Alkalimetry

 ALKALIMETRY- STANDARIZATION OF BASES

        The determination of the amount of base in a sample by titration with a standard acid solution is called alkalimetry

Principle  :

            Equal volumes of all acids / bases containing their gram equivalent weights per litre of the solution react completely with each other.

  2 KOH + 2 HCl -->2 KCl + 2 H2O

                In other words, the product of volume (V1) and normality (N1) of one solution (acid) is equal the product of volume (V2) and normality (N2) of another solution (alkali / base)

V1N1 =  V2N2

Molecular weight of KOH=56 g

             Equivalent weight of KOH =  = 56        

Therefore, one equivalent weight of KOH = 56 g

            56 g dissolved in 1 litre gives 1 N solution

            5.6 g dissolved in 1 litre gives 0.1 N solution

            1.4 g dissolved in 250 mL gives 0.1 N solution

Procedure:                                

• Weigh 1.4 g of KOH

• Transfer this to a 400 mL beaker and add 100 mL of distilled water and dissolve it by stirring with a glass rod.

• Add the remaining 150 mL of water to the beaker. This is approximately 0.1 N solution of KOH.

• Now, fill up a burette with 0.1 N HCl (secondary standard) and mount it in the burette stand.

• Pipette 10 mL of the approximately 0.1 N solution of KOH into a 250 mL conical flask and add a few drops of phenolphthalein indicator.

• Titrate against 0.1 N HCl up to an end point of disappearance of pink colour.

• Note down the burette readings and find out the volume of 0.1 N HCl used in the titration.

• Repeat the titration until you get concordant titre values.

• Now, using the relationship, V1N1 = V2 N2, find out the normality of the KOH solution.

• Calculate the volume of water to be added in case the normality is more than 0.1N or the amount of KOH to be added in case the normality is less than 0.1 N.

• Add either the volume of water or the amount of KOH, as the case may be and mix it thoroughly by stirring.

• Repeat the titration and calculation processes until the normality of the KOH prepared reaches 0.1 N.