I N S T R U M E N T A T I O N An electronic potentiostat, fluid magnetic clutches, and precision potentiom­eters, recently made commercially available, are called to analysts' attention

by R. Η. Muller THE revival of widespread interest in electrochemical methods

of analysis is largely a consequence of new and improved techniques and the development of improved instruments. At the turn of the century, the researches of Classen, Erich Millier, and Edgar Pahs Smith seemed to presage an era in which much of inorganic analysis would yield to the clean-cut, smooth tech­niques of electrodeposition. Much was learned in those days about the possibilities of separation by a suitable control of poten­tial and the extent to which this could be supplemented or ex­tended by appropriate complex-ion formation. The precise de­termination of alkalies, by depositing them in a mercury cathode, followed by titration of the equivalent amount of alkali, was a commonplace in 1910. Smith's students of that era were de­positing 0.1 gram of metal with the requisite analytical precision in 7 or 8 minutes by means of the rapid deposition methods that he developed.

These techniques were destined to be temporarily over­shadowed by alternative methods such as spectrophotometry, colorimetry, and electrometric titrations. With the advent of Heyrovsky's polarographie methods, new criteria for the effective separations could be established, and many of the modern proce­dures depend upon critical information so derived.

The current literature is rich in new methods and applications dealing with coulometric analysis, the preliminary deposition of bulk constituents into a mercury cathode, organic oxidations and reductions, and the titration procedures of Swift and his asso­ciates, wherein reagents are electrolytically generated. The latter methods and those of coulometric analysis show great promise of a general approach to automatic analysis. To a large extent, these developments are paced by instrumental improve­ments. The development of suitable potentiostats has been classified by Lingane in this journal, and he has been responsible for the development of some of the more successful types. As he has pointed out, every investigator has been required to design and build his own potentiostat.

Electronic Potentiostat What appears to be the first commercially available instrument

is the Model 301 electronic power supply for controlled electrode potential electrolysis of the Sanford Development Laboratories, 3001 Upshur St., Mount Rainier, Md.

This potentiostat is all-electronic in operation and has no mov­ing parts. I t will maintain an electrode at constant potential with respect to a standard reference electrode independent of the load current. The instrument contains an amplitude-controlled oscil­lator, a power amplifier and rectifier, and two direct-coupled stages of amplification. The oscillator is a multivibrator operat­ing at approximately 2000 cycles per second, which produces a variable amplitude square wave which is amplified by a pair of beam power tubes in push-pull. The output of these tubes is passed through a step-down transformer and thence to a low voltage, high current rectifier. A filter section reduces ripple and noise to a very low value. This filtered voltage is the output of the instrument. Regulation is achieved by comparing a portion of the output voltage with a voltage stabilized by a gaseous regu­lator tube. The difference between these voltages is amplified by the two direct-coupled stages and used to control the amplitude of the oscillator. The effective internal impedance of the instru­ment is very low as a consequence of the high degree of regulation. I t is approximately 0.01 ohm at full output, and to achieve the equivalent performance with a storage battery at an output of 1 volt, it would be necessary to employ a voltage divider with 200-ampere fixed drain.

Three terminals are provided, for output, control electrode, and ground. The output is varied by means of a control located on the front panel. A meter and meter selector switch are provided, so that either the output voltage or the control voltage can be read.

The output voltages are 0 to 2 volts d.c. at 2 amperes, 4 volts at

1.5 amperes, or 6 volts at 0.5 ampere. The control voltage input impedance is approximately 10 megohms. The ripple voltage is less than 10 millivolts peak to peak. The instrument is housed in a 9 X 9 X 14 inch cabinet and weighs 21.5 pounds. The power consumption is 20 watts from a 110-volt 60-cycle line.

We hope we are not mistaken in identifying this instrument with one developed several years ago a t the National Bureau of Standards, where it was successfully used in electroanalytical de­terminations. The manufacturer's bulletin affords no information on this point.

F^luitl Magnetic Clutches An elegant series of fluid magnetic clutches is available from

Duncan & Bayley, Inc., 785 Hertel Ave., Buffalo 7, Ν . Υ. These range from 8 inch-ounce torque up to 3 hp. In several configura­tions, their standard models include a single clutching means, a single braking means, a geared twin clutch bidirectional control means, a twin clutch-brake providing uni direction drive and brake means, and a geared twin clutch, bidirectional control with, integral no backlash step-down gearing.

