AIDS FOR THE ANALYST Polarographic Electrode Assembly. Rolf K. Ladisch and Stanley L. Knesbach, Pioneering Research Laboratories, U. S. Army Quartermaster Corps, Philadelphia 45, Pa.
,YOSG the various dropping mercury electrodes for polarographic use are the ones recommended by Mueller (4),
I
AIcReynolds (3),and Cooper and T r i g h t (1). Mueller's electrode is compact and automatically maintains a constant head of mercury over a long period of time. However, i t is affected by inconveniences in its operation ( 3 ) . The assembly illustrated in the drawing is simple, convenient to manipulate, and extremely reliable. With a given dropping mercury capillary i t maintains a constant pressure, h, for several weeks in continuous operation with routine analyses. The assembly consists of glass except for the rubber bulb, a Teflon plug in the cock, and a short length of Tygon t,ubing for --RUB&R BULB attaching the dropping mercury capillary. The rubber bulb has a capacity of 15 ml. and is the type commonly used for large pipets and springes. A lead wire (omitted in the drawing) may be placed anywhere in the stand tube a p usual. To operate the electrode assembly, mercury for polarographic use is introduced through the inlet to a height corresponding approximately to half the length of the capillary side arm. By squeezing the rubber bulb, air is removed from the mercury reservoir through the capillary side arm. The rubber bulb is then released and mercury replaces the lost air by streaming into the mercury reservoir through its bottom hole and through the capillary side arm. The out.sid9 level of the mercury falls until it comes into balance a t height. h in agreement with the of the capillary side arm. 13 tip acts as a valve, permitTYGON TUBING ting air to flow into the free space of the mercury reservoir until the mercury, oncoming from the reservoir, seals off the tip. During the course of an analysis, mercury IS being nithdrawn st,eadily through the dropping mercury capillary, but its level is maintained a t height h by the described mechanism. When
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'"t;
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~ ~ { ~ ~ supply of mercury is introduced through the inlet as described.
The constancy in the height of the mercury level with this assemhl). has been checked during a series of test runs, by means of
Table I.
Drop Time and Mass Flow of iMercury
(Obtained from 3 capillaries in combination with electrode assembly. 0 . 1 N ICCl solution, 25' i 0.05' C. Room temperature not controlled) Dropping M e r c u r y Capillary m KO. Length, rnm. h. 1Im. Sec.?Drop MMp.jSec. m2/811/0 1 48.0 307.0 f 0 . 3 3.29 2.25 2.10 3.30 2.25 2.10 22 .. 22 33 22 .. 00 99 33 .. 33 32 2 71.8 331.210.2 5.23 1.33 1.60 5.22 1.34 1.61 5.22 1.34 1.61 5.24 1.33 1.60 3 90.8 346.3AO 2 6.33 1.09 1.44 6.29 1.09 1.44 -
a cathetometer. I n no case were deviations greater than 0.5 mm. observed. The drop times, t , as well as the mass flows, m, of various dropping mercury electrodes as determined in thermostated (25" =I=0.05" C.) 0.1N potassium chloride solution remained constant within &0.5%. Typical data taken a t random during a day's operation are reproduced in Table I. From these data the radii of the dropping mercury capillaries were calculated (2) to be 29.9 microns for capillary 1, and 28.5 microns for the other two capillaries, which had been cut from the same length of tubing. The drop times, t , were checked by using the radii in the formula suggested by Muller ( 5 ) : t =
5.253 X lo6 X L
P X ra
The values calculated for capillaries 1, 2, and 3 were 3.24, 5.18, and 6.26 seconds, respectively. This is in good agreement with the drop times actually measured (Table I). LITERATURE CITED
(1) Cooper, W. C., and Wright, 11. RI., ANAL. CHEW, 22, 1213
(1950). Kolthoff, I. Ll., and Lingane, J. J., "Polarography," 2nd ed., Vol. I, pp. 80, 81, New York, Interscience Publishers, 1952. (3) JIcReynolds, R. C., IND.ENG.CHEV., ANAL. ED., 14, 586 (1942). (4) Nueller, E. F.,Ibid., 12, 171 (1940). (5) Aluller, 0. H., "Polarographic Method of rlnalysis," 2nd ed., p. 102, Easton, Pa., J . Chem. Education, 1951. (2)
Small Glass Circulating Evaporator for Laboratory Use. Morton Beroza, Bureau of Entomology and Plant Quarantine, U. S. Department of Agriculture, Beltsville, Md. evaporation of solvents from plant extracts containing Isimilar heat-sensitive ingredients, a small glass circulating evaporator to the one described by Mitchell, Shildneck, and Dustin N THE
[IsD. ENG.CHEM.,ANAL. ED., 16, 754(1944)] was found very useful. Evaporation was carried out quickly, a t a low temperature and with no interference from foaming. However, continuous operation of the apparatus required frequent attention and careful adjustment, especially when a small evaporator and liquids that evaporated rapidly were used. A modification of this apparatus eliminates these difficulties by including a reservoir in the system. Until this reservoir is drained, no attention is required to maintain a working level of ~liquid in the ~evaporator. ~When the level ~ finally drops ~ below the heating jacket, very little further evaporation takes place. Figure 1 drawn to scale, shows the apparatus, which consists of a circulating evaporator, a reservoir, a connecting adapter, a condenser, a vacuum adapter, and a receiver. The glass tubing connecting the evaporator and the reservoir is 7 mm. in inside diameter, the upper connection being reinforced against breakage by being fastened to the adjacent 2.5-cm. tube with a notched stopper and wire strapping (not shown). The vapor inlet to the bowl must deliver the vapor a t a tangent to the periphery of the bowl. Stopcock B should have a 2-mm. bore and be a t least 3 cm. above the bottom of the steam jacket. A solid glass rod supports the inlet tube holding this stopcock. Plastic tubing (such as Tygon) was used to connect the reservoir with the evaporator, although these connections may be glasssealed or ball and socket glass joints may be employed. The level of the liquid in the reservoir should be below the level of the vapor inlet to the bowl. The entire apparatus is clamped to a ringstand (not shown). After stopcocks A and B are shut, the reservoir is filled. Stopcock Cis then shut, and the vacuum, steam, and condensing water are turned on; stopcock B is opened and stopcock A is cracked to permit the entry of a small stream of air or inert-gas bubbles. 1251
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