Determination of Orthophosphate in Ammonium Polyphosphate by Ion Chromatography for Fertilizer Quality Control
2026-11-11
Background
Ammonium polyphosphate (APP), also called polymerized or condensed ammonium phosphate, is widely used as a compound fertilizer. Low-polymerization grades are highly water-soluble, carry high nitrogen and phosphorus content, and release their phosphorus slowly. Compared with conventional phosphorus fertilizers, polymerized phosphate offers better solubility, a longer lasting effect, higher nutrient use efficiency, greater safety for soil and crops, and support for balanced fertilization.
Because the slow-release behaviour of APP depends on how much of its phosphorus is still in polymerized form, the polymerization rate — the mass fraction of polymerized phosphate in total phosphorus — is a key quality indicator. The Chinese chemical industry standard GB/T 5939-2021 (Fertilizer grade ammonium polyphosphate) describes two determination routes: an ion exchange resin method and an ion chromatography method. Both separate orthophosphate from polymerized phosphate on a resin or IC column before measuring the orthophosphate content. This case reports the results obtained with a Wayeal ion chromatograph applying the IC approach.
Instrument Configuration
| Item | Description | Qty |
|---|---|---|
| Ion chromatograph | IC6600 series with conductivity detector | 1 |
| Autosampler | AS2800 | 1 |
| Software | SmartLab CDS 2.0 chromatography workstation | 1 |
| Analytical column | HS-5A-P3, 4.0 × 250 mm | 1 |
| Consumables | 0.22 µm disposable syringe filters; orthophosphate standard 1000 mg/L | — |
Sample Preparation and Chromatographic Conditions
Approximately 0.02 g of ammonium polyphosphate sample is weighed, dissolved in ultrapure water and made up to 100 mL. The solution is passed through a 0.22 µm disposable syringe filter and injected directly — no derivatisation or complex clean-up is required.
| Parameter | Setting |
|---|---|
| Column | HS-5A-P3, 4.0 × 250 mm |
| Eluent | Isocratic, 40 mmol/L KOH |
| Flow rate | 1.0 mL/min |
| Run time | 45 min |
| Injection volume | 25 µL |
| Column temperature | 30 °C |
| Cell temperature | 30 °C |
| Suppressor current | 120 mA |
Results
The determination of orthophosphate in the ammonium polyphosphate sample was completed within 45 minutes, with well-shaped peaks and good response. A blank injection showed no orthophosphate (0 mg/L), confirming a clean baseline.
Linearity: a series of diluted orthophosphate standards was prepared. The linear range was 10–50 mg/L with a correlation coefficient R² of 0.99972, and the deviation from the known concentrations stayed within the maximum permitted tolerance.
Sample results: three parallel determinations were run on a sample with 60.48% available phosphorus.
| Sample | Weight (g) | Volume (mL) | Measured (mg/L) | Orthophosphate (%) |
|---|---|---|---|---|
| Parallel 1 | 0.0215 | 100 | 20.879 | 9.71 |
| Parallel 2 | 0.0227 | 100 | 20.922 | 9.22 |
| Parallel 3 | 0.0239 | 100 | 21.926 | 9.17 |
| Average | — | — | — | 9.367 |
Taking the average of the three parallels, the orthophosphate content was 9.367%. The polymerization rate — the difference between available phosphorus and orthophosphate relative to available phosphorus — was 84.5% by parallel average and 84.8% for the reported sample result, showing good repeatability across injections.
Conclusion
The Wayeal ion chromatograph, configured with a conductivity detector, AS2800 autosampler and HS-5A-P3 column, provides a precise and efficient route for monitoring orthophosphate in ammonium polyphosphate. The method supports fertilizer-grade product quality control, and the polymerization rate it delivers helps manufacturers assess hydrolysis behaviour in soil, evaluate phosphorus release capability, optimise fertilization programmes and improve overall fertilizer performance.
