INDUSTRIAL A N D ENGINEERING CHEMISTRY
52
Vol. 18, No. 1
Potassium Xanthate as a Soil Fumigant’ By E. R. deOng UNIVERSITY
OF
CALIFORNIA, BERKELEY, CALIP.
Potassium xanthate changes in an acid medium to xanthic acid, which at moderate temperatures decomposes into carbon disulfide and alcohol. Sodium xanthate, a slightly cheaper compound, gives a similar reaction. It has been found that potassium xanthate on account of its solubility in water may be distributed in the soil at any convenient depth and at any concentration. This makes possible the treatment of soils at lower concentrations of carbon disulfide than is possible where the latter is used pure. By this means a uniform dosage may be distributed through large bodies of soil and with less danger to the living plant.
*.
.. . ,
T
,HE killing of injurious insects and plant-infesting nematodes that live in the soil is a serious problem, upon which little progress has been made. Flooding has been practiced to a limited extent, but is restricted by the supply of water, type of soil, and resistance of the insect. Heat sterilization is very effective, but restricted to small areas. Of the chemicals used for soil treatment, calcium cyanideZhas been found successful for many purposes, and is apparently superior to potassium and sodium cyanide. The two latter compounds evolve hydrocyanic acid gas only in the presence of an acid, but the gas itself is subject to adsorption, particularly by clay, to such an extent that slight diffusion is ~ e c u r e d . ~Paradichlorobenzene is now being used generally around trees, but not for acreage treatment. For general soil fumigation work carbon disulfide, alone or in the form of an emulsion4 or as a salt, has been found most satisfactory. The French, while fighting to save their vineyards in the Phylloxera campaign of 1870 to 1885, studied the action of potassium xanthate (KS2COCB6) and sulfocarbonate of potash. Both of these materials proved effective and the latter salt is now being used to a certain extent in France, especially in vineyards of great value, as the vine is very tolerant to this salt. Potassium xanthate may be used similarly to potassium sulfocarbonate or carbon disulfide. Its stability when exposed to the air gave it some superiority over the sulfocarbonate compound, but its use did not become general on account of its high costa6 Improvements in the preparation of this material by the Great Western Electro-Chemical Company of San Francisco have so reduced the cost of manufacture as to place it in the realm of agricultural poisons. This company has been sufficiently convinced of its value as an insecticide as to finance this investigation to date. Properties of Xanthate
The xanthate is a compound of alcohol, carbon disulfide, and an alkali, either potassium or sodium being commonly used. With the addition of sulfuric or hydrochloric acid, xanthic acid (CsH,OSz) is formed-a colorless, heavy, volatile 1 Presented as a part of the Insecticide and Fungicide Symposium before the joint session of the Divisions 01 Agricultural and Food Chemistry and Biological Chemistry at the 70th Meeting of the American Chemical Society. Los Angeles, Calif., August 3 to 8 , 1925. Moore, J. Econ. Enlomol., 17, 104 (1924). 8 deOng, J. Agr. Research, 11, 421 (1917). 4 Leach, Fleming, and Johnson, J. Econ. Enfomol., 17, 361 (1924). 6 Bourcart, “Insecticides, Fungicides, and Weedkillers,” 1918, p. 130.
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Combining potassium xanthate with varying proportions of calcium phosphate, ferric nitrate, or sulfur will give a variable rate of release of xanthic acid, depending on the proportion of the chemicals used and the soil medium. (Citric and tartaric acid were tried, but the cost was prohibitive.) This offers the possibility of an immediate fumigating action followed by a slow release over a period of days or possibly weeks, with varying concentrations of carbon disulfide. This is now being tried as a means of controlling the root knot nematode which in its encysted form is very difficult to kill with chemicals.
