🚧 Under construction — VoTechWorks is being built. A free tech program — complimentary, as our commitment to get America working.
voTechWorksGrade 13 · a vocational technology learning center, by urbanicity
newSchoolWorks.com › VoTechWorks › Agriculture, Environment & Public Service › Aquaculture & Marine Science Tech › Water Quality Monitoring and Management for Aquaculture › Lesson 5
AGR-AQUA50-03 · Lesson 5 · 20 clock hours · lab

Nitrogenous Wastes: Ammonia, Nitrite, and Nitrate Testing and Management

Objective. Students use spectrophotometric and colorimetric methods to measure total ammonia nitrogen (TAN), nitrite, and nitrate, calculate un-ionized ammonia fraction from pH and temperature, and recommend a corrective strategy for an elevated ammonia reading.

⚠ Pending professional review

This lesson, its worked examples and its practice questions has not yet been verified by a licensed professional in the field or by a certification or approval body. It is a study draft and may contain errors. Do not use it as instructions for work on a job site or on live equipment. Always follow your instructor, your supervising professional, and current official references and regulations.

VoTechWorks program approval and accreditation are still in progress. Credentials are issued only by the certifying bodies named in each program, after their own exams.

Why Nitrogenous Wastes Matter in Aquaculture

Fish and shrimp excrete nitrogen primarily as ammonia through their gills and in urine. In a pond or recirculating aquaculture system (RAS), uneaten feed and fecal matter also decompose into ammonia. Left unmanaged, nitrogenous wastes — ammonia, nitrite, and nitrate — accumulate to concentrations that stress or kill stock. Understanding how to measure these compounds accurately and respond to dangerous readings is a core competency for any aquaculture technician.

The Nitrogen Cycle in Brief

Ammonia (NH₃) and its ionized form ammonium (NH₄⁺) coexist in water; together they are reported as total ammonia nitrogen (TAN). Nitrifying bacteria (Nitrosomonas spp.) oxidize TAN to nitrite (NO₂⁻), and a second group (Nitrobacter spp.) oxidizes nitrite to nitrate (NO₃⁻). In a healthy, established biofilter this two-step process runs continuously. When the biofilter is stressed — by a power outage, disinfectant carryover, temperature shock, or overfeeding — TAN and nitrite can spike before nitrate rises, which is exactly the RAS failure pattern you will diagnose in Activity 3.

Un-ionized Ammonia: The Toxic Fraction

Only the un-ionized form, NH₃, crosses gill membranes easily and is acutely toxic. The proportion of TAN present as NH₃ depends on pH and temperature: higher pH and higher temperature shift the equilibrium toward NH₃. A TAN of 2.5 mg/L is harmless at pH 7.0 and 15 °C but potentially lethal at pH 8.0 and 28 °C. Always calculate un-ionized NH₃ before deciding whether a TAN reading requires action.

Published fraction tables (Boyd & Tucker, Pond Aquaculture Water Quality Management, or equivalent) list the proportion of TAN that is NH₃ at each pH–temperature combination. Multiply TAN by that fraction to get un-ionized NH₃ concentration in mg/L.

General guidance from the literature places acute toxicity thresholds roughly below 0.05 mg/L NH₃ for salmonids (which are sensitive) and somewhat higher — commonly cited around 0.3 mg/L — for tilapia, though exact values vary by species, life stage, and exposure duration. Always confirm thresholds against current peer-reviewed references or regulatory guidance for the species you are culturing.

Colorimetric and Spectrophotometric Testing Methods

Colorimetric test kits use pre-measured reagent pillows or drops that produce a color proportional to analyte concentration. The technician compares the sample color to a printed chart or uses a portable meter such as the HACH DR 900. These kits are fast and field-portable but have limited sensitivity at very low concentrations.

Benchtop spectrophotometers measure absorbance at a specific wavelength with greater precision and a wider linear range. By preparing a standard curve — a series of solutions at known concentrations — you verify that instrument readings track true concentrations linearly. If your standards read within ±10 % of their known values, the instrument and reagents are performing acceptably.

Common methods:

Safety in the Chemistry Lab

Diagnosing a RAS Failure: The Elevated TAN Scenario

When TAN rises while nitrite also rises but nitrate remains stable, the first-stage nitrifiers are overwhelmed or inhibited. The second stage cannot convert nitrite it is not receiving, so nitrate stays flat. This pattern points to biofilter under-performance, not complete failure.

