1850
ANALYTICAL CHEMISTRY
have been encountered whenever canned goods prepared frcm treated crops have been analyzed. As, in general, the spheres of optimum applicability of dimethyldithiocarbamates and zinc ethylenebis(dithi0carbamate) do not overlap, only limited comparisons have been made of the active ingredient residues obtained when each type was applied t o the same crop. The comparative data available, summarized in Table X, indicate, in general, that the more stable dimethyldithiocarbamates are retained t o a somewhat greater extent than zinc ethylenebis(dithiocarbamate). However, this should not be used as a criterion of their relative fungicidal activities. Table XI is included as a summary of typical data on dithiocarbamate residues obtained during the course of this investigation on several crops which were not included in any of the previous tables. SUMMARY
A modification of the conventional Dickinson-Viles technique for the determination of dithiocarbamates in micro quantities has been employed for the determination of residues on a variety of fruits and vegetables. Recoveries of 100 and 75% of theoretical were obtained with dimethyldithiocarbamate and zinc ethylenebis(dithiocarbamate), respectively, in the absence of foreign materials. Intercomparisons indicated that one of the three general sampling techniques (stripping, scrubbing, or pulping) was applicable t o each residue problem encountered during this work, although the recoveries from large quantities of food pulps were considerably below theoretical. In general, dithiocarbamate residues were significantly reduced by allowing several days bet\-;een h a 1 treatment and harvest,
and by washing or canning. No significant build-up of zinc ethylenebis(dithi0carbamate) residue was encountered when the frequency and number of treatments were increased t o afford protection during long growing seasons. As anticipated, crops having a leafy or pubescent surface retained considerably higher residues of dithiocarbamates, and some indications were found that the more stable dimethyldithiocarbamates resulted in crop residues somewhat greater than those obtained with zinc
ethylenebis(dithiocarbamate). ACKNOWLEDGMENT
The author wishes to express his appreciation to Gerrjt Dragt and K. W. Greenan for their important contributions and suggestions during the course of this investigation, and t o the agricultural field representatives of the Grasselli Chemicals Department of E. I. du Pont de Nemours & Co., Inc., for the collection and submittal of samples, without which the data presented in this paper could not have been assembled. LITERATURE CITED
(1) Clarke, D. G., Baum, H., Stanley, E. L., and Hester, W. F., ANAL.CHEW,23, 1842 (1951). (2) Dickinson, D., Analyst, 71, 327 (1946). (3) Jacobs, M. B., “Industrial Poison, Hazards and Solvents,” l s t , od., p. 262, New York, Interscience Publishers, 1941. (4) Viles, F. J., 2nd. Hug. Tosicol., 22, 188-96 (1940). RECEITEDApril 10, 1951. Presented before the Divisions of Agricultural a n d Food Chemistry and Analytical Chemistry, Symposium on Methods of Analysis for Micro Quantities of Pesticides, a t the 119th Meeting of the AJIERICAN CHEMICAL SOCIETY, Boston, Mass.
Polarographic Determination of Iodine in Water, Soil, and Plant Material P. R. GODFREY’, H. E. PARKER, AND F. W. QUACKENBUSH Purdue University Agricultural Experiment Station, Lafayette, I n d . A search for a sensitive method for determination of the small amounts of iodine present in biological materials led to the application of the polarographic technique. Materials high in organic matter were subjected to dry combustion, while samples low in organic matter were oxidized in a chromic acidsulfuric acid solution. The iodine was separated by a distillation procedure and oxidized with ozone. The amount of iodate present was then determined
D
URING the past decade several procedures for the microdetermination of iodine in biological materials, soils, and water have appeared in the literature. These methods have been used successfully for the analysis of some materials. The small percentage of iodine in most of the materials, however, has presented the challenge to develop more sensitive methods or procedures that simplify the treatment of large samples. The chromium trioxide oxidation procedure of Matthews, Curtis, and Brode (6) previously used in this laboratory gives a satisfactory solution for polarographic analysis. For plant materials, hocrever, a preliminary dry combustion was necessary, as the method was not capable of handling the large amounts of organic matter present. The dry combustion procedures and equipment of McHargue and Offutt ( 4 ) and of von Kolnitz and 1
Present address, Louisiana College, Pineville. La.
polarographically. The method is sensitive because 6 electrons are involved in the reduction of the iodate ion. A s little as 0.5 microgram of iodine can be determined with amaximum error of &lo%. This method was used in a survey of the iodine in Indiana soils, crops, and water. Its sensitivity permits determination of the minute amounts of iodine present in bioIogical material and avoids the necessity of handling large samples.
