Automated Impedance Measurements in Electrochemical Cells
Abstract In this paper a system is described for impedance measurements in electrochemical cells, integrated by a fast potentiostat, signal generator, digital transient recorder and a computer interface. The treatment of the signal is made by obtaining the correlation between the response cell signal with the in-phase and in-quadrature signals, as is made in the frequency response analysis systems, but the quadrature component being numerically generated. This allows one to obtain the d.c. first and second harmonic cell responses in a single experiment, as well as the a.c. phase selective response by programming the reference signal phase angle. The results obtained with a dummy cell showed the validity of the procedure since the deviation of the measured results from the theoretical data is low. The C/E curve of the interface 1 M KCI/Hg at 25°C is concordant with that previously reported in the literature. Moreover, the system has been tested with the reduction of Cd(II) ion.
Development of a Spectroelectrochemistry Assembly (SNIFTIRS) Based on a Commercial Spectrophotometer. Test with the Ferrocyanide/Ferricyanide Redox Couple
The development of a spectroelectrochemistry assembly for substractively normalized interfacial FTIR spectroscopy (SNIFTIRS), based on the commercial spectrophotometer Unicam Research Series 1, is presented. The sample chamber has been modified to achieve a vertical orientation of the cell and a special support was constructed for the same purpose. The original path of the IR radiation has been modified by using two black anodized aluminium 30° wedges, both supporting a parabolic mirror. This allows the radiation to reach the working electrode forming 60° with the vertical. The cell holder allows vertical and horizontal displacement of the cell. The electrochemical cell was made of Pyrex glass and permits an easy manipulation even in the sample chamber of the spectrometer, making it unnecessary to open this chamber in the manipulation of either the solution or the electrodes. The working electrode was a polished polycrystalline gold electrode. A computer-based potentiostat was also constructed. To control the system, a set of 16-bits Visual-Basic programs was developed allowing the synchronization between the spectra acquisition and the application of potentials. A variety of sequences of operation can be selected. The system was checked with the ferrocyanide/ferricyanide redox couple, concluding that the system here described is useful to make SNIFTIRS experiments, being versatile enough to design new operational sequences that help improve the knowledge of the electrode processes.
Hygroscopic Expansion of some Casting Investments
CREDIT is generally given to Taggart1 for introducing the cast gold inlay to the dental profession. Although he realized that gold shrinks when cooled from molten state to solid state of room temperature, the investment available to him did not supply sufficient compensation. In 1910, Van Horn2 published a paper dealing with a method by which he obtained the compensation by means of wax expansion. The next major research in connection with this problem was the work of Coleman3 at the National Bureau of Standards. He determined the gold shrinkage to be 1.25 per cent and presented a casting technic with compensation values of quantitative nature. The total compensation for the gold shrinkage was obtained by the combination of the setting and thermal expansion of the mold. This procedure is the general practice followed at the present time for compensation. There are some disadvantages to this method. Skinner has indicated the difficulty of obtaining smooth castings and the improper compensation for the casting shrinkage.4 It has been found by Hollenback5 and Scheu6a that castings made at a high temperature are rougher than those made at a low temperature (450° C.), depending upon the great difference in thermal expansion between plaster of Paris and silica, which causes a poor union of the fine particles. This may also result in friability and there may be some warpage due to the differential expansion. A method which provides sufficient compensation at a low mold temperature is the hygroscopic casting technic.b Wilson7 showed that certain plasters expand as much as 0.0044 inch to the inch by taking up moisture from the air. With this information at hand, Scheula, b tested some commercial investment to determine what results the addition of moisture would have. He found6b: (1) All of the casting investments that were tested expanded. (2) The thicker the mix, the greater the expansion. (3) A mix with hot water expanded more than one with cold water. (4) The time at which the mix is placed in the water is of great importance. It was found that if the mix is placed in the water early, it would not expand so much as when placed in a little later. It was also found that if the mix was placed in the water after a certain time, it could not expand to the maximal amount. (5) The temperature of the water in which the mix is placed is important, for two reasons; (a) the temperature of the water controls slightly the expansion of the investment, and (b) water at a temperature of 1000 F. slightly softens the wax pattern so that the fine margins are not marred while expansion is taking place. The mix should remain in the water at least 20 minutes to obtain the full expansion, after which time no further expansion is obtained.