Colorimetric determination of acidity constant using a paper-based microfluidic analytical device

Acid dissociation constant is an important chemical characteristic of organic and inorganic compounds and it affects both chemical properties and biological activities of the molecules. Herein, a very simple, fast and cost-effective method based on microfluidic technology has been reported for colorimetric determination of acidity constants. The designed device works based on pH-metric titration of colorful indicators followed by colorimetric measurements by a smart phone mobile device or a flatbed scanner. So, it does not need sophisticated instrumentation and is accomplished in a very short time (about 1 min). All titration steps are transferred on a star-like designed A mu PAD device: (1) spotting 0.3 A mu L buffers of different pHs at the end of the channels (reaction zones), (2) spotting a 30 A mu L portion of the indicator on the center of A mu PAD followed by movement of indicator solution toward the reaction zone by capillary action of the paper. The measured color change of the indicators at the reaction zone is fitted to the Henderson-Hasselbalch equation, through which acidity constants are calculated. The performance of the device was evaluated by measuring acidity constant of 4 indicators including bromothymol blue, bromocresol green, bromocresol purple and phenolphthalein. A very close agreement was achieved between those measured by the suggested device here and the previously reported values. The reproducibility of this method was lower than 5% for relative standard deviation of three replicate measurements.