A N A L Y T I C A L CHEMISTRY
1842
The claimed specificity for ethylene oxide or other low boiling oxides is supported by the data in Table XI1 with the apparatus and procedure as recommended. Other cyclic oxides may respond to the color test, but the sensitivity of this reaction is primarily dependent upon the number of substituents on the cyclic carbon atoms. Details of this study will be rpported elsewhere. Ammonia and simple amines-e.g., triethylamine-seem to prevent the formation of color even in the presence of large amounts of ethylene oxide. It is suggested that this method could be utilized as an extremel) sensitive test for the spectrophotometric characterization of epoxides, some halohydrins, and some cyclic tertiary amines. ACKNOWLEDGMENT
The authors wish to thank L. R. Jeppson for providing the treated citrus fruits recorded in Table XI. The sample of commercial orange oil was generously provided by W. E. Baier, California Fruit Growers Exchange, Ontario, Calif.
LITERATURE CITED
(1) Deckert, Walter, Z . Angew. Chem., 45,458, 559 (1932). (2) Eastham, A. M., and Latremouille, G. A., Can. J . Research, 28, 264 (1950). (3) Guia, &I.,Gazz. chim. iLaZ., 52 I , 349 (1922). (4) Gunther, F. X., and Barkley, J. H., Standardized Procedures 11, University of California Citrus Experiment Station, mimeo., revised 1951. 15) . . Gunther. F. A., Barnes, M.hI., and Carman, G. E., Adcances in C h e k . Series, No. 1, 141 (1950). (6) Gunther, F. A . , and Blinn, R. C., Ibid., No. 1, 72 (1950). (7) Gunther, F. A , , Kolbezen, M. J., and Blinn, R. C., unpublished data. (8) Jeppson, L. R., Jesser, M. J., and Complin, J. O., Calif. Citrograph, 36, 50 (1950). (9) Lohmann, H., J . prakt. Chem., 153,57 (1939). (10) Naugatuck Chemical Division, U. S. Rubber Co., unpublished communications. (11) Ridgeway Color Standards, Author, Washington, D. C., 1912. RECEIVEDMarch 22, 1951. Presented in essential part before the Divisions of Agricultural and Food Chemistry and Analytical Chemistry, Symposium on Methods of Analysis for Micro Quantities of Pesticides, a t the 119th Meeting of the QMERICAXCHEiwcAL SOCIETY,Boston, Mass. Paper No. 696, University of California Citrus Experiment Station, Riverside, Calif.
Determination of Dithoi carbamates D. G . CLARKE, HARRY BAUhf, E. L. STANLEY, AND W. F. HESTER' Rohm & Haas Co., Philadelphia, Pa. Commercial production of dithiocarbamates for use as agricultural fungicides has necessitated methods for their determination. A macroprocedurehas been developed which is based on acid decomposition, scrubbing the evolved carbon disulfide with lead acetate, absorbing in alcoholic potassium hydroxide, and titrating with iodine. This procedure is also applicable on a semimicro scale. A microprocedure is based on colorimetric determination of the evolved carbon disulfide as cupric dimethyldithiocarbamate. The methods are suitable for assaying the dithiocarbamates themselves, for determining them in dust or spray formulations, and for determining spray residues.
Callan and Strafford ( 1 ) reported obtaining quantitative (99.6 t o 100.2%) results for piperidinium piperidyldithiocarbamate, for zinc dimethyldithiocarbamate, and zinc diethyldithiocarbamate in pure specimens, and also in known mixtures with kaolin. The authors have adapted the method t o dithiocarbamate fungicide analysis by washing the evolved carbon disulfide with a lead acetate solution in order t o remove hydrogen sulfide and sulfur dioxide arising from sulfide and thiosulfate impurities in most samples. Although the alkyl monodithiocarbamic acids can decompose in only one way (Equation l ) , the bis(dithiocarbamic) acids undergo two types of decomposition:
H S
I
E
ARLY attempts a t determining dithiocarbamates used as
agricultural fungicides quickly showed that elemental analysis was practically useless, while bioassay was not sufficimtly accurate or precise. Various methods of precipitation of the dithiocarbamate radical, direct titration with iodine, precipitation and titration of the ditliiocarbamic acid, and decomposition to insoluble products for gravimetric measurement were unsatisfactor3 because of lack of specificity in the presence of interfering substances normally present. Linch (3)has recently discussed some of these difficulties. I n the literature on the analysis of vulcanizing accelerators ( 1 ) is a method which the authors have modified and applied t o the analysis of dithiocarbamate fungicides. Acid decomposition of the dithiocarbamate yields carbon disulfide quantitatively
II
N-C-SH
A /--+
H~S--CH~-CH~-NHZ
+ 2 CS,
(4)
H
RZN-CSSH +RzNH CSz (1) Absorption of the carbon disulfide in alcoholic potassium hydrouide forms the xanthate, CSz KOH CHaOH CHsOCSSK Hz0 (2)
Fortunately, decomposition A is quantitative in dilute acid a t 100' C. Decomposition B has not been thoroughly studied, but is slower and occurs a t lover temperatures. The method is specific for dithiocarbamates. I n modificatione described herein, it is applicable t o assay of the commercial products, t o analysis of spray strength solutions and sprayed plants (semimicromethod), and to determination of trace amounts of dithiocarbamates (micromethod).
