NEWS105
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A typical blackwater pool in the rainforest,habitat of numerouspopular aquarium fishes.
Measuringelectrical conductivity
Distilled water doesn’t conduct electricity
(importantwarningtoallchildren:DON’TTRY
THE FOLLOWING!). Fatal accidentswithhair-
driers in thebathtubhappenonly if ionsare
in solution in the water (see part 1 of this
article; not "How can I do away with my
spouse"onpage49,but"What isan ion?"on
page 36). Because the total and carbonate
hardness are the result of ions in thewater,
they canbemeasured indirectly electrically.
This involves passing a current through an
electrode submerged in the water. The
quantity of ions present in the water will
affecthowwell thiscurrent isconducted.This
conductivity is measures in Micro-
siemens/centimeter(=µS/cm).Distilledwater
producesazeroreadingas itdoesn’tconduct
at all. The lower the reading, the softer the
water. The conductivity in Lake Malawi
measures around 200-260 µS/cm, in typical
South American blackwater habitats it is
around 20 µS/cm, and in the Rhine at
Düsseldorf on21.11.2012 itwas 557µS/ cm.
BUT: the conductivity is ameasureof all the
ions in solution in the water, ie the total
dissolved salts, not just the ions that are
responsible for hardness. But all the
freshwatersof theworldhavea ratherhandy
characteristic: the ratioof thedissolved salts
they contain is practically the same
worldwide, only the overall concentration
varies.To put it another way: the same salts
are present in the Amazon and in the
(unpolluted) Rhine and in LakeMalawi,only
indifferentamounts.So it isnormallypossible
to calculate a good approximation of the
hardness from the conductivity. But the
whole thingworks only in completely fresh
water. As soon as common salt (sodium
chloride)enterstheequationthewholething
falls apart. Seawater contains some 33 g of
salts per liter, around 85% of this total is
common salt (NaCl), and the water has a
conductivity of around 53,000 µS/cm. The
littlebitofhardnessplaysnosignificantroleat
thisscale.But incompletely freshwater itcan
be reckoned that 100 µS/cm is roughly
equivalent to3°dGH.
Dirty tricks
Sometimes the results of harness
measurement are quite absurd, andwewill
look at possible causes of this shortly. In the
first part of this series it was stated that the
non-carbonate hardness has hardly any
influence on the aquarium. If onemeasures
the total hardness then in natural waters
normally 80% of this will be carbonate
hardness. So it is generally sufficient to
measure the totalhardnessandcalculate the
carbonate hardness from it. But this doesn’t
alwayswork.This is usually the result of the
way water is treated tomake it potable for
humans.Becausecarbonatehardnesscanbe
removed by boiling the water, and
precipitatesoutas insoluble“fur”(seepart1).
This fur not only looks unsightly, but it also
ruins washing and washing-up machine
pumps and precipitates out in hot-water
pipes,eventuallyblockingthem.Sothewater
is frequently softened. Innormal households
this is often achieved by so-called ion
exchange.This involvesthecalcium ionofthe
calcium bicarbonate (reminder: this is what
causes the carbonate hardness) being
exchanged for a sodium ion. The result is
sodium bicarbonate,which isn’t affected by
heat and remainsdissolved in thewater.This
Rapids in theRioXingu inBrazil,habitat vonL-number catfishes,cichlids,and characins.
