Probes, Spectrometers and Chemical Test Kits

TDS The Variable Parameter

Total Disolved Solids is used as an indicator of Water purity. Traditionally TDS was determined by taking an accurately known volume of water, placing it in an accurately weighed pan, drying it, and weighing the pan again. The mass gain of the dry pan in milligram is then calculated to a per litre of sample basis, and TDS is expressed as mg/l.

Today most TDS measurements are done with an electronic conductivity meter. The conductivity of the water is then converted into TDS by means of a calculation using a conversion factor. Many conductivity meters allow the user to change this conversion factor. This is a very useful feature if you know the true Ionic composition of the water you are dealing with. The big problem is that many TDS meters do not have this facility and have a fixed conversion factor programmed into it. The common conversion Factors are : 0.65 for meters designed for Agricultural Fertigation use; 0.5 for some meters designed for potable water; 1.0 for some meters designed for effluent analysis.

Therefore, a potable Water unit of this type, used to determine the TDS of Effluent will yield a reading of only 50% of the TDS reading generated by a meter programmed for effluent.

The reason for the variation of the conversion factor is due to:

  1. Variation in the Atomic Mass of the Ion.
  2. Variation in the Valence Values of the Ion (Larger charge Faster movement for a given Potential Difference)
  3. Variation in the size of the Ion. Smaller ions move faster than larger ions due to fewer collisions with water molecules and therefore conducts electricity better.

Therefore:

  1. For concentrations with the SAME conductivity reading, solutions containing NaCl and BrCl will show the same TDS. However if determined under the traditional gravimetric method NaCl will display a TDS of 48.7% of that of KBr (atomic mass 58 vs 119)
  2. Similarly, Ferrous ions++ vs Ferric+++ ions will result in a 33% difference in Conductivity or electronic TDS Reading compared to a "same as" result derived from an analysis by means of the traditional gravimetric method. And so on.

A further distorting factor is CO2. Under certain conditions, Carbon Dioxide dissolves in water to form Carbonic acid, this is conductive and results in a conductivity reading. Under the traditional TDS determination process this gas will disappear resulting in a zero TDS reading.

It is for these reasons that we recommend that TDS and it's concomitant values, ppm. and ppt. are only used in respect of water with extreme circumspection.

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