which is then quantitatively determined by titration with iodine.
ASSAY METHOD
+
+
2CHsOCSSK 1
Deceased
+
+
12
+
+ CHIOC(S)SSC(S)OCH~+ 2KI
(3)
Procedure. The apparatus shown in Figure 1 is used. The sample containing about 0.004 equivalent of dithiocarbamate 1s placed in the reaction flask. (Aliquots of liquid samples are
V O L U M E 23, NO. 12, D E C E M B E R 1 9 5 1 pipetted in; solid samples are weighed into vials which are then placed in the flask.) The first absorber is charged with 10% lead acetate, and 25 ml. of 2 N methanolic potassium hydroxide are pipetted into the second. Aspiration is started (approximately 150 ml. per minute) and 50 ml. of hot 1.1 N sulfuric acid are poured into the air intake. Mild boiling is maintained and aspiration is continued for 15 minutes. The second absorber is disconnected and its contents are rinsed into a beaker with 75 to 100 ml. of water. The solution is just neutralized to phenolphthalein with 30y0 acetic acid, and then titrated with 0.1 N iodine; 5 ml. of starch indicator and 250 ml. of distilled water are added just before the end point, which is a faint but definite color change in the white suspension formed.
1843 Ethylenethiourea did not affect the analysis, but sodium thiosulfate caused low results. A study of this showed a linear relationship between the amount of disodium ethylenebis(dithiocarbamate) found and the amount of thiosulfate present (Figure 2). Investigations have shown that thiosulfate causes the decomposition to go to.some extent according to Equation 5 , giving some ethylenethiourea and less carbon disulfide. The same result is caused by introducing sulfur dioxide (an acid decomposition product of thiosulfate) when thiosulfate is not present. Elimination of this thiosulfate effect has not been attempted, but it amounts t o only 0.55% disodium ethylenebis(dithi0carbamate) for each part of thiosulfate in 100 parts of dithiocarbamate. Results of Assays. Biological tests showed high assay materials to be active while those of low assay were inactive, but these tests were not sensitive enough to differentiate further. The proof of this method depends upon analysis of a high purity sample. Disodium ethylenebis(dithi0carbamate) forms a hexahydrate, When the authors were able to isolate this compound in a highly refined state and of reasonably sharp melting point (82-83" C.) and repeated preparations showed nearly 100% purity by this analysis, they were convinced of the accuracy of the method. Table I gives the analyses on separate preparations, and shows that the average deviation of the procedure is 320.2% n-hen applied to the pure substance. The standard deviation of the procedure, uhen applied to the zinc salt, was 0.6% absolute.
