A substance that conducts electricity in solution form is called electrolyte.

Its example is sodium chloride (NaCI). Many fluids contain strong electrolytes hat break down (ionize) into ions. Most acids and bases are electrolytes.


n acid is a substance that releases hydrogen ions (I-1) when dissolved in ater. A hydrogen atom without its electron is only a proton. Thus an acid can be escribed as a proton donor:

HCI _________________ H + CI

Dne molecule of hydrogen chloride dissolves in water to produce hydrochloric acid. It dissociates into one hydrogen ion and one chloride ion.

Base (or alkali)

A substance that releases hydroxyl ions (OH’) when dissolved in water is called base. One molecule of sodium hydroxide dissolves in water. It produces one sodium ion and one hydroxyl ion. When a. base dissolves in water, it removes free protons (H”) from the water. Therefore, it is a proton acceptor.

NaOH______________ Nil + Cl”


The negative log (antilog) of hydrogen ion concentration is called pH. Hydrogen and hydroxyl ions affect the chemical reactions in the cells. Therefore. the concentrations of these ions in body fluids are important. Higher the concentration of hydrogen ions (He), the more acidic the solution is. Higher the concentration of hydroxyl (OH-) ions, the more basic (alkaline) the solution is. A neutral solution has equal number of hydrogen ion and hydroxyl ions. The pH scale measures acidity and alkalinity. It runs from 0 to 14.

  • Acidic solutions have a pH less than 7.
  • The basic (alkaline) solutions have a pH above 7.
  • A pH of 7 is neutral.

Each whole number on the pH scale represents a tenfold change (logarithmic) in acidity. Therefore, a solution with a pH of 3 is 10 times is more acidic than a solution with a pH of 4. A pH of 9 is 10 times more basic than a pH of 8.


The substance which resists change in pH is called buffer. Stable internal environment is maintained in animals. It requires a relatively constant pH of the body fluids. A strong acid or base can destroy cell stability. Similarly opposite charges attract each other and form the ionic bond.



Change in pH may be destructive. The fluid systems of most animals contain chemicals. These substances help to regulate the acid base balance. These substances are called buffers. They resist changes in pH. They accept H ions present in excess. They donate H ions when their amount is reduced.

The most important buffers are the bicarbonates, phosphates, and organic molecules, such as amino acids and proteins. The carbonic acid bicarbonate ion system is important in buffering the blood of many vertebrates:

H2CO3 __________________ H+ + HCO3

Carbonic acid dissociates to form hydrogen ion and bicarbonate ion. In this example, if H. ions are added to the system, they combine with HCO3 to form H2CO3 (the reaction goes to the. left). This reaction removes H+ ions. So it does not allow changing the pH. If excess OH” ions are added, they react with the H’ ions to form water. So more H2CO3 ionize and replace the H+ ions. Thus the reaction goes to the right. Again, pH stability is maintained.

The relationship between hydrogen ion concentration, Hydroxyl
ion concentration and pH


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