Remington (3) were modified to suit conditions. Vycor tubing was used instead of specially fabricated alloys to construct a onepiece combustion tube. A chromic acid-sulfuric acid mixture replaced the usual alkaline absorbents in the scrubbing tower. This eliminated the difficult task of preparing an alkaline absorbent of low iodine content, and the mixture served to oxidize any organic matter carried into the absorption tower. The polarographic technique has been used to determine iodine by Rylich (7), Cizek (I), and Orlemann and Kolthoff ( 6 ) , but it has not been adapted to the analysis of such materials as plant tissue and soils. The method is sensitive because 6 electrons are involved in the reduction of the iodate ion. The most difficult problem encountered in adapting the polarographic method w-as to avoid high salt concentrations in the small volume of final solution. .4 drop or less of solution can be
1851
V O L U M E 23, NO. 1 2 , D E C E M B E R 1 9 5 1 analyzed polarographically if the salt concentration is low. This difficulty was circumvented by the use of ozone instead of chlorine as an oxidizing agent; with ozone the iodine is oxidized in the alkaline distillate, whereas with chlorine the alkaline distillate must be made acidic for oxidation and then basic for further concentration.
Figure 2. Polarizing Cell a n d Assembly
ceivers were unsatisfactory because of the accumulation of a siliceous material in them during evaporation of the alkaline distillate. The siliceous material subsequently interfered with the polarographic determination of iodine. Ozone Generator. To produce ozone, oxygen gas was subjected to a silent discharge between the walls of two vertical coaxial borosilicate glass cylinders ( B and C, Figure 1). C, a tube 25 mm. in outside diameter and 35 em. long, was sealed to tube B , 29 mm. in inside diameter and 45 cm. long, a t the top. Cylindrical sheets of thin aluminum, A and D,were placed inside C and outside B and connected to the terminals, I and J , of a gas tube transformer having the following specifications: 115 primary volts; 5000 secondary volts; 0.105 kv.-amp.; and 60 cycles. The lower end of cylinder B was connected through a 10/30 joint, E, to a 15-em. length of 8-mm. glass tubing, F , which was drawn out to a 0.5-mm. opening, G. In operating the generator, oxygen gas enters through the side arm, H , and the ozone which forms as it passes between the cylinder, B and C, flows out through the capillary, G. Polarographic Equipment. The current-voltage curves were obtained by means of a Leetis 8: Northrup Electro-Chemograph. Polarizing Cell. The cell (C, Figure 2) was made by sealing a piece of 10-mm. borosilicate glass tubing into a piece of 14-mm. borosilicate glass tubing and making a U-shaped bend a t the point of union. The ends of the tubes were then cut off 25 mm. above the bend. A little mercury, D,was placed in the cell, the solution to be analyzed was added on the small side, and a platinum wire, E , connected to the Electro-Chemograph, was placed in the mercury on the large side. The cell was supported by a slotted rubber stopper, G, placed in the bottom of a round-bottomed tube, B , 31 mm. in outside diameter and 70 mm. long, which contained water to aid in heat transfer. B was placed in a constant temperature water bath (25” & 0.1”C.) underneath the stationary dropping mercury electrode. The water bath was then elevated until the dropping mercury electrode, A , extended through a hole in rubber stopper F , and into the solution t o be analyzed. To prevent excessive contact of the solution with oxygen during transfer of the sample from the distillate receiver to the cell, the receiver, the cell, and the 25-ml. flask containing the sulfite-gelatin solution were placed in a 2-liter beaker into which nitrogen was passed. The beaker was covered with a thin sheet of plywood containing a 10 X 50 mm. hole in the center, through which the pipets and the tube carrying the nitrogen were placed. The halfwave potential for the iodate wave with this apparatus and conditions described in the procedure was found to be -0.98 volt. The half-nave potential for thallium was found to be -0.32 volt with the same apparatus and in the same sulfite-gelatin solution. .4 drop time of 2 seconds was used in obtaining the standard curves and the values recorded in Table I. The value f o r m with this capillary was 2.35. Combustion Apparatus. The apparatus is shonn diagrammatically in Figure 3. A Vycor glass tube, B (25 mm. in outside diameter, 19 mm. in inside diameter, and 1 meter long), was sealed to a smaller tubr, J ( 10 mm. in outside diamrter and 500 mm. long) iyhich was inserted into the borosilicate glass absorbing tower K. A Vycor glass tube E (17 mm. in outside diameter, 13 mm. in inside diameter, and 250 mm. long) drawn out to a 1-mm.
