514
INDUSTRIAL AND ENGINEERING CHEMISTRY
and the o-chlorofluorobenzene listed in the tables yielded substantially correct values for the chlorine. This indicates that the decomposition of the samples containing fluorine is accomplished satisfactorily, and that the only remaining problem is that of recovering the fluorine quantitatively. The time required for the analysis of a sample varies considerably, depending upon the physical characteristics of the substance. Excluding the time required for preparation of apparatus and standard solutions, but including the sampling and calculation of results, the analysis of a very volatile substance such as methyl chloride may be completed in 35 to 40 minutes, and may be turned out in a routine manner a t the rate of one analysis every 15 minutes. Other hubstances may require as much as 30 to 45 minutes
Vol. 15, No. 9
for the vaporization and burning of the sample. depends upon the experience of the analyst.
Much also
Literature Cited (1) Cadenbach, G., Angea. Chem., 46, 130 (1933). (2) Caldwell, J. R., and Moyer, H. V., IND.ENG.CHEM.,A N . ~ LED., . 7, 38 (1935). (3) Elving, P. J., and Ligett, W. B., Ibzd., 14, 449 (1942). (4) McEke, E. T., Hass, H. B., Xeher, C. AM., and Strickland, H., IND. ENG.CHEM..34.296 (1942). ( 5 ) Martinek, M. J., and Marti, W. C., I N D .ENG. CHEM.,Ax.4~. ED.,3, 408 (1931). (6) Universal Oil Products Co., “U. 0. P. Laboratory Test Methods for Petroleum and Its Products”, p. H-27, Chicago, 1940.
PRESENTED before the Division of Analytical and Micro Chemistry a t the 105th Meeting of the . ~ M U E R I C A NCHEMICAL S O C I E T Y , Detroit, Mich.
Determination of Sulfur Residues from Sulfur Application on Citrus Foliage F. A. GUNTHER, R. L. BEIER’, AND J. P. LADUE University of California Citrus Experiment Station, Riverside, Calif.
Elemental sulfur (98 per cent sulfur by weight) mixed with a sulfur-free spray oil was applied to lemon leaves and then stripped off with purified carbon disulfide. After removal of the solvent, the sulfur was oxidized to inorganic sulfate by alkali fusion, and the sulfate ion was determined gravimetrically by precipitation as the barium salt. No special apparatus was required, and 10 to 200 micrograms of sulfur per square centimeter of leaf surface were determined rapidly and accurately in field practice.
procedures for the estimation of the sulfate ion have been proposed, but these methods often require special equipment or are otherwise unsuited to the rapid determination of a small amount of elemental sulfur, in the presence of petroleum spray oils and spreading (wetting) agents, which are contained in a large amount of bulky material. The present method is an outgrowth of a study initiated by Boyce et aZ. ( I ) , who devised a sulfur-residue method involving a mechanical stripping of the sulfur from treated leaves in the absence of spray oils, and it combines the solvent extraction or stripping technique of Thatcher and Streeter (11) with an alkaline fusion similar to that of White (12). Wet-oxidation methods using nitric acid and bromine were found to give variable and incomplete oxidation (65 to 85 per cent) of elemental sulfur in the presence of a petroleum oil.
Experimental Procedure
E
LEMENTAL sulfur has long been used for the suppression of various insect and mite pests and diseases of agricultural crops. It has been applied to the plants b y different methods and in various combinations with inert materials and with other insecticides. The practice of applying elemental sulfur and petroleum spray oils simultaneously has brought u p the difficulty of rapidly determining this elemental sulfur quantitatively in the presence of low concentrations of an oil of low volatility. The usual procedure (4, 11) has been to oxidize the elemental sulfur to sulfate, which is then precipitated with a soluble barium salt and weighed as barium sulfate. T o simplify the procedure in work with spray residues, a preliminary extraction (3, 4, 8, 11) of the sulfur from the plant material, followed b y an oxidation process, is the most logical method. If the whole sample is oxidized in an alkaline fusion (2, l a ) , the time element involved becomes prohibitive. This same objection applies, generally, to methods in which the elemental sulfur is reduced to sulfide (6). Even when simplified as t o size and heterogeneity of sample, the gravimetric methods are usually time-consuming. Several volumetric (7, 9, 10, 13, 14) and photometric (15) 1 Present
address, 16th Bombardment Wing, Biggs Field, El Paso, Texas.