Figure 1. Apparatus for Assay of Dithiocarbamate
A blank is run daily with omission of the decomposition step, and is usually 0.2 to 0.3 ml. The calculation is: dithiocarbamate = (sample titration - blank) (11normality)(eq. aft.of dithiocarbamate) wt. of sample X 10 70
Diluted samples must be analyzed immediately, because air oxidation destroys certain types of dithiocarbamates in dilute solution. Aspiration must be controlled t o prevent foaming of the contents of one part of the system into another. Sufficient time must be allowed for complete decomposition. Decomposition Time. When a sample of disodium ethylenebis(dithi0carbamate) hexahydrate was analyzed, allowing 15 minutes for decomposition and aspiration, the average of three results was 93.79y0. When the time was extended to 45 minutes for this sample the analysis (average of two results) was 93.64%. A solution of the same material which analyzed 31.00% gave as the average of three results 31.10% when analyzed in the presence of excess zinc sulfate allowing the same 15-minute period. The conclusion is that 15 minutes are adequate for decomposition of the sodium and zinc salts. Limited experience with potassium and calcium salts shows 15 minutes t o be adequate for thwe, while the nickel, manganese, copper, and iron salts seem to require longer times. Acid Concentration. Callan and Strafford (1) used 30% sulfuric acid (approximately 7.5 iV) for the decomposition. Because the ethylenehis(dithiocarbamates) can decompose under certain conditions to ethylenethiourea and only 1 mole of carbon disulfide, acid strength was investigated. Two of four samplps analyzed more than 1% higher when 50 ml. of 1.1 AVsulfuric acid R ere wed than they did with 25 ml. of 30% sulfuric acid. ,is the other two samples gave essentially the same results ~ i t both h acid concentrations, the dilute acid was chosen for general use. Effects of Impurities. The chief reason for the failure of other methods of dithiocarbamate determination is the presence of impurities. Among these are sulfides and thiosulfates, and, in the case of ethplenebis(dithiocarbamate), ethylenethiourea, and other less known impurities. The lead acetate absorption tube removes hydrogen sulfide, and experiments shon ed that amounts of sodium sulfide less than 5% (20% as the normal hydrate, S a z S 9H20)do not interfere.
2
91 90 89 88 87 86 85
e 84 83 82 81
"0
2
6 8 IO 12 14 16 18 PARTS THIOSULFATE PER HUNDRED PARTS DITHIOCARBAMATE 4
20
Figure 2. Effect of Added Thiosulfate on Amount of Disodium Ethylenebis(dithiocarbamate) Found Slope = 10.7/19.4 = 0.55
This method has been used to analyze various commercial dithiocarbamate fungicides (Table 11),and the manufacture of Dithane D-14 and Dithane 2-78 has been controlled by this analysis since these products were placed on the market.
Table I. Decomposition Analyses of Purified Disodium Ethylenebis(dithiocarbamate) Hexahydrate Sample A R C D E F G
An a1y se s
99.84,99.47
Av. 99.9 99.7 99.2 99.1 100.0 99.6 99.7
Deviation 10.3 fO.0 10.02 zt0.2 10.4 10.2 10.2 Av. 1 0 . 2
Table 11. Assays of Various Commercial Dithiocarbamate Fungicides Fungicide Dithane D-14 Dithane 2-78 Fermate Parzate Liquid Paraate Zerlate
Active Ingredient
Active I n redient Founj, % 22.2 74.8 68 6 71.6 20.4 73.9
ANALYTICAL CHEMISTRY
1844 Table 111. Comparison of Assay and Semimicromethods Analysis No.
Dithiocarbamate, % Assay method Semimicromethod 0.180 ... o i76 ... 0.168 0.166
o:iii
SEMIMICROMETHOD
This modification of the assay method permits analysis of spray strength solutions [O. 18% disodium ethylenebis(dithb carbamate) hexahydrate] and of plants that have been sprayed with dithiocarbamates. Figure 3 shows the apparatus used, which differs only in details from that shown in Figure 1. The air intake trap containing methanolic potassium hydroxide is provided to prevent contamination from carbon disulfide in the atmosphere. The condenser reduces the amount of water carried over and, with its bulbs, aids in breaking the foam produced when plant slurries are heated. To improve the starch-iodine end point, potassium iodide is added and the solution is cooled to 25' C. before titrating with 0.01 N iodine ( 8 ) , using a microburet. This titration should be carried out as soon as possible after the absorption, which is done in only 5 mi. of 2 N methanolic potassium hydroxide. Other features of the procedure are the same aa in the assay method.
intensity, proportional to the amount of carbon disulfide absorbed, was measured spectrophotometrically. Hydrogen sulfide is known t o interfere, but is removed by the lead acetate trap. The absorption curve of cupric diethyldithiocarbamate indicated that the optimum wave length for determination is 430 mp. Calibration curves were prepared from standard carbon disulfide solutions and a reagent blank. The reagent blank has negligible absorption [O st 0.004 absorbancy (optical density) unit 1. The reliability of this micromethod is indicated in Table IV, which shows the recovery of known amounts of disodium ethylenebis(dithi0carbamate) in water solution. The recovery averages 79%. This compound decomposes slowly in dilute solution, and 79% recovery is surely in keeping with the degree of stability a t these dilutions. When the zinc salt was analyzed, the recovery averaged 79.6%, essentially the same.
