General Chemistry/Titration and pH

Ionization of Water

Water is a very weak electrolyte. It will dissociate into hydroxide and hydronium ions, although only in a very small amount. Because pure water is completely neutral, it always dissociates in equal amounts of both hydroxide and hydronium. Once acidic or basic substances have been added to pure water, the concentration of the ions will change. Regardless of which acid-base theory is used, acids and bases all have one important thing in common:

• All acids increase the H⁺ concentration of water.
• All bases increase the OH^- concentration of water.

Furthermore, the concentration of hydrogen ions multiplied by the concentration of hydroxide ions is a constant. This constant is known as the ionization constant of water, or K_w. At room temperature it equals 10^-14 mol²/L². Thus:

$K_{w}=[{\hbox{H}}^{+}]\times [{\hbox{OH}}^{-}]=1.00\times 10^{-14}mol^{2}/L^{2}$

In a neutral solution, the concentrations of H⁺ and OH^- are both equal to 10^-7. Using the above equation, the concentration of one ion can be determined if the concentration of the other ion is known. This equation further demonstrates the relationship between acids and bases: as the acidity (H⁺) increases, the basicity (OH^-) must decrease.

The pH Scale

To measure the acidity or basicity of a substance, the pH scale is employed.

The pH Scale

• A completely neutral substance has a pH of 7.
• Acids have a pH below 7
• Bases have a pH above 7.

pH usually ranges between 0 and 14, but it can be any value. Battery acid, for example, has a negative pH because it is so acidic.

Various pH values.

Definition of pH

The pH scale is mathematically defined as:

$pH=-\log {[{\hbox{H}}^{+}]}$

Substances that release protons or increase the concentration of hydrogen ions (or hydronium ions) will lower the pH value.

pOH

There is also a less common scale, the pOH scale. It is defined as:

$pOH=-\log {[{\hbox{OH}}^{-}]}$

Substances that absorb protons or increase the concentration of hydroxide ions will lower the pOH value.

The sum of pH and pOH is always 14 at room temperature:

$pH+pOH=14$

Calculating pH

A strong acid or strong base will completely dissociate in water, so the concentration of the acid/base is equal to the concentration of H⁺ or OH^-. If you know the concentration of the acid or base, then you can simply plug that number into the pH or pOH formula. The sum of pH and pOH will always equal 14 at room temperature, so you can interconvert these two values.

If you know the H⁺ concentration and need to know the OH^- concentration (or vice versa), use the definition of K_w above. The product of the two ion concentrations will always equal 10^-14 at room temperature.

Titration

Titration is the controlled mixing of a solution with known concentration (the standard solution or titrant) to another solution (called the analyte) to determine its concentration. One solution is acidic and the other is basic. An indicator is added to the mixture. An indicator must be selected so that it changes color when equal amounts of acid and base have been added. This is known as the equivalence point. This does not necessarily mean that the pH is 7.0.

Polyprotic acids have multiple equivalence points.

Once the equivalence point has been reached, the unknown concentration can be determined mathematically.

Practice Questions

1) 5.00g of NaOH are dissolved to make 1.00L of solution.

a What is the concentration of H⁺?

b What is the pH?

Answers for Titration and pH