Ammonia & Ammonium Sensor

Ammonia in water is not one interchangeable number. Unionized ammonia (NH3), ammonium ion (NH4+), total ammonia nitrogen (TAN) and a result reported as mg/L as N describe different species or reporting bases. A defensible result identifies the analyte, method, units, sample conditions and intended decision.

In water, NH3 and NH4+ exist in a pH- and temperature-dependent equilibrium. At the same TAN value, higher pH generally produces a larger unionized NH3 fraction; in freshwater, higher temperature also generally increases that fraction. This matters in aquaculture and aquatic-life interpretation because NH3 is generally the more toxic form. It does not create one universal “safe ammonia” value for every species, water use or jurisdiction.

APURE’s listed NHN-206 uses a PVC-membrane ammonium ion-selective electrode (ISE). An ammonium ISE responds directly to NH4+ ion activity; it is not automatically a direct NH3 or TAN measurement. Confirm whether the supplied system reports NH4+, NH4-N, NH3-N, TAN or another calculated value before comparing its output with a laboratory result or limit.

Product evidence note: Keep the current NHN-206 product card. Do not add or propagate its range, accuracy, concentration resolution, pH/TAN calculation, interference compensation or regulatory conformity until a current APURE manual/datasheet defines the measured species, reporting basis and test conditions.

Ammonia, ammonium and TAN: what is the difference?

Ammonia is NH3; ammonium is NH4+. TAN is the combined nitrogen contributed by both forms when reported on a nitrogen basis.

Term	Chemical/reporting meaning	What must accompany a number
Ammonia, NH3	Unionized ammonia molecule	State whether the result is mg/L NH3 or mg/L NH3-N
Ammonium, NH4+	Positively charged ammonium ion	State whether the result is mg/L NH4+ or mg/L NH4-N
Total ammonia	Combined NH3 + NH4+ under the named analytical definition	State method and whether the result is as N or as a full species
TAN	Total ammonia nitrogen: nitrogen contributed by NH3 and NH4+	Normally mg/L as N under the applicable definition; verify the program/method
Ammonia nitrogen	A method/market/regulatory phrase, often for combined ammonia reported as N	Do not infer the exact species, fraction or instrument output from the phrase alone
NH3-N	Nitrogen mass contained in NH3	Not the same numerical basis as mg/L NH3
NH4-N	Nitrogen mass contained in NH4+	Not the same numerical basis as mg/L NH4+

The equilibrium is commonly written:

NH3 + H2O ⇌ NH4+ + OH−

This equation describes the acid-base relationship in water. Avoid treating NH4OH as a separate dominant dissolved compound that an instrument necessarily measures.

What does mg/L as N mean?

mg/L as N reports only the mass of nitrogen per litre, not the mass of the complete NH3 or NH4+ species. Numbers on different bases cannot be compared directly.

Using nitrogen atomic mass 14.0067, NH3 molecular mass 17.03052 and NH4+ molecular mass 18.03846:

• NH4-N (mg/L as N) = NH4+ (mg/L) × 14.0067 / 18.03846
• NH4+ (mg/L) = NH4-N (mg/L as N) × 18.03846 / 14.0067
• NH3-N (mg/L as N) = NH3 (mg/L) × 14.0067 / 17.03052
• NH3 (mg/L) = NH3-N (mg/L as N) × 17.03052 / 14.0067

Approximate factors are:

Convert from	Convert to	Multiply by
mg/L NH4+	mg/L NH4-N	0.7765
mg/L NH4-N	mg/L NH4+	1.2878
mg/L NH3	mg/L NH3-N	0.8225
mg/L NH3-N	mg/L NH3	1.2159

Worked reporting-basis example

If a verified result is 5.00 mg/L NH4+, the nitrogen-basis result is:

5.00 × 14.0067 / 18.03846 = 3.88 mg/L NH4-N

This changes the mass basis only. It does not convert ammonium into ammonia and does not determine TAN or toxicity.

Assumptions: the source value and species are correctly identified; the same sample volume is used; the operation is only a molecular-mass conversion; and no pH/temperature speciation calculation is implied.

How do pH and temperature change the NH3/NH4+ balance?

Higher pH shifts a larger fraction of the combined ammonia system toward unionized NH3. In freshwater, higher temperature also generally increases the NH3 fraction at the same pH.