Table IV.
Recovery of Disodium Ethylenebis(dithi0carbamate) b y Micromethod
Time after Dilution, Min. 8-10 8-10 8-10 8-10 15 8-10 8-10 8-10
Taken, y 5 98 12.0
7.04 49 8 50.1 50.1 5.95 16.6
Found, y 6.0 6.0
Recovery, '?& 100
6.0 42 42 41 4.8 11 Av.
50 85 84 84 82 80 67 79
RESULTS OF ANALYTICAL STUDIES ON ETHY LENEBIS(DITHI0CARBAMATES)
REACTION FLASK
,
R ASPIRATOR BOTTLE
IOcm.
,
Figure 3. Apparatus for Determination of Microgram Quantities of Dithiocarbamate
The favorable comparison of the semimicro and assay methods
is shown in Table 111. Consecutive analyses were run on the same dithiocarbamate spray solution, using 250 ml. for the assay method and 1 ml. for the semimicromethod. The fall in assay with time is known to be real. MICROMETHOD
For the determination of amounts of dithiocarbamates that might be expected to remain as residue on fruit and vegetables, or to be absorbed by growing plants sprayed with dithiocarbamates, this micromethod was developed. The primary difference from the other methods is the manner of absorbing and measuring the carbon disulfide. The apparatus of Figure 3 is used. The technique differed from that of the semimicromethod as follows:
Stability of Sodium Salt. 4 study of the effect of air on spraystrength solutions of disodium ethylenebis(dithiocarbamate) by the semimicromethod revealed that such a solution is stable when aerated with purified nitrogen, but decomposes rapidly under the influence of air or oxygen. Figure 4 presents the ~ approxidata obtained when these cases were passed a t s l o but mately similar rates into quart bottles practically filled with such dilute solutions. Spray strength solutions are 0.18% disodium ethylenebis(dithiocarbamate) hexahydrate. That air (oxygen) is not the only requirement for decomposition was shown by treating each of three series of filter papers with the same amount of 4 X spray-strength solution and storing them under the desired conditions of humidity. At intervals samples from each series were analyzed. The data given on Figure 5 show clearly the importance of water in the decomposition of disodium ethylenebis (dithiocarbamate). Once dry, and kept dry, the residue is stable for as much as 6 days, and probably much longer.
The aspiration rate was controlled a t 8 ml. per minute. Higher flow rates gave low recoveries. Decomposition time was 1 hour, with 15 minutes of additional as iration. %he reagent and procedure for determining the carbon disulfide are discussed below. The colorimetric method of Viles ( 4 ) was used with slight modification. The reagent contains 20 ml. of technical triethanolamine, 1.0 m]. of diethylamine, and 0.05 gram of cupric acetate uer liter in ethyl alcohol. Absorption of carbon disulfide forms cipric diethyldithiocarbamate, whjch is yellow. This color
-
.
-
TIME IN HOURS
Figure 4. Effect of Air, Nitrogen, and Oxygen on Dilute Solutions of Disodium Ethylenebis(dithiocarbamate)
1845
V O L U M E 23, N O . 1 2 , D E C E M B E R 1 9 5 1 Stability of Residue of Zinc Ethylenebis(dithi0carbamate)
Table V.
% Dithiocarbamate Remaining after: Sample Zn salt From ZnSOd From ZnClz
Table VI.
1 day
2 days
3 days
4 days
89 87
89 84
90 84
80 84
7 days I
73 79
Recovery of Disodium Ethylenebis(di thiocarbamate) from Potato Foliage
Run
Spray Taken, M1.
4 5 6 7 8
Recovery, % 73.4 83.1 78.4 78.4 83.0 77.2 77.6 89.1 Av. 8 0 . 0
Stability of Zinc Salt. In contrast to the stability of the sodium salt of ethylenebis(dithiocarbamic) acid is that of the zinc salt. The results in Table V were obtained by a procedure similar to that used to get the previous set of data, but with uncertain humidity control. These conditions result in an average loss of the zinc salt by decomposition of only 24% after 7 days. This stability, much greater than that of the sodium salt, is probably due t o the very low solubility in water. Recovery of Disodium Ethylenebis(dithiocabamate) from Potato Foliage. When unsprayed potato foliage was analyzed by the semimicromethod, using 20 grams of foliage per test, the titration averaged 0.065 ml. whereas the average blank was 0.096 ml. of 0.01 N iodine solution (Table VI). When k n o m amounts of disodium ethylenebis(dithiocarbamate) were placed in the decomposition flask x-ith 20 grams of potato foliage, an average of only 80% of the dithiocarbamate was found by analysiq. Apparently the plant materials change the course of the decomposition in some mannw.
