Harr Clinical Chemistry Practice Test 2026 – All-in-One Study Guide for Exam Success!

Persistent noise from an ion-selective electrode is most commonly associated with a blocked junction at the salt bridge. The salt bridge serves as a crucial interface for ion flow and measurement stability. When the junction is blocked, it can lead to erratic signals and noise because the flow of ions that facilitates a stable potential measurement is impeded. This blockage can result from crystallization of salts, protein buildup, or samples with high viscosity.

Although contamination of the sample, overrange conditions, and improper calibration can all lead to issues with measurement, they are less likely to result in persistent noise. Contamination often produces inaccurate readings rather than consistent noise, while overrange conditions typically manifest as a flat-line or maximal signal response instead of noise. Improper calibration affects the accuracy and reliability of the measurements but does not directly generate persistent noise in the manner a blocked junction would. Therefore, the significance of a blocked junction stands out as the primary issue leading to consistent disturbances in the signal of an ion-selective electrode.