I have acid erosion damage to my teeth and use a bicarb mouth rinse (half tspn or so in glass of water). My dentist told me to stop doing this, claiming Sodium bicarbonate is an amphoteric substance with both acidic and basic (alkaline) properties. The correct concentration in a solution is important in order to reduce any likelihood of acidity". Is it possible that mixing bicarb with water can ever be acidic? If so, how would it occur as I have to be careful not to cause further acid damage by using an acidic rinse! Thanks so much for any help.

When dissolved in water, the NaHCO3 splits into Na+ and HCO3-. I think the latter can then react with water as:

HCO_3\;^- + H_2O \to H_2CO_3 + OH^-

OH- is, of course, the hydroxyl ion, which is what makes it predominantly alkaline. H2CO3, however, is carbonic acid. Perhaps this is the starting point for a potentially acidic solution?

Hopefully Dr. Bob or Unknown008 will chime in and provide some better insight here. They're the real chemistry experts.

It is to be noted that HCO3 is both acidic and alkaline, as shown by this equation:

HCO_3/ ^1 \rightarrow H^+ + CO_3\ ^{2-}

An acid is defined as a proton donor, and HCO3 gives a proton (hydrogen cation).
It can also accept a proton (to act as a base/alkali) depending on the situation.

And of course, since H2CO3 can give two protons, it's even more acidic than HCO3 :rolleyes:

But the equation I posted above is what is the easiest to occur.

Your dentist is correct. Maybe you should try something else? I know for instance that Mint is alkaline.

I googled for a site and found this:
http://www.angelfire.com/az/sthurston/acid_alkaline_foods_list.html

It's a good site to read and if you do a quick search for mint, you'll get a small paragraph containing alkaline food. And scrolling further downs gives you a huge table of foods classified into alkaline and acidic :)

My blonde teacher mixed a teaspoon of bicarb and 2 capfuls of water and expected a reaction. How did she ever become a ks2 teacher?

As the salt of a weak acid, bicarbonate is a weak base. In the presence of a strong base (OH-) it can act as an acid, but as far as your mouth is concerned it has no acidic properties whatsoever.
I'd be concerned that its basic nature could perhaps cause erosion, but my dentists have always said it is OK.