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CHEMICAL ANALYSIS & IDENTIFICATION TESTS
Doc Brown's Chemistry Qualitative
Methods of Analysis Revision Notes
PART 1 INTRODUCTION and chemical identification test index (repeated on each
page)
Introduction to qualitative analysis tests to
identify inorganic gases & ions (cations/anions) and organic molecule functional
groups
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Chemical Calculations and Quantitative
Analysis Index
QUALITATIVE CHEMICAL TEST INDEX
 Part 1
Introduction to chemical testing and analysing substances (this page)
Part 2
Qualitative tests to identify organic molecule functional
groups of homologous series
Part 3 TESTS for
Metal cations (positive ions), metal carbonates, ammonium ion, hydrogen ions (acids)
Part 4
TESTS for Gases,
water and non–metallic elements
Part 5
TESTS for Anions (negative ions) including hydroxide
(alkalis)
APPENDIX 1. IDENTIFYING ELEMENTS from LINE SPECTRA
(non-chemical test method)
PART 1 INTRODUCTION to CHEMICAL TESTS
and ANALYSING SUBSTANCES
Of what use is chemical analysis
and chemical testing?
Well, rather a lot, as it happens!
The production of many products we use in our society involved some kind of
chemical analysis at some stage or other. It might analysing mineral ores for
their metal content or testing the final purity of some drug/medicine from the
pharmaceutical industry.
For chemical analysis analysts have developed
a wide range of qualitative tests to detect specific chemicals which may be
molecules or ions. Chemical tests are based on reactions that produce a gas with
distinctive properties, or a colour change produced by adding a reagent or the
production of an insoluble solid that appears as a precipitate. Modern
instrumental methods provide a fast, sensitive and accurate means of analysing
chemicals, and are particularly useful when the amount of chemical being
analysed is small. Forensic scientists and drug control scientists rely heavily
on such instrumental methods in their work.
Environmental agencies monitor
levels of oxygen and pollutants in rives and lakes. Hospitals carry out complex
blood analysis for iron, proteins, cholesterol etc. and this is very important
diagnostic information for clinicians and doctors seeking to get you well again
prevent diseases developing. Blood can be analysed for specific proteins to
indicate particular medical conditions e.g. cancer, and ions such as sodium,
chloride and iron compounds. At water treatment plants test are done to
determine the levels of metal ions, insecticides and lots of other substances to
check that their levels are not high enough to be harmful to humans.
Apart from a multitude of forensic
tests for DNA, powder burns from guns, explosives etc. the police use
breathalyser kits to test for alcohol levels in your breath and may request a
blood sample for analysis too.
Elements and compounds can be detected and
identified using instrumental methods which are accurate, sensitive and rapid
and are particularly useful when the amount of a sample is very small
There are two types of chemical
analysis
Qualitative tests and quantitative
analysis. Whatever the nature of the investigation, all tests of analyses should
be carried out by using 'tried and tested' standard procedures. They should be
the most accurate, reliable and safest methods that can be devised. It means,
whatever laboratory you work in, anywhere in the world, you should get the same
results as long as the sample is collected, stored and analysed by the same
standard procedures.
(a)
QUALITATIVE CHEMICAL ANALYSIS
Qualitative chemical analysis
indicates whether a particular substance is present or not. It does not tell how
much of the substance is there or its concentration. However, if a substance is
potentially harmful, even toxic, its a good idea to know whether the substance
is there at all. The larger the sample you have, the better. With more to work
with you are more likely to able to detect minute traces of substances with
qualitative tests AND have spare material to repeat tests several times if the
results seem uncertain at first.
At school/college level, the simple
tests you learn enable to identify the cation (+ve ion e.g. metal ions) and anion (-ve ion,
e.g. chloride, sulfate) in a
salt, and the salt usually does contain only two ions, but some salts do have
three ions e.g. iron(II) ammonium sulfate which might take a bit of sorting out.
Most tests at this level are done on soluble substances so that you can dissolve
the substance in water and carry out tests on the aqueous solution. Aqueous
means a solution in water. You may come across a non-aqueous solution using a
non-aqueous solvent like alcohol or hexane.
