[GeoffLondon]

This question concerns the solids dissolved in the hard tap water found in London, and across southern England.

I use this tap water in a warm-mist humidifier that continuously boils about 30 ml of water to create the water vapor.  I've been filtering London's hard water with a household Brita filter, which is effective in reducing the amount of residue left in the boiling chamber.

However, every couple of weeks, a significant amount of "limescale" can be seen creating a rock-like layer of minerals in the boiling chamber, reducing the effectiveness of the humidifier's heating element.

I've researched this limescale, and learned it is likely to consist of the following solids:

CaSO4 Calcium sulphate
MgSO4 Magnesium sulphate
Ca(HCO3)2 Calcium hydrogencarbonate
Mg(HCO3)2 Magnesium hydrogencarbonate

The limescale mixture can be mostly dissolved with a solution of CITRIC ACID, and then rinsed away.  Perhaps two treatments are necessary.

Occasionally, after removal with citric acid, there is still a hard substance, which is talc-like on the surface, and yet resistant to removal, and seemingly impervious to further attempts at removal with fresh citric acid.

It defies reason that this remaining mineral arrived via the water, but the residue seems to be incapable of being dissolved.  Considering this is London tap water, what is this final residue likely to be?  What would be worth trying to attempt to dissolve it? 

Thanks,
Geoff

[Borek]

Silicates perhaps.

[GeoffLondon]

It's true that talc is a silicate. 

Would your experience suggest anything I could try as a solvent?

Thanks again!