rainwater tanks, in the hope that these rods will react with the carbonic acid instead of my copper pipes. Does that make sense? Would this work?

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    $\begingroup$ Then you'll have white residue all over the place. Admittedly, Mg is less harmful than Cu. Then again, are you sure the blue stains were there because of Cu? $\endgroup$
    – Ivan Neretin
    Feb 27, 2020 at 9:19
  • $\begingroup$ You may try. But pure water in contact with air dissolves CO2 extremely quickly. So even with magnesium rods, I'm afraid water will quickly redissolve the CO2 from the air to compensate the losses due to the action of magnesium $\endgroup$
    – Maurice
    Feb 27, 2020 at 10:16
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    $\begingroup$ Mg could help if it would be in direct contact with Cu (sacrificial cathodic protection) but then it would dissolve pretty quickly. $\endgroup$
    – Ivan Neretin
    Feb 27, 2020 at 10:31
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    $\begingroup$ Distilled water ( rain) plus CO 2 is pretty aggressive to many metals. You could try adding calcium as adding limestone gravel to your storage tank; one source is aquarium gravel ,aragonite/crushed coral. It will add some hardness to the water and lower the acidity . I can't think it would create any problem and is cheap and easy. $\endgroup$
    – blacksmith37
    Mar 29, 2020 at 15:39
  • $\begingroup$ I wonder if other items show blue stains: kitchen or bathroom sink, toilet? perhaps detergent items used in the shower (body wash, soap, shampoo, conditioner) might be responsible for increasing the blue color by complexing or precipitating the color. Changing one or more of these items might help. Or you could investigate a cleaning system better adjusted to removing the blue, like one which contains a chelating agent, or is more acid than your rainwater. $\endgroup$
    – James Gaidis
    Jan 19, 2022 at 15:50

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Hot water heaters normally come with anodes. Aluminum is cheaper than magnesium so aluminum is more common. Replacement magnesium and aluminum water heater anodes are readily available online. That seems a handy source for either one. Magnesium has a higher potential and is consumed more quickly. In addition, I see some ads say magnesium has health benefits.

Both will react with carbonic acid.

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First, thanks for reporting some obvious unexpected chemistry occurring. Others in your community may also have similar metal leaching issues, which especially for children, may be a health concern (see, for example, this article).

My speculation, on what is occurring, is that the iron pipe in your shower is displacing (upon standing over time) some dissolved copper (possibly from created bicarbonate). The likely acidic rainwater may also contain chloride or nitrate salts, which may further complex with the fresh active copper precipitate. Here is a related illustrative source, per an article 'Electrochemical detection of nitrate and nitrite at a copper modified electrode' to quote:

The development of a reagentless electrode system for the determination of nitrate is presented. The approach is based upon the deposition of a macroporous copper deposit which shows marked selectivity for nitrate ion under mildly acidic conditions (pH 3) with a linear range extending from 10 to 200 mM nitrate.

So, do not employ Magnesium, address the iron presence. The coloration means there is some, but in actual amounts, most likely limited copper ion presence along with nitrate.

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