The leaf sample to be analyzed (usually 125 average-sized mature leaves) was collected in a 2-quart Mason jar, which was capped and stored a t 0” C . Just before the analysis, the leaves were removed from the jar with specimen forceps, and their surface areas were measured photoelectrically ( 5 ) . For the extraction of the sulfur, 100 ml. of recently redistilled carbon disulfide were added to the jar, and the sample waa shaken vigorously for 2 minutes. -4fter decantation of the supernatant liquid throu h a Buchner funnel, another 100-ml. portion of carbon disulfife w m added; the sample was shaken for 1 minute, and the supernatant liquid was decanted through the same funnel into the same receiving flask. The filtrate was transferred to a 250-ml. volumetric flask, and the funnel and receiving flask were washed carefully with 50 ml. of fresh carbon disulfide. This wash solution was then added to the previous filtrate, and the volume was adjusted to 250 ml. with additional carbon disulfide. I n the meantime, a fusion melt consisting of 8 parts of pure potassium hydroxide, 1 part of potassium nitrate, and 0.3 part of distilled water (all parts by weight) was prepared. This melt was crushed while warm and stored in a bottle tightly stoppered with a rubber stopper. For each sample, 3 grams of this crushed melt were weighed into a 20-ml. silver crucible, and an accurately measured 5-ml. aliquot of the carbon disulfide extract was pipetted onto the melt in the crucible. The carbon disulfide was evaporated from the sample, under vacuum, by placing the crucible containing the sample on a thin iron plate; a bell jar connected to the laboratory vacuum line was placed over the sample, and the iron plate was warmed gently by means of a light bulb
September 15, 1943
ANALYTICAL EDITION
or very sensitive hot plate. (If the rate of evaporation was too high, spattering was likely to occur.) Under these conditions of evaporation, the carbon disulfide apparently did not react with the fusion melt. When all the solvent had been removed, the sample was fused over a shielded microburner with constant stirring with either a silver or a platinum wire. Considerable care had to be exercised in increasing the temperature of the melt during fusion, since spattering was sure to occur if the temperature was raised too rapidly. Sfter bubbling had ceased, even over a raised flame, the crucible was heated strongly until t’he black deposit disappeared and the melt became water-clear. After cooling, the melt was transferred t o a 250-ml. beaker and dissolved in 10 ml. of distilled water, and the excess potassium hydroxide in it mas neutralized with 6 N hydrochloric acid, phenolphthalein being used as indicator. The resulting solution was filtered quantitatively into a 400-ml. beaker, and the filter paper was washed with four 50-ml. portions of distilled water. The combined filtrate and washings were brought t o a volume of 300 ml. viith distilled wat,er, and 1 ml. of 6 N hydrochloric acid was added. Aft’er the acidified solution had been heated to boiling, 0.1 N barium chloride solution was added dropm-ise from a buret, with constant stirring, until no new precipitate of barium sulfate formed when the mixture vias allowed to settle for a minute and a drop of the barium chloride solution as added to a clear region of the sample solution. The solution T T ~ kept S just a t the boiling point during this addition. An excess of 10 ml. of the barium chloride solution \?as then added, and the sample solution was kept hot for 1 hour to alloJ5- time for complete precipitation of the barium sulfate. 4 watch glass was used to cover the beaker a t all times. A Gooch crucible was prepared in the usual manner and dried at 160” to 180” C. to comtant weight. Immediately before filtering, the sample was again tested for complete precipitation. If precipitation was complete, the sample was filtered t,hrough the Gooch crucible, care being taken that no precipitate should adhere to the beaker or stirring rod. The precipitate was washed a t least ten times with hot water, until the washings were free from chloride ion. The crucible and its contents were dried approximately 1 hour a t 160” to 180” C. to constant weight, and the samole was weighed as barium sulfate. Blanks were run on both untreated leaves and solvent, and any ne-essary corrections were applied.
