Hi all,
Conductivity will carry on rising until ions begin to precipitate out of solution as salts. It is true that N2 could be out-gassed, but that is a special case. The only way that you could lose (and gain) Ca++ or Mg++ ions is as compounds precipitate out of/go into solution. This may be a pH mediated effect (with carbonates for example). You see this effect every day if you add CO2, as the added CO2 changes the CO2 ~ HCO3 equilibrium. You can also see this if you add calcium hydroxide, sodium carbonate or sodium hydroxide to carbonate rich water as calcium carbonate is precipitated, there is a more complete explanation here:<
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cheers Darrel