21 Titration: Reactions of Acids and Bases

The Self-ionization of Water

In pure water at 25oC, both H3O+ and OH- ions are found at concentrations of 1.0 x 10-7 M

Kw is called the ion-product constant

Kw = [H3O+] [OH-]

In pure water at 25oC, Kw = 1.0 x 10-14

The Self-ionization of Water

Kw is very useful because it applies not only to pure water, but to every water solution at 25oC, even acidic or basic solutions.

Thus, if a solution has an H3O+ concentration of 1.0 x 10-2 M, then the OH- concentration must be 1.0 x 10-12 M. i.e. [OH-] = Kw / [H3O+]

pHIn 1909, Danish biochemist Soren

Sorensen proposed a simple way to express the concentrations of H3O+ ions based on logarithms. This scale is known as the pH scale.

The pH of a solution is -1 times the Log of the H3O+ concentration in moles per Liter.

pH = -log[H3O+]

pH = -log[H3O+]Because the pH scale is a logarithmic

scale, each one-unit change in pH represents a 10-fold change in the concentration of H3O+ ions.

The pH of a solution can be measured by using acid-base indicators such as litmus paper, or by using an electronic device called a pH meter.

pH [H3O+] Concentration

.1 M HCl 0 100 M

1 10-1 M

Lemon juice 2 10-2 M

3 10-3 M

Banana 4 10-4 M

Coffee 5 10-5 M

Saliva 6 10-6 M

Pure water 7 10-7 M

Blood 8 10-8 M

9 10-9 M

Borax 10 10-10 M

Lime water 11 10-11 M

12 10-12 M

Bleach 13 10-13 M

1.0 M NaOH 14 10-14 M

pH Scale

BuffersA buffer is a mixture that is able to

release or absorb H+ ions, keeping a solution’s pH constant.

Most common buffers are mixtures of weak acids with their conjugate bases. i.e. a buffer of acetic acid and the acetate anion keeps the pH near 4.74

The amount of acid or base that a buffer can neutralize is called the buffer capacity. [See examples on page 634]

Acid-Base TitrationThe concentration of a weak acid or a

weak base can be easily calculated from the results of a procedure called an acid-base titration.

An acid-base titration is a carefully controlled neutralization reaction.

Acid-Base TitrationTo conduct a titration, a standard

solution is slowly added to the unknown solution until neutralization is complete – called the equivalence point.

The point at which the indicator changes color is called the end point of the titration.

Performing a TitrationTo run a titration, the standard solution

is slowly added to the unknown solution.

As the two solutions mix, the base in one solution neutralizes the acid in the other solution, a reaction that runs nearly to completion.

Performing a TitrationThe reaction between acetic acid and

sodium hydroxide is:

HC2H3O2 + NaOH H2O + NaC2H3O2

The point at which the indicator changes color is called the end point of the titration.

Total # mol of H+ = Total # mol of OH-

This equation is key to calculating the concentration of an acid or base using data from a titration.

Performing a TitrationThe reaction between oxalic acid and

sodium hydroxide from Lab #57 is:

H2C2O4 + 2 NaOH 2H2O + Na2C2O4

Note that it takes 2 mol of base to neutralize each 1 mol of acid.

This equation is key to calculating the concentration of an acid or base using data from a titration.

TitrationStrong Acid / Strong Base

HCl + NaOH H2O + NaCl

Weak Acid/ Strong Base

HC2H3O2 + NaOH H2O + NaC2H3O2

Weak Base / Strong Acid

NH3 + HCl NH4Cl

[Fig 19-14, 19-15, & 19-16 on pgs. 640-642]