Periodic Table of Elements

Titration

Titration is the process of slow addition of one solution of a known concentration (titrant) to a known volume of another solution of unknown concentration (analyte) until the reaction reaches neutralization, which is often indicated by a color change. The word titration comes from the Latin "titalus" meaning inscription or title.

Elements of titration:

  • Titrant - the solution of known concentration.
  • Analyte - the solution of unknown concentration but known volume.
  • Equivalence point – the volume of titrant required to neutralize the sample.
  • End point – the pH at the equivalence point of a titration.
  • Indicator – a chemical which is added to the sample that changes colour at the equivalence point of a titration.
  • Buffering region – a horizontal region of the pH curve where pH is not changing significantly.
  • Titration curve - is a plot of pH vs. the amount of titrant added. These curves are useful for determining end points and dissociation constants of weak acids or bases.
  • Neutralization - a chemical reaction where an acid is reacted with an equivalent amount of base.

Types of titrations

  • Acid-Base Titrations
  • Redox Titrations (oxidation-reduction)
  • Complexometric Titrations or complex formation or chelometry
  • Precipitation titrations

Acid-Base Titrations:

An acid–base titration is the determination of the concentration of an acid or base by exactly neutralizing the acid or base with an acid or base of known concentration.

Types of acid-base titrations:

Strong acid-strong base titrations:

  • A strong acid will react with a strong base to form a neutral (pH = 7) solution.
  • Strong acid-strong base titrations usually have an equivalence point around pH 7.
  • This is due to the fact that in solution, strong acids and strong bases will both completely dissociate, so there is an approximate 1:1 ratio of moles of acid to moles of base at the equivalence point.

Strong base-strong acid titrations:

  • Strong base-strong acid titrations are just like the strong acid-strong base titrations.
  • A strong base will react with a strong acid to form a neutral (pH = 7) solution.
  • Strong base-strong acid titrations usually have an equivalence point around pH 7.

Strong acid-week base titrations:

  • A strong acid will react with a weak base to form an acidic (pH < 7) solution.
  • Strong acid-weak base titrations reach their equivalence point at a pH less than 7.
  • More moles of the weak base are required to completely react with the amount of strong acid present, because the base is not entirely dissociated.

Week acid-strong base titrations:

  • A weak acid will react with a strong base to form a basic (pH > 7) solution.
  • Weak acid-strong base titrations reach their equivalence point at a pH greater than 7.
  • More moles of the weak acid are required to completely react with the amount of strong base present, because the acid is not entirely dissociated.

Week acid-week base titrations:

  • Weak acid-weak base titrations are difficult because each solution is in equilibrium.
  • The pH of the equivalence point is around 7.

Redox Titrations (oxidation-reduction):

Redox titrations are based on a reduction-oxidation reaction between an oxidizing agent and a reducing agent. Redox titration may involve the use of a redox indicator and/or a potentiometer. A redox reaction is a reaction which involves transfer of electrons. Oxidizing agents are reduced as they take electrons from reducing agents. Reducing agents are oxidized as they give electrons to oxidizing agents. An example of a redox titration is treating a solution of iodine with a reducing agent and using starch as an indicator. Common oxidising agents are permanganate, dichromate.
Purple permanganate ion is reduced to colorless manganese ion.

MnO4-(aq) + 8H+ + 5e Mn2+(aq) + 4H2O
Reduction potential/redox potential/oxidation reduction potential Eo = +1.51V

Orange dichromate ion is reduce to green chromium ions.

Cr2O72-(aq) + 14H+ + 6e 2Cr3+(aq) + 7H2O
Reduction potential/redox potential/oxidation/reduction potential Eo = +1.23V

The redox titration curve is a plot of electrode potential in volts vs volume of titrant or analyte.

Complexometric Titrations or complex formation or chelometry

Complexometric titration is a form of volumetric analysis in which the formation of a colored complex is used to indicate the end point of a titration. This technique involves titrating metal ions with a complexing agent or chelating agent or ligand. Ethylenediaminetetraacetic acid (EDTA) is a common chelate because it makes 6 bonds with metal ions to form 1:1 complexes with large formation constants. The two nitrogen atoms can donate their lone pairs to form two bonds and the four -OH groups can lose thier protons. Calmagite indicator is used for EDTA titrations.

Precipitation titrations

A reaction in which the analyte and titrant form an insoluble precipitate is known as precipitation titration. Precipitation titration is useful to determine halogens and some metal ions, for example Cl- can be determined when titrated with AgNO3. As it is based on silver nitrate as a precipitating agent, it is also called as "argentimetric processes". End point in precipitation titration can be determined by several methods:

  • Chemical:
    • Precipitation Type - Mohr’s method
    • Adsorption – Fajan’s method
    • For silver analyses – Volhard method
  • Sensors –Potentiometric or amperometric

Mohr titration:

Chloride is titrated with AgNO3 solution. In this method Sodium chromate can be used as an indicator for determining chloride, bromide, cyanide ions. When the precipitation of the chloride is complete, the first excess of Ag+ reacts with the indicator to precipitate brick-red silver chromate. The Mohr method must be performed at a pH about 8. This method is useful for determining Cl- in neutral or unbuffered solutions such as drinking water.

2Ag+(aq) + CrO42-(aq) Ag2CrO4(s)

Volhard titration:

This method is used for the determination of the halogen content of a solution by titration with a standard thiocyanate solution. A measured excess of AgNO3 is added to precipitate the anion. The excess Ag+ formed is determined by back titration with standard potassium thiocyanate solution. The end point is detected by adding iron (Fe3+).

Ag+(aq) + Cl-(aq) AgCl(s) + excess Ag+
excess Ag+(aq) + SCN-(aq) thiocyanate AgSCN(s)
Fe3+(aq) + SCN-(aq) thiocyanate FeSCN2+(aq)

Fajan titration:

In this method at end point, a colored indicator is absorbed onto the precipitate. The difference between the color of the adsorbed indicator and un-adsorbed indicator determines the completion of the titration. Flourescein is a typical absoption indicator used in this method of titration.

Potentiometric titration:

Potentiometric titration are based on the difference in potential between two electrodes (indicator electrode, reference electrode) in an electrochemical cell. The indicator electrode forms an electrochemical half cell with the interested ions in the test solution. The reference electrode forms the other half cell, holding a consistent electrical potential. Potentiometric titrations are preferred to manual titrations, since they are more accurate and precise.

The potential developed in a cell Ecell = Eind - Eref + Ej

Where Ecell is the potential of the cell
Eref is the potential of the reference electrode
Eind is the potential of the indicator electrode
Ej is the liquid junction potential
The liquid junction potential is a potential generated at the junction of two solutions with unequal concentration separated by a porous membrane due ionic transfer.