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
ANALYTICAL CHEMISTRY
1196
effect was removed by boiling the acid mith the calculated equivalent quantity of 3% hydrogen peroxide. Thallous perchlorate solutions were prepared in order to study the titrations in solutions in which chloride or bromide was the only complex-forming anion present in relatively high concentration. Five grams of pure thallium metal were dissolved in hot concentrated nitric acid and the solution was evaporated nearly to dryness. Ten milliliters of 600/, perchloric acid were added and the solution was evaporated to fuming; a second portion of the perchloric acid was added and the fuming repeated. The thallium salts which precipitated on cooling were dissolved in boiled water containing sufficient perchloric acid to give a solution approximately 0.1 F in thallic perchlorate and 1 F in perchloric acid. A stock solution of thallous perchlorate \yas prrpared from a portion of the thallic perchlorate by reduction \* ith sulfur dioxide followed by boiling to remove the excess sulfur dioxide. The thallous perchlorate was standardized against a standard solution of potassium iodate x i t h the iodine nionochloride end point being used (12). APPARATUS
The titration apparatus used was that described by bIeier, Myers, and Swift (6) with the modifications of Ramsey, Farrington, and Swift (8). The generator cathode was isolated within a glass tube open a t the top and terminating a t the bottom in a “fine” porosity sintered-glass disk; the tube was kept filled above the level of the surrounding solution with 2 F perchloric acid. Shielding the cathode from the body of the solution prevented reduction of thallium(II1) and eliminated indicator currents caused by the presence of hydrogen gas in the titrated solution. Potential differences of 200 and of 300 mv. were impressed across the indicator electrodes when making titrations with bromine ( 7 ) and with chlorine (5),respectivelf Titrations were made in 40 X 80 mm. weighing bottles and a t a solution volume of 40 ml. Two rates of generation were used corresponding t o approximately 10 X 10-8 and 1 X 10-8 equivalents per second. The generator circuit was calibrated by measuring with a student potentiometer the potential drop across a standard 199.8-ohm coil from a resistance box. The standard cell was checked occasionally against a Weston cell calibrated by the U. S. Bureau of Standards
the high rate. This solution was removed and the electrodes were rinsed with water. The electrodes were stored in 1 F hydrochloric acid when not in use. DISCUSSION OF THE METHOD
Indicator Current Behavior. A plot of the indicator current against time of generation when thallium(1) is titrated in either perchloric acid-sodium bromide or hydrochloric acid solutions shows a small residual indicator current before generation is begun, a current increase to a flat maximum, a minimum at the equivalence point for the oxidation of thallium( I ) to thallium(111) and finally a linear current increase. When the indicator potential is first applied, there is an initial surge of indicator current (to nearly 40 pa.), the current then drops off to a small residual value of 1 to 3 pa. As is seen in Figure 1, which shows a titration in bromide solution, there may he an initial indicator current of as much as 5 pa. in the perchloric acid-sodium bromide solution if the sample is acidified several minutes before beginning the titration. It is believed that this current is due to bromine formed by air oxidation of the bromide in the acid solution. I n most cases the samples were not acidified until immediately before they were to be titrated.
