Chemical Analysis

1
Chemical Analysis

• E. Bottari, Chemistry Department,
• La Sapienza University of Rome

Malta, Summer School 2007, 20th August 9th
September
2
Analytical chemistry - Methods

• Qualitative analysis
• Quantitative analysis
• Traditional analysis
• Instrumental methods of analysis
• Suitable reactions for analysis

3
Qualitative analysis

• Recognition of chemical species by means of
  colour, reaction producing a colour, reaction
  producing a precipitate, reaction involving a
  change of a physical parameter.
• Colored ions are Cu2 (blue), Cr3 (green),
  CrO4(yellow), Cr2O7
• (orange), MnO4-(viole), MnO4(green),
  Ni2(green), Co2(pink, or blu), Mn2(pink), and
  generally ions of transition metals.
• Precipitates (slight soluble compounds) sulphurs
  of eavy metals (like As, Sb, Hg, Cu, Pb, Cd, Sn,
  Bi, Zn, Ni, Co, Mn), BaSO4, Hg2Cl2, AgCl, PbCl2,
  Ag2CrO4, many hydroxides of eavy metals.

4
Traditional - Quantitative analysis

• Volumetric analysis
• A B C.
• A solution of B, at known concentration is added
  to a known volume of A, until the number of
  equivalent of B is equal to those of A. The added
  volume of B is measured and the concentration of
  A can be calculated.
• Gravimetric analysis
• A B C.
• An excess of B, at unknown concentration, is
  added to A so that A is completely transformed in
  C, that can be weighed.

5
Quantitative analysis Volumetric 1

• Titration (traditional)
• To perform a volumetric analysis (titration) is
  necessary to have a solution at known
  concentration a glass vessel, a burette
  (calibrated tube able to measure volume, equipped
  with a tap) and an indicator (chemical species
  able to change colour when change the composition
  of the solution.
• A titration is a chemical operation which allows
  to obtain the unknown concentration of a reagent,
  A, by adding the reagent B and by stopping the
  addition when the equivalent number of A and B
  are exactly equal.
• The point corresponding to meq A meq B is
  called equivalent point or point of equivalence.
• meq milli equivalents VACA VB CB

6
Quantitative analysis Volumetric 2

• Reaction must be
• Quick
• Stoichiometric with known coefficients
• Univocal
• Collateral reactions must be absent
• Complete
• In correspondence to the equivalent point, a
  sharp change of the followed parameter must occur
• This change can be put into evidence by the
  presence of a colorimetric indicator or by means
  of an instrumental method

7
Quantitative analysis Volumetric 3

• Standard
• It is very hard to perform a volumetric analysis
  by comparing a solution with another, without a
  reference point. It is necessary to have
  solutions at known concentration, prepared by
  dissolving a known weight in a measured volume.
  Compounds having such property are called mother
  substances or primary standards.
• A standard must be very pure, stable, not
  reacting with air or water (the solvent). It must
  be possible to dry it at 110C. It must follow
  all the rules of volumetric analysis and have a
  high equivalent weight.

8
Volumetric Analysis Kind of reactions

• Solutions of electrolytes
• Acid base
• Precipitimetric (formation of a precipitate)
• Chelometric (formation of a chelate)
• Redox

9
Electrolytes

• Electrolytes are all the compounds dissociable in
  ions. For ex. NaCl ? Na Cl- or Na2SO4 ? 2 Na
  SO4
• Electrolytes can be strong or weak.
• Strong electrolytes are completely dissociated,
  weak ones are only partially dissociated and
  generate an equilibrium.
• Salts are strong electrolytes, like NaCl. It
  exists as Na and Cl-
• Acids and bases can be strong or weak according
  their properties. Mineral acids are generally
  strong, organic acids are weak. H3PO4 is a weak
  acid and many inorganic acids are weak. NaOH and
  KOH are strong bases, while NH3 is a weak base.
• Dissociation of weak acids or bases is regulated
  by a constant.

10
What is an acid, what is a base?

• An acid is a compound able to yield protons
• A base is a compound to catch protons
• A compound able to yield or catch protons,
  depending
• on the experimental conditions, is called
  ampholite or
• amphiprotic substance.
• Free protons in a condensed matter do not exist,
  so that
• in a solution the proton comming from an acid
  must be
• taken by a base contemporanily present. Often the
  solvent
• can carry out such task.