Table I . Figure 1. Ozone Generator
Sample Water I
In selecting a method for the determination of iodine in soils, the most attractive procedure appeared to be that of Fraps and Fudge ( 2 ) . Although some of the well-established principles of their method have been retained, substitution of the polarographic measurement has permitted a reduction in the size of the sample with a resultant saving of time.
Water I1 Soil I
Soil I1 Corn I Barley I
APPARATUS
Distillation Apparatus. The apparatus of Matthews, Curtis, and Brode ( 5 ) was modified slightly. A tube having an inside diameter of 2 mm. instead of 3 mm. was used below the trap, and the dew cup on the condenser was eliminated as unnecessary. The flasks used for digestion and distillation were made by sealing 29/42 joints on 800-ml. Kjeldahl flasks. Distillate Receivers. The receivers were made from 125-ml. flat-bottomed flasks (Corning Brand alkali-resistant glass KO. 728) by blowing out the entire bottom and sealing off the neck a t the mouth with a slightly rounded tip. Borosilicate glass re-
Alfalfa I Kelp Ia Iodized salt I
Sample Size Grams 500 500 500 500 1.00 1.00 1.00 1.00 21.14 19.42 17.85 18.20 12.70 14.10 0 00474 0.00604
Analyses of Various Materials Wave Height Divisions 11.5 10.7 16.4 16.0 8.2 8.1 11.5 11.5
Sensitivity
Iodine
l/5
4.8
l” 1/1
1.3 1.3 3.5 3.5 2.3 2.3
Y
l” 1 /5
”*1/2
4.4 3.9 6.6 6.7
21.2 21.3 14.5 17.0
1/1
:$: 1/1 l/5
lj5
4.5
0.31 0.27 0.50 0.51
1.70 1.70
6.2 7.5
Iodine Content
P.P.B. 9.6 9.0 0.26 0.26 3500 3500 2300 2300 14.7 13.9 as.0 28.0 134 120
%
0.131 0.124
1.2 0,0080 0.015 14.3 l’l 1.1 0.0073 0.015 13.7 1/1 a Sample obtained from hIcHargue ( 4 ) and reported t o have an iodine cont e n t of 0.12770.
ANALYTICAL CHEMISTRY
1852 opening a t C served as a container for the sample. A small glass hook, D , on tube E facilitated its removal from the combustion tube. A small indentation, F , in tube B held tube E in place. Combustion was effected by a series of Bunsen burners, A , and by the combustion tube furnaces, G (32 mm. in inside diameter and 30 em. long, hinge design), and H (32 mm. in inside diameter and 10 em. long, hinge design). The absorbing tower, K (45 mm. in outside diameter and 425 mm. long), was filled with glass beads. The stopcock, L (4-mm. bore), was closed and the tower filled with chromic acid solution during combustion of a sample.
Figure 3.
Combustion Apparatus
REAGENTS
Concentrated Sulfuric Acid, C.P. Baker's Analyzed, special. Low in nitrogen and arsenic. Water. Distilled water was redistilled from alkaline solution in - ' - - -9: a glass still. Phosphorus Acid Solution, 5 M , prepared from reagent grade phosphorus acid obtained from the General Chemical Co. Chromium Trioxide Solution, 10 M , prepared from Du Pont Flake. Potassium Carbonate Solution, 1 M , prepared from analytical reagent potassium carbonate. Oxygen. Commercial oxygen was scrubbed in a gas-washing bottle filled with glass beads and half filled with 1 M sodium hydroxide solution. Ozone, generated with equipment described above. Sodium Sulfite-Gelatin Solution, 1.25 grams of analytical reagent sodium sulfite plus 2.5 mg. of gelatin (Difco) made up to 25 ml. Potassium Iodide, Analytical Reagent. Standard solutions of potassium iodide were used in preparing the standard curves. These solutions were checked against standard solutions of potassium biiodate obtained from G. Frederick Smith Chemical Co.
