College of PREPARED BY

University of Babylon

College of PREPARED BY

Ali Jassim Al-Zuhairi Dr. Engineering / Al-Musayab

Chemistry Manual / First stage M.sc. Mohammed Thamer Al-Zubaidi

Classification of Titrimetric or Volumetric Methods

There are found general classes of titrimetric method:

1- Acid- Base titration

2- Reduction - Oxidation titration

3- Precipitation titration

4-Complex metric titration

1- Acid- Base titration

Experiment No.1

preparation and standardization of approximately (0.1N) HCl

Theory: -

The laboratory solution of HCl is not primary standard substance (p.s.s) because it is volatile, and not has high M.wt so it usually standardized against borax or sodium carbonate.

Borax is the more convenient, and, it is easily obtained in a chemically pure state (Na2B4O7.10H2O), it is quite stable and it has high gram -equivalent (190.7 gm).

In contrast, sodium carbonate is more difficult to purify and it is gram -equivalent is smaller (52.9gm) and anhydrous, Na2CO3 is hygroscopic, so that it must be heated before use to remove water, we shall therefore use borax as the primary standard.

Borax undergoes hydrolysis when it dissolved in water

Na2B4O7 + 7H2O ↔ 2NaOH + 4H3BO3

Then the below reaction is taken place.

2NaOH + 2HCl ↔ 2NaCl + 2H2O

Adding the two equations:-

Na2B4O7 + 2HCl + 5H2O ↔ 2NaCl + 4H3BO3

The solution PH is determined by the presence of H3BO3 therefore, the most suitable indicator for the titration is methyl orange .

Procedure:-

1- Prepare (0.1N) Borax in (100ml) (D.W)

(wt = N × eq.wt× vml)

2- Prepare (0.1N) HCl in (100ml) (D.W)

{N = (sp.gr×%×1000) / eq.wt}

then N1V1 = N2V2

3- Take (5ml) of borax and put it in the conical flask.

4- Add (2-3) drops of methyl orange(M.O).

5- Fill the burette with HCl.

6- Titrate with HCl until the change of the color of indicator.

7- Calculate the concentration of HCl.

8- Take (1gm) of Borax and dissolve in water and repeat the step (3) above and ably

{N1V1 = wt (1gm) / eq.wt (192.7)}

Questions:-

1- Why (HCl) solution in lab. is not (p.s.s) ?

2- Which prefer Na2CO3 or Borax in standardized HCl and why?

3- Which indicator can be used in titration HCl against Borax and why?

4- Why we standardized HCl?