of carrying 6 amperes controlled the potential, which was maintained at -1.70 volts us. S.C.E. In one experiment 9 grams of cyclo-octatetraene were reduced in 10 hours, and another sample of 10 grams took 12.5 hours. The supporting electrolyte consisted of 750 ml. of a 1.OM lithium chloride solution. The cell resistance rose from an initial 20 ohms to 60 ohms or more as the silver anode became coated with silver chloride. Anode replacement brought it down to about 20 ohms. At room temperature 3.9 X 10-9 mole per liter of cyclo-octatetraene is soluble in water (S), so that most of the sample was suspended in droplets in the solution. This undissolved portion did not interfere with the electroreduction. Its final disappearance indicated visually the approaching end of the reaction, because, in this case,
the reaction products were sufficiently water soluble to stay in solution. This procedure should be applicable whenever a solvent or solvent mixture combines limited solubility for the substance to be electrolyzed with ample solubilit,y for a suitable supporting electrolyte. Though not as fast and convenient as the automatic version with a potentiostat, it should interest the occasional user and thus improve the situation deplored by Lingane ( 7 ): “Why have both preparative electrochemists and analytical chemists been so indifferent for a half century to the demonstrated capabilities and advantages of controlled-potential electrolysis?” ACKNOWLEDGMENT
The authors express gratitude to the
Research Corp. for financial assistance. LITERATURE CITED
(1) Craig, L. E., Elofson, R. M., Ressa, I. J., J. Am. Chem. Soc. 75, 480-2
(1953).
( 2 j Diehl, H., IND.ENG.CHEM.,ANAL.
ED. 16, 532 (1944). (3) Gilbert, T. W., Jr., senior research thesis, Massachusetts Institute of Technology, 1951. (4) Haber, F., 2. Eleklrochem. 4 , 506 (1898). (5) Hickling, A., Trans. Faraday SOC.38, 27 (1942). (6) Lassieur, A , , “Electroanalyse Rapide SBparations par Potentieh GraduBs,” thesis, University of Paris, 1925. (7) Lingane, J. J., “Electroanalytical Chemistry,” p. IV, Interscience Publishers. New York-London. 1953. (8) Lingane, J. J., I N D .~ N G . CHEM., ANAL.ED. 16, 147-52 (1944). (9) Sand, H. J. S., “Electrochemistry and Electrochemical Analvsis,” Vol. 11, Blackie & Son, London, le40.
Variable Solvent Programs for Column Chromatography
R. P. Harpur, institute of STEPWISE
Parasitology, McGill University, Macdonald College, P.Q., Canada
elution, preferably with a
A graduation from one step to another, may be desirable for separation of the acids associated with the tricarboxylic acid cycle (1). This operation may be performed automatically with the apparatus described by Roberts and Mason (3), but a change in the solvent program requires construction of a new apparatus. The modified apparatus, shown in Figure 1, can be changed from one program to another in a few minutes. The principle is the same, but the glass diaphragms are replaced by removable disks and the volume of the free space, between these diaphragm disks, is varied by changing the diameter of the spacer disks. Although the size of the apparatus depends upon the types of program required, the dimensions given here provide for 12 steps (13 if the chromatographic column contains the solvent for the first step) and each step may be 10, 15, 20, 22.5, 25, 27.5, 30, 32.5, or 35 ml. The results obtained are similar to those given by the original authors (3) and in the elution of a known mixture of organic acids the separations are the same as those obtained by manual additions of the solvents (1). With chloroform solutions under pressure. corks are more satisfactory than ball joints for sealing the side arms, and tetrafluoroethgrlene tubing with a special hose clamp (2) is used instead of a conventional stopcock. A capillary tube carries the solvents to the top of the chromatographic column and allows a
21 12
ANALYTICAL CHEMISTRY
more compact mounting of the apparatus.
Construction. The internal diameter of the glass tubing selected is 46 mm. The tolerance for tubing of
Figure 1.
Complete apparatus
this size is =k 1 to 1.6 mm., but the variation within 4-foot lengths is less than 0.2 mm. The upper end of the tubing is flanged and flared to take a rubber stopper and clamp. The side arms are also flanged and flared to take size 0 corks, covered with a thin sheet of saran (Saranwrap, Dow Chemical Co.), held in place with metal disks and ball joint clamps. The stainless steel shaft, A (diameter 1.27 cm.) , is threaded a t either end to take the retaining nuts and also an internally threaded T pipe (not shown) which serves to remove the assembly from the tube. SPACERDISKS. The height of each compartment is 2.34 cm., but by employing two stainless steel spacers, 1.17 cm. high, for each compartment the specified variation is possible. Each spacer has a center hole reamed to a slide fit on the shaft. A wide choice of solvent program is possible with 48 spacers, 12 of each diameter shown in the table. The values given were calculated to include a side arm volume of 4.2 ml. and by using appropriate pairs, any of the volumes listed above may be obtained. DIAPHRAGM DISKS. These are made of tetrafluoroethylene (3 mm. thick) and are 45.9 mm. in diameter. The edges are beveled as shown in Figure 2 and four
Figure 2.
