Salinity - what do those figures mean?

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How is salinity measured? A quick way is to use a conductivity meter and read off the electrical conductivity. The idea being that a salty solution, because it is full of charged particles will conduct electricity. Most conductivity meters give readings in micro Siemens per cm (ÁS/cm).

So what's a micro Siemen? Well most fresh drinking water will have less than 100 ÁS/cm conductivity eg Melbourne. However in WA the statewide standard for drinking water is nearer 821 microS/cm. Some slightly salty drainage water found on salt affected farms in Victoria will be around 1800 ÁS/cm. In Western Australia salt drainage water conductivity could range from 8000 to 23000 microS/cm **. Very brackish water could be around 27000 ÁS/cm. By the way seawater has conductivity of around 54000 ÁS/cm.

If you are growing salt sensitive crops make sure the conductivity is less than about 700 ÁS/cm. If your irrigation water is around 5000 ÁS/cm then make sure you are growing salt tolerant crops.

Sometimes just to confuse matters, conductivity is given in some textbooks in deci Siemens per meter (dS/m). The conversion is relatively easy 1 dS/m = 1000 ÁS/cm. Likewise if you come across milliSiemens per cm (mS/cm) just remember that 1 mS/cm = 1000 ÁS/cm. In some USA texts millimhos is used. The conversion is 1 millimho/cm = 1 milliSiemen/cm.

Now some salinity meters read off parts per million (ppm). This is an approximation - the problem is that ppm is a measure of dissolved solids and its usually on a weight for volume basis. For example 50 ppm in water means there are 50 milligrams of solids per litre. How does a conductivity meter know how many ppm to show? It just uses its inbuilt conversion factor. This means that you need to choose a meter with either an appropriate factor or get one with an adjustable factor. Some meters display salts as ppt (parts per thousand). 1000 ppm = 1 ppt so that 4300 ppm = 4.3 ppt.

An open channel supplying towns and farms in the Mallee in Victoria. By the time this water gets to the Mallee from the Grampians it's conductivity is up to about 1100 ÁS/cm.

Salinity vs TDS. What's the connection? This is a commonly asked question. To answer it we have to delve into a little chemistry theory.

First the short answer.

What's the connection between conductivity and TDS? TDS is more precisely measured in the laboratory by evapourating a measured sample gently to dryness then calculating how much solids are left. Conductivity is usually given as ÁS/cm which measures the ability of the sample to conduct an electric current.

There is no exact relationship between conductivity as ÁS/cm and TDS as ppm. So why are both measurements used? It has been discovered experimentally that for particular types of water there is an approximate relationship. In water with a higher proportion of sodium chloride to get to ppm just multiply the ÁS/cm reading by 0.5. For most other water for example in hydroponics solutions use a factor of 0.67 or 0.7 instead.

Sometimes it is useful to have some sort of comparison for values measured on a conductivity meter. Remember that conductivity of seawater is around 54000 ÁS/cm. This is approximately 35000 ppm TDS. Seawater has a high proportion of sodium chloride and this is around 28000 ppm.

Now the more complete answer.

Different salts in water have a different ability to conduct electricity. This is because of the differences in charge and size / weight and mobility of the different ions. This difference is quantified as a property called the specific conductance. The specific conductance is a value based on the theoretical conductivity of ions at very low concentrations. Although it is possible to calculate the conductivity for any electrolyte at any temperature and concentration, see Conductivity calculator, the exact contribution of individual ions is difficult to determine due to interactions between the ions. This means that it is difficult to work out what the conductivity of a particular salt mix should be, and hence it is difficult to establish the theoretical relationship between conductivity and TDS for a given mixture.

A simple experiment was carried out at Apps Laboratories. Three salt solutions all at the same concentration 0.01 mol/l were made up. There were sodium chloride NaCl, calcium chloride CaCl2 and sodium bicarbonate NaHCO3. Conductivity of the solutions was meaasured using the Hanna Instruments HI 9835 conductivity meter. See this meter at HI9835. The good thing about this meter is that it automatically compensates for temperature and reports the conductivity at a standard 25 degrees C. Here are the conductivities of each individual solution.

 
Salt (all 0.01 mol/l) mg/l * Conductivity ÁS/cm TDS factor - mg/l / Cond
NaCl 584 1156 0.51
CaCl2 1110 2310 0.48
NaHCO3 840 865 0.97

* For none chemists this is how many mgms needed per litre to make up a 0.01 mol/l solution.

There is already a trend and it is explained like this: To get mg/l for a NaCL solution multiply the conductivity by 0.51 etcetera for each solution. Interestingly it seems that conductivity in ÁS/cm and mg/l are very close for NaHCO3. However in nature there are never pure salt solutions like these.

Measurements of conductivity of mixtures of these three base solutions should be interesting - watch this space.

** my thanks to Dr Lazarus Leonhard, WA Dept. of Water for these comparative figures.

Please let us know if you have any comments or questions about this explanation or if you have suggestions about how it could be improved. Comments / questions.


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