. Tartar Emetic on Leaf and Fruit Surfaces Distributional and Semiquantitative Analysis, Using an Iodine-Starch Paper DONALD, STARR, Bureau of Entomology and Plant Quarantine, United States Department of Agriculture, Washington, D. C.
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TTEMPTS to determine the quantity of tartar emetic remaining on citrus foliage during a spraying program led to the realization that in insect control it is sometimes more important to know how the poison is distributed than t o know the exact amount present on a given surface. A simple and rapid analytical method was needed if the large number of analyses required in control work were to be handled. A sensitive iodine-starch test paper was devised which gave a picture of the distribution of the tartar emetic over the leaf surface. The iodometric titration of tartar emetic is a wellknown analytical procedure (1) in which the following reaction is involved :
and the citrus leaves were tested with the same iodine-starch paper, and by roughly integrating and comparing the whitened area due to the standard with that due to the unknown the approximate quantities of tartar emetic on the citrus leaves were obtained. The test may also be used for roughly estimating tartar emetic residues on citrus fruit surfaces.
Preparation of the Test Paper The iodine-starch mixture used in coating the paper had the following composition: 6.4 grams of iodine, 13 grams of potassium iodide dissolved in a small portion of the water, 54 grams of soluble starch, 3.4 grams of sodium bicarbonate, and 1 liter of water. It is conveniently prepared by adding the starch and sodium bicarbonate t o 1 liter of a stock solution of 0.05 N iodine. The effect of varying the proportions has not been studied extensively, but the above amounts may be altered without markedly affecting the results. Larger quantities of sodium bicarbonate increased the fading of the paper; however, when sodium bicarbonate was absent the sensitivity was decreased. The amount of water used depends upon the sensitivity of the reaction desired. The more concentrated mixtures give darker and better coats of iodine, but the reaction is less sensitive.
I2 + K(SbO)C4H40e+ 6NaHCO8 + NasSb04
+ 2NaI + KNaCnH406 + ~ H z O+ 6COz
The iodine-starch paper used was dark blue or brown, and the reaction with tartar emetic reduced the iodine and whitened the paper. A semiquantitative estimation of the tartar emetic was made possible by preparing, as a standard of comparison, small slips of paper containing a known quantity of tartar emetic on the surface. The tartar emetic paper
FIGURE 1 . PRINTB OF TARTAR EMETICON GRAPEFRUIT LEAVES USING THE MORESENSITIVE TEBTPAPER Numbers indicate tartar emetic concentration in micrograms per square centimeter.
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Each circle represents 10 micrograms.
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FIGURE 2.
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PRINTS O F
VOL. 11, NO. 9
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TARTAR EMETICON GRAPEFRUIT LEAVESUSING COATING O F IODINE
THB PAPER WITH 1 H E
HEAVIER
Numbers indicate tartar emetic conaentration in milligrams ger square centimeter. Each circle represents 0.02 mg.
The paper which gave the best results in these studies was manifold typewriting paper of about 9 pounds per ream, 17 by 22 inches, containing 500 sheets. The methylene blue size test (as used at the Institute of Paper Chemistry) was about 30 seconds. Some heavier, harder sized sheets took a thinner and less uniform coat. A lighter paper took a fair coat, but the uniformity and opacity were poor. Filter paper did not take a uniform coat. The aper was dip ed into the iodine-starch mixture until thorouglly wet, and tgen removed, drawn over a 12-mm. glass rod to remove the excess, and hung up to dry. The iodine-starch mixture may be used or stored for 3 or 4 weeks, but the coat obtained gradually becomes lighter in color. The aper fades noticeably after it has been kept for 1 or 2 montis. Increasing the sodium bicarbonate content and exposure to sunlight accelerates fading. Iodine-starch paper kept in a small air-tight container away from the light had faded slightly after a month, but it was in better condition than when ke t i n the open, exposed to air and light. en general, the strength of the coating mixture should be varied to suit individual needs, depending on the time the paper is to be stored before use, on the sensitivity of the reaction desired, and on the way the paper takes up the iodine-starch mixture.
Preparation of the Standard Tartar Emetic Paper Different amounts of tartar emetic were used to prepare the standard paper, but 0.01 or 0.02 mg. was found to be satisfactory. The standard papers were prepared using a microburet which delivered a drop of distilled water having a volume of 0.045 cc. at 22" C. a t a speed of 1 drop in 3 to 5 seconds. Using 0.00133 N tartar emetic solution to prepare 10-microgram standards and 0.00266 N for the 20-microgram standards, 80 drops represented 3.595 cc. in the first case and 3.615 cc. in the second. The average differences are 0.14 and 0.42 per cent from the volume on which the calculations were based.
The paper was such that the solution would stand up well and upon drying leave a ring of tartar emetic 5 to 7 mm. in diameter. Alcohol was added to the solution in attempts to produce a spot of tartar emetic rather than a ring. Up to 50 per cent by volume of alcohol, with 20 micrograms contained in a drop of 0.02 cc. of the solution when applied to the paper still gave a ring, but 10 to 14 mm. in diameter. Sodium thiosulfate standards equivalent to 10 micrograms of tartar emetic also gave ringed deposits.