AND don't forget, negative test
results are just as important as positive results, you may need to eliminate
possibilities as well as confirm the presence of a particular ion or gas etc.
UNFORTUNATELY, not all tests are
unique for a particular ion, but this shouldn't be a problem in school
chemistry!
Associated qualitative analysis
links
See index at
top of page Typical qualitative tests
are described in Parts 2 to 5.
Summary of some cation and anion
tests for GCSE/IGCSE/O Level students
TOP OF PAGE
(b)
QUANTITATIVE CHEMICAL
ANALYSIS
Quantitative analysis gives you
(hopefully) a precise measure of how much of a substance is present or its
concentration in a sample being analysed e.g. ore analysis tells you whether it
is worth exploiting for a metal, the purity of drug ensure no harmful impurities
in it, blood sample analysis for alcohol allows the police to decide to
prosecute for 'being over the limit'. With a large sample you have spare
material to repeat the quantitative analysis several times to get the most
statistically valid result.
In schools and colleges you can do
quite accurate titrations to illustrate quantitative analysis.
Associated quantitative links
% purity of a product (GCSE/IGCSE/O level, introduction for Advanced Level)
Volumetric titration analysis
methods and calculations (GCSE/IGCSE/O level, introduction for Advanced Level)
Various non-redox titration
methods and questions (Advanced Level, acid-alkali, EDTA, silver nitrate etc.)
Various redox titration
methods and questions (Advanced Level, potassium manganate(VII),
thiosulfate/iodine etc.)
Other Associated Qualitative or
Quantitative Analysis LINKS
(GCSE/IGCSE Level)
Instrumental Methods of Chemical Analysis
Chromatography
- Paper/Thin Layer Chromatography and Gas Chromatography
GCSE/IGCSE
Revision QUIZ on chemical tests for
identifying ions, gases and compounds
Part 1 contd. Alphabetical TEST INDEX
for cations, anions, organic functional groups etc.
Full list of KEYWORDS
for inorganic/organic
identification methods in alphabetical order
e.g. test/reagent for: * acid ==> H+ *
acid/acyl
chloride RCOCl * alcohols – general ROH/prim
RCH2OH/sec R2CHOH/tert
R3COH) * aldehyde RCHO
* prim aliphatic amine R–NH2 *
aliphatic/aromatic carboxylic acids * alkali
==> OH–
* alkane/alkene >C=C</alkyne –C C– (saturated versus unsaturated)
* aluminium/aluminum ion Al3+ *
amide RCONH2
* prim aliphatic amines R–NH2 *
ammonia gas NH3
*
ammonium ion NH4+ *
prim aromatic
amine C6H5–NH2 etc. *
barium ion Ba2+ * Benedict's
solution * Brady's reagent *
bromide ion Br– * bromine
Br2
* caesium ion Cs+ * calcium ion Ca2+ by flame
or hydroxide
ppt.
* carbonate CO32–/hydrogencarbonate
HCO3– with acid or
effect of heating metal carbonate e.g. MCO3 *
carbon dioxide gas CO2
* carboxylic
acid RCOOH * carboxylic acid (aliphatic) salts e.g. RCOO–Na+
* chloride ion Cl– * chlorine
gas Cl2 *
Chromate(VI) ion CrO42–
* copper(II) ion Cu2+ by flame or hydroxide
ppt. * 24DNPH (for aldehydes/ketones
test)
* esters RCOOR
* Fehlings
test/solution * flame test for metal ions
* fluoride ion F– * haloalkanes/halogenoalkanes R–X
* hydrogen gas H2 * hydrogen sulphide
H2S * hydrogen ion,
acids H+
* hydrogen bromide gas/hydrobromic acid HBr * hydrogen
chloride gas/hydrochloric acid HCl * hydrogen
iodide gas/hydriodic acid HI * hydroxide ion, alkali OH– *
hydroxy/alcohol/phenol
(organic) * iodide ion I– *
iodine I2
* iodoform test – formation of CHI3 *
iron(II) ion Fe2+
* iron(III) ion Fe3+ *
ketone R2C=O
* lead(II) ion Pb2+ * lithium ion
Li+ *
lime water Ca(OH)2(aq) *
magnesium ion Mg2+ * metal
carbonates–heating e.g. MCO3 * metal ions via
hydroxide precipitate * nitrate or nitrate(V) NO3– *
nitrite
or nitrate(III) NO2– * nitrogen dioxide or
nitrogen(IV) oxide NO2 * oxygen gas O2 *
phenols C6H5OH
etc.
