ZingPath: pH, Acids, and Bases

The term pH stands for "power of hydrogen" and is a measure of the concentration of hydrogen ions (H+) in a solution, measured in terms of molarity (M), or moles per liter. The pH value is defined as the negative base 10 logarithm of the molar hydrogen ion concentration, pH = .

However, because hydrogen ions associate with water molecules, the hydronium ion is commonly used.

The greater the molar concentration of hydronium ions, the larger the logarithm and the lower the pH. Acidic solutions, which have high hydronium ion concentrations, have low pH values.

For example, coffee has a pH of about 5, which translates to a . Tomato juice, with a pH of 4, has ten times as many hydronium ions per volume: .

However, the chemical dynamics of acidic and basic solutions are due to the relative concentrations of both hydronium and hydroxide (OH-) ions. A neutral solution (pH = 7) contains equal concentrations of the two, and a base (pH > 7) has more hydroxide than hydronium ions. Hydroxide ion concentration is reflected by pOH, which is the negative base 10 logarithm of the molar concentration of hydroxide. The sum of the pH of an aqueous substance at 25°C and its pOH always equals 14. In other words, the product of the ion concentrations is constant, . (In pure water, the concentrations of hydroxide and hydronium ions are both Knowing the pH of a substance provides a path for determining its hydronium ion concentration, pOH, hydroxide ion concentration, and solute concentration.

A solute may dissociate in water to form hydronium or hydroxide ions. For example, sodium hydroxide (NaOH) dissociates into Na+ and OH- ions in water. Its ionization equation tells us the molar concentrations of the resulting ions when a known amount of the solute is dissolved. For example, if moles of NaOH are dissolved in exactly 1 liter of water, the molar concentration of each resulting ion will be . From this information, we can determine that the solution's pOH is 5. The resulting molar concentration of hydronium ions of the solution is , resulting in a pH of 9.