This paper presents a thread-based microfluidic device (μTAD) that includes ionophore extraction chemistry for the optical recognition of potassium. The device is 1.5 cm × 1.0 cm and includes a cotton thread to transport the aqueous sample via capillary wicking to a 5 mm-long detection area, where the recognition chemistry is deposited that reaches equilibrium in 60 s, changing its color between blue and magenta. A complete characterization of the cotton thread used as well as the sensing element has been carried out. The imaging of the μTAD with a digital camera and the extraction of the H coordinate of the HSV color space used as the analytical parameter make it possible to determine K(I) between 2.4 × 10−5 and 0.95 M with a precision better than 1.3%