* potassium ion K+ * rubidium
ion Rb+
* reducing sugars * saturated/unsaturated *
silver nitrate AgNO3 (see
chloride, bromide,
iodide tests)
* sugars (reducing) * sodium ion Na+
*
strontium Sr+ * 'sulphate/sulfate' or sulphate(VI)
SO42–
* sulphide S2– * 'sulphite/sulfite'
or sulphate(IV) SO32– * sulphur dioxide gas
SO2 *
Tollen's Reagent *
unsaturated/saturated * water H2O
* zinc ion Zn2+ *
Use
the list above for identifying anions, cations, gases,
molecules etc. to find what you require! for your KS3–KS4 Science–GCSE–IGCSE–
Chemistry and GCE–AS–A2–IB–US
grades 9–12 K12 advanced subsidiary chemistry course etc. and help you to identify unknown inorganic and organic
compounds–molecules for qualitative analysis. Its also a good idea to read the
brief notes after the alphabetical list.
EMAIL
query? comment? test missing? * Gas Preparations * Hazard warning signs/symbols–examples
of labelling
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PLEASE NOTE:
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Most of the tests
describe use simple apparatus like test tubes, teat pipette, wire for flame
test (nichrome, platinum best but costly) and standard chemical reagents
accessible in most school or college laboratories.
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Where possible balanced
symbol equations are given for the reactions occurring in doing the test.
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Sometimes a precipitate
(ppt) initially forms with a limited amount of a reagent, it may then dissolve
in excess of reagent to give a clear solution. Both observations will be
crucial for a positive id.
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There are no tests
specific to identify a compound e.g.
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(i) there is no test for calcium chloride, but
there are tests for the calcium ion and the chloride ion, i.e. using
specific ion tests.
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(ii) Similarly, in
organic tests, all you can do is identify a functional group i.e. a particular bit of
the molecular structure of a member of a homologous series, rather a
particular unique molecule.
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Not all the reactions
are good definitive tests, but they may well be important reactions of
cations or anions you need to know about.
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The first tests in the
'inorganic' section are typical of GCSE Science level, but finally these
overlap and extend into those needed for GCE Advanced AS or A2 level. In the
organic section, only the alkene test is in GCSE double award science, but
some others might be found in a full single or coordinated triple award GCSE syllabus.
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If
any GCSE/IGCSE/GCE/AS/A2/IB/US grade 8–12 K12 test seems missing, just let me know
by email
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These days more emphasis is
given to modern spectroscopic methods of analysis such as NMR, Infrared,
Mass spectrometry, Atomic Emission etc. Quite correctly, though updating
A level chemistry is intellectually challenging at times, it isn't always as much fun!
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The methods described give no
recipe details or risk assessment, just basically what is needed, what you see and what you can or cannot
deduce. Consult teacher, 'practical' text books and
Hazcards before attempting any analysis.
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Most tests involve 'standard'
chemical reactions and few tests are totally specific so observations should be viewed in context,
i.e. is this a
realistic deduction in that particular situation?
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Please remember each syllabus
has its own 'list' of required tests – so do not
'over learn' – check out what is needed!
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There is a web page covering the methods
some safety aspects of "Preparing
and collecting gases".
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Use the
alphabetical list to find the test you need.
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Biohazard:
Biohazardous materials
include anything that may cause disease in living organisms or cause significant
impact to the environment or community.
If
the atoms of an element are heated to a very high temperature in a flame they emit
light of a specific set of frequencies (or wavelengths). These are due
to electronic changes in the atoms, the electrons are excited and then
lose energy by emitting energy as photons of light. The high temperature promotes
electrons to higher energy levels and the electrons re-emit the energy
as photons of light as they go back to their normal stable level. These
emitted frequencies can be recorded on a photographic plate, or these
days, more likely onto a sensitive photocell (like in a digital camera)
and displayed on a high resolution computer screen.
