I'm an analytical chemist troubleshooting a persistent issue with our automated titration system for determining the acid number in used lubricating oils. We're following a standard ASTM method using potassium hydroxide, but our endpoint detection with the potentiometric probe is inconsistent, leading to high variability between replicates. For others who run similar high-throughput titrations, have you encountered problems with electrode drift or sample matrix interference from additives in degraded oils? What steps did you take to validate your electrode response and cleaning procedure between samples, and did you find switching to a different titrant or a mixed solvent system improved reproducibility? I'm also considering implementing a more rigorous system suitability test.
End-point drift is a common hurdle in high-throughput titrations. Set up a strict calibration regime: run a fresh standard KOH on a clean solvent blank, monitor the slope and intercept over the day, and document every drift. A stable reference electrode is crucial; replace or condition it if drift exceeds a few mV per run. Also ensure samples are fully dissolved and well-mixed to avoid localized pH shifts near the electrode. If you can, run a quick 'check-standard' between samples to catch drift early.
Matrix interference from additives in degraded oils is real. Pre-dissolve using a compatible solvent system that maintains conductivity and minimizes emulsions; filter if necessary; degas if CO2 evolves. Consider adding a small amount of a background electrolyte to stabilize ionic strength. Keep the sample prep consistent across runs to avoid variability. Also screen a subset of samples with different solvents and evaluate effect on endpoint.
Endpoint detection: consider using Gran plots or second-derivative plots to robustly identify the equivalence point; ensure stable stirring and temperature; auto-zero drift can be corrected by baseline subtraction. If you're using potentiometric titration with a glass electrode, watch for drift from the reference; consider a differential titration approach to reduce offset; you could evaluate dynamic pre-titration to approach end-point gradually.
About titrant: switching to a different titrant or a mixed solvent system can help. Some labs move to a non-aqueous system (THF/EtOH or toluene/MeOH) with KOH to improve solubility and reduce water interference and emulsions. If you switch solvents, standardize the solvent purity and prepare fresh solutions; consider using a co-solvent that improves oil dissolution while maintaining reaction equivalence.
Implement system suitability: run a standard oil sample with known acid number to establish baseline; track precision (RSD across replicates), accuracy (recovery vs known value), linearity across concentration ranges; monitor drift over time; include blanks; implement control charts; set acceptance criteria: e.g., replicate RSD < 2-3%, measured value within ±0.5 mg KOH/g across days; require at least two consecutive passes before releasing data.