Conductivity- Background
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How can you tell whether a solution has been made correctly and has the proper components, at the
proper concentrations? It is possible to check the pH, clarity, and volume of a solution, but these
properties are not sufficient to determine if a solution has the correct constituents. The conductivity of a
solution is another property that can be used as a quality control test. Conductivity is a measure of a
solution's ability to conduct electrical charge. Salts, like those we will use in this laboratory, are able to
carry charge in solutions. Therefore, conductivity is a measure that can be used to determine whether the
concentration of salts is correct in a solution.
Conductivity refers to the ability of a solution to conduct electrical current. Conductivity is the reciprocal
of electrical resistance, an index of difficulty of flow of electric current. Conductivity is measured in
S/cm, siemens per centimeter.
Electrolytes are substances which dissolve in water to produce a solution that conducts an electrical
current. Such substances produce ions when dissolved in water, and it is the ions which carry the current
through the solution. Nonelectrolytes are substance whose aqueous solutions do not contain ions and hence
do not conduct an electrical current. Electrolytes are classified as either strong electrolytes or weak
electrolytes. Strong electrolytes when dissolved in water react completely to produce ions. For example,
when NaCl is dissolved in water the reaction is
NaCl(s)  Na+(aq) + Cl-(aq)
All of the NaCl(s) reacts to give ions. There are no dissolved NaCl molecules present in the solution.
Solutions of strong electrolytes are good conductors of electricity because they contain a relatively high
concentration of ions.
Weak electrolytes are only partially converted to ions in aqueous solution. Acetic acid, CH3CO2H, the
active ingredient of vinegar, is a weak electrolyte. In water it undergoes the reaction
CH3CO2H(l)  CH3CO2 -(aq) + H+(aq)
Only about 1% of the dissolved acetic acid molecules are converted to ions. The rest remain unchanged.
This is in contrast to NaCl, which is completely converted to ions in water. Thus, a solution of a weak
electrolyte contains a relatively low concentration of ions and its conductivity would be expected to be
low.
Nonelectrolytes do not conduct an electrical current because they do not produce ions when dissolved in
water. An example is sucrose or table sugar, C12H22O11. A solution of sucrose contains molecules with the
formula C12H22O11, but no ions.
Adapted in part from http://wwwchem.csustan.edu/CHEM2000/EXP6/exp6a.htm and
http://matcmadison.edu/biotech/resources/methods/labManual/unit_3/exercise_6.htm LI-T 2008
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