INSTRUMENTATION

---

INSTRUMENTATION The importance of polarographic analysis makes timely the description of an instrument capable of reproducing polarograms at relatively high speed by Ralph analysis is a well established branch of analytical chemistry. Several well designed instruments are commercially available : the Sargent series for both photographic and pen-and-ink recording, the improved Leeds & Northrup Electrochemograph, and the manually operated Fisher-Eimer & Amend and American Instrument Company instruments.

H.

Müller

Although the theoretical foundations of polarography are sufficiently well developed to put applied polarography on a perfectly rational basis, there are continuing questions which interest the serious student of the subject. Anyone familiar with the Kolthoff-Lingane classic on polarography will be inclined to raise an eyebrow when the statement appears in one of the bestknown texts in physical chemistry that "there are other factors involved, but simple satisfactory analyses have been made on a great many different substances using empirical calibrations with known concentrations under conditions which are the same as those existing in the 'unknown' solution." We are far beyond this primitive stage of empiricism, yet curiously enough, some recent investigations have thrown some doubts on the validity of the time function in the Ilcovic equation, according to which the current-j^me function should be a sixthorder parabola. A thSfcough discussion of this matter is given in the last i^ihual review by Lingane [ANAL.

POLAROGRAPHIC

CHEM.,21,45(1949)1].

Figure 1.

It is therefore advisâbfetp draw attention to an instrument which has been available for two years and is capable of reproducing polarograms at relatively high speed. The Polaro-Analyzer shown in Figure 1 is manufactured by the Rutherford Instrument Co., Inc., P. O. Box 7406, Benjamin Franklin Station, Washington, D. C. It is based upon a design described by J. H. Schulman, H.'B. Battey, and D. G. Jelatis [Rev. Sci. Instruments, 18, 226 (1947) ]. It uses a current-recording system of negligible damping, and the half-wave potentials so determined are quite unaffected by high scanning rates. Although conventional damping can be employed, if so desired, this instrument emphasizes some of the advantages which can be realized if the peak currents are recorded. For one thing, the envelope of the current peaks is a precisely determined curve and the operator is not required to make a visual estimate of the average value of an envelope of lines.

Polaro- Analyzer 23 A

ANALYTICAL

24 A

Now—anyone can make accurate determinations with

INSTRUMENTATION

Designed to measure both

DIFFUSE REFLECTION (whiteness,

bright­

ness, color) a n d SPECULAR REFLECTION (shininess, gloss) A carefully constructed and highly versatile instrument for fast and accurate determinations on solid surfaces of every descrip­ tion, powders, pastes and opaque liquids. To the basic instru­ ment can be attached any of a number of search units, each engineered especially for one or more specific purposes. *

Paper brightness tests

*

Detergency tests

*

Paint gloss tests

•

line or battery operated models

•

12 i n t e r c h a n g e a b l e search units A variety of color filters available, in­ cluding those for ICI Tristimulus values.

MURGATROYD 2428/3 — R e ( l e c t i o η M e t e r Model 610, foi 110 volt, 60 cycle, A.C. Current $1 35.00 2428/19—Same u 2 4 2 8 / 3 , but for 6 volt storage battery opera­ tion. Without battery SI 20.00 2428/32—Same as 2428/3, but for cither 110 volt, 60 cycle A.C. or 6 volt storage battery operation. Without battery 1150.00

2421/12—6 volt storage battery. $19.75 2428/4—Search unit for general use, with green tristimulus Rltei $65.00 2428/5—Search unit for paper brightness tests, with built-in Wrattcn filter #'49 $55.00 2428/8—Search unit for 60° gloss tests $75.00

Write for Full Information on These and Other Units

SCHAAR & COMPANY Complete Laboratory 7 5 0 W . LEXINGTON STREET

Equipment CHICAGO

CHEMISTRY

7

It has been shown t h a t this instrument records the true maximum current within a few per cent [Taylor, J. K., Smith, R. E., and Cooter, I. L., J. Research. Natl. Bur. Standards, 42, 387 (1949)]. These investigators have made a complete study of the time-current rela­ tionship using a photographic-cathode ray oscillo­ graphic technique. I n this procedure, the ratio of the average to the maximum current was found to be 0.80 whereas the value found by integration of the Ilcovic equation requires 0.859 (6/7). In their hands, the re­ sults obtained with the Polaro-Analyzer gave this ratio as 0.82. T h e Polaro-Analyzer is housed in a single cabinet 30 X 25 X 15 inches. A standard Esterline-Angus recorder is employed and a complete scan is designed to require 5 minutes, although chart drive at half t h a t speed can be selected, if greater resolution along the voltage axis is desired. Six polarizing ranges can be selected: 0 t o 1. 0 t o 2, 0 t o 3, 1 t o 2, 1 to 3, and 2 to 3 volts. Less fre­ quently used ranges can be chosen by an auxiliary switch, which permits the polarity of the cell to be re­ versed one third of the way through the run. The recorder is turned on automatically when scan­ ning begins and shuts off a t its termination. T h e ap­ plied voltage is a t all times indicated by Veeder count­ ers, one for each direction of rotation. T h e appropriate counter is illuminated; the other remains dark. AVhen records a t a fixed potential are required as a function of elapsed time, the desired voltage is very readily set in by means of those counters. The amplifier stage is de­ scribed in intimate detail in the original paper. I t pro­ vides current sensitivities of 0.5, 1, 2, 5, 10, 20, 50, 100. and 200 microamperes full scale. A built-in indicating milliammeter is in series with the recorder and the latter can be switched off whenever a mere visual indication is desired. Both damping and a compensation circuit artprovided and, although the usefulness of compensation has been questioned by m a n y investigators, it is in­ cluded for those who m a y wish to use it. T h e original design called for a storage b a t t e r y source for the polariz­ ing circuit. In this instrument a built-in, nonspill stor­ age b a t t e r y has been provided with an integral trickle charger for the battery. Thus, effectively, the system is line-operated. This is a well organized instrument and particularly suited to rapid analyses. I t would also seem to be use­ ful in studying some of the residual uncertainties in the Ilcovic equation. In this connection it should not be inferred t h a t the latter is seriously in error or t h a t this has anything t o do with sound, practical analysis. I t is also evident t h a t systematic oscillographic studies are still in order, if the finer details of drop behavior are to be unraveled, especially in the region of zero time.