Important physical and chemical parameters of water are influenced by temperature, pH, salinity, dissolved oxygen, and redox potential. Others are total suspended and dissolved solids, nutrients, heavy metal contaminants, etc [1].

Correlation between Conductance and TDS

Conductivity or electrical conductivity (EC) and total dissolved solids (TDS) are frequently used as water quality parameters. The value of EC and TDS are correlated. EC is the measure of liquid capacity to conduct an electric charge. Its ability depends on dissolved ion concentrations, ionic strength, and temperature of measurements. The dissolved ions concentration is usually measured as TDS. EC can be measured easily and inexpensively in situ by a portable water quality checker. On the other hand, the analysis of TDS is more difficult and expensive as it needs more equipment and time. Researchers have done various investigations to find out the precise mathematical correlation between these two parameters, so TDS concentration can be simply calculated from the EC value. The correlation of these parameters can be estimated by the following equation:

TDS ( mg L ) = k x EC ( μS cm) (1)

The value of k will increase along with the increase of ions in water. However, the relationship between conductivity and TDS is not directly linear; it depends on the activity of specific dissolved ions average activity of all ions in the liquid, and ionic strength [2].

Temperature and TDS

The conductivity of ions in water depends on the water temperature. Ions move faster when the water is warm. Hence, the apparent conductivity is increased when the water has a higher temperature [3].

It works pretty well but conductivity is temperature-dependent. The conductivity of ions in water depends on the water temperature. Ions move faster when the water is warm. Hence, the apparent conductivity is increased when the water has a higher temperature [4]

Temperature is also important because of its influence on water chemistry. The rate of chemical reactions generally increases at higher temperatures. Water, particularly groundwater, with higher temperatures can dissolve more minerals from the surrounding rock and will, therefore, have a higher electrical conductivity [5].

Evaporation and TDS

If it is exactly the same source of water, and no evaporation has occurred, TDS will be the same. If some evaporation has occurred, then the solute is more concentrated, so TDS is higher. As temperature increases, both the water molecules and the dissolved ions become more mobile and the conductivity increases, so even for exactly the same sample at different temperatures, with no evaporation to concentrate the solids, the reading will be different. It is because of the measurement method. As long as the temperature and composition remain the same, the conductivity of the water will not change. So the rate of diffuse is directly proportional to TDS [6].

Two things are at play when you measured your water’s TDS: first, warm water evaporates faster than cooler water. This leads to the minerals being concentrated. Thus, higher TDS reading. Second, the way the meter operates is by measuring and calculating based on the specific conductance of the water. It is measuring the salt content. Warmer water has higher conductance than cooler water. Again, higher conductance equal higher TDS. Temperature changes will often change the conductivity of various materials, and this may be interpreted by the meter as a higher reading [7].

Velocity (turbulence) and TDS

Stirring a solute into a solvent speeds up the rate of dissolving because it helps distribute the solute particles throughout the solvent. For example, when you add sugar to iced tea and then stir the tea, the sugar will dissolve faster

The rate of dissolving of a solute in a solvent is faster when the solute and solvent are stirred, the solvent is warmer, or the solute consists of smaller particles with more surface area. If there is turbulence or there is the velocity in water then there is an increase in the rate of solubility and TDS will be increased [8].

Surface water (velocity containing water) supplies such as lakes and rivers that are exposed to the sun are richer in organics may or may not have biological oxygen demand. Surface waters also contain more fine silt and more dissolved minerals because they have had more water movement and been exposed to more minerals. Water from deep wells is almost always colder than surface waters. It has been filtered naturally through layers of stone and thus has fewer minerals. The definition of the deepwater well is one that does not contain organics. Shallow wells often contain organics [9].

The solubility of solutes and TDS

Increasing the temperature increases the solubility of solids and liquids.

The solubility of solutes is dependent on temperature. When a solid dissolves in a liquid, a change in the physical state of the solid analogous to melting takes place. Heat is required to break the bonds holding the molecules in the solid together. At the same time, heat is given off during the formation of new solute-solvent bonds.

CASE I: Decrease in solubility with temperature: If the heat given off in the dissolving process is greater than the heat required to break apart the solid, the net dissolving reaction is exothermic (energy is given off). The addition of more heat (increases temperature) inhibits the dissolving reaction since excess heat is already being produced by the reaction. This situation is not very common where an increase in temperature produces decrease insolubility.

CASE II: Increase in solubility with temperature: If the heat given off in the dissolving reaction is less than the heat required to break apart the solid, the net dissolving reaction is endothermic (energy required). The addition of more heat facilitates the dissolving reaction by providing energy to break bonds in the solid. This is the most common situation where an increase in temperature produces an increase in solubility for solids.