1194
ANALYTICAL CHEMISTRY
A sample evaluation is shown in Table IV. Because of normal variations in sample size and instrument sensitivity, the total of the components determined may vary from 100%. Provided that the difference is not too great, i t is common practice t o normalize the results t o 100%. This normalizing can be kept t o a minimum by the use of an automatic sample metering system t o secure reproducibility of sample size and installation of amplifier feedback control t o permit adjustment of instrument sensitivity. OBTAINING INVERSE MATRIX
It is sometimes advantageous to be able to determine one z value independently of the others. To accomplish this end, the above reduction may be followed until Matrix I11 is obtained and the inversion completed by use of the Crout method. This is followed by reconversion of the diagonals to unity by multiplying each row by the reciprocal of its respective diagonal and multiplying the factors 1,611, l / b 2 2 , and l / b 3 3 of Matrix I11 by the re-
spective diagonals. Any x value may be calculated independently, then, in terms of mole per cent, by the usual cumulative multiplications followed by multiplying by the corresponding corrected factor. While obtaining the inverse matrix is much more time-consuming than the reduction step of the principal procedure described herein, its use may be justified where repeated determinations for one component (say 50 or more) are required. Conversion of the diagonals t o unity results in a more rapid procedure, which is less subject t o error than the procedure generally used. ’
LITERATURE CITED
(1) Crout, P. D., Am. Inst. Elec. Engrs. Trans., 60, 123540 (1941). (2) Daigle, E. C., and Lee, J. H., Petroleum Refiner, 27, S o . 9, 492-5
(1948). (3) Milne, IT. E., “Xumerical Calculus,” pp. 15-25, Princeton, N. J., Princeton University Press, 1949. RECEIVED for review December 17, 1951. Accepted March 17, 1952.
Determination of Insecticide Residues Analysis of Flour f r o m Pyrethrum-Treated Cotton Bags ALBERT A. SCHREIBER AND DONALD B. MCCLELLAN McLaughEin Gormley King Co., Inc., Minneapolis, Minn.
HE protective treatment of cotton flour bags achieved by ‘weaving the cloth from a warp treated with a pyrethrum extract size ($) left open the question of whether any of the pyrethrins would migrate into the flour and could be detected there after storage. Even though pyrethrum extract is considered as one of the insecticides least harmful to warm-hlooded beings ( 7 ) , definite information was needed. Samples of wheat flour in cambric and Osnaburg bags had been drawn from a flour layer about 0.5 inch thick nearest to the fabric. An equal number of samples was available from untreated bags (group C ) and experimental bags, the warp of which had been treated so that the fabric contained originally 5 mg. (group B) and 10 mg. (group A) of pyrethrins per square foot. All the filled bags of three groups from the same source had been stored in a flat position in the same location. Upon inspection of the groups of samples received in tin cans with airtight lids from mills in three different states, the flour from untreated bags was found t o contain up t o 7 live specimens of confused flour beetle per pound of flour. None was found in the flour from the treated bags. About 3 months later the number of live insects had increased t o about 30 and about 20 larvae were found in the same infested samples; very few, if any were found in the samples from treated bags, confirming Cotton’s and other experiments demonstrating the effectiveness of the treatment. The flour samples were then examined chemically for pyrethrins, after having been sifted t o avoid contamination of the extractives by the body fat of the insects, which might interfere with the determination. Sixty-gram portions of the flour samples were extracted with 200 ml. of low boiling petroleum ether, first by shaking at room temperature and then by refluxing for 3 hours. The combined extracts were filtered after cooling and then evaporated on the water bath (about 75” C.), The concentrates Jvere subjected t o the usual analysis of pyrethrum extract by the Seil method, which allows separate determination of pyrethrins I (pyrethrolone and cinerolone esters of chrysanthemum monocarboxylic acid) and pyrethrins I1 (equivalent esters of chrysanthemum dicarboxylic acid). No pyrethrins I could be detected but, in all cases, small values were found for “pyrethrins 11”(Table I). ils the latter are present in most commercial pyrethrum extracts t o the extent of approximately 80 t o 90% of the pyrethrins I and never in the complete
absence of pyrethrins I, the values shown in Table I must be attributable t o constituents of the flour which had been extracted and interfered with the normal analytical procedure by reacting as pyrethrins I1 would do. It could be suspected that unsaturated fatty acids in flour (8, 9 ) , such as linoleic acid, ivhich occur in pyrethrum extract in a small proportion (j), might give more soluble barium salts than saturated fatty acids and thus be washed out of the pyrethrins I fraction t o appear in the fraction not volatile with steam in the Sei1 procedure-i.e., that containing the chrysanthemum dicarboxylic acid of pyrethrins 11. Hence, a comparative material of vegetable origin was examined t o determine if such an explanation were possible. I n place of an extended check on the solubility of barium salts of unsaturated fatty acids, samples of linseed oil rvere saponified and then subjected t o the Sei1 procedure. T o obtain comparable data, the quantities of linseed oil were chosen to have as closely as possible the same acid and ester content as the u-beat berry and flour (8, 9 ) . In each case values for pyrethrins I1 obtained from the pyrethrum-free linseed oil were practically identical r i t h those obtained from the equivalent quantities of flour from treated as n-ell as untreated bags. It appears that the Seil procedure, without modification, is not strictly applicable to the determination of very small amounts of pyrethrins in the pres-
Table I.
