high ohmic resistance systems could not be studied effectively. CONCLUSIONS
The most important practical advantage of electronically controlled a x . voltammetry is the application t o high resistance systems, where conventional a x . voltammetry is inapplicable. Phase selective detection of the current, as reported by Smith and Reinrnuth (7), extends the utility even more. LilERATURE CITED
(1) Bauer, H. H., Elving, P. J., ANAL. CHEM. 30. 334 (1958). (2)-Bauer, H. H., Elving, P. J., Australian J.Chem. 12,335 (1959). (3) . . Booman, G. L. ASAL. CHEM. 29, 213 (1957). (4) Breyer, B., Gutmann, F., Hacobian, S.. Australian J. Sci. Research Ser. A 4,‘597 (1951). (5) Kelly, M. T., Fisher, D. J., Jones, H. C., Maddox, W. L., Stelzner, R. JV., Laboratory Instruments Session, I.S.A. Conference, Sen- York, Sept. 26, 1960. (6) Philbrick, George A., Researches, Inc., Boston, Mass., “GAP/R Electronic Analog Computers,” ‘‘L4pph.tions Manual for Philbrick Octal Plug-In Computing Amplifiers,” and catalog data sheets. ( 7 ) Smith, D. E., Reinmuth, W. H., ANAL.CHEM.32, 1892 (1960). DONALD E. WALKER RALPHS. ADAMS Department of Chemistry University of Kansas Lawrence, Kan. JOHN R. ALDEN School of Electrical Engineering University of Kansas Lawrence, Kan. RECEIVED for review October 21, 1960. Accepted December 7 , 1960. I - - -
0.9
0.8
b.6
0.7
E
vs.
0.4
0.S
0.3
0.2
0.1
S. C. E.
Figure 3. A.c. voltammetric scan of 2 X 1 O-3M KI in 75% acetone-25% water with 0.25M HzS04 background electrolyte using controlled potential (4) 4600 ohms cell resistance
10,000 ohms or less, and no difference in the phase angle of the current was noted. Resistances up to 100,000 ohms did not change the peak height, but the total a x . current increased during part of the alternating cycle, which indicates some loss of control. Resistances above 500,000 ohms caused overloading of the amplifiers, and resulted in clipping of the sine-wave current. The charging of the double layer capacitance for the electrode system in I N KC1 with an applied square wave was studied as an indication of the ability of the instrument to control the
ax. potential. The square wave response is shown in Figure 2. Potential control results in rapid charging of the double layer, and better separation of capacitive and faradaic currents than in noncontrolled systems. A solution of 2 i< 10-3AVKI and 0.25M H2S04in a mixed solvent composed of 7 5 7 , acetone-25% water was oxidized using potential control and the scanning conditions previously described. The cell resistance as determined by an a x . bridge was 4600 ohms. The resulting scan is shown in Figure 3. Without potential control, such
~
Determination of Cesium as Permanganate SIR: The solubility of cesium permanganate in water a t temperatures a few degrees above 0” C. is lower than that of any other metal permanganate. At 1O C. a saturated solution has only a faint violet tint, and the solubility product of the salt a t this temperature is 1.5 X This indicates the possibility of practically quantitative preripitation, a t least in a sufficiently concentrated cesium solution a t a low temperature in the presence of a sufficiently high concentration of permanganate. EXPERIMENTAL
Evaporate the neutral solution, containing not less than 5 mg. nor more than 50 mg. of cesium, to dryness in a 50-ml. beaker or Erlenmeyer flask. Dis-
solve the residue in 1 ml. of water and half immerse the vessel in a bath of salted cracked ice. Add dropwise from a buret 6 ml. of 0.1X ammonium permanganate while swirling the liquid in the vessel. Allow the vessel to stand in the bath a t least 10 minutes. Prior to beginning the precipitation, half immerse in the same bath a micro wash bottle containing about 25 ml. of propionic acid. At the same time place a weighed filter crucible in a small container near some dry ice so that the entire crucible is cooled by carbon dioxide vapor. At the end of the cooling period of a t least 10 minutes fit the cold crucible to a suction apparatus, and transfer the precipitate and solution rapidly to the crucible with the aid of successive I-ml. portions of the propionic acid. Wash the precipitate with a few additional 1-ml. portions of pro-
pionic acid and allow the suction to continue until the precipitate appears to be dry. Remove the crucible, touch the bottom of it to a piece of filter paper to remove any adhering propionic acid, and place it in a drying oven maintained a t 110’ to 120’ C. Dry for about an hour, cool, and weigh. Multiply the weight of the precipitate by 0.5270 to obtain the weight of cesium or by 0.5593 to obtain the weight of cesium as oxide. Instead of drying and weighing the cesium permanganate, some time may be saved by dissolving the precipitate and titrating the solution. To do this, first dry the crucible containing the precipitate briefly, place it in a funnel leading to a 250-ml. beaker, and dissolve the precipitate by adding in successive small portions a solution prepared by adding 2 ml. of concentrated sulfuric acid to 25.00 ml. of VOL. 33, NO. 2, FEBRUARY 1961
309
standard 0.1N oxalic acid. Heat this solution to about 90" C. before use. Transfer the last portions of oxalic acid
Table 1. Effect of Concentration of Reagent and Concentration of Cesium on Completeness of Precipitation
VolRatio umeof of"( CsCl Differ~ 1 1 0 solu~ Cesi~m,Mg. ence to tion, PresError, CsCl M1. ent Found Mg. 2 5 9
5.00 5.00 5.00
50.0 50.0 50.0
49.9 50 4 50.4
-0.1 +0.4 +0.4
2
son
246
-n
25:2 25.0
+0.2 0.0
9
5.00
250 25.0 25.0
2 7 9
2.00 2.00 2.00
10 0 10.0 10 0
9.9 10.2 10.0
-0.1 +0.2 0.0
2 5 7
1.00 1.00 1.00
10.0 10.0 10.0
10.0 9.9 10.0
0.0 -0.1 0.0
2 5
1.00 1.00 1.00
5.0 5.0 5.0
4.6 4.8 5.1
-0.4 -0.2 t0.1
2 5
1.00 100 1.00
2.0 2.0 2.0
0.3 0.4 1.1
-1.7 -1.6 -0.9
7
7 7
5100
4
solution by washing with 1-ml. portions of water. Heat the solution in the beaker to 90' C. and back-titrate with standard 0.1N potassium permanganate. Thedifference between the volume of permanganate required and that required for the blank titration of the oxalic acid solution corresponds to the amount of cesium present. Each milliliter of O.1OOON permanganate equals 2.66 mg. of cesium or 2.82 mg. of cesium oxide. The results are as accurate as those obtained gravimetrically.
