The pH Scale (DP IB Chemistry)

The pH Scale

• The acidity of an aqueous solution depends on the number of H⁺ (H₃O⁺) ions in the solution

• pH is defined as:

pH = – log₁₀[H⁺]

• Where [H⁺] is the concentration of H⁺ in mol dm^–3

• The pH scale is a logarithmic scale with base 10

• This means that each value is 10 times the value below it

  • For example, pH 5 is 10 times more acidic than pH 6.

• pH values are usually given to 2 decimal places

• The relationship between concentration is easily seen in the following table:

pH & [H⁺] Table

• The pH scale is a numerical scale that shows how acidic or alkaline a solution is

• The values on the pH scale go from 0 - 14 (extremely acidic substances have values of below 0)

• All acids have pH values below 7, all alkalis have pH values above 7

  • The lower the pH then the more acidic the solution is

  • The higher the pH then the more alkaline the solution is

The pH scale

The pH scale showing acidity, neutrality and alkalinity

pH of acids

• Acidic solutions (strong or weak) always have more H⁺ than OH^- ions

• Since the concentration of H⁺ is always greater than the concentration of OH^- ions, [H⁺] is always greater than 10^-7 mol dm^-3

• Using the pH formula, this means that the pH of acidic solutions is always below 7

• The higher the [H⁺] of the acid, the lower the pH

pH of bases

• Basic solutions (strong or weak) always have more OH^- than H⁺ ions

• Since the concentration of OH^- is always greater than the concentration of H⁺ ions, [H⁺] is always smaller than 10^-7 mol dm^-3

• Using the pH formula, this means that the pH of basic solutions is always above 7

• The higher the [OH^-] of the base, the higher the pH

pH of water

• Water at 298K has equal amounts of OH^- and H⁺ ions with concentrations of 10^-7 mol dm^-3

• To calculate the pH of water, the following formula should be used:

pH = – log₁₀[H⁺ (aq)]

[H⁺ (aq)] = concentration of H⁺/H₃O⁺ ions

pH = -log (10^-7) = 7

• Thus, water has a pH of 7 at 298 K      

How to measure pH

• The most accurate way to determine the pH is by reading it off a pH meter

• The pH meter is connected to the pH electrode which shows the pH value of the solution

Using a pH meter

The diagram shows a digital pH meter that measures the pH of a solution using a pH electrode

• A less accurate method is to measure the pH using universal indicator paper

• The universal indicator paper is dipped into a solution of acid upon which the paper changes colour

• The colour is then compared to those on a chart which shows the colours corresponding to different pH values

Using universal indicator

The diagram shows the change in colour of the universal indicator paper when dipped in a strong (HCl) and weak (CH₃COOH) acid. The colour chart is used to read off the corresponding pH values which are between 1-2 for HCl and 3-4 for CH₃COOH