Bromide Content of Fruits and Vegetables Following Fumigation with Methyl Bromide H. C. DUDLEY, National Institute of Health, Washington, D. C.
T
HE use of methyl bromide as a fumigant for certain food-
analysis or the times of sampling after fumigation. Their results are in fair agreement with the findings of this investigation. The following method of analysis has been used in determining the bromide content of a variety of foodstuffs up to 48 hours after fumigation with methyl bromide.
stuffs in order to control insect pests has increased markedly in the past two years, and gives promise in many types of fumigation procedures because of its effectiveness a t moderate concentrations @,S, 4). I n order to study the effect of methyl bromide on foodstuff s intended for human consumption it was considered necessary to determine the amount taken up by the produce during fumigation and the rate of volatilization after completion of the fumigation. The analytical method herein described for the determination of total bromides in vegetable products is based on the work of Baughman and Skinner (1). Modifications have been used as microchemical procedures in studying the bromide content of vegetable and animal products by Keufeld (6) and Yates (7). The macroanalytical procedure here described requires 50to 100-gram samples of the produce. Samples were taken in their usual state, and no correction is made for water content. The material being analyzed was covered with 1 per cent alcoholic potassium hydroxide solution, which serves to dissolve and hydrolyze the methyl bromide, converting the volatile bromide to potassium bromide. The treated sample was ashed three times a t 500' C., being extracted with hot water between ashings. Mackie (4) and McLaine and Munro (6) give the results of analyses of fruits and vegetables following methyl bromide fumigation, but present no details as to the methods of
Reagents Required Alcoholic potassium hydroxide, 10 grams per liter of 95 per cent ethyl alcohol. Dilute sulfuric acid, 2 to 6 N . Stock sulfuric acid, 1400 CC. of water plus 650 cc. of concentrated sulfuric acid. Chromic acid solution, 1600 cc. of water lus 200 grams of chromic anhydride and 600 CC. of concentratezsulfuric acid. Potassium iodide solution, 100 grams of potassium iodide plus 1000 cc. of water (make fresh daily). Standardized sodium thiosulfate solution, about 0.01 N . Prepare above reagents and allow to come to room temperature.
Procedure Place a 50- or 100-gram sample of fruit or vegetable in a IO-cm. Pyrex or porcelain evaporating dish, cover with about 100 cc. of alcoholic potassium hydroxide, and allow to stand overnight a t room temperature. Evaporate the alcohol and dry thoroughly on a low hot late. Place the dish in a heated muffle furnace, ignite, and hoPd a t 500" C. for 2 to 3 hours. Extract with two 50-cc. portions of hot water, filtering through No. 2 filter paper. The filtrate may be caught in a 400-cc. beaker. Remove the filter paper, add to dish, and dry dish and contents on a hot plate. Place the dish in a heated muffle furnace controlled a t 500' C., ignite the paper, and heat until charcoal ceases t o glow. Remove,
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FIGURE1. APPARATUS FOR ASPIRATINQ BROMINE A . All connections glass t o glass, rubber sleeved B . Spray trap, sealed-in sintered-glass disk C,D. All-glass bubblers having sintered-glass foot, E , on bubbler stem. Air flow 500 to SO0 00. per minute 259
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INDUSTRIAL AND ENGINEERING CHEMISTRY
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AND REPRODUCIBILITY OF RESULTS TABLE I. ACCURACY
AverBr Found in 5 Determinationa age Br MiniMaxi- AverRecovSample Taken mum mum age ery Mg. Mg. Mg. Mff. % 100.0 1.50 1.48 1.47 1.48 KBr added direotly t o bub99.7 3.72 3.70 3.69 bler oontainin H2804- 3 . 7 1 100.0 7.43 7.41 7.38 CrOs. Aspirate%as shown 7 . 4 1 in methods 1.12 75.7 1.02 1.18 1.48 KBr added t o dish. Heated 3.51 94.6 3.45 3.58 3.71 a t 500' C. for 2 hours. 9 9.0 7 . 3 4 7.49 7.22 Residue rinsed, direotly 7.41 into bubbler with stock HnS015 --_. -. 68.9 1.08 1.02 0.93 1.48 KBr 100 00. of 1% aloo3.40 91.7 3.37 3.42 h o l i o K O H , T a k e n 3.71 7.45 100.5 7.56 7.25 through entlre proce- 7.41 dures 98.0 1.50 1.45 1.39 1.48 KBr 10 gram8 of char100.0 3.78 3.71 3.59 ooal j-100 ao. of 1 o al- 3 . 7 1 102.1 7.57 7.71 7.39 7.41 ooholio KOH. T a % e n through entire prooedure5 0.30 ... 0.35 0.23 KBr added to 100 grams. of 0 90.1 1.71 1.75 1.65 1.78, potato pulp. Carried 102.9 4.13 4.19 3.97 4.01b through entire procedure' 103.8 8.01 8.12 7.95 7.71) 0 Combined reagents give no oolor when K I solution is tested with starch, after carrylng reagents through approprlate blank procedure. b Value of bromine as taken includes average Br content of 100 grams of potato pulp = 0.30 mg. of Br.