a 1-ml. pipet. Pass nitrogen into the beaker, into the cell, and into the distillate receiver. Add 0.5 ml. of the sulfite-gelatin solution to the residue in the distillate receiver and wash the residue down from the sides of the receiver by forcing the solution in and out of a small pipet with the aid of a rubber bulb. Allow 4 minutes for the residue to dissolve completely, then transfer the solution to the polarizing cell. Place the holder carrying the cell in the constant temperature vessel underneath the dropping mercury electrode. Insert the platinum wire in the mercury and elevate the vessel until the dropping electrode extends into the solution. Record the polarogram between -0.6 and -1.2 volts, using a drop rate of 2 seconds per drop. Soil. Place 1 gram of soil in the distillation flask, add the reagents and antibump, and proceed with the digestion and distillation as described under the procedure for water. It is necessary to agitate the flask slightly during the distillation to prevent bumping. Treat the distillate in the same manner as for water. Plant Material. Place a weighed Dortion of the plant material in the ;ampie tube. Stopper the large end of thesample tube with a quarter of asheet of 9-em. filter paper, and plug the 1-mm. hole with a small piece of filter paper rolled up in cylindrical shape. Moisten the small piece of filter paper and insert the sample tube in the combustion
: : u) ; -1.0
e
-0.
-0.7 0.4
0.2
0
0
0.2
0.4
1.0
0.6 0
2.0
M I C R O A M P E R E S
Figure 4. Typical Current-Voltage Curves I
I
I
24
I
I
I
@s=
Y2
I
I
22
20
ANALYTICAL PROCEDURE
c
Water. Place 500 ml. or less of the water sample in the distillation flask, and add 5 ml. of chromic acid and 50 ml. of concentrated sulfuric acid. Place an antibump in the flask and concentrate the solution. If more than 500 ml. of water is needed to give the necessary amount of iodine, add more water during the concentration procedure. Continue heating the solution until a temperature of 220 C. is reached, and hold a t that temperature for 5 minutes. It is usually necessary to shake the flask slightly during this 5-minute period to decompose most of the excess chromic acid. Allow to cool to 100" C., add 75 ml. of water with thorough stirring, place another antibump in the flask, and attach for distillation. Heat the flask a t high heat on a 500-watt flask heater (beveled opening, 90 mm. in diameter). Place 4 ml. of the phosphorous acid in the funnel of the still. In the distillate receiver place 5 ml. of 0.015 M potassium carbonate solution. As soon as the first drop of distillate issues from the tip of the condenser, insert the latter into the solution of the distillate receiver. With the aid of a rubber bulb placed in the mouth of the funnel, force the phosphorous acid into the distillation flask. As soon as 50 ml. of distillate have been collected, lower the distillate receiver and wash down the outside of the tip with a small stream of viater. Remove the still from the heater. Place the distillate receiver under the ozone generator and raise it until the tip of the ozone discharge tube extends well down into the solution. Force oxygen through the ozone generator a t a rate of 75 ml. per minute for 10 minutes. Concentrate the ozonetreated distillate to 2 or 3 ml. on a steam bath, wash down the sides of the receiver, and evaporate to dryness. Place the receiver in the 2-liter beaker, with the polarizing cell, its holder, and the 25-ml. volumetric flask containing the sulfite-gelatin solution and
2
m
18
ml6 0
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Figure 5. Calibration Curves
V O L U M E 23, NO. 12, D E C E M B E R 1 9 5 1
1853
Table 11. Recovery Experiments Sample Size Grams
Sample Water I
500 500 Red clover I 4.123 4.125 4.008 8.134
Soil I
1.0
Soil I1
0.5 0.5
1.0
Iodine Content
Iodine Added
Iodine Found
Y
Y
%
1.50 1.50 1.00 1.00 1.00
2.90 2.85 2.02 2.02
1.00
3.05 8.61 8.53 3.50 3.60
101 98 10 1 101 105 106 98 96 95 100
9
1.38 1.38 1.01
1.01 0.98 1.99
3.80 3.80 1.60 1.60
4.90 4.90 2.00 2.00
2.03
Recovery
tube up to the point of constriction. Plug in the tube furnaces. Add to the tower of beads 5 ml. of the chromic acid solution and then 50 ml. of concentrated sulfuric acid. Connect the combustion tube to an oxygen line and introduce.oxygen slowly. As soon as the tube furnaces attain a dull red color, light the burner nearest the tube furnace. When the filter paper in the mouth of the sample tube catches &e, increase the flow of oxygen t o about 2 liters per minute. Light the burners in as rapid succession as possible without getting smoke over in the absorption tower. Volatile matter will burn a t the mouth of the sample tube until the wet filter pa er plug burns out. The charred residue will then burn, leaving onyy an inorganic ash in the sample tube. The time required for the combustion of a plant sample depends upon its nature and size. For a 20-gram sample of whole soybeans, the time required is 30 to 40 minutes. For a 20-gram sample of whole corn kernels, 20 to 30 minutes are usually required. Draw off the liquid in the absorption tower into a distillation flask. Fill the tower with water and draw it off. Then wash down the beads with two 50-ml. portions of water. Add the ash in the sample tube to the combined solution and washings by drawing the liquid up in the tube several times and washing out with a stream of water. Concentrate this solution and proceed as for water or soil.
