Hydrogen Peroxide Measurement: Online H2O2 Sensors and Selection
Continuous monitoring of hydrogen peroxide concentration with a reagent-free amperometric sensor, a bare-gold electrode multi-parameter sensor, a controller, and a dosing pump.
Three Decisions in H2O2 Measurement
- Which range? The Walchem H2O2 sensor is selected in a 0–200, 0–2,000, or 0–20,000 ppm range; the Pyxis ST-766SS-H2O2 is 0–200 ppm.
- Which method? An interference-free amperometric sensor if only H2O2 will be monitored and oxidants such as chlorine, PAA, or ozone may be present; the Pyxis bare-gold electrode sensor if pH, ORP, and conductivity are also required.
- How will it be controlled? The sensor connects to a Walchem controller whose relay drives an Iwaki dosing pump, with pump materials selected for chemical compatibility.
Benefits of Continuous Measurement
Closed-loop dosing
The sensor reading is compared against the setpoint in the controller, which opens or closes the dosing pump via its relay. Dosing stays on target without waiting for spot measurements.
Off-gassing chemical
Hydrogen peroxide solutions release gas, as noted for high concentrations in chemical compatibility tables. Gas locking silently reduces actual dosing; online measurement makes this deviation visible.
Reagent-free measurement
The Walchem amperometric sensor consumes no reagents, and the sample can be returned to the process after measurement. Calibration is weekly.
For other oxidants and disinfection analyzers (free chlorine, ozone, chlorine dioxide, and peracetic acid), see our online analyzers page.
Hydrogen Peroxide Sensors
| Feature | Walchem amperometric | Pyxis ST-766SS-H2O2 |
|---|---|---|
| Method | Amperometric, reagent-free | Membraneless bare-gold electrode |
| H2O2 range | 0–200 / 0–2,000 / 0–20,000 ppm (part no. 103985 / 103987 / 103989) | 0.00–200.00 ppm |
| Resolution and accuracy | Resolution 0.1 / 1 / 10 ppm | Plus/minus 0.5 mg/L or 1% of reading (compensated up to pH 9.0) |
| Additional measurements | None | pH 0–14, ORP plus/minus 1,500 mV, conductivity 0–10,000 µS/cm |
| Sample flow | 30–100 L/h | ST-007 / ST-009 tee 200–400 mL/min; FR-306-PLUS 200–800 mL/min |
| Sample pH and conductivity | pH 2–11; at least 10 µS/cm | pH is measured and used in compensation |
| Temperature and pressure | 0–60 °C; sensor body 1 bar | 4–49 °C; inlet 0.05–0.6 MPa |
| Response time | 8 minutes; conditioning 5 hours | Oxidant T95 at most 240 seconds |
| Interference | Unaffected by chlorine, PAA, and ozone | Non-selective; cross-sensitive to other oxidants |
| Output | W100 / W600 / W900 and Intuition controllers | 2 x 4-20 mA, RS-485 Modbus; uPyxis via Bluetooth (MA-CR) |
| Maintenance | Weekly calibration | Replaceable electrode block, typical life 2 years |
What Happens If Another Oxidant Is Present?
The interference specification in the Walchem brochure is bidirectional, and the direction matters. The H2O2 sensor is unaffected by chlorine, peracetic acid, and ozone; conversely, for the peracetic acid sensor, ozone (2,500 percent), chlorine dioxide (100 percent), and H2O2 are listed as interferences. Because peracetic acid solutions are synthesized from acetic acid and hydrogen peroxide, a line containing both PAA and H2O2 requires using two separate sensors and interpreting the readings together.
Pyxis ST-766SS sensors are non-selective. According to the manufacturer datasheet, the same electrochemical platform is used for H2O2, PAA, chlorine, and other oxidants, and each model is cross-sensitive to other oxidizing species in the water. When multiple oxidants are present in the water, this sensor yields a single combined reading, which is interpreted with knowledge of which oxidant is being dosed.
Mounting and Sample Conditions
- Walchem amperometric: sample flow 30–100 L/h; sample pH 2–11; conductivity at least 10 µS/cm; temperature 0–60 °C; response time 8 minutes, conditioning 5 hours. Flow cell inlet is 1/4 inch and outlet is 3/4 inch NPTF.
- Pyxis ST-766SS: in clean water, ST-007 or ST-009 stainless steel tee (200–400 mL/min) or FR-50 chamber (600–1,800 mL/min); in fouled water, self-brushing FR-306-PLUS chamber (200–800 mL/min, 200 rpm motorized brush).
- Cable and power supply: Walchem sensor signal 0 to -2,000 mV DC; extension cable 22 AWG shielded twisted pair, at most 30 m. The Pyxis sensor requires a 22–26 V DC supply and approximately 2 W power.