A structured corrective-action response has three components:

1. Emergency water exchange: Dilute TAN and nitrite immediately. Calculate the volume to exchange based on target concentration and current system volume using a dilution equation.

2. Biofilter assessment: Check dissolved oxygen at the biofilter inlet (nitrifiers need DO > 4 mg/L), verify temperature is within range, confirm no disinfectant (chlorine, ozone residual) has entered the system, and inspect media for channeling or biofilm sloughing.

3. Feeding reduction: Cutting feed by 25–50 % immediately reduces the nitrogen load entering the system, buying time for biofilter recovery.

Document all actions, readings, and decisions on the facility corrective-action report template. Good records protect the business legally and allow root-cause analysis later.

Plotting a Standard Curve

Prepare at least four standards spanning the expected sample range (for example, 0, 0.5, 1.0, and 2.0 mg/L TAN). Read each on the spectrophotometer and plot absorbance (y-axis) versus concentration (x-axis). Fit a straight line. The slope and intercept let you back-calculate concentration from any unknown absorbance. If a point deviates more than ±10 % from the line, remake that standard and re-read before reporting results.

Key terms8

Total Ammonia Nitrogen (TAN)The combined concentration of un-ionized ammonia (NH₃) and ionized ammonium (NH₄⁺) in water, expressed as mg/L of nitrogen.
Un-ionized ammonia (NH₃)The toxic, uncharged form of ammonia that readily crosses gill membranes; its proportion of TAN increases with rising pH and temperature.
NitrificationThe two-step biological oxidation of ammonia to nitrite (by Nitrosomonas spp.) and then nitrite to nitrate (by Nitrobacter spp.) carried out by aerobic bacteria in a biofilter.
Standard curveA graph of instrument response (absorbance) versus known analyte concentration used to calculate unknown sample concentrations from spectrophotometer readings.
Colorimetric methodAn analytical technique in which a chemical reagent reacts with an analyte to produce a color whose intensity is proportional to concentration, measured visually or by spectrophotometer.
BiofilterA unit in a recirculating aquaculture system that provides surface area for nitrifying bacteria to colonize and convert toxic ammonia and nitrite to less toxic nitrate.
Corrective-action reportA standardized facility document recording an observed problem, measurements taken, actions implemented, and follow-up monitoring results.
DiazotizationThe colorimetric chemistry used to measure nitrite, in which sulfanilamide reacts with NO₂⁻ under acidic conditions and couples with NED reagent to form a pink color.

Worked examples4

1. Prepare a 1.0 mg/L TAN standard from a 100 mg/L TAN stock solution using a 100 mL volumetric flask. How many mL of stock do you need?
  1. Use the dilution equation: C₁V₁ = C₂V₂
  2. C₁ = 100 mg/L (stock), C₂ = 1.0 mg/L (target), V₂ = 100 mL (final volume)
  3. Solve for V₁: V₁ = (C₂ × V₂) / C₁ = (1.0 × 100) / 100 = 1.0 mL
  4. Transfer 1.0 mL of stock into the 100 mL volumetric flask and dilute to the mark with deionized water.
Answer: 1.0 mL of the 100 mg/L stock is needed.
2. A water sample has a TAN of 2.5 mg/L at pH 8.0 and 28 °C. A published un-ionized ammonia fraction table gives a fraction of 0.094 at these conditions. What is the un-ionized NH₃ concentration, and does it exceed a commonly cited acute threshold of 0.05 mg/L for salmonids?
  1. Identify values: TAN = 2.5 mg/L, fraction = 0.094
  2. Calculate un-ionized NH₃: NH₃ = TAN × fraction = 2.5 × 0.094 = 0.235 mg/L
  3. Compare to salmonid threshold: 0.235 mg/L > 0.05 mg/L
  4. Compare to tilapia threshold (commonly cited ~0.3 mg/L): 0.235 mg/L < 0.30 mg/L
Answer: Un-ionized NH₃ = 0.235 mg/L. This exceeds the salmonid threshold (0.05 mg/L) but is below the commonly cited tilapia threshold (~0.3 mg/L). Immediate action is required for a salmon system.
3. A RAS tank holds 10,000 L of water with a measured TAN of 4.0 mg/L. You want to reduce TAN to 1.0 mg/L by water exchange with clean water (TAN = 0 mg/L). How many liters must you replace, assuming complete mixing?
  1. Use the dilution/exchange formula: Volume to exchange = V_total × (C_initial − C_target) / C_initial
  2. V_total = 10,000 L, C_initial = 4.0 mg/L, C_target = 1.0 mg/L
  3. Volume to exchange = 10,000 × (4.0 − 1.0) / 4.0 = 10,000 × 3.0 / 4.0 = 10,000 × 0.75
  4. Volume to exchange = 7,500 L
Answer: Replace 7,500 L (75 % of system volume) with clean water to dilute TAN from 4.0 to 1.0 mg/L.
4. You prepared a 2.0 mg/L nitrite standard and the spectrophotometer reads an absorbance that back-calculates to 1.82 mg/L using your standard curve. Is this within the acceptable ±10 % tolerance?
  1. Calculate the allowable range: 10 % of 2.0 mg/L = 0.20 mg/L
  2. Acceptable range: 2.0 − 0.20 = 1.80 mg/L to 2.0 + 0.20 = 2.20 mg/L
  3. Compare the reading: 1.82 mg/L falls within 1.80–2.20 mg/L
Answer: Yes — 1.82 mg/L is within ±10 % of 2.0 mg/L. The reading is acceptable.

