have been alkali halides, Irtran, borosilicate glass, and quartz. Cell bodies and window materials may be used in all possible combinations. The cylindrical cell bodies in most instances are constructed from borosilicate glass tubing cut to the desired length with a glass saw. The ends of the tubing as cut are usually satisfactory for the application of the form-fitting sealisg material. As shown by the accompanying illustration, the sealant is worked to the consistency of a well kneaded bread dough. The sealant is drawn out to a ribbon and applied to the end of the cell cylinder, overlapping a t the joining point; numbers 1, 2, and 3, Figure 1. The window material is then centered and pressed firmly into position; number 4, Figure 1. The valve or stopcock of the gas cell is then opened and after applying the ribbon to the opposite end of the cylinder, the opposing window is put in place. The completed cell is then evacuated and is invariably found to be vacuum tight; number 5,
Figure 1. A “see-the-seal” test can be used as a guide at this point. The pressure of one atmosphere against the windows causes the perfluorocarbon material to thin out at the point of seal to a tough, adhesive, transparent seal. After baking out with a heatgun a check with a vacuum gauge over an appropriate period can be made for vacuum tightness. The cell may now be used for samples in the infrared, visible, or ultraviolet regions of the spectrum. External heat may be applied when desired in the spectrometric examination of materials of low vapor pressure. The completed gas cell is demountable and requires no supporting framework. Because of the ease of construction, cleaning of window materials and the cell interior presents no problem. A large inventory of window materials is also considerably reduced because of the ease with which they may be exchanged from one cell type to another. Windows of 3-mm. thickness or larger may
be removed by applying a twisting force. Window materials less than 3-mm. thickness are safely removed by pulling out the sealant between the cell body and window with the aid of tweezers. Figure 2, illustrates an infrared and/ ormass spectrometry gas cell that is used a t the exit end of a gas chromatograph in the identification and confirmation of pure fractions. It may also be used in conjunction with a vacuum system to increase sample concentration by condensing a sample contained in a larger vessel at low pressure onto the microbeads in the U-tube and then analyzed after bringing to room temperature. ACKNOWLEDGMENT
The author thanks Lester P. Kuhn and associates a t the Ballistic Research Laboratories for their cooperation, confidence, and flexibility in adapting this new material to their work.
Three-Compartment Cell for Polarography and Coulometry Jose M. Costa, Michael S. Spritzer, and Philip J. Elving, The University of Michigan, Ann Arbor, Mich.
and coulometric p studies frequently must be made using small quantities of test solution. OLAROGRAPHIC
This is particularly important when nonaqueous media are employed, since solvent purification is often laborious and time consuming; high purity solvents, when available-e.g., in spectroscopic grade-are expensive. In addition, cell dimensions are important in determining solution resistance. It is preferable in most electrochemical work to separate the reference and indicating electrodes by some type of membrane or bridge. The agar plug, commonly used with an H-cell containing a sintered glass disk in the case of aqueous solutions, cannot be applied to nonaqueous solutions since agar will usually not gel in such media. As a result, a third compartment is frequently introduced to separate the refTable 1.
Solvent‘ Water Acetonitrile Pyridine 85y0 Dioxane15% water
-
0
I
2
3 c m
Figure 1. Three-compartment cell for polarography and coulometry
Resistance of Typical Solutions in Three-Compartment Cell Resistance in Kohms for 0.1M solutions LiClOd KC1 n-PrrNBr Ab CC Ab Cc Ab C C 0.4 1.7 0.3 1.1 0.5 2.0 0.4 1.7 0.4 1.7 1.3 6.0 4.0 16
3.0
20
Dielectric constants of solvents used: water, 78; acetonitrile, 36; pyridine, 12; p-dioxane, 8. b Resistance between DME and mercury pool in compartment A . c Resistance between DME in compartment A and mercury pool in compartment C. a
698
ANALYTICAL CHEMISTRY
erence and indicating electrode compartments, in spite of the resulting increase in resistance. Although the use of a mercury pool reference electrode is more convenient in some cases than use of an external reference electrode, it is still necessary t o measure periodically the pool potential us. an external reference electrode. In addition, the electrochemical phenomena to be studied and the convenience of operation must be taken into consideration when designing an electrochemical cell. A considerable number of cells have been proposede.g., the extensive review by Zagorski ( 2 ) . An insulated, constant temperature H-cell ( I ) has been used in the authors’ laboratory for many years for studies in aqueous solutions. Description of Cell. The cell (Figure 1) consists of three sections or compartments separated by two medium-porosity fritted-glass disks: ( A ) test solution, ( B ) bridge solution, and (C) external electrode. Compartment A has a capacity of 5 to 10 ml.; a platinum contact a t the bottom permits the use of a mercury pool electrode. A stirred pool electrode can be obtained by placing the flat bottom of this compartment over a magnetic stirrer; coulometric determinations can thus be conveniently performed. Deaerating gas is introduced through a fine-porosity fritted disk for a better gas dispersion. The