Example HA1 B2 HA2 B1. There are two
conjugated couples acid base HA1 H B1
and B2 H HA2
11
What is Water?
Water is able to yield or to acquire a protons,
according the following relation H2O H2O ?
H3O OH- (1) Eq. (1) is the sum of the two
following H2O ? H OH- (acid behaviour of
water) H2O H ? H3O (basic behaviour of water)
As eq. (1) is an equilibrium reaction, it is
regulated by a constant k H3O OH-H2O-2,
but H2O is constant in the solvent water and
by involving it in the constant k, it can be
written Kw H3O OH- 10-14 at 20C. In
a neutral solution H3O OH- Kw½ 10-7.
12
What is pH?
pH is an usual expression suitable to indicate
the acidity of a solution. Practically it is the
negative decimal logarithms of the free hydrogen
ionic concentration, i.e. pH -log H3O
In the above slide, it was shown that in neutral
solution, H3O OH- Kw½ 10-7 It can be
deduced that pH -log H3O -log 10-7 7.
A neutral solution has pH 7, an acid one has
pHlt7 and a basic solution has pH gt 7.
Similarly, it can be defined pOH and pKw and of
consequence pKw pH pOH
13
Calculation of pH 1

• Solution of strong acids or bases free
  concentration is equal to the analytical one CH
  cH H3O. COH cOH OH-.
• Solution of HCl 0.1 M has pH 1. Solution of
  NaOH 0.1 M has pH 13.
• For solution of weak acid, like acetic acid
  CH3COOH, 0.1 M, the free concentration H3O
  must be calculated on the basis of the following
  equilibrium (0.1 M it the total concentration)
• CH3COOH H2O ? CH3COO- H3O (2)
• Acid1 Base2 Base1 Acid2

14
Calculation of pH 2

• The constant of eq. (2) is ka CH3COO-H3O
  CH3COOH-1, where CH3COO- H3O, then it
  follows
• ka H3O2 CH3COOH-1 H3O2 (0.1 -
  H3O)-1. If H3O is negligible with respect
  to 0.1, it can be written
• H3O (ka CHA)½, where CHA represents the
  generalization of acid.
• For a base solution, i.e. acetate ions, CH3COO-
  0.1 M, H3O must be calculated by means of the
  equilibrium
• CH3COO- H2O ? CH3COOH OH- (3), with a
  constant
• Kb CH3COOHOH- CH3COO--1. By combining (2)
  and (3), it
• can be obtained ka Kb Kw and pOH (kb CA-)½
  (approx. formula).
• In this case Kb is also called hydrolysis
  constant, indicated by Kh.

15
Buffer solutions
Solutions are called buffer when their pH does
not change appreciably, by adding little amounts
of strong acid or bases.
Buffer solutions can be those having pH lt 3 and
pH gt 11. They can be also formed by a weak acid
in the presence of Its conjugated base. In this
case in eq. (2) it is not possible To write
CH3COO- H3O, then it follows H3O ka
CCH3COOH CCH3COO--1
They can be also formed by a weak base in the
presence of its conjugated acid. For example NH3
and NH4. OH- can be calculated as
follows OH- kb CNH3 CNH4 -1
16
Procedure for acid base titrations
If the concentration H0 of the solution of a
reagent HA must be determined, with accuracy of
0.1 , a measured volume V0 of the HA solution is
transferred in a titration vessel and two drops
of indicator are added. A solution of NaOH, at
exactly known concentration OHT, is gradually
added by means of a Burette, till the indicator
changes colour for the addition of a drop more.
The final point of the titration is reached. The
added volume of NaOH is measured by the burette,
VT. In this point the number of equivalent of
both reagents is equal. The following realtion
can be written H0 V0 OHT VT. VT is measured,
OHT and V0 are known, H0 can be calculated.
17
Determination of olive oil acidity
This analysis allows to classify oil in a
particular category, with different commercial
value according the acidity content. Olive oil
can be classified in the first value category, if
its Content of acidity (expressed as oleic acid)
is ? 0.80 .
Analysis is performed similarly to that
previously described. V0 measured of oil is
transferred in a vessel for the titration. A
solution of OHT standard is gradually added till
the change of the colour of a suitable indicator
is reached. At this point, the volume VT of added
titrant OHT, is read on the burette and the
initial acidity of oil H0 VT OHT V0-1 can be
calculated.
18
Reaction involving a slight soluble compound
(precipitate)
By mixing a solution of AgNO3 with one of NaCl, a
precipitate of AgCl takes place, according to the
reaction AgCl- AgCl, or AgNO3 NaCl AgCl
NaNO3. The equilibrium in this case is shifted
to right because a slight soluble substance is
formed.
According to the equilibrium rules, it can be
written k AgCl- AgCl-1, but AgCl is
solid in equilibrium with The ions in solution
and its free concentration can be assumed as
constant, which involved with k, gives Ks
AgCl- . Ks is called solubility product.
The reaction between Ag and Cl- frequently used
to determine The quantity of silver present in a
sample, in a similar way seen For the reaction
acid base.
19
Chelometric reactions Hardness of water 1
It was previously described the dative bond,
i.e. a covalent bond, where a couple of electron
coming from the same ion or compound (for example
NH3) is put in common between the transition ion
(for example Cu2) and the donor (NH3) to form
a complex ion or a molecule.
Next to NH3 which is able to give only a couple
of electrons, many compounds exist containing
several atoms able to give each a couple of
electrons, so that a molecule of such compounds
is able to bind a metallic ion with several
bonds, forming five membered rings particularly
stable. The most used compound having this
property is called EDTA.
The most important application of EDTA is the
water hardness determination, i.e. the
calcium(II) magnesium (II) dosage.
20
Hardness of water 2 - Chelometry
The knowledge of hardness of water is very
important in many practical cases beer industry,
metallic tubes for water, formation of limestone
in caldrons, water heaters, and in general, in
washing machines. Hardness is due to the
formation of calcium or magnesium carbonate,
slight soluble compound forming limestone.
EDTA standard solution is able to titrate
solutions containing Calcium and magnesium ions
to determine their concentration.
Analysis is carried out similarly to those
previously described, but with different
indicator and at well defined and buffered pH. It
is possible to know separately the calcium and
magnesium present in the same solution. The same
analysis is performed to know the calcium and
magnesium concentration in milk, or cheese.
21
Redox reactions 1

