IL91674A - Methods for producing thiocarbonates and thiocarbonate solutions, stabilized thiocarbonate aqueous solutions and methods for controlling pests utilizing the same - Google Patents

Description

91674/2 METHODS FOR PRODUCING THIOCARBONATES AND THIOCARBONATE SOLUTIONS, STABILIZED THIOCARBONATE AQUEOUS SOLUTIONS AND METHODS FOR CONTROLLING PESTS UTILIZING THE SAME m>n>>o nio>nn ,o>DN_n.npiN>n bv> nio'on *m»»¾i οϊυκη.ηρ'ΐΝ'η -n >> tno>.j im mu/nniynn D>pin mp:.i> πΐϋ'ϊπ .KTD DKj iptN>n by - 1 - 91674/2 TECHNICAL FIELD This invention relates to the field of stabilized thiocarbonate compositions and, in particular, to stabilized, aqueous thiocarbonate solutions and to methods of using such compositions.

INTRODUCTION Among the more economically serious plant parasites are nematodes, which are roundworms, comprising as many as 10,000 species, of which at least 150 are known to adversely affect plant life. Plant parasitic nematodes have been known since about the year 1750. Most of the nematodes which cause crop damage do so by feeding on plant roots, and therefore are found primarily in the upper few inches of soil in the roots or in close proximity to the roots.

Nematode feeding causes hypertrophy or gall formation, and the evidence of heavy infestation is plant stunting, pale foliage, wilting, and even plant death in extreme cases.

Virtually all of the world's crops and ornamental plants can be attacked by parasitic nematodes. Important destructive nematode species include the root knot nematodes which are hosted by tomatoes, alfalfa, cotton, corn, potatoes, citrus and many other crops, the golden nematode of potatoes, the sugar beet cyst nematode and the citrus nematode. These, and a few other species, are described in "The soil Pest Complex", Agricultural and Food Chemistry, Vol. 3, pages 202-205 (1955). Also described therein is a further complication resulting from nematode infestation, namely a lowered resistance to the effects of plant attack by bacteria and pathogenic soil fungi.

Except for small volumes of soil which can be sterilized, it has not been found possible to eliminate nematodes. Parasite populations can, however, be kept at A82272EP.APA -1- - 2 - 91674/2 levels which economically permit agricultural operations by soil fumigation, crop rotation using non-hosting plant varieties, and (to a much lesser extent) the development of plants which are resistant to infestation. In many instances, control of nematodes is achieved only by combinations of these techniques, and most control programs have proven quite costly.

Another serious problem in agriculture is the attack of plants by pathogenic microorganisms, particularly fungi. Such pathogens are normally controlled by fumigation, prior to crop planting, using broad spectrum biocides, many of which are no longer regarded as environmentally safe. Certain narrow spectrum fungicides are available, but are extremely expensive and lose effectiveness against successive generations of fungi, due to genetic adaptability.

Carbon disulfide is the first reported soil fumigant, used in Europe during the 1870's to control the sugar beet nematode. This agent is commercially impractical, however, since very large quantities must be applied, due to its high volatility. Further, the material is quite flammable, reportedly being ignited even by static electricity resulting from pouring the material out of drums. In addition, carbon disulfide possesses a very objectionable odor, and its vapors are toxic to humans.

When sold for fumigant use, the carbon disulfide is normally mixed with an inert fire retarding compound, such as carbon tetrachloride, and occasionally also with another fumigant. Typically, these compositions do not contain over about 20 percent by weight of carbon disulfide.

In addition to soil uses, carbon disulfide has been proven effective in the fumigation of commodities, as an insecticide, as a rodenticide, and for controlling certain weeds. > - 3 - 91674/2 Numerous compositions possessing nematocidal properties have been developed, including active ingredients such as the polyamlnes of U.S. Patent 2,979,434 to Santmyer, the heterocyclic compounds of U.S. Patent 2,086,907 to Hessel, and various halogenated compounds. Among the useful halogen-containing nematocides are 1,2-dibromoethane, methyl bromide, 3-bromopropyne, 1,2-dichloropropane, ethylene dichloride and others, all of which are quite phytotoxic, therefore restricting their utility to mostly preplant treatments .

One compound which enjoyed considerable commercial success is 1 , 2-dibromo-3-chloropropane (DBCP), which can be used to control nematodes in soils with growing perennial plants. However, use of this material has been limited due to a finding of undesirable reproductive system effects in workers exposed to the chemical, and the possibility that the compound is a carcinogen. The unavailability of DBCP has been a serious setback to growers of perennial crops, such as grapes, stone fruits and nuts, since these crops experience more severe cumulative nematode population increases, and most replacement soil fumigants are phytotoxic. U.S. patents concerned with the use of DBCP as a soil fumigant include 2,937,936 to Schmidt and 3,049,472 to Swezey.

A further class of materials used to control nematodes includes some thiocarbonates . U.S. Patent 2,676,129 to Bashour describes the preparation of lower aliphatic disubsti uted trithiocarbonates having the structure as in (1): SR, S = C (1) SR, wherein R, and R~ are alkyl radicals having from three to - 4 - 91674/2 nine carbon atoms. The compounds were dissolved in acetone and added to nematode-infested coils, resulting in control of the nematodes.

Other compounds have been reported by Seifter in U.S. Patents 2,836,532 and 2,836,533, the former relating to the use of sodium and potassium tr ithiocarbonate, and the latter pertaining to alkali metal and ammonium salts of tetrathioperoxycarbonic acid. Both are described as effective in nematode control. These references state that "not all carbon disulfide derivatives are effective nematode toxicants." Furthermore, U.S. Paten 2,636,532 points out that sodium tr ithiocarbonate is unexpectedly superior to potassium tr ithiocarbonate as a nematocide.

The chemistry of thiocarbonic acids and salts has been studied in some detail, as indicated in the papers by O'Donoghue and Kahan, Journal of the Chemical Society. Vol. 89 (II), pages 1812-1818 (1906); Yeoman, Journal of the Chemical Society. Vol. 119, pages 38-54 (1921); and Mills and Robinson, Journal of the Chemical Society. Vol. 1928 (II), pages 2326-2332 (1928). According to O'Donoghue and Kahan, derivatives of thiocarbonic acid were prepared by Berzelius, who reacted aqueous solutions of hydrosul ides with carbon disulfide, the reactions occurring as in (2): 2 KHS + CS2 *- K2CS3 + H2S (2) giving unstable solutions which yielded unstable crystalline salts.

