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CHEMISTRY FORM THREE STUDY NOTES TOPIC 5: VOLUMETRIC ANALYSIS & TOPIC 6: IONIC THEORY AND ELECTROLYSIS

TOPIC 5: VOLUMETRIC ANALYSIS 

Standard Volumetric Apparatus
The Concept of Volumetric Analysis
Explain the concept of volumetric analysis
Volumetric
analysis is a quantitative analysis involving the measurement of
different solutions. These solutions are made to react completely and
the completion of the reaction is indicated by certain substances called
indicators. The quantitative composition of the solution is then determined.
Important steps of volumetric analysis include:
  • Weighing;
  • Preparation of the solution;
  • Titration; and
  • Calculation
In
volumetric analysis, we deal with volumes of solutions. That is why
this quantitative determination of solutions of substances is called volumetric analysis.
The
amount of a substance present in a solution is given in terms of its
volume and its concentration. The volume of a solution is usually given
in litres (dm3). The concentration of a solution is given in moles per litre (mol/dm3) or grams per litre (g/dm3).
Volumetric
analysis is a means of finding the concentration of an unknown
solution. For example, the concentration of an unknown solution of an
acid can be found if it is reacted with a standard solution of an
alkali. A standard solution is one whose concentration is well known and does not change with time.
In
volumetric analysis, the reaction is carried out in a carefully
controlled way. The volumes are measured accurately using a pipette and
burette. The method is to add a solution of one reactant to the solution
of another reactant until the reaction is complete. When the reaction
is complete, we say the end-point has been
reached. If the reactants are acids and bases, completion (end-point) is
determined by the change in colour of an acid-base indicator. The
method is called titration. In other
reactions, completion is determined by a colour change of reactant(s).
The concentration of one of the reactant solutions must be known in
order to be able to find the concentration of unknown solution.
Significance of Volumetric Analysis
  1. Volumetric
    analysis is used to quantify the amount of substances present in
    solutions by analytical procedure, which involves precise measurements
    of volumes of solutions and masses of solids.
  2. Volumetric
    analysis helps in the determination of the accurate volumes and
    concentrations of the reacting substances, often solutions.
  3. Volumetric analysis (titration) helps in the preparations of standard solutions.
  4. Volumetric analysis knowledge helps in the standardization of acids and bases.
Volumetric Apparatus
Use volumetric apparatus
We
have seen that volumetric analysis involves determinations of
quantities of substances, usually acids and alkalis, present in volumes
of solutions. This is usually done by using measuring apparatus.
Apparatus
used in volumetric analysis is based on volume measurements and since
the analysis demands high accuracy, the apparatus has to be calibrated
with the highest possible accuracy. It is for this reason that all
apparatus for volumetric analysis are specifically for this and not
other purposes.
Apparatus
used for volumetric analysis include, burette, pipette, burette stand,
white tile, conical flask, filter funnel, reagent bottle, watch glass,
beaker, measuring cylinder and measuring flask (or volumetric flask).
For approximate measurements, measuring cylinders may be used. For
accurate measurements of volumes, volumetric flasks are used.
Burette
This
is a long glass tube with a narrow lower part, which is fitted with a
tap that controls the amount of solution let out of the burette. This
instrument is calibrated from 0 to 50 cm3.Before measuring
the solution, rinse the burette with distilled water, then with the
solution it is going to hold. It has to be filled to the tip and all gas
bubbles removed. Thus, the burette is an apparatus used for
transferring the solution to the titration vessel (normally a flask).
Pipette
This
apparatus has a wider middle part with narrow parts at either ends. The
upper narrow part has a mark which marks the volume of all the space
below it. If, say, the pipette is one that is marked 25 cm3, we can say that a solution, when filled in the pipette up to this mark, will have a volume of 25 cm3.
The
pipette is used in transferring a standard solution to the titration
flask. There are many types of pipettes depending on their volume
capacity. The common ones are the 25-cm3 and 20-cm3 capacity pipettes. Less common ones are the 10-cm3
capacity.Before measuring the solution, rinse the pipette several times
with distilled water and then with the solution to be measured; suck
the rinsing solution above the graduated mark, then discard the rinsing.
The
pipette is commonly filled by mouth suction but the use of pipette
fillers is highly recommended. When using a pipette, never blow out the
last drop.

