We report the fabrication and characterization results of a simple and low-cost pH sensor fabricated using a graphite pencil to define a working electrode and silver paste to define a reference electrode on Scotch tape. The sensor operation is based on potentiometric measurement and thereby is insensitive to fabrication variations in the shape of the electrode, unlike amperometric and chemiresistive measurement techniques. The substrate of the disposable sensor is prepared by pasting Scotch tape atop a piece of chart paper, and two types of sensors fabricated with 6B and 2B graphite pencils are tested with three solutions with different pH values. The sensor functions as a passive sensing tag without requiring any external power or stimulus, and the measured sensitivities of the pH sensors fabricated using 2B and 6B pencil carbon electrodes are -4.54 and -4.09 mV/pH, respectively.