What is the TDS of DM water
The TDS value is a chemical parameter and represents - as an abbreviation for the English expression 'total dissolved solids' - the totality of all inorganic and organic substances dissolved in a solvent.
Common units for specifying the TDS value are ppm (parts per million parts) and mg / L (mg per liter).
A TDS value is therefore a purely quantitative value for the amount of substances that are dissolved in a water sample, for example; however, it does not allow any conclusions to be drawn about the nature and properties of these substances.
The TDS measurement is often used to assess the water quality of bodies of water, drinking water, industrial water, wastewater, etc. Each of these waters naturally or for technical reasons carries a certain amount of substances with it. In natural waters, the amount of substance depends, among other things, on geological factors; Due to the salt dissolved in it, seawater has a significantly higher TDS value than freshwater. And drinking water should contain as few foreign substances as possible.
Table: TDS values
The following table shows some TDS values or TDS ranges for various types of water.
|Pure water||0 mg / L|
|Freshwater||from natural sources||30 to 6000 mg / L||depending on the geological conditions|
|Rio Grande||in the spring||510 mg / L||near El Paso, Texas|
|in the autumn||610 mg / L||near El Paso, Texas|
|Sea water||Salt water||> 5000 mg / L||WHO |
|35000 mg / L||Frequently encountered TDS concentration in the oceans|
|Baltic Sea||~ 7000 mg / L||average value|
|Red Sea||~ 45000 mg / L||average value|
|Drinking water||Very good taste assessment||<300 mg / L||WHO |
|Good taste assessment||300 - 600 mg / L||WHO |
|Bad taste assessment||600 - 900 mg / L||WHO |
|Assessment of taste unacceptable||> 1200 mg / L||WHO |
|United States||<500 mg / L||Recommendation according to the US drinking water standard, 1996|
The most reliable and precise, but also the most complex method for determining the TDS value is gravimetry: the solvent (water) is removed from the sample by evaporation or drying and the residue is determined by weighing. If necessary, the behavior of highly volatile substances must be taken into account.
Example: If a residue of 1.205 g remains from a sample of half a liter of water, the TDS value (based on 1 liter of water sample) is 1205 mg * 2 = 2410 mg / L.
The indirect determination of TDS values via the conductivity of the water sample is widespread. However, there are a few things to consider when using this measurement method:Basics:
Pure water has only a very low electrical conductivity. If you add substances to the water that release free-path, electrically landed ions (electrolytes), such as salts, acids or alkalis, the conductivity increases depending on the type and concentration of these ions as well as the temperature (mobility of the Ions).
The following table shows the dependence of the conductivity - given in μS / cm - on the type and concentration of some selected ions at a given temperature (25 ° C):
An alternative option for pure salt solutions (sodium chloride, NaCl) is to determine the salt content via density; a corresponding table is listed under sodium chloride solution density table. Measuring devices for determining the density of liquids are commercially available under the name densimeter.
Overall, the determination of the TDS value is very specific to the sample, application and objective. The measurement is usually only suitable in connection with a known setpoint. If the TDS value suddenly deviates from a normally measured value, this is an indication that something is wrong with the sample and that further measures and measurements are required.
Sources and further information
TDS measuring devices, salinity measuring devices: sources of supply, purchasing
* Salinity Meters - Product Selection.
* TDS meter - selection of TDS meters in the ChemLin chemistry shop.
* Conductivity Meters - Selection.
Sources and Articles:
 - Total dissolved solids in drinking water. WHO Guidelines for Drinking-water Quality, 2nd ed. Vol. 2., Geneva, (1996).
Updated on December 05, 2017.
© 1996 - 2021 Internetchemie ChemLin
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