Smooth curves and well defined waves were obtained even a t full sensitivity. Calibration curves were obtained by carrying known quantities of iodine through the entire procedure (Figure 5). Deviations of individual samples are within &lo% a t full sensitivity. The method has been used to determine the iodine content of a large number of soil, water, and plant samples. Some typical analytical results are shown in Table I. As little as 0.5 microgram of iodine can be determined with a maximum error of +10%. Determinations with a precision of within 5% of the amounts of iodine from 2 to 40 micrograms can be made. Experiments in which known amounts of potassium iodide were added to soil, water, and plant samples before combustion showed that recovery of iodine was quantitative within the limits of reproducibility of the method (Table 11). The sensitivity of this method permits the determination of iodine in the minute amounts present in plant materials without the necessity of handling large samples. LITERATURE CITED
Cizek, V., “Polarographie,” by Heyrovskg, p. 352, Ann Arbor, Mich., Edwards Brothers, Inc., 1944. Fraps, G. S., and Fudge, J. F., Texas .4gricultural Experiment Station, Bull. 595 (November 1940). Kolnitz, H. von, and Remington, R. E., IND.EKG. CHEM., ANAL.ED.,5, 38 (1933). h‘IcHarguc, J. S.,and Offutt, E. R., J . Assoc. Ojic. Agr. Chemista, 2 2 , 4 7 1 (1939).
Matthews, N. L., Curtis, G M., and Brode, W. R., IND. ENQ. C H E M .ANAL. , ED.,10, I312 (1938). Orlemann, E. F., and Kolthoff, I. M., J . Am. Chem. S O C 64, , 1044, 1970 (1942).
Rllich, A., Collection Csechoslov. Chem. Communs., 7, 288 (1935)
RESULTS
Typical polarograms are presented for solutions derived from wheat, water, and soil ( A , B, and C, respectively, Figure 4).
RECEIVED March 31, 1951. Journal Paper Number 517. Supported in part by funds furnished b y the Chilean Iodine Educational Bureau, Inc.. 120 Broadway, New York, N.Y.
Indirect Colorimetric Micro-oxidimetry of Organic Compounds H. T. GORDON, Division of Entomology a n d Parasitology, University of California, Berkeley, Calif. The methods described in this paper are applicable to a great variety of organic compounds, in trace amounts (1 to 10 micrograms) and dilute solutions (0.001%). The analytical reaction is a stoichiometric oxidation with an excess of bromine, hypochlorite, permanganate, or ceric sulfate; the excess oxidant is then measured colorimetrically by reaction with an oxidizable dye. Each oxidation reaction is applicable to all compounds having certain reactive groups. Examples are given of reactions for cho-
V
ERY low concentrations of many oxidizing agents, of the order of 1 t o 10 micronormal, can be detected by using oxidizable dyes as reagents ( 7 , 8). The quantitative decolorization of a dye by various oxidizing agents is the basis of the analytical methods described in this paper. From 1 to 10 micrograms of many organic compounds can be determined oxidimetrically. The first step in an analysis is the reaction of the organic compound with an excess of oxidizing agent, and the second step is the reaction of the excess oxidant with a standard amount of oxidizable dye. The excess dye is then measured in a colorimeter.
lesterol (bromine addition to double bonds), amino acids and carbohydrates (hypochlorite oxidation of amino and aldehyde groups), citric acid (permanganate oxidation of alpha-hydroxy acids), glycerol and palmitic acid (ceric sulfate oxidation of C-C bond), and others. These micro reactions may be useful for identification of oxidizable structures in unknown compounds, for molecular weight determinations, and for analysis of very dilute solutions (distillates, extracts, chromatographic eluates).
All analytical results are expressed in terms of micrograms of dye protected from Oxidation by 1 microgram of organic compound. REAGENTS
Fast Green FCF (National Aniline and Chemical Co., Inc., New York, N. Y.), a water-soluble triphenylmethane dye (molecular weight as disodium salt 809), 0.1% solution in 1 M sulfuric acid. The dye is obtainable 94% pure and used without purification. The solution fades slightly in the first few days, but is then stable for months. Bromine, 1.2y0solution in carbon tetrachloride. This gradu-
