Determination of phosphoric acid in liquid fertilizer with 859 Titrotherm

Importance of the Topic

Liquid fertilizers are widely used in agriculture, and precise quantification of phosphoric acid content is essential for quality control and optimal nutrient management.
Traditional titration methods can be influenced by interfering ions such as calcium, necessitating robust approaches to ensure accurate and reproducible results.

Objectives and Study Overview

The primary goal of this study is to establish a thermometric titration protocol for quantifying H3PO4 in liquid fertilizers using the 859 Titrotherm system.
Key tasks include standardizing a 2 mol/L NaOH titrant, eliminating calcium interference via potassium oxalate addition, and integrating automated data evaluation with tiamoTM software.

Methodology and Instrumentation

Theory of Thermometric Titration:

• Constant titrant addition rate produces uniform heat effects until the endpoint, detected as an inflection in the temperature curve.
• The 859 Titrotherm with Thermoprobe sensor monitors solution temperature continuously without requiring insulated vessels.

Reagents and Conditioning:

• Standard: Potassium hydrogen phthalate (PHP), dried at 105 °C for titer determination.
• Titrant: 2 mol/L NaOH solution standardized via regression of volume consumed vs. PHP mass.
• Conditioning solutions: 300 g/L NaCl and saturated potassium oxalate monohydrate to precipitate calcium.

Sample Preparation:

• Weigh 0.6–1.1 g PHP or 0.61–0.68 g fertilizer sample into the titration vessel.
• Add 20 mL NaCl solution, allow precipitation for 30 min, then add 10 mL deionized water and 2 mL potassium oxalate; stir and titrate to the third endpoint.

Instrumentation:

• 859 Titrotherm equipped with one 10 mL and one 5 mL Dosino dosing units, 3-way stopper, and reagent organizer.

Data Evaluation:

• TiamoTM software performs linear regression on titration endpoints to calculate titrant concentration, method blank, and sample analyte content.

Main Results and Discussion

Titrant Standardization:

• NaOH titration of PHP yielded a molarity of 1.9488 mol/L and a titer of 0.9744, with R² = 0.9994.

Method Blank Determination:

• Blank titrations across different aliquots produced a slope of 2.3142, an intercept of 0.0499 mL, and R² = 0.9996.

Sample Analysis:

• Analysis of a liquid fertilizer sample (0.6576 g) resulted in 44.93 % H3PO4, with s(abs) = 0.263 and RSD = 0.59 % (n=5).

These findings demonstrate high precision, accuracy, and linearity for the thermometric titration method in fertilizer matrices.

Benefits and Practical Applications of the Method

• Fast and automated determination without the need for insulated vessels.
• Effective removal of calcium interference via oxalate addition.
• Seamless integration with tiamoTM for on-the-fly data processing and blank correction.
• Suitable for routine quality control in fertilizer production and research laboratories.

Future Trends and Applications

• Integration with automated sample preparation and flow-through analysis for higher throughput.
• Extension of thermometric titration protocols to other agricultural analytes (e.g., nitrogen, potassium).
• Implementation in inline process control for real-time monitoring of fertilizer synthesis.
• Potential use in mobile field units for on-site fertilizer testing.

Conclusion

The thermometric titration method using the 859 Titrotherm provides a robust, accurate, and efficient approach for quantifying phosphoric acid in liquid fertilizers. Automatic endpoint detection and software-driven data evaluation enhance reproducibility and streamline routine analyses. This protocol meets the demanding requirements of industrial quality control and offers opportunities for further automation and expansion into related analytical applications.