CONDUCTIVITY OF METAL SOLUTIONS. ELECTROLYTES

1. CONDUCTIVITY OF METAL ION IN SOLUTION
BY
C.U.EBONG
SUMMARY
Conductivity (or specific conductance) of a metal ion in solution is a measure of its
ability to conduct electricity. The SI unit of conductivity is siemens per meter
(S/m). Conductivity measurements are used routinely in many industrial and
environmental applications as a fast, inexpensive and reliable way of measuring
the ionic content in a solution.

2. TABLE OF CONTENTS
Title page
Summary
Introduction
Electrolytic conductors
CONDUCTION IN ELECTROLYTIC SOLUTIONS
Units of conductivity
STRONG AND WEAK ELECTROLYTES
FACTORS INFLUENCING THE DEGREE OF METAL ION CODUCTIVITY
OF SOLUTION
MECHANISM OF ELECTROLYSIS
TEMPERATURE AND CONDUCTIVITY
Applications Of Conductivity
INTRODUCTION
Conductivity is a measure of how well a solution conducts electricity. To carry a
current a solution must contain charged particles, or ions. Most conductivity
measurements are made in aqueous solutions, andthe ions responsible for the
conductivity come from electrolytes dissolved in the water. Salts (like
sodiumchloride and magnesium sulfate), acids (like hydrochloric acid and acetic
acid), and bases (like sodium hydroxide and ammonia) are all electrolytes.

3. If we place a metal plate in a solution containing its ions (e.g. a solution of one of
its salts), some of the atoms from the metal plate will ionize and go into solution as
positively charged ions, leaving behind their valence electrons on the surface of the
plate. For a univalent ion, we have;
M(s)→ M+
(l)(aq) + e-
At the same time, some of the metallic ions in solution will take up electrons from
the metal plate and deposit themselves as neutral atoms on the plate, leaving
behind an excess of anions in the salt solution.
M+(aq) + e- → M(s)
If reaction (1) is favoured, the metal plate or electrode becomes negatively charged
with respect to the solution or electrolyte. If reaction (2) is favoured, the electrode
becomes positively charged with respect to the electrolyte. Potential difference,
known as the electrode potential for the metal ions/metal system is set up between
the metallic electrode and the electrolyte solution.
Although water itself is not an electrolyte, it does have a very small conductivity,
implying that at least some ions are present. The ions are hydrogen and hydroxide,
and they originate from the dissociation of molecular water. See Figure 1.
Salt:NaCl →Na+
+ Cl-
Acid:HCl→ H+
+ Cl-
Base:NaOH→ Na+
+ OH-
Water: H2O →H+
+ OH-
Metals are the best conductor and it remains unchanged with the passage of
current. A metallic conductor behaves as if it contains electrons which are
relatively free to move. So electrons are considered as charge carrier in metals.
Therefore, these conductors or electronic conduction is the property possessed by
pure metals, most alloys, carbon and certain solid salts and oxides.
ELECTROLYTIC CONDUCTORS

4. These are conductors through which passage of an electric current through them
results in transfer of matter or bring about a chemical change in them, which are
called electrolytic conductors or electrolytes. Electrolytic conductors are of two
types:
i. Electrolytic conductors, which conduct electrolytically in the pure state.
Such as acids, bases and salt in water. E.g. NaCl, NaNO3, K2SO4 etc.
ii. Electrolytic conductors which consists of solutions of one or more
substances.
In general, electrolytic solutions are prepared by dissolving a salt, acid or base in
water or other solvents. There is a special class of conductors, which conduct
partly electronically and partly electrolytically, they are known as mixed
conductors. For example, solution of the alkali and alkaline earth metals in liquid
ammonia are mixed conductors. Fused cuprous sulphide conducts electronically,
but a mixture with sodium or ferrous sulphide also shows electrolytic conduction.
CONDUCTION IN ELECTROLYTIC SOLUTIONS
The passage of current through solutions of salts of metals such as zinc, iron,
nivkel, cadmium, lead, copper, silver and mercury resultd in the liberation of these
metals at the cathode and from solutions of salts of the metals. If the anode consists
of an attackable metal, the flow of the current is accompanied by the passage of the
metal into solution. When the anode is made of an inert metal, e.g. platinum, an
element is generally set free at this electrode; from solutions of nitrates, sulphates,
phosphates etc., oxygen is liberated, whereas from halide solutions, other than
fluorides, the free halogen is produced.
The decomposition of solutions by the electric current, resulting in the liberation of
gases or metals, is known as electrolysis.
Ionic theory

