Soap Bubbles in Analytical Chemistry. Conductometric Determination of Sub-Parts Per Million Levels of Sulfur Dioxide with a Soap Bubble

Abstract

Soap bubbles provide a fascinating tool that is little used analytically. With a very low liquid volume to surface area ratio, a soap bubble can potentially provide a very useful interface for preconcentration where mass transfer to an interfacial surface is important. Here we use an automated system to create bubbles of uniform size and film thickness. We utilize purified Triton-X 100, a nonionic surfactant, to make soap bubbles. We use such bubbles as a gas-sampling interface. Incorporating hydrogen peroxide into the bubble provides a system where electrical conductance increases as the bubble is exposed to low concentrations of sulfur dioxide gas. We theoretically derive the conductance of a hollow conducting spherical thin film with spherical cap electrodes. We measure the film thickness by incorporating a dye in the bubble making solution and laser transmission photometry and find that it agrees well with the geometrically computed thickness. With the conductance of the bubble-making soap solution being measured by conventional methods, we show that the measured values of the bubble conductance with known bubble and electrode dimensions closely correspond to the theoretically computed value. Finally, we demonstrate that sub-ppm levels of SO(2) can readily be detected by the conductivity change of a hydrogen peroxide-doped soap bubble, measured in situ, when the gas flows around the bubble.

Micro gas analyzers for environmental and medical applications

“…Soap film is very thin and so has a very large surface to volume ratio. Soap film conductivity has been monitored directly during the collection of gas and the concentration of SO 2 determined [11]. Gaseous NH 3 has also been determined by liquid film in situ conductivity monitoring, and applied to continuous breath analysis [12].…”

Conductometric determination of ammonium ion with a mobile drop

“…As breath ammonia flowed over the drop it reduced the conductance by neutralizing the acid; the rate of conductivity decrease was a direct measure of the breath ammonia concentration. Although a liquid film/bubble [31] can provide an even greater surface/volume ratio than a drop, a liquid drop based system is simpler to set up.…”

Nanometer-Thick Newton Black Film for Selective Formaldehyde Gas Detection

“…Li et al 28 faster assay we need a system that can efficiently take up formaldehyde in the gas phase, which means high surface area to volume ratio. Recently, Kanyanee et al 31 utilized a soap bubble to detect subppm levels of SO 2 by monitoring the bubble's conductance after analyte uptake from the gas phase. They also used doped soap films for chiral pinene selective transport.…”