[Chemistry Class Notes] on Volumetric Analysis Pdf for Exam

The analytical method wherein the concentration of a substance in a solution is estimated by adding exactly the same number of equivalents of another substance present in a solution of known concentration is called volumetric analysis.
This is the basic principle of titration. Another name for volumetric analysis is titrimetric analysis.

The volumetric analysis can be classified into three types:

The main aim of simple titration is to find the concentration of an unknown solution with the aid of the known concentration of another solution. Simple titration can again be classified into four different types:

An acid-base indicator gives different colors of different compounds. An indicator is chosen in a particular titration based on the pH-range of the indicator and the pH change near the equivalence point.

Redox Titrations

An oxidant can be estimated by adding reductant in redox titrations (or vice-versa). An example of this is; Fe2⁺ ions can be estimated by titration against acidified KMnO₄ solution when Fe²⁺ ions are oxidized to Fe³⁺ ions and KMnO4 is reduced to Mn²⁺ in an acidic medium. Also, KMnO₄ is a self-indicator. At the equivalence point, it can be noticed that it discharges a purple color.

Mn + 8H⁺ + 5e^– → Mn²⁺ + 4H₂O
Fe²⁺ → Fe³⁺ + e^–] × 5
Mn + 8H⁺ + 5Fe²⁺ → Mn²⁺ + 5Fe³⁺ + 4H₂O
(n=5) (n=1)

Acidified KCr2O7 can also be employed instead of KmnO₄. Other redox titrations are iodimetry, iodometry etc.

I₂ + 2Na₂S₂O₃ → 2NaI + Na₂S₄O₆

This is known to be an indirect method of estimation of iodine. Here, an oxidizing agent is made to react with an excess of solid KI. The oxidizing agent oxidizes I^– to I₂. The next step is to make the liberated I₂ to react with Na₂S₂O₃ solution.

Precipitation Titrations

In a titration of this form, a compound of very low solubility is formed by combining cations and anions. A solid residue can be separated out.

AgNO₃ + NaCl → AgCI¯ (white) + NaNO₃
BaCI₂ + H2SO4 → BaSO₄¯ (white) + 2HCI

CuSO₄ + 4NH₄OH → [Cu (NH₃)₄] SO4 + 4H2O
AgNO₃ + 2KCN → K [Ag (CN) ₂] + KNO₃

Certain conditions have to be satisfied for back titrations to work:

• Compounds ‘A’, ‘B’ and ‘C’ which are provided should satisfy the condition that ‘A’ and ‘B’ react with each other.

• ‘A’ and pure ‘C’ should also react with each other but the impurity present in ‘C’ is not supposed to not react with ‘A’.

• Another important condition is that the product of ‘A’ and ‘C’ should not react with ‘B’.

Now, a certain volume of ‘A’ in a flask is taken out (the equivalents of ‘A’ taken has to be greater than equivalents of pure ‘C’ in the sample) and then a simple titration using ‘B’ is performed.

Na₂CO₃ + HCI → NaHCO₃ + NaCI ……. (ii)
NaHCO₃ + HCI → NaCI + CO₂ + H₂O …… (iii)

When HCI is added to the alkaline solution, alkali is neutralized. This results in the pH of the solution getting decreased. The pH decreases initially at a rapid pace since the strong base (NaOH) is neutralized completely. Whereas when Na₂CO₃ is converted to NaHCO₃ completely, the solution remains weakly basic. This is due to the presence of NaHCO₃ (which is weaker as compared to Na₂CO₃). It is at this point, where it is noticed that phenolphthalein changes color since it requires a weakly basic solution to show the color change.

Thus, in conclusion, phenolphthalein changes color when the solution contains weakly basic NaHCO₃ along with other neutral substances while methyl orange changes color when the solution contains weakly acidic H₂CO₃ along with other neutral substances.