5. The ionic theory was first prepared by Arrhenius in 1887 to explain the
conductivity of metal ion solution (electrolyte). The theory proposed that when an
electrolyte is melted or dissolved in water, some, if not all of the molecules of the
substance dissociateinto freely moving charged particles callled ions. The process
of dissociation into ions is known as ionization. The metallic ions, ammonium ions
and hydrogen ions are positively charged while the non-metallic ions and
hydroxide ions are negatively charged. The number of electrical charges carried by
an ion is equal to the valency of the corresponding atom or group. Due to
electronic charges carried by these ions, their properties are quite different from
those of their corresponding atoms which are electrically neutral.
+ - + - +
- - + - -
+ + + -
- + - + -
+ - + - +
- - + - -
+ + + -
- + - + -
+ - + - +
- - + - -
+ + + -
- + - + -
+ - + - +
- - + - -
+ + + -
- + - + -
+ - + - +
- - + - -
+ + + -
- + - + -
+ - + - +
- - + - -
+ + + -
- + - + -
+- +-+
- - +--
+++-
-+-+-
+ - + - +
- - + - -
+ + + -
- + - + -
random movement of metal ions in an electrolyte
When we pass an electric current through an electrolyte, the free ions lose their
random movement. The positive ions become attracted to the cathode (negative
electrode) and are known specifically as cations (i.e. cathode ions). The negative
ions move towards the anode (positive electrode) and are called anions (i.e. anode

6. ions). Therefore, the current through the electrolyte is carried by the movement of
ions to the electrodes, and not by a flow of electrons in the electrolyte.
During the electrolysis of a given electrolyte, the products formed at the electrodes
depend on the nature of the electrolyte. Where the electrolyte is a solution, the
electrolytic products formed at the electrodes may vary because the solvent, which
is usually water, will also ionize. In such a case, the cations and anions of bothvthe
electrolyte and the solvent will migrate to the cathode and the anode respectively
where they will compete with one another to be discharged. The product which is
formed at an electrode will depend on which ions are preferentially discharged-the
ions from the electrolyte or those from the solvent.
The discharge of ions is due to the relative position of ions in the series: the
position of ions in the electrochemical series is similar to that of the corresponding
elements.
If all other factors are constant, a cation which is lower in the series (less
electropositive) will show a greater tendency to be discharged than another which
is higher up (more electropositive). This is because the former gains electrons more
readily from the cathode and so becomes discharged as a neutral atom while the
latter tends to persists in solutions as a positive ion.
Also the concentration of the metal ion; increasing the concentration of a given ion
tends to promote its discharge from solution.

7. K
Na
Mg
Al
Zn
Fe
Sn
Pb
H
Cu
Hg
Ag
Au
+
+
2+
3+
2+
2+
2+
2+
+
2+
2+
+
+
Increasingpreferencefordischarge
Decreasingelectropositive To cathode
ELECTROCHEMICAL SERIES
UNITS OF CONDUCTIVITY
The units of conductivity are siemens per cm (S/cm). Derived units are μS/cm (one
millionth of a S/cm) and mS/cm (one thousandth of a S/cm). S/cm is the same as
the older unit mho/cm. Certain high purity water industries, primarily
semiconductor and pharmaceutical,use resistivity instead of conductivity.
Resistivity is the reciprocal of conductivity. The units are MΩ cm.

8. + -
Cathode
Anode
Electrolyte
cathode
anode
+ -
electron
flow
A simple Electrolytic cell
STRONG AND WEAK ELECTROLYTES
Solutes giving conducting solution in a suitable solvent are called electrolytes. On
the basis of degree of ionization, these electrolytes have been divided into two
categories.
 Strong electrolytes
 Weak electrolytes
STRONG ELECTROLYTES
Strong electrolytes are compounds like NaCl, which are nearly 100% dissociated
in solution. This means that nearly every sodium chloride formula unit exists as
sodium ions surrounded by water molecules and chloride ions surrounded by water
molecules. We can represent this by the following equation:

