Semi-quantitative analysis by spot counting on origami paper-based device for endpoint detection in titrimetric analysis

Abstract

Use of spot counting on an origami paper-based device (SC-PAD) was proposed for end point detection in titrimetric analysis. The multi-folded device allows sample solution to flow from the top layer to the bottom layer. Each layer contains a circular reaction zone by which its hydrophobic barrier was made by wax printing. Reagents used for titration (titrant and indicator) were added onto the circular pattern of the origami PAD. The sample solution was added onto the top layer of the origami PAD. During the solution flow, the analyte then reacted with the pre-deposited reagents on the circular reaction zone of each paper layer. After the reaction was completed, the folded paper was then unfolded for counting the numbers of circular spot that exhibited the color change. This SC-PAD was applied for end point detection for determination of various analytes in water samples by various types of titrimetric analysis. These included the determination of alkalinity by acid-base titration, water hardness by complexometric titration, chloride by precipitation titration, and thiosulfate by redox titration. This SC-PAD could be used for on site test of water quality for the determination of alkalinity in the range of 25 – 400 mg CaCO3 L−1, water hardness in the range of 50 – 425 mg CaCO3 L−1, chloride in the range of 75 – 2500 mg L−1, and thiosulfate in the range of 10 – 200 mg L−1. The SC-PAD was applied to determine hardness and chloride in drinking water and natural water samples. The results obtained by SC-PAD were in good agreement with those from conventional titration in the laboratory. Not only that the SC-PAD platform minimizes the use of chemicals, it enables visual quantitative readout without the use of any instrument.