PROCEDURE
Corrections for Impurities. The oxidizing or reducing impurities in the reagents were determined by titrating blank solutions. For each bromide blank, 5 ml. of 1 F sodium bromide, 5 ml. of 60% (9 F ) perchloric acid, and 30 ml. of water were placed in a titration cell. For each chloride blank, 5 ml. of 8 F hydrochloric acid and 35 ml. of water were taken. The indicator potential was set a t 200 or 300 mv. and the generation current passing through the dummy resistance was adjusted to the desired value. Bromine or chlorine was generated for short intervals of time depending upon the rate of generation used (0.5 second for the high rate, 5 seconds for the low rate). Ten seconds were allon-ed after each interval of generation before the current and the generation time were recorded. The linear portion of the plot of indicator current us. time of generation (from 10 t o 40 pa.) was extrapolated to zero current. The value of the generation time intercept was designated as the blank time. Several blanks were run and the average value used for the impurity correction. The Titration. When titrating with bromine, 25 ml. of a standardized thallous perchlorate solution was pipetted into a titration cell and 5 ml. of 1 F sodium bromide, 5 ml. of 9 F perchloric acid, and 5 ml. of water were added. When titrating n i t h chlorine, 5 ml. of 8 F hydrochloric acid and 10 ml. of water were added The generation current was adjusted and the titration was begun. When the approximate time for the titration s a s known, the generation was continued to within a few seconds of the end point. The generation was then continued in short intervals until the indicator current passed through a minimum, after which the current value was noted a t each pause. A plot of the indicator current us. generation time was constructed from the data and the linear portion of this plot was extlapolated to zero indicator current. The intercept was designated as the titration time. The blank time was subtracted from the titration time to give the corrected titration time. The corresponding weight of thallium was calculated from the values of the corrected titration time and the rate of generation. The sensitivity of the indicator electrodes was maintained by shorting them to the generator anode after each blank and titration, and generating bromine or chlorine for 50 seconds a t
I
I
I
I
20
40
60
80
GENERATION TIME
1
1
I
1
00
120
140
160
I
SECONDS
Figure 1. Titration in Bromide Solution The initial rise, the flat maximurn, and the decrease of the indicator current to a minimum a t the time calculated for the complete oxidation of the thallium(1) are attributed to the reversibility of the thallous-thallic half-cell a t the indicator electrodes. Presumably the current is controlled during the initial linear rise by diffusion of the thallium(II1) to the indicator cathode, and by diffusion of thallium(1) to the indicator anode during the linear current decrease before the equivalence point. The broad maximum occurs near the calculated time for equal concentrations of thallium(1) and thallium(II1). However, the point of maximum indicator current would not he expected to occur precisely at the time corresponding to equal concentrations unless the diffusion coefficients for both the thallous and the thallic species viere the same and the indicator electrodes were the same size. In the vicinity of the equivalence point, the concentrations of thallium(1) and bromine are very small; after the equivalence point the bromine concentration increases and its diffusion to the indicator cathode determines the magnitude of the indicator current. Titration of Thallium(1) with Bromine or with Chlorine. The standard potential of the thallous-thallic couple (5), T l f = TI++’
+ 2e-
EO = -1.25 volts
in a noncomplexing medium such as perchloric acid appears to be too negptive for one to expect quantitative oxidation of thallium(1) nith either chlorine or bromine. However, the greater stability
1197
V O L U M E 2 4 , NO. 7, J U L Y 1 9 5 2 Table I. s o . of
>ample So.
Titrations
-
Thallium* Y Taken Found Err& .4. Titrations with Brorninc
Average Deviation
Error,
%
7
Generation Rate: 1 0 . 3 8 5 X 10-8 Equiv./Sec. 6 1873.7 1874.1 +0.4 1.3 3 1555.1 1555.2 +O.l 0.3 0.9 7 935.9 936.1 +0.2 Generation Rate: 1.0393 X 10-8 Equiv./Sec. 6 187.4 187.4 0.0 0.5 5 93 59 93.44 10.15 0.2 -B. Titrdtions with Chlorine
1 7
3
,-
~~
Generation Rate: 5 1873.7 4 1555.1 6 935.9 Generation Rate: 6 187 4 6 93.59
1
P
R 4 5
I t is helieved that the factors limiting the accuracy of these titrations are the preparation and dilution of the standard solutions, and the determination of the end point time. The accuracy u i t h which the generator current could be determined was about 0.037,.