Seil .-inalysis of 60 Grams of Flour from Bags of Cotton Fabric Pyrethrins I hlg. %
Group A
Cambric 4 Osnahurg Cambric B Group B Cambric A Osnahurg Cambric B Group C Cambric 4 Osnaburg Cambric B 0 . 8 4 g. linseed oil Saponified by hydrogenolysis By AOAC method (6th ed.)
Pyrethrins I1 Mg. 7%
0
3.37 0 2.24
0 0056
0
0 0 0
0
5,60
2.25 2.24
0.0093 0.00375 0.0037
0 0 0
0 0 0
2.26 2.62 1.87 5.7 3.13
0.00376 0.0044 0.0031 0.68 0.37
0 0
... ...
18.5
0
0
0 0
... ...
2.2
Group A , 10 mg. pyrethrins per square foot Group B. 6 mg. Group C, none.
0
0.0037
...
1195
V O L U M E 2 4 , NO. 7, J U L Y 1 9 5 2 Table 11.
Seil Analysis after Hydrogenolysis (Quantities as in Table I) Pyrethrins I Mg. -//&a
Group A Cambric A Balance over blank, p.p.m. As f a t t y acids, p.p.m. Osnaburg Balance over blank, p.p.m. Group B Cambric A Balance over blank Osnaburg Balance over blank Group C Cambric A Osnaburg Av p.p.m. Av:’of 10 samples of
Pyrethrins I1 hlg. y/g.a
0
0
1.01
0
0
0.56
0
0
0
0
0.9 0 1.01 0
0
0
0.9 1.01
0
0
commercial flour, p.p.m.
16.83 1.87 1.37 9.53 -7.48 14.96 0
16.83 0 14.96 16.83 15.89 15.80
All values in milligrams, micrograms, and parts per million calculated as pyrethrins 11, if not stated otherwise. 0 Or p.p.m.