(a) A pipette (b) A pipette and pipette filler (used to fill and empty pipettes)
Measuring (Volumetric) flask
The
flask is made of glass and has a mark at the upper part of the narrow
tube. The space in the flask up to this mark represents a certain
volume. If a solution is filled up to this mark, the volume of the
solution is equal to the volume indicated by inscriptions on the flask
e.g. 50 cm3, 100 cm3, 150 cm3, 250 cm3, 500 cm3, etc.
Filter funnel
A
filter funnel is required for effective transfer of the weighed solid,
liquid or solution into the volumetric flask or burette.

A filter funnel
Wash bottle
Wash
bottle contains water and when squeezed, water squarts out. This is
used in washing down the remains of the weighed solid into the
volumetric flask.

A wash bottle
A weighing bottle
This is used in weighing the solute. It is a stoppered bottle. A watch glass can also be used to serve the same purpose.
Retort stand
A burette stand is used for holding the burette in place while carrying out volumetric analysis experiments.

A burette stand
Dropper
A dropper is used to add the indicator dropwise into the solution.
White tile or paper
A
white tile or piece of paper is placed under the flask to give a clear
background for accurate observation of the colour change at the end of
the reaction (end point).
The Application of Volumetric Analysis in Real Life Situations
Explain the application of volumetric analysis in real life situations
Volumetric
analysis has a variety of laboratory and industrial applications in
everyday life. The following are just a few of the applications (uses)
of volumetric analysis in daily life:
  • Use in preparation of standard solutions:Standard
    solutions are prepared by applying the knowledge of volumetric
    analysis. Volumetric analysis is used in school, college and university
    chemistry laboratories to determine concentrations of unknown
    substances. The titrant (the known solution) is added to a known
    quantity of analyte (unknown solution) and a reaction takes place.
    Knowing the volume of the titrant allows one to determine the
    concentration of the unknown substance.
  • Use in environmental and water safety:Titration
    is important in environmental chemistry, where scientists can use it to
    analyze acid rain or contaminants in surface water samples.
    Environmental studies usually involve an analysis of precipitation and
    its response to pollution. To quantify the degree of contamination in
    natural rainwater or snow, titration is used. The process is quick and
    results are reliable. Since most titration processes do not require
    expensive or specialized equipment, the test can be performed often and
    in different areas with relatively little effort.The safety of water is
    based on its chemical ingredients. By analyzing wastewater, the extent
    of contamination and the requirements for filtering and cleaning can be
    determined. Titration is a key mechanism in this analysis. Often, more
    specialized titration equipment is used in this application, which
    measures ammonia levels in combination with other reactants to quantify
    other chemicals present.
  • Use in food and beverage industry:In
    the food and beverage industry, manufacturers must ensure their
    products meet certain quality criteria or contain standard
    concentrations of specific additives, so titration is often used to
    analyze the products before sale. Wine is often affected by its degree
    of acidity. It is possible to improve wine production by measuring
    acidity using titration. Simple, inexpensive titration kits are
    available to winemakers for this purpose. The results of a titration
    test on wine can suggest if additional ingredients are necessary to
    maintain its quality.In general, all brewing industries and distilleries
    apply the knowledge of volumetric analysis (titration) to determine the
    acidity and alcohol contents of their beers and other alcoholic
    beverages.The process also finds ample use in food industry. The
    compounds which make up food products help determine their nutritional
    implications. Titration is one technique that assists in these studies.
    The acidity of orange juice, for example, is easily determined using a
    standard titration process. In this process, an electrode is added to a
    solution made up of orange juice and deionized water. The titrant
    catalyst then measures the acidity of the juice. Manufacturers can use
    the technique to vary this quality to satisfy customers or those with
    special nutritional needs.
  • Use in agriculture:Volumetric
    analysis technique is used to determine the soil pH. This is important
    because, if the pH of a certain soil is found to be extremely low or
    high, corrective measures are taken by adding the correct quantity of
    agricultural limes or other chemicals to make the soil suitable for
    plant growth.The method is also used by agronomists and farmers to
    analyse the kind and amount of plant nutrient elements present in a
    particular sample of soil, the knowledge of which helps determine soil
    fertility.
Industrial and Laboratory Skills of Volumetric Analysis
Compare industrial and laboratory skills of volumetric analysis
The
knowledge of volumetric analysis (titration) is used in hospitals and
medical laboratories to carry out such duties as preparation of
solutions and suspensions, blood analysis, and diagnosis of certain
diseases and health problems. For example, when dissolving a solid drug
to make a solution for injection, utmost precision is required to
measure the correct volume of liquid to be used to dissolve a correct
amount of solid drug to prepare the solution of a given concentration to
inject to a patient.
Also
titration is very important in the pharmaceutical industry, where
precise measurements of quantities and concentrations are essential
throughout the manufacturing process. Titration is thus an important
part of the pharmaceutical industry to ensure quality control. Many
variations of the titration technique are used, and specialized
equipment for pharmaceutical titration is often developed to make the
process more efficient.

