Lowering alkalinity raises TDS - What is it?

[chem geek]

Tom,

I'll oversimplify this a bit, but essentially all that the lowering alkalinity procedure does is to replace bicarbonate ion ( (HCO3)- ) with chloride ion (Cl-). Technically, the TDS in terms of numbers of molecules of dissolved solids doesn't change at all, though in terms of the normal ppm (mg/liter) measurement it actually declines because chloride has a lower molecular weight than bicarbonate.

Why did you say that lowering alkalinity raises TDS? Why did you think that was the case? If the TDS is measured using a conductivity meter then it will measure slightly higher conductivity and report higher TDS because of my oversimplification since some of the carbonate is in the form of the neutral carbonic acid H2CO3 and does not get measured in conductivity, but this is a failure of the conductivity test to accurately determine TDS since it does not account for any neutral (uncharged) substances.

Since there will already be sodium ion in the water from the sodium bicarbonate that was initially added, removing the alkalinity will simply result in salt (NaCl) dissolved in your water. In fact, if you start with pure water and add calcium chloride and sodium bicarbonate and a little acid so that you are at a starting point with pool water, then if you remove most of the alkalinity you will end up with the calcium chloride still there plus sodium chloride. This of course ignores any "organic junk" that gets into your pool water and doesn't fully oxidize (hopefully most of it does).

Oh, I just realized that you are probably starting with some well water source that is high in TA, rather than starting with pool water. So the basic principle is the same, but the well water probably has significant amounts of calcium and magnesium in addition to high TA. So when you lower the TA you will produce salt (NaCl) dissolved in the water, but there will also be calcium and magnesium leftover as well and they will be charge balanced with choride ion (usually the calcium and magnesium original come from dissolved calcium carbonate and magnesium carbonate from rock that contains such substances).

Richard