EPA’s 2013 freshwater ammonia criteria cite the Emerson et al. equilibrium relationship. For zero/low-salinity freshwater:

pKa = 0.09018 + 2729.92 / (273.2 + T)

fNH3 = 1 / (1 + 10^(pKa − pH))

where:

• T is the water temperature in °C;
• pH is measured for the same water and time;
• fNH3 is the fraction of total ammonia present as unionized NH3 on a consistent molar or nitrogen basis.

If TAN is reported as mg/L as N:

• NH3-N = TAN × fNH3
• NH4-N = TAN × (1 − fNH3)

To express the NH3 portion as full-species mass, multiply NH3-N by 17.03052 / 14.0067.

Equilibrium limitations

This is a freshwater equilibrium calculation, not a universal compliance or toxicity calculator. Ionic strength and salinity affect the relationship. The calculation also fails if TAN, pH and temperature do not represent the same sample/time or if their analytical definitions are mismatched. Use the equation and criteria specified by the applicable method, authority or aquaculture program.

Is TAN the same as toxic ammonia?

No. TAN includes both NH3 and NH4+ on a nitrogen basis; it does not directly equal the unionized NH3 concentration.

Unionized NH3 is generally the more toxic form to aquatic organisms, but toxicity depends on species, life stage, exposure duration, pH, temperature and other water chemistry. EPA freshwater aquatic-life criteria are expressed as TAN and use pH- and temperature-dependent equations; the published values are scientific recommendations for a defined freshwater aquatic-life context, not universal limits for drinking water, wastewater, aquaculture or every jurisdiction.

Do not use one TAN or NH3 number to declare water safe, toxic or compliant. Identify the protected use, organism/population where relevant, exposure duration, method, reporting basis and governing source.

What can ammonia measurement tell you?

It can help indicate

• changes in nitrogen loading;
• influent, process-stage or effluent trends;
• possible nitrification breakthrough when interpreted with process data;
• the need to review aeration, pH, temperature or alkalinity;
• aquaculture nitrogen stress risk when TAN is interpreted with pH, temperature, species and exposure;
• when a confirmatory/reference analysis is needed.

It cannot determine by itself

• complete water safety;
• exact toxicity in every water or species;
• nitrate, nitrite, total nitrogen or organic nitrogen;
• full nitrification health or microbial activity;
• the identity/source of a pollutant;
• universal treatment compliance;
• whether an online result is equivalent to a required laboratory method.

How is ammonia or ammonium measured?

Different methods respond to different species or method-defined reactions. “Ammonia sensor” does not identify one universal principle.

Method	Direct response	Typical strength	Main limitation
Ammonium ISE	NH4+ ion activity at a selective membrane	Continuous/in-situ process trend	Ionic strength, competing ions, temperature, membrane condition and matrix matter; not automatically TAN
Gas-sensitive ammonia electrode	NH3 diffusing through a gas-permeable membrane after method-specific sample conditioning	Potentiometric laboratory determination across a defined method range	Conditioning, volatile amines and method handling matter; not the same as a PVC NH4+ ISE
Colorimetric/spectrophotometric method	Method-defined color response after reagent chemistry	Reference/laboratory analysis with defined calibration and QC	Reagents, interferences, sample handling and method range apply
Flow-analysis spectrometric method	Automated method-defined ammonium-nitrogen response	Repeatable laboratory throughput	Requires the named method, reagents and QC; not an in-situ ISE

EPA Method 350.1 is a semi-automated colorimetric ammonia-nitrogen method. ISO 7150-1 defines a manual spectrometric ammonium method, and ISO 11732 covers flow analysis with spectrometric detection. Their ranges and interferences belong to those methods and must not be applied to an online ISE by analogy.

For compliance reporting, use the method named by the permit, regulation or authority. A method’s presence in this guide is not an APURE product-conformity claim.

How does an ammonium ion-selective electrode work?

An ammonium ISE develops a membrane potential related logarithmically to NH4+ ion activity relative to a reference electrode.

A typical measurement chain is:

NH4+ activity at selective membrane → membrane potential → reference comparison → temperature-aware calibration/model → displayed result

The Nernst-type relationship can be written in simplified form as:

E = E0 + S log10(aNH4+)

where:

• E is the measured electrode potential;
• E0 combines reference/system constants under the calibration conditions;
• S is the electrode slope, which depends on temperature and electrode behavior;
• aNH4+ is ammonium-ion activity, not automatically concentration.

Calibration relates the activity response in standards to the selected displayed concentration basis. Differences in ionic strength, interfering ions and the sample/reference junction can make a real sample behave differently from a simple standard. The membrane is selective, not exclusive.

