US2889347A - Process for producing alkali metal methyl arsonates - Google Patents

Description

June 2, 195

A. SCHWERDLE PROCESS FOR PRODUCING ALKALLI METAL METHYL ARSONATES Filed March 2, 1956 6 ARSENIOUS SODIUM DIMETHYL OXIDE HYDRQXIDE SULPHATE STORAGE STORAGE STORAGE HOLDING TANK FOR CYCLE MIXER REACTOR CRYSTALLIZER CRYSTALLIZATION MOTHER LlQUlD 52 CENTRIFUGE 55 DRYER INVENTOR: ARTHUR SCHWERDLE BY W ATTYS.

a Sres PROCESS FOR PRODUCING ALKALI METAL METHYL ARSONATES This invention relates to a method for making alkali metal methyl arsonates, and more particularly to a relatively inexpensive method for manufacturing alkali metal methyl arsonates in which the alkali metal may be sodium or potassium, which arsonates are in a form suitable for use as herbicides, by methylating an alkali metal arsenite, in which the alkali metal may be sodium or potassium, with dimethyl sulphate in the presence of an alkali metal hydroxide including sodium and potassium hydroxides.

Alkali metal methyl arsonates, such as sodium and potassium methyl arsonates are excellent herbicides for use in selectively controlling the growth of crabgrass. Although not as strong herbicides, as for example dimethyl arsonate, however, they are more selective in controlling the growth of crabgrass without adversely affecting or harming the growth of other grasses or plants. To be acceptable for herbicidal use, these alkali metal methyl arsonates should be relatively free of byproducts of reaction and/or reactants, which may be injurious to plants or toxic to humans or animals, may adversely affect the selective herbicidal properties of the alkali metal methyl arsonates, or attack metallic containers in which the arsonates are stored or apparatus by means of which they are applied to the area to be treated.

In the past, alkali metal methyl arsonates, such as sodium methyl arsonate have been formed by the addition of a methylating agent to an alkali metal arsenite solution. Several processes have been evolved to carry this out, one of which employs methyl iodide as the methylating agent, a second which uses methyl chloride, and a third which employs dimethyl sulphate for the formation of alkali metal methyl arsonate. These prior methods possess disadvantages, such as the requirement of the use of expensive starting materials, high pressure equipment, and other disadvantages. In addition, since these processes are directed to the obtaining of a relatively pure arsonate product suitable for pharmaceutical use, alkali metal methyl arsonates can be produced by these processes only at considerable expense.

For example, the prior process employing dimethyl sulphate as the methylating agent possesses the disadvantage in that the reaction is never quantitative, the maximum yield of alkali metal methyl arsonate, based on dimethyl sulphate employed, generally being less than about 60%. This has been found to be true even if a substantial excess, for example, one molar excess of dimethyl sulphate methylating agent is employed. Since the arsonate product is not readily separable from reactants and by-products of this process, the removal of these contaminants from the alkali metal methyl arsonate product represents probably the greater problem and expense in its manufacture, and particularly so if yield of arsonate is low. At lower yields of arsonate, correspondingly greater quantities of sodium sulphate are formed at the expense of depleting the most expensive reactant, dimethyl sulphate.

The principal object of this invention is the provision atent 2. Y V of a process for the formation of alkali metal methyl arsonates which is of an inexpensive nature and affords a practical method for their manufacture.

Another object of this invention is the provision of an improved process for the formation of alkali metal methyl arsonates which employs dimethyl sulphate as the methylating agent.

Still another object of the invention is the provision of an improved process involving the formation of alkali metal methyl arsonates by methylation of an alkali metal arsenite solution with dimethyl sulphate in the presence of alkali metal hydroxide, and in which substantially increased yields of readily separable alkali metal methyl arsonates, in a form readily adaptable for herbicidal use, are obtained.

Still another object of the invention is the provision of an improved process involving the formation of alkali metal methyl arsonates by methylation of an alkali metal arsenite solution with dimethyl sulphate in the presence of alkali metal hydroxide, wherein substantially complete conversion of alkali metal arsenite is achieved by recycle of methylated arsenite solution subsequent to separation of arsonate product.

