ANALYTICAL CHEMISTRY
610
510 mp filter, for measuring changes in voltage output from the regulator by measurement of the changes in galvanometer reading. As it was found experimentally that the galvanometer reading varied as the fifth power of the lamp supply voltage, changes of approximately 0.02% in output voltage could be detected. Errors due to drift of the measuring instrument were minimized by substituting a resistance box for the series resistors of the colorimeter, and by allowing an overnight warm-up period, during which an auxiliary poxer source was used, for elimination of drift due to fatigue of the colorimeter lamp and the barrier layer photocell. The external load on the regulator was 0.18 ampere. The output voltage of the regulator decreases by 0.2% when the line voltage is raised from 90 to 130 volts. The change with line voltage is approximately uniform throughout this range This overcorrection is apparently caused by the small amount of wave-form distortion produced by the regulator with the attendant change in the ratio of the RMS voltage (which determines the light output of the 82 lamp, and hence the voltage drop through the phototube load resistor) to the peak voltage (which to a largp extent determines the D.C. voltage to which this drop must be equal). Drift of the regulator output voltage after narm up was found to be just detectable (0.1% change in light output, corresponding to 0.02% change in output voltage) over a 4-hour period. A 150 ampere-hour storage battery was capable of equally good performance after overnight operation. Forty minutes are required after the regulator is turned on to reach complete stability (less than 0.02% voltage drift per hour). Duiing the first 5 minutes, downward voltage drift is a t the rate of 0.04% per minute. Comparison with battery warm-up characteristics is difficult because these characteristics vary greatly with the type of battery, its state of charge, and the length of time of disuse before the trial. Thirty minutes were required for a 150 ampere-hour storage battery, charged, connected to the colorimeter for 20 hours, then allowed to stand idle 13 hours before the test, to reach the same degree of stability. The response of the regulator to line voltage changes is not instantaneous. If the line voltage is changed from 90 to 130 volts in less than 0.1 second, when the regulator is serving as a power supply for an Evelyn photoelectric colorimeter, an upward excursion of the galvanometer spot, amounting to about 10% of the total deflection, occurs. Approximately 2 seconds are required for return to normal. As the galvanometer time constant is longer than that of the regulator, only large sudden changes in voltage are apparent as temporary excursions of the galvanometer from its equilibrium reading If a galvanometer of very short period were used, flutter would probably be apparent. The effect of supply-voltage wave-form variations on output voltage has not been studied, In the author’s laboratory no variations ascribable to this source have been detected. However, in a few laboratories, with devices drawing heavy currents and having nonlinear current-voltage characteristics-e.g., thyratron-controlled devices-the waveform distortion encountered is sufficient to make the voltage regulation poor. h modified version of the voltage regulator described here, involving the use of voltage regulator tubes in the direct current voltage supply for elimination of wave-form difficulties where they exist, is being marketed by the Rubicon Co.
Applying Spectrochemical Analysis to 2 M Perchloric Acid Solutions. John G. Conway and Milton F. Moore, University of California Radiation Laboratory, Berkeley 4, Calif. HE spectrographic analysis of perchloric acid solutions presents T m a n y difficult problems not usually encountered in the analysis of solutions, The main difficulty is the chemical reaction between the electrodes and perchloric acid. Platinum or gold electrodes in sufficient quantities for such work are prohibitive in cost. The copper spark method modified by the use of acidresistant paint and evaporation in vacuum offers a solution to
this problem (1, 8). The copper electrodes are not expensive and are available in sufficient quantities. Electroplating a thin film of gold on the end of the machined copper electrode was tried, but even the best plated electrodes had small pinholes through which the acid would attack the copper. This etching became greater upon concentration of the acid by evaporation in the drying process. Salts were formed in such quantities that mechanical dislodgment occurred when the spark first &truck. The salts also increased the background to such an extent that it vas possible to densitometer the lines. Several acid-resista n t p a i n t s were T O V A C PUMP f o u n d satisfactory, the best of which is a vehicle f r o m a paint known as Prufcoat (Prufcoat Laboratories, Inc, 50 East 42nd St., New York, ,, c12 N. Y.). For use in this work the Prufcoat is diluted with 6 parts of benzene.
A freshly machined copper electrode is dipped into the diluted paint to a depth of 0.5 inch, withdrawn, and placed upright in a holder to dry. Drying may be accelerated by heating. The dried electrodes are then placed in a copper block and the samples and standards are discharged onto the protective coat. If the sample or standard is too large to be placed on the electrode a t one time, it may be reduced by partial evaporation in air. The solution should not be permitted to go to dryness in air, because the action of the perchloric acid is enough to decompose the protective coat. The partially dried samples and standards are transferred to a copper electrode holder and this is placed in a vacuum desiccator which is then evacuated to betn-een 0.2 and 0.5 mm. of mercury and held there until the solution dries. This drying in vacuum may be hastened by irradiation of the copper block and electrodes with an infrared lamp. Perchloric acid will distill undecomposed a t reduced, but not a t atmospheric, pressures (3). Calcium chloride is used as a drying agent; it is considered adequate because under under the conditions of sample preparation-heating vacuum-the desiccant is maintained close to its maximum efficiency. The dried electrodes are kept in the desiccator until ready to use because of the hygroscopic nature of the sample. Once dry they are sparked by the copper spark method. The protective coat increases the background to some extent. This method of handling samples was originally worked out for the analysis of hafnium and zirconium in perchloric acid and haB been very satisfactory. LITERATURE CITED
Bachelder, M. C., Conway, J. G.. Nachtrieb, N. H., and Wildi, B. S Atomic Enernv Commission Declassified Document. MDDC 511 (Nov. 6, “945). (2) Fred, M., Nachtrieb, N. H., and Tomkins, F. S., J. Optical SOC. Am., 37,279 (1947). (3) Prescott and Johnson, “Qualitative Chemical Analysis,” p . 536, Xew York, D. Van Nostrand Co.. 1932. (1)
WORKperformed under auspices of
U. 5. Atomic
Energy Commission.
Increased Life for Sodium Arc Lamp. Norman G. Adams, Bureau of hfines, Petroleum Experiment Station, Bartlesville, Okla. useful life of a General Electric laboratory sodium arc lamp In most cases the useful life of the lamp is determined when an excessive time is required before the discharge begins. It is possible, with a lamp of this age, t o induce the discharge by illuminating it for a few seconds with an intense tungsten lamp such as a Westinghouse 150-watt reflector spot lamp. This laboratory is a t present using a sodium arc lamp which would otherwise have been discarded 6 months ago. HE
Tcan be greatly increased by a very simple procedure.