Confirmatory Titrations
f0.02
COMPARISON OF COULOMETRIC TITRATIOY WITH OTHER METHODS
0.0
+ O 02
Sill and Peterson ( I f ) determined thallium by an iodometric titration. Samples as small as 40 micrograms were determined with an accuracy of 2.5y0. Samples of from 1000 to 5000 micrograms were determined with an accuracy of 0.1 to 0.3%. I n a colorimetric method proposed by Shaw ( I O ) , 500 micrograms could be determined with an accuracy of 3 to 5%.
0.00 10.16 7
1 0 . 3 8 5 X 10-8 E q u i r . 'Sec. 1873.6 +0.1 3 0 1555.3 f0.2 1.2 936.6 +0.7 2.4 1 .0:193 X 10-8 Equiv./Sec. 187.3 -0.1 0.3 93.43 -0.16 0.2
0.0 io.01 f0.07
-0.05
LITERATURE CITED
-0.17
Benoit, R., Bull. soc. chim. France, 1949, 518-24. Brown, R. .I.,arid Swift, E. H., J . Ani. Chem. Soc., 71, 2717 (1949).
of l)oth the bromide and chloride complexes of thallium(II1) relativr to any thallium(1) complexes so shifts the formal potentials ( t o -0.78 volt in hydrochloric acid, 4)that the titration of thallium(1) with bromine, or chlorine, in solutions of these halides is possible. The formal potential calculated in bromide solutions from the data of Benoit ( I ) for the dissociation constants of the thallic bromide complexes is slightly more positive than that given above for chloride solutions. I n the procedure described here no apparent advantage was found in titrating with chlorine rather than with bromine. Data obtained from confirmatory titrations made as described above are shown in Table I. I n solutions 2 F in hydrochloric acid, or 2 F in perchloric acid and approximately 0.1 F in sodium bromide, thallium(1) has been titrated in quantities ranging from 93 to 200 pg. with a maximum deviation from the mean of about &0.8 pg. Samples of 93 pg. have been titrated with an average error of less than 0.2%. The average error has been less than 0.1 0;. foi quantities of from 200 t o 1900 pg.
Farrington, P. S.,and Swift, E. H., ANAL. CHEM.,22, 889 (1950).
Hughes, R. H., and Garner, C. S., J . Am. Chem. Soc., 64, 1644 (1942).
Latimer, W. M., "Oxidation States of the Elements," p. 153, New York, Prentice-Hall, 1935. Meier, D. J., Myers, R. J., and Swift, E. H., J . A m . Chem. Soc., 71, 2340 (1949).
Myers, R. J., and Swift, E. H., Ibid., 70, 1047 (1948). Ramsey, W. J., Farrington, P. S., and Swift, E. H., ANAL. CHEM.,
22, 332 (1950).
Sease, J. IT,,Niemann, C., and Swift, E. H., Ibid., 19, 197 (1947).
Shaw, P. A., IND.ENG.CHEM.,ANAL.ED.,5, 93 ( 1 9 3 3 ) . ' Sill, C. IT,, and Peterson, H. E., ANAL.CHEW,21, 1268 (1949). Swift, E. H., and Garner, C. S.,J . A m . Chem. Soc., 58, 113 (1936).
Wooster, W. S., Farrington, P. S., and Swift, E. H., ANAL. CHEIM., 21, 1457 (1949). RECEIT-ED for review January 2 , 1952. Accepted March 24, 1952. Contribution h-0. 16.51, from the Gates a n d Crellin Lahoratories of Chemistry, California Institute of Technology. Pasadena 4 , Calif.
Infrared Absorption Spectrum of Toxaphene Identification in Agricultural Products WESTCOTT C. KENYON H e r c u l e s E x p e r i m e n t S t a t i o n , H e r c u l e s P o w d e r Co., W i l m i n g t o n 99, D e l .
OR some time there have been attempts to find analytical Fmethods which are specific for toxaphene, an agricultural toxicant-a chlorinated camphene of 67 to 69c7, chlorine. KO 3atisfactory assay method for this material has been published. HoLvever, one method of identifying tosaphene is by means of its too,
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