Recommended Testing Procedure The degree of reaction obtained between the iodine-starch test paper and the tartar emetic standard paper increases with time, temperature, and pressure. A nearly complete reaction is obtained in 15 minutes a t 20" to 25" C. with a pressure of 100 to 150 grams per square centimeter, but qualitative tests on grapefruit leaves and fruit have been very distinct in a minute or less. It hag been determined experimentally that the reaction from grapefruit leaves, dorsal surface, and from glass surfaces is faster than from the standard paper. If the pressure used is less than 100 grams per square centimeter, it may be necessary to increase the time in order to get a complete reaction. The test is run by placing the standard paper beside the leaf and pressing the test paper against them with a wellmoistened pad made of about eight thicknesses of filter paper backed by about eight thicknesses of muslin toweling and four thicknesses of turkish toweling. (In the laboratory a weight of about 20 kg. was applied to thick glass plates of 160-sq. cm. area, but in the field A. C. Baker found it convenient to use a small photographic printing frame.) The
SEPTEMBER 15,1939
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ANALYTICAL EDITION
leaf can be tested on both top and bottom surfaces by placing it between two test papers with moist pads above and below. If permanent records of the tests are desired, brown or blue prints may be made from the iodine-starch test paper. The prints must be exposed accurately to bring out the tartar emetic spots without smudging the paper. Iodine-starch prints fade considerably after one or two months. The sensitivity of the test has been increased by diluting the coating mixture. A paper prepared as described above gave a distinct reaction with 0.0005 cc. of a solution containing 2.0 grams of tartar emetic per liter, or 1.0 microgram of tartar emetic. A more sensitive paper prepared from the above formula with the sodium bicarbonate doubled, and diluted with seven parts of water reacted to 0.1 microgram of tartar emetic. Figures 1and 2 are photographs of prints taken from grapefruit leaves dusted with varying known amounts of tartar emetic. It will be noted from Figure 1 that the tartar emetic concentration can be tested within the range of 0.9 to 10 micrograms per square centimeter using the more sensitive paper mentioned above. With the higher concentrationsnamely, 0.02 to 0.13 mg. per square centimeter-paper prepared from the undiluted mixture may be used. The results are shown in Figure 2. The photographs indicate the degree of precision obtainable under the prescribed conditions. The distribution of tartar emetic on citrus leaves after being
sprayed with tartar emetic solutions is much less uniform than that obtained by dusting.
Interfering Substances The materials that may be expected to interfere with the test are reducing substances in general. Some of the materials found to interfere are sodium arsenite, glue, and molasses. The paper is not very sensitive to arsenious oxide or Paris green, probably owing to the low solubility of these compounds. Sucrose, glucose, fructose, galactose, arabinose, and xylose were found t o be nonreactive, or so nearly so that there would be no interference. Strong ammonia solution, calcium hydroxide, and potassium carbonate reacted with the paper. Strong alkalies in general react with iodine to give hypoiodites, Summary Iodine-starch test papers have been prepared for the purpose of obtaining distributional and semiquantitative analyses of leaf surfaces for tartar emetic. The test is sensitive to 0.9 microgram per square centimeter. Known quantities of tartar emetic have been deposited on paper as a standard of comparison. Substances that reduce iodine interfere with the test for tartar emetic. Literature Cited (1) Treadwell and Hall, “Analytical Chemistry,” 7th Ed., Vol. 11, p. 581, New York, John Wiley & Sons, 1930.
Ejector-Type Evacuator for Wet Assay Systems EDGAR J. POTH, Stanford University School of Medicine, San Francisco, Calif.
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HE elimination of corrosive fumes evolved in wet assay
methods such as Kjeldahl digestions frequently presents a problem, especially in poorly planned laboratories. One is faced with the problem of disposing of corrosive fumes as well as removing them from a given system. For the elimination of fumes created in many microanalytical procedures, a small digestion chest is presented (Figure 1, G). It is made of stainless steel, has a removable top, and, after use, can be washed out thoroughly. It can be placed on a hot plate. The ejector or jet pump is constructed of Pyrex glass and operates with high efficiency on low-pressure steam. The fumes are brought into direct contact with live steam, which in turn is condensed by a spray of cold water. I n this way the corrosive materials either react with the steam or are dissolved in the condenser water. Furthermore, they are highly diluted and can be dumped directly into the ordinary laboratory draina,ge system. The pump will handle liquids as readily as vapors and so it is not necessary to have traps for liquids condensing in or introduced into the system. I n the construction of this pump it is well to determine experimentally the position where any particular jet gives the greatest efficiency with the steam pressure available. A.
T-i
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Steam inlet and jet
B. Vacuum outlet t o oonneot with digestion system C . Condenser
D. Spray condenser G . Digestion box made of stainless steel H. Hot plate I. Fume and spillway t o be oonneoted with B X. Male half of steam jet, 6 mm. in inside diameter Y . Female half of steam jet, 12 mm. in inside diameter, slightly tapered. Gap between X and Y measures 4 mm. Dimensions in millimeters. Design on right best suited for system of small capacity; design on left, illustrated with sump and overflow water trap, best for systems of large capacity.
FIGURE1. EJECTOR-TYPE EVACUATION PUMP (See designation8 a t left)