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liquid, which decomposes readily into alcohol and carbon disulfide. The formation of the latter substance produces the fumigating effect. The xanthates and xanthic acid do not seem in themselves to be more efficient as fumigants than carbon disulfide alone. The superiority of the xanthates lies in their solubility in water, making possible any degree of dilution; stability when exposed to air; safety to the operator, for although inflammable, they are nonexplosive; and the possibility of a graduated release of carbon disulfide by the addition of slow-acting materials such as sulfur or calcium phosphate. I n this way a portion of the xanthate may be changed to xanthic acid with a consequent quick decomposition to carbon disulfide, and the remainder changed over slowly, thus giving a continuous fumigation action over a period of days, or possibly weeks if so desired. The gas from carbon disulfide or cyanide is released over a period of a few hours or at most, days, and if further fumigation is desired a second treatment is necessary. The prolonged action with potassium xanthate offers the possibility of killing nematode or insect larvae which hatch from eggs long after the material has been applied. Such action would be especially desirable on the root knot nematode [Caconema (Heterodera) radicicola ], since the encysted females buried in fleshy roots or the potato tuber are difficult to reach with chemicals. Some data have been gathered on the value of prolonged exposures, and further experiments are now being made to determine the value of such treatment. The solubility of potassium xanthate, shown in Figure 1 , 6 is a property of considerable value for soil fumigation. It makes dilution possible to any desired degree and offers the means of carrying the fumigant as deep in the soil as insects or nematodes live without the expense of disturbing the soil or making holes for injecting the chemical. Two methods of application have been followed-dissolving the xanthate in irrigating water and applying this in the desired amount, or broadcasting it and plowing it under. Where the latter plan is followed it is best to irrigate soon after the application unless the soil is quite moist. An excessive amount of soil moisture is not desirable, as it hinders the diffusion of the gas very much as in the case of carbon disulfide fumigation. I n treating small plots it is possible to distribute the material in shallow furrows in which water is afterwards run, but this will lead to inaccuracy in dosage if the water runs through a long furrow. 6 Data contributed by the research laboratory of the Great Western Electro-Chemical Company.
INDUSTRIAL AND ENGINEERING CHEMISTRY
January, 1926
Potassium xanthate begins decomposing at about 20" C., the rate below this being very slow. This has been checked in field experiments where a t a range of soil temperature of 7' to 10" C. the'salt remained almost unchanged for weeks with a correspondingly low rate of toxicity. I n these experiments 400 cc. of moist sand were placed in a flower pot, the dry xanthate mixed with the sand or the solution poured over it, and then a hundred ladybird beetles (Hippodamia conuergens Guerin) were placed in each pot. The percentage of mortality was determined after 24 hours' exposure. With a n exposure of 96 hours 100 per cent mortality was secured in almost all cases, except a t 0" C. and at 0.1 per cent dosage. The amount of xanthate used corresponded roughly to 600 pounds per acre. The concentration of the solutions was by weight (5 grams per liter, etc.) and when applied dry a corresponding amount was used. Table I shows the action of potassium xanthate at different temperatures. At 0' C. the mortality dropped to almost nothing a t some concentrations, but increased rapidly in direct proportion to the rising temperature. Field experiments also showed a slow rate of decomposition. Heavy dosages of 1200 to 1500 pounds per acre applied in August at a soil temperature of 32" C. gave a distinct odor of xanthate 5 months afterward. of Potassium Xanthate as a Soil Fumigant at Different Temperatures (x indicates more than 47 per cent; exposure 24 hours)
Table I-Effect
-12OC. O O C . 0 . 5 % solution . . 0 0 . 3 % solution . 0.1% solution Check ,. 0 0 . 5 % dry 0 0 0.3% dry X 0.1% dry 0 Check 0 0
. ..
Per cent Dead l l a C . 2 O o C . 30'C. 0
-
37OC.
