Determination of sulfate in phosphoric acid (liquid fertilizer samples) with 859 Titrotherm

Importance of the Topic

• Sulfate content in phosphoric acid fertilizers must be accurately determined to ensure product quality and compliance with industry standards.
• Thermometric titration provides a rapid, robust alternative to traditional potentiometric methods without requiring insulated vessels or electrode maintenance.
• Reliable sulfate quantification supports process control and customer specification verification in fertilizer production.

Objectives and Study Overview

This application bulletin demonstrates a thermometric titration procedure using the 859 Titrotherm to quantify sulfate in wet-process phosphoric acid samples. Key aims include establishing titrant titer, determining method blank, and validating the procedure against expected sulfate levels in commercial fertilizer samples.

Methodology and Instrumentation

Titration is based on the exothermic reaction between sulfate ions (from Na₂SO₄ standard or sample) and Ba²⁺ (as BaCl₂ titrant), producing insoluble BaSO₄ and a measurable temperature inflection at the endpoint. A constant titrant addition rate ensures a uniform heat signal leading to a clear endpoint detected by the Thermoprobe sensor. Regression analysis of titrant volume versus standard mass yields titrant molarity and method blank via intercept.

Used Instrumentation

• 859 Titrotherm with integrated Dosino dosing units (5 mL and 50 mL)
• 800 Dosino for precise solvent delivery
• 3-way stopper with anti-diffusion tip and SGJ glass stoppers
• Reagent organizer and stacking frame for sample handling

Key Results and Discussion

Titrant standardization with dried Na₂SO₄ produced a BaCl₂ concentration of 1.0019 mol/L (R² > 0.9996). Method blank intercepts remained under 0.10 mL, indicating negligible background heat. Three replicate determinations of two fertilizer samples yielded sulfate contents of 2.38 ± 0.03% and 1.85 ± 0.02% SO₄, which align with the customer’s specified ranges (2.15–2.45% and 1.53–1.83% respectively).

Benefits and Practical Applications

• Automated endpoint detection and regression analysis minimize operator intervention and calculation errors.
• Short analysis time and high reproducibility suit routine quality control in fertilizer plants.
• Method requires only standard laboratory glassware without specialized insulation.
• Built-in blank and titer corrections enhance accuracy across varying sample matrices.

Future Trends and Opportunities

• Integration of online thermometric titration for continuous process monitoring in industrial plants.
• Expansion of the technique to other anions and complex sample matrices using tailored reagent systems.
• Development of advanced signal processing and chemometric endpoint detection algorithms.
• Miniaturized and portable thermometric titration devices for field analysis.

Conclusion

The thermometric titration method using the 859 Titrotherm enables accurate, efficient, and automated sulfate determination in phosphoric acid fertilizers. Its robustness, ease of use, and minimal sample preparation make it ideal for industrial quality control, delivering reliable results that meet customer and regulatory requirements.