Acid-Base Titration and pH

Aqueous Solutions and the Concept of pH In the self-ionization of water, two water molecules produce a hydronium ion and a hydroxide ion by the transfer of a proton

At 25 degrees C, the concentration of [OH] and [H 3 O] equals 1.0 x The ionization constant for water is written as K w and is equal to [H3O]x[OH] K w = 1.0 x Since the concentrations of hydroxide and hydronium are equal in water, water is neutral –If the hydronium concentration is greater, it is acidic, if the hydroxide concentration is greater it is basic

Since the ionization constant is known, the hydronium or hydroxide concentration can be calculated if the other is known

The pH of a solution is defined as the negative of the common logarithm of the hydronium ion concentration –pH – French for pouvoir hydrogene “hydrogen power” –pH = -log [H 3 O] –pOH is the negative log of the hydroxide ion concentration

To calculate the pH simply take the –log of the concentration –If only the OH concentration is known, divide this from 1.0 x and take the –log of this number To calculate the hydronium ion concentration from pH, take the antilog of the pH –(2 nd /log/(-)/pH/enter) The molarity and ion concentration are only equal in strong acids and bases (weak ones must be calculated)

Determining pH and Titrations Acid base indicators are compounds whose colors are sensitive to pH They are weak acids or bases

A pH meter determines the pH of a solutions by measuring the voltage between the two electrodes that are placed in the solution

A titration is the controlled addition and measurement of the amount of a solution of known concentration required to react completely with a measured amount of a solution of unknown concentration

The point at which the two solutions used in a titration are present in chemically equivalent amounts is the equivalence point The point in a titration at which an indicator changes color is called the end point of the indicator

In titrations of strong acids and strong bases, the equivalence point will be 7 In titrations of strong acids and weak bases, the equivalence point will be less than 7, since the salt formed is a weak acid In titrations of strong bases and weak acids, the equivalence point will be greater than 7, since the salt formed is a weak base