How are total dissolved solids (TDS), salinity and electrical conductivity (EC) different?

This is one of those details that we don't pay much attention to. We have a vague idea that TDS, salinity and EC are somewhat different yet most of the time, we treat them as the same creature. Well folks, they may be in the same happy family but they are certainly not the same family member.

Ok, here is a quick introduction.

Electrical conductivity (EC)
EC is easily and cheaply measured by a sensor consisting of 2 electrodes. A potential difference is applied, a current flows between the 2 electrodes and the conductance (reciprocal of resistance) in Siemens (formerly mho) is measured. The conductivity is determined by multiplying conductance by the cell constant (a property of the sensor; = electrode distance / surface area) in cm-1. Since a Siemen is a pretty large value, most fresh waters have EC in uS/cm while brackish and salt waters have EC in mS/cm.

Naturally, the higher the amount of ions (i.e. dissolved salts) in the water, the easier it is to conduct electricity by ion transport and the higher the EC.

Total dissolved solids (TDS)
Theoretically, TDS refers to the total mass of dissolved solids (most inorganic and some organic stuff) in a given volume of water.

One way to measure TDS is to filter a known volume of water through a standard glass fibre filter, vaporise the water in a steam bath, dry at 180oC and weigh the remaining solid residue. If you are testing fresh waters, you are going to need a lot of water (figure at least 1L). If all these steps sound tedious, you are right. You also need to wait while the drying takes place. Definitely not suitable for achieving instantaneous results. Also, note that the drying step may remove a portion of some substances e.g. chloride, organics.

Nowadays however, TDS is commonly measured using sensors (easily, cheaply and instantaneously). Or is it? If you religiously go through the instruction manual for your TDS instrument (meter + sensor/probe), you realise there is usually a user defined parameter known as TDS constant ranging between 0.3 - 1.0 (range varies depending on instrument brand and model). OMG, what is this? What value should I use? Thankfully, the instrument is usually set at a default, say 0.65. Not so thankfully, this TDS constant can be awfully important as its value depends on a the type of water sample. Unfortunately, most people will not go into such details when measuring TDS.

Basically, TDS (mg/L) = TDS constant * EC (uS/cm)

The above formula only works if you use the correct TDS constant for your water. Fresh water may have a constant of 0.65-0.7 while seawater is better off at 0.5 because they have different compositions! Seawater is mostly sodium and chloride. Fresh waters can have vastly different compositions depending on the geology and other factors. How good is the value you are using or do you even know what value you are using?

Salinity
To make matters worse, some smart guy came up with salinity as a water quality parameter. Even without complicating TDS, salinity by itself is slippery to define.

Originally, it refers to the amount of dissolved substances (including gases) in a given volume of water. But in practice, this definition is difficult to quantify e.g. gases are lost easily, some chloride can be lost during drying.

Therefore salinity is redefined using chlorinity.

S = 1.80655 Cl
where C = "the mass of silver required to precipitate completely the halogens in 0.3285234kg of the sea-water sample"

With the advent of EC sensors (did I say that they are easy and cheap to use?) which far surpass the cumbersome nature of chemical methods, salinity of seawater was once again redefined based on EC.

S = 0.0080 - 0.1692 K15 1/2 + 25.3851 K15 + 14.0941 K15 3/2 -7.0261 K15 2 + 2.7081 K15 5/2

K15 = C (S,15,0) / C (KCl,15,0)

where C (S, 15, 0) is the conductivity of the sea-water sample at a temperature of 15°C and standard atmospheric pressure, and C (KCl, t, 0) is the conductivity of the standard potassium chloride (KCl) solution at a temperature of 15°C and standard atmospheric pressure. The standard KCl solution contains a mass of 32.435 6 grams of KCl in a mass of 1.000 000kg of solution.   If all the above sounds profound, don't worry. Your isntrument should be able to perform the calculations for you automatically behind the scenes without your intervention.   Please note that the historical development of salinity above is a simplification. There are more intermediate development stages not described above.     What is the verdict then? My verdict is...... just measure in EC!   EC is the basic parameter measured by your instrument. Though many instruments proudly proclaim that they can measure EC, TDS and salinity at the same time, TDS and salinity are simply derived parameters and not measured directly.   How would I know whether my peat swamp should be using the same TDS constant as my storm water canal? They probably should not since the composition of each is likely to be different.   For that matter, the salinity above works because it is designed for seawater which amazingly does not vary much in the ratio of its consituent ions (sodium, chloride, sulfate, magnesium, calcium, potassium) from location to location. You definitely can't bet the same for fresh waters.   Basically comparisons of TDS and salinity become meaningless when the compositions of the water samples are quite different. Therefore, why go to the extra trouble of measuring TDS and salinity when you can do with EC? (Of course, salinity is still quite suitable for measurement in seawaters for reasons mentioned above.)  

Figure: A typical hand-held EC probe (meter + sensor (right end)) often used for my water quality monitoring activities. This particular model is only good for fresh waters.

Figure: A more elaborate set-up that can "measure" EC, TDS & salinity. The meter is at the bottom with the probe/sensor on top. This is suitable for fresh to brackish and salt waters