Titration•Titration is the quantitative measurement of an analyte (the substance whose quantity or concentration is to be determined) in solution by completely reacting it with a reagent.•The point at which all of the analyte is consumed is called the endpoint and is determined by some type of indicator that is also present in the solution.•For acid-base titrations, indicators are available that change color when the pH changes. •When all of the analyte is neutralized, further addition of the titrant causes the pH of the solution to change causing the color of the indicator to change.•The analyte concentration is calculated from the reaction stoichiometry and the amount of reagent that was required to react with all of the analyte.

Instrumentation• Manual titration is done with a burette, which is a long

graduated tube to hold the titrant. • The amount of titrant used in the titration is found by

reading the volume of titrant in the burette before beginning the titration and when the endpoint is reached, and taking the difference.

• The most important factor for making accurate titrations is to read the burette volumes reproducibly.

• For repetitive titrations, autotitrators with microprocessors are available that deliver the titrant, stop at the endpoint, and calculate the concentration of the analyte.

• The endpoint is usually detected by some type of electrochemical measurement.

There are many types of titrations in common use in the analytical chemistry laboratory. Each type uses a different kind of chemical reaction. E.g.

• acid-base titration,

• reduction-oxidation titrations,

• precipitation titrations, and

• complexometric titrations.

In a typical acid-base titration experiment, the solution containing the analyte (an acid of unknown identity and/or concentration) is placed into a container, and the titrant (a base of accurately-known concentration) is slowly added from the buret in small increments

This is a typical titration graph plotting volume of titrant added versus pH of solution.

Only a small change in pH occurs as most of the titrant is added, then near the end of the titration there is a sudden, rapid change in pH.

The sudden change occurs at the equivalence point.

The location of the equivalence point is, determined by identifying the point of fastest increase.

• The chemical reaction that occurs between the acid and the base allows one to calculate the initial concentration (or amount) of the acid.

• E.g. in the titration of hydrochloric acid (HCl) with a base such as sodium hydroxide (NaOH), The reaction is as follows:

• HCL + NaOH  ↔ NaCL + H20 • This allows calculation of the concentration of a solution of HCl by

titration with the base NaOH (where the concentration of NaOH is accurately known)

• E.g. solution is 0.02500 L of an unknown concentration of the acid, HCl

• Titrate with 0.1000 M NaOH • find that the equivalence point occurs at 0.04398 L of NaOH • at equivalence point, moles HCL = moles NaOH. • calculate the number of moles of NaOH at the equivalence point by

multiplying its concentration by the volume of NaOH added in order to reach the equivalence point:

calculate the unknown concentration of HCl:

Complexometric titrationa type of titration based on complex formation between the

analyte and titrant.• Complexometric titrations are particularly useful for

determination of a mixture of different metal ions in solution. • An indicator with a marked color change is usually used to

detect the end-point of the titration. • is used to find the total calcium and magnesium content of

milk and sea water. It can also be used to determine the total hardness of fresh water provided the solutions used are diluted.

• The combined concentration of calcium and magnesium ions is considered to be the measure of water hardness.

• EDTA (ethylenediaminetetraacetic acid) forms a complex with calcium and magnesium ions.

• A blue dye called Eriochrome Black T (ErioT) is used as the indicator.

• The blue dye also forms a complex with the calcium and magnesium ions, changing colour from blue to pink in the process.

• The dye–metal ion complex is less stable than the EDTA–metal ion complex.

• For the titration, the sample solution containing the calcium and magnesium ions is titrated with EDTA.

• The indicator is added to sample and goes pink as all the Ca2+ and Mg2+ ions present.

• A titration is carried out. • The Calcium and Magnesium form a complex with the EDTA

molecules until the end-point, when all the Calcium has been complexed.

• ErioT indicator, immediately changing its colour from pink to blue

The main reaction is:

• Ca 2+ + EDTA4- --> [Ca-EDTA]2-

• EDTA4- + Mg 2+ --> [Mg-EDTA]2-

Indicator reaction: Note:

ErioT-Mg/Ca is pink

The Ca/ Mg gets complexed with the EDTA and is unavailable to the dye and the dye goes blue