Ammonia is highly toxic to aquatic organisms, particularly fish, at concentrations as low as 0.2 mg/L. It can cause gill damage, reduce growth rates and lead to mass mortality events (“fish kills”). Thus environmental protection is a major driver for ammonia monitoring.

As discussed above (in “How does ammonia enter water?”), various sources of runoff and industrial effluent can carry ammonia into natural waterways. Monitoring ammonia in surface water can point to sources of pollution nearby and inform management practices to protect water quality and aquatic life.

Another driver is public health, achieved by protecting drinking water sources and monitoring treatment processes. Naturally-occurring bacteria in soil and water can convert ammonia into nitrate (NO₃^-) through nitrification. Nitrate in excess of 50 mg/L in drinking water is a known factor in “blue baby syndrome” in infants (though disputed as a direct cause). “Blue baby syndrome” is so named for the bluish skin tone that indicates the blood’s ability to carry oxygen has been compromised, which can obviously have severe health consequences.

The efficiency of water treatment processes–in both drinking water and wastewater applications–can be affected by ammonia concentrations.

Ammonia is the primary nutrient that allows bacteria to break down organic matter. For this reason it is a critical component of secondary wastewater treatment processes, particularly nitrification. During nitrification, bacteria convert ammonia to nitrite and then nitrate. High ammonia measurements can indicate factors impeding the process such as insufficient aeration, inadequate sludge age, toxic shock to nitrifying bacteria or improper temperature/pH conditions. Monitoring ammonia allows operators to assess whether complete nitrification has occurred and make adjustments to improve process efficiency, ensure complete treatment and meet discharge permits. Monitoring ammonia to meet discharge requirements ensures natural waters remain habitable for aquatic life and prevents damage to ecosystem health.

The relationship between ammonia and chlorine in drinking water treatment is complex. The chloramination process involves adding calculated doses of chlorine and ammonia to water to form chloramines. The formation of monochloramine, the desired disinfectant, requires a chlorine-to-ammonia ratio of around 4.5:1 (by weight). Excess or insufficient ammonia can cause improper chloramine formation, which can reduce disinfection effectiveness or lead to the formation of undesirable disinfection byproducts. Any ammonia that does not combine with chlorine to form chloramines is referred to as free ammonia. Total ammonia refers to all ammonia ions present in the water in any form, including free ammonia, ammonium and the ammonia component of chloramines.

If too much chlorine is added relative to ammonia, free chlorine may form instead of chloramines, leading to breakpoint chlorination. This also reduces disinfection efficiency and increases the risk of harmful disinfection byproducts like trihalomethanes (THMs). Monitoring ammonia during chloramination helps prevent breakpoint chlorination and its side effects. If too much ammonia is added relative to chlorine, the excess ammonia can promote the growth of nitrifying bacteria, which convert ammonia into nitrite and nitrate. This can lead to loss of disinfectant residual, biofilm growth, and water quality issues in the distribution system.

Many utilities are required to monitor ammonia to comply with drinking water quality regulations. Regulations aim to minimize contaminants and limit taste and odor issues caused by excess ammonia or improper chloramine formation. Ammonia monitoring allows for efficient dosing of both chlorine and ammonia during disinfection, reducing unnecessary chemical use and saving costs through treatment optimization.

Ion-selective Electrodes 

Ion-selective electrodes (ISEs) have become a standard tool for continuous ammonia monitoring. There are two types of ISEs used in ammonia monitoring: those that measure ammonia (NH₃) and those that measure ammonium (NH₄+). 

Ammonia ISEs

ISEs that directly measure ammonia use a hydrophobic gas-permeable membrane that allows only dissolved NH₃ to diffuse through. Inside the membrane, the NH₃ dissolves in an ammonium chloride solution (NH₄Cl) of a specific concentration, changing the pH of the solution. This pH change is measured by an internal pH electrode and used to calculate an ammonia concentration. As with all pH measurements, a temperature sensor is also contained within the probe for temperature compensation.

Advantages of this type of ISE include that there are no reagents required (as there are with Nessler’s method, for example), the probes are simple to maintain relative to colorimetric analyzers, and probes from different manufacturers can have surprisingly wide ranges of detection (0.1 to 1000 mg/L NH₃-N). However, interference from volatile amines and fouling in complex sample types (like sludge) are a real challenge. Due to its volatile and reactive nature, ammonia concentrations can change rapidly in a sample, so sample handling is another challenge. For best results, samples are alkalinized to a pH >11 for complete conversion to ammonia prior to making the measurement.

