Re: RO water bad for coffee machines?
The lack of ions in pure water will cause the native oxide to dissolve even with a vanishing (but not zero) solubility. Dissolved oxygen and CO2 may help both the removal of native oxide and the oxidation process. In a boiler there should be much less dissolved gas, but the high temperature will increase solubilities and speed up chemical reactions.
In short the chemistry of water is very complex. Take away the ions and many things that are virtually insoluble will dissolve. There is a lot of zinc in brass, which is a very electro-active metal, so expect brass to be more strongly affected.
The aluminium example was meant just to point out that metal can react with water. The aluminium reaction is vigorous, but zinc will behave similarly and copper less so. The only thing protecting the metal surface is a native oxide. Once this is removed (by dissolving in water with low ion content) the metal can corrode. Continual use of ion-less water will continue this process by replacing the water and preventing ion biuld-up.
That said, Im not sure just how ion-less commonly available RO water is. At work we can achieve a resistivity of 18.2 MOhm-cm. I doubt that home systems or RO drinking water is close to that grade. Ive been using Pureau RO water in my machine with no problems so far. However I often use filtered water as well and the residual ions in the boiler should keep things fine. Certainly there is no metallic smell to the steam, so I doubt there is any adverse water chemistry occurring. This water may only be from a single pass RO system, not the ultrapure water that a high end Millipore laboratory system produces.
The bottom line is that the manufacturers dont recommend it and no cafe uses it. Therefore youre not gaining anything by using it, but may in fact be losing something, be it taste or a slowly dissolving machine.
At least this thread has raised some awareness.
Cheers,
Mark.
Originally posted by Bill link=1209725970/15#23 date=1209866233
In short the chemistry of water is very complex. Take away the ions and many things that are virtually insoluble will dissolve. There is a lot of zinc in brass, which is a very electro-active metal, so expect brass to be more strongly affected.
The aluminium example was meant just to point out that metal can react with water. The aluminium reaction is vigorous, but zinc will behave similarly and copper less so. The only thing protecting the metal surface is a native oxide. Once this is removed (by dissolving in water with low ion content) the metal can corrode. Continual use of ion-less water will continue this process by replacing the water and preventing ion biuld-up.
That said, Im not sure just how ion-less commonly available RO water is. At work we can achieve a resistivity of 18.2 MOhm-cm. I doubt that home systems or RO drinking water is close to that grade. Ive been using Pureau RO water in my machine with no problems so far. However I often use filtered water as well and the residual ions in the boiler should keep things fine. Certainly there is no metallic smell to the steam, so I doubt there is any adverse water chemistry occurring. This water may only be from a single pass RO system, not the ultrapure water that a high end Millipore laboratory system produces.
The bottom line is that the manufacturers dont recommend it and no cafe uses it. Therefore youre not gaining anything by using it, but may in fact be losing something, be it taste or a slowly dissolving machine.
At least this thread has raised some awareness.
Cheers,
Mark.

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