Determination of Available Oxygen in Manganese Ores by Cerate Oxidimetry SIR: Examination of the literature shows no record of the use of cerate solutions in the well known determination of the oxidizing power of manganese ores by treatment with an excess of arsenite solution and back-titration with an oxidizing agent. Since this method ought t o work, i t was tried on a Bureau of Standards manganese ore and on four ores supplied by a well known manufacturer of student unknonm with entirely satisfactory results. PROC E DU RE
A determinate 0.1.V solution of arsenious acid in sulfuric acid and an approximately 0.1N solution of ammonium sulfato cerate in sulfuric acid
Table I.
Ore S B S 25c No 61 No. 73 No. 97 Yo. 98
%O
16 69 16 70 16 70 2 506 2 509 6.27 6.28 11.99 12.00 11.02 11.04
were used ( 2 ) . The cerate solution was standardized by comparison with the arsenite solution, using 3 drops of 0.0161 osmium tetroxide solution as a catalyst and 1 drop of Ferroin indicator solution. Independent standardizations with solid ferrous ammonium sulfate of known purity were also run. The results of the two methods agreed within 0.2%. Suitable samples of ore were weighed into 1-liter conical flasks. About 45 ml. (roughly twice the theoretical quantity) of arsenious acid solution was measured accurately into the flask from a buret, 10 ml. of concentrated sulfuric acid added, and the whole heated to gentle boiling until no colored particles remained, or until no further dissolution seemed to occur after about 10 minutes. The solution was then
Oxygen Determination
Mean 16 70 2 508
6.28 12.00 11.03
Check Values 16 70 2 498 2 509 6.27 6.27 11.96 11.95 11.08 11.oo
Mean
Deviation, % 0
2 504
0 16
6.27
0.16
11.96
0.3
11.04
0.1
diluted with 100 ml. of water and cooled, three drops of osmium tetroxide and one drop of Ferroin indicator were added, and the whole was back-titrated with the cerate solution to the pale blue end point. The results of the four student unknowns varied considerably from the manufacturer’s published values. These were consequently analyzed by the permanganate method ( 1 ) . The results agreed well with those obtained by the cerate method. All analyses were run in duplicate except those on the Bureau of Standards ore, which were run in triplicate. Results are given in Table I, in which the column headed “Check Values” contains the published value for the Bureau of Standards ore and the values obtained by the permanganate method for the others. LITERATURE CITED
(1) Pierce, W. C., Haenisch, E. L.,
Sawyer, D. T., “Quantitative Analysis,” 4th ed., p. 285, W i k g New York, 1958. (2) Smith, G. F., erate Oxidimetry, Further Applications of the Use of Cerium in Volumetric Analysis,” p. 41, G. Frederick Smith Chemical Co., Columbus, Ohio, 1942. DUNCAN G. FOSTER Swarthmore College Swarthmore, Pa. RECEIVED for review September 28, 1960. Accepted October 20, 1960.
Gas Chromatographic Determination of Trace Amounts of the Lower Fatty Acids in Water SIR: The flame ionization detector is well suited for use in the gas chromatographic determination of trace organics in water. This detector is insensitive to the large amount of water eluted, allowing detection of the trace organics present. It has been reported (3) that the sensitivity of the argon ionization detector is diminished by the presence of water in the carrier gas, thereby making it less suitable for such an application. An example of the use of the flame ionization detector for this purpose is in the determination of the lower free fatty acids in water. Such solutions are encountered in the determination of the 146
ANALYTICAL CHEMISTRY
fatty acid levels in aqueous biological systems. Smith (4) recently reported the gas chromatographic separation of aqueous mixtures of formic, acetic, propionic, isobutyric, and n-butyric acids using 5y0Tween 80 on Celite 545 with a conventional thermal conductivity detector, but the method is limited to mixtures containing only small amounts of water due to tailing of the water peak into the acids. Very recently Hunter, Ortegren, and Pence (2) described a scheme for determining such acids in the presence of water in amounts up to 50% of the mixture. This involves combustion of the eluted acids t~ carbon dioxide
and removal of the water in a drying tower ahead of a thermal conductivity detector. It was thought that the direct determination of low levels of the lower fatty acids in water solution could be accomplished by using a flame ionization detector which would be completely insensitive to the large amount of &-ater present. Further, the presence of this water would tend to deactivate temporarily the residual adsorptive sites on the solid support, thus minimizing tailing of the polar components. The apparatus used was a PerkinElmer Model 154-C Vapor Fractometer
fitted with the flame ionization accessory kit. An aluminum block containing a 75-aatt cartridge heater was attached to the back of the liquid injection block aiid a thermocouple inserted in a hole in the injection block. The cartridge heater power was supplied by a separate 1-ariac to allow independent control over the temperature of the sample introduction system. A 1-meter column of l, .,-inch stainless steel tubing packed n i t h 20 weight % T ~ e e n80 on acidnashed 60- t o 80-mesh Chromosorb K \\as used a t a temperature of 110" C. a i t h a nitrogen carrier gas flow rate of 40 cc. per minute. The injection block nas maintained a t 210" C. to provide prompt vaporization of the relatively large amount of n-ater introduced. Only a portion of the effluent from the column was sent to the flame detector. A Perkin-Elmer Yo. 0 restriction needle nas attached to the bypass line around the detector resulting in attenuation of the 0 0 t ~ o the detector. The hydrogen flow rate t o the flame jet was 8 cc. per minute and the air flow rate to the detector chambermas 430 cc.per minute. A direct current potential of 300 volts n as used across the flame. The output of the Perkin-Elmer ionization detector amplifier was transmitted t o a 5-mv. rcc-order.
