This article details the principle of boiler water phosphate (radical) detection, focusing on the operating steps, national standard requirements, and instrument recommendations for the ammonium molybdate spectrophotometric method. Includes practical cases and application scenarios of the ERUN-SP7-E3 portable phosphate analyzer to help industrial users accurately monitor water quality.
The detection principle for phosphate (radical) in boiler water involves the reaction of ammonium molybdate with phosphate in an acidic environment to form yellow phosphomolybdic acid, which is then reduced by ascorbic acid to molybdenum blue. The phosphate concentration is calculated precisely by measuring its absorbance with a spectrophotometer. This method complies with national standards and is a core technology for industrial boiler water quality monitoring.
Boilers are the "heart" of industrial facilities such as power plants, chemical plants, and paper mills. Their water quality directly impacts operational efficiency and safety. Phosphate treatment is a key part of boiler water chemical control: by dosing chemicals like trisodium phosphate, phosphate radicals can combine with calcium and magnesium ions to form loose sludge, preventing hard scale from adhering to heating surfaces.
Data shows: A 1 mm increase in boiler scale thickness leads to a 2%~5% rise in fuel consumption (Source: "Technical Specification for Industrial Boiler Water Treatment" TSG G50002-2010). If phosphate concentration is out of control:
Concentration too low (<2 mg/L): Unable to effectively prevent scale, leading to decreased heat transfer efficiency.
Concentration too high (>15 mg/L): Can induce "phosphate hiding," accelerating corrosion.
Therefore, accurate detection of phosphate radical content is central to achieving the dual goals of "zero scale loss, zero corrosion."
In an acidic medium (typically sulfuric or hydrochloric acid environment), orthophosphate (PO₄³⁻) in the water sample reacts with ammonium molybdate ((NH₄)₂MoO₄) to form phosphomolybdic acid:
PO₄³⁻ + 12MoO₄²⁻ + 27H⁺ → H₃[P(Mo₃O₁₀)₄] + 12H₂O
Subsequently, ascorbic acid (C₆H₈O₆) is added as a reducing agent, converting the yellow heteropoly acid into a blue complex – molybdenum blue:
H₃[P(Mo₃O₁₀)₄] + 4C₆H₈O₆ → (MoO₂·4MoO₃)₂·H₃PO₄ (Molybdenum Blue)
Key Point: Molybdenum blue has a maximum absorption peak at 880 nm wavelength. The concentration is proportional to the absorbance, conforming to the Lambert-Beer law.
According to "GB/T 6913-2023 Analysis of water used in boilers and cooling systems - Determination of phosphates", the specific steps are:
Take a 100 mL water sample in a colorimetric tube.
Add 5 mL of acidic ammonium molybdate reagent.
Shake well, then add 2 mL of ascorbic acid solution.
Let it stand for 10 minutes for color development.
Measure the absorbance with a spectrophotometer.
Calculate the phosphate radical concentration using a standard curve.
Accuracy Requirements: The method's detection limit is 0.01 mg/L, with a relative error of <5% in the 0~20 mg/L range.
| Boiler Pressure Grade | Phosphate Radical Control Range (mg/L) | Detection Frequency | Reference Standard |
|---|---|---|---|
| Low-Pressure Boiler (<2.5 MPa) | 5~15 | Once per shift | GB/T 1576-2018 |
| Medium-Pressure Boiler (2.5~5.8 MPa) | 2~10 | Every 4 hours | GB/T 12145-2016 |
| High-Pressure Boiler (>5.8 MPa) | 0.5~5 | Continuous Monitoring | DL/T 912-2005 |
Note: Specific limits should be adjusted based on parameters like feedwater hardness and silicon content.
Sampling and cooling can take up to 30 minutes.
Reagent preparation errors affect accuracy.
Data recording relies on manual entry, prone to errors.
Taking the ERUN SP7-E3 Portable Phosphate Analyzer as an example, this device is optimized for field detection:
Fast Measurement: Completes the process from sampling to reading in 3 minutes.
Built-in Curves: Pre-loaded with national standard curves, automatic temperature compensation.
Anti-contamination Design: Dedicated cuvettes prevent cross-contamination.
Data Management: Stores 500 data sets, supports Bluetooth printing.
Practical Application Scenario: After a chemical power plant started using the ERUN-SP7-E3, they reduced detection time from 45 minutes to 8 minutes and successfully provided an early warning for a phosphate hiding phenomenon, preventing an unplanned shutdown.
Background: A 2×300 MW thermal power plant in Shandong, with boiler pressure at 12.5 MPa, using an AVT (All Volatile Treatment) water regimen.
Problem: In April 2023, frequent drum pressure fluctuations occurred. Traditional detection showed a phosphate concentration of 3.2 mg/L (within the standard range), but corrosion signs still appeared in the thermal system.
Solution:
Used the ERUN-SP7-E3 for intensive spot testing (every 2 hours).
Discovered phosphate concentration fluctuated sharply between 2.8~7.5 mg/L.
Combined with pH monitoring, confirmed the presence of phosphate hiding.
Result: By adjusting the phosphate dosing strategy, the concentration was stabilized within the 2.5~3.5 mg/L range. After 3 weeks, the corrosion rate decreased from 0.15 mm/a to 0.02 mm/a.
Q1: Why must the measurement be completed within 10 minutes after color development?
A: The molybdenum blue complex gradually oxidizes in air, leading to decreased absorbance. The national standard specifies that measurement within 10 minutes after color development yields the smallest error.
Q2: Does silicate interfere with the determination?
A: Yes. However, the national standard method effectively suppresses the formation of silicon molybdenum blue by controlling the acidity (pH=1.0~1.2), ensuring detection specificity.
Q3: How to verify the accuracy of detection results?
A: It is recommended to perform curve calibration monthly using standard samples (e.g., 0.5 mg/L, 5.0 mg/L, 10.0 mg/L). The deviation should be <3%.
Boiler water phosphate detection is not merely a simple chemical test but a critical line of defense for industrial safety. The ammonium molybdate spectrophotometric method, as a nationally recommended standard, combined with portable equipment like the ERUN-SP7-E3, brings accurate detection from the laboratory to the field, providing a solid guarantee for the long-term operation of boilers.
The technical content of this article refers to "GB/T 6913-2023" and "DL/T 912-2005". Product information is based on publicly available technical data from Yingrun Environmental Protection.
