Download determination of electrolyte molar conductivity at infinite dilution

SILESIAN UNIVERSITY OF TECHNOLOGY
FACULTY OF CHEMISTRY
DEPARTMENT OF PHYSICAL CHEMISTRY
AND TECHNOLOGY OF POLYMERS
DETERMINATION OF ELECTROLYTE
MOLAR CONDUCTIVITY AT INFINITE DILUTION
Exercise supervisor: Krzysztof Karoń
Place of exercise: room 210
LABORATORY OF PHYSICAL CHEMISTRY
DETERMINATION OF ELECTROLYTE MOLAR CONDUCTIVITY AT INFINITE DILUTION
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I. THE AIM OF THE EXERCISE
The aim of the exercise is to determine the molar conductivity at infinite dilution
(limiting molar conductivity) of chosen strong electrolytes.
II. INTRODUCTION AND THE THEORETICAL BACKGROUND
Molar conductivity Λm is the efficiency of given electrolyte electricity conduction in
solution. Its units are siemens per meter per molarity, or siemens meter-squared per mole.
The value of molar conductivity strongly depends on the concentration of ions, which are
charge carriers in the electrolyte solution. While the electrolyte solution is being diluted its
degree of dissociation and so the concentration of charge carriers increases and therefore the
also increase the molar conductivity of the solution. In the infinite dilution (i.e. when the
concentration of the electrolyte tends to zero) the molar conductivity reaches a limit value
Λ0m , called limiting molar conductivity or conductivity at infinite dilution.
The molar conductivity of strong electrolytes in high dilutions decreases linearly
according to Kohlrausch equation:
Λ m = Λ0m − a C
(1)
In this case, the limiting conductivity may be determined directly from the molar conductivity
measurement. Designation by the graphical method involves the preparation of a graph. of
Λm = f( C ) dependence and extrapolation to concentration of zero.
In the case of weak electrolytes the molar conductivity changes in more complex way
with dilution and hence conductivity boundary cannot be determined directly from the
measurements. In this case the limiting conductivity can be calculated from the Kohlraucsh
law of independent migration of ions:
Λ0m = ν + λ0+ + ν − λ0−
(2)
where:
λ0n – limiting molar conductivity of n-th ion
νn –numer of n-th ions in solution
Ionic conductivity is determined from the limiting molar conductivity of strong electrolytes
using their additivity. Limiting conductivity of weak electrolytes can be calculated from the
limiting conductivity of several strong electrolytes, containing the same ions.
DETERMINATION OF ELECTROLYTE MOLAR CONDUCTIVITY AT INFINITE DILUTION
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III. PROCEDURE
Aparatus:
Reagents:
E - 316 a universal conductivity bridge
Deionised water, salt given topic of the
with the option of ac measurements,
exercise.
thermostat, conductivity cell.
•
Turn on the thermostat and set the temperature of 298 K
•
Rinse thermostated dish and electrodes several times with distilled water; for final rinse
use the deionized water.
•
Determine the conductivity of deionized water. The resistance value should be greater
than 100,000 ohms, otherwise you need to download the new deionized water.
•
Prepare the starting solution of specified by the teacher salt of 2.0⋅10-3 mol/dm3
concentration. The solution must be precisely prepared in volumetric flask of 250 cm3. For
solution preparation use deionized water.
•
Prepare a series of diluted solutions of eight concentrations (0,2 ⋅10-3, 0,4⋅10-3, 0,6⋅10-3,
0,8⋅10-3... 1,6⋅10-3 mol/dm3) by measuring respectively 5, 10, 15, 20, 25, 30, 35, 40 cm3 of
starting solution to the volumetric flasks of 50 cm3, and complementing the level with
deionized water up to the mark.
•
Measure the resistance of a series of solutions beginning with the most dilute solution and
finishing with the starting solution. Remember that each time while new solution is
poured to the measuring vessel you should wait for a while until until the solution will
reach correct temperature.
•
From 0.1 M solution of KCl prepare 50 cm3 of 0.001 M solution and measure its
resistance in order to determine the cell constant. Electrical conductivity of 0.001 M KCl
solution at a temperature of 298 K is given in the topic of this exercise.