gas can be directed over the solution with a stopcock. The upper part of the compartment has a wider diameter to allow use of a stopper large enough to hold the capillary, gas exit tube, etc. If desired, a ground-glass stopper can be used to give an all-glass cell. Compartment B if3 provided with a ground-glass stopper to minimize flow of the bridge solution between compartments. Compartment C contains a platinum contact at the bottom for use with an exterc.al mercury pool, massive calomel, or similar reference electrode. Resistance of Celll. Resistance of the cell when it was filled with a number of aqueous and nonaqueous solutions of typical background elec-
trolytes was measured with a conductance bridge t o indicate the magnitude of the resistance t h a t might be expected under the usual conditions of operation. The values, tabulated in Table I, represent the resistance between a dropping mercury electrode and a mercury pool electrode in compartment A (1.5-cm. separation), and a dropping electrode in A and a mercury pool in C. When the resistance was measured between a dropping mercury electrode in compartment A and a mercury pool in compartment C, all three cell compartments were filled with the same solution. No attempt was made to use a particular reference electrode in compartment C for the resistance measurements. I n general, use of a
reference electrode solution in compartment C would result in a lower cell resistance because of the high electrolytic concentration in such solutions. ACKNOWLEDGMENT
The authors thank D. I. Myers for his cooperation in manufacture of the cell. LITERATURE CITED
Komyathy, J. C., Malloy, F., Elving, P. J., ANAL.&EM. 24, 431 (1952). (2) Zagorski, 2. P., in “Progress in Polarography,” P. Zuman, ed., pp. 549-68, Interscience, New York, 1962. WORK supported in part by the U. S. Atomic Energy Commission. (1)
A Versatile Solubility Apparatus
R. L. Coffin, L. Stoller, and D. B. Wetlaufer,’,
Department of Biochemistry, Indiana University Medical School,
Indianapolis 7, Ind. EXPERIMENTAL arrangements have been described for equilibrating solids with their saturated solutions (2). While the present apparatus was designed f i x this purpose, it has also proved convenient for gasliquid and liquid-liqui d solubility equilibrations. It permits isothermal phase separation for samp,ing and is easily adapted for use with inert atmospheres. I n addition, it is compact, inexpensive, and easily contructcd in borosilicate glass. As shown in Figure 1, the apparatus is composed of an upper and lower chamber, joined by a 24/40 ’$ joint, appropriately sealed. Equilibration of solid or liquid with solvent occurs in the lower chamber, when the assembled apparatus is immersed in a thermostated bath to a depth such that only about 3 cm. of the upper chamber is above the surface of the bath liquid. Agitation is by magnetic stirring-such a stirrer with external rheostat can easily be encased in a submersible, water-tight copper sheath. As many as six such equilibration vessels c m be stirred simultaneously with one h i v e unit below. It is also sometimes convenient to stir single samples from :he side, with the drive unit outside the (nonmagnetic) thermostat wall. The upper chamber and sidearm may be closed with serum stoppers during equilibration. After separate tests have proved equilibrium, saturated solution is forced through the filter stick into the upper chamber by UMEROUS
To whom correspcindence should be addressed. * Present address, Department of Biochemistry (Medical Sciences), University of Minnesota, Minneapolis 14, Minn.
applying gentle air pressure to the sidearm. The shaft of the filter stick is of thick wall capillary for ruggedness. I n the case of volatile solvents or solutes, the saturated solution can be forced directly into a rubber-tipped pipet placed snugly against the opening in the
T
23mm
bottom of the upper chamber. A pipet such as that used by Eberz and Lucas ( I ) is helpful in sampling volatile materials. For measuring the solubilities of gases in liquids, we lead the gas into the solvent through the filter stick, which now serves as a gas disperser. The gas efflux escapes through the side-arm. At moderate bubbling rates, hydrocarbon gases saturate aqueous solvents in less than half an hour. The saturated solution can be forced up into the sampling pipet by causing the gas in question to flow in the side-arm. When one is equilibrating two partly miscible liquids, the stirring must be controlled so that none of the less dense phase becomes attached to the sintered glass, or the liquid forced into the upper chamber is likely to contain microdroplets of the other phase. With the dimensions indicated in the sketch, a sample volume of about 15 ml. is conveniently handled. The design can easily be modified for special requirements. LITERATURE CITED
(1) Eberz, W. F., Lucas, H. J., J . Am.. Chem. SOC. 56, 1230 (1934). ( 2 ) Mader, W. J.,. Vold, R. D., Vold, M. F.. “Solubility.” in “Physical
Methods of Organic’ Chemistry,” 3rd ed., Part I, A . Weissberger, ed., p. 655, Interscience, Kew York, 1959.
INVESTIGATION supported in part by U. S. Public Health Service Research Grant GM 10900, Division of General Rledical Sciences. One of us (L.J.S.) was supported on an USPHS Medical Student Figure ratus
Exploded view
Of
appa-
Summer Research fellows hi^ (1961). Another (D.B.W.)gratefully achodedges support by an USPHS Senior Fellowship, SF-505. VOL. 36, NO. 3, MARCH 1964
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