March 1, 2013
Researchers at the Warsaw-based Institute of Physical Chemistry of the Polish Academy of Sciences have developed a method for producing antimony microelectrodes that make it possible to measure pH changes directly above the metal surface on which chemical reactions take place.
For several decades antimony electrodes have been used to measure acidity/basicity—and thus to determine the pH value of solutions. However, these only make it possible to measure pH changes in solutions at a certain distance from electrodes or corroding metals.
Changes in solution acidity/basicity provide important information on the nature of chemical reactions occurring at metal surfaces. This data is particularly important for a better understanding of electrochemical and corrosion processes. However, the measurement methods used to date in research laboratories did not make it possible to observe the changes with sufficient precision.
The pH value for pure (inert) water is 7, for hydrochloric acid 0, and for sodium hydroxide (one of the strongest bases) 14.
“Previously we were not able to measure pH changes in places where the most interesting things occur: at the very metal surface. The measurements had to be carried out at a certain distance,” says IwonaFlis-Kabulska, Ph.D., from the Institute of Physical Chemistry. “It’s obvious that the data collected under such circumstances did not always accurately reflect what was really going on at the metal surface.”
In an attempt to better understand the mechanisms governing the electrochemistry and corrosion of metal surfaces, researchers from the institute have developed a new measurement tool. It is an antimony microelectrode with a design allowing for easy and reproducible measurements directly above the metal surface—at a distance of just one-tenth of a millimeter.
The new microelectrode is made of a glass capillary filled with liquid antimony. The microelectrode enables measurements at hard surfaces, in a liquid environment. It is thus suitable for monitoring electrochemical reactions and corrosion processes resulting from the interaction between the metal and the solution or a thin water film.
A major strength of the microelectrode developed at the institute is that measurements can be easily performed. The designs previously available on the market required the use of micromanipulators for precise placement of electrodes at the surface. “We rely on plain geometry,” says Flis-Kabulska. “We just move a flat cut glass microelectrode tip closer to the surface of the tested metal, at an appropriate angle.”
During the measurements, the flat microelectrode tip is tilted to the surface of the tested metal, which means that it does not come into contact with the metal surface along its entire surface. This fact provides additional benefits. Protons produced in reactions on the surface do not disperse quickly in the solution. Their diffusion is slowed down, and this significantly increases the sensitivity of the instrument and the accuracy of the measurements.
The antimony microelectrode from the institute shows the highest sensitivity in measuring pH changes ranging from 3 to 10.
There is a broad range of potential applications of the new microelectrode. The instrument was constructed with laboratory research applications in mind. Due to the low manufacturing cost and the simplicity of measurements, the microelectrode could also be used in field tests, for instance as a component of sensors monitoring the condition of reinforced concrete structures, the institute says.