Other thiocarbonates were prepared and further characterized by O'Donoghue and Kahan. Their paper, at page 1818, reports the formation of ammonium thiocarbonate by reacting liquid ammonia with cold alcoholic thiocarbonic acid, prepared by dropping a solution of "calcium thiocarbonate" into concentrated hydrochloric acid. The "calcium thiocarbonate" utilized by the authors is described - 5 - 91674/2 as a double salt, including the calcium cation in combination with both hydroxide and tr ithiocarbonate anions.

The noted paper by Yeoman reports the further study of thiocarbonates (called tr ithiocarbonates therein) and also reports the preparation and properties of perthiocarbonates (or tetrathiocarbonates ) , derivatives of tetrathiocarbonic acid, HjCS^. Yeoman prepared ammonium tr ithiocarbonate by saturating an alcoholic ammonia solution with hydrogen sulfide, and then adding carbon disulfide; dry ether was added to precipitate the product salt. Ammonium perthiocarbonate was prepared in a similar manner, except that after reacting the ammonia and hydrogen sulfide, elemental sulfur was added to form the disulfide, (NH4)2S2; adding carbon disulfide immediately precipitated the product.

Yeoman states that "solutions of both ammonium tr ithiocarbonate and perthiocarbonate are very unstable" due to both decomposition to form thiocyanate as a product, and to "complete dissociation into ammonia, hydrogen sulfide, and carbon disulfide." Considerable explanation is provided concerning the stability of thiocarbonates, as exemplified by sodium tr ithiocarbonate and perthiocarbonate. Sodium tr ithiocarbonate solutions in water are said to remain stable only if oxygen and carbon dioxide are "rigidly excluded"; the presence of oxygen causes decomposition to form carbon disulfide and thiosulfates , while carbon dioxide decomposes the solution to give a carbonate and carbon disulfide. Similarly, solutions of sodium perthiocarbonate are reported to be stable for a considerable time in the absence of oxygen, the presence of air causing decomposi ion into thiosulfate and carbon disulfide, while carbon dioxide decomposes the compound to form a carbonate, elemental sulfur, carbon disulfide, and hydrogen sulfide. The potassium thiocarbonates behave similarly, according to Yeoman. - 6 - 91674/2 Yeoman also attempted to prepare and characterize the stability of thiocarbonate salts of four of the alkaline earth metals. Yeoman was unable to prepare a "pure" calcium tri- or tetrathiocarbonate, but observed that the double salt of calcium tr ithiocarbonate that he prepared was more stable (probably because it was less hygroscopic) than the sodium or potassium thiocarbonates. The barium tetrathiocarbonate could not be isolated, although Yeoman believed that it existed in solution. Barium tr ithiocarbonate was found to be stable, although it was alleged to behave like sodium tr ithiocarbonate when dissolved in water. The preparation of aqueous solutions of the tri- and tetrathiocarbonate of magnesium and strontium was alleged, but the magnesium thiocarbonates were not characterized. However, the stability of none of the magnesium or strontium salts or solutions was determined.

The previously noted paper by Mills and Robinson discusses the preparation of ammonium thiocarbonate by digesting ammonium pentasulfide (obtained by suspending sulfur in aqueous ammonia, then saturating with hydrogen sulfide) with carbon disulfide. A crystalline residue from this digestion was found to be ammonium per thiocarbonate.

These authors prepared a "better" ammonium perthiocarbonate product, however, by extracting the ammonium pentasulfide with carbon disulfide in a Soxhlet apparatus.

Another serious problem in agriculture is that of low nitrogen use-efficiency, since crops have been found to recover only 30 to 70 percent of the total amount of expensive fertilizer nitrogen which is applied to the soil.

Most of the lost nitrogen is due to nitrite and nitrate ions, which are exceptionally mobile in a soil environment, and therefore are readily lost by surface runoff and also by leaching from the plant root zone into deeper soil. Other losses of these ions are due to denitr i ication, which is reduction to elemental nitrogen or gaseous nitrogen oxides - 7 - 91674/2 under conditions of limited aeration. In addition to the direct economic losses, these nitrogen forms constitute environmental pollutants when runoff enters surface and ground water systems.

Although some nitrogen is applied to soil in the form of nitrate (e.g., ammonium nitrate-containing fertilizers), most nitrogen fertilization is with ammonia, ammonium compounds other than nitrate and urea materials.

Ammonium nitrogen is fairly tightly bound by various physical and chemical processes in a soil environment and, therefore, is much less subject to losses. Unfortunately, the bound ammonium nitrogen is also less available to plants .

The process of nitrification results in conversion of ammonium ions into nitrate Ions. Microbial species known as nltrosomonas oxidize ammonium to nitrate; nitrobacter species oxidize nitrite to nitrate. This more mobile ion is easily taken up by plant roots and is also readily assimilated by plant. In this regard, the nitrification process is desirable, but control of the rate at which conversion occurs has not been easily obtained. Inhibition of nitrification would tend to make the applied nitrogen available to plants over a longer period of time, resulting in an increased plant uptake efficiency.

Various compositions have been offered as inhibitors of nitrification, including expensive organic materials such as 2-chloro-6-( tr ichloromethyl )-pyr idine, 2-amlno-4-chloro-6-methyl-pyr imidine, sul athiazoles , alkanoylsulfathiazoles, and others. A paper by J. M.

Bremner and L. G. Bundy in Soil Biology and Biochemistry.

Vol. 6, pages 161-165 (1974) describes the efficacy of various volatile organic sulfur compounds, including methyl mercaptan, dimethyl sulfide, dimethyl disulfide, carbon disulfide, and hydrogen sulfide. Carbon disulfide in very small amounts is described as having "a remarkable 8 - 91674/2 inhibitory effect on nitrification of ammonium in soils incubated in closed systems." Carbon disulfide was tested in the field by J. Ashworth et ai., Chemistry and Industry, September 6, 1975/ pages 749-750, and found to be effective as a nitri ication inhibitor. Hawkins, in U.S. Patent 4,078,912, describes the use of sodium, potassium and ammonium tr i thiocarbonates , and of xanthates, either alone or in fertilizer mixtures, to inhibit nitrification; the mode of operation is attributed to a release of carbon disulfide by the compounds.