9. NaCl(s) →Na+
(aq) + Cl–
(aq)
These substances are highly dissociated and gave solutions with high conductance
in water. Due to the high degree of dissociation of strong electrolytes they are good
conductor of electricity i.e. aqueous solutions of these substances have high value
of molar conductance and on dilution the increase in their molar conductance is
very small. This is due to the fact that such electrolytes are completely ionized at
all dilutions therefore on further dilution the number of current carrying particles
does not increase in the solution. Thus, a soluion of electrolytes that has high molar
conductance, and increase very slowly on dilution has a high degree of dissociation
and is called strong electrolyte.
During the passage of an electric current through solutions, the ions carying
positive charges and moving in the direction of the current, i.e. towards the
cathode. Are refered to as cations and those carrying a negative charge and moving
in the opposite direction, i.e. towards the anode, are called anions.
WEAK ELECTROLYTES
Weak acids and weak bases, e.g. amines, phenols, most carboxylic acids and some
inorganic acids and bases, such as ammonia and a few salts, e.g. mercuric chloride
and cyanide dissociate only to a small extent at reasonable concentration; this
group of compounds in general are called weak electrolytes.
The molar conductance of the solutions of these electrolytes increases rapidly on
dilution. The reason of this is that more molecules ionuze on dilution inspite of this
they are never completely ionized.

10. FACTORS INFLUENCING THE DEGREE OF METAL ION
CODUCTIVITY OF SOLUTION
i. The nature of solutes: Thisnis the chief factor which determines the
degree of dissociation in solution. Strong electrolytes almost completely
dissociated in solution, while weak electrolytes feebly dissociated.
ii. The nature of solvent: This affects the dissociation to a marked degree.
It weakens the electrostatic forces of attraction between the two ions and
separates them. This effect is measured by its dielectric constant. The
dielectric constant is defined as its capacity to weaken the force of
attraction between the electrical charges immersed in that solvent.
iii. Concentration: The extent of dissociation of an electrolyte is inversely
proportional to the concentration of its solution. The less concentrated the
solution, the greater will be the dissociation of the electrolyte.
iv. Temperature: The higher the temperature the greater is the dissociation.
At high temperature the increased molecular velocities overcome the
forces of attraction between the ions and consequently the dissociation is
great.
MECHANISM OF ELECTROLYSIS
Electrolysis is the chemical change brought about by the passage of a direct current
through an electrolyte via electrodes.
Before the passage of current, the ions move about randomly in the electrolyte.
When electrolysis begins, the battery or generator of electric current pumps
electrons from its negative terminal (anode) to the cathode of the electrolytic cell.
The negatively charged cathode then attracts cations in th electrolyte to itself.
The cations accept electrons to become electrically neutral and are eventually
discharged. The positive terminal (cathode) of the battery draws electron from the
anode of the electrolytic cell. Anions in the electrolyte are then attracted to the

11. positively charged anode, where they give up their electrons to become electrically
neutral and are also finally discharged. Hence, an electric current passes through
the complete circuit.
+ -
Cathode
Anode
Electrolyte
cathode anode
+ -
electron
flow
A simple Electrolytic cell
-
-
-
-
+
+
+
+
TEMPERATURE AND CONDUCTIVITY
The conductivity of a metal solution is a function of temperature. Generally the
conductivity of a metal solution increases with temperature, as the mobility of the
ions increases. The increase issignificant, between 1.5 and 5.0% per °C. To
compensate for temperature changes, conductivity readingsare commonly
corrected to the value at a reference temperature, typically 25°C. All process
conductivitysensors have integral temperature sensors that allow the analyzer to

12. measure the process temperature and correct the raw conductivity. Three
temperature correction algorithms are in common use.
i. Linear temperature coefficient
ii. High purity water or dilute sodium chloride
iii. Cation conductivity or dilute hydrochloric acid
No temperature correction is perfect. Unless the composition of the process liquid
exactly matches the model used in the correction algorithm, there will be an error.
In addition, errors in the temperature measurement itself will lead to errors in the
corrected conductivity.
Linear temperature coefficient
The linear temperature correction is widely used. It is based on the observation that
the conductivity of anelectrolyte changes by about the same percentage for every
°C change in temperature.
High purity water
The high purity water correction assumes the sample is pure water contaminated
with sodium chloride(NaCl). The measured conductivity is the sum of the
conductivity from water and the conductivity from thesodium and chloride ions.
Cation conductivity
The cation conductivity temperature correction is unique to the steam electric
power industry. Cation conductivity is a way of detecting ionic contamination in
the presence of background conductivity caused by ammonia or neutralizing
amines, which are added to the condensate and feedwater to elevate the pH and
reduce corrosion. In cation conductivity, the amines are removed and the ionic
contaminant is converted to the equivalent acid, for example sodium chloride is
converted to hydrochloric acid. The cation conductivity model assumes the sample
is pure water contaminated with hydrochloric acid. The