cause they react rapidly with both precipitate and reagent. Pure propionic acid is a fairly satisfactory wash liquid. It does not react appreciably in a shoi t period of time a t a low temperature with either precipitate or reagent, and cesium permanganate is practically insoluble in it. Its somewhat disagreeable physiological properties were not found to be a serious obstacle. Unfortunately, this method is applicable only when the ratio of cesium to rubidium is high, because rubidium causes a serious positive error due to coprecipitation, as would be expected from its low solubility. Potassium interferes to a much smaller degree. Other metal ions except silver do not interfere unless they reduce perinanganate in neutral solution.
RESULTS AND DISCUSSION
Table I shows that satisfactory results can be obtained if the concentrations of reagent and cesium are both sufficiently high. Such results cannot be obtained by precipitation and filtration a t room temperature under any conditions of concentration. Solutions of sodium or lithium permanganate are also suitable as reagents. A solution of cesium permanganate in water saturated a t the working temperature may be used as the wash liquid, but its use leads to slightly high results because of the precipitation of cesium permanganate from it on contact with reagent still adhering to the precipitate. The usual organic washing solvents such as acetone or alcohol are entirely unsuitable be-
EARLER. CALEY WALLACE H. DEEBEL~ Department of Chemistry The Ohio State University Columbus 10, Ohio RECEIVED for review May 2, 1960. Accepted November 18, 1960. Division of Analytical Chemistry, 137th Meeting, ACS, Cleveland, Ohio, April 1960. Taken from the Ph.D. thesis of Wallace H. Deebel, Ohio State University, 1957. 1 Present address, Pittsburgh Plate Glass Co., Pittsburgh, Pa.
Influence of Column Support on Separation of Fatty Acid Methyl Esters by Gas Chromatography SIR: We reported the use of a poly (vinyl acetate) (PVA) liquid phase on Chromosorb, 30-60 mesh, for the separation of fatty acid methyl esters (4). Subsequently, batches of Chromosorb R and SV (both diatomaceous earth products) when used as supports for PVA gave separations of widely varying efficiencies. Homing, Moscatelli, and Sweeley (S), following the procedure of Howard and Martin (6), reported that screened Celite 545, 60-80 and 80-100 mesh, acid-washed, dried, treated with vapors of dimethyldichlorosilane, and then washed with methanol, gave a superior column support for polyestertype liquid phases. More recently, Hishta et al. (1, 2 ) suggested the use of microbeads as the solid support for liquid substrates, in the order of 0.25% by weight, to separate high boiling compounds at a rapid rate and a t relatively low column temperatures. A comparison of the separations obtained for several fatty acid methyl 310
ANALYTICAL CHEMISTRY
esters on PVA, supported on "silanized" 30-60 mesh Chromosorb R and Celite 545 and on waterproofed glass microbeads, 30-60 mesh, is given in Table I. Although the Celite 545 fractions can be pretreated satisfactorily by passing
Table 1.
nitrogen saturated with dimethyldichlorosilane through a 100-gram batch of the support for approximately one hour, Chromosorb R required a very much longer exposure time to the vapors of dimethyldichlorosilane. Best results
Separation Factors, Retention Volumes, and Theoretical Plates (9-Foot Column) for Fatty Acid Methyl Esters on Poly(viny1Acetate)
Column Support Chromosorb R, T = 205' C. Reten-
Glass Microbeads, Celite 545, T = 168" C. T = 205" C. Retention Retenvol- Separa- Theoret- tion Separa- Theoret-' tion Separa- Theoretical ume, tion ical volume, tion ical volume, tion ml. factor plates ml. factor plates ml. factor plates ~
Methyl Ester Laurate Myristate Palmitate Margarate Stearate Oleate Linoleate
452 798 1438 1962 2580 2890 3370
0.23 0.41 0.73 1.00 1.32 1.47 1.72
1190 1600 1760 1820 1925 2050 2120
248 444 803 1097 1450 1633 1913
0.23 0.41 0.73 1.00 1.32 1.48 1.75
640 910 950 1470 1500 1735 2035
85 177 366 530 750 850
0.1G 0.33 0.69 1.00 1.41 1.60
16 32 34 73 94 139
