The pH Scale and Universal Indicator

1How acidic is the soil?

A farmer in the southern highlands notices that her beans are yellow and weak although she weeds and fertilises them. An agronomist shakes a spoonful of her soil with distilled water, dips in a strip of indicator paper and reads a number from a colour chart: 4.5. The soil is too acidic for beans, which grow best between about 6 and 7. The advice is to spread agricultural lime, a base, before the next planting. "Acidic" was not enough; the farmer needed to know how acidic. That is what the pH scale measures.

Figure 4.1: The pH scale, coloured as universal indicator would show it, with everyday substances placed on it.

2Reading the scale

• pH 7 is neutral: pure water and salt solution.
• Below 7 is acidic. The further below 7, the stronger the acid.
• Above 7 is alkaline. The further above 7, the stronger the alkali.
• Each step is a factor of ten. A solution of pH 3 has 10 times as many hydrogen ions as one of pH 4, and $10\times 10=10010\; \backslash times\; 10\; =\; 100$ times as many as one of pH 5.

In general, a difference of $nn$ pH units means a factor of ${10}^{n}10^\{n\}$ in the concentration of hydrogen ions.

For exact work, chemists use a pH meter: an electronic probe dipped into the solution gives the pH on a screen, often to one or two decimal places.

Table 4.1: Universal indicator colours

Try it yourself

Open the pH Scale simulation. Pour in milk, coffee, soap or drain cleaner and read the pH, then add water and watch the pH move towards 7 but never cross it.