Ammonium ISEs

The second ISE type measures ammonium rather than ammonia. Ammonium ISEs measure the concentration of ammonium ions (NH₄+) as a difference in the electrical potential between an ammonium-sensitive electrode and a reference electrode. The relationship between concentration and potential is defined by the Nernst equation. Most ammonium sensors contain both the sensing electrode and reference electrode in a single unit. 

Within the ammonium sensor, the electrode in the ammonium half-cell consists of a glass bulb that is only permeable to NH₄+ ions, which gives the sensor ion selectivity (thus making it an ion-selective electrode, or ISE). The solution inside the bulb is at a constant ion concentration. Therefore, a potential difference develops across the glass due to the concentration difference of NH₄+ ions on either side of the bulb. To measure this potential, a small amount of current must pass through, so the glass must be permeable to the selected ion.

The reference electrode, or reference junction, provides a stable value for comparison to the ammonium electrode. The reference electrode consists of a silver wire coated in silver chloride sitting in an internal reference solution. Unlike the ammonium-sensitive electrode, however, the reference system does not include an ion-selective glass bulb. Instead, it has a porous frit allowing reference solution migration to the analyte solution, the solution being measured. This is necessary to ensure electrical continuity of the measurement system. When kept at the correct levels, the internal reference solution maintains a consistent electrical potential regardless of changes in ammonium outside the sensor.

The ammonium sensor yields a voltage of the difference in electrical potential generated by the two electrode systems within the sensor. For this reason, ISEs can also be referred to as electrochemical or potentiometric sensors because their measurements are based on electrical potential. The Nernst equation gives us the ion concentration (Q) from the measured voltage difference. Because the glass bulb is selectively permeable to ammonium ions, and we know the potential at which the reference electrode sits, we can directly relate the potential difference to ammonium ion concentration.

As temperature is required for the Nernst equation, the measured potential is temperature dependent, so accurate ammonium measurement almost always requires accurate temperature measurement as well.

This type of ISE operates across the full pH range, so there is no need for sample prep or alkalinization of the sample. Like all ISEs, the necessity of the porous frit and slow leakage of the reference solution gives them a relatively short lifespan. Portable ISEs may contain a single or double reference junction. Another design difference is whether the reference junction is refillable. Refillable sensors have an opening to pour new reference solution into the system, so the entire sensor doesn’t need to be replaced. Nonrefillable reference junctions typically last longer between replacements but require replacing the entire sensing element when reference solution runs out.

In both types of ISEs the electrode response follows a Nernst equation for ammonium measurement:

E_cell = E⁰ - (RT/nF)lnQ

Where

• E_cell = potential of the ammonium electrode
• E⁰ = potential of the reference electrode
• R = Gas Law constant
• T = temperature (in Kelvin)
• n = ionic charge (for NH₄+, n = 1)
• F = Faraday’s constant and
• Q = reaction quotient

Ammonium ISEs in particular are subject to interference from other univalent cations, the most challenging being potassium (K+). Because of the potential for sodium interference, these ISEs are not as well suited to brackish or ocean environments. With this type of ISE, it is necessary to measure the sample’s pH with an independent probe to extrapolate the concentration of ammonia in the sample.

Gas-Sensing Membrane Probes

Gas-sensing membrane probes are designed to work in air/gaseous environments. To assess ammonia in water, they can be placed in a sealed container above the surface of a water sample (called a “headspace analysis”). The dissolved ammonia creates a barrier between the water and the headspace. The probe measures the gaseous ammonia concentration in the headspace, and Henry’s Law can be used to calculate the concentration of ammonia in the water.

The probes themselves are electrochemical. They typically use a metal-oxide semiconductor (usually tin dioxide or tungsten oxide) that changes its electrical conductivity when exposed to ammonia gas molecules. When ammonia molecules come into contact with the sensor’s surface, they undergo a chemical reaction with the metal oxide, causing electrons to be either donated to or withdrawn from the semiconductor material. This change in electron density alters the electrical resistance of the sensing material, which is then measured by the probe's circuitry and converted into a corresponding ammonia concentration reading. Many modern probes also incorporate temperature and humidity compensation to ensure accurate readings across varying environmental conditions, as well as selective membranes that help filter out interfering gases while allowing ammonia molecules to pass through.

