instrumentation - ACS Publications

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INSTRUMENTATION The Type Ε Electroehemograph offers the analyst precision stability and oper­ ational convenience for routine work as well as for research in polarography

by Ralph L I K E many of our readers, we are compelled to judge and appraise performance claims of new instruments in terms of current practice, published data, and whatever new and original features seem apparent. We have, wherever possible, hurried out to buy one to see if our initial enthusiasm was warranted. I t is a cheering commentary on progress in instrumentation that our hopes have been sustained in most cases. I t has been our privilege, along with several other investigators, to use and study the Electroehemograph Type Ε in its late developmental stage, as a result of which we can confirm the advantages and operational features claimed by Leeds & Northrup. Modern polarography is in such an advanced state of develop­ ment and is used by so many investigators for research and rou­ tine analyses that the design of a new instrument is a very difficult matter. To satisfy all the prospective users and meet their re­ spective desires would make the price of the instrument pro­ hibitive and require a B-36 for its transportation. Even the choice of chart paper can start a lively argument. Some want finely ruled paper, others want it plain, some want it green, and others, who are publication-minded, want it blue. There are general instrumental criteria such as precision, reliability, stabil­ ity, ease of servicing, and ruggedness. In our opinion, these are well met in the Type Ε Electroehemograph. The instrument permits one to measure all the fundamental characteristics of a dropping mercury electrode system in absolute terms and without arbitrary conversion factors. This holds for half-wave potentials, diffusion currents, and the influence of damping. Beyond this, the operational speed and convenience are such that one can cut

Figure 1-

Electroehemograph

H.

Müller

corners and obtain fast routine checks or monitoring measure­ ments to suit practical problems in which absolute values are of no particular importance. A general view of the instrument is shown in Figure 1.

The lower sloping panel contains the voltage-scanning drive, range and damping selectors, and all electrical controls. Prac­ tically every necessary instruction for standardization and diffu­ sion current measurement is engraved in proper and numbered sequence at the lower center of this panel. The motor-driven slide wire is controlled by an on-off switch and a reversing switch. A well-engineered clutch mechanism provides very fast reversal and manual change of the slide-wire setting can be made at any time. The prevailing slide-wire voltage is visible at all times through a window at the upper right. This is an indication of the slide-wire scale position and not in terms of a voltmeter of doubtful precision and accuracy. A current range selector can be set to eleven ranges affording 1, 2, 3, 5, 7, 10, 15, 20, 30, 50, or 100 microamperes full scale. These are both precise and accurate and can be verified within the least perceptible line on the chart recording. The polarizing range is selected by the switch at the lower right for scanning from 0 to —2, —1 to —3, or + 1 to —1 volts at a rate of 200 mv. per minute. Another selector switch at the lower left affords four degrees of damping, the respective influence of which can be seen in the typical polarogram of Figure 3. An operation selector switch is located in the center of the panel, directly below the meter. From it one can obtain anodic or cathodic polarization, or disconnect the recorder from the polarizing slide wire and cell for the purpose of setting the recorder zero, or connect a calibrated resistor into the circuit for a linearity check. By mean? of the latter feature, one can record Ohm's law lines at various current ranges and check both uniformity of voltage rise or fall and current sensitivities. Although the manufacturer provides an alternate power supply unit using a 6-volt 100-ampere-hour storage bat­ tery, our experience with the 115-volt 60-cycle electronically controlled power supply was ex­ tremely gratifying. This furnishes the working current for the polarizing- network. Its correct value can be verified by pressing a button and noting balance on the zero-center meter. Any necessary adjustment can be made by the knob to the right of the meter. This comparison is against a standard cell. After preliminary warm-up, we found such corrections to be negligible and practi­ cally ignored the adjustment. By Variac control on the line, satisfactory operation was obtained over wide limits, approximately 90 to 135 volts. Moderate transients caused by switching heavy motors on and off the same line had no effect. This may be of interest to investigators in industrial laboratories where lines are heavily and intermit­ tently loaded or in communities where the subtle­ ties of frequency and voltage regulation have not yet penetrated. There was no opportunity to check the effect of poor frequency regulation of the line. Recording of polarograms in this instrument is accomplished by the integrally mounted Type G Speedomax microampere recorder. This is a true current-measuring instrument and does not involve measurement of an RI drop across a series resistor in the polarographic circuit. Distinctive charac­ teristics, other than high speed, are a convenient chart tear-off device, a fast solenoid-operated penlifting device, and a V-shaped index at the edge of Figure 2. Dropping the chart for noting the exact point on the chart a t Mercury Electrode which the pen is to be lifted or dropped. Assembly 23 A

24 A

ANALYTICAL

METALLURGICAL ESTING EQUIPMENT foïs*peed'andaccu/uicy

CHEMISTRY

The manufacturers have not ignored the electrochemical aspects of polarographic analysis. While no two polarographers will agree on a cell arrangement, manufacturers' offering in acces­ sory equipment is an excellent compromise and is well built. Figure 2 shows a dropping mercury electrode assembly conven­ tional in some respects, but holding the glass standpipe in a rectangular aluminum channel. An appropriate length of marine barometer capillary tubing is sealed in the lower end and the cell assembly is carried by a sheet-metal carriage which can be slid parallel to the channel and locked in place by a knurled knob. This is convenient for mounting in a small thermostated bath. Η-cells of the Lingane type with fritted-glass connection to the saturated calomel electrode reference section are available and also a single-compartment cell for routine work with a mercury-pool reference or external calomel reference electrode. Shielded leads are also provided which can be plugged into Amphenol receptacles on either side of the Electrochemograph control panel. Figure 3 shows typical results obtained in the Leeds & Northrup laboratories with this instrument. The effect of various degrees of damping is illustrated, as well as the convenience which is inherent in the use of the continuously adjustable recorder zero, which has a plus or minus full scale current range for any of the current settings. This enables one to offset a series of recordings and with the help of the pen-dropping mechanism and fast-acting drive clutch to start all curves at the same point. We have confirmed the ability of this equipment to obtain accurate values for diffusion currents and half-wave potentials. Even at high degrees of damping, the apparent shift of half-wave potentials can be corrected by reverse scanning. The mean of the two values, so obtained, agrees within 1 or 2 mv. of accepted values.

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r i n m i i H i p

METALLURGICAL APPARATUS 165 West Wacker Drive, Chicago 1, Illinois

Figure 3.

Typical Polarogram

The ruggedness, freedom from vibration, and well-engineered design of this instrument should appeal to the professional polarographer who has little time to fiddle with adjustments. The electrical and mechanical components are typical of L. & N. standards and the electronic design reflects modern develop­ ments and a notable absence of "radio-store" components. Perley, Eans, and their engineering associates deserve much credit for placing a new and dependable tool in the hands of the analyst. Saturable Reactors Last month we contended that saturable reactors are interest­ ing and useful devices but generally unavailable. J. R. Lilienthal of the Los Alamos Scientific Laboratory tells us that saturable reactors and magnetic amplifiers for numerous applications are available from the Vickers Electric Division, Sperry Corporation, 1815 Locust St., St. Louis 3, Mo.