Detection of ultratrace levels of metallic ion impurities in hydrofluoric acid solutions was demonstrated using a silicon-based sensing electrode. The sensor’s operation principle is based on direct measurements of the silicon open-circuit potential shift generated by the charge-transfer reaction between metallic ions and the silicon-based sensing surface. For instance, the silicon-based sensor is capable of detecting parts per-trillion to parts per billion level of Ag+ ions in HF solutions with a detection sensitivity of ca. +150 mV shift per decade change of [Ag+]. The new sensor can have practical applications in the on-line monitoring of microelectronic chemical processing.