Microscale titration of acetic acid using digital colorimetry and paper-based analytical devices

A quantitative method for acid-base titrations in paper-based devices (PADs) is described to analyze acetic acid in vinegar samples. In this work, two different types of PADs were developed: a device for individual spot testing and a microfluidic device. Digital colorimetry was used as the detection method, and the images were acquired using a smartphone and a homemade box with LED lights for controlled image acquisition. Titration curves were built with just eight points, using the R channel based on the gradual color transition from red to blue of litmus, a natural indicator. The endpoint was accurately determined by second derivative calculations. Both systems were applied to fifteen vinegar samples of different types, and good concentration results were obtained in comparison to the reference method. The proposed methodology is simple, fast, environmentally friendly, and surpasses the need for calibration curve construction. Moreover, the subjective endpoint identification is eliminate, and the method was automated to provide a high throughput workflow, suitable for quality control processes and real-time measurements.