A proton combined with a number of water molecules. It is often written as H3O+ and called the hydronium ion. However, this species is best considered as an excess proton on a tetrahedral group of four water molecules and so would be designated as H9O4+. For simplicity, it is most commonly written as H+(aq).
Since it is formed by the self-ionization of water, the hydrogen ion is present in all aqueous solutions. This formation also means that H+(aq) is always found in the company of the hydroxide ion, OH−(aq). The equilibrium relationship between the concentrations of these two species is a very important property of water. See also
The H+(aq) and OH−(aq) concentrations in pure water are equal to each other with a value of 10−7 mole/liter. Any aqueous solution with this concentration of H+(aq) is called a neutral solution. If the H+(aq) concentration is greater than 10−7 mole/liter, the solution is called acidic. Basic solutions are those in which the H+(aq) concentration is less than 10−7 mole/liter. As the H+(aq) concentration increases the OH−(aq) concentration must decrease, and vice versa. In the most straightforward system, acids are substances that can donate an H+(aq), and bases are substances that can accept one. See also Acid and base.
Hydrogen ion concentration determines the course of many chemical reactions that occur in living organisms and in the chemical industry. The control of hydrogen ion concentration is achieved in living organisms and in the laboratory by buffer systems. These are chemical mixtures designed to resist change in hydrogen ion concentration. See also Buffers (chemistry).
Another property of the H+(aq) is important in both theoretical and practical ways. H+(aq) is the best conductor of electricity of any ion in aqueous solution. Its conductance at 77°F (25°C) is almost five times as large as the next-most-conducting ion. See also Electrolytic conductance.
The hydrogen ion concentration can vary over fourteen powers of 10. To avoid dealing with such exponentials, the concept of pH is used. Since all aqueous solutions contain both hydrogen ion and hydroxide ion, it is possible to define all degrees of acidity and basicity on the pH scale. See also pH.
Two general methods are used for the determination of hydrogen ion concentrations. For relatively crude work, colorimetric methods are commonly used. These methods depend on the fact that certain natural and synthetic dyes have colors that depend on the hydrogen ion concentration. At times, paper is impregnated with such an indicator. In most precise work, a potentiometric method is used for the determination of hydrogen ion concentration. This method depends on an electrode whose potential is sensitive to hydrogen ion concentration. The only electrode commonly in use for practical pH measurements is the glass electrode. See also Acid-base indicator; Colorimetry; Titration.