The math in this lesson2

Learners multiply TAN by the un-ionized fraction to find toxic NH₃ concentration.
7.RP.3A-CED.4
Brush up in the math halls at newSchoolWorks.com:
In class: Prepare reagent-grade standards at known concentrations of TAN and nitrite; run them through a colorimetric test kit and a benchtop spectrophotometer, plot a standard curve, and evaluate whether kit sensitivity is adequate for the target species threshold. · Using a published un-ionized ammonia fraction table (Boyd & Tucker or equivalent), calculate the un-ionized NH3 concentration from a TAN of 2.5 mg/L at pH 8.0 and 28°C and determine whether the value exceeds the threshold for tilapia and for salmon. · Diagnose a simulated RAS failure scenario (elevated TAN, rising nitrite, stable nitrate): propose water exchange volume, biofilter troubleshooting steps, and a feeding reduction, then write a one-page corrective-action report using a standard facility template.
Materials: HACH DR 900 or equivalent portable spectrophotometer, ammonia nitrogen reagent set, nitrite reagent set, nitrate reagent set, volumetric glassware for standards preparation, un-ionized ammonia fraction table, facility corrective-action report template, Boyd & Tucker Pond Aquaculture Water Quality Management
Check: Spectrophotometer readings on prepared standards within ±10% of known concentration; un-ionized ammonia calculation correct within ±0.05 mg/L; corrective-action report includes water exchange volume calculation and biofilter assessment; pass = all three criteria met.

Practice check8

Answer each question, then check it. Nothing is recorded; this is your own practice.

Water Quality Monitoring and Management
1. A technician measures a TAN of 3.0 mg/L in a tilapia RAS at pH 7.5 and 26 °C. The un-ionized ammonia fraction from a published table is 0.038. What is the un-ionized NH₃ concentration?
Recirculating Aquaculture Systems
2. In a RAS, TAN is rising rapidly, nitrite is also rising, but nitrate concentration is unchanged from yesterday. What does this pattern most likely indicate?
Safety and Hazard Management
3. Which personal protective equipment is the minimum required when preparing Nessler's ammonia reagent, which contains mercury compounds?
Water Quality Monitoring and Management
4. A 5,000 L tank has a TAN of 5.0 mg/L. Using clean water (TAN = 0) and complete mixing, what volume must be exchanged to reach a target TAN of 2.0 mg/L?
Laboratory Methods and Instrumentation
5. When constructing a standard curve for nitrite using a spectrophotometer, what is the recommended maximum acceptable deviation of a standard reading from its known concentration?
Water Chemistry
6. Why does un-ionized ammonia (NH₃) toxicity increase at higher pH even when total ammonia nitrogen (TAN) stays the same?
Aquaculture Operations and Record-Keeping
7. A corrective-action report for an elevated ammonia event must include which of the following to meet the lesson's pass criteria?
Safety and Hazard Management
8. Which of the following is the correct procedure when a cadmium-reduction nitrate reagent contacts your skin during testing?
Record this lesson on my transcript. Sign in with your key and enrollment email to record lessons (enroll free if you have no key). This page stays open to read and practise.
← Previous lessonProgram planNext lesson →