WTW NH500/TC Ammonia Ion-Selective Electrode (Order No. 821210)

Overview

The WTW NH500/TC Ammonia Ion-Selective Electrode (Order No. 821210) is a high-stability, gas-diffusion-based potentiometric sensor engineered for continuous, on-line measurement of ammonium nitrogen (NH₄–N) and total ammonium (NH₄⁺) in aqueous environmental and wastewater matrices. Designed exclusively for integration with WTW’s TresCon+ OA110 and TresCon UNO A110 online analyzers, the NH500/TC operates on the principle of selective ammonia gas diffusion across a hydrophobic membrane, followed by pH-dependent detection via an internal pH-sensitive glass electrode. In alkaline sample conditions—achieved through automated reagent dosing (NaOH)—ammonium ions (NH₄⁺) are converted to gaseous ammonia (NH₃), which diffuses freely through the membrane into a thin, buffered internal electrolyte layer. This causes a measurable shift in the pH of the internal solution, proportional to the logarithm of the NH₃ partial pressure—and thus to the original NH₄⁺ concentration. The electrode’s architecture eliminates direct contact between the sample and the sensing element, minimizing fouling, drift, and interference from heavy metals, sulfides, or organic matter commonly present in raw sewage or industrial effluents.

Key Features

• Gas-sensing configuration with integrated pH transducer and sealed internal reference system
• Stable, low-drift response over extended operational periods (typical calibration interval: 7–14 days under routine wastewater monitoring)
• Three replaceable ammonia-selective membranes included per kit—designed for consistent permeability and minimal aging effects
• Pre-filled internal electrolyte solution optimized for long-term pH buffering and thermal stability (operating range: 5–45 °C)
• Direct compatibility with WTW TresCon platform firmware; automatic temperature compensation (via integrated Pt1000 sensor in TC variant)
• No moving parts or optical components—robust mechanical design suited for unattended operation in harsh municipal or industrial environments

Sample Compatibility & Compliance

The NH500/TC electrode delivers reliable performance across a broad spectrum of real-world water matrices, including activated sludge return streams, nitrification/denitrification basin effluents, surface water influents, and pre-treated industrial discharges. It meets functional requirements aligned with ISO 14911 (water quality — determination of ammonium — ion-selective electrode method) and supports compliance workflows under EU Urban Wastewater Treatment Directive (91/271/EEC) and U.S. EPA Method 350.1 (ammonia by ion-selective electrode). While not a standalone analytical instrument, its use within validated TresCon analyzer systems enables data traceability, audit-ready calibration logs, and adherence to GLP principles when deployed with proper SOPs and maintenance records. Membrane integrity and electrolyte volume must be verified during scheduled preventive maintenance per WTW Service Bulletin SB-NH500-2023.

Software & Data Management

When installed in a TresCon+ OA110 or TresCon UNO A110 analyzer, the NH500/TC electrode interfaces seamlessly with the embedded WTW TresCon Control Software (v4.2+). All sensor diagnostics—including membrane status flagging, electrolyte level estimation, response time monitoring, and slope/stability validation—are logged automatically with timestamps and user-assignable event tags. Data export supports CSV and XML formats compatible with SCADA, LIMS, and cloud-based environmental dashboards (e.g., Siemens Desigo CC, Schneider EcoStruxure). Full audit trail functionality satisfies FDA 21 CFR Part 11 requirements when configured with electronic signatures and role-based access control on networked systems.

Applications

• Real-time feedback control of nitrification processes in biological nutrient removal (BNR) plants
• Monitoring of ammonia breakthrough in tertiary treatment stages prior to discharge
• Compliance reporting for NPDES permits requiring hourly or sub-hourly NH₄–N data
• Early warning detection of process upsets in anaerobic digesters or sidestream treatment units
• Long-term trend analysis of seasonal ammonia loading in combined sewer overflow (CSO) monitoring stations

FAQ

What is the recommended replacement interval for the NH500/TC membrane?
Membranes should be replaced every 3–6 months depending on suspended solids load and cleaning frequency; visual inspection for cloudiness or physical damage is mandatory before each recalibration.
Can the NH500/TC be used with non-WTW analyzers?
No—it requires proprietary electrical interface, fluidic coupling, and firmware-level handshake protocols exclusive to the TresCon platform.
Is temperature compensation automatic?
Yes—the TC variant includes an integrated Pt1000 RTD sensor that feeds real-time temperature data directly to the analyzer’s compensation algorithm.
What maintenance steps are required between calibrations?
Daily visual check of membrane clarity and electrolyte fill level; weekly flush with deionized water; monthly verification of zero/slope using certified NH₄–N standards (e.g., NIST-traceable 10 mg/L and 100 mg/L solutions).
How does the NH500/TC handle samples with high chloride or sulfide content?
The gas-diffusion barrier inherently suppresses interference from Cl⁻, SO₄²⁻, S²⁻, and most common cations; however, samples with >5000 mg/L Cl⁻ may require dilution to maintain measurement linearity and membrane longevity.