The gradual disappearance of the dithiocarbamate from the plant is shown by the data in Table VII. This disappearance is advantageous in that it leaves no dithiocarbamate residue on edible above-ground crops, but disadvantageous in that loss of dithiocarbamate surely means loss of fungitoxicity. Recovery of Disodium Ethylenebis(dithi0carbmate) from Potato Slurry. As a part of the problem of determining whether or not disodium ethylenebis(dithiocarbamate) was absorbed by the potato plant, with the possibility of translocation to the tubers, the recovery of this compound from potato slurries was studied. The soluble sodium salt was used because it seems the dithiocarbamate would have to be water-soluble for absorption and translocation. Microgram quantities were used because only small amounts, if any, were expected in potatoes. Tubers from untreated plants, found to give the usual blank by the micromethod, were slurried with added amounts of disodium ethylenebis(dithiocarbamate), then analyzed. The average recovery from potato slurry was 13.3% (Table VIII). Disodium Ethylenebis(dithi0carbamate) in Potato Tubers. A total of eight different samples of potatoes (Pontiacs) which had been protected with an ethylenebis (dithiocarbamate) fungicide plus D D T were analyzed by the micromethod in duplicate. .4lthough the plants had had from six to twelve applications of the spray, none gave a significantly positive test for dithiocarbamate. Ethylenebis(dithiocarbamates) in Celery. The presence of spray residues on above-ground crops such as celery is undoubtedly more likely than on potatoes. Some studies were made on celery plants. 100
90
z 80 0 70 c 60 50
6 DAYS
8 40
x 30
UIRS'
* 20 IO
Table 1'11. Determination of Zinc Ethylenebis(dithi0carbamate) in Potato Foliage over Several Sprayings
+
Disodium ethylenebis(dithi0carbamate) (0.5 lb.) ZnSO4 (1 lb.) Disodium Ethylenebis(dithiocarbamate) Hexahydrate, RIg. per 20 Grams Foliage after: Befofe 1 2 1 2 3 4 7 9 S p r a y S o . spraying hour hours day days days days days days 1 9.04 4 . 2 8 1.50 0.85 0 . 3 9 .. .. 2 0:OO 5.48 . . 1.28 0.82 0.41 0.18 . . , . 3 .. 5,13 3.03 0 23 4 .. 4 : 5 6 3 . 2 8 i:ii i:o7 o:i7 o : i 4 o:Oo Spray:
..
..
Table VIII. Recovery of Disodium Ethylenebis(dithi0carbamate) Added to Potato Slurry Potato Variety Cobblers
Method of Mixing Waring Blendor
Pontiacs
Shaking
Dithiocarbamates Taken Found 50.0 6.0 50.0 4.8
Recovery, yo 12 10
Residue Changes in the Field. The data presented show that zinc ethylenebis(dithi0carbamate) is reasonably stable under laboratory conditions. I n the field the complicated conditions of weathering, which include dew, rain, wind, and sunlight, come into play, and one might expect entirely different results. To investigate this, sprays were applied in the field with conventional high pressure machinery and representative samples of foliage were collected from that part of the plant which had matured and had received full benefit of the treatments. A leaf was selected a t random from each of fifteen t o twenty different plants per treatment and submitted to analysis by the semimicromethod.
TIME IN HOURS
Figure 5.
Effect of Moisture on Disodium ethplenebis(dithi0carbamate) Residue
Although potato tubers from untreated plants gave a normal blank upon analysis, untreated celery plants gave a positive value. It was not determined whether the material causing the color was carbon disulfide, as is the case when dithiocarbamates are present. The amount of this substance, calculated as disodium ethylenebis(dithiocarbamate), mas only 0.5 p.p.m. Table IX. Sample
Zinc Ethylenebis(dithi0carbamate) on Celery
C2-2 C1-3
History Sprayed-unwashed Sprayed-washed Sprayed-unwashed
'22-3
Sprayed-washed
(21-2
Type Slurry Slurry Slurry Laboratory washings of (21-3 Slurry Laborator washings of 6 2 - 3
Dithiocarbamate Found, P P.SI. 0 7 0 4 0 4
< 0.1
0 3
'