Is potassium an interference for an ammonium ISE?

Potassium can produce a positive bias for many ammonium-selective membranes, but the size of the effect is electrode- and matrix-specific.

The practical importance depends on the NH4+:K+ relationship, the membrane/selectivity coefficient, ionic strength, calibration and any verified compensation channel. Do not borrow an interference coefficient or compensation claim from another manufacturer and apply it to NHN-206. Obtain the NHN-206 selectivity and compensation documentation, then verify the installed system against representative reference samples.

Is an ammonium ISE the same as a gas-sensitive ammonia electrode?

No. An ammonium ISE responds to NH4+ activity at an ion-selective membrane; a gas-sensitive ammonia electrode measures NH3 that diffuses through a gas-permeable membrane after method-specific conditioning.

The gas-sensitive analytical method commonly raises sample pH so the equilibrium shifts toward NH3. The diffusing gas changes an internal solution response sensed by the electrode system. This laboratory method description does not establish the principle or output of every commercial “ammonia sensor,” and it does not describe the listed PVC-membrane NHN-206.

Online ammonium ISE vs laboratory/reference analysis

Dimension	Online ammonium ISE	Laboratory/reference method
Direct response	NH4+ ion activity at the membrane	Method-defined colorimetric, potentiometric, chromatographic or other analytical response
Response time	Rapid/continuous when installed correctly	Batch result after sampling, preparation and analysis
Continuous monitoring	Yes	No
Matrix sensitivity	Competing ions, ionic strength, pH/speciation, temperature, fouling, flow and membrane/reference condition	Method-specific sample preservation, reagents, interferences, range and QC
Calibration	Standards plus model/manufacturer procedure; may need matrix adjustment	Method calibration, blanks, controls, spikes and other specified QC
Regulatory role	Process trend/control unless explicitly accepted for the use	Use the method required by the permit/rule/authority
Main strength	High-frequency process visibility	Method-defined, traceable result
Main limitation	Output depends on electrode/matrix and may not be TAN	Delayed result and method-specific labor/reagents

Online and laboratory values can support one another, but they are not automatically interchangeable.

Ammonium ISE interferences and operating conditions

Interference / condition	Relevant method	Possible effect	Practical check
Potassium and other competing cations	Ammonium ISE	Usually positive bias or distorted response, depending on membrane/selectivity and ratio	Obtain model-specific data; compare K+ and NH4+ ranges; verify with matched samples
Ionic strength / salinity	Ammonium ISE	Activity differs from concentration; calibration transfer may fail	Match/adjust matrix only by the approved procedure; validate saline use separately
pH change	NH4+ ISE and TAN interpretation	Changes NH3/NH4+ distribution; an NH4+ reading can change even if total ammonia does not	Measure matched pH and temperature; do not treat pH compensation as universal TAN conversion
Temperature	ISE response and speciation	Changes electrode slope/response and NH3 fraction	Allow temperature equilibrium; use verified compensation and record temperature
Fouling/biofilm/solids	Online ISE	Drift, slow response or unstable reference/membrane behavior	Inspect/clean by the current manufacturer procedure; verify before recalibration
Membrane aging/conditioning	Online ISE	Slope loss, slow stabilization or calibration failure	Review slope/history; condition or replace only by the approved procedure
Flow/bubbles/installation	Online ISE	Noise, lag, poor representativeness or intermittent contact	Inspect immersion/flow and sampling point; compare a simultaneous sample
Volatile amines	Gas-sensitive ammonia electrode	Positive interference in applicable methods	Follow method interference controls; do not transfer to NH4+ ISE without evidence
Residual chlorine/color/matrix chemistry	Colorimetric methods	Method-specific positive/negative bias or reaction change	Follow the named method’s pretreatment and QC

The direction and magnitude of an effect depend on method and instrument. This table is a diagnostic framework, not an NHN-206 performance specification.

How should an ammonium sensor be calibrated and verified?

Calibration establishes the sensor response under defined conditions; verification checks whether it still produces an acceptable result for the intended sample and decision.

