Dissolved Salts
Confusion can arise from the fact that salinity is sometimes equated to one of the terms ‘total dissolved salts’ or ‘total dissolved solids’ – both are sometimes abbreviated TDS.
Total Dissolved Salts is determined by calculation from the results of analysis for common ions (sodium, calcium, chloride).
Total Dissolved Solids is determined by filtering a sample, drying at a specified temperature, and weighing the residue. It includes non-ionised substances if present (example: sugars, other organics, colloidal particles too small to be retained by filter medium), so it can be greater than Total Dissolved Salts.
TDS should properly be interpreted as Total Dissolved Salts, because non-ionised species do not contribute to electrical conductivity of the sample. Total Dissolved Salts (TDS) is a measurement of the total amount of dissolved salts in water and expressed in ppm of NaCl. TDS is essentially the same as conductivity. For aquarium applications it is recommended that the conversion formula TDS (in ppm) x 0.68 = EC (in µS/cm) be used. The conversion formula is only an approximation. TDS meters are calibrated with a Sodium chloride solution.
Conductivity or specific conductance is the measure of the water’s ability to conduct an electric current. Electrical conductivity is an inherent property of most materials, and ranges from extremely conductive materials like metals to very non-conductive materials like plastics or glass. About halfway between the two extremes in conductivity are aqueous solutions, such as aquarium water. In metals, the electrical current is carried by electrons, while in water it is carried by charged ions. In both cases, the conductivity is determined by the number of charge carriers, how fast they move, and how much charge each one carries.
Thus, for most aquarium waters, the higher the concentration of dissolved salts, which will lead to more ions, the higher the conductivity. This effect continues until the solution gets “too crowded”, restricting the freedom of the ions to move, and the conductivity may actually decrease with increasing concentration. This can result in two different concentrations of a salt having the same conductivity.
The specific conductance is measured by passing a current between two electrodes (one centimetre apart) that are place into a sample of water. The basic unit of conductance is the Siemens (S) and was formerly called the mho. The normal unit of measurement for conductivity is expressed in microsiemens per centimetre (µS/cm) or millsiemens per centimetre (mS/cm). Conductivity meters are calibrated with a Potassium chloride solution.
In natural waters the distributions of freshwater macrophytes, invertebrates and fish are not noticeably affected by variations in conductivity if concentrations remain below 1000 µS/cm (roughly equivalent to total dissolved salt concentration of 500 mg/L). Monitoring of intermittent inland streams has shown that conductivity levels fall dramatically during storm events, and rise gradually during periods of base flow, achieving maximum values when flows stop.