Photometric determination of acidity in ethanol according to ASTM D7795
Applications | 2018 | MetrohmInstrumentation
Denatured fuel ethanol often contains acidic species such as acetic acid and additives that can accelerate corrosion and reduce storage stability. Monitoring acidity is therefore critical for maintaining fuel quality and preventing equipment damage.
This application note presents a photometric titration method following ASTM D7795 to determine total acidity in ethanol. The method was validated by analyzing denatured ethanol samples spiked with 30 mg/kg acetic acid to assess accuracy and precision.
The procedure involves weighing 55–65 g of sample, adding 2 mL of phenolphthalein indicator solution, purging with nitrogen at 400 mL/min for 120 s, and titrating with 0.01 mol/L NaOH. Endpoint detection at 574 nm is achieved with an Optrode sensor integrated into an automated OMNIS titration system. Rinsing steps between runs ensure system cleanliness and reproducibility.
Analysis of six replicate samples yielded an average acidity of 31.3 mg/kg with a relative standard deviation of 2.5%, closely matching the spiked concentration. The photometric endpoint at 574 nm provided clear detection of the phenolphthalein color shift, demonstrating high repeatability and sensitivity.
Future developments may include miniaturized optical sensors for inline monitoring, expanded acid speciation through multiwavelength detection, and real-time data analytics integration for process control. Such enhancements could further streamline fuel quality assessment and regulatory compliance.
The photometric titration method according to ASTM D7795 provides a robust, automated, and precise approach to assess acidity in denatured ethanol, supporting reliable quality control and operational efficiency.
Titration Application Note T-199, Version 1, November 2018
Titration
IndustriesEnergy & Chemicals
ManufacturerMetrohm
Summary
Importance of the Topic
Denatured fuel ethanol often contains acidic species such as acetic acid and additives that can accelerate corrosion and reduce storage stability. Monitoring acidity is therefore critical for maintaining fuel quality and preventing equipment damage.
Objectives and Study Overview
This application note presents a photometric titration method following ASTM D7795 to determine total acidity in ethanol. The method was validated by analyzing denatured ethanol samples spiked with 30 mg/kg acetic acid to assess accuracy and precision.
Methodology and Instrumentation
The procedure involves weighing 55–65 g of sample, adding 2 mL of phenolphthalein indicator solution, purging with nitrogen at 400 mL/min for 120 s, and titrating with 0.01 mol/L NaOH. Endpoint detection at 574 nm is achieved with an Optrode sensor integrated into an automated OMNIS titration system. Rinsing steps between runs ensure system cleanliness and reproducibility.
Used Instrumentation
- OMNIS Sample Robot S with Pick&Place and pump modules
- OMNIS Titrator Advanced (without stirrer)
- OMNIS 10 mL cylinder unit and analog measuring module
- Optrode photometric sensor
- Supporting accessories: ETFE stir propeller, electrode cable, adapter cable, software license
Main Results and Discussion
Analysis of six replicate samples yielded an average acidity of 31.3 mg/kg with a relative standard deviation of 2.5%, closely matching the spiked concentration. The photometric endpoint at 574 nm provided clear detection of the phenolphthalein color shift, demonstrating high repeatability and sensitivity.
Benefits and Practical Applications
- High accuracy and precision suitable for QA/QC in fuel ethanol production
- Automated titration reduces manual intervention and potential errors
- Low reagent consumption and rapid throughput enhance laboratory efficiency
Future Trends and Potential Applications
Future developments may include miniaturized optical sensors for inline monitoring, expanded acid speciation through multiwavelength detection, and real-time data analytics integration for process control. Such enhancements could further streamline fuel quality assessment and regulatory compliance.
Conclusion
The photometric titration method according to ASTM D7795 provides a robust, automated, and precise approach to assess acidity in denatured ethanol, supporting reliable quality control and operational efficiency.
References
Titration Application Note T-199, Version 1, November 2018
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