AIDS FOR THE ANALYST

Polarographic Electrode Assembly. Rolf K. Ladisch and Stan- a cathetometer. In no case were deviations greater than 0.5 ley L. Knesbach, Pioneering Research Laboratories, U. S. mm. observed. The drop times, t , as well as the mass flows, m, Army Quartermaster Corps, Philadelphia 45, Pa. of various dropping mercury electrodes as determined in ther-

,YOSG the various dropping mercury electrodes for polaro- I graphic use are the ones recommended by Mueller (4) , AIcReynolds (3), and Cooper and Tright (1). Mueller's elec- trode 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

mostated (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) :

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 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 cor- responding approximately to half the length of the capillary side arm. By squeezing the rubber bulb, air is removed from the

Mor- mercury reservoir through the capillary side arm. The rubber bulb is then released and mer- ton Beroza, Bureau of Entomology and Plant Quarantine, U. S.

Department of Agriculture, Beltsville, Md. cury replaces the lost air by streaming into the mercury reser-

N THE evaporation of solvents from plant extracts containing voir through its bottom hole and through the capillary side arm. The out.sid9 level of the mercury I heat-sensitive ingredients, a small glass circulating evaporator falls until it comes into balance similar to the one described by Mitchell, Shildneck, and Dustin a t height. h in agreement with the [IsD. ENG. CHEM., ANAL. ED., 16, 754(1944)] was found very

useful. Evaporation was carried out quickly, a t a low tempera- ;E. of the capillary side arm.

ture and with no interference from foaming. However, continu- ting air to flow into the free space of the mercury reservoir until ous operation of the apparatus required frequent attention and the mercury, oncoming from the careful adjustment, especially when a small evaporator and liq-

uids that evaporated rapidly were used. reservoir, seals off the tip. Dur- ing the course of an analysis, mercury IS being nithdrawn A modification of this apparatus eliminates these difficulties by st,eadily through the dropping mercury capillary, but its level is including a reservoir in the system. Until this reservoir is maintained a t height h by the described mechanism. drained, no attention is required to maintain a working level

of liquid in the evaporator. When the level finally drops below supply of mercury is introduced through the inlet as described. 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 rein- forced against breakage by being fastened to the adjacent 2.5-cm.

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.

1 4 8 . 0 307 .0 f 0 . 3 3 . 2 9 2 . 2 5 2 . 1 0 Plastic tubing (such as Tygon) was used to connect the reservoir 3 . 3 0 2 . 2 5 2 . 1 0 with the evaporator, although these connections may be glass- 3 . 3 2 2 . 2 3 2 . 0 9 sealed or ball and socket glass joints may be employed. The

level of the liquid in the reservoir should be below the level of 1 . 3 3 1.60 the vapor inlet to the bowl. The entire apparatus is clamped to

5 . 2 2 1.34 1 . 6 1 a ringstand (not shown). 5 . 2 4 1 . 3 3 1.60 After stopcocks A and B are shut, the reservoir is filled. Stop-

3 9 0 . 8 346.3AO 2 6 . 3 3 1 . 0 9 1 . 4 4 cock Cis then shut, and the vacuum, steam, and condensing water 6 . 2 9 1 . 0 9 1 . 4 4 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.

5.253 X lo6 X L P X ra t =

The values calculated for capillaries 1, 2, and 3 were 3.24, This is in good agreement 5.18, and 6.26 seconds, respectively.

with the drop times actually measured (Table I). --RUB&R BULB

LITERATURE CITED

(1 ) Cooper, W. C., and Wright, 11. RI., ANAL. CHEW, 22, 1213

(2) Kolthoff, I. Ll., and Lingane, J. J., "Polarography," 2nd ed.,

(3) JIcReynolds, R. C., IND. ENG. CHEV., ANAL. ED., 14, 586

(4) Nueller, E. F., Ib id . , 12, 171 (1940). (5) Aluller, 0. H., "Polarographic Method of rlnalysis," 2nd ed., p.

(1950).

Vol. I, pp. 80, 81, New York, Interscience Publishers, 1952.

(1942).

102, Easton, Pa., J . Chem. Education, 1951.

Small Glass Circulating Evaporator for Laboratory Use.

13 tip acts as a valve, permit- TYGON TUBING

When

,f"ll',i The constancy in the height of the mercury level with this as-

semhl). has been checked during a series of test runs, by means of

~ ~ { ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ e ~ ~ ~ ~ f '"t;

Table I. Drop Time and Mass Flow of iMercury (Obtained from 3 capillaries in combination with electrode assembly. 0.1N tube with a notched stopper and wire strapping (not shown).

ICCl solution, 25' i 0.05' C. Room temperature not controlled) Dropping Mercury

Capillary KO. Length, rnm. h. 1Im. Sec.?Drop MMp.jSec. m2/811/0

m

3 . 3 3 2 . 2 3 2 . 0 9

5 . 2 2 1 . 3 4 1 . 6 1 2 71.8 3 3 1 . 2 1 0 . 2 5 . 2 3

-

1251