The Water Course

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Dissolved Solids

TWC 1.05 – Total Dissolved Solids (TDS)

Total Dissolved Solids (TDS) – What Are They?


Water is a well-known conductor of electricity. If a thunderstorm develops, everyone quickly jumps out of the swimming pool — for good reason. One curiosity is that the fewer minerals water has in it, the worse it is at conducting electricity. The electricity travels through water via the ions the water contains. Seawater is so effective at conducting electricity that swimmers have been known to survive nearby lightning strikes because the electric current will often take the more efficient route, around a human body, passing along between the masses of ions dissolved in the salty water.

So far in this chapter we have discussed hydrogen and hydroxide ions, as well as magnesium and calcium ions. Many other substances can also be dissolved in water. Even if these substances don’t cause limescale, they will affect the readings of a TDS meter. The most notable amongst these substances are sodium and chlorine. They develop strong ionic bonds to form NaCl (table salt) but will immediately dissociate when they come into contact with water, producing the free sodium cation (Na+) and chloride (Cl-) anion in the water. We will revisit both of these important ions when we discuss water treatment.


How to Measure Total Dissolved Solids



A handheld total dissolved solids (TDS) meter measures the conductivity of water — which is determined by the concentration of ions in the water. The TDS meter is calibrated to present an estimated reading in parts per million (ppm), which is the same as milligrams per litre. The device has two electrodes located a specific distance apart, and it measures the speed of a current that runs between them. The more ions in the water, the faster the current travels between the electrodes.

This system of measurement has its limitations because the rates of conductivity between different types of ions can be quite different. A TDS meter will be programmed to assume a certain ratio of all the ions present in water,