Calling All ChemE and Chemists Out There
 

Basic question, but it is BMW related. I just had a basic sodium ion exchange water softener installed on my house. The tap I was my bike from now runs softened water. The good news is the bike will no longer get hard water stains. The million dollar question (and possibly the bad news) is, will the softened water cause corrosion on my bike because of the brine used during regeneration?

-Jeff

The Brine might attack the magnesium and aluminum parts. But so long as most of the water is stripped off these parts in good time. You should not be any worse off than the chemicals used in most rapid car washes today. I usually run my engine after washing my bike. Ensuring the heat from the engine helps evaporate any water left behind from the washing process.

Cheers
Tomas

Oy. Well, a couple of things here. First, the "brine" never makes it near the motorcycle, it stays in the softener. The softened water is simply water that previously had a lot of calcium ion, the concentration of which is conventionally lowered by replacing some ("ion exchange" if anyone cares, water with high calcium is placed adjacent a membrane that has pores which pass small ions but not large water molecules. On the other side of the membrane is the "brine" and the ion concentrations equalize from side to side.) with another cation. I am assuming sodium, but maybe not. I will check tomorrow. (Do you have a model no. for your softener so I can check on its specs?) It is entirely possible that no ions are introduced (there is no need to start out with a high ion concentration in the brine, it is OK for ion passage to work one way too), in which case the resulting water is far safer (for the bike) than what you had before.

Then theoretically, yeah, it really would be better not to use that water for washing depending on the parameters of your particular softener. So the particular replacement ion is important, some (obviously) are more reactive than others. As to stripping the water off, forget about it. When the water evaporates, the salts stay behind. And using heat, engine or otherwise, simply makes any reaction proceed quicker.

If you want to be trick, back in the lab we had (I think it was a Culligan system) tap for deionized water. I schlepped home a giant bucket of the stuff after rebuilding a car engine. Cut it 50/50 with antifreeze and four years later, the coolant was still dead clear. Neat stuff, and absolutely nonreactive.

People on sodium restricted diets should not consume softened water straight from the tap, so indeed, there is some sodium that would make it to the bike. An alternative for those on sodium restricted diets, with water softeners, is to use Potassium Chloride rather than Sodium Chloride for the ionic exchange. Is the Potassium less likely to cause corrosion?

-Jeff

Thanks. Now I do not have to search the 'net.

Well, there you go then, it is a two way system and sodium is added. (Which frankly, I would have known if I had just read the original post more carefully; it plainly said sodium ion exchange. I got lost thinking about the technology. Sorry.) The sodium level is not especially high, but it is a bit like washing your bike in salt water. (For the same reason, avoid car washes, even the manual ones, which conventionally recycle water.)

As to sodium vs. potassium, sodium is ca. 13% more electronegative (more reactive) than potassium. So if you can use a potassium-based brine (which would certainly be more costly) it would be better for washing the bike.

Better yet to go next door and borrow the neighbor's hose.