13. correction algorithm is more complicated than the high purity water correction
because the contributionof water to the overall conductivity depends on the amount
of acid present. Hydrochloric acid suppressesthe dissociation of water, causing its
contribution to the total conductivity to change as the concentration
of hydrochloric acid changes.
APPLICATIONS OF CONDUCTIVITY
Measured conductivity is a good indicator of the presence or absence of conductive
ions in solution, and measurements are used extensively in many industries. Some
important applications are described below:
i. Water treatment: Raw water as it comes from a lake, river, or the tap is
rarely suitable for industrial use. The water contains contaminants,
largely metal in form of ions, that if not removed will cause scaling and
corrosion in plant equipment, particularly in heat exchangers, cooling
towers, and boilers. There are many ways to treat water, and different
treatments have different goals.Often the goal is demineralization, which
is the removal of all or nearly all of the metal contaminants. In other
cases the goal is to remove only certain contaminants, for example
hardness ions (calcium and magnesium). Because conductivity is a
measure of the total concentration of ions, it is ideal for
monitoringdemineralizer performance. It is rarely suitable for measuring
how well specific ionic contaminants arebeing removed.
ii. Conductivity is also used to monitor the build up of dissolved ionic solids
in evaporative cooling water systems and in boilers. When the
conductivity gets too high, indicating a potentially harmful
accumulationof solids, a quantity of water is drained out of the system
and replaced with water having lower conductivity.

14. iii. Leak detection: Water used for cooling in heat exchangers and surface
condensers usually contains large amounts of dissolved ionic solids.
Leakage of the cooling water into the process liquid can result
inpotentially harmful contamination. Measuring conductivity in the outlet
of a heat exchanger or in the concondenser hotwell is an easy way of
detecting leaks.
iv. Clean in place:In the pharmaceutical and food and beverage industries,
piping and vessels are periodically cleaned and sanitized in a procedure
called clean-in- place (CIP). Conductivity is used to monitor both the
concentration of the CIP solution, typically sodium hydroxide, and the
completeness of the rinse.
v. Interface detection: If two liquids have appreciably different
conductivity, a conductivity sensor can detect the interface between
them. Interface detection is important in a variety of industries including
chemical processing and food and beverage manufacturing.
vi. Desalination: Drinking water desalination plants, both thermal
(evaporative) and membrane (reverseosmosis), make extensive use of
conductivity to monitor how completely dissolved ionic solids are
beingremoved from the brackish raw water.
vii. To Study Solutions: Substances that completely dissociate into ions
(strong electrolytes) produce solutions with high conductivity.
Substances that partially dissociate (weak electrolytes) will produce
solutions with low conductivity. Substances that dissolve but do not
dissociate produce solutions that have the very low conductivity of pure
water.
viii. To Study Chemical Reactions: Since conductivity is a function of both
the concentration and the composition of the solution being measured,

15. we can use conductivity to follow chemical reactions. Consider an acid
base titration of a solution of KOH with HBr. The reaction is represented
in the following: balanced chemical equation:
KOH(aq) + HBr(aq) →KBr(aq) + H2O
Complete ionic equation : K++ OH–+ H++ Br- →K++ Br– + H2O or the
net ionic equation: OH–+ H+ →H2O
CONCLUSION
Conductivity is a measure of how well a solution conducts electricity. To carry a
current a solution must contain charged particles, or ions. Most conductivity
measurements are made in aqueous solutions, and the ions responsible for the
conductivity come from electrolytes dissolved in the water.
Metals are the best conductor and it remains unchanged with the passage of
current. A metallic conductor behaves as if it contains electrons which are
relatively free to move. So electrons are considered as charge carrier in metals.
Therefore, these conductors or electronic conduction is the property possessed by
pure metals, most alloys, carbon and certain solid salts and oxides.
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