• This kind of reaction takes place involving the
  electrons transfer from the reducing to oxidant,
  as follows
• Ox1 n1 e Red1 and Red2 Ox2 n2 e, if n1
  n2, the complete reaction can be written Ox1
  Red2 Red1 Ox2
• Oxidant and reducing compounds have different
  straight, which can be experimental proved.
• A solution of Cu2 is blue. If you put a piece of
  Zn or of Pb inside the copper solution, you can
  observe that after some minutes solution becomes
  colourless, Zn or Pb are dissolved and a red
  slight soluble compound is formed.
• This means that Zn or Pb are oxided by Cu2
  reduced to Cu0
• Red. The following reactions occur
• Cu2 Zn Cu0 Zn2 or Cu2 Pb Cu0 Pb2

22
Redox reactions 2

• If you repeat the same experiment by using Pb2
  and Zn, you
• find that Zn is oxidized by Pb2 to Zn2
  according to the
• reaction Pb2 Zn Pb0 Zn2

• If you repeat the same experiment by using Ag
  and Cu, you
• find that Cu is oxidized by Ag to Cu2 according
  to the
• reaction Ag Cu Ag0 Cu2

From the above presented examples, you can deduce
the following sequence, as straight of
oxidation Ag gt Cu2 gt Pb2 gt Zn2, vice versa
the straight as reducing
The straight as OX of a couple (i.e. Ag e
Ag0) is represented by the redox potential. The
table of standard potentials collects all the
couples with their values.
23
Redox reactions 3
Nernst equation shows the dependence of the redox
potential on the reagents (ox and red form) free
concentration
Nernst equation for a generic couple Ox1 n1 e
Red1 E1 E1 RT (n1 F)-1 ln Ox1
Red1-1, which at 25C is E1 E1 0.05916
(n1)-1 log Ox1 Red1-1

• The following Redox couple are frequently used in
  analysis
• MnO4- 5 e 8 H Mn2 4 H2O
• Cr2O7 6 e 14 H 2 Cr3 7 H2O
• I2 2 e 2 I- IO3- e 6 H I- 3 H2O
• 2S2O3 S4O6 2 e C2O4CO2 2 e S S
  2 e
• Sn2 Sn4 2 e Fe2 Fe3 e
• NO3- 3 e 4 H NO 2 H2O 2HgCl2 2 e
  Hg2Cl2 2Cl-

24
Instrumental Analysis

• Electro analytical analysis
• Optical analysis (spectroscopy)
• Thermal analysis
• Chromatography

25
Electro analytical analysis

• Electrolysis (electro gravimetric analysis).
• Electromotive Force Measurements (Galvanic
  elements Piles) (pH measurements). Direct and
  Titrations.
• Conductometry.
• Coulometry (direct and indirect).
• Polarography.

26
Spectroscopy

• Emission (flame, voltaic arc, sparkly).
• Absoption (Molecular UV violet, IR).
• Absorption (Atomic flame, furnace).
• X Ray or more sophisticated methods.

27
Chromatography

• On column
• On paper
• TLC (Thin layer chromatography)
• GC (gas chromatography)
• HPLC (High performance liquid chromat.)

Many applications analysis of fat fraction of
many substances, Like Milk, Different kinds of
oil, Butter, Meet, Eggs, Vitamins Cosmetic,
Pesticides, Dioxins, Herbicides, etc