A further object of the invention is the provision of an improved method for forming an alkali metal methyl arsonate product by methylating a solution of alkali metal arsenite with dimethyl sulphate in the presence of alkali metal hydroxide from which product substantially pure alkali metal methyl arsonate can be obtained by simple and inexpensive purification and recovery procedures.

, A still further object of the present invention is the provision of an improved process for the production of alkali metal methyl arsonates which overcomes the disadvantages of the prior processes heretofore described.

Other objects will be apparent from a consideration of the following specifications and claims, and also from a consideration of the accompanying drawing, which illustrated by way of a flow sheet a preferred form of the invention.

The present invention relates to a relatively inexpensive method for making an alkali metal methyl arsonate, in which the alkali metal is selected from the group consisting of sodium and potassium, which arsonate is in a form suitable for use as a herbicide, which comprises methylating by means of dimethyl sulphate, in the presence of an alkali metal hydroxide selected from the group consisting of sodium hydroxide and potassium hydroxide, a solution of an alkali metal arsenite, in which the alkali metal is selected from the group consisting of sodium and potassium, the initial concentration of the alkali metal arsenite in the arsenite solution being not less than about 35%, by weight of total solution, the molar ratio of dimethyl sulphate to alkali metal arsenite employed being from about 0.221 to about 0.8:1 and the amount of alkali metal hydroxide present being from about 1% to about 10%, based on methylated arsenite solution, said percentage amount of hydroxide being in excess of that amount required to react stoichiometrically with all arsenious oxide present to form alkali metal arsenite.

As was stated previously, the prior processes for producing an alkali metal methyl arsonate, such as sodium methyl arsonate, are directed to obtaining an arsonate product of exceptionally high purity for employment in the pharmaceutical field. Thus, these processes are extremely costly by reason of the relatively low yields of product produced and considerable expense involved in the purification and recovery of a relatively pure arsonate product. Although these processes apparently are satisfactory for producing economically a product for pharmaceutical use, obviously they are not suitable v Patented June 2, 1959.

for producing an alkali metal methyl arsonate product at a cost level which would make such arsonate product available in large quantity as an agricultural chemical, i.e. a selective herbicide for the control of crabgrass. According to the process of my invention, however, I am able to manufacture in an economical manner alkali metal methyl arsonates, in whichthe alkali metal may be selected from the group consisting of sodium and potassium, which arsonates are in a form ready for use as a herbicide without further chemical processing.

The alkali methyl arsonate product resulting from the methylation of an alkali metal arsenite with dimethyl sulphate according tothe process of my invention comprises from about 50% to about 67%, by weight, of alkali metal methyl arsonate, the remainder being substantially entirely sodium sulphate. An arsonate product comprising about 67%, by weight, alkali metal methyl arsonate, in the form of the hexahydrate, and about 33% by weight, alkali-metal sulphate represents approximately 100% conversion of dimethyl sulphate. Since, in the use of an alkali metal methyl arsonate as a herbicide it is the general practice to combine the active ingredient with a diluent, such as pumice or the like, the sodium sulphate merely acts as a diluent, for it does not adversely efiect the selective herbicidal properties of the alkali metal methyl arsonate, neither is it harmful to plants nor is it toxic to humans during ordinary handling of the herbicide product. Although the herbicide product formed according to the process of my invention may contain traces of reactants, the amounts of these materials are extremely small, thus their presence is neither harmful to plants nor a hazard to health. For example, the alkali metal methyl arsonate produced according to my invention will generally contain no more than about 0.5%, by weight, of alkali metal arsenite, such as sodium arsenite. Thus, although the alkali metal arsenite may be quite toxic, and in suflicient concentrations may be injurious to plants and harmful to health, its concentration in the herbicide product produced according to'this invention is so small that the effect of its presence is negligible.

Although the process of my invention provides directly a relatively low cost alkali metal methyl arsonate herbicide product, substantially pure alkali metal methyl arsonate can be obtained in an inexpensive and'relatively simple manner from this impure product containing alkali metal sulphate. The method by which substantially pure alkali metal methyl arsonate can be obtained according to my invention will be more fully described hereafter.

