Pure Water as Electrolyte
Date: Fall 2009
Is pure water a weak electrolyte or a nonelectrolyte?
Pure water (100.000% H2O) is a non-conductor. You may want to demonstrate that
water is composed of two elements, gaseous Hydrogen and gaseous Oxygen. You can do
this demonstration by sending an electric current through the water -- electrolysis.
If you use pure water the reaction will be really slow. However, if you add several
drops of sulfuric acid to the water the reaction bubbles right along. Why? Because
the sulfuric acid breaks into ions (H+, HSO4- and SO42-) ions. The ions will conduct
the electricity to break down the water.
Strictly speaking the term electrolyte applies to substances dissolved in water and whether
the addition of these substances to water allows the solution to be a good or poor conductor
of electricity. So, technically, the term should not apply to water itself.
On the other hand, if we take the broad definition of an electrolyte as something that is a
good conductor of electricity, then we can say that water is nonelectrolyte since pure water
does not contain enough ions to allow for good electrical conductance.
Greg (Roberto Gregorius)
Pure water does not conduct electricity. A dissolved ion must be
present in order to make it an electrolyte solution.
It is difficult, however, to produce water that is so pure that it will
not conduct a charge. Tap water and other ordinary water sources
frequently contain sufficient dissolved materials that they will conduct
a small electrical charge.
I have not heard it discussed, but I would think it was a weak electrolyte.
Maybe a "VERY weak" electrolyte.
The resistivity of pure water is roughly 18 meg-ohm.cm at room temperature.
If you had a plastic tube 1cm inside diameter, and 1 cm long,
with good electrodes at both ends, the resistance between them would be 18 megohms.
That is enough conductivity to grow a medium-thin anodic oxide on aluminum if
100vdc was applied between two electrodes for a long time, like a week.
The conductivity is ionic; the charge-carriers are H+ ions and OH- ions.
All electrolytes have ions of both polarities, just like that.
That is also enough conductivity to dissipate static charges
or communicate chemically-caused voltages to electrodes.
Such as neural impulses, ECG's & EEG's.
Pure water does it, but does not do as good a job for small signals
as stronger electrolytes
because almost all metal electrodes will either:
a) react with water, making metal and OH- ions,
making a stronger electrolyte, then it is not "pure water"; or,
b) develop dipole layers in the water adjacent to the metal,
which are more insulating than the bulk of the water.
Then the measuring instrument might actually have difficulty
getting onto its wire the exact same voltage
that exists in the water nearby.
c) grow solid (precipitated) oxides or hydroxides on the metal,
which are also more insulating than the water itself.
There are usually some electrochemical noise sources
on every electrode, that need to be shorted out
to allow the weak signal voltages to show clearly.
A strong electrolyte usually does that better.
A good electrode assembly tends to need
strong electrolyte inside, next to the metal.
Then it is connected to the weak electrolyte under test by a very narrow salt bridge,
that lets just enough ionic conductivity happen for the meter,
but slows down contamination of the weak by the strong
as much as can be gotten away with.
It is a theoretical issue in every practical situation:
can your solution be allowed this amount of contamination?
Even the best pH probes will contaminate your best de-ionized water
if left soaking in it a long time.
You might find that overnight the resistivity has gone down
from 18M-Ohm.cm to 10 or 1 M-Ohm.cm.
So electrode assemblies are only used in temporary-exposure modes:
either the electrode is used then removed,
or the solution is large and/or flowing or otherwise renewed continuously.
I hope you were not asking with a different definition of :"electrolyte" in mind.
The physicists' definition would be the bipolar ionic conductivity.
Y'know, one electron stuck in a non-electrolyte like propane liquid
would probably be considered a static charge.
It could migrate by convection,
or by its host molecule being dragged through the fluid
by the electric-field, which would be called "drift",
or maybe even by hopping from molecule to molecule.
The difference in an electrolyte is that
the number of mobile-charges present greatly outnumbers
the number of charges you would imagine travelling between electrodes.
So one charge entering the liquid then travels as a million charges in parallel,
each travelling one-millionth of the distance between electrodes.
Then, voila, one charge pops out onto the other electrode,
and nobody has travelled very far at all.
I am sure this greater number business is true for pure water.
Oops! Another case of type casting. An electrolyte is a substance
(solid, liquid, gas) that will conduct electricity, that is, allow electrons
to flow. Nothing, not even a vacuum, is a nonelectrolyte, if the applied
voltage is large enough. We put substances into electrolyte/non-electrolyte
"boxes" which should be labeled "more or less". Pure water (which is very
difficult to prepare, by the way) has a concentration of 10^-7 mols/liter of
H[+] and OH[-] each. Now that is not very much, but it is not zero. In
contrast, add a bit of salt and the conductivity, a measure of
"electrolyte-ness", is very large and one would classify that solution as an
We have to be careful when we put things into categories. We should add
the caveat "more or less" or "approximately". Too often the categories
themselves get a "life" of their own and the classifications becomes
absolute, and become more important than the data they are intended to
classify and organize.
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Update: June 2012