Potassium ion batteries (KIBs) are gaining attention as attractive, low-cost alternatives to lithium ion batteries (LIBs). Emerging KIB materials are not yet fully understood, so in situ characterization techniques are being developed to address the similarities and differences to the operation of LIB materials, including aspects of interfacial ion transfer and solid electrolyte interphase (SEI) formation. Here, we introduce the use of Hg disc-well microelectrodes as probes in scanning electrochemical microscopy (SECM) for the detection of K+ gradients on an operating graphitic material. Electrochemically controlled amalgamation and stripping reactions on these probes permit their accurate positioning near a conductive surface, and the detection of local concentration changes once the substrate is biased to intercalate K+. K+ reduction into the Hg phase follows a behavior similar to that of Li+ and Na+ and yields an electrochemical response that is used to evaluate local substrate reactivity. Using these probes in situ, we demonstrate the reversible intercalation of K+ on a surface site of patterned highly oriented pyrolytic graphite (HOPG), a model interface for carbonaceous KIB materials. Our method affords a direct measurement of localized K+ fluxes, which are not resolvable through bulk electroanalytical techniques, thus making our approach potentially informative about reaction mechanisms for nascent KIB-based energy storage technologies.