 TOPIC 6: IONIC THEORY AND ELECTROLYSIS

Ionic Theory
To
account for the phenomena of electrolysis the Ionic Theory was put
forward by Arrhenius in 1880. The theory states that electrolytes are
made up of ions, which are built up in certain patterns called crystal
lattice. When these substances dissolve in water, the structure is
destroyed and the ions are set free to move.Concentrated mineral acids
such as sulphuric acid, hydrochloric acid and nitric acid do not contain
ions but they consist of molecules. However, when they are diluted, the
molecular structure is destroyed and ions are formed.
Electrolytes and Non-electrolytes
Distinguish electrolytes from non-electrolytes
The
main purpose of this chapter is to investigate the effects which
electricity has on a range of substances, and to develop a thorough
explanation of those effects in terms of our present knowledge of atomic
structure. Before we begin, it is important that we familiarize
ourselves with different terms that we are going to use to explain
different phenomena. It is crucial that the definitions and meanings of
these terms be understood at the outset in order that concepts defined
in this chapter are easily and clearly apprehended. These terms are
given hereunder:
  • Electrolysis: decomposition of a compound in solution or molten state by passing electricity through it.
  • Conductor: a solid substance that allows electricity to pass through it. All metals are included in this class.
  • Non-conductor or insulator: a solid substance that does not allow electricity to flow through it. All non-metals fall in this class.
  • Electrolyte: a substance which, when dissolved or molten, conducts electricity and is decomposed by it.
  • Non-electrolyte: a compound which cannot conduct electricity, be it in molten or solution state.
  • Electrode: a graphite or metal pole (rod) or plate through which the electric current enters or leaves the electrolyte.
  • Cathode: a negative electrode which leads electrons into the electrolyte.
  • Anode: a positive electrode which leads electrons out of the electrolyte.
  • Ion: a positively or negatively charged atom or radical (group of atoms).
  • Cation: a positive ion which moves to the cathode during electrolysis.
  • Anion: a negative ion which moves to the anode during electrolysis.
Electrolytes and non-electrolytes
Liquids
such as ethanol, paraffin, petrol and methylbenzene do not conduct
electricity. The bonding in these compounds is covalent. These
substances consist of molecules. There are no free electronsor charged
particles to flow through them. Solutions of covalent compounds, for
example sugar solution, do not conduct electricity.
These compounds are non-electrolytes. Non-electolytes exist only in the form of molecules and are incapable of ionization.
Ionic
compounds contain charged particles (ions), but in solid state, the
ions are firmly held in place and they are not free to move. An ionic
solid does not conduct electricity. However, the ions present can become
free to move if the solid is melted or dissolved in water. Then they
can conduct electricity. For example, solid sodium chloride cannot
conduct electricity but when melted or dissolved in water, the ions, Na+ and Cl- are set free. Then these ions are free to move in solution and hence conduct electricity. These compounds are called electrolytes.
Weak and Strong Electrolytes
Categorize weak and strong electrolytes
Weak electrolytes
are compounds that are only partially or slightly ionized in aqueous
solutions. Some substances, for example, ethanoic acid solution ionize
partially.
CH3COOH(aq) ⇔CH3COO-(aq) + H+(aq)
Most
of the electrolytes exist in solution in the form of unionized
molecules. For example, in ordinary dilute (2M) ethanoic acid, out of
every 1000 molecules present, only 4 are ionized and 996 are unionized.
A solution of ammonia water is also a weak electrolyte, containing a relatively small proportion of ammonium and hydroxyl ions.
NH4OH(aq) ⇔NH4+(aq) + OH-(aq)
Most of the organic acids are weak electrolytes, e.g. tartaric, citric and carbonic acids.
However,
there is no sharp dividing line between weak and strong
electrolytes.Water is also a weak electrolyte. It ionizes only slightly.
H2O(l)⇔H+(aq) + OH-(aq)
Study shows that for every molecule of water ionized, there are 6 million molecules of water not ionized.Strong electrolytes
are compounds that are completely ionized in aqueous solutions. When
sodium chloride is dissolved in adequate water it ionizes completely
into Na+ and Cl- ions. There are no NaCl solid
particles left unionized. All strong electrolytes (salts, the mineral
acids and caustic alkalis) ionize completely in solutions.
Electrolysis
has several uses in industry. Its main application has been in the
fields of manufacture of chemicals and in the purification of metals for
which other purification methods prove either too difficult or highly
expensive to apply. Some applications of electrolysis are as discussed
below:
The Industrial Purification of Copper by Electrolysis
Outline the industrial purification of copper by electrolysis
Some
metals can be purified by means of electrolysis. This process is used
in industry to purify copper, which must be very pure 99.9% for
electrical wiring. Copper made by roasting the sulphide ore is about
99.5% pure (so it has an impurity level of 0.5%). This level of impurity
cuts down electrical conductivity significantly.
This
is how the electrolytic purification (refining) process is carried
out:The anode is made of a large block of impure copper. The cathode is a
thin sheet of pure copper. The electrolyte is copper (II) sulphate
solution.During the refining process, the copper atoms of the impure
block become ions (the anode dissolves).Cu → Cu2+ + 2e-
The ions from the solution become atoms.
Cu2+ + 2e- → Cu(s)
They
stick onto the cathode. A layer of pure copper builds up on the
cathode. As electrolysis takes place, the cathode gains mass as copper
is deposited on it. As a result, the cathode gets smaller while the
cathode gets bigger as electrolysis proceeds. Eventually the whole
cathode dissolves.