Colorimetric 

Colorimetric analyzers contain a colorimeter set to detect the intensity of colored light within a specific wavelength range (typically 630-660 nm). Adding reagents to the sample produces color; the intensity of these colors indicates the concentration of the selected parameter (free ammonia or monochloramine) in the sample. Reagents added to the sample serve two purposes. Some adjust the pH of the sample to transform ammonium ions into free ammonia. Some transform free ammonia into monochloramine. Other reagents react with either free ammonia or monochloramine to produce a color specific to that substance. The colorimeter then measures the color intensity and determines the concentration of the substance as compared to a reference solution.

Different colorimetric processes exist. Some colorimetric analyzers employ the Berthelot reaction to measure ammonia while others use the Phenate method. Both rely on the fundamental reaction of ammonia with reagents to form a colored compound whose intensity is then measured by the analyzer. The main difference lies in which reagents are added and how environmental conditions may affect the process.

Spectrophotometry

Spectrophotometers measure the absorption of ultraviolet (UV) light waves as they pass through a sample to determine the concentration of ammonia or monochloramine present. As with the colorimetric method, reagents are added to the sample to transform compounds present in the sample into the desired form (ammonia or monochloramine). These compounds absorb some of the UV light as it passes through the sample. The analyzer compares the amount of initial UV light emitted to the amount of UV light received through the sample. The instrument then calculates the concentration of ammonia or monochloramine in the sample based on this difference. At times, substances in the sample may interfere with or otherwise effect the absorption of UV light. Some spectrophotometers may employ multiple UV wavelengths to account for high-turbidity samples or other characteristics that could affect measurements.

In-Situ ammonium sensors are ion-selective electrodes (ISEs). An ISE measures the difference in electrical potential between a stable reference electrode and an electrode sensitive to the ion being measured. In the case of ammonium, the difference in electrical potential yields the concentration of ammonium ions (NH₄⁺), which is then converted to an ammonium value via the Nernst equation, seen below. 

E = E⁰-(RT/nF)InQ

In-Situ ammonium sensors include the ammonium electrode and reference electrode as half-cells in one unit. The ammonium electrode sits within a potassium chloride (KCl) solution, which conducts electricity between the electrode and a glass bulb selectively sensitive to ammonium ions. The glass bulb changes voltage proportionally to the concentration of ammonium ions in the analyte solution.

The reference electrode, on the other hand, contains a replaceable reference junction, which is porous to allow electrical contact with the sample. The reference electrode is filled with a saturated mixed potassium chloride/silver chloride (KCl/AgCl) solution, which maintains a stable value regardless of the ammonium of the analyte solution when kept at the necessary level. 

In any ammonium sensor, the reference junction must contain a porous frit in order to maintain electrical continuity of the measurement system. This allows a small but consistent flow of reference solution to pass out of the junction. Over time, this will change the concentration of the internal reference solution, interfering with its ability to maintain a consistent electrical potential within the reference half-cell. This is a necessary process, as slowing diffusion out of the reference junction can cause unstable ammonium readings and slow sensor response times to changes in ammonium. However, when ammonium readings drift or crash, it is usually due to issues with the reference solution and the frit.  

In-Situ’s line of ChemScan mini Analyzers use different methods based on the form or forms of ammonia they measure. ChemScan mini Analyzers for free and total ammonia both employ the colorimetric method, using reagent-assisted optical absorbance with sample zero correction. There are three options of mini Analyzer to measure ammonia: one measures free ammonia, one measures total ammonia and one measures total ammonia in low-ammonia environments. The mini ChlorAm Analyzer also uses the colorimetric method, tracking reagent-assisted optical absorbance at 660 nm with sample blank correction. This analyzer, designed for the chloramination process, monitors free ammonia, total ammonia, monochloramine and the chlorine-to-ammonia ratio. In-Situ’s ChemScan UV Analyzers use spectrophotometry based in the Berthelot reaction to measure free ammonia, total ammonia, monochloramine and other parameters, depending on the analyzer model. They analyze samples via UV absorbance technology using pattern recognition of spectral data and employ multiple wavelengths of UV light to account for high turbidity and other characteristics in a sample that could influence UV absorption.