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This series was carried out by procedures described in the table, in order to show the accuracy and reproducibility of results. I n Table I1 are shown the results of analyses for total bromide of fruits and vegetables following fumigation (at atmospheric pressure) in the laboratory with concentrations of methyl bromide approaching those used in commercial practice. These analyses show that appreciable quantities of methyl bromide are taken up by the produce during fumigation, In most cases a large part of the adsorbed gas is volatilized within 48 hours, escaping to the surrounding atmosphere. In Table I11 is shown the bromide content of dried fruits following fumigation by commercial methods.
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cool, and extract with two 50-cc. portions of hot water, filtering the extracts into the beaker containing the previous extracts. Again add filter paper to evaporating dish, dry on hot plate, and ash in muffle at 500" C. To ash from third ignition, add sufficient dilute sulfuric acid to react with alkali and carbonate present in the ash; have little excess acid present. Filter the extract, wash twice with cold water, and rinse funnel into combined extracts. Discard filter paper and residue. The combined extracts should be 300 t o 350 cc. and show a strongly basic reaction. To test for basicity, use drop on spot plate. Do not add indicator to solution. If not sufficientlybasic, add a pellet of potassium hydroxide. Evaporate combined extracts t o dryness on steam bath or low hot plate, takin care at the final stages to prevent spattering. To the dry resijue from the evaporation of the water extracts add 25 cc. of stock sulfuric acid solution, dissolve residue as much as possible, and rinse into a bubbler using the stock sulfuric ac!d in a wash bottle as rinsing solution. Add 25 cc. of the chromlc acid solution t o the mixture (final volume in bubbler, 75 to 80 cc.). Add 80 cc. of 10 per cent potassium iodide solution to second bubbler, and connect in series as shown in Figure 1. Aspirate for 20 minutes at 500 to 800 cc. of air per minute. Titrate liberated iodine immediately with standard sodium thiosulfate 0.01 N , using soluble starch indicator.
TABLE 11. BROMIDE CONTENT OF FRUITS AND VEGETABLES Following fumigation with CHaBr, in laboratory' Before Fumiga- Immedi24 48 tlon ately Hours Hours Fumigation Sample (Control) After After After Proaedureb Mg. Br per 100 grams White potatoes 3.66 3.02 2.58 4.22 Peel 1.29 1.001 Puln 0.79 1.28 SGeetrpotatoes Peel 1.66 3.16 3.20 3.16 2 Ib. of CHaBr Pulp 0.55 0.99 0.98 0.90 per 1000 CU. ft. 4.20 4.08 7.22 0.54 Green beans for2hours 1.11 0.91 1.26 Traoe Tomatoes 2.11 1.72 Eggplant 0.10 2.38 0.62 0.61 0.80 Onions Traoe 0.27 0.30 0.31 None Apples (fresh) Traae None Pears (fresh) None 0.28
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Results I n Table I are shown the result of a series of five duplicate determinations for bromide content of standard samples.
