PH lab

Water occasionally behaves as an ionic compound. A very small number, about 2 in 1 billion water molecules, dissociate as shown below, to form H+ ions, also called protons, and OH^- or hydroxide ions.

H₂O        H⁺    +    OH^-

hydrogen ion    hydroxide ion

When water dissociates, an equal number of H⁺ and OH^- ions are produced. 

Ionic compounds dissolve readily in water. As they dissolve in water, the positive and negative ions separate or dissociate. For example, when table salt is dissolved in water, it dissociates into sodium and chloride ions:

NaCl    →    Na⁺    +    Cl^-

Some compounds, when dissolved in water, may produce either H⁺ or OH^- ions.  Substances that produce H⁺ ions when dissolved in water are acids.  An acid as a substance that increases the number of H⁺ ions in a solution. An example of such a compound is hydrogen chloride (HCl),  which dissociates into a hydrogen ions and chloride ion. HCl dissociation is shown below.

HCl    →    H⁺    +    Cl^-

Substances that produce OH^- ions when dissolved in water are bases. When dissolved in pure water, base molecules will accept a hydrogen ion from a water molecule, leaving behind an increased concentration of OH⁻ ions in the solution. Consequently, we can define a base as a substance that decreases the number of H⁺ ions in a solution. For example, when sodium hydroxide is dissolved in water will dissociate in sodium ions and hydroxide ions.  

NaOH    →    Na⁺     +    OH^-

However, when ammonia is placed in water, it will pick up a hydrogen ion from water and form ammonium and hydroxide ions.

NH₃ + H₂O → NH₄⁺ + OH^-

The pH of a solution is a measure of the molar concentration of hydrogen ions in the solution and it measures the acidity or basicity of the solution. The numerical value is defined as the negative base 10 logarithm of the molar concentration of hydrogen ions. 

pH = -log₁₀[H⁺]

The pH scale ranges from 0 to 14.   A solution of pH =7 is neutral (the numbers of H⁺ and OH^- ions are equal), a solution of pH lower than 7 is acidic (there are more H⁺ ions than OH^- ions)and a solution of pH greater than 7 is alkaline or basic (there are more OH^- ions than H⁺ ions). The pH of a solution and the H⁺ concentration are inversely proportional.  The number of H⁺ ions increases as the pH number decreases. Likewise, the number of H⁺ ions decreases as the pH number increases. A log scale is used with pH to avoid using very small numbers, such as 0.00000001.  These very small concentrations can be more easily expressed as logarithms. 

Because the pH scale is a logarithmic scale, the difference between two successive numbers on the pH scale represents a ten-fold difference in the H⁺ ion concentration (log of base 10). For example, a solution with a pH of 2 has 10 times more H⁺ ions as a solution with a pH of 3. A solution with a pH of 2 has 100 times more H⁺ ions as a solution with a pH of 4.  Weak acids and bases do not dissociate readily into their ions, while strong acids and bases dissociate almost completely. Adding a strong acid or a strong base to a solution will have a greater impact on pH than adding the equivalent amount of a weak acid or a weak base. A buffer is a solution whose function is to minimize the change in pH when a base or an acid is added to the solution. Most buffers in the human body consist of a buffer pair, with a weak acid (which releases H⁺ ions) and a weak base (which binds H⁺ ions). If an acidic solution is added to a buffered solution, the buffer combines with the extra H⁺ ions and helps to minimize the change in pH. If a basic solution is added to a buffered solution, the buffer will release H⁺ ions to help minimize the change in pH.