0 0 X
X
0
Neutralization with Sulfuric Acid
The first series of experiments was with potassium xanthate alone without attempting to hasten the rate of decomposition. But since the change from xanthate to xanthic acid is favored by an acid medium, a series of experiments similar to those already conducted was begun using neutralizing acids with potassium xanthate. Fumigation tests in tight containers showed that beetles confined in close proximity to potassium xanthate alone would live almost as long as those in the control, but with the addition of the theoretical amount of sulfuric acid necessary for neutralization a mortality similar to that of carbon disulfide resulted and in the same length of time; conversely, the addition of a n alkali in soil fumigation would reduce the toxicity of potassium xanthate from 20 to 35 per cent. Since many soils are decidedly alkaline, the results would be similar to those where an alkali had been mixed with the xanthate. The curve of the rate of mortality (Figure 2) is seen to follow closely that of partial and complete neutralization with sulfuric acid. The test was made in pure sand, using 0.187 gram of potassium xanthate to 400 cc. of sand. The xanthate in solution was combined with the theoretical amount of sulfuric acid and then added immediately to the sand in which the beetles were buried, the exposure being for 24 hours, except in the first column marked "72-hour exposure." This longer exposure was given as being more typical of field conditions. This experiment included five duplicate sets of the chemical treatments-in the first two, the beetles were placed in the treated soil immediately after adding the xanthate and acid; the remaining three were exposed to the air for 3, 6, and 12 days, respectively, before placing the beetles in the soil, where they remained for 24 hours. Reading from left to right, the first series is the record of beetles
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placed in the sand, when the chemicals were added, and exposed for 72 hours; the second, a t 24 hours' exposure; and the third, fourth, and fifth varying intervals of time before the beetles were subjected to the chemicals. A very striking contrast will be noted between the curve of xanthate used alone and where it is completely neutralized. The latter curve also shows that the fumigating action was almost exhausted a t the end of 6 days-the dry, porous sand releasing the gas almost as rapidly as it was generated, the same effect as from carbon disulfide fumigation. An increase in mortality for the 3-day exposure was noted in all instances of partial neutraliaation. This would indicate an accumulation of gas in the soil, for this period of time, resulting from a delayed or partial neutralization. This has been dissipated a t the end of 6 days, but in a heavy or wet soil it would persist much longer. A much higher mortality would have been shown in the last four series if exposed for 3 or 4 days. These curves show variations which are due largely to the degrees of resistance of the individual beetle. This was overcome to some extent by using about fifty beetles for each test, but larger numbers would still further obviate this error. Neutralization with Acid Salts
The use of a liquid form of acid as the neutralizing medium was quite satisfactory in laboratory work where the chemicals were combined in the pure form, but for soil tests, particularly field trials, i t had many disadvantages. If the xanthate is dissolved in irrigating water i t is a simple matter to add an acid solution, but it is frequently necessary to apply the xanthate in a dry form, either broadcasting and plowing in or by means of a fertilizer drill. I n applications of dry potassium xanthate to the soil it is difficult to determine the amount of sulfuric or hydrochloric acid to use for neutralization, for unless a large excess is present the alkalinity of the soil will consume much of the acid before the xanthate is even dissolved. To overcome this difficulty recourse was had to dry organic acids, including citric and tartaric acids. These, 1250,
Figure I-SpedEc
Gravity of Potassium Xanthate in Water
when pulverized and mixed thoroughly with potassium xanthate, caused rapid decomposition, but the cost of such acids is prohibitive. Sodium diacid phosphate (NaHQPOd), ferric nitrate [Fe(NO&], the commercial form of superphosphate (CaHPOd), which is used as a fertilizer, and sulfur were then tried, mixed in the powdered form with the xanthate, using equal weights of the latter and the combining chemical. Each of the combinations gave decidedly greater efficiency than the xanthate alone, as will be seen from Figure 3. The sodium phosphate varied in its effectiveness, but considering its cost was inferior to calcium phosphate, and since the latter can be bought anywhere and at a very reasonable price i t seems to be one of the most desirable. The combination of phosphate and potash is also desirable from the standpoint of fertilizing value. Ferric nitrate proved very desirable and also offers the advantage of a nitrogenous fertilizer. It is
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Vol. 18, No. 1
INDUSTRIAL AND ENGINEERING CHEMISTRY
Figure 2-Comparative Toxicity of Potassium X a n t h a t e Alone a n d w i t h Varying Degrees of Neutralization w i t h Sulf uric Acid
Figure 3-Comparative Toxicity of Potassium X a n t h a t e Alone a n d i n Combination w i t h Other Reagents
the neutralized potassium xanthate stood for one hour before exposing the beetles, while but little difference was seen between 3 and 20 exposures, indicating that the greater part is lost within 3 hours. It will be noted also that the results from potassium xanthate alone were almost identical to those of the control. Longer exposure will, however, give better results from xanthate alone. The numbers listed as “very feeble” would usually be killed in actual practice, as they show motion only by heat stimulation. Under this test all beetles showing any motion are counted alive.