TABLE 111. BROMIDE CONTENT OF DRIED FRUITS FUMIGATED UNDEB COMMERCIAL FUMIGATION CONDITIONS' After Fumigation Before After Followed by Hot Fumi- FymiWater Washing. Fumigation Sample gationb gationb Air-Driedb Procedure Mg. Br per 100 grams Seedless raisins 0.56 0.86 3 lb. of CHaBr per 3100 ou. ft. for 15.5 hours Dried unprocessed 0.39 0.48 0.47 4 lb. of CHtBr prunes per 19880 ou. ft.for l5hours Dried processed 0.40 1.97 1.20 4 lb. of CHaBr er 19880 ou. peaches t,for 15hours 5 Samples were in usual state. Values are not oorreoted for moisture content of samde. Results are average of 5 determinations on samDles from same lot of material. b Random samples taken from lot and plaaed in sealed aans. Shipped t o laboratory for analysis. Boxoar fumigation.
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Discussion The method herein outlined for the analysis of fruits and vegetables in order to determine their bromide content is applicable to most vegetable products; by a critical survey of many other methods, including various colorimetric procedures, it was found most suitable for analyzing the usual foodstuffs for total bromides. Other procedures were unsuitable either because they were inaccurate a t the concentrations of bromide present, or the amount of sample required was inadequate for the author's purpose. The success of this method of analysis depends on the complete carbonization of the sample during the first ashing. A colorless filtrate should be obtained when extracting with hot water. If organic matter, charcoal, or indicators are present in the extract, low results will be obtained by reason of adsorption or combination of the bromine liberated by the chromic anhydride-sulfurio acid solution. This factor is particularly troublesome in the case of fruits of high sugar content, or dried fruits, where the material carbonizes slowly, forming a compact mass. Longer heating during the initial ashing a t 500" C. is the only successful method so far devised to overcome this difficulty. By raising the temperature of the initial ashing to 600" C. much quicker carbonization takes place, but there is lower average recovery of bromides from standard samples. The separation of bromine from solutions containing chlorides and iodides described herein is based on the facts that chlorides are not affected, bromides are converted to bromine, and iodides are oxidized to iodates, by the chromic anhydride-sulfuric acid mixture. The concentrations of sulfuric acid and chromic anhydride together with the temperature determine the completenes8 and selectivity of this method. The procedure must be carried out a t room temperature (20' to 25' C.), and the acid mixtures must be cooled to this tempera-
MAY 15, 1939
ANALYTICAL EDITION
ture before mixing with the chlorine-bromine-iodine mixture. High bromine values will be obtained if the temperature of the room or reagents is too high, due to the liberation of chlorine. Studies of the influence of relatively high concentrations of chlorine have been made by Baughman and Skinner (1) and others. Losses of iodine from the potassium iodide solution are negligible if 10 per cent potassium iodide solution is used. When a second potassium iodide bubbler was connected in series with the bubbler system (Figure l),sufficient iodine was lost through aspiration to give only a weakly positive test with starch, when 7.41 mg. of bromine were present in the sulfuric acid-chromic anhydride mixture. If 1 per cent potassium iodide solution is used the iodine lost approximates 4 per cent of the total present in the sample. It has been found that the amount of chromic anhydride
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adsorbed on glass and porcelain ware after washing in the usual chromic anhydride-sulfuric acid cleaning solution causes a loss of bromine through oxidation of the soluble bromides. All apparatus used in this analytical procedure should be washed with scouring powder and soap, and rinsed several times in hot water, followed by a distilled water rinse.
Literature Cited 11, (1) Baughman, W. F., and Skinner, W. W., J. IND.ENG.CHEM., 954 (1919).
(2) Lepigre, A. M., Bull. SOC. encow. ind. nat., 135, 458-62 (1936). (3) Lindgren, D. L., J. Econ. Entomol., 29, 1174 (1936). (4) Mackie, D. B., Ibid., 31, 70-9 (1938). (5) McLaine, L. S., and Munro, H. A. U., Ontario Dept. Agr., 67th Ann. Rept., p. 15 (1936). (6) Neufeld, A. H., Can. J. Research, B14, 160 (1936); B15, 132
(1937). (7) Yates, E. D., J. Chem. SOC.(London), 27, 1763 (1933).
