Membrane Material for a Galvanic Cell Gordon L. Emleton. John J. Williamson, and Donna K. Johnson -Southeastern Oklahoma State University, Durant, OK 74701 We read with interest a recent1 article on the construction of mini-galvanic cells. The authors described the problem of unsteady voltage readings when the cotton plug in the hottom of the electrode tube created circuit-breaking air hubhles. We found a similar difficulty using a plug of agar gel made with potassium nitrate solution. Our newest design eliminates these frustrations and uncertainties by replacing "plugs" with a membrane material. The membrane allows diffusion without mixine but does not trau air bubbles. It also introduces the students to some useful materials and techniauesthat a ~ ~ etoatheir l interest in both the miniscale experi&entationand biomedical applications. The cost per student for the consumable memhrane materials is about ~~
~
$0.07.
The tubes for each electrode are prepared from a disposable polystyrene serological pipet. The tip and end are removed, and the remainder is cut in half (scored and broken like glass tubing). The membrane material is cellulose strips prepared from dialysis tubing (we have used 12-14,000 molecular weight cutoff (MWCO) range for the cells because that was available). Before the activity l-in. lengths of the memhrane material are soaked in the salt bridge solution. The moist membrane tubing pieces are opened, cut in half to give a single thickness, placed over the end of the polystyrene tuhe and sealed to the side with a 5- X %in. strip of wrapping film. The tube is filled with a solution of the metal salt and a wire or strip of clean metal is secured in the tuhe with another small piece of wrapping film. (See figure.) The millivolt readings are taken on a standard pH/mv meter. The connections to the electrodes are made by alligator clips attached to the ends of a coaxial cable which has heenscavenged from a hrokenpH electrode. In the cable, the
center wire is used for the measured electrode and the outer wire is connected to the "reference" electrode. In our activity four students make a total of eight different electrodes. These are arranged in a test tube rack in a beaker that bas heen filled with the salt bridge solution to cover the ends of the tubes. Each student makes two~setsof seven measurements using first one and then the other of their assigned electrodes as the "reference" electrode. After arranaine each set of measurements from most negative to most p k t i v e (including zero for the reference), thestudents within each mouu are asked to make an eiaht-bv-eight . . matrix of this d a t a . - ~ nexamination of this information illustrates that the choice of the "reference" does not effect the order (from - to +) of the cell potentials nor the interval between any two cell potentials.
me membrane materiel held on the end of a section of disposable pipet by 'Craig, N. C.: Ackermann. M. N.; Renfrow, W. 8. J. Chem. Educ. I W S , 66.85.
wrapping film allows contact with an electrolyte sdutian in the beaker lo complete the circuit in these galvanic cells. Electrical connection to the pH/ mV meter Is made by adapting the wire scavenged from a broken electrode.
Volume 67 Number 6 June 1990
527
