conductivity value of a water based
solution containing a single salt is determined by the concentration of that
salt, the solution temperature and the type of salt.
i. Concentration effect
With relatively dilute solutions of soluble
salts (i.e. up to 100ppm or so), if the concentration is doubled, its
conductivity also doubles. At higher salt concentrations however, this
linearity) deteriorates. For example, Table 18.20 demonstrates that there is
a better linear relationship between concentration and conductivity from 1
to 2 g/L compared to 10 and 20 g/L.
A consequence of this linearity feature is that
simple arithmetic can be used to calculate the approximate conductivities
which would result from soluble mixtures between solutions of known
conductivity. For example:
a) If a 2.0 mS/cm water is diluted with an
equal amount of 'distilled' water (zero mS/cm), the result would be
approximately 1.0 mS/cm.
b) If 100ml of a 4.8 mS/cm nutrient solution is
diluted with 900ml of 0.40 mS/cm water (i.e. 1 + 9), the expected result
would be about 0.84 mS/cm (i.e. 100 x 4.8 ÷ 1,000 + 900 x 0.4 ÷ 1,000).
ii. Temperature effect
The temperature of the solution causes the
conductivity value to rise by about 2% (compounded) for each 1OC
/ 1.8OF increase in temperature. However, the software of most
conductivity meters automatically apply a correction factor to compensate
for solutions not at 25OC / 77OF. Hence the displayed
value is what the conductivity would be if the solution temperature was at
25OC / 77OF.
iii. Effect of salt type
The conductivity or mobility of different salts
varies widely and is determined by factors such as the 'size' of the ions,
the number of ions and the 'charge density' on these particles in solution.
For example, the conductivities at 25OC (77OF) of
500ppm water based solutions of sodium chloride, potassium chloride and
potassium phosphate are 1.02 mS/cm, 0.95 mS/cm and 0.40 mS/cm respectively
(Chart 18.30). Note that the 500ppm solution of potassium chloride has about
30% fewer ions to carry the current than a 500ppm solution of sodium
chloride – due to the fact that the combined mass of potassium and chloride
is 30% heavier than sodium chloride. Similarly, a 500ppm solution of
potassium phosphate has only 40% of the number of ions than in the sodium
The impact of salt ‘type’ upon the EC value is
further emphasized when the EC of typical ‘natural’ waters (i.e.
uncontaminated water) is compared with that of an inorganic nutrient
solution of equal concentration. For example, an uncontaminated bore water
containing 1,000ppm of salt will typically yield an EC of ~1.8mS/cm.
However, an inorganic nutrient solution of the same EC will in fact contain
~1,600ppm of salts. The reason for this is inorganic nutrient mixtures have
much higher concentrations of the heavier substances like potassium and
phosphate compared to natural water and therefore have fewer ions to carry
the electric current. Bore waters however, typically contain numerically
more ions of lighter salts like sodium and chloride. Thus, the electrical
mobility of these individual ions in water are not that different. Rather,
it is the total number of ions present that determines the conductivity.
Hence, when following EC recommendations in
hydroponics, consider the composition of all additives. Flowering additives
that contain a large proportion of phosphate yield a relatively low
conductivity. Consequently, their addition to the nutrient solution will
produce a smaller increase in conductivity than a normal inorganic nutrient
mixture. Also, note that additives that claim to be 100% organic should
contain no salts, and their addition would therefore produce no increase in
conductivity when added to a nutrient solution.