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.

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.

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

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

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.

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

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.

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.

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:

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.

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

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.

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

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

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.

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,

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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