Simple Paper Device Titrates Acids And Bases

ACE OF BASE A paper-based device titrates basic solutions of NaOH. The numbers at the end of each spoke indicate the concentrations of potassium hydrogen phthalate, which neutralizes NaOH, in the reservoirs in the middle of each spoke. The reservoirs at the tip contain phenolphthalein, which turns magenta if the concentration of NaOH exceeds that of potassium hydrogen phthalate. Credit: Anal. Chem.

Now when scientists want to titrate an environmental sample in the field, they can leave their volumetric flasks back at the lab. Researchers have developed a simple and inexpensive paper-based device that can determine the concentration of an acid or a base in less than a minute (Anal. Chem. 2014, DOI: 10.1021/ac5039384).

Chemists have long used titrations to determine the amount of a chemical dissolved in a sample. As any chemistry student knows, the procedure requires measuring precise volumes, which can be somewhat tedious and involves volumetric flasks, burettes, and several standard solutions. “Sure, we can do titrations anyplace if we have a lot of glassware and solutions, but that’s very heavy,” says Takashi Kaneta of Okayama University, in Japan. To make it easier to perform titrations in the field or on a factory floor, Kaneta came up with a paper-based microfluidic device that changes color to indicate the concentration of a chemical of interest.

In a proof-of-principle experiment, Kaneta and colleague Shingo Karita designed the device to measure the concentration of NaOH. They used a printer to apply a wax layer to a piece of filter paper, creating a system of channels in a sunburst-like design, with 10 radial spokes in a 30- by 30-mm area. At the tip of each spoke were two reservoirs: a reaction reservoir and a detection reservoir. They added phenolphthalein, which turns magenta in the presence of excess hydroxide ions, to all of the detection reservoirs. To each of the 10 reaction reservoirs, they added solutions of potassium hydrogen phthalate, an acid that neutralizes NaOH, in concentrations ranging from 0.1 to 1.0 M in intervals of 0.1 M.

After drying the paper devices, the researchers applied 30 µL of a 0.4 M NaOH test solution to the center of the device. The test solution traveled up each spoke through the reaction reservoirs and into the detection reservoirs. The three spokes containing less than 0.4 M of potassium hydrogen phthalate had detection reservoirs that turned magenta as excess hydroxide ions reacted with the dye. The device worked in a similar way for any solution of NaOH between 0.1 and 1.0 M.

By changing the concentration range of potassium hydrogen phthalate on the paper device, the researchers were then able to measure NaOH concentrations between 0.01 and 0.1 M. In another experiment, they created even finer distinctions, correctly differentiating among 0.25, 0.27, and 0.29-M NaOH solutions. The team also used different neutralizing chemicals and indicators to successfully titrate nitric acid, sulfuric acid, acetic acid, and ammonia.

Andres W. Martinez of California Polytechnic State University, San Luis Obispo, says the work lends the ease and portability of a paper-based device to a stalwart laboratory technique. However, he says, the paper device isn’t as precise as a standard titration, which may be fine for certain applications. He’d also like to see the approach extended beyond acid-base titrations.

In future experiments, Kaneta wants to apply the device’s design to other types of titrations, such as those involving redox chemistry.