Redox Titration

As their name implies, these reactions make use of the reactivity of the oxidizing/reducing pair. During the reaction, the oxidizing ion, whether it is the analyte or the titrant, is reduced by gaining one or more electrons as the reducing ion is oxidized, losing one or more electrons. 

These reactions are less common than acid/base reactions but involve a wider range of titrants including:

Oxidizing agents

• Iodine, potassium dichromate, potassium permanganate solutions

• Cerium IV salts, hydrogen peroxide, oxidized chlorine (e.g. ClO^- and ClO₂)

Reducing agents

• Sodium thiosulphate solutions, oxalic acid, ammonium iron (II) sulphate (Mohr’s salt), hydrogen peroxide, phenyl arsine oxide (PAO)

This titrimetric method is mainly based upon the change of the oxidation number or electrons transfer between the reactants, that is, these reactions are mainly based upon the oxidation-reduction reactions. In oxidation-reduction titration method, a reducing substance is titrated with standard solution of an oxidizing agent (e.g., ceric ammonium sulphate) or an oxidizing substance is titrated with the standard solution of the reducing agent.

PRINCIPLE

The principle involved in the oxidation-reduction titrations is that the oxidation process involves the loss of electrons whereas the reduction process involves the gain of electrons.

Oxidant + ne ↔ Reductant

These tend to take place in strongly acidic media and consume H+ ions. A medium containing sulphuric acid (H₂SO₄) or phosphoric acid (H₃PO₄) is therefore required, as can be seen in the examples below:

KMnO4 reduction (potassium permanganate) to Mn²⁺ by the oxalate ion (COO^- ) ²

2 MnO₄- + 5 C₂O₄ ^- + 16 H⁺ ----------> 10 CO₂ + 2Mn²⁺ + 8 H₂O

Fe(II) oxidation to Fe(III) by the dichromate ion (Cr₂O₇ ) 2-

6 Fe2+ + (Cr₂O₇ ) ^2- + 14 H⁺ ---------> 2 Cr³⁺ + 6 Fe³⁺ + 7 H₂O

When dealing with an unknown reaction, we recommend writing the equation to discover the stoichiometric coefficients and obtain the analyte/titrant consumption.

Fields of application

A.    Environment : COD of water & Oxidation capacity of water by permanganate

B.   Determining the analyte / titrant equivalence

C.   Food and beverage: Determination of free and total SO2  in water, wine, alcohol, dried fruit, etc.

D.   Pharmaceuticals: Synthetic & herbal medicines assay

E.    Surface treatment: Titration of copper or tin using iodine, Titration of chromium VI

F.    Petrochemicals: Determination of water in hydrocarbons

Acid-base titration

Acid-Base titrations are usually used to find the the amount of a known acidic or basic substance through acid base reactions. The analyte (titrand) is the solution with an unknown molarity. The reagent (titrant) is the solution with a known molarity that will react with the analyte.

Definitions

Acid-base titration : Determines the concentration of an acid or base by exactly neutralizing it with an acid or base of known concentration.

Equivalence point: The point at which an added titrant's moles are stoichiometrically equal to the moles of acid/base in the sample; the smallest amount of titrant needed to fully neutralize or react with the analyte.

Titrant: The standardized (known) solution (either an acid or a base) that is added during titration.

Analyte: The unknown solution whose concentration is being determined in the titration.

Procedure

An acid-base titration is an experimental procedure used to determine the unknown concentration of an acid or base by precisely neutralizing it with an acid or base of known concentration. This lets us quantitatively analyze the concentration of the unknown solution. Acid-base titrations can also be used to quantify the purity of chemicals.

Alkalimetry, or alkimetry, is the specialized analytic use of acid-base titration to determine the concentration of a basic (alkaline) substance; acidimetry, or acidometry, is the same concept applied to an acidic substance.

