As mentioned earlier, a hydrogen atom has a single proton in its nucleus orbited by a single electron. A hydrogen ion (H+), formed by the loss of the electron, is thus a single free proton. Hydrogen ions are formed when the proton of a hydrogen atom in a molecule is released, leaving behind its electron. Molecules that release protons (hydrogen ions) in solution are called acids, for example:
HCl hydrochloric acid
Conversely, any substance that can accept a hydrogen ion (proton) is termed a base. In the reactions above, bicarbonate and lactate are bases since they can combine with hydrogen ions (note the double arrows in the two reactions). It is important to distinguish between the un-ionized acid and ionized base forms of these molecules and to note that separate terms are used for the acid forms, lactic acid and carbonic acid, and the bases derived from the acids, lactate and bicarbonate. By combining with hydrogen ions, bases lower the hydrogen-ion concentration of a solution.
When hydrochloric acid is dissolved in water, 100 percent of its atoms separate to form hydrogen and chloride ions, and these ions do not recombine in solution (note the one-way arrow above). In the case of lactic acid, however, only a fraction of the lactic acid molecules in solution release hydrogen ions at any instant. Therefore, if a 1 mol/L solution of hydrochloric acid is compared with a 1 mol/L solution of lactic acid, the hydrogen-ion concentration will be lower in the lactic acid solution than in the hydrochloric acid solution. Hydrochloric acid and other acids that are 100 percent ionized in solution are known as strong acids, whereas carbonic and lactic acids and other acids that do not completely ionize in solution are weak acids. The same principles apply to bases.
It must be understood that the hydrogen-ion concentration of a solution refers only to the hydrogen ions that are free in solution and not to those that may be bound, for example, to amino groups (R—NH3+). The acidity of a solution refers to the free (unbound) hydrogen-ion concentration in the solution; the higher the hydrogen-ion concentration, the greater the acidity. The hydrogen-ion concentration is frequently expressed in terms of the pH of a solution, which is defined as the negative logarithm to the base 10 of the hydrogen-ion concentration (the brackets around the symbol for the hydrogen ion in the formula below indicate concentration):
Thus, a solution with a hydrogen-ion concentration of 10-7 mol/L has a pH of 7, whereas a more acidic solution with a concentration of 10-6 mol/L has a pH of
Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition
Chemical Composition of the Body CHAPTER TWO
Chemical Composition of the Body CHAPTER TWO
6. Note that as the acidity increases, the pH decreases; a change in pH from 7 to 6 represents a tenfold increase in the hydrogen-ion concentration.
Pure water, due to the ionization of some of the molecules into H+ and OH~, has a hydrogen-ion concentration of 10~7 mol/L (pH = 7.0) and is termed a neutral solution. Alkaline solutions have a lower hydrogen-ion concentration (a pH higher than 7.0), while those with a higher hydrogen-ion concentration (a pH lower than 7.0) are acidic solutions. The extracellular fluid of the body has a hydrogen-ion concentration of about 4 X 10~8 mol/L (pH = 7.4), with a normal range of about pH 7.35 to 7.45, and is thus slightly alkaline. Most intracellular fluids have a slightly higher hydrogen-ion concentration (pH 7.0 to 7.2) than extracellular fluids.
As we saw earlier, the ionization of carboxyl and amino groups involves the release and uptake, respectively, of hydrogen ions. These groups behave as weak acids and bases. Changes in the acidity of solutions containing molecules with carboxyl and amino groups alter the net electric charge on these molecules by shifting the ionization reaction to the right or left.
For example, if the acidity of a solution containing lac-tate is increased by adding hydrochloric acid, the concentration of lactic acid will increase and that of lac-tate will decrease.
If the electric charge on a molecule is altered, its interaction with other molecules or with other regions within the same molecule is altered, and thus its functional characteristics are altered. In the extracellular fluid, hydrogen-ion concentrations beyond the tenfold pH range of 7.8 to 6.8 are incompatible with life if maintained for more than a brief period of time. Even small changes in the hydrogen-ion concentration can produce large changes in molecular interactions, as we shall see.
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