A defensible workflow is:

1. Define the required output: NH4+, NH4-N, TAN or another stated basis.
2. Identify the exact probe revision, controller/register units and current manufacturer procedure.
3. Inspect and condition the membrane/reference system as specified.
4. Prepare or obtain traceable standards on the required species/reporting basis.
5. Allow standards and sensor to reach the required temperature equilibrium.
6. Calibrate over the intended range by the approved one- or multi-point procedure.
7. Review slope, stability and acceptance criteria from the actual manual; do not invent universal limits.
8. Verify with an independent standard and a representative sample/reference method.
9. Record standards, lots, units, temperature, slope/result, maintenance, firmware/register settings and operator.
10. Reassess after membrane replacement, failed verification, material matrix/process change or unexplained drift.

There is no universal calibration interval. Use the current manufacturer procedure, process risk, matrix stability, fouling history, quality-control results and regulatory/contract requirements.

Why do online and laboratory ammonia values disagree?

Disagreement can come from different reporting bases, different species/methods or different samples—not only sensor failure.

Check in this order:

1. Are both values on the same basis: NH4+, NH4-N, NH3-N or TAN as N?
2. Were the samples taken at the same location and time?
3. Did pH or temperature change between the in-situ reading and sampling?
4. Which laboratory method and sample fraction were used?
5. Were preservation, transport, filtration and holding time appropriate?
6. Did potassium, salinity/ionic strength or another matrix constituent change?
7. Is the sensor fouled, insufficiently conditioned or outside its verified range?
8. Did both the laboratory QC and sensor verification pass?

Do not force agreement by changing calibration after one unmatched comparison.

How is ammonia used in wastewater process monitoring?

Ammonia/ammonium trends can show nitrogen loading and nitrification breakthrough, but ammonia alone does not diagnose the biological process.

Useful wastewater context includes:

Monitoring objective	Relevant ammonia result	Additional context required	Limitation
Influent load trend	Method-defined TAN/NH4-N concentration plus flow	Flow, sampling time and wastewater source	Concentration alone is not a mass load
Nitrification performance	NH4-N/TAN before and after the aerobic stage	DO, pH, temperature, alkalinity, SRT/process conditions, nitrite/nitrate	Low ammonia alone does not prove complete nitrogen removal
Breakthrough/alarm	Continuous verified NH4+ or NH4-N trend	Sensor status, process events and confirmatory sample	Alarm basis must match the configured output
Aeration/process control	Fast ammonium trend	DO, energy/control logic, pH/alkalinity and site validation	Do not infer one universal control setpoint
Effluent reporting	Required method and basis	Permit, sample type and laboratory QC	Online value is not automatically the compliance result

Read Ammonia in wastewater for the process-focused guide.

How should ammonia be interpreted in aquaculture?

Use the sequence TAN → matched pH and temperature → unionized NH3 fraction → species/life-stage/exposure guidance.

1. Confirm whether the analytical result is TAN as N, NH4-N, NH4+ or another basis.
2. Measure pH and temperature at the same time and location.
3. Use an equation/table appropriate to freshwater or salinity conditions and the governing program.
4. Convert to unionized NH3 on the required basis without mixing mg/L as N and full-species mass.
5. Compare only with guidance for the relevant species, life stage, duration and jurisdiction.
6. Review DO, nitrite, stocking/feeding, biofilter condition and recent process changes.

Do not publish “ammonia above X mg/L kills fish” without those conditions. A project-specific alarm is not a universal safe level.

Ammonia vs nitrite, nitrate and total nitrogen

Parameter	What it represents	Monitoring implication
NH3 / NH4+ / TAN	Reduced inorganic nitrogen species/system	Tracks ammonia loading and nitrification input/breakthrough when properly defined
Nitrite, NO2− / NO2-N	Intermediate oxidation state	May indicate incomplete nitrification or another process condition; requires its own method
Nitrate, NO3− / NO3-N	More oxidized inorganic nitrogen form	Can support nitrification/denitrification assessment; not a substitute for ammonia
Total nitrogen	Method-defined sum/conversion of multiple inorganic and organic nitrogen forms	Broader nitrogen balance/compliance parameter; an ammonium-only sensor does not measure TN

One result cannot universally substitute for another. Always state whether the value is the full ion or as N.

