The pH scale measures the acidity or alkalinity of a solution, ranging from 0 to 14, with 7 being neutral. The term pH refers to the concentration of hydrogen ions (\(H^+\)) present. When a solution’s pH is “brought up,” it moves away from acidity (below 7) toward neutral or into the alkaline range (above 7). This adjustment is common in chemistry, biology, and environmental management to maintain chemical balance.
The Core Chemical Mechanism
Increasing the pH of a solution fundamentally reduces the concentration of free hydrogen ions (\(H^+\)). A substance that raises pH, known as a base or an alkaline compound, achieves this effect through one of two primary chemical actions.
The first mechanism involves releasing hydroxide ions (\(OH^-\)) into the solution. These \(OH^-\) ions combine with free \(H^+\) ions to form a neutral water molecule (\(H_2O\)). This removal of \(H^+\) directly lowers their concentration, causing the pH value to rise.
The second mechanism involves the base directly absorbing, or accepting, the free \(H^+\) ions already present. This action, characteristic of a Brønsted-Lowry base, decreases the available \(H^+\) concentration. Both methods shift the solution toward alkalinity.
Common Alkaline Compounds
Compounds used to raise pH are classified based on their chemical structure and reactivity. Hydroxides, such as sodium hydroxide (lye) or calcium hydroxide (slaked lime), are strong bases that readily dissociate in water to release \(OH^-\) ions. While highly effective, these strong bases must be handled carefully due to their caustic nature and rapid reaction speed.
Carbonates and bicarbonates, including sodium bicarbonate (baking soda) and calcium carbonate (limestone), are generally weaker bases. They consume \(H^+\) ions to form carbonic acid and water. These compounds are often preferred as buffers because they resist sudden pH changes, allowing for a more stable adjustment.
Ammonia-based compounds, like ammonium hydroxide, also function as bases by reacting with water to produce hydroxide ions. These are frequently used in commercial and industrial applications. The choice depends on the desired speed of reaction and the need for sustained buffering capacity.
Biological pH Regulation
The human body maintains the pH of blood and interstitial fluid within a narrow, slightly alkaline range, typically between 7.35 and 7.45. This stability is maintained primarily by the buffering system, most importantly the bicarbonate buffer system, which uses a carbonic acid-bicarbonate pair to neutralize excess acid.
When metabolic processes generate excess acid, the bicarbonate ion (\(HCO_3^-\)) acts as a base, accepting extra \(H^+\) ions to form carbonic acid (\(H_2CO_3\)). This conversion prevents a significant drop in pH. The body manages this buffer system using the lungs and kidneys.
The lungs provide a rapid response by controlling the amount of carbon dioxide (\(CO_2\)) exhaled. Since \(CO_2\) is in equilibrium with carbonic acid, increasing the breathing rate reduces the acid load and raises the pH within minutes. The kidneys offer a slower, long-term mechanism by adjusting \(H^+\) ion excretion into the urine and regenerating bicarbonate ions.
Adjusting pH in External Systems
Raising pH is a common intervention in many external environments, particularly soil and water systems. In agriculture, soil acidity often increases due to heavy rainfall, organic matter decay, and nitrogen-based fertilizers. When soil pH drops below the ideal range (typically 6.0 to 7.0), essential plant nutrients like phosphorus and calcium become less available for uptake.
Farmers raise soil pH by applying agricultural lime, a finely ground form of calcium carbonate or calcium hydroxide. This compound neutralizes acidic components, improving nutrient availability and creating a healthier environment for beneficial microbes. Effectiveness depends on the powder’s fineness and thorough mixing, often taking several months for a full reaction.
In controlled water environments, such as aquariums and swimming pools, raising pH prevents corrosive conditions and maintains aquatic health. Biological processes in aquariums naturally generate acids, causing a slow pH drop. To counteract this, alkaline buffers like sodium bicarbonate or crushed coral (calcium carbonate) are added. The goal is to raise the pH and increase the water’s alkalinity, which resists future pH changes and prevents sudden shifts known as pH shock.