Diaphragm disk
Size of Spacer Disks
Free mace requireh, cc.
Diameter, cm.
5
4.24 3.54 2.66 2.09
10
15 17.5
slits, 90" apart and 1 mm. deep, are cut with a scalpel as shown. The bottom diaphragm disk (Figure 1) has large
slots cut and is trimmed to seat smoothly on the bottom of the glass tube. Operation. After the compartments are filled, pressure is applied t o the system with valves B and E closed and the rubber hose clamp, D, open. D is then closed and B opened. The needle valve, C, acts as a temporary vent if necessary and the flow through the whole system is controlled by the column valve, E. ACKNOWLEDGMENT
Appreciation is expressed for the tech-
nical assistance of R. C. Verney. The financial assistance of the National Research Council of Canada is gratefully acknowledged. LITERATURE CITED
(1) Harpur, R. P., Can. J. Biochem. and Physiol. 36? 707 (1958). (2) Harpur, R. P., J. Chem. Educ. 36, 149 (1959). ( 3 ) Roberts, E. J., hlason, A., ANAL. CHEM.28, 1063 (1956).
Modified Absorption Cell and Cell Compartment Cover for the Beckman Model DU Spectrophotometer Samuel R. Henderson and Louis J. Snyder, Research and Development Department, Ethyl Corp., Baton Rouge 1,La.
TOtions
the number of manipulaand improve the over-all accuracy of the determination of heavy metal ions (lead, bismuth, copper, mercury, zinc, etc.) by the colorimetric dithizone method, a special absorption cell was developed. Its use eliminates indeterminate errors arising from leaky, plugged, or frozen stopcocks, emulsions due to temperature changes in immiscible solvents, contamination during filtration or excessive handling, and increase in concentration of metal dithizonates due to evaporation of volatile solvents during transfers. The cell permits the analyst to carry out the color development, extraction, and photometric measurement rapidly and accurately in a single container. I t is a modification of a color comparator tube previously described [Snyder, L. J., Barnes, W. R., Tokos, J. V., AKAL.CHEM.20, 772 (1948); Griffing, M. E., Rozek, A., Snyder, L. J., Henderson, S. R., Ibid., 29, 190 (1957)l for visual estimations. Its wide application to other accurate photometric measurements employing different photometers makes it a valuable laboratory tool. The cell (Figure 1) may be fabricated by attaching a square precision (borosilicate glass) absorption cell (10.0 X 10.0 X 95 mm.) to the upper part of a globe-shaped 200-ml. separatory funnel. A shorter cell (10.0 X 10.0 X 48 mm.) REDUCE
i , i t . Figure 1. tion cell
u
I
I
Modified absorp-
*Absorption cell and glass bulb fabricated from borosilicate glass or comparable material
may be used, but requires the insertion of a piece of borosilicate glass tubing (11 mm. in outside diameter X 48 mm.) between the cell and the glass bulb. The dimensions given here are for a cell to fit either the Bcckman Model DU spectrophotometer or the Leitz Rouy photometer. This modified absorption cell is now available on special order from E. Leitz, Inc., 304 Hudson St.,NewYork14, N . Y . Otherphotometers may require slight changes in the dimensions. The Hellige Chromatron requires a 13 X 13 X 88 mm. cell. A special cell compartment cover
Figure 2. Cell compartment cover for Beckman Model DU spectrophotometer
(Figure 2) is required for the Beckmnn Model DU spectrophotometer. This cell has been found to have great usefulness in all types of photometric measurements: determination of lead, bismuth, mercury, copper, zinc, etc., by dithizone techniques; aluminum by extraction of the oxinate with chloroform; chlorine in organic solvents by extraction with aqueous o-tolidine reagent; and iron in organic solvents by extraction with o-phenanthroline reagent. In any case, the colored phase is obtained 2s the lower layer by adjusting the density of the organic solution by diluting with a more dense solvent (chloroform) or a less dense solvent (hexane) as the case may require.
VOL. 31, NO. 12, DECEMBER 1959
21 13