The solution in the flask contains an unknown number of equivalents of base (or acid). The burette is calibrated to show volume to the nearest 0.001 cm3. It is filled with a solution of strong acid (or base) of known concentration. Small increments are added from the burette until, at the end point, one drop changes the indicator color permanently. (An indication of the approaching equivalence point is the appearance, and disappearance after stirring, of the color that the indicator assumes beyond neutralization.) At the equivalence point, the total amount of acid (or base) is recorded from the burette readings. The number of equivalents of acid and base must be equal at the equivalence point.

Sublimation

Sublimation is a purification technique, in which a solid is directly converted to vapor phase without passing through liquid phase. However, the compound must have a relatively high vapor pressure, and the impurities must have significantly lower vapor pressures. By heating, the solid will be vaporized and become solid again when the vapor contacts with the cold surface. Some solid compounds, such as iodine, camphor, naphthalene, acetanilide, benzoic acid, can be purified by sublimation at normal pressure. Several compounds will sublime when heating under reduced pressure. In this experiment, the impure acetanilide and impure naphthalene will be purified using a suction flask with cold finger at atmospheric pressure.

Distillation

Distillation is a widely used method for separating and purifying a mixture of liquids by heating the liquids to boiling at different temperatures to transform them into the vapor phase. The vapors are then condensed back into liquid form in a sequence from lower to higher boiling points. Distillation is used for many industrial processes, such as production of gasoline and kerosene, distilled water, organic solvents, and many other liquids. There are 4 types of distillation including simple, fractional, steam and vacuum distillations. In simple distillation, all the hot vapors produced are immediately passed into a condenser to cool and condense the vapors back to liquid. Therefore, the distillate may not be pure depending on the composition of the vapors at the given temperature and pressure. Simple distillation is usually used only to separate liquids whose boiling points differ greatly (more than 25°C), or to separate liquids from nonvolatile solids or oils. In case of very close boiling points, fractional distillation must be used in order to separate the components well by repeated vaporization-condensation cycles within a fractionating column. Steam distillation is a method for distilling compounds which are heat-sensitive by bubbling steam through a mixture. After the vapor mixture is cooled and condensed, a layer of oil and a layer of water are usually obtained. Some compounds have very high boiling points and may boil beyond their decomposition temperatures at atmospheric pressure. It is thus better to do vacuum distillation by lowering the pressure to the vapor pressure of the compound at a given temperature at which the compound is boiled, instead of increasing the temperature.

Complexometric Titration

Precipitation Titration

Organic Chemistry: 6th Edition by Robert T. Morrison (Author), Robert N. Boyd (Author)

Organic Chemistry Vol. 1  by I. L. Finar (Author)

A Guidebook to Mechanism in Organic Chemistry for the JEE

Organic Chemistry by T. W. Graham Solomons  (Author), Craig B. Fryhle  (Author)

March's Advanced Organic Chemistry: Reactions, Mechanisms, and Structure 6th Edition by Michael B. Smith (Author), Jerry March (Author)

Aromatic Hydrocarbons

THEORY OF INDICATORS:

 An indicator is a substance which is used to determine the end point in a titration. In acid-base titrations, organic substances (weak acids or weak bases) are generally used as indicators. They change their colour within a certain pH range. The colour change and the pH range of some common indicators are tabulated below:

Theory of acid-base indicators: Two theories have been proposed to explain the change of colour of acid-base indicators with change in pH.

1.     Ostwald's theory:

According to this theory:

(a) The colour change is due to ionisation of the acid-base indicator. The unionised form has different colour than the ionised form.

(b) The ionisation of the indicator is largely affected in acids and bases as it is either a weak acid or a weak base. In case, the indicator is a weak acid, its ionisation is very much low in acids due to common H+ ions while it is fairly ionised in alkalies. Similarly if the indicator is a weak base, its ionisation is large in acids and low in alkalies due to common OH- ions.

Considering two important indicators phenolphthalein (a weak acid) and methyl orange (a weak base), Ostwald theory can be illustrated as follows:

Phenolphthalein:

It can be represented as HPh. It ionises in solution to a small extent as:

                                        HPh ↔ H+ + Ph^-

                                Colourless            Pink

Applying law of mass action,

                                         K = [H+][Ph-]/[HpH]

The undissociated molecules of phenolphthalein are colourless while Ph- ions are pink in colour. In presence of an acid the ionisation of HPh is practically negligible as the equilibrium shifts to left hand side due to high concentration of H+ ions. Thus, the solution would remain colourless. On addition of alkali, hydrogen ions are removed by OH- ions in the form of water molecules and the equilibrium shifts to right hand side.