One additional potential problem, which could be presented to the agricultural industry in the very near future, is the loss of the widely used, effective fumigant, 1 , 2-dibromoethane, i.e. ethylene dibromide (EDB), due to environmental concerns. This agent is approved for use on the same crops as is carbon disulfide, and is additionally used extensively in chambers for fumigating fruits and vegetables to control various insects.

In view of the above, it is clear that the chemical behavior of the alkaline earth metal thiocarbonate salts is unpredictable. Moreover, it is clear that there is no method taught in the art for preparing either the trithio- or tetrathio-salt of calcium.

While aqueous solutions of thiocarbonates provide a method for delivering and using equivalent carbon disulfide in a much safer form than is the use of carbon disulfide, per se, both the dilute and concentrated, aqueous thiocarbonate solutions have significant carbon disulfide vapor pressures with the consequence that significant CS2 concentrations can occur in the equilibrium vapor space overlying such solutions. For instance, we have found that a stoichiometric, aqueous solution of sodium tr ithiocarbonate in a concentration corresponding to 12.9 weight percent equivalent carbon disulfide has a CS2 vapor pressure corresponding to an equilibrium CS concentration in the vapor phase of 27 volume percent at 24 °C. the somewhat more stable stoichiometric, aqueous solution of sodium tetrathiocarbonate, also containing about 12.9 weight percent equivalent CS2, has a vapor pressure corresponding to approximately 14 volume percent CS2 in the vapor phase overlying the solution at 2 °C. Such compositions can be very hazardous, particularly in view of the fact that the explosive range of carbon disulfide in air is from 1 to 50 volume percent; i.e. an air-CS2 mixture having a CS2 concentration between 1 and 50 volume percent is explosive. In addition, CS2 is very toxic, and the presence of such high volumes of CS2 in the vapor phase results in significant loss of active, equivalent CS2 in the aqueous solution. Significant CS2 vapor pressures also can occur over more dilute, stoichiometric thicarbonate solutions.

International Publication WO 84/04230 discloses fumigation of soil, etc., by the use of thiocarbonates (ammonium, alkali metal, alkaline earth metal) in aqueous solutions. While this document discloses (a) stabilizing ammonium thiocarbonate solutions by the presence of excess H2S or sulfur and (b) retarding the decomposition of dilute solutions of the metal thiocarbonates by increased pH, it does not suggest that solutions of the metal thiocarbonates can be stabilized by the addition of sulfides and/or polysulfides of alkali metals or alkaline earth metals. Further, this document does not disclose that alkali metal and alkaline earth metal tetrathiocarbonates of enhanced ..it stability can be obtained from solutions containing such a thiocarbonate and also a sulfide or polysulfide of alkali or alkaline earth metal.

A need exists for a fluid which can release carbon disulfide for fumigation and nitrification inhibiting purposes, but which can be stored and handled safely and without significant loss of effectiveness during a reasonable commercial storage and delivery cycle.

It is therefore an object of the present invention to provide a stabilized liquid composition which can be caused to release fumigants, including carbon disulfide.

It is a further object to provide a stabilized composition which is miscible with water to form a fumigant and nitrification inhibitor which can be applied to soils by means of fluid handling equipment or introduced into irrigation water.

Another object is a provision of concentrated and dilute, aqueous thiocarbonate solutions having reduced CS_ vapor pressures useful for industrial and agricultural applications.

Other objects and advantages of the instant invention will be apparent from a careful reading of the specification below. - 10 - 91674/4 The invention is directed to the fumigation of soils, enclosed spaces, agricultural products and other commodities, etc., using compositions which decompose to form carbon disulfide and certain other biocidal materials. Such fumigation can be used to control bacteria, fungi, insects, nematodes, rodents, and weeds, all of which are included herein in the term "pests," and it can be used to i nhibi ni rification. umigant compositions are described herein as " thiocarbonates" , including, without limitation, salts of trithiocarbonic acid and tetrathiocarbonic acid, compositions having empirical formulae intermediate to these acid salts {such as MCS3.7, wherein M is a divalent cation), and compositions containing substances in addition to thi ocarbonates . Stabilized, aqueous thiocarbonate solutions useful for industrial and agricultural applications are also provided which contain an amount of added sulfide and/or polysulfide sufficient to reduce the carbon disulfide vapor pressure of the solution, and compositions are also provided which contain combinations of added base and added sulfide and/or polysulfide. Thus, the stabilized aqueous thiocarbonate solutions of this invention involve aqueous solutions of thiocarbonates, soluble in the solution, and having the general formula AaCSto, wherein Λ is selected from the group consisting of alkali metals and alkaline earth metals; b is 3 to 4; a is 2 when A is an alkali metal, and a is 1 when Λ is an alkaline earth metal; and a base and/or a sulfide and/or polysulfide of the formula M,,S„, wherein M is selected from the group consisting of alkali metals and alkaline earth metals, and combinations thereof; x is at least 1/ n is 2 when M is an alkali metal; and n is 1 when M is an alkaline earth metal. The aqueous ~ 11 - 91674/2 solutions can comprise mixtures of tri- and tetrathiocarbonates having the same or different cations as well as mixtures of sulfides and polysulfides of the same or different cations.

The compositions are generally water soluble and can be prepared, stored, and used in aqueous solutions.

Thiocarbonate solutions of the invention are stable during prolonged periods of storage in a closed container, exhibit a low vapor pressure, and are not flammable. For soil fumigation, thiocarbonates can be mixed with fertilizers to provide a multi- unctional application.

DETAILED DESCRIPTION The process of soil fumigation requires the movement of gaseous chemicals through the soil which is treated, and the readily apparent necessity for a sufficient concentration of gas at a given temperature and pressure condition to be lethal to the pest which would be controlled. Volatility of the chemical agent is critical to successful fumigation, since a very volatile substance will disperse too readily and not develop an effective concentration except for locations very close to the point of introduction to the soil. Substances having a very low volatility are also undesirable, since they will not disperse in the soil, and will be effective only at locations near the point of introduction.

