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