Thus, the concentration of Ph- ions increases in solution and they impart pink colour to the solution

Methyl orange:

It is a very weak base and can be represented as MeOH.

 It is ionized in solution to give Me⁺ and OH^- ions.

                             MeOH ↔ Me⁺ + OH^-

                                 Yellow       Red
Applying law of mass action,
                            K = [Me+ ][OH- ]/[MeOH]

In presence of an acid, OH- ions are removed in the form of water molecules and the above equilibrium shifts to right hand side. Thus, sufficient Me+ ions are produced which impart red colour to the solution. On addition of alkali, the concentration of OH" ions increases in the solution and the equilibrium shifts to left hand side, i.e., the ionisation of MeOH is practically negligible.

 Thus, the solution acquires the colour of unionised methyl orange molecules, i.e., yellow.
This theory also explains the reason why phenolphthalein is not a suitable indicator for titrating a weak base against strong acid. The OH" ions furnished by a weak base are not sufficient to shift the equilibrium towards right hand side considerably, i.e., pH is not reached to 8.3. 

Thus, the solution does not attain pink colour. Similarly, it can be explained why methyl orange is not a suitable indicatorfor the titration of weak acid with strong base.

Quinonoid theory:

According to this theory:

(a)   The acid-base indicators exist in two tautomeric forms having different structures.Two forms are in equilibrium. One form is termed benzenoid form and the other quinonoid form.

(b)    The two forms have different colors. The color change in due to the interconversation of one tautomeric form into other.

(c)    One form mainly exists in acidic medium and the other in alkaline medium.

Thus, during titration the medium changes from acidic to alkaline or vice-versa. The change in pH converts one tautomeric form into other and thus, the colour change occurs.

Phenolphthalein has benziod form in acidic medium and thus, it is colourless while it has quinonoid form in alkaline medium which has pink colour.

Methyl orange has quinonoid form in acidic solution and benzenoid form in alkaline solution. The color of benzenoid form is yellow while that of quinoniod form is red.

Synthesis of Methyl orange

Organic Chemistry

Column Chromatography

NMR Spectroscopy

Hantzsch widman nomenclature of heterocyclic compounds

In 1887 and 1888, Hantzsch and Widman independently introduced methods for naming five- and six-membered nitrogen monocycles. Although differing in details, such as expressing the order of the heteroatoms and indicating their positions in the ring, both methods were based on the same underlying principle, i.e., the combination of appropriate prefixes, representing heteroatoms, with stems, representing the size of the ring. At first, only the heteroatoms oxygen, sulfur, and selenium, in addition to nitrogen, and the stems -ol (-ole) and -in (-ine) denoting five- and six-membered rings, respectively, were used.

A heterocyclic compound or ring structure is a cyclic compound that has atoms of at least two different elements as members of its ring(s). Heterocyclic chemistry is the branch of chemistry dealing with the synthesis, properties and applications of these heterocycles. In contrast, the rings of homocyclic compounds consist entirely of atoms of the same element.

Hantzsch-Widmann nomenclature may be applied in the naming of unsaturated, as well as saturated, monocyclic heterocycles. According to this nomenclature system, the name of a heterocycle is composed of a prefix that denotes the heteroatom and a suffix (see table below) that determines the ring size and the degree of the ring's saturation. In addition, the suffixes distinguish between nitrogen-containing heterocycles and heterocycles that do not contain a nitrogen ring atom. The prefixes applied in Hantzsch-Widman nomenclature are "aza" for nitrogen, "oxa" for oxygen, and "thia" for sulfur. If the prefixes are combined with the suffixes, the last letter of the prefix is left out. Thus, tetrahydrofuran is called oxolane and not oxaolane, for instance. Hantzsch-Widman nomenclature may also be used in connection with various other heteroatoms.