I have been reading this thread with disbelief. As Bill Warner says, the situation is very straightforward. Total dissolved solids is a term which did not originate with aquarists OR water treatment personnel - it is a concept of considerable antiquity in chemistry, indeed, a matter of daily experience. It means essentially the same as 'dry weight' and 'total non-volatile solids' and 'solid residue', terms used in other areas of applied chemistry. As Bill says, it is defined operationally by drying out a sample. It does NOT include dissolved liquids or gases. The person who said that dissolved HCl has to be considered a solid because the HCl-derived chloride ions can't be distinguished from those due to dissolved chlorides appears to have no understanding of chemistry. The examples chosen to illustrate zero TDS, high conductivity (HCl) and conversely, high TDS with zero conductivity (sugar) are good examples because they refer to everyday substances. Examples more relevant to aquaria might be adding free organic acids for acidification (not used in the hobby, to my knowledge, but would have some advantages) and dissolved organic compounds (as found in blackwater aquaria), respectively. Quite obviously, TDS has no relation to conductivity in general. However, if we know 1. that all the dissolved solids are ionic; 2. the relative concentrations of all the ionic species then conductivity is proportional to TDS - but only if the relative concentrations of the various ionic species stay constant. If, in addition, we look up the electrophoretic mobilities of the various ions, we can predict the conductivity. The point is that we cannot relate TDS to conductivity unless we know concentrations, and with this specific knowledge there is no longer any point in measuring conductivity. TDS should not be used at all. We should just measure conductivity and quote the result as such. Ross Drewe
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