Hydrogen-Ion Activity and Buffer Capacity of Natural and Treated Waters A. P. BLACK, University of Florida, Gainesville, Fla.,
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EDWARD BARTOW, State University of Iowa, Iowa City, Iowa
HE electrometric measurement of the hydrogen-ion activity of natural and treated waters by means of the hydrogen and quinhydrone electrodes is often a difficult problem. The high resistance of the solutions, loss of carbon dioxide, and polarization of the electrodes may result in serious errors, particularly in the case of waters low in dissolved solids. Furthermore, waters are often weakly buffered, which introduces possible inaccuracies in both electrometric and colorimetric methods. The glass electrode seems to be well suited to such solutions. Burton, Matheson, and Acree (4) have shown that the pH values of very dilute buffer solutions and of distilled water obtained by its use check closely with values obtained by the isohydric colorimetric method. When first introduced, the cost of the necessary apparatus and the exacting technique required tended to limit its widespread use. Further developments lowering the cost and simplifying the technique have resulted in its widespread use as a new and valuable tool for the analytical chemist. It has, however, certain limitations and errors. The relationship of e. m. f . to pH is linear over the pH range 1to 9 when the sodium-ion concentration does not exceed 0.1 molar. Above pH 9, however, the alkali cations, particularly sodium and lithium, exert errors which become very large a t high pH values. Asymmetry potentials must be guarded against by frequent calibration of individual electrodes. I n highly unbuffered solutions the solubility of the glass of the electrode itself is sufficient to introduce errors, From the standpoints of both speed and ease of manipulation the quinhydrone electrode is well adapted to the determination of the p H value of natural and treated waters. However, the rapidity with which equilibrium potentials are attained and their ready reproducibility may inspire a feeling of confidence in results very seriously in error. Although this electrode has been investigated by many workers, results obtained by the authors have indicated certain factors in connection with its use with waters and other weakly buffered solutions which merit additional study. The order of precision is limited to that which may be obtained by the average careful worker reasonably familiar with the theoreti-
cal principles involved. The authors have investigated (1) effect of method of preparation of quinhydrone, (2) usefulness of quinhydrone reference electrodes, and (3) effect of buffer capacity of waters or diluted buffer solutions a t various p H values.
Experimental POTENTIOMETER ASSEMBLY.A Leeds & Northrup studenttype potentiometer was used in conjunction with a sensitive lamp and scale galvanometer. Readings were accurate to 0.1 millivolt. Standard cells were checked at frequent intervals, and the entire assembly was checked daily, using freshly prepared 0.05 molar acid potassium phthalate solution. QUINHYDRONE ELECTRODES. These were prepared by fusing pieces of platinum foil 1 cm. square onto 5-cm. lengths of platinum wire which in turn were sealed in glass tubes, using a special sealing-in glass. Electrical contact was made in the usual manner by filling the glass tube with mercury. The electrodes were cleaned twice daily in hot dichromate cleaning mixture. They were then immersed for 3 minutes in boiling 10 per cent potassium bisulfite solution, after which they were thoroughly washed with distilled water and ignited t o redness in the flame of an alcohol lamp before each determination. Electrodes so treated are very sensitive and yield results capable of du lication with great exactness. Since the painstaking work of borgan, Lammert, and Campbell (8) has shown that exceedingly minute cracks may produce wide variations in potential, all determinations were made using either duplicate or triplicate electrodes which gave readings differing by not more than 0.02 H unit. ‘[ISOHYDRIC” INDICATOR SOLUTIONS.iolutions of bromophenol blue, bromocresol green, chlorophenol red, bromocresol purple, bromothymol blue, phenol red, cresol red, and thymol blue were prepared and adjusted according t o the method of Acree and Fawcett ( 1 ) . Five solutions of each indicator were used, adjusted in steps of 0.3 pH unit over its useful range. They were stored in Pyrex bottles provided with Pyrex droppers. BUFFERCOLORSTANDARDS. All buffer color standards were prepared from Clark and Lubs buffer solutions whose pH values were carefully adjusted using the hydrogen electrode. COLORCOMPARATORS. Two LaMotte Roulette comparators were used for all colorimetric determinations. With the special lighting conditions employed it was found possible to hake readings t o 0.05 pH unit. The technique developed by Acree and Fawcett (1)was used throughout. Because of possible loss of small amounts of carbon dioxide from some waters and as salt corrections were not applied, it is probable that the accuracy did not exceed 0.1 pH unit.
