Conductance

1

• Conductance
• In electrolyte solutions, fused electrolytes or
  suspensions of charged colloidal particles,
  electric conductivity occurs as a result of
  transport of the charged particles in these
  systems
• The magnitude of the current in such systems
  depends on
• For weak electrolytes, the concentrations of ions
  must be calculated from equilibrium constant
  considerations
• Number and type of ions present
• The mobility of the ions
• The type of solvent
• The magnitude of the applied potential
• Definitions
• Ohms Law most electrolyte solutions are ohmic
• For a conductor of univorm composion a cross
  sectional area

2

• Conductance
• Definitions
• Specific resistance, r, is the resistance of a 1
  cm cube of matter
• Units are ohm cm
• Reciprocal of resistance is conductance, G
• (ohm-1 or mho)
• Reciprocal of specific resistance is specific
  conductance
• (ohm-1 cm-1 or mho cm-1)
• (ohm-1)

3

• Conductance
• Definitions
• Specific conductance of some materials

• Equivalent conductance
• Specific conductance depends on concentration,
  need a parameter corrects for concentration
  effects
• The Equivalent conductance is the conductance of
  a hypothetical solution containing one
  equivalent of solute between two parallel
  electrodes 1 cm apart
• N
  is the normality calculate from equiv
  mass determined by dividing the
  molar mass by the charge on the ion

4

• Conductance
• Definitions
• Equivalent conductance, L the conductance of 1
  equiv of solute between electrode plates 1 cm
  apart
• For a 1 N solution,one has 1,000 cm3 of solution,
  so the electrodes are 1,000 cm2 in area
• For a 0.1 N solution, one has 10,000 cm3 of
  solution, so the electrodes are 10,000 cm2 in
  area
• These kinds of cells are impractical
• Theyre huge!!!
• Electrodes dont have to be 1 cm apart
• Use standard solutions of KCl to standardize
  cells of any size or shape
• For any electrode kkR
• kcell constant in cm-1
• if k 1 cm-1,
• Measure k for R using a solution of known k
• From k and R for an analytical solution,
  calculate k and L

5

• Conductance
• Definitions
• Equivalent conductance
• For electrolyte solutions having a single
  dissolved salt
• L l l- where the li are the equivilant
  conductance of the anion or cation of the
  ionic substance
• For a mixture, L would be calculated from the sum
  of all the equivalent conductances of all the
  cations and anions in solution
• Specific conductances (k) decreases with
  decreasing concentration but the equivalent
  conductance (L) increases with decreasing
  concentration
• Ions become more effective charge carriers at
  lower concentrations
• As concentration decreases, interionic
  interactions decrease
• Electrophoretic effect solvent molecules move in
  the opposite direction to the solvated central
  ion which decrease an ions mobility
• The ionic atmosphere around a charged species in
  solution moves slower than the central ion
  creating a charge separation retarding the
  central ions transport
• As the solutions become more dilute, the ionic
  atmosphere becomes weaker

6

• Conductance
• Definitions
• Equivalent conductance
• Ions become more effective charge carriers at
  lower concentrations
• These two forces decrease with
• At infinite dilution there are no such disturbing
  effects on ionic mobilities
• Lo lo l-o
• Onsager has shown L Lo -(A BLo)C0.5
• A is the electrophoretic term
• B is the time of relaxation term
• Instrumentation

AC Wheatstone bridge - no polarizing
effects Capacitor around R1 balances out phase
shifts in AC signal caused by capacity effects
at the electrode surfaces
7

• Conductance
• Conductance titrations - use conductance
  measurements to determine the endpoints for
  titrations in which the conductivity of the
  solution changes with position of the titration
• Depending on the reaction, find the intersection
  of two lines
• For best accuracy, correct the observed
  resistance for the effect of dilution from the
  volume of added titrant
• Rs corrected
  solution resistance
• V original
  analytical solution volum
• v volume of
  added titrant
• Ro observed
  solution resistance
• Strong Acid - Strong Base titrations
• The high mobilities of H and OH- produce sharp
  endpoints
• Na OH- H Cl- H2O Na Cl-

8

• Conductance
• Conductance titrations
• Strong Acid - Strong Base titrations

Titration of 0.001 M HCl with 0.100 M NaOH Dashed
line is the conductance due to NaCl conc. Salt
line if NaCl high, observe errors
Titration of a monoprotic weak acid with
NaOH Titration curve goes through a
minimum Decrease H reduces G more than the
increase in G from added Na and A- At the
minimum, H becomes very small and Na and A-
contribute to G the curve follows the salt
line Just before end point, A- H2O HA
OH- and titn curve becomes rounded After
endpoint, the titration follows the same general
shape as for strong acid/base case
9

• Conductance
• Applications
• Direct measurements are not selective because any
  ionic substance in solution produces
  conductivity
• Very sensitive to the presence of ionic
  impurities
• Used to measure the purity of distilled or
  deionized water
• Specific conductance of water 5 x 10-8 ohm-1cm-1
• Specific resistance of water 20 x 106 ohm cm
• Traces of ionic impurities decrease specific
  resistance by 10x
• Used to measure ions in ion chromatography
• Measure extent of association and dissociation
  equilibria in solution
• Follow the kinetic progress of chemical reactions
  
• Characterize ions from the equivalent conductance
  and correlation with known charge types