The pH scale

An aqueous solution can be acidic, alkaline, or neutral. It has to do with the number of these ions - hydrogen ions. The more hydrogen ions there are per litre, the more acidic the solution. And the other way around, the fewer hydrogen ions a solution contains, the more alkaline it is. We use a scale to measure the acidity - or the alkalinity of a solution.

It's called the pH-value of the solution. The pH scale - lower case p, capital H, is a way of counting the number of hydrogen ions in a solution. But the scale has some unusual properties... One special thing with the pH scale is - that it is reversed. The more hydrogen ions a solution contains - the lower the pH value gets.

A solution with a pH of two... ... contains more hydrogen ions than a solution with a pH of four. This means that the lower the pH gets... The more acidic the solution is. Another peculiar thing is that the mid-point of the scale, is around a pH of seven.

A solution with a pH-value of seven is neither acidic nor alkaline - it is neutral. Pure water for instance. If the pH is lower than seven, the solution contains more hydrogen ions than pure water. A pH under seven means that the solution is acidic. If the pH is higher than seven, there are fewer hydrogen ions than in pure water.

That means that the solution is alkaline. Let's look at a few examples: Carbonated water is a little more acidic than pure water. The pH is around 5. Each step downwards on the pH scale means a ten-fold increase in the number of hydrogen ions. Carbonated water is two pH steps lower than pure water, so it contains a hundred times more hydrogen ions.

Lemon juice is even more acidic. About 2.5 on the pH scale. Your stomach contains diluted hydrochloric acid. The pH there is usually between 1 and 2. This bottle also contains hydrochloric acid, but concentrated.

Now, we'll have to extend the scale below zero. It's so acidic that the pH is minus one. Yes, the pH value can become negative. Substances that acidic are highly corrosive. Let's look at some alkaline solutions as well, with pH higher than seven.

Here's an aqueous solution of sodium bicarbonate, a substance found in baking soda. If we dissolve as much as we can in the water, the pH goes up to about nine. Dishwasher detergent is even more alkaline, it has a pH value of about 11. Can we go higher? Yes, a concentrated solution of sodium hydroxide can increase the pH to a little over 15.

Substances that alkaline are also highly corrosive, just like strongly acidic substances. In order to determine an approximate pH-value, you can use a substance that changes its colour depending on the acidity of a solution - a pH indicator. Different indicators change colour at different pH-intervals. If you want a more exact pH-measurement, you can use a pH-strip. That's a piece of paper with several different indicator substances.

On the box, there is a colour chart showing what colour combination corresponds to each pH value. An even more precise tool is an electric pH-meter. One of these can often show the pH value to one or two decimal places. The pH scale measures the acidity or alkalinity of an aqueous solution. The lower the pH, the more hydrogen ions are present, and the more acidic the solution is.

The mid-point is at a pH of seven. Acidic solutions have a pH lower than seven. Alkaline solutions have pH-values above seven. At a pH value of seven, the solution is neither acidic nor alkaline, but neutral.