more expensive than the calcium phosphate and difficult to secure in quantity. There was also some evidence of the inhibition of plant growth where it was used. Sulfur has proved quite desirable, especially in combination with the quicker acting phosphates. This seems to offer the greatest possibility of any combination tried for a regulated release of xanthic acid over a long period of time. By this means a rather quick decomposition into carbon disulfide can be secured from part of the xanthate for an immediate fumigating action, followed by the slower action of the sulfur giving a release of carbon disulfide over a long period of time. Further work is being done along this line to determine the practicability of this theory. The use of sulfur as a neutralizing medium would probably depend on the rate of conversion of sulfur into sulfuric acid which varies in different soils, but would usually extend over a period of weeks or months. The tests with dry neutralizing chemicals shown in Figure 3 were conducted the same as those with sulfuric acid, 0.187 gram of potassium xanthate being used per 400 cc. of sand, and an equal weight of the combining chemical. A remarkable similarity will be noted, particularly in the rise in mortality for the third series-another example of a delayed generation of gas.
of Xanthic Acid as Shown b y Its Toxicity t o Beetles (Temperature range, 10’ to 24’ C.;exposure 24 hours) Potassium Time of -CONDITION OF BEETLES-xanthate standing AcVery Per cent Hoursb tive Feeble feeble Dead dead Expt. useda 1 Alone 0 27 .. .. 1 3.5 2 Plus HzSOt to 0 . .. 3 29 90.5 neutralize 3 0.5 1 1 46 95.9 4 1 18 30 62.4 5 2 2 20 7 24.1 6 3 1 21 5 18.5 7 20 1.5 4 7 2 7.1 Check .. 32 ,. 1 3.2 8 0.15 gram potassium xanthate in solution, total 4 cc., plus 1 cc N HsSOa in all but (1). b Beetles added after these time intervals.
Volatility of Xanthic Acid on Standing
Effect on Nematodes
A rapid loss of xanthic acid has been noted when potassium xanthate is combined with any acid. To determine the rate of loss, a bio-assay was made, as there are no accurate means of determining minute quantities of this volatile material. The tests were in the nature of a fumigation experiment using glass cylinders of 300 cc. capacity, tightly corked, and with the test insects (ladybird beetles, Hippodamia convergens) exposed in the tube to the gas as evolved. The xanthate was used in an acid solution except in Expt. 1. A decided reduction of efficiency was noted (Table 11) where
Field and laboratory experiments on the action of xanthate on the root knot nematode, Caconema radicicola, are still in progress. Soil counts of living nematodes in treated plots have always shown a less number than the checks, as do counts of infested plants in the different plots. However, the earlier experiments were with xanthate alone, the use of neutralizing substances being tried only recently. Laboratory tests of xanthate solutions alone and neutralized have all shown the superiority of the latter plan both in the killing of larvae and in preventing the eggs from hatching.
Table 11-Volatility
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INDUSTRIAL AND ENGINEERING CHEMISTRY
January, 1926
Effect on Plants
Much work has been done to determine the danger of potassium xanthate inhibiting the germination of seeds. Retardation of the growth of seedlings has been noted for over a month, after the treatment of the soil where the xanthate was used alone. This length of time may be greatly reduced, however, if the material is neutralized to the point where xanthic acid is given off immediately. But where incomplete or partial neutralization is accomplished a correspondingly longer time must elapse between fumigation and planting. Large quantities of undecomposed xanthate in the soil will
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inhibit germination of seeds and retard the growth of young plants. At the present state of experimentation it seems best to incorporate with the xanthate a t the time of application, calcium phosphate, sulfur, or other materials which will decompose it and thus lessen the time of danger to growing crops. There have been no bad results noted to citrus or dormant deciduous trees in any plots. Applications of 600 to 1000 pounds of potassium xanthate per acre on bearing lemon trees and 600 to 2400 pounds on nursery orange trees caused no injury. Similar treatment on dormant peach trees has also been found safe.
