Basics of Biology

Basic Chemistry For Biology

What is meant by pH?

One easy-to-find food can be used to determine if a solution is acidic or basic— the red cabbage. This vegetable contains a water-soluble pigment called flavin—also found in plums, apple skins, and grapes—which is also called an anthocyanin. If you chop some red cabbage into small pieces, cover them with boiling water, and allow the mixture to sit for about ten minutes, you can use the cabbage juice to discover the pH of a solution. Basic solutions will turn the anthocyanin in the cabbage juice a greenish-yellow, neutral solutions will turn purple, and acidic solutions will turn red.

The term “pH” is taken from the French phrase l’puissance d’hydrogen, meaning “the power of hydrogen.” Scientifically, pH refers to the -log of the H (positive hydrogen). The mathematical equation to determine pH is usually written as follows: pH = -log [H ]. For example, if the hydrogen ion concentration in, say, a solution is 1/10,000,000 or 10-7, then the pH value is 7.

The composition of water can also be used to understand the concept of pH: Water is composed of two hydrogen atoms bonded covalently to an oxygen atom. In a solution of water, some water molecules (H2O) will break apart into the component ions—H and OH- ions; it is the balance of these two ions that determines pH. When more H ions than OH- ions exist, the solution is an acid, and when more OH- ions than H ions exist, the solution is a base.


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