Troubleshooting online ammonium readings

Symptom	Possible causes	Practical checks
Unstable reading	Bubbles, poor immersion/flow, loose connection, moving solids, unstable reference, real process pulses	Observe installation and trend; inspect cable/reference; compare a simultaneous grab sample
Persistent high bias	Potassium/competing-ion effect, matrix mismatch, wrong unit conversion, contamination, lab basis mismatch	Confirm units; review K+/matrix; run independent verification and matched reference sample
Persistent low bias	Membrane response loss, poor conditioning, range issue, NH3 shift at high pH, sample mismatch	Review slope/conditioning; record pH/temperature; confirm sample and basis
Gradual drift	Fouling, membrane aging, reference/junction change, process chemistry shift	Inspect/clean; review maintenance and slope history; verify before recalibration
Step change after pH shift	Real NH3/NH4+ redistribution, process addition or compensation/configuration effect	Check matched TAN, pH and temperature; do not assume total ammonia changed
Poor lab correlation	Different time/location, preservation, method, fraction, basis or matrix	Build a paired-sample record with method metadata and QC
Slow response	Fouling, cold/unequilibrated sensor, membrane conditioning or low concentration	Allow stabilization; inspect/clean/condition by manual; verify range
Failed calibration	Wrong/degraded standard, unit error, temperature mismatch, damaged/aged membrane, reference problem	Prepare independent standard; inspect procedure and slope; replace only when justified
Seasonal change	Temperature/speciation, salinity/ionic-strength shift, source/load or biological-process change	Trend pH, temperature, conductivity/salinity and supporting process parameters

How should I choose an ammonia/ammonium monitoring method?

Need	Recommended approach	Why	Limitation/check
Permit/compliance reporting	Method accepted by the permit/rule/authority	Produces the defined legal analytical result	An online sensor is not automatically acceptable
Continuous wastewater control	Online ammonium ISE validated for the matrix plus reference checks	Rapid process visibility	Interference, fouling and reporting basis must be controlled
Aquaculture risk interpretation	TAN/reference result plus matched pH/temperature; continuous monitoring where validated	Supports NH3-fraction interpretation	Species/exposure and salinity context remain necessary
Drinking-water/chloramination operation	Method/instrument selected for the treatment objective and required range	Supports chlorine-ammonia control and nitrification monitoring	No universal target or method fits every system
Variable industrial wastewater	Paired online/reference program across process states	Tests matrix robustness	Revalidate after composition changes
Ammonium ion specifically	Verified NH4+ method/ISE with correct unit basis	Directly targets ammonium response	Does not automatically provide TAN or NH3

Confirm before specifying an APURE system:

1. monitoring objective and required reference/regulatory method;
2. exact species and reporting basis;
3. minimum/normal/maximum expected range;
4. pH, temperature, potassium, salinity/ionic strength and other matrix conditions;
5. solids, fouling, flow and maintenance access;
6. calibration standards and reference-data availability;
7. installation, materials, pressure and temperature limits;
8. output/register units, RS-485 settings, logging and alarms;
9. current model manual/datasheet and revision;
10. acceptance and ongoing verification plan.

APURE product path

The NHN Online Ammonia Nitrogen Sensor is listed as an NHN-206 ammonium-ISE product. Before quotation or configuration, state the required result (NH4+, NH4-N or TAN), expected matrix/range, pH/temperature context, interfering ions, installation and receiving-device units. APURE must confirm the supplied model documentation and output basis before the value is used for control or comparison.

Frequently asked questions

Is ammonia the same as ammonium?

No. Ammonia is unionized NH3; ammonium is charged NH4+. They coexist in a pH- and temperature-dependent equilibrium.

Is TAN the same as NH3?

No. TAN is the nitrogen contributed by NH3 and NH4+ together under the stated definition. NH3 is only the unionized portion.

Can mg/L NH4+ be compared directly with mg/L as N?

No. Convert the reporting basis using molecular masses and keep the species label. The numerical values differ.

Does an ammonium ISE measure TAN?

Not directly. It responds to NH4+ activity. TAN requires an analytically valid definition/calculation or a suitable reference method with matched pH/temperature context.

Is ammonium harmless?

No absolute statement is defensible. NH4+ is generally less acutely toxic than unionized NH3, but effects and criteria depend on organism, exposure and water chemistry.

How often should an ammonium sensor be calibrated?

There is no universal interval. Follow the exact current manual and adjust the plan to matrix stability, fouling, verification results, process risk and regulatory/contract requirements.

Can an online sensor replace laboratory ammonia analysis?

Not universally. Online monitoring is valuable for trend and control after validation. Use the method required by the applicable permit, rule or authority for compliance reporting.

Authoritative references

• EPA 2013 Freshwater Aquatic Life Ambient Water Quality Criteria for Ammonia
• USGS: Ammonia or Ammonium? What’s the difference?
• EPA approved inorganic non-metal methods
• WHO ammonia drinking-water fact sheet
• ISO 7150-1:1984
• ISO 11732:2005