Determination of Fluoride, Chloride, Bromide and Sulfate in Zinc Borate Flame Retardants by IC
2026-11-15
Background
Zinc borate is one of the earliest inorganic flame retardants, developed in the 1970s. It offers good thermal stability, low toxicity and smoke suppression, works well in combination with other flame retardants, and noticeably reduces smoke density when added to a formulation. The white to pale yellow crystalline powder acts through several mechanisms: it forms a glassy inorganic intumescent coating, promotes char formation, hinders the escape of volatile combustibles, and releases water at high temperature, providing heat absorption, foaming and dilution of combustible gases.
In halogen-free systems, zinc borate combined with halogen-free flame retardant packages achieves excellent flame retardancy while reducing the dosage of other additives and improving processing performance. Because zinc borate is a core component of environmentally friendly flame retardants, its purity directly affects flame retardant efficiency and product safety. Excess halide ions disrupt the synergistic flame retardant action between zinc borate and the polymer and reduce smoke suppression; chloride and bromide corrode metal parts such as cable sheathing at high temperature and shorten service life; and excessive sulfate can crystallise out, blocking production piping and contaminating wastewater. A fast method for these anions is therefore essential.
This case establishes an ion chromatography method in which zinc borate is dissolved in dilute nitric acid and fluoride, chloride, bromide and sulfate are separated on a Wayeal Nuopu HS-5A-P3 anion-exchange column. The method is sensitive and delivers accurate, reliable results.
Instrument Configuration and Test Conditions
| Item | Description |
|---|---|
| Ion chromatograph | Wayeal IC system with conductivity detector |
| Autosampler | AS3100 fully automatic autosampler |
| Suppressor | 4 mm hydroxide anion suppressor |
| Analytical column | Hydroxide P3 anion column, Nuopu HS-5A-P3 |
| Guard column | General-purpose anion guard column, HS-5AG |
| Parameter | Setting |
|---|---|
| Eluent | 18 mmol/L KOH, isocratic |
| Flow rate | 1.0 mL/min |
| Column temperature | 30 °C |
| Cell temperature | 35 °C |
| Injection volume | Full-loop injection, 25 µL |
| Run time | 20 min |
| Suppressor current | 54 mA |
Sample Preparation
Approximately 0.1 g of sample is weighed, dissolved in 0.8 mL of 25% nitric acid, and made up to 100 mL with ultrapure water (18.25 MΩ·cm at 25 °C). The solution is filtered through a 0.22 µm aqueous syringe filter and injected directly. Dissolution is fast and quantitative, so no ashing or extraction step is required.
Results
Linearity: five-level calibration curves were prepared for fluoride, chloride and bromide at 0.01, 0.05, 0.1, 0.15 and 0.2 mg/L, and for sulfate at 0.1, 1, 5, 10 and 15 mg/L. All four analytes showed correlation coefficients above 0.999.
| Ion | Linear equation | R |
|---|---|---|
| F− | y = 35.01936x − 0.14805 | 0.99960 |
| Cl− | y = 26.84452x − 0.19272 | 0.99938 |
| Br− | y = 9.12204x − 0.05199 | 0.99918 |
| SO42− | y = 18.79192x − 3.89925 | 0.99959 |
Sample results: two parallel determinations were run on a zinc borate sample. Fluoride averaged 38.5 mg/kg, chloride 75.5 mg/kg, bromide was not detected, and sulfate averaged 1346 mg/kg — with good agreement between the parallels for every analyte.
| Analyte | Measured (mg/L) | Content (mg/kg) | Average (mg/kg) |
|---|---|---|---|
| Fluoride | 0.038 / 0.041 | 37.3 / 39.7 | 38.5 |
| Chloride | 0.074 / 0.081 | 72.7 / 78.3 | 75.5 |
| Bromide | ND | ND | ND |
| Sulfate | 1.376 / 1.386 | 1352 / 1340 | 1346 |
ND = not detected. Results are blank-corrected; values may vary between methods and laboratories.
Conclusion
Using a Wayeal ion chromatograph with conductivity detection, dilute nitric acid dissolution, anion-exchange separation and external standard quantification, fluoride, chloride, bromide and sulfate in zinc borate can be determined qualitatively and quantitatively. The method is simple, shows good parallelism, and its sensitivity and accuracy meet the requirements — making it suitable for routine anion control of inorganic flame retardant raw materials.
