When studying biology, you must grasp how reactions can form acids or bases, as well as what effect that has on pH. A basic understanding of chemistry is important in biology, because living things are composed of matter.

When water splits apart (which doesn’t happen to every water molecule all at the same time), hydrogen and hydroxide ions are created. The hydrogen ion H(+) can combine with negatively charged elements to form acids. Acids are molecules that can split apart in water and release hydrogen ions. A common example is hydrochloric acid (HCl). When HCl is added to water, it splits apart into H(+) and Cl(-), increasing the number of hydrogen ions in the water–HCl solution.

Bases are molecules that can split apart in water and release hydroxide ions. The most common example is sodium hydroxide (NaOH). When NaOH is added to water, it splits apart into Na(+) and OH(-). The hydroxide ions can combine with hydrogen ions (therefore, decreasing the number of hydrogen ions in the solution) to form more water. The principles of how acids and bases react in water form the basis of the pH scale.

The pH scale

The term pH symbolizes the hydrogen ion concentration in a solution (for example, what proportion of a solution contains hydrogen ions). The pH scale goes from 1—14. A pH of 7 is neutral, meaning that the amount of hydrogen ions and hydroxide ions in a solution are equal. For example, water has a pH of 7 because when water breaks up, the split is equitable into one hydrogen ion for every hydroxide ion.

If a solution contains more hydrogen ions than hydroxide ions, it is said to be acidic, and the pH of the solution is less than 7. If a molecule releases hydrogen ions in water, it is an acid. The more hydrogen ions it releases, the stronger the acid, and the lower the pH value. The table below shows you the pH of some common substances and may visually help you to figure out the pH scale.

The situation is reversed for bases. If a solution contains more hydroxide ion than hydrogen ion, it is said to be basic, and its pH is higher than 7. Remember that bases dissociate (break apart) into hydroxide ions and a positive ion. The hydroxide ions can combine with H(+) to create water. Because the hydrogen ions are used, the number of hydrogen ions in the solution decreases, making the solution less acidic and therefore more basic. So, the more hydroxide ions a molecule releases (or the more hydrogen ions it takes in), the more basic it is.

The pH of Some Common Substances
Increasing pH (Decreasing Acidity) Substances
0 (most acidic) Hydrochloric acid (HCl)
1 Stomach acid
2 Lemon juice
3 Cola, beer, vinegar
4 Tomatoes
4.5 Fish die if water is this acidic
5 Coffee
5.5 Normal rainwater
6 Urine
6.5 Saliva
7 (neutral) Water, tears
7.5 Human blood
8 Seawater
9 Baking soda, antacids
10 Great Salt Lake
11 Ammonia
12 Bicarbonate of soda
13 Oven cleaner
14 (most basic) Sodium hydroxide (NaOH)

Buffing up on buffers

In living organisms, blood or cytoplasm are the “solutions” in which the required ions (for example, electrolytes) are floating. That is why most substances in the body hover around the neutral pH of 7. However, the body has a backup system in case things go awry. A buffer system exists to help neutralize the blood if excess hydrogen or hydroxide ions are produced.

The buffers help to keep the pH in the normal range by “taking up” (combining with) the excess hydrogen or hydroxide ions. If something is wrong with the buffer system, an organism, such as you, can develop acidosis if the pH drops too low (blood becomes too acidic) or alkalosis if the pH gets too high (blood becomes too basic).

The most common buffers in the body are bicarbonate ion and carbonic acid. Bicarbonate ion is prevalent in the bloodstream. It carries carbon dioxide through the bloodstream to the lungs so that it can be exhaled. It also combines with excess hydrogen ions to keep the pH of the blood in the normal range.

When bicarbonate ion takes up the extra hydrogen ions, it forms carbonic acid, which keeps the pH of the blood from going too low. However, if the pH of the blood gets too high, carbonic acid breaks apart to release some hydrogen ions, which brings the pH back into balance. The pH of the body is fine-tuned by actions in the kidneys.