Discussion The method reported in this paper is a combination of several of the standard procedures for the determination of sulfate ion, modified to eliminate certain errors introduced b y the extraneous materials commonly found in oil-spray preparations, and to expedite the oxidation of sulfur to sulfate. About 30 minutes, exclusive of drying periods, are required for one complete analysis; this value includes the time required to measure the total surface area of the sample. B y running series of samples simultaneously and progressively, the authors have had no difficulty in analyzing 15 samples per man per 8-hour day. I n all test analyses, a knon-n weight of yellow dusting sulfur (98 per cent sulfur b y weight), comparable to that found in actual practice in the field on a unit leaf surface, was added to a volume of a so-called “light-medium” sulfur-free spray oil comparable to that applied in actual field work. This oil-sulfur mixture was then applied to average-sized mature lemon leaves, the leaf surface areas were measured photoelectrically, and the sample was analyzed for micrograms of sulfur per square centimeter of leaf surface. A small correction factor from blanks run on the solvent (1.1 mg. of sulfur per 5 ml. of freshly distilled solvent) was applied; blanks run on untreated leaves indicated that a small correction factor (average value of 1.5 X loF2mg. of sulfur per sq. cm. of leaf as used in these test analyses) was usually necessary per unit leaf area, the magnitude of this correction factor depending upon the previous spray history of the sampled leaves. Blanks run on the oil indicated i t to be sulfur-free. Results of analyses to show the accuracy and reproducibility of the method are presented in Table I.
575
TAB^ I. ANALYSESOF ELEMENTAL SULFUR ON LEAFSAMPLES (125 leaves each, containing known quantities of ure sulfur and a sulfurfree spray oiln, to show the accuracy and reprozuoibility of the method) Sulfur Recovered (Corrected) Sample Sulfur Applied Grams Grams %b 0.980 0.974 99.4 0.980 0,985 100.5 0.980 0.945 96.4 0.980 0,944 96.3 1.558 1.558 100.0 0.941 0.922 98.0 0.613 0.606 98.8 a Each sample contained 0.5 ml. of a so-called “light-medium” spray oil which had previously been shown t o contain no sulfur, as determined by the present method. Coefficient of variation (per cent), 1.69.
It has been the experience of workers a t the Citrus Experiment Station that a coefficient of variation of 1.69 per cent, as found in this method, indicates a low variability in methods to be applied to the removal of spray residues in field experiments. This method has been extensively used a t the University of California Citrus Eyperiment Station for the determination of sulfur residues resulting from field applications of various forms of sulfur in combination with other materials in controlstudies of the citrus thrips, Scirtothrips citri (Moult.), and the citrus bud mite, Eriophyes sheldoni Ewing. Greater precision, with a consequently increased time requirement, obviously may be attained in this method b y increasing the number of extractions and thus the volume of extracting solvent. However, the degree of accuracy of the method as reported is thoroughly suitable for the purpose for which the method was devised. Interfering Materials The sulfur-free spray oil added to each sample (usually
0.5 ml. per 125 leaves) apparently did not interfere with the accuracy of this method. Excessive amounts of salts formed during neutralization, leading to coprecipitation, probably constitute the main source of positive error, while those forms of sulfur insoluble in carbon disulfide probably constitute the main source of negative error. Careful technique is essential in obtaining reproducible results with this method.
Literature Cited Boyce, A. M.,
Hansen, J. W., “Field Experiments with Sulfur for Control of Citrus Thrips”, unpublished manuscript on file at University of California Citrus Experiment Station, Riverside, Calif., 1937. Emerson, H., J . Am. Chem. Soc., 52, 1291 (1930). Fitch, H. W.,Phytopathology, 15, 351 (1925). (4) Ibid., 16, 427 (1926). (5) Gunther, F. A., and Barnhart, C. S.,“Photoelectric Device for Measuring Surface Areas of Irregular Planar Objects”, in preparation. ENG.CHEM. ANAL. ( 6 ) Heinemann, G., and Rahn, H. W., 1x1~. ED.,9, 458 (1937). (7) Krause, W.,Chemist-Analyst, 27 ( l ) , 14 (1938). (8) Mack, G. L., and Hamilton, J. M., IND. ENG.CHEM.,AXAL. ED., 14, 604 (1942). (9) Mahoney, J. F., and Michell, J. H., Ibid., 14, 97 (1942) (10) Manov, G. G., and Kirk, P. L., Ibid., 9, 198 (1937). (11) Thatcher, R. R., and Streeter, L. R., New York Agr. Expt. Sta., Tech. B u l l . 116 (1925). (12) White, R. P., New Jersey Agr. Expt. Sta., B u l l . 611, 7 (1936). EXG.CHEX.,ANAL. (13) Wilson, C. W.,and Kemper, TV. A., IND. ED., 10, 418 (1938). (14) Woodward, G., I b i d . , 1, 117 (1929). (15) Zahn, V., I b i d . , 9, 543 (1937). Kagy, J. F., and
PAPER No. 490, University of California Citrus Experiment Station, Riverside, Calif. This investigation was initiated by Lieutenant Beier. who was unable t o develop the work into its final form
