🫧 Dissolved Oxygen in Water
In short
Electrochemical determination of dissolved oxygen concentration in water using membrane probe. The test is performed according to PN-EN ISO 5814:2013-04; Measurement range: 0–20 mg O₂/L (0–200% saturation). The procedure comprises 9 steps (about 31–33 min in total); it is used for: Surface water monitoring — rivers, lakes, reservoirs (WFD), Aeration process control in wastewater treatment plants, BOD5 testing — DO measurement on day 0 and after 5 days incubation.
At a glance
- Standard: PN-EN ISO 5814:2013-04
- Category: Physicochemistry
- Procedure steps: 9
- Total time of stages: 31 min–33 min
- Measurement range: 0–20 mg O₂/L (0–200% saturation)
- Resolution: 0.01 mg/L (0.1% saturation)
- Accuracy: ±0.1 mg/L or ±1% saturation
Overview
Dissolved Oxygen (DO) is one of the most important natural water quality parameters. It conditions aquatic organism life, river and lake self-purification processes, and its deficiency leads to fish kills and ecosystem degradation. Oxygen saturation concentration in water at 20°C at normal pressure is approximately 8.84 mg O₂/L.
DO measurement is crucial in: surface water monitoring (Water Framework Directive), wastewater treatment process control (activated sludge aeration), BOD5 testing (oxygen measurement on day 0 and after 5 days), aquaculture and fisheries.
The ISO 5814 method uses electrochemical probe with gas-permeable membrane — it is faster and more convenient than classical Winkler titration method (ISO 5813), requires no reagent addition, and can be used in situ. Two probe types are available: galvanic (no power supply, e.g., WTW CellOx) and polarographic (with polarization, e.g., Clark).
Method principle
The electrochemical probe consists of cathode (gold, silver, or platinum) and anode (silver, lead, or zinc) immersed in electrolyte (KCl, KOH, or NaOH), separated from tested water by gas-permeable membrane (PTFE, PE, FEP). Oxygen diffuses through membrane and is reduced at cathode:
Cathode: O₂ + 2H₂O + 4e⁻ → 4OH⁻ Anode: Ag → Ag⁺ + e⁻ (or Pb → Pb²⁺ + 2e⁻)
Current generated is proportional to oxygen partial pressure in sample. DO meter converts current to DO concentration [mg/L] or saturation [%] accounting for temperature and atmospheric pressure.
Modern optical probes (luminescent, e.g., WTW FDO 925) — based on luminescence quenching by oxygen — do not consume oxygen, require no flow, and have longer lifespan.
Applications
- Surface water monitoring — rivers, lakes, reservoirs (WFD)
- Aeration process control in wastewater treatment plants
- BOD5 testing — DO measurement on day 0 and after 5 days incubation
- Aquaculture and fish farming — oxygen condition control
- Drinking water testing (groundwater and treated)
- Water ecological status assessment — water quality classes
Key parameters
| Parameter | Value |
|---|---|
| Measurement range | 0–20 mg O₂/L (0–200% saturation) |
| Resolution | 0.01 mg/L (0.1% saturation) |
| Accuracy | ±0.1 mg/L or ±1% saturation |
| Response time (T90) | 30–180 s (depends on probe type) |
| Measurement temperature | 0–50°C (with automatic compensation) |
| Saturation at 20°C | 8.84 mg O₂/L (pressure 1013 hPa) |
Standard
- Standard number
- PN-EN ISO 5814:2013-04
- Title (PL)
- Jakość wody -- Oznaczanie tlenu rozpuszczonego -- Metoda z czujnikiem elektrochemicznym
- Title (EN)
- Water quality — Determination of dissolved oxygen — Electrochemical probe method
Step-by-step procedure
-
Probe preparation
Check membrane (no damage, bubbles under membrane). For electrochemical probe — check electrolyte level, replace membrane if needed. For optical probe — check sensor cap.
-
Probe polarization
Turn on DO meter min. 15 minutes before measurement (polarographic probe requires polarization). Galvanic and optical probes — ready immediately.
-
Calibration — water-saturated air
Place probe above water surface (in water vapor-saturated chamber) or in laboratory air. Enter current atmospheric pressure. Confirm calibration at ~100% saturation.
-
Zero verification (optional)
Immerse probe in Na₂SO₃ solution (DO = 0 mg/L). Check reading is <0.2 mg/L. Rarely required — mainly for two-point calibration.
-
Sample preparation
Analyze sample immediately after sampling (oxygen undergoes rapid changes!). Do not aerate, do not mix vigorously. Record sample temperature and atmospheric pressure.
-
Dissolved oxygen measurement
Immerse probe in sample (min. 3 cm below surface). For electrochemical probe — ensure gentle flow (stirrer, probe movements). Optical probe — no flow required. Wait for stabilization.
