The pH scale and neutralisation

What you'll learn:

• What specific ions make a solution an acid or an alkali.
• How to measure pH using Universal Indicator and pH probes.
• The chemical reaction that happens when acids and alkalis cancel each other out.

What makes an acid or an alkali?

When we talk about acids and alkalis in chemistry, we are almost always talking about aqueous solutions. An aqueous solution is simply a mixture where a substance has been dissolved in water.

The properties of acids and alkalis come down to the specific ions they release when they dissolve in water.

For example, when hydrogen chloride gas dissolves in water, it splits apart to form hydrochloric acid, releasing $H^{+}$ ions into the water.

Similarly, when solid sodium hydroxide dissolves in water, it releases $O{H}^{-}$ ions. It is the presence of these $O{H}^{-}$ ions that makes the solution alkaline.

The pH scale

We measure how acidic or alkaline a solution is using the pH scale. The scale runs from $0$ up to $14$.

• Acids have a pH of less than 7. The closer the pH is to $0$, the more strongly acidic the solution is.
• Alkalis have a pH of greater than 7. The closer the pH is to $14$, the more strongly alkaline the solution is.
• Neutral solutions (like pure liquid water) have a pH of exactly 7.

Measuring pH

There are two main ways to measure the pH of a solution in the laboratory:

1. Universal Indicator (or wide range indicator) This is a mixture of dyes that changes to a specific colour depending on the pH of the solution. You add a few drops to your solution, look at the colour it turns, and compare it to a printed colour chart to find the approximate pH.

• Strong acids turn red.
• Weak acids turn orange or yellow.
• Neutral solutions turn green.
• Weak alkalis turn blue.
• Strong alkalis turn purple.

2. A pH probe A pH probe is an electronic device attached to a pH meter. You place the probe directly into the solution, and it reads the pH as a number on a digital display.

Neutralisation

When you mix an acid and an alkali together, they react to form water. This is called a neutralisation reaction.

In terms of the ions involved, the hydrogen ions ($H^{+}$) from the acid join together with the hydroxide ions ($O{H}^{-}$) from the alkali to create molecules of water ($H_{2}O$).

Because water is neutral (pH $7$), the acid and the alkali effectively cancel each other out. The overall reaction for any acid neutralising any alkali can be represented by a single, standard ionic equation:

$$H^{+}(aq)+O{H}^{-}(aq)\to H_{2}O(l)$$