Understanding the pH scale

Yasmin Jayathirtha

One of the earliest classification schemes students of chemistry learn is that of acid/base/neutral. Along with that they also learn about the pH scale. There is no explanation given for the origin of the scale or its name. A typical passage in a secondary level textbook is as follows:

“A much more useful measure of the strength of an acid solution was worked out by the Danish biochemist S.Sorensen. He worked in the laboratories of the Carlsberg breweries and was interested in checking the acidity of beer. The scale he introduced was the pH scale. The scale runs from 1 to 14, and the following general rules apply.
1. Acids have a pH less than 7
2. The more acidic a solution, the lower the pH
3. Neutral substances, such as pure water, have a pH of 7
4. Alkalis have a pH greater than 7
5. The more alkaline a solution, the higher the pH

The pH of a solution can be measured in several ways. Universal indicator papers that are sensitive over the full range of values can be used…” – Chemistry by Richard Harwood

There will also be a table containing pH values of some common substances and a figure showing the range of colours shown by the universal indicator.

The author works with Centre for Learning, Bengaluru. She can be reached at yasmin.cfl@gmail.com.

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