Acids, bases, and salts are three classes of biologically important compounds (table 2.3). Their characteristics are determined by the nature of their chemical bonds. Acids are ionic compounds that release hydrogen ions in solution. A hydrogen atom without its electron is a proton. You can think of an acid, then, as a substance able to donate a proton to a solution. Acids have a sour taste, such as that of citrus fruits. However, tasting chemicals to see if they are acids can be very hazardous, because many are highly corrosive. An example of a common acid is the phosphoric acid—H3PO4—in cola soft drinks. It is a dilute solution of this acid that gives cola drinks their typical flavor. Hydrochloric acid is another example:
A base is the opposite of an acid, in that it is an ionic compound, which, when dissolved in water, removes hydrogen ions from solution. Bases, or alkaline substances, have a slippery feel on the skin. They have a caustic action on living tissue by converting the fats in living tissue into a water-soluble substance. A similar reaction is used to make soap by mixing a strong base with fat. This chemical reaction gives soap its slippery feeling. Bases are also used in alkaline batteries. Weak bases have a bitter taste—for example, the taste of broccoli, turnip, and cabbage. Many kinds of bases release a group of hydrogen ions known as a hydroxide ions, or an OH-group. This group is composed of an oxygen atom and a hydrogen atom bonded together, but with an additional electron. The hydroxide ion is negatively charged; therefore, it will remove positively charged hydrogen ions from solution. A very strong base used in oven cleaners is sodium hydroxide, NaOH. Notice that ions that are free in solution are always written with the type and number of their electrical charge as a superscript.
Acids and bases are also spoken of as being strong or weak (Outlooks 2.2). Strong acids (e.g., hydrochloric acid) are those that dissociate nearly all of their hydrogens when in solution. Weak acids (e.g., phosphoric acid) dissociate only a small percentage of their hydrogens. Strong bases dissociate nearly all of their hydroxides (NaOH); weak bases, only a small percentage. The weak base sodium bicarbonate, NaHCO3, will react with acids in the following manner:
NaHCO3 + HCl → NaCl + CO2 + H2O
Notice that sodium bicarbonate does not contain a hydroxide ion but it is still a base, because it removes hydrogen ions from solution.
The degree to which a solution is acidic or basic is represented by a quantity known as pH. The pH scale is a measure of hydrogen ion concentration (figure 2.14). A pH of 7 indicates that the solution is neutral and has an equal number of H-ions and OH-ions to balance each other. As the pH number gets smaller, the number of hydrogen ions in the solution increases. A number higher than 7 indicates that the solution has more OH- than H+. Pure water has a pH of 7. As the pH number gets larger, the number of hydroxide ions increases. It is important to note that the pH scale is logarithmic—that is, a change in one pH number is actually a 10-fold change in real numbers of OH-or H+. For example, there is 10 times more H+ in a solution of pH 5 than in a solution of pH 6 and 100 times more H+ in a solution of pH 4 than in a solution of pH 6.
Salts are ionic compounds that do not release either H+ or OH-when dissolved in water; thus, they are neither acids nor bases. However, they are generally the result of the reaction between an acid and a base in a solution. For example, when an acid, such as HCl, is mixed with NaOH in water, the H+ and the OH-combine with each other to form pure water, H2O. The remaining ions (Na+and Cl- ) join to form the salt NaCl:
The chemical reaction that occurs when acids and bases react with each other is called neutralization. The acid no longer acts as an acid (it has been neutralized) and the base no longer acts as a base.
As you can see from figure 2.14, not all acids or bases produce the same pH. Some compounds release hydrogen ions very easily, cause low pHs, and are called strong acids. Hydrochloric acid (HCl) and sulfuric acid (H2SO4) are strong acids (figure 2.15a). Many other compounds give up their hydrogen ions grudgingly and therefore do not change pH very much. They are known as weak acids. Carbonic acid (H2CO3) and many organic acids found in living things are weak acids. Similarly, there are strong bases, such as sodium hydroxide (NaOH) and weak bases, such as sodium bicarbonate—Na(HCO3)-.