PREPARATION OF STANDARD SOLUTIONS (TITRATION)

PREPARATION OF STANDARD SOLUTIONS (TITRATION)

A standard solution is a solution whose concentration is known, commercial acids such as HCL, H2SO4, and base/alkalis such as NaOH, KOH e.t.c. However such solutions that are not standard/ standardized can be standardized using titration method/volumetric analysis. The process of standardizing is called Standardization.

Standard solutions can be prepared in two ways:

•          From stock solution

•        From a prepared solution of higher concentration

1     From stock solution: Standard solutions can be prepared from stock solutions using the formulas;

For liquids (Acids):

Molecular weight x Molarity x 100

Specific gravity x Percentage purity

For solids or powder (Base/ alkalis):

Molar concentration x Molar mass

Parameters for liquid solution:

Molecular weight: This is the total weight of the compound.

Molarity: This is the required concentration to be prepared.

Specific gravity: This is the density of the stock solution. Density is ratio between mass & volume.

Percentage purity (%):

 Percentage purity and Specific gravity of some acids

Acid	Percentage purity	Specific gravity
HCL	32% - 39%	1.13 -1.19
H2SO4	95% - 98%	1.83 – 1.84
HNO3	63% - 67%	1.43 – 1.44

CALCULATIONS

Prepare

•          0.1M HCL in 1000ml volumetric flask

•          0.1M HNO₃ in 1000ml volumetric flask

•          0.1M H₂SO₄ in 1000ml volumetric flask

SOLUTION

•           0.1M HCL

Molecular weight of HCL= 1+35.5= 36.5 (H=1, CL= 35.5)

36.5 x 0.1 x 100   =       365

1.18 x 35.4                    41.77

= 8.74ml in 1000ml volumetric flask

•           0.1M HNO₃

Molecular weight of HNO3= 1+14+48= 63(H=1, N=14, O=16)

63 x 0.1 x 100      =       630

1.43 x 65                       92.95

= 6.78ml in 1000ml volumetric flask

3

•              0.1M H₂SO₄

Molecular weight of H2SO4=2+32+64=98 (H=1, S=32, O=16)

98 x 0.1 x 100      =       980

1.83 x 97                       177.51

= 5.52ml in 1000ml volumetric flask

Parameters for solid solution:

Molar concentration: This is same thing as the molarity.

Molar mass: This is same thing as the molecular weight.

CALCULATION

Prepare

0.1M KOH in 1000ml volumetric flask

SOLUTION

Molar mass of KOH=56

0.1 x 56

= 5.6ml in 1000ml volumetric flask

After your calculations, what you have is what you will weigh and dissolve with water.

2  From a prepared solution of higher concentration:  A lesser concentration of a standard solution can be prepared from a higher concentration of solution using serial dilution formula.

C₁V₁ = C₂V₂

C₁– Concentration of assumed stock (solution with higher concentration)

V₁ – The amount (volume) to be collected from the assumed stock

C₂ – Concentration of the solution to be prepared

V₂ – The volume of the volumetric flask to be used in the preparation

NOTE: Distilled or Deionized water is to be used in preparing your standard solution.

Distilled water: Water that is free from organic matter

Deionized water: Water that is free from surrounding ions

CALCULATION

From a solution of 2M HNO₃, prepare;

•        0.1M HNO₃ in 1000ml volumetric flask

•        0.25M HNO₃ in 500 ml volumetric flask

•        0.001M HNO₃ in 250ml volumetric flask

SOLUTION

•           0.1M HNO₃ from 2M HNO₃

           V₁      =       0.1 x 1000            =             100

                                   2                                        2

         
            V₁      = 50ml in 1000ml volumetric flask

•          0.25M HNO₃ from 2M HNO₃

           V₁      =       0.25 x 500            =       125

                                      2                               2

           
            V₁      = 62.5ml in 500ml volumetric flask

•          0.001M HNO₃ from 2M HNO₃

            V₁      =       0.001 x 250                   =       0.25

                                         2                                       2

             V₁      = 0.125ml in 250ml volumetric flask