•
Using the formula (4) calculate the conductivity of each solution.
Due to the possibility of fluctuations in demineralized water quality its conductivity should
be taken into account in the calculations:
κ real = κ measured − κ water
(3)
Received the real conductivity values can be used to calculate the solution molar
conductivity.
DETERMINATION OF ELECTROLYTE MOLAR CONDUCTIVITY AT INFINITE DILUTION
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IV. SAFETY AND WASTES UTILIZATION RULES
NOTE: In the case of the undesirable contact with hazardous substance, immediately notify the teacher.
Risks and safety precautions:
Before starting any work read in annex the risks and safety precautions associated with the
substances listed in the exercises subject
Waste Treatment:
Highly dilute aqueous solutions of salts used in the exercise introduce to drains. Solutions of
higher concentration dispose in proper container for inorganic waste.
Classification:
ammonium nitrate
zinc nitrate
sodium nitrate
potassium nitrate
ammonium chloride
zinc chloride
copper (II) chloride
potassium chloride
sodium chloride
calcium chloride
potassium chromate
sodium acetate
ammonium sulfate
zinc sulfate
copper (II) sulfate
potassium sulfate
sodium sulfate
ammonium thiocyanate
potassium thiocyanate
potassium carbonate
Potassium bicarbonate
oxidising substance (O)
oxidising substance (O), harmful (Xn)
oxidising substance (O), harmful (Xn)
oxidising substance (O)
harmful (Xn)
corrosive (C), dangerous for the environment (N)
harmful (Xn)
irritant (Xi)
toxic (T), dangerous for the environment (N)
irritant (Xi), dangerous for the environment (N)
harmful (Xn)
harmful (Xn)
harmful (Xn)
harmful (Xn)
-
DETERMINATION OF ELECTROLYTE MOLAR CONDUCTIVITY AT INFINITE DILUTION
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Risks:
ammonium nitrate
zinc nitrate
sodium nitrate
potassium nitrate
ammonium chloride
zinc chloride
copper (II) chloride
potassium chloride
sodium chloride
calcium chloride
potassium chromate
sodium acetate
ammonium sulfate
zinc sulfate
copper (II) sulfate
potassium sulfate
sodium sulfate
ammonium thiocyanate
potassium thiocyanate
potassium carbonate
Potassium bicarbonate
R: 8-9 Contact with combustible material may cause fire,
explosive when mixed with - combustible material
R: 8-22-36/37/38 Contact with combustible material may cause
fire, harmful if swallowed, irritating to eyes, respiratory system
and skin
R: 8-22-36 Contact with combustible material may cause fire,
harmful if swallowed, irritating to eyes
R: 8 Contact with combustible material may cause fire
R: 22-36 Harmful if swallowed, irritating to eyes
R: 34-50/53 causes burns, very toxic to aquatic organisms; may
cause long-term adverse effects in the aquatic environment
R: 22-36/37/38 Harmful if swallowed, irritating to eyes,
respiratory system and skin
R: 36 irritating to eyes
R: 49-46-36/37/38-43-50/53 may cause cancer by inhalation may
cause heritable genetic damage, irritating to eyes, respiratory
system and skin, may cause sensitization by skin contact, very
toxic to aquatic organisms; may cause long-term adverse effects in
the aquatic environment
R: 36/38-50/53 Irritating to eyes and skin, very toxic to aquatic
organisms; may cause long-term adverse effects in the aquatic
environment
R: 22-36/38 Harmful if swallowed, irritating to eyes and skin
R: 20/21/22-32 Harmful by inhalation, in contact with skin and if
swallowed, in contact with acids liberates very toxic gas
R: 20/21/22-32 Harmful by inhalation, in contact with skin and if
swallowed, in contact with acids liberates very toxic gas
R: 22-36/37/38 Harmful if swallowed, irritating to eyes,
respiratory system and skórędziała Harmful by inhalation, in
contact with skin and if swallowed, in contact with acids liberates
very toxic gas
-
DETERMINATION OF ELECTROLYTE MOLAR CONDUCTIVITY AT INFINITE DILUTION
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Safety precautions:
ammonium nitrate
zinc nitrate
sodium nitrate
potassium nitrate
