The
actual conductivity value of an aqueous solution containing a single salt is
determined by the concentration of that salt, the solution temperature and
the nature of the particular 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 usually also
doubles. At
higher
concentrations however, this strict proportionality (i.e. linearity)
deteriorates (e.g. see Table 2.20b).
Note 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
mixing different solutions of known conductivities. For example, 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. Similarly, 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. 100/1,000 x 4.8 + 900/1,000 x 0.40).
ii.
Temperature effect
The effect of solution temperature on conductivity is such that its value
rises by about 2% (compounded) for each 1 deg C temperature increase.
However, most meters automatically apply a correction factor to the
determined value such that the displayed value is as if the solution
temperature was at 25oC / 77oF.
iii.
Effect of salt type
The conductivity of different salts varies widely and is determined by such
factors as the 'size' of the ions, and the 'charge density' on these particles
whilst in
solution. For example, the conductivities at 25oC (77oF)
of 500ppm aqueous 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 2.1).
Notably, the potassium phosphate solution has less than half the
conductivity of a sodium chloride solution of equal concentration.
Further, notice how potassium when combined with chloride (as potassium
chloride) has a lower conductivity than what sodium does when combined with
chloride (as sodium chloride). This is mainly because a 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 chloride solution.
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 salt. The reason for this is
inorganic nutrient mixtures have much higher concentrations of the heavier
substances like potassium and phosphate. Bore waters however,
typically contain
numerically more ions of lighter salts like sodium and chloride. The
important point here is that the electrical mobility of these ions in water
are not that different – it is the total number that are 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
you need to be aware that their addition will produce less 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.
It should be emphasized that conductivity measurements determine total
dissolved 'salts' only - not total dissolved 'solids'. This is
because the presence of solids and substances such as organics, clay,
tannins, algae particles, precipitates, colour etc., will not affect the
measured conductivity. Total dissolved solids can only be determined by
evaporation methods.