Purification of copper by electrolysis
Only
pure copper sticks to the cathode. Most impurities fall to the bottom
of the electrolytic cell. They form a solid material (anode sludge or slime)
which contains small quantities of precious metals such as silver, gold
and platinum. The precious metals recovered from the slime are purified
and sold.
An Experiment on Electroplating of Metallic Materials
Carry out an experiment on electroplating of metallic materials
Electroplating
is the coating of a metal with a layer of another metal by means of
electrolysis. Electrolysis can be used to coat a thin layer of a less
reactive metal onto a more reactive metal. The thin layer of less
reactive metal will provide protection from corrosion for the more
reactive metal underneath. It may also make the product more attractive.
The
object to be coated should be made the cathode and the coating material
should be the electrolyte. The most commonly used metals for
electroplating are copper, chromium, silver and tin.
Steel
can be electroplated with chromium or tin. This prevents the steel from
rusting and gives it a shiny, silver finish. This is also the idea
behind chromium-plating articles such as car bumpers, kettles, bath
taps, etc. Chromium does not corrode, it is a hard metal that resists
scratching and wear, and can also be polished to give an attractive
finish.
Nickel
can be electroplated with silver. This will make nickel more
attractive.The diagram below shows how a steel jug is electroplated with
silver. The jug becomes the cathode of an electrolytic cell. The anode
is made of silver. The electrolyte is a solution of a silver compound,
for example silver nitrate.

Silverplating a steel jug
At the anode: The silver dissolves, forming ions in solution:Ag → Ag+ + e-
At the cathode: The silver ions receive electrons, forming a coat of silver on the jug:Ag+ + e-→Ag (s)
When the layer of silver is thick enough, the jug is removed.In general, to electroplate any object with metal M, the set up is:
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