--A 54
32
:C
28
i6
24
2'2
18
2b
TIVE IT
Figure 1. acids
_____ ;1
I4
IO
lh
4
6
4
FAUS
~ b - e s
Chromatogram of aqueous mixture of 0.1% each of C?to
Csorganic
w
01
0 z
Figure 1 shows the chromatogram requlting from the injection of a 20-pl. sample of an aqueous solution containing 0.1 weight yo each of acetic, propionic, isobutyric, n-butyric, isovaleric, and n-valeric acids. The signal from the amplifier has been attenuated by a factor of four. The hydrogen flame is extinguished when this large quantity of n ater vapor first is eluted from the column, but the flame can be reignited easily after most of the n ater has passed through the detector within 2 minutes after injection and well before the first acid is eluted. The sharp peaks a t the very beginning of the chromatogram are apparently due to trace organics in the distilled water used. The relative areas (and, thereby, responses since equal neights of the various acids rvere used) of the other acids t o that of the acetic acid are propionic 1.6, isobutyric 2.1, n-butjric 1.9, isovaleric 2.2, and n-\-aleric 2.0. The results for the n-acids confirm earlier observations ( 1 ) that the flame ionization detector response is proportional to the carbon content of the compounds. Honever, the different values obtained for the iso-acids shornthat structure also is a factor in determining relative responre. Figure 2 shows the chromatogram of a 25-pl. sample of aqueou': qolution containing only 0.01% rach of the tame acids with no attencation of the aniiilifier signal. This chromatogram u as obtained at a later occasion than that in Figure 1 and with a repacked column a t a slightly lower temperature (108" C.), accounting for the somen h a t longer retention times. An attempt to in( rease the sensitivity
n c
L
6
TIME imiiutesl
Figure 2. acids
Chromatogram of aqueous mixture of 0.01
of the method by passing the total effiuent from the column to the flame ionization detector n as unsuccessful due t o increased base-line noise apparently caused b y column instability and slight bleeding of the Tween 80 packing a t 110" C. Some other polar column packing might have greater stability for use in this analysis, but no further search has been made. The use of the flame ionization detector with a polar packed column as in the present method should have wide application for the determination of similar trace organic materials in water. I n the case of very polar organic components the presence of the large amount of TT ater is actually beneficial in greatly decreasing the tailing of these materials. We have used a related application of the principle shonn here. When using the flame ionization detector in the analysis of solid and viscous liquid samples which must be dissolved in a n organic solvent for introduction to the gas chromatographic column, the tailing of a
yoeach of C2to Cr organic
large solvent peak into the sample components can be avoided by the use of carbon disulfide as the solvent. =is in the case of n-ater, the flame ionization detector is insensitive to this material and only detects the trace organic impurities in this solvent which usually give only a few sharp peaks a t the very beginning of the chromatogram. LITERATURE CITED
(1) Ettre, L. S., Claud?;, H. S . , Chemical
Institute of Canada, Symposium on Gas Chromatography, Toronto, Ontario, February 1960. ( 2 ) Hunter, I. R., Ortegren, V. H., Fence, J. W., A N A LCHEM. . 32, 682 (1960). (3) Lovelock, J. E., Third Intern. Syrnposium on Gas Chromatography, Edinburgh, Scotland, June 1960. (4) Smith, Bengt, dcta Chem. Scund. 13, 480 (1959).
EDW.4RD SI.EMERY K . E. KOERNER
Research Department Organic Chemicals Division Monsanto Chemical Co.
St. Louis 77, Mo. RECEIVED for review September 19, 1960.
Accepted October 21, 1960.
VOL. 33, NO. 1, JANUARY 1961
147