Since fumigants typically are effective against a pest only during specific phases in the life cycle of the pest, some measures must be taken to ensure that the fumigant is present during the proper phases. This requirement normally has been met by either applying highly persistent chemicals, applying large enough doses of the chemicals so that the normal decomposition, leaching, volatilization, and other processes will have a lesser effect upon pesticide concentration in the treated - 12 - 91674/2 environment, or, for highly volatile chemicals, enclosing the treated area (such as by covering soils) for sufficient time to achieve control of the pest. Unfortunately, most of the persistent chemicals are now environmentally undesirable and the noted application methods are sometimes prohibitively expensive.

Stabilized, aqueous thiocarbonate solutions are also provided which contain an amount of added base and¾¾ sulfide or polysulfide sufficient to reduce the carbon disulfide vapor pressure of the solution. Such reduction of CS2 vapor pressure results in and is reflected by a reduction in the CS2 concentration in the equilibrium vapor phase overlying the solutions. Such compositions are particularly useful in agricultural and industrial applications and in the manufacture, storage and transportation of thiocarbonate solutions in that they reduce the hazards associated with CS2 evolution and inhibit thiocarbonate decomposition and consequent CS2 release.

The term "stability", as used herein, can be regarded as a composite of two concepts: chemical stability and physical stability. Since the effectiveness of a composition depends, at least in part, upon its ability to release carbon disulfide during decomposition, chemical stability is expressed accordingly; this can be quantified by, for example, chemically decomposing the composition and measuring the amount of carbon disulfide which evolves.

Alternatively, an indication of the amount of available carbon disulfide can be obtained by spectrophotometr ically determining the presence of the thiocarbonyl bond ( =C=S) in a sample of the composition. The absorbance at wavelengths corresponding to those at which thiocarbonyl is known to absorb energy can be used for a quantitative analysis.

Symptomatic of chemical stability, but having an independent significance, is physical stability. This concept is important due to the nature of the products - 13 - 91674/2 formed during decomposition of the composition, particularly the ammonia, hydrogen sulfide, and carbon disulfide, which each have a high vapor pressure. It is readily apparent that a change in the physical form of the composition from a solution of low vapor pressure into a mixture of compounds, each possessing a high vapor pressure, imposes some rather stringent requirements upon storage containers. Vapor pressure above the composition of the invention, therefore, will be used herein as an indicator of physical stability; a condition of maintained low vapor pressure is the desired property. Another index of physical instability is the formation of undesirable insoluble precipitates, which frequently comprise sulfur, or of an immiscible liquid phase, such as carbon disulfide. The more general description of physical stability, then, is the maintenance of only a single phase in the composition.

Assessment of the stability of a particular composition must involve consideration of both the chemical stability and the physical stability over a period of time during which stability is desired. Certain formulations do not form precipitates and do not develop high vapor pressures during a reasonable storage period and, therefore, may be preferred over a formulation which has a greater chemical stability, but develops objectionable physical characteristics during storage. As a further example, a composition which is intended to be used as an additive to irrigation water is likely to be selected for its freedom from precipitate formation upon dilution; to obtain this property, a composition having a lower chemical stability could be necessary.

The useful thiocarbonates include, without limitation, salts of tr ithiocarbonic acid and tetrathiocarbonic acid, compositions having empirical formulae intermediate to these acid salts (such as MCS^, wherein M is a divalent metal ion), and compositions containing substances in addition to thiocarbonate: -G-g^abl-ll-.cd-Qmmonlum_tc.tr.a.thi.ooc &nunonlum--3tX£ dey= These compositions are generally water soluble and. can be prepared, stored, and used in aqueous solutions. The solutions are stable during prolonged periods of storage a closed container, exhibit low vapor pressure, and are flammable.

We have also found that the stability of both the concentrated and dilute thiocarbonate solutions can be markedly increased, with respect to CS2 evolution and physical stability, by the addition of a base, sulfide and/or polysulfide. Increases in stability are particularly evident in the more concentrated solutions, i.e. solutions having equivalent CS2 concentrations in excess of 1 weight percent. Aqueous solutions that can be stabilized by the addition of base, sulfide and/or polysulfide include - solutions of alkalia$dalkaline totrathiooarboaa-feos and combinations of these, and very stable alkali metal and alkaline earth metal tetrathiocarbonate solutions can be obtained. Significant stability enhancement can be achieved even in the most concentrated solutions. Thus, significant stability enhancement can be achieved in compositions having equivalent CS2 concentrations of about 1 weight percent or more or even 5 weight percent or a more equivalent CS2 up to the solubility limit of the thiocarbonate in the solution. Typically, the more concentrated solutions (as opposed to the dilute solutions employed in most agricultural practices) have thiocarbonate, concentrations corresponding to about 1 to about 20 weight percent equivalent carbon disulfide. The stability and safety of concentrates containing 10 weight percent or more equivalent CS2 can be markedly improved by these procedures.

A82272EP.APA -14- - 15 - 91674/3 Stability enhancement can be achieved by providing, in the solution, an organic or inorganic base which, preferably, is soluble in the solution, and more preferably, has significant solubility in water. Presently preferred bases include water-soluble inorganic bases, and the most preferred are alkali metal and ammonium hydroxides, and combinations of these. Alternatively, similar increases in stability can be achieved by providing, in the solution, a sulfide and/or. polysulfide which preferably is soluble in the solution, and more preferably, has significant solubility in water. Illustrative sulfides include alkali and alkaline earth metal sulfides and polysulfides having the general, empirical formula MnSx, wherein M is alkali or alkaline earth metal; x is at least about 1, preferably greater than 1 in the case of polysulfides, and usually within the range of 1 to about 5, most preferably greater than 1 to about 5; n is 2 when M is alkali metal, and n is 1 when M is an alkaline earth metal. Combinations of different sulfides and/or polysulfides can be employed. Thus, combinations of alkali and/or alkaline earth metal sulfides and/or polysulfides can be used to stabilize the thiocarbonate compositions, and combinations of the described bases, sulfides and/or polysulfides can be used to achieve further enhanced stability, and are presently preferred. Presently, the most preferred stabilized, thiocarbonate compositions contain added base in addition to one or more of the described sulfides or polysulfides .

Any amount of added sulfide or combination of base and sulfide, enhances the solution's stability. Thus, the novel compositions comprise aqueous solutions of thiocarbonates containing added base, sulfide and/or polysulfide. Generally, the amount of added base, sulfide or polysulfide will correspond to about 0.01, usually about 0.02, preferably at least about 0.04, and most preferably \-'·about 0.08 equivalent^' of base, sulfide or polysulfide per equivalent of carbon disulfide in the solution.