Examination of Some -Methodsfor the Determination of Sulfites and of Ferrous Iron' By Walter D. Bonner and Don M. Yost UNIVERSITY OF UTAH SALT LAKS CITY, U l A H
URING the progress of some work in this laboratory it became necessary to determine accurately sulfurous acid in the presence of ferric and ferrous iron. This problem has proved a stubborn one, and has not yet reached a solution. However, it led to a careful consideration of the various volumetric methods available for determining sulfites and iron, with the discovery that, although the analysis of sulfites has received considerable attention, there is nowhere available a critical comparison of the various methods. Moreover, i t was not possible, from the published data, to make such a comparison. The writers have therefore made the following examination of volumetric sulfite determinations, and have added to i t a short discussion of two unusual reagents for ferrous iron.
D
Sulfite De terminations Since solutions of sulfites or of sulfurous acid are unstable, the procedure of Ferguson2 was adopted. which consists in adding weighed quantities of a finely pulverized sample of the sulfite to the solutions containing the oxidizing agent. I n all cases a weighed sample of sodium sulfite was added to a known excess of oxidizing agent in an Erlenmeyer or glassstoppered flask. After the reaction was complete, solid potassium iodide was dissolved therein and the whole allowed to stand for about 10 minutes. The liberated iodine was then titrated w%h 0.1 N thiosulfate. The strength of the known volume of oxidizing agent was determined in the same manner, being recorded in cubic centimeters of thiosulfate. I n making these comparisons the iodine method is assumed to give accurate results, and the merit of any other method is judged by its agreement with the iodine method. Bureau of Standards burets were used throughout. Comparison of Oxidizing Agents
STANDARD IODINE SOLUTION-Twenty-five cubic centimeters of potassium triiodide solution were pipetted into a 250-cc. Erlenmeyer flask, and followed by 2 oc. of 6 N sulfuric acid and 50 CC. of water. To the resulting solution 0.1260 gram of sodium sulfite was added, this amount being used in all subsequent experiments. The contents of the flask were shaken until the salt was completely dissolved, and 1
Received July 6, 1925. J . A m . Chem. SOC.,39, 372
(1917).
after 2 to 5 minutes the excess iodine was titrated with approximately 0 1 N thiosulfate. The average of seven titrations gave 18.80 cc. * 0.05 as the thiosulfate corresponding to 0.1260 gram of sodium sulfite. The effect of acid on the titrations is very marked. Omitting the 2.0 cc. of 6 N sulfuric acid lowers the thiosulfate value from 18.80 to 18.65 cc. I n order to be certain that the assumption of accuracy for the iodine method is justifiable, the foregoing results were compared with a set of gravimetric determinations. 0.1260 gram of the same sodium sulfite was oxidized with the same potassium triiodide solution, and weighed as barium sulfate. The average of five determinations, after correcting for sulfate already present in the sulfite, was 0.2135 * 0.0003 gram barium sulfate, corresponding to 0.0586 gram sulfur dioxide in 0.1260 gram sodium sulfite. The weight of sulfur dioxide in the Fame weight of sodium sulfite was, by the above volumetric determination, 0.0587 gram. The thiosulfate solution was standardized by liberating iodine from a neutral solution of potassium iodide and iodate and by means of standard acid. During these gravimetric determinations i t was noticed that the barium sulfate settled very quickly in the solutions containing iodine, and was ready for filtering in less than half the time needed for those solutions containing no iodine. It has since been the practice of this laboratory to add a small amount of iodine solution to barium sulfate precipitates, with quite uniform success in facilitating the settling and filtering. POTASSIUM PERMANGANATE-It is well known that the reaction between potassium permanganate and sulfurous acid gives rise to dithionic acid in varying amounts, depending upon experimental conditions. As a consequence a volumetric sulfite determination using permanganate always gives a low result. The average of a number of concordant titrations gave 17.23 cc. of thiosulfate corresponding to 0.1260 gram of sulfite. Since permanganate oxidizes sulfite to both sulfate and dithionate, it seemed possible that increasing the permanganate concentration might decrease the experimental error. It was found, however, that a sevenfold increase in permanganate concentration did not alter the experimental result. POTASSIUM DICHROMATE-since the completion of this work, the paper by Hendrixson3 has appeared, making i t a J . A m . Chern. SOC.,47, 1319 (1925).