I have found, that to a considerable degree, the success of my process for producing an alkali metal methyl arsonate by methylating a solution of an alkali metal arsenite with dimethyl sulphate is dependent upon the fact that the methylation is conducted in the presence of free alkali metal hydroxide selected from the group consisting of sodium hydroxide and potassium hydroxide. Not only does the presence of alkali metal hydroxide provide substantially increased yields, in addition it enables simple and inexpensive product separation, whereby is obviated one of the prohibitive expenses of the prior processes.

In the methylation of an alkali metal arsenite solution with dimethyl sulphate, only one methyl group of the dimethyl'sulphate is highly reactive in the formation of the alkali metal methyl arsonate, the other methyl group generally entering into the formation of by-products such.

as alkali metal methyl sulphate and methanol. The reaction contributing to the formation of the desired alkali metal methyl arsonate may be represented by the following equation in which the alkali metal is sodium. Of course, potassium may be substituted for sodium in this and other equations set'forth herein,

It is believed that'one of the reasons for the rather poor yields of alkali metal methyl'arsonate obtainable by the prior process for its production in which alkali metal arsenite solution is methylated with dimethyl sulphate, is that there exists a competing, undesirable reaction which may be expressed by the following equation:

The disodium acid arsenite formed by this reaction is extremely difficult to methylate to form sodium methyl arsonate. At least, the disodiurn acid arsenite is considerably more difficult to methylate than the sodium arsenite. Thus, because of the side reaction represented by Equation II, the yield of alkali metal arsonate is maintained at a relatively low figure, generally below about 60% conversion to the desired arsonate, based on dimethyl sulphate employed.

I have found that by carrying out the methylation of alkali metal arsenite with dimethyl sulphate in the presence of free alkali metal hydroxide, such as sodium hydroxide, particularly under certain specific process conditions more completely described hereafter, that yields of alkali metal methyl arsonate as high as based on available dimethyl sulphate, can be obtained. Apparently, the surprisingly high yield of product obtainable by my process may be attributed at least in part to the fact that the alkali metal hydroxide neutralizes the alkali metal acid arsenite formed by the reaction expressed by Equation 11 above. Thus, the alkali metal hydroxide present reconverts the difficultly methylated alkali metal acid arsenite to the more easily methylated alkali metal arsenite with the resulting higher yields of arsonate product. Regardless of theory, higher yields of arsonate product are obtainable by my process conducted in the presence of alkali metal hydroxide than heretofore have been obtainable with prior processes.

The quantity of free or stoichiometrically unbound alkali metal hydroxide present may be from about 1% to about 10%, by weight, based onthe total quantity of methylated arsenite solution, i.e. the total amount of reaction solution resulting from the addition of dimethyl sulphate to the original alkali metal arsenite solution. The expression free or stoichiometrically unbound alkali metal hydroxide as used herein and in the appended claims means that quantity of alkali metal hydroxide in excess of the amount required stoichiometrically to react with all arsenious oxide present to form tn'sodium arsenite. Thus, the amount of alkali metal hydroxide employed is from about 1% to about 10%, by weight, of methylated arsenite solution, the stated percentage amount being that quantity in excess of any amount of alkali metal hydroxide required stoichiometrically to react with all arsenious oxide that may be present. Since the reaction by which the alkali metal methyl arsonate is formed, i.e. that represented by Equation I above, is highly exothermic, all of the dimethyl sulphate is not added at one time to the arsenite solution but rather is added substantially continually in small amounts. Thus, the total quantity of solution, i.e. methylated arsenite solution, is continually increasing, addition of alkali metal hydroxide to the reaction mixture from time to time is necessary to maintain the concentration of alkali metal hydroxide at the desired strength. Also, sodium hydroxide present is consumed by reaction with dimethyl sulphate in the formation of by-product methanol. Further, since alkali metal hydroxide is consumed during the process, as for example by neutralization of the alkali metal acid arsenite formed by the reaction represented by Equation II, the addition of alkali metal hydroxide to replace that consumed, thereby maintaining the desired concentration, or excess of alkali metal hydroxide is necessary, In other words, the quantity of alkali metