WHAT MEANS pH?

pH is a number used to measure the degree of acidity of a solution. It is used on a pH scale that ranges from 0 to 14, with the difference between each number being a factor of 10. In the life sciences, as well as in chemistry, many chemical reactions depend on the pH of a solution. pH is also used to analyze body secretions, to test soil suitability, and for industrial purposes.

pH refers to the amount of acid in a substance. The letters are said to have come from the French for “hydrogen power,” meaning how many hydrogen atoms are concentrated in a solution. The lowercase "p" means its “power,” or its logarithmic value. This means that each time a number is raised to another power (from a 2 to a 3), it increases by a factor of 10. Another explanation for pH is that it stands for “potential of hydrogen.” Either way, it is known that the pH symbol was first used by the Danish chemist, Soren Sorenson (1868–1939), in 1909. He used the pH symbol on what he called a Sorenson scale.

Today, however, it is called a pH scale, and it is a 0 to 14 scale that tells us exactly how acidic a substance is. This scale uses as a reference point the number 7 which is the midpoint between the scale’s two extremes of 0 and 14. A pH of 7 is considered to be neutral—or neither acid nor its opposite, base. Acids and bases are two types or classes of biological compounds. They affect every living cell as well as the habitats of organisms.

The pH of a solution can be measured with an electronic pH meter or by various paper or liquid indicators. These change color depending on the pH of the mixture. A pH meter will give a digital readout, or number, indicating the pH of a solution. A treated paper indicator turns darker pink for more acid and darker blue for more base. The paper color is checked against a standard chart that indicates the pH number. The scale itself tells the exact degree of acid in a solution. Starting with the lowest number, the strongest acid, a pH of 0, would be concentrated nitric acid. Following that, in approximate values, stomach acid has a pH of 1, lemon juice 2, vinegar 3, fresh tomatoes 4, black coffee 5, and peas 6. Distilled water is neutral and has a pH of 7.

After this, the base part of the scale begins. Baking soda has a pH of 8, borax 9, ammonia 10, lime 12, oven cleaner 13, and lye 14. Since these values are logarithmic, the difference between each one is a factor of 10. Thus a solution of pH 5 is 10 times more acidic than a solution of pH 6. In the living world, almost all biological processes take place in a pH environment between 6 and 8. There are, however, a few exceptions such as digestive acids that are extremely powerful (with a pH of 1). Many organisms have built-in regulators that act as buffers and either soak up or join with small amounts of excess acid or base.

In "Complete Life Science Resource", U.X.L (Gale Group), USA, 2001,excerpts v.III, p.445-446. Adapted and illustrated to be posted by Leopoldo Costa.