# The pH scale

## The pH scale

The concentration of specific ions in solution determines whether the solution is acidic or basic. Acids and bases can be described as substances that either increase or decrease the concentration of hydrogen ($$\text{H}^{+}$$) or hydronium ($$\text{H}_{3}\text{O}^{+}$$) ions in a solution. An acid increases the hydrogen ion concentration in a solution, while a base decreases the hydrogen ion concentration.

pH is used to measure the concentration of $$\text{H}^{+}$$ ions ($$[\text{H}^{+}]$$) and therefore, whether a substance is acidic or basic (alkaline). Solutions with a pH of less than seven are acidic, while those with a pH greater than seven are basic (alkaline). The pH scale ranges from $$\text{0}$$ to $$\text{14}$$ and a pH of $$\text{7}$$ is considered neutral.

#### Fact:

The universal indicator changes colour from $$\color{red}{\textbf{red}}$$ in $$\color{red}{\textbf{strongly acidic}}$$ solutions through to $$\color{purple}{\textbf{purple}}$$ in $$\color{purple}{\textbf{strongly basic}}$$ solutions.

#### Fact:

The term pH was first used by in 1909 by Søren Peter Lauritz Sørensen (a Danish biochemist). The p stood for potenz and the H for hydrogen. This translates to power of hydrogen.

### Definition: pH

pH is a measure of the acidity or alkalinity of a solution.

At the beginning of the tutorial we mentioned that we encounter many examples of acids and bases in our day-to-day lives. The pH of solutions of some household acids and bases are given in the table below.

#### Fact:

Fizzy cooldrinks often have very low pH (are acidic).

 Drink pH Coke $$\text{2.5}$$ Diet coke $$\text{3.3}$$ Pepsi $$\text{2.5}$$ Diet pepsi $$\text{3.0}$$ Sprite $$\text{3.2}$$ 7 Up $$\text{3.2}$$ Diet 7 Up $$\text{3.7}$$
 Molecule Found in pH Type phosphoric acid fizzy drinks $$\text{2.15}$$ acid tartaric acid wine $$\text{2.95}$$ acid citric acid lemon juice $$\text{3.14}$$ acid acetic acid vinegar $$\text{4.76}$$ acid carbonic acid fizzy drinks $$\text{6.37}$$ acid ammonia cleaning products $$\text{11.5}$$ base ammonium hydroxide cleaning products $$\text{11.63}$$ base sodium hydroxide caustic soda $$\text{13}$$ base

Table: The pH of solutions of acids and bases as found in common household items.