The Total Dissolved Solids (TDS) is a rough proxy for what is known as ionic strength. My spreadsheet accurately calculates the actual chemical species including all of the ions and the TDS number is just used to compute any excess ions which are assumed to be salt (sodium chloride). I also make some assumptions that the calcium in the pool was added via calcium chloride (Cal-Hypo would end up the same) and that the carbonates in the pool were added via sodium bicarbonate or sodium carbonate. The main error in these assumptions would be for water that is initially very hard with lots of magnesium in it since the ionic strength in that case would be higher than I calculate. The spreadsheet calculates the actual ionic strength of the solution which is the sum of the ion concentrations multiplied by the square of their charges with this sum divided by two.
The ionic strength changes the equilibrium for all chemical equations that contain ions (neutral chemicals are only very slightly affected by ionic strength). Essentially, ions in water are partially "shielded" by other ions in the water and this shielding effect reduces the effective concentration of these ions (aka their "activity"). So if you have a chemical equation that has more ion species on one side of the equation or where those species are more highly charged, then a higher ionic strength will make those effective concentrations lower and will tend to drive the equation towards producing more of those ions.
The net effect is that the higher TDS, implying higher ionic strength, is more corrosive. This is because the dominant equations that are affected by ionic strength AND that affect the calcium carbonate saturation the most are the following:
HCO3- <--> CO32-+ H+
Bicarbonate Ion <--> Carbonate Ion + Hydrogen Ion
which is moved to the right and is less corrosive (more scaling) due to the higher carbonate. The other affected equation from the TDS being the following
CaCO3(s) <--> Ca2+ + CO32-
Calcium Carbonate <--> Calcium Ion + Carbonate Ion
which is moved towards the right at higher TDS and is therefore more corrosive. This latter equation is more affected by ionic strength by a factor of about 2.6 than the first equation so it is the one that dominates and is why higher TDS is more corrosive. The implication is that salt pools (those with an SWG, for example) are (all else equal) more corrosive and should have higher TA, pH or CH to compensate. The difference is rather small, but not insignificant, at about -0.3 between a fresh pool with TDS of 525 ppm (350 ppm salt) and a salt pool with TDS of 3200 ppm (3025 ppm salt).
Though the higher TDS implies more saturation with chemical substances (mostly charged substances, aka ions), the thing to keep in mind is that this TDS saturation is NOT extra calcium or carbonate. The calcium is accounted for in CH while the carbonate is accounted for in TA and pH and though calcium and carbonate make up part of the TDS, any "extra" TDS is presumed to be salt and that does not saturate the water with calcium carbonate, but rather "shields" the calcium and carbonate ions so that it is as if they are reduced in concentration (they are technically reduced in "activity") so that the water is effectively less saturated in calcium carbonate.
I hope that made some sense.
Richard
