The pH Concept

Changes in the acidity or basicity of some solutions can be critical in some systems such as the following:

• a body of water supporting aquatic plants and animals
• our stomachs digesting ingested food
• cells of living organisms where vital life processes are carried out
• some industrial processes where reactions take place at a desired rate

As discussed earlier, the acidity of a solution is measured by the hydronium ion concentration. For most solutions this is a very small number, usually expressed using exponential notation. In 1909, Soren Sorensen, a Danish biochemist, proposed the pH scale as a more convenient way of expressing hydronium ion concentration. The symbol pH stands for some German words which literally mean ' the power of the hydrogen ion.'

The pH of a solution is defined as the negative of the logarithm of the hydronium ion concentration.

pH = -log [H₃O⁺]

Pure water has a hydronium ion concentration of 1 x 10 raised to -7. Thus, using the formula given above, we can now say that pure water has a pH of 7.

 

pH Box

pH of Common Substances

• 1.0 M HCl-pH 0
• Battery acid-pH 1
• Gastric juice-pH 1.6-1.8
• Calamansi juice-pH 2.2
• Vinegar-pH 2.8
• Apple juice-pH 3
• Carbonated drink-pH 3.1
• Orange juice-pH 3.5
• Tomato juice-pH 4.2
• Urine-pH 6.1
• Milk-pH 6.5
• Saliva-pH 6.5
• Pure water-pH 7
• Blood-pH 7.4
• Seawater-pH 8.4
• Milk of Magnesia-pH 10.5
• 1.0 M NaOH-pH 14