Controller and Dosing Pump
| Component | Option | Documented feature |
|---|---|---|
| Controller | Walchem Intuition-6 | 6 relays; universal sensor input (including disinfection sensors); data logging and e-mail alarms |
| Controller | Walchem Intuition-9 | Up to 24 analog inputs and 12 relays depending on model code; Ethernet, BACnet/Modbus-TCP |
| Ready-made panel | OxiPanel IK-765SS-BP PLUS | H2O2 format 0–200 ppm; FR-300-PLUS brush chamber, FS-100 ultrasonic flow control, UC-80 touchscreen |
| Off-gassing chemical pump | Iwaki EWN-A | Automatic air vent valve; hydrogen peroxide dosing listed as an application |
| High-compression type | Iwaki EWN-R (type C) | Reduces dead volume in the pump head; for gas-bearing liquids such as sodium hypochlorite or hydrogen peroxide |
| Flow verification | Iwaki EWN-Y and EFS | Measures actual flow rate; restores dosing volume in the event of gas locking |
| Semiconductor transfer | Iwaki CFD | PTFE/PFA wetted end; high-vapor-pressure chemicals such as ammonia water and hydrogen peroxide; 8 mL/stroke |
See dosing pumps for pump selection and materials, and dosing pump maintenance for servicing.
By Application
| Application | Highlighted solution in documentation | Selection considerations |
|---|---|---|
| Cooling tower biocide control | Walchem amperometric H2O2 sensor and Intuition controller; Pyxis ST-766SS-H2O2 | Other oxidants in the water; the Pyxis sensor is non-selective |
| Food and beverage process water | Pyxis ST-766SS; Walchem amperometric sensor | Combined use with PAA in CIP and disinfection lines |
| Wastewater and industrial water | Pyxis ST-766SS-H2O2; Walchem amperometric sensor | Sample pH 2–11 (Walchem) and conductivity at least 10 µS/cm |
| Drinking water, swimming pool, and spa | Walchem amperometric sensor family | For pools, see pool chlorine measurement |
Frequently Asked Questions About Hydrogen Peroxide Measurement
Range, interference, sample conditions, controllers, dosing pumps, and calibration.
How is hydrogen peroxide measured online?
Sample water is passed through a flow cell and the sensor continuously reads the H2O2 concentration. Two options in our documentation: the Walchem amperometric H2O2 sensor (requiring no reagents or consumable tubing, with the sample returnable to the process after measurement) and the Pyxis ST-766SS-H2O2 (membraneless bare-gold electrode that also measures pH, ORP, and conductivity). The sensor signal goes to a controller, and the controller drives the dosing pump.
Which measuring range should be selected?
The Walchem H2O2 sensor is offered under separate part numbers in ranges of 0–200, 0–2,000, and 0–20,000 ppm, with resolutions of 0.1, 1, and 10 ppm respectively. The Pyxis ST-766SS-H2O2 range is 0.00–200.00 ppm. The range is selected according to the expected concentration; an overly wide range causes loss of resolution at low concentrations.
Is the H2O2 reading affected if chlorine, peracetic acid, or ozone is present?
The Walchem brochure states that chlorine, peracetic acid, and ozone do not interfere with the H2O2 sensor. The reverse is not true: for the peracetic acid sensor, ozone, chlorine dioxide, and H2O2 are listed as interferences. The Pyxis ST-766SS-H2O2, on the other hand, is non-selective; it is cross-sensitive to other oxidizing species in the water and yields a combined reading when multiple oxidants are present.
What are the sample and mounting conditions?
For the Walchem H2O2 sensor, sample flow is 30–100 L/h, sample pH is 2–11, conductivity is at least 10 µS/cm, temperature is 0–60 °C, and body pressure is 1 bar; response time is 8 minutes and conditioning of a new sensor takes 5 hours. For the Pyxis ST-766SS, operating temperature is 4–49 °C, inlet pressure is 0.05–0.6 MPa, and response time for the oxidant is T95 at most 240 seconds.
Which controller does it work with?
The Walchem H2O2 sensor operates with W100, W600, and W900 controllers; in installation, the W600-DS-PN panel-type flow switch or W600-DS-FN flow cell is selected. Because the W600 and W900 have been discontinued, Intuition-6 and Intuition-9 are used in new installations. The Pyxis ST-766SS sensor provides 2 x 4-20 mA and RS-485 Modbus outputs; for display and data logging, the OxiPanel IK-765SS-BP PLUS panel includes an H2O2 format.
Is hydrogen peroxide measurement related to peracetic acid measurement?
Yes. According to technical documentation, peracetic acid is synthesized from acetic acid and hydrogen peroxide, so H2O2 may be present in PAA solutions. Consequently, H2O2 interference is taken into account in PAA measurement.
Which pump is selected for hydrogen peroxide dosing?
With off-gassing chemicals such as hydrogen peroxide, the Iwaki EWN-A prevents gas locking via its automatic air vent valve. The high-compression type of the EWN-R is also designed for gas-bearing liquids such as sodium hypochlorite or hydrogen peroxide. Pump head, valve ball, and seal materials are selected according to hydrogen peroxide chemical compatibility, which notes off-gassing at high concentrations.
How often is calibration performed?
For the Walchem amperometric H2O2 sensor, the manufacturer brochure specifies weekly calibration. On the Pyxis ST-766SS, the reference electrode block is replaceable with a typical life of 2 years; calibration can be performed via the uPyxis app using the MA-CR Bluetooth adapter.
Send the make and model of your current instrument to [email protected] — our engineers will propose the equivalent with the same connection and measuring range.
Planning a project? Send us your flow rate, feed water source and target water quality to [email protected] — our engineers will reply with a technical proposal.