The pH scale takes its name from the words potential of hydrogen. It is a scale used to measure the acidity or alkalinity of a solution. The pH scale uses a range from 0 to 14, with 7.0 indicating neutrality. Numbers beginning at 7.0 and moving toward 0 indicate acidity, while the numbers beginning at 7.0 and moving toward 14 indicate alkalinity, so the scale divides acids from bases. We owe the concept of pH to Danish chemist S. P. L. Sørensen, who introduced it in 1909.

There are several categorizations for solutions as measured by the pH scale. A common one is this:

It is interesting to note that while some people associate corrosion with acid, a number of organizations define pH’s of both 0-2 and 11.5-14 as corrosive.

While litmus paper is used to indicate whether a solution is acidic or basic with a simple red/blue color indication, sometimes more fine-tuned results are necessary. To find the equivalence point in Acid-Base titration, for example, choosing the proper indicator is crucial to getting suitable results. The indicators used, such as methyl violet, Bromthymol blue, and Alizarin yellow, have very specific bands of color reaction that can be matched to the equivalence point.

A number of plants are natural pH indicators, including red cabbage, a fact you may have seen revealed in your kitchen. You may also see soil pH revealed by plants in your garden, because the pH of the soil is one of the factors that determines the success or failure of gardening ventures. Generally, plants experience their best growth in soils with a pH near neutral, though certain plants, notably blueberries and azaleas, need acid soils to grow well. One of the interesting gardening tricks accomplished by altering pH is to encourage the blossoming of blue – if the soil has a pH of 5.5 or below – or pink – if the soil has a pH of 6.5 or higher – hydrangea flowers from identical plants. Soil with a pH in between may yield purple blossoms — a mixture of pink and blue.

The pH of different substances is also of interest in other fields. The proper pH is important in determining the quality of wine, best determined using a pH meter. The pH of wine ranges from 2.9 to 4.2 and is considered best at about 3.0 to 3.5 — different wine makers give slightly different figures. The pH scale is also the measure used to determine whether paper qualifies for description as acid-free.