The acidity or alkalinity of water, measured as pH, significantly influences the chemical form and resulting toxicity of nitrogen compounds. Ammonia is a common byproduct of metabolism and the decomposition of organic matter in aquatic systems. This nitrogen compound exists in water in two forms: un-ionized ammonia ($\text{NH}_3$) and the ionized form, ammonium ($\text{NH}_4^+$). Understanding the relationship between pH and these two forms is important for maintaining healthy aquatic environments.
The Relationship Between pH and Ammonia’s Form
In any aqueous solution, ammonia and ammonium exist in a state of chemical equilibrium, constantly converting back and forth. This balance is determined by the concentration of hydrogen ions ($\text{H}^+$), which the pH scale measures. The point at which the concentrations of $\text{NH}_3$ and $\text{NH}_4^+$ are equal is known as the pKa value, which is approximately 9.25 for this system.
The water’s pH dictates which form dominates the total ammonia nitrogen (TAN). When the pH is below 9.25, the water is more acidic, meaning high concentrations of $\text{H}^+$ ions bond with $\text{NH}_3$ molecules. This pushes the balance toward the relatively harmless $\text{NH}_4^+$ form. Conversely, in alkaline water with a pH above 9.25, the lower concentration of $\text{H}^+$ ions favors the creation of the un-ionized $\text{NH}_3$ form.
Why Ammonia Toxicity Depends on pH
The chemical shift is significant due to the vastly different biological impacts of the two forms. The un-ionized ammonia ($\text{NH}_3$) is the highly toxic form, while the ionized ammonium ion ($\text{NH}_4^+$) is relatively non-toxic to most aquatic life. This difference is a direct result of their molecular structure and electrical charge.
Un-ionized ammonia is lipid-soluble and carries no electrical charge, allowing it to easily pass through the biological membranes of aquatic organisms, such as the gills of fish. Once inside the bloodstream, $\text{NH}_3$ disrupts the fish’s ability to excrete metabolic waste, causing a toxic buildup in the blood and tissues. This accumulation impairs the central nervous system and reduces the blood’s capacity to carry oxygen.
In contrast, the ammonium ion ($\text{NH}_4^+$) carries a positive electrical charge, which prevents it from easily diffusing across cell membranes. Because it cannot readily enter the organism’s system, $\text{NH}_4^+$ presents a much lower risk to aquatic inhabitants. Therefore, the total toxicity of ammonia is determined almost entirely by the small fraction of un-ionized $\text{NH}_3$ present, which is directly proportional to the $\text{pH}$ level.
Practical Measurement and Management of pH and Ammonia
Accurately measuring and controlling these parameters is a routine requirement for managing water quality in environments like aquaculture systems, home aquariums, or wastewater treatment facilities. The first step involves determining the Total Ammonia Nitrogen (TAN), which is the combined concentration of both the toxic $\text{NH}_3$ and the non-toxic $\text{NH}_4^+$. Standard methods for measuring TAN include the use of colorimeters or spectrophotometers.
Since analytical methods measure only the total concentration, the $\text{pH}$ must be measured simultaneously to calculate the amount of toxic $\text{NH}_3$. This is typically done using a $\text{pH}$ meter or test kit. Once the TAN and $\text{pH}$ are known, specialized charts or equations, often factoring in water temperature, are used to determine the exact percentage of un-ionized ammonia.
The most effective management strategy for mitigating ammonia toxicity involves controlling the water’s $\text{pH}$ level. If a spike in TAN occurs, lowering the $\text{pH}$ is a primary mitigation tool because it immediately shifts the equilibrium away from the toxic $\text{NH}_3$ form toward the safer $\text{NH}_4^+$ ion. For instance, in fish cultivation, water $\text{pH}$ is often maintained in a neutral range of 6.5 to 7.5. If the $\text{pH}$ rises, a weak acid may be channeled into the water to balance the $\text{pH}$ and convert the ammonia to its less harmful form.