In Situ Ultrasonic Monitoring of Aluminum Ion Hydrolysis in Aqueous Solutions: Instrumentation, Techniques, and Comparisons to pH-Metry

A cross-disciplinary experimental study related to both ultrasonic instrumentation and analytical chemistry is reported. The hydrolysis process was conducted by time-resolved titration in a fully automated manner. Acquired ultrasonic records were processed in order to estimate the propagation delay of the ultrasonic pulse in the evolving medium. The limited hardware resolution of two different ultrasonic instruments employed was improved by calculating the center of gravity of the recorded pulses. Application of signal averaging to the acquisition of raw records in the custom-built instrument eliminated spurious records almost completely. The estimated ultrasonic delays were corrected for temperature changes that were measured independently. This procedure transformed the ultrasonic titration curves into two almost straight lines that intersected at the equivalence point. The results obtained showed that it was possible to detect changes as small as 200 ppm by using the ultrasonic instrument in situ at significantly lower setting times compared to a conventional pH-meter.