ence of other natural unsaturated esters such as occur in flour and linseed oil. Even when applied t o pyrethrum extract itself, values for pyrethrins I1 can be expected to be slightly higher than those corresponding t o the actual content of chrysanthemum dicarboxylic esters alone. This would mean that the values obtained also embrace material \vith much lower insecticidal values than the pyrethrolone and cinerolone chrysanthemum esters. As a matter of interest the mercury reduction, or AOAC, method for the assay of pyrethrum extracts was checked against linseed oil. The unsaturated acids of linseed oil did not give the vividly colored precipitate with Denigbs reagent which occurs with the chrysanthemum monocarboxylic acid of pyrethrins I but a white precipitate which, obviously, would not be detected in the presence of the colored precipitate. It can be assumed, therefore, that the BOAC method will give higher values for the more valuable pyrethrins I than those obtained by the Seil method. This has been confirmed recently, a t least for extracts from pyrethrum flowers by warm extraction, by Campbell and Mitchell ( 2 ) and corresponds t o substantial recent commercial experience. [The latest modification ( 1 ) of the AOAC method has been checked since on fresh pyrethrum flowers and found to give results almost identical with those obtained by the Seil method.] Regardless of the rather far-reaching implications of these results for the analysis of pyrethrum extract itself, it was hoped
that a conversion of the unsaturated fatty acids into saturated acids by hydrogenation might prevent interference from the former in either analytical procedure. Accordingly, petroleum ether extracts of the flour sample were evaporated, taken up in anhydrous ethyl alcohol and hydrogenated a t room temperature and a t about 28 pounds per square inch for 45 minutes with palladium oxide on barium sulfate prior t o analysis by the Sei1 method as originally suggested by Haller and Acree (4,6). Even then, very small values for pyrethrins I1 were found in extracts of flour samples from both treated and untreated bags (Table 11). That these values are within the limit of error appears from the negative balance in Table I1 for flour from bags treated with the higher amount of pyrethrum extract. CONCLUSION s
The determination of small amounts of pyrethrin residues in flour, and consequently other vegetable materials containing similar, noticeable proportions of unsaturated fatty acids, by the conventional Seil and .40AC methods should be preceded by conversion of the fatty acids by hydrogenation t o reduce the error caused by their interference. Though this treatment reduces the interference to at least 1/100 of the original, comparative tests should be run with samples of the same materials which are certain not to contain any pyrethrins. Upon deduction of the values of these blank tests, the values obtained are negligible and well within the limits of physical errors. LITERATURE CITED
(1) Assoo. Official Agr. Chemists, “Methods of Analysis,” 7th ed., p. 72, 1950. (2) Campbell, A,, and Mitchell, Wm., J . Sci. Food Agr., 5, 137-9 (1950). (3) Cotton, R. T.,and Frankenfeld, J. C., U. S. Dept. Agr., Bull. E783 (July 1949). (4) Haller, H. L., and LaForge, F. B., J . Org. Chem., 2 , 49 ff. (1935); J . Am. Chem. SOC.,57, 1893 ff. (1935). (5) LaForge, F. B.,and scree, F., Jr., J. Org. Chem., 2, 208-13 (1937). (6) LaForae. F. B.. and Acree, F.. Jr.. Soav,. 17. 95 ff. (1941). (7) Lehman, A. J., J . Assoc. Food & Drug Oficiala, U.S.A., 13 (2), 66-70 (1949). (8) Sullivan, B.,and Bailey, C. H., J . Am. Chem. SOC.,58, 383-90 (1936). (9) Sullivan, B.,and Howe, M., Cereal Chem., 15 ( 5 ) , 716-20 (1938). RECEIVED for review May 11. 1961. Accepted March 20, 1952. Presented before the Division of Food a n d Agricultural Chemistry, Symposium on Methods of Analysis for Micro Quantities of Pesticides. 119th Meeting of the AMERICINCHEMICAL SOCIETY, Boston, Mass.
Coulometric Titration of Unipositive Thallium with Either Bromine or Chlorine RICHARD P. BUCK, PAUL S. FARRINGTON, AND ERNEST H. SWIFT California Institute of Technology, Pasadena, Calif. COULOMETRIC titration is described in which unipositive
A thallium is oxidized to the tripositive state by electrolytically
generated bromine or chlorine. The end point is determined amperometrically by measuring the current between two platinum electrodes having a n impressed potential difference of 200 mv. Confirmatory titrations have shown average errors of less than 0,2y0for samples ranging from 93 to 1900 micrograms. Procedures and apparatus for the coulometric titration of various substances by means of electrolytically generated bromine or chlorine have been developed in these laboratories and described (2, S, 7 , 9, IS). These methods have involved passing a known constant current for a measured time between two platinum electrodes immersed in a solution containing the substance to be determined and a soluble bromide or chloride; the end point has been determined by observing the current flow between a second
pair of platinum electrodes (the indicator electrodes), which have a small potential difference impressed across them. 8 s pointed out by Swift and Garner (1%’) certain of the volumetric methods for the titration of thallium are not entirely satisfactory; therefore there seemed justification for a study of the titration of thallium( I ) with electrolytically generated bromine or chlorine, and of the reversibility of the thallous-thallic half-cell at the indicator electrodes. The results of these studies are presented below. REAGENTS
Reagent grade sodium bromide, perchloric and hydrochloric acids, and freshly boiled distilled water were used throughout for the preparation of solutions. Electrolytic oxidation of the hydrochloric acid solutions indicated reducing impurities; the
