In this inquiry lab, students use a paper microfluidic device to measure the acetic acid content of vinegar.
By Marya Lieberman, Kimberley Frederick, Vincent Remcho, and Renee Cole
Description: The titrator consists of 13 paper squares separated by hydrophobic wax barriers; the back of the titrator is also hydrophobic. Each square is loaded with 50 microliters of NaOH solution of a different concentration (indicated on the titrator). The challenge to the student is to use this device to determine the acetic acid concentration of vinegar. This experiment relies on familiar stoichiometry and dilution calculations, but it’s conceptually different from glassware titrations that students may have encountered previously, and students must design their procedures thoughtfully to get good data.
The lab is set up so students must think through their approach and make decisions about how to perform the experiment and analyze the data. Students must calibrate the volume delivered from their pipet. There are multiple ways to do this (eg, filling a known volume whilst counting drops delivered, or weighing the drops with an inexpensive mg balance). A consultation with the vinegar label and a quick calculation will show that a reasonable number of drops delivered to each square will totally overwhelm the quantities of OH- available–this means that the vinegar must be diluted before analysis. There are different experimental approaches for using the pH paper to find the endpoint. The titrator is discontinuous, but multiple measurements can be used to get a more accurate estimate for the concentration of the vinegar.
Materials: Each student will need 4-6 titrators, a pipet (could be a Beral pipet or a fixed-volume 20 ul micropipetor), and 4-6 sheets of pH paper with universal indicator (pH 1-12 works well). Any type of vinegar can be used.
If you have access to a Xerox ColorQube or similar wax printer, you can print your own titrators from this print file:
You can request printed sheets of titrators from the MICRO team via this form: https://docs.google.com/forms/d/e/1FAIpQLScnUHpznY3MSkiTZR3DYwjag-gqV4OrBvXiQldyx_T59j1xmg/viewform