Skip to main content
Medicine LibreTexts

2.4: Acids and Bases

  • Page ID
    11104
  • Learning objectiveS

    • Define and differentiate the terms acid and base
    • Define the terms pH, neutral, acidic, and basic (or alkaline)
    • Define the term buffer, and compare the response of a regular solution with a buffer solution to the addition of acid or base

    An acid is a substance or compound that releases hydrogen ions (H+) when in solution. In a strong acid, such as hydrochloric acid (HCl), all hydrogen ions (H+), and chloride ions (Cl-) dissociate (separate) when placed in water and these ions are no longer held together by ionic bonding. In a weak acid, such as carbonic acid (H2CO3), only some of the ions dissociate into hydrogen ions (H+) and bicarbonate ions (HCO3-), while others are still held together by ionic bonding.

    A base is a substance that releases hydroxyl ions (OH-) when in solution. The hydroxyl ions (OH-) released will combine with any hydrogen ions (H+) in the solution to form water molecules (OH- + H+ = H2O), so we can also define a base as a substance that takes or accepts hydrogen ions (H+) already present in the solution.

    Sodium hydroxide (NaOH) is a strong base because when placed in water, it dissociates completely into sodium ions (Na+) and hydroxyl ions (OH-), all of which are now released and dissolved in water.

    Acids, bases and salts, dissociate (separate) into electrolytes (ions) when placed in water. Acids dissociate into H+ and an anion, bases dissociate into OH- and a cation, and salts dissociate into a cation (that is not H+) and an anion (that is not OH-).

    clipboard_efda3d4487b86da98cf7af7d73d1f40df.png Figure \(\PageIndex{1}\) (a) In aqueous (watery) solution, an acid dissociates into hydrogen ions (H+) and anions. Every molecule of a strong acid dissociates, producing a high concentration of H+. (b) In aqueous solution, a base dissociates into hydroxyl ions (OH) and cations. Every molecule of a strong base dissociates, producing a high concentration of OH.

    When an acid and a base react (combine) releasing equal quantities of H+ ions and OH- ions, neutralization results. H+ ions and OH- ions combine (neutralize each other) to regenerate water.

    Concepts, terms, and facts check

    Study Questions Write your answer in a sentence form (do not answer using loose words)

    1. What is an acid?
    2. What is a base?

    pH is a unit of measurement of the concentration of hydrogen ions (H+) and hydroxyl ions (OH-) in an aqueous (water) solution. Pure water is said to be neutral with a pH of 7, because there are very few H+ and OH- ions in equal concentrations (only 1 in 10,000,000 water molecules dissociate to H+ and OH-, which gives a pH of 7). Adding equal amounts of H+ and OH- to water will also be neutral with a pH of 7, because most of these ions combine to form water molecules and the remaining H+ and OH- ion concentration is equal and very low.

    When H+ concentration is higher than OH- concentration, the solution is acidic, and the pH of the solution is indicated with a number below 7. Saliva, coffee, lemon juice, tomato juice, and the acid in a battery are all acidic, so in all of them the concentration of H+ is higher than the concentration of OH-. The more H+ in a solution the more acidic and the lower is its pH (See Figure \(\PageIndex{2}\) below).

    When H+ concentration is lower than OH- concentration, the solution is basic or alkaline, and the pH of the solution is indicated with a number above 7. Blood, baking soda, ammonia and bleaches are all basic, so in all of them the concentration of H+ is lower than the concentration of OH-.

    clipboard_e3d38045a88c1f51b6d89fda066432410.png

    Figure \(\PageIndex{2}\) pH of various solutions. The lower the pH, the more
    hydrogen ions (H+) the solutions has. The higher the pH, the less hydrogen ions the solution has.

    Concepts, terms, and facts check

    Study Questions Write your answer in a sentence form (do not answer using loose words)

    1. What is pH?
    2. What is a neutral solution?
    3. What is an acidic solution?
    4. What is a basic (or alkaline) solution?

    Chemical reactions in the body, the food we eat, medication we take, and the effects of some diseases can add or remove hydrogen or hydroxyl ions in or from our body fluids. Levels of these ions, especially H+ since body cells are constantly producing H+ as a waste product of cell activity, must be maintained within a normal range (slightly alkaline pH between 7.35 and 7.45,). Then, all cells in our body depend on homeostatic regulation of acid-base balance to maintain pH within optimal living conditions.

    There are several homeostatic mechanisms to maintain pH within optimal conditions. It can be regulated by the internal availability of substances (chemicals), by adjusting breathing rate, and by eliminating chemicals in urine. Chemical buffers in the body are substances that can absorb extra hydrogen ions preventing a change in pH. For example, during exercise muscle cells can produce excess lactic acid, which increases hydrogen ions (acids release hydrogen ions). These hydrogen ions tend to make our body fluids more acidic, but chemical buffers in the body absorb them preventing a pH change. See below a table comparing what happens when acid or base are added to a plain solution (no buffer), or to a solution that absorbs hydrogen ions or hydroxyl ions (buffer).

    Table \(\PageIndex{1}\) Comparison between a regular solution without buffering properties and a buffer solution with buffering properties
    Regular solution (without buffering properties) Buffer solution (with buffering properties)
    When acid is added, it releases hydrogen ions and... the pH drops and the solution becomes more acidic the pH does not drop
    When base is added, it absorbs hydrogen ions (or releases hydroxyl ions) and... the pH rises and the solution becomes more basic (alkaline) the pH does not drop

    Bicarbonate, phosphates, and proteins work as a chemical buffer in our body fluids. They absorb extra hydrogen ions or extra hydroxyl ions released from the things we make or eat.

    Concepts, terms, and facts check

    Study Question Write your answer in a sentence form (do not answer using loose words)

    1. What is a buffer?
    2. What happens to the pH of a plain solution when acid is added to it?
    3. What happens to the pH of a buffer solution when acid is added to it?