Determination of Orthophosphate in Ammonium Polyphosphate by Wayeal Ion Chromatography.

Ammonium polyphosphate is also known as ammonium polypolyphosphate or condensed ammonium phosphate. Ammonium polyphosphate with a low polymerization degree features good water solubility, high nitrogen and phosphorus nutrient content, and a slow-release effect of its phosphorus element, making it widely used as a compound fertilizer.

Compared to conventional compound phosphate fertilizers, polymeric phosphorus offers advantages such as better solubility, longer-lasting fertilizer efficiency, and higher nutrient utilization rate, along with greater safety for soil and crops, and contributes to balanced fertilization. The phosphorus in ammonium polyphosphate exhibits a slow-release effect, and the polymerization ratio, which represents the mass fraction of polymeric phosphate in total phosphorus, serves as a critical indicator for evaluating this slow-release performance.

China's chemical industry standard "GB/T 5939-2021 Ammonium Polyphosphate for Fertilizer Use" specifies two determination methods: the ion exchange resin method and the ion chromatography method. These methods separate orthophosphate from polymeric phosphorus through an ion exchange resin column or an ion chromatography column, respectively, followed by determination of the orthophosphate content.This application note describes the determination of orthophosphate content in ammonium polyphosphate using Anhui Instrument's ion chromatograph.

Keywords: Ion chromatography, Ammonium Polyphosphate, Orthophosphate, Chemical Industry.

1. Instruments and Reagents

1.1 Configuration List of Ion Chromatography

Table 1 Configuration List of Ion Chromatography

1.2 Reagents and Standards

Table 2 Reagents and Standards

1.3 Experiment Material and Auxiliary Equipment

Prefilled syringe filter (0.22μm)

2. Experiment Method

2.1 Sample Pretreatment

Weigh approximately 0.02g of ammonium polyphosphate sample, dissolve it in ultrapure water, dilute to a final volume of 100mL, and filter the sample through a 0.22μm disposable syringe filter before injection into the ion chromatograph for analysis.

2.2 Experiment Conditions

Chromatographic Conditions for Anions

3. Experiment Result

3.1 Chromatogram of Standards

The determination of orthophosphate in the ammonium polyphosphate sample was completed within 45 minutes, showing excellence peak shape and good response, which met the analytical requirements.

Fig 1 Overlap Chromatogram of Orthophosphate Standard Curve

3.2 Linear Range

Take appropriate amount of orthophosphate standard solution and sequentially diluted to a series of concentrations to prepare the calibration curve. The linear range was 10–50mg/L, with deviations of the measured results from known concentrations being less than the maximum permissible deviation. The R² values were above 0.999, indicating excellent linearity for all components.

Table 5 Orthophosphate Linearity Range Table

Fig 2 Linear Regression Results for Orthophosphate

3.3 Sample Analysis

Sample Analysis (Available Phosphorus Content: 60.48%)

3.3.1 Blank Sample

3.3.2 Ammonium Polyphosphate Sample Testing

Chromatograms of Orthophosphate Ions in Ammonium Polyphosphate Parallel Samples 1, 2, and 3

3.4 Sample Analysis Result

The data indicate that the method yields satisfactory chromatographic peak shapes, with linear correlation coefficients all greater than 0.999. The sample contents are as follows (the analyte content in the ammonium polyphosphate sample is calculated as the average of Parallel Samples 1, 2, and 3; the polymerization ratio is determined by the difference between the available phosphorus and orthophosphate contents in the sample, divided by the available phosphorus content):

4. Conclusion

The determination of orthophosphate content in ammonium polyphosphate was performed using an Wayeal ion chromatograph. This method enables precise monitoring of orthophosphate levels in ammonium polyphosphate, ensuring compliance with quality control requirements for fertilizer-grade products. It further allows evaluation of the hydrolysis rate of ammonium polyphosphate in soil and its phosphorus release capacity, thereby supporting the optimization of fertilization strategies to improve utilization efficiency and enhance overall fertilizer performance.