Valve-switching Enrichment – Determination of Sulfate and Phosphate Ions in Boric Acid by Ion Chromatography

Boric acid is a pure and versatile raw material containing boron oxide, which can be used as a chemical reagent and insecticide. In medicine, it serves as an external bactericide, disinfectant, astringent, and preservative. In industrial production, it is widely used in industries such as glass, ceramics, metallurgy, dyes, electroplating, and textiles. Particularly in the nuclear industry, pressurized water reactor (PWR) power stations often use boric acid to control nuclear reactions. Boric acid is an excellent neutron absorber. In pressurized water reactor (PWR) power stations, trace amounts of sulfate and phosphate ions at concentrations as low as μg/L can cause corrosion of stainless steel components in the station's equipment. Therefore, it is essential to establish a reliable analytical method for detecting anionic contaminants in boric acid.

Valve-switching technology is a commonly used online technique in ion chromatography, primarily employed for the elimination of matrix ions. By switching between chromatographic columns with different selectivities, it utilizes the differences in stationary phase selectivity to enrich target ions and eliminate interference from matrix ions. Alternatively, switching can occur between two columns with identical selectivity. By analyzing the retention behavior of trace ions in the presence of a large number of matrix ions, different strategies can be adopted for various samples, allowing the determination of the core switching time window directly based on the retention time of the target ion standard solution.
The valve-switching method proposed in this study uses a rejection column to separate boric acid from the target ions and an enrichment column for online enrichment. This approach enables the online enrichment of anions in samples with a large injection volume (1 mL) while eliminating interference from the matrix borate ions. Finally, a KOH eluent gradient (generated by an EG) is used for elution, and separation is achieved using the Wayeal NovaChrom HS-5A-P3 analytical column, thereby accomplishing the separation of sulfate and phosphate ions.

Keywords: Ion chromatograph, Boric acid, Anion ions.

1. Experiment Method

1.1 Instrument Configuration

Table 1 Configuration List of Ion Chromatograph System

1.2 Experiment Materials and Auxiliary Equipment

Sulfate ion standard solution in water: 1000mg/L

Phosphate ion standard solution in water: 1000mg/L

Boric acid sample

Aqueous syringe filter (0.22μm)

Electronic balance with a precision of 0.1mg

The experimental water was prepared by using the Wayeal ultrapure water purification system, with a conductivity of 18.25 MΩ·cm (25°C).

1.3 Test Conditions

Table 2 Ion Chromatography Conditions

1.4 Sample Pre-treatment

Weigh 1.0g of the sample into a 50mL volumetric flask, add ultrapure water to the mark, shake well, and let it stand. Take an appropriate amount of the solution, filter it through a 0.22μm membrane filter, and proceed with instrumental analysis.

2. Result and Discussion

2.1 Linear Test

Sequentially pipette a series of standard working solutions of sulfate ions at concentrations of 0.01mg/L, 0.02mg/L, 0.03mg/L, 0.04mg/L, 0.05mg/L, and 0.1mg/L, as well as phosphate ions at concentrations of 0.002mg/L, 0.004mg/L, 0.006mg/L, 0.008mg/L, 0.01mg/L, and 0.02mg/L. Under the working test conditions specified in Section 1.3, the overlapped multi-point chromatograms of the standard curves are obtained as shown in Figure 1. The linear equations are showed in Table 3. Under these chromatographic conditions, the linear correlation coefficients (R) for sulfate ions and phosphate ions are both above 0.999, indicating excellent linearity.

Fig 1. Overlaid Chromatograms of Standard Curves

Fig 2 Standard Curves of Sulfate Ions and Phosphate Ions

Table 3 Linear Equations of Standard Curves

2.2 Sample Test

2.2.1 Sample Content Analysis

Under the working test conditions specified in Section 1.3, the pretreated samples from Section 1.4 were analyzed. The sample chromatogram is shown in Figure 3, and the concentrations of sulfate and phosphate ions in the samples are presented in Table 4.

Fig 3 Sample Test Chromatogram

Table 4 Sample Result Analysis

Note: Blank values have been subtracted from the test results; variations may exist between different methods and inter-laboratory analyses.

3. Conclusion

This analysis by using Wayeal’s ion chromatograph equipped with a conductivity detector, a valve-switching enrichment-ion chromatography method was established for the determination of sulfate and phosphate ions in boric acid. The sample is separated through an exclusion column, enriched via a switching valve on an enrichment column, and then further separated on an analytical ion chromatography column. Quantitative analysis is performed using the external standard method, enabling both qualitative and quantitative determination of sulfate and phosphate ions in boric acid. This method is simple, practical, and meets the requirements for both sensitivity and accuracy, making it suitable for the determination of sulfate and phosphate ion content in boric acid.