Home News CHEMISTRY FORM FOUR STUDY NOTES TOPIC 5: QUALITATIVE ANALYSIS

CHEMISTRY FORM FOUR STUDY NOTES TOPIC 5: QUALITATIVE ANALYSIS

The Concept of Qualitative Analysis

The Meaning of Qualitative Analysis
Explain the meaning of qualitative analysis
Qualitative
analysis involves determining metals and non metals (or metallic and
non-metallic elements) present in a given sample. It is a process used
to determine what elements (or radicals) are present in the sample.
Whereas
quantitative analysis involves determining how much (quantity) of a
substance is present in a sample, qualitative analysis involves
identifying what (quality) substances are present in a sample.
Qualitative analysis may be divided into examination of:
  • acidic or non-metallic radicals or negative ions;
  • basic or metallic radicals, including ammonium ion.
The Importance of Qualitative Analysis in Real Life
State the importance of qualitative analysis in real life
Qualitative
analysis has numerous applications. The following are some applications
(importance) of qualitative analysis in real life.
Detection of poisonous substances in the environment
Sometimes
it may be necessary to find out the chemical composition of
contaminants in the environment. This may require the application of
qualitative analysis procedures to identify the chemical composition of a
given contaminant. Qualitative analysis methods are highly employed by
environmental scientists to detect and identify different contaminants
in the environment.
Determination of ions present in a chemical substance
This
is achieved through dissolution of a substance in distilled water. Then
the characteristics of the solution formed serve as a clue to establish
the type of elements present in a tested sample. The mixture formed
following dissolution of a solid sample may be a clear solution, an
emulsion or a precipitate. The solution or emulsion is further analysed
to detect the ions present in it. The precipitate is then separated from
the filtrate and both are subjected to further tests to identify the
kind of elements present.
Determination of the type of gases produced in a chemical reaction
Many
chemical reactions are accompanied with evolution of gases, as one of
the products of the reaction. In some cases, the smell of the gas may
not suffice to detect the gas, especially if the gas is colourless and
odourless. In such cases, the gas is subjected to various qualitative
analysis tests in order to establish its identify.
Finding the nature and identity of chemical substances
The
nature of a chemical substance such as its solubility in water,
characteristic smell, flame colour, and the characteristics of its
reaction products can be used to identify the chemical substance under
test. In this way, the nature and identity of unknown substance can
ultimately be known.
Soil analysis
Determination of soil pH
Soils
of given pH give specific colours when their solutions are added to
certain types of indicators. This procedure is purely qualitative
because it involves observation of the change in colour of indicators to
determine the pH of the soil. For further details on the measurement of
soil pH, read a topic on Soil Chemistry (Chapter Three) in this book.
Determination of soil composition
Qualitative
tests are performed to determine the type of minerals contained in a
particular soil. Such tests include test for nitrate, sulphate, chloride
and phosphate ions. Determination of soil composition gives soil
scientists information necessary for conservation.
Application in medical and criminology fields
Qualitative
analysis techniques are applied in medical field, for example in
carrying out various tests such as testing blood and urine samples,
determining the level of blood sugar, pregnancy diagnose and blood
grouping. Most of these analytical tests are done to diagnose a wide
range of diseases and medical conditions.
For example:
  • in
    detecting the causative agents for typhoid (salmonella typhi), the
    blood is left to clot, or it is centrifuged in order to separate blood
    corpuscles from plasma. The plasma is then subjected to various
    qualitative tests to detect the presence of salmonella typhi.
  • in
    pregnancy diagnosis, a certain chemical is added to urine, where a
    specific change in colour of the urine confirms whether one is pregnant
    or not.
Qualitative
analysis is also applied in blood grouping, whereby antibodies are
added to the blood to determine the blood group. Agglutination of the
blood corpuscles when antibodies are added help detect the group of the
blood.
Forensic
scientists may use qualitative chemical analysis to identify substances
left at crime scenes, which can be vital in convicting criminals.
Application in industries
Many
industries use qualitative analysis in their efforts to perform better
and make quality products.In chemical industries, the products often get
contaminated by unknown contaminants. Qualitative tests are done to
detect the contaminants. This may be followed by quantitative analysis
aiming at finding out the quantity (amount) of the contaminant present.
Qualitative analysis techniques
It
is very important to be very keen when performing qualitative analysis
tests. Just a small error can cause misinterpretation of the contents of
a tested sample. The following are few but important measures that
should be observed when carrying out qualitative analysis experiments.
Adding the reagent
After
adding the test reagent, make sure the solution is stirred thoroughly.
Before making any inference, wait until the solution has completely
settled down. Often, additional quantities of the reagent must then be
added so that an excess is present.
Avoid contamination
Contamination
of the sample or reagent should strictly be avoided. For instance, if
the contaminant contains a cation, the test can give misleading positive
results. To avoid contamination of the sample and apparatus, the
following precautions should be observed:
  1. All the apparatus should be cleaned and dried thoroughly and must remain clean throughout the experiment.
  2. Do not lay a glass rod on a dirt laboratory bench as it can get contaminated easily.
  3. Avoid
    touching the side of a test-tube with the tip of a dropper. The
    contaminant can be picked up and transferred to another solution, a fact
    that would contaminate the solution, thus producing false results.
  4. Only
    distilled water from the wash bottle should be used to dissolve the
    solids. Spring, rain or tap water contains chemicals that can lead to
    wrong results and conclusions.
Warming up the solution
The
solution is warmed by dipping the test-tube containing the solution in a
beaker of warm or boiling water (water bath). The test-tube is kept in a
water bath for some time, with constant stirring. Then, the solution is
tested for the presence of anions or cations contained in it.
Testing the pH of a solution
This
is conducted by placing a litmus paper on a clean, dry surface and then
pouring a drop of the solution to be tested on it. The colour of the
paper is observed and noted while the paper is wet. Once dry, the colour
may change or disappear, resulting in wrong interpretation.
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