-
Result reading
Record DO value in mg O₂/L and/or saturation in %. Record measurement temperature. Repeat measurement (duplicate) — difference max 0.2 mg/L.
-
Quality control
Measure air-saturated water — reading should be 100±5% saturation. Check Na₂SO₃ blank once weekly.
-
Post-measurement maintenance
Rinse probe with deionized water. Store in humid environment (cap with wet sponge) or immersed in water. Do not store dry!
Required equipment and apparatus
| Equipment | Example | Indicative price |
|---|---|---|
| Laboratory DO meter | WTW inoLab Oxi 7310, Hach HQ40d, YSI ProODO | 4 000–15 000 PLN |
| Electrochemical oxygen probe | WTW CellOx 325 (galvanic), WTW StirrOx G (self-stirring) | 2 000–5 000 PLN |
| Optical oxygen probe (LDO) | Hach IntelliCAL LDO101, WTW FDO 925 (luminescent) | 3 000–8 000 PLN |
| Magnetic stirrer | IKA C-MAG MS 4, to ensure flow at electrochemical probe | 1 500–4 000 PLN |
| Barometer | For atmospheric pressure correction (built into modern DO meters) | 200–500 PLN |
| Glass beakers 250 mL | DURAN borosilicate | 20–40 PLN/pc |
Reagents, media and consumables
| Reagent | CAS | Details |
|---|---|---|
| Probe electrolyte (KCl) | 7447-40-7 | KCl solution (1–3 mol/L) for galvanic and polarographic probes. Replace every 1–6 months. |
| Probe membrane | — | PTFE or PE membrane, replace every 2–4 weeks (galvanic probe) or when damaged. |
| Na₂SO₃ solution (oxygen-free) | 7757-83-7 | Sodium sulfite — for preparing DO = 0 mg/L solution (zero verification). 10 g/L + trace CoCl₂. |
| Air-saturated water | — | For calibration — water aeration with diffuser for min. 30 minutes (DO ≈ saturation at given temperature and pressure). |
Health and safety (OHS)
- Sodium sulfite Na₂SO₃ — irritating, avoid skin and eye contact
- Cobalt chloride CoCl₂ — toxic, carcinogenic (cat. 1B). Use under fume hood, nitrile gloves.
- Wastewater samples — potentially infectious, use gloves
- Safety goggles and laboratory coat mandatory
- KCl electrolyte — low risk, but avoid eye contact
Training and video materials
Frequently asked questions
Which standard describes this test?
The test is performed according to PN-EN ISO 5814:2013-04 — “Water quality — Determination of dissolved oxygen — Electrochemical probe method”.
How does this method work?
The electrochemical probe consists of cathode (gold, silver, or platinum) and anode (silver, lead, or zinc) immersed in electrolyte (KCl, KOH, or NaOH), separated from tested water by gas-permeable membrane (PTFE, PE, FEP). Oxygen diffuses through membrane and is reduced at cathode: Cathode: O₂ + 2H₂O + 4e⁻ → 4OH⁻ Anode: Ag → Ag⁺ + e⁻ (or Pb → Pb²⁺ + 2e⁻) Current generated is proportional to oxygen partial pressure in sample.
What is the measuring range and accuracy?
Measurement range: 0–20 mg O₂/L (0–200% saturation); Resolution: 0.01 mg/L (0.1% saturation); Accuracy: ±0.1 mg/L or ±1% saturation; Response time (T90): 30–180 s (depends on probe type).
How long does the test take?
The times given for the individual stages add up to approximately 31 min–33 min. The procedure comprises 9 steps.
What equipment is required?
Laboratory DO meter, Electrochemical oxygen probe, Optical oxygen probe (LDO), Magnetic stirrer, Barometer, Glass beakers 250 mL. Indicative cost of the core instrument (Laboratory DO meter): 4 000–15 000 PLN.
Where is this test used?
Surface water monitoring — rivers, lakes, reservoirs (WFD); Aeration process control in wastewater treatment plants; BOD5 testing — DO measurement on day 0 and after 5 days incubation; Aquaculture and fish farming — oxygen condition control; Drinking water testing (groundwater and treated); Water ecological status assessment — water quality classes.
What safety precautions apply?
Sodium sulfite Na₂SO₃ — irritating, avoid skin and eye contact; Cobalt chloride CoCl₂ — toxic, carcinogenic (cat. 1B). Use under fume hood, nitrile gloves.; Wastewater samples — potentially infectious, use gloves; Safety goggles and laboratory coat mandatory; KCl electrolyte — low risk, but avoid eye contact.
Which laboratory can perform this test?
The test is performed by laboratories accredited to ISO/IEC 17025. On LabCoda you can find them by the standard number PN-EN ISO 5814.