ammonium chloride
zinc chloride
copper (II) chloride
potassium chloride
sodium chloride
calcium chloride
potassium chromate
sodium acetate
ammonium sulfate
zinc sulfate
copper (II) sulfate
potassium sulfate
sodium sulfate
ammonium thiocyanate
potassium thiocyanate
potassium carbonate
Potassium bicarbonate
S: 15-16-41 Keep away from sources of heat, not Keep away from
sources of ignition - no smoking, do not breathe fumes from fire or
explosion
S: 26 of contact with eyes, rinse immediately with plenty of water
and seek medical advice
S: 22-24-41 Do not inhale dust, avoid contact with skin, do not
breathe fumes from fire or explosion
S: 16-41 Keep away from sources of ignition - no smoking, do not
breathe fumes from fire or explosion
S: 2-22 keep away from children, do not breathe dust
S: 1/2-7/8-28-45-60-61 Keep out of the reach of children and keep
the container tightly closed and dry, contaminated skin, wash
immediately with plenty of (to be specified by the manufacturer),
in case of accident or if you feel unwell, seek medical advice
immediately - if possible, show the label, product and container
must be disposed of as hazardous waste avoid release to the
environment, comply with the instructions or the data sheet
S: 26 of contact with eyes, rinse immediately with plenty of water
and seek medical advice
S: 2-22-24 keep away from children, do not breathe dust, avoid
contact with skin
S: 53-45-60-61 Avoid exposure - before use, read the instructions,
in case of accident or if you feel unwell, seek medical advice
immediately - if possible, show the label, the product and the
container must be disposed of as hazardous waste Avoid release to
the environment, comply with the instructions or card sheet
S: 2-22-25-60-61 keep away from children, do not breathe dust,
avoid contact with eyes, the product and packaging disposed of as
hazardous waste Avoid release to the environment, comply with
the instructions or the data sheet
S: 22 Do not breathe dust
S: 13 Do not store together with food, drink and animal feeding
stuffs
S: 13 Do not store together with food, drink and animal feeding
stuffs
S: 22-26 Do not inhale dust contact with eyes, rinse immediately
with plenty of water and seek medical advice
-
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DETERMINATION OF ELECTROLYTE MOLAR CONDUCTIVITY AT INFINITE DILUTION
V. STUDY RESULTS
Electrical conductivity of the electrolyte is given by the formula:
κ=
kn
R
(4)
where:
kn – cell constant
R – the electrical resistance of the solution
The molar conductivity of the electrolyte is given by:
Λm =
κ
(5)
C
Fill the measurement table with measurements and calculation results:
Lp.
RKCl = ..........[Ω]
κKCl = ..........[S/m]
cell constant
Rwater = ..........[Ω]
κwater = ..........[S/m]
kn = ..........[1/m]
C [mol/dm3]
C
R [Ω]
κmeas [S/m]
κreal [S/m]
Λm
[m2/Ωmol]
Make the plot of Λm = f( C ) and determine the molar limited conductivity Λ0m which is an
intersection of the extrapolated line of experimental data with the ordinate axis. Calculate the
coefficients of the Kohlraucsh equation using the least squares method. Carry out an analysis
of measurement uncertainty.
VI. REVIEW QUESTIONS EXAMPLES
1. Give the definitions of conductivity, solution conductivity and limiting molar conductivity.
2. Why alternating current is used for solution conductivity measurement.
3. Discuss the equation (1) and (2) from the instruction.
4. Give examples of the use of electrolyte solutions conductance measurements.
5. Discuss in detail how to determine the limiting molar conductivity of CH3COOH?
6. Discuss the Weaston bridge operation principle and methods of conductivity
measurements.
DETERMINATION OF ELECTROLYTE MOLAR CONDUCTIVITY AT INFINITE DILUTION
VII. LITERATURE
1. Praca zbiorowa; Chemia fizyczna; PWN Warszawa 1965
2. G. Kortum; Elektrochemia; PWN Warszawa 1970
3. A. Kisza; Elektrochemia I; WNT Warszawa 2000
4. P. W. Atkins, Physical Chemistry, Oxford University Press
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