Chapter U4.22, Problem 1E

Interpretation Introduction

Interpretation: The method to determine the concentration of potassium hydroxide in a water sample by titration needs to be explained.

Concept Introduction: An acid-base titration is defined as a neutralization reaction where an acid (containing proton, H⁺) reacts with a base (containing hydroxide, OH^-) to form a salt and water, H₂O. In terms of the ions present in the solution, an acid-base titration reaction is defined as the neutralization of H+ by OH- to form undissociated H₂O molecules as per the reaction below.

  $H^{+}\left(aq\right)+O{H}^{-}\left(aq\right)\to H_{2}O\left(l\right)$

The concentration of one of the reagents (either the acid or the base) is known and the concentration of the other (unknown) can be obtained by simple stoichiometric calculations.

The completion of the neutralization reaction can be determined by the color change of an acid-base indicator. An acid-base indicator is a pH indicator which shows different colors in acidic and basic pH.

Answer to Problem 1E

The concentration of potassium hydroxide, KOH in a sample of water can be easily determined by titrating the water sample with a solution of hydrochloric acid, HCl having known concentration in presence of phenolphthalein indicator.

Explanation of Solution

Potassium hydroxide, KOH reacts with hydrochloric acid, HCl in a 1:1 stoichiometric manner as per the reaction below.

  $KOH\left(aq\right)+HCl\left(aq\right)\to KCl\left(aq\right)+H_{2}O\left(l\right)$

In terms of the ions present in the solution, the net ionic equation can be written as

  $O{H}^{-}\left(aq\right)+H^{+}\left(aq\right)\to H_{2}O\left(l\right)$

A measured volume (say 10 mL or 25 mL) of HCl is taken in an Erlenmeyer flask. The concentration of the HCl solution is known. Since both the volume and the concentration of the HCl solution are known, hence, the amount of HCl can be easily determined. A few drops of phenolphthalein indicator are added to the flask when the solution is colorless.

The water sample containing the KOH is added from the buret until a permanent color change to pink is observed. The permanent color change indicates the solution contains equal amounts of KOH and HCl and that, the added KOH has been neutralized by the HCl present. By reading the volume of the water sample added from the buret, the concentration of KOH can be easily determined.

The concentration of the HCl solution can be determined by standardization against a standard KOH solution using phenolphthalein as the indicator.

Conclusion

The concentration of KOH in the given water sample can be determined by titration against a known HCl solution with phenolphthalein indicator.