This graph shows the output from a pH probe during a long automated titration run. A solution of sodium gold chloride (NaAuCl) dissolved in acid is slowly neutralized by adding dilute sodium hydroxide (NaOH). An accurate peristalic pump performs the titration. A computer reading a sensitive pH probe controls the pump.

A peristalic pump is often used in the medical field because it can be accurately controlled. It does not introduce contamination from its mechanical parts because the pumped fluid never touches them. In the pump a pliant plastic tube is squeezed by rollers which flatten the tube at regular intervals. This captures the fluid in the tube between the rollers. When the rollers move, the fluid in the tube moves with them, much the same as a liquid moves in your eusophagus when you swallow.

The two vertical spikes in the graph above occurred during calibration pauses in the run where the probe is removed, rinsed in distilled water, and immersed in a buffered standard pH solution. This sudden pH change caused the graph to make the large shift. A buffered solution of pH 4.00 was used to produce the last spike. Any errors are mechanically corrected at this time. The probe is rinsed again and placed back in service.

The hardware used to introduce the NaOH has been modified to increase the accuracy of the titration. When the pump stops, so must the NaOH. Only with equipment like this can such an accurate titration be accomplished over such a long time.

A setup like this can be seen in the Virtual Lab. Take this link to see an..

Automatic Titration Setup