F igu re 1. H igh Precis ion P o t e n t i o m e t e r

These devices are all based upon the pioneer work of Jacob Rabinow of the National Bureau of Standards, in which the coupling between two rotating members is effected by the exter­nal magnetization of an iron powder in oil dispersion. The rate of response of these devices is of the order of 10 milliseconds and a magnetizing current of as little as 20 milliamperes can control 0.25 hp. a t 3400 r.p.m. Duncan and Bayley have eliminated many of the earlier difficulties associated with these devices, one of which was isolation of bearings from the iron particles in the fluid, and the other a tendency for the iron particles and the fluid to separate because of centrifugal loading at high rotational speeds.

These devices enable one to control relatively large mechanical loads with the output of small vacuum tubes a t high speed. As such, they are useful components in countless servosystems for measurement and control.

Precision Potentiometers Among the various instruments and components manufactured

by the Technology Instrument Corp., Waltham 54, Mass., analysts may be interested in the extensive line of precision potentiometers which is offered. These embody all the latest ad­vances in construction, such as the use of Paliney contact ma-

25 A

26 A A N A L Y T I C A L C H E M I S T R Y

Varicell THE IDEAL SOURCE OF LOW DC VOLTAGE F R O M AC LINES

A-C I N P U T : 95-135 volts, 60 cycles, single phase

DC OUTPUT: 0-30 volts, 15.0 amperes

STABILIZED, REGULATED, VARIABLE The D-C outpu t of the VARICELL can be set to any value from 0 to 30 volts . . . is unaffected by A-C line voltage fluctua­tions or by any changes in load current. With the VARICELL, any D-C voltage in the range of 0 to 30 volts is at your Fingertips. Rotation of the handwheel provides the D-C voltage you require — and the output is stabi­lized and regulated. Stabilization and regula­tion is ± 0.25 volts for any output voltage setting in the range of 6 to 30 volts. R. M. S. ripple voltage never exceeds 0.1 volts for the 6-30 volt range. The VARICELL is easy to oper­ate. It plugs into any output receptacle of the proper voltage, frequency and phasing. The load is connected to either of two sets of Su­perior 5-WAY Binding Posts. A voltmeter and ammeter indicate output voltage and load current. For detailed information on VARICELL, request Form 2504 Write THE SUPERIOR ELECTRIC CO., 812 Meadow St.,

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I N S T R U M E N T A T I O N

terials, centerless ground stainless steel shafts, and the guarantee of long trouble-free life. A wide assortment of nonlinear tapers is available and in conjunction with multiple fixed taps, sine and secant functions can be furnished.

A general view of a typical combination is shown in Figure 1. This is a 3-gang assembly of precision potentiometers. A dis­tinctive feature is the peripheral clamping rings, which permit standard units to be ganged with accurate and convenient phasing of each potentiometer in the assembly.

F i g u r e 2 . L a b o r a t o r y P o t e n t i o m e t e r

The torque values are nominally 2 inch-ounces, but may be adjusted to as low as 0.75 inch-ounce when specified.

A convenient model, designated as the RVL3 laboratory potentiometer, is direct reading in ohms and forms convenient voltage dividers of 1% accuracy. Each dial knob boars the value of the total resistance and current carrying capacity. An exam­ple is illustrated in Figure 2. Standard units are offered in eleven resistance values ranging from 100 to 200,000 ohms.

F igure 3. T rans l a to ry P o t e n t i o m e t e r

The Type RVT translatory potentiometer as shown in Figure 3 is actuated by longitudinal instead of rotary motion. In the unit shown here the stroke is 2.5 inches for a total resistance of 6000 ohms. The linearity is ± 0 . 1 5 % of total resistance. The pressure necessary to move the sliding element when the unit is in afhorizontal position is 17 grams. A pressure of 70 grams is re­quired when the unit is in the vertical position.

Special^modcls can be obtained with stroke lengths from 0.5 to 15 inches'and resistances ranging from 20 to 100,000 ohms. In addition, dual units can be supplied without dimensional changes in which two independent resistance elements and take-offs are provided with a common slider.

Complete information, quotations, and suggestions on applica­bility of these devices as well as the Duncan and Bayley instru­ments are available from the manufacturer's representative, Harold H. Everett, 490 Plandome Road, Manhasset, L. Ι., Ν . Υ.