Concentrated/ aqueous tetrathiocarbonate solutions having CS2 vapor pressures corresponding to CS2 concentrations in the equilibrium vapor phase below about 1 volume percent at 24* C, i.e. below the explosive limit for carbon disulfide, can be achieved with base concentrations of about 0.02 equivalent of base per equivalent of carbon disulfide.

Somewhat higher base concentrations, i.e. at least about \ 0.08 equivalent^ of base per equivalent of carbon disulfide, are presently preferred for producing aqueous, tr ithiocarbonate solutions having CS2 partial pressure corresponding to about 1 volume percent or less carbon disulfide in the equilibrium vapor phase at 24* C. While significant improvements in solution stability and reductions in CS2 partial pressure can be achieved by the use of sulfides and/or polysulfides in the absence of added base, the concentration of sulfide and/or polysulfide required to achieve the desired reduction in CS2 partial pressure (and consequent increase in stability) is generally somewhat higher than the concentration of base required to achieve a similar stability improvement. Thus, in order to obtain a CS2 partial pressure corresponding to a CS2 concentration in the equilibrium vapor phase overlying the solution below 1 volume percent of 24" C, it is presently preferred to employ concentrations of sulfide and/or polysulfide of about 0.04 or more equivalent of sulfide and/or polysulfide per equivalent of carbon disulfide. As in the case of added base, greater solution stability and lower CS2 partial pressures can be achieved by using even higher concentrations of sulfides and/or polysulfides, or by employing combinations of base and sulfide and/or polysulfide. Typically, the concentration of sulfide, polysulfide or combination thereof will correspond to at least about 0.02, preferably at least about 0.04, and most A82272EP.APA -16- preferably at least about 0.08 equivalent of sulfide and/or polysulfide per equivalent of carbon disulfide. However, when combinations of base and sulfide are employed, the respective concentrations of each can be reduced by approximately 1/2 to obtain a comparable degree of stability improvement and CS2 partial pressure reduction. In other words, the degree of stability enhancement achieved by the use of 0.02 equivalent of base per equivalent of carbon disulfide, can be achieved by using approximately 0.01 equivalent of base in combination with about 0.01 equivalent of sulfide or polysulfide. The term "equivalent," as employed herein, is used in its conventional sense. Thus, one mole of carbon disulfide constitutes 2 equivalents, and the same is true for the sulfide and polysulfide and for the alkaline earth metal bases and other bases which can be employed in which the cation is divalent. However, one mole of the ammonium and alkali metal bases, wherein the cation is monovalent, constitute only 1 equivalent. Therefore, on a molar basis, as opposed to an equivalent basis, 2 moles of an alkali metal hydroxide, e.g. sodium hydroxide, are equivalent to 1 mole of carbon disulfide.

Accordingly, the amount of base, sulfide and/or polysulfide employed should be sufficient to reduce the carbon disulfide partial pressure of the solution by the desired amount, and the amount of additive required to achieve that effect can be easily determined by adding different, known quantities of base, sulfide and/or polysulfide to the desired thiocarbonate solution, confining the vapor space over the solution at 24* C. for a sufficient period of time, e.g. about 24 hours, and analyzing the vapor phase by gas chromotography for carbon disulfide. Lower additive concentrations will result in somewhat higher CS2 equilibrium concentrations (e.g. higher CS2 partial pressures), and higher additive concentrations will result in lower CS partial pressures.

A82272EP.APA -17- The most preferred compositions, presently, are those in which the carbon disulfide partial pressure has been reduced to a level corresponding to about 1 volume percent or less carbon disulfide in the equilibrium vapor phase at 24* C. A greater safety factor, with regard to CS2 partial pressure, toxicity, handling difficulty, etc., can be realized by reducing CS2 partial pressure even further. Thus, more preferred thiocarbonate solutions are those in which the carbon disulfide partial pressure corresponds to less than about 0.5, most preferably less than about 0.2 volume percent carbon disulfide in the equilibrium vapor phase overlying the solution at 24* C. - 19 - 91674/2 —Te at-ive—amounts—o—the—var--otis—eempenenfca— r-e9en - 20 - 91674/2 Alkaline earth metal (i.e., magnesium, calcium, strontium, and barium) thiocarbonates are somewhat more stable against loss of carbon disulfide than is an ammonium thiocarbonate. Moreover, neither alkaline earth metal nor alkali metal (lithium, sodium, potassium and cesium) thiocarbonate solutions form the phytotoxic thiocyanate species upon decomposition, so such solutions generally are more suitable for long-term storage.

Alkaline earth metal thiocarbonates can be prepared by reacting alkaline earth metal sulfides, either alone or mixed with elemental sulfur (when tetrathiocarbonate is to be prepared), with carbon disulfide, preferably in aqueous media, to directly form - 21 - 91674/2 aqueous fumigant composi ions. Alkaline earth metal sulfides can be generated in situ, by reaction of hydrogen sulfide with an aqueous solution or dispersion of alkaline earth metal salts, oxides, hydroxides, and the like. This same procedure is applicable to preparation of alkali metal thiocarbonates.

The preparation is conveniently carried out at temperatures of about 15* C. to about 35' C. , but may be conducted between about 0' C. and the boiling point of carbon disulfide, preferably under an inert or reducing gas atmosphere, to avoid oxidation of sulfur compounds to sulfur oxide moieties such as thiosulfates . Reactants are preferably provided in approximately stoichiometric amounts: one mole of alkaline earth metal sulfide per mole of carbon disulfide, to form alkaline earth metal tr ithiocarbonate, and one additional mole of elemental sulfur added to form alkaline earth metal tetrathiocarbonate. Products have the AaCS. a Λ empirical formula -M^€i>^/ wnereln T/is 1 when Μ/is alkaline earth metal, i'/is 2 when ( is alkali metal, and ^/is 3, 4 or values between 3 and 4.

The solubility limit for alkali and alkaline earth metal trithlocarbonates in water is approximately 55 percent by weight; the limit for corresponding tetrathiocarbonates is about 45 percent by weight. Solutions are normally diluted with water to concentrations less than about 33 percent by weight, to avoid precipitation at low temperatures .

The base-containing compositions of further enhanced stability and reduced CS2 partial pressure can be readily obtained by providing the desired amount of base in the thiocarbonate solution. Base can be introduced into the thiocarbonate solution before, during or after preparation of the thiocarbonate, it being necessary only that the final composition contain additional base. Preferably, such added base is provided either during or after preparation of the - 22 - 9.1674/2 thiocarbonate. Similar techniques can be employed to prepare the sulfide- and polysul ide-containing compositions. Thus, the sulfide and/or polysulf ide can be introduced into the thiocarbonate solution before, during or after preparation of the thiocarbonate, although such sulfides are preferably added either during or after preparation of the thiocarbonate. Sulfide and polysulfide can be provided in the composition by direct addition of such compounds, or they can be formed in situ. Thus, an amount of base, e.g. sodium hydroxide, can be added followed by addition of an equivalent quantity of hydrogen sulfide to convert the base to the corresponding sulfide, e.g. sodium sulfide (Na2S). The polysulf ides can be formed in situ by addition of elemental sulfur with adequate agitation to promote the reaction of the elemental sulfur with the sulfide already present in the composition. Thus, 3 equivalent weights of sulfur can be added to a solution containing 1 equivalent weight of sodium sulfide to produce a composition nominally containing sodium tetrasulf ide, i.e. Na2S4. Similar preparation techniques can be employed with all Salts may be recovered from the aqueous solutions by evaporation of the water and filtration of the resulting precipitate (under an inert or reducing atmosphere) if it is desirable to store the thiocarbonate for extremely long periods prior to use as a fumigant. However, the aqueous solution is substantially stable in and of itself; therefore, there is usually no need to recover the salt as a substantially anhydrous solid. Moreover, it is generally easier to handle the liquid solution than the solid thiocarbonate.

The above-described thiocarbonates , and in particular the aqueous, thiocarbonate solutions of enhanced stability and reduced CS~ partial pressure containing added - 23 - 916 base# sulfide and/or polysulfide, can be used as fumigants or In industrial applications involving the use of thiocarbonate compounds. The stabilized/ concentrated compositions are particularly useful for manufacture/ storage and transport of concentrated thiocarbonate compositions/ particularly when it is desired to avoid the hazards associated with carbon disulfide evolution.

While the above-described thiocarbonates are active fumigants and therefore may be used in any form (e.g., as a powder admixed with inert solids, as solution or dispersion in an organic solvent, etc.), it is preferred to use the aqueous solutions directly as fumigants. Therefore, the fumigation method of the invention may be carried out by the application of aqueous solutions of the thiocarbonates.

The above aqueous reaction solutions may be diluted prior to application as fumigants to provide a solution concentration of as low as 0.01 percent by weight of the thiocarbonate. The aqueous solution may incorporate surfactants to assist in application as a fumigant.

Preferably, a strong base, e.g., an alkali metal hydroxide such as sodium hydroxide/ is added to the aqueous solution to increase the stability thereof during application.

The alkaline earth metal thiocarbonates (like the ammonium and alkali metal analogues) decompose upon exposure to the atmosphere, at ambient temperatures and humidities, to yield carbon disulfide. Therefore, the aqueous solution will yield (upon evaporation of the water) a solvated alkaline earth metal thiocarbonate which decomposes to carbon disulfide, in the presence of atmospheric gases at ambient temperatures.

The aqueous thiocarbonate solutions utilized in the methods of this invention are stable against significant increases in vapor pressure, and significant solid phase formation, during storage periods. These solutions also maintain acceptable chemical stability during such periods, - «^4 - 91674/2 as measured by their ability to decompose to carbon disulfide upon application as a fumigant.

The stabilized compositions containing added base, sulfide and/or polysulfide have even greater stability, particularly with regard to CS2 evolution, and they are even more preferred in many applications due to that desirable property.

Soil application of a thiocarbonate composition can be accomplished either prior to planting or after plant growth is established. It should be noted, however, that different plant species exhibit differing tolerances to chemical agents. In addition, phytotoxicity to a particular plant can be dependent upon its growth stage. Germination is not inhibited for most plant seeds after soil treatment, and growth of established plants is not significantly altered. Some seedlings, though, show phytotoxicity symptoms. Postplant applications of the composition to such diverse crops as corn, cotton, tomatoes, potatoes and grapes have given no indications of phytotoxicity at effective nematocidal application rates, but cucumber plants have been shown to be somewhat sensitive to thiocarbonate.

The compositions can be applied as fumigants in undiluted form (to minimize the amount which is required per acre) by spraying onto the soil surface, preferably followed within several hours by water application to move the composition into the soil before a significant amount of free carbon disulfide is released. Injection into the soil, using a shank or knife, is also a useful method for applying the compositions. This application can either be "flat," wherein the injectors are closely spaced to treat essentially the entire field area, or can be "localized" by spacing the injectors such that only the plant growing bed is treated, in bands.

Alternatively, those forms of the compositions which are physically stable upon dilution can be mixed into - 25 - 9167 irrigation water and applied by any customary manner, such as through sprinklers, in the water for furrow or flood irrigation, and in drip irrigation systems. The compositions will move into the soil with the water, and decompose to accomplish their fumigation functions, including nitrification inhibition.

Decomposition of the thiocarbonates in the diluted solutions, prior to movement into the soil, can be retarded by increasing the pH of the solutions. With waters having a high total hardness, however, even the inherent alkalinity of thiocarbonate salts can lead to the precipitation of insoluble carbonates, i.e., of calcium, which tend to plug drip irrigation emitters or sprinkler nozzles. Such precipitation can be retarded by the addition of a hardness-complexing agent, such as sodium hexa etaphosphate, to the water.

The thiocarbonates can be combined with other agricultural chemicals to provide a multifunctional product. For example, the stable salts may be combined with solid or liquid fertilizers such as urea, ammonia, ammonium nitrate, calcium nitrate, etc. and other sources of plant nutrients. Since the described thiocarbonates inhibit nitrification, they reduce the rate at which ammoniacal compounds, such as fertilizers, are nitrified in the soil. Ammoniacal fertilizers are well known in the art, and as that term is used herein, it includes ammonia and ammonium-containing compounds as well as ammonia and ammonium compound formers such as urea, biuret, etc. Illustrative ammonium-containing compounds include ammonium nitrate, ammonium sulfate, etc.

The compositions also can be used in non-soil fumigation procedures, such as in the chamber fumigation of commodities which are introduced into commerce. In this type of procedure, dilution of a composition or the application of heat, or both, can be used to promote a rapid decomposition into the fumigant components. Upon - 26 - 9167 termination of the fumigation procedure, vapors in the chamber can be drawn through a scrubbing system, e.g., one containing an alkaline aqueous solution, to remove the fumigant and prevent atmospheric pollution when the chamber is opened.

Another important use of the compositions is as a fumigant for stored grains and other agricultural products. If applied to products which are to be stored, a fumigant composition can be applied simply by spraying into the product as it is being transported to the storage enclosure with a conveyor , auger or other device. The composition can be applied to agricultural products which are already in storage, by spraying onto the exposed products and sealing the storage enclosure.

It is also possible to use the thlocarbonate compositions for fumigating rooms or storage enclosures; this is accomplished by spraying the floor and walls with the composition, and sealing the space until the desired fumigation is accomplished. As an alternative to spraying, a technique similar to chamber fumigation can be used, wherein heat decomposes the composition in an enclosed space.

The fumigating ability of compositions described herein has been expressed primarily in terms of the available carbon disulfide content. It should be noted, however, that other components can contribute to efficacy as a fumigant. Ammonia, for example, is a fungicide for use on harvested grapefruit, lemons, oranges, and on grain for feed use. In addition, sulfur is very widely used as a fungicide- acar icide-insecticide, so any of the compositions of the invention which decompose to form sulfur will have similar properties in addition to the properties attributable to the carbon disulfide content.

Upon dilution, acidi ication, heating or introduction into the soil (which is a form of dilution), - 27 - 91674/3 the compositions of the invention break down into their components by a process which can be conceptualized as a physical dissociation. In a soil environment, the inorganic cation, sulfur, and hydrogen sulfide components are rapidly withdrawn into soil particles, and thereby rendered more or less immobile, depending upon soil characteristics, moisture, ambient temperature and the like. Certain of these species will be used as plant nutrients. Carbon disulfide, however, is not tightly bound to the soil and readily migrates to perform the fumigation function.

The invention is further described by the following examples, which are illustrative of various aspects of the invention and are not intended as limiting the scope of the invention as defined by the appended claims.

Example 1 A calcium tetrathiocarbonate solution was prepared by mixing 115.8 grams of calcium oxide with 585 grams water, and adding, with vigorous stirring, 71.6 grams of hydrogen sulfide, forming a dark green slurry. When 67.4 grams of sulfur had been added, the slurry became dark yellow in color. Addition of 180.7 grams of carbon disulfide produced a deep yellow solution, containing 36.5 % by weight calcium tetrathiocarbonate.

Example 2 A series of solutions containing 12.9 wt% equivalent carbon disulfide as sodium tri- or tetrathiocarbonate were prepared by combining sodium hydroxide, deionized water, sulfur (in the case of tetrathiocarbonate only), hydrogen sulfide and carbon disulfide. In the stoichiometric - 28 - - 91574/2 solutions, the reactants were combined in proportions sufficient to provide sodium tri- or tetrathiocarbonate solutions containing 12.9 wt% equivalent carbon disulfide without any excess reactant. In addition to the stoichiometric solutions, solutions were prepared with systematically increasing sodium hydroxide concentrations at constant equivalent carbon disulfide concentrations. The sodium sulfide was introduced by adding additional amounts of hydrogen sulfide equivalent to the amount of excess sodium hydroxide in the non-stoichiometric solutions, to convert the excess sodium hydroxide to sodium sulfide in situ.

The respective solutions were prepared by combining the appropriate amounts of base, water, and elemental sulfur (when used to form the tetrathiocarbonate), in 250 ml. bottles. The contents were then tared, and the appropriate amounts of hydrogen sulfide gas were bubbled in with cooling as necessary to form 100 grams of each of the test solutions. The bottles were then capped with Mininert valves, and the appropriate amount of carbon disulfide was added by injecting with a syringe. All sample bottles were shaken overnight to complete the reaction and were then allowed to equilibrate 3 days at 24°C, and the vapor phase was then sampled and analyzed for carbon disulfide by gas chromotography . The results are reported in the following table : - - REDUCTION OF CS- PARTIAL PRESSURE WITH SULFIDE ADDITION in Equilibrium Vapor ah 24· C. Vol. ¾ S_uJJLid£a Trlthlocarbonate Tetrathiocarbonabe 0.00 27.5 14.4 0.02 45.9 6.60 0.04 1.39 0.74 0.08 0.57 0.12 0.12 0.42 0.13 a Equivalent of sulfide per equivalent of CS These results demonstrate that the carbon disulfide vapor pressure of both tri- and tetrathiocarbonate solutions can be significantly reduced by providing a sulfide in the solution, that the tetrathiocarbonate solutions have consistently lower CS2 vapor pressures than the corresponding trlthlocarbonate solutions, and that the CS2 partial pressure of concentrated tri- and tetrathiocarbonates containing 12.9 weight percent equivalent carbon disulfide can be reduced to a level significantly below the explosive limit, i.e. below the level that would form 1 volume percent CSj in the equilibrium vapor phase at 24* C.

The operation described in Example 2 can be repeated using potassium tri- and tetrathiocarbonate solutions containing 12.9 weight percent equivalent carbon disulfide, 0.04 equivalent sodium hydroxide per equivalent of carbon disulfide, and 0.04 equivalent sodium hydroxide per equivalent carbon disulfide. The thiocarbonate solutions are prepared as described in Example 2 and the amount of sodium hydroxide added in excess of that required - 30 - 91674/3 to form the thiocarbonate corresponds to 0.08 equivalent sodium hydroxide per equivalent of carbon disulfide (keeping in mind that 1 mole of sodium hydroxide corresponds to 1 equivalent of sodium hydroxide, while 1 mole of carbon disulfide corresponds to 2 equivalents of that component). 0.04 equivalent of hydrogen sulfide is then sparged into the solution, and pressure is maintained on the system with adequate mixing to assure complete reaction of the hydrogen sulfide with a portion of the excess sodium hydroxide to convert 0.04 equivalent of the sodium hydroxide to sodium sulfide (Na2S). The resulting composition will have a significantly lower CS2 partial pressure than an otherwise identical thiocarbonate composition in the absence of the combination of excess base and sulfide under otherwise identical conditions.

While particular embodiments of the invention have been described, it will be understood, of course, that the invention is not limited thereto, since many obvious modifications can be made, and it is intended to include within this invention any such modifications as will fall within the scope of the appended claims.

Claims (32)

- 31 - 91674/2 WHAT IS CLAIMED IS:

1. An aqueous solution comprising: (a) one or more thiocarbonates having the formula AaCSb; and (b) one or more sulfides having the formula MnSx, wherein A and M are independently selected from the group consisting of alkali metals and alkaline earth metals; a is (i) 2 when A is an alkali metal and (ii) 1 when A is an alkaline earth metal; b is 3 to 4; n is (i) 2 when M is an alkali metal and (ii) 1 when M is an alkaline earth metal; and x is at least 1.

2. A stabilized aqueous solution comprising one or more thiocarbonates having the formula A^S^, wherein A is selected from the group consisting of alkali metals and alkaline earth metals; a is (i) 2 when A is an alkali metal and (ii) 1 when A is an alkaline earth metal; b is 3 to 4, the stabilized aqueous solution being prepared by the method comprising providing in the solution a sulfide soluble in the solution, the sulfide having the formula nSx, wherein M is selected from the group consisting of alkali metals and alkaline earth metals; n is (i) 2 when M is an alkali metal and (ii) 1 when M is an alkaline earth metal; and x is at least 1.

3. The composition of any one of claims 1-2 comprising at least about 0.02 equivalent of the sulfide per equivalent of carbon disulfide in the thiocarbonate .

4. The composition of any one of claims 1-2 comprising at least about 0.04 equivalent of the sulfide per equivalent of carbon disulfide in the thiocarbonate. - 32 - — 91674/2

5. The composition of any one of claims 1-2 comprising at least about 0.08 equivalent of the sulfide per equivalent of carbon disulfide in the thiocarbonate.

6. The composition of any preceding claim having a carbon disulfide partial pressure corresponding to about 1 volume percent or less carbon disulfide in the equilibrium vapor phase overlying the solution at 24 °C.

7. The composition of any preceding claim wherein the concentration of the thiocarbonate in the solution corresponds to at least about 1 weight percent equivalent carbon disulfide.

8. The composition of any preceding claim wherein M is selected from the group consisting of sodium, potassium, calcium, and combinations thereof.

9. The composition of any preceding claim wherein A is selected from the group consisting of sodium, potassium, calcium, and combinations thereof.

10. The composition of any preceding claim wherein x is greater than 1.

11. The composition of any preceding claim wherein b is 3.

12. The composition of any preceding claim wherein b is 4.

13. The composition of any preceding claim further comprising a water-soluble base.

14. The composition of claim 13 comprising at - 33 - _ 91674/2 least about 0.02 equivalent of the base per equivalent of carbon disulfide in the thiocarbonate.

15. A method for preparing a stabilized, aqueous solution of one or more thiocarbonates having the formula AgCS^, wherein A is selected from the group consisting of alkali metals and alkaline earth metals; a is (i) 2 when A is an alkali metal and (ii) 1 when A is an alkaline earth metal; and b is 3 to 4, the method comprising the step of providing in the solution one or more sulfides soluble in the solution and having the formula MnSx, wherein M is selected from the group consisting of alkali and alkaline earth metals; x is at least 1; and n is (I) 2 when M is alkali metal and (II) 1 when M is an alkaline earth metal.

16. The method of claim 15 comprising providing in the solution at least about 0.02 equivalent of the sulfide per equivalent of carbon disulfide in the thiocarbonate. ,

17. The method of claim 15 comprising providing in the solution at least about 0.04 equivalent of the sulfide per equivalent of carbon disulfide in the thiocarbonate . v

18. The method of claim 15 comprising providing in the solution at least about 0.08 equivalent of the sulfide per equivalent of carbon disulfide in the thiocarbonate .

19. The method according to any one of claims 15 to 18 comprising providing in the solution an amount of the sulfide to provide a carbon disulfide partial pressure corresponding to about 1 volume percent or less - 34 - 91674/2 carbon disulfide in the equilibrium vapor phase overlying said solution at 24°C.

20. The method according to any one of claims 15 to 19 wherein M is selected from the group consisting of sodium, potassium, calcium, and combinations thereof.

21. The method according to any one of claims 15 to 20 wherein A is selected from the group consisting of sodium, potassium, calcium, and combinations thereof.

22. The method according to any one of claims 15 to 21 wherein x is greater than 1.

23. The method according to any one of claims 15 to 22 wherein the concentration of the thiocarbonate in the solution corresponds to at least about 1 weight percent equivalent carbon disulfide.

24. The method according to any one of claims 15 to 22 wherein the concentration of the thiocarbonate in the solution corresponds to at least about 5 weight percent equivalent carbon disulfide.

25. The method according to any one of claims 15 to 22 wherein the concentration of the thiocarbonate in the solution corresponds to at least about 10 weight percent equivalent carbon disulfide.

26. The method according to any one of claims 15 to 25 wherein b is 3.

27. The method according to any one of claims 15 to 25 wherein b is 4. - 35 - 91674/3

28. The method according to any one of claims 15 to 27 wherein the solution further comprises a water-soluble base.

29. A method for controlling pests, which ' comprises applying to the location occupied by the pests the solution as claimed in any of claims 1 to 14 or as prepared by the method defined in any of claims 15 to 28.

30. The method of claim 29 wherein the pests are located in soil.

31. A method for producing a solid thiocarbonate having the formula AaCSb, wherein A is selected from the group consisting of alkali petals and alkaline earth metals; a is (i) 2 when A is an alkali metal and (ii) 1 when A is an alkaline earth metal; and b is 3 to 4 , the method comprising the step of removing water from an aqueous solution comprising one or more of the thiocarbonates and one or more sul ides having the formula MnSx, wherein M is selected from the group consisting of alkali and alkaline earth metals; x is at least 1; n is (I) 2 when M is alkali metal and (II) 1 when M is an alkaline earth metal; the concentration of the sulfide in the solution is sufficient to increase the stability of the thiocarbonate in the solution; and the water is removed from the aqueous solution under conditions sufficient to obtain the solid thiocarbonate of improved chemical stability.

32. The method of claim 31 wherein the aqueous solution further comprises a water-soluble base. for the Applicant: WOLFF , BREGMA AND GOLLER