  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 insensitive
to fabrication variations in 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 (PCEs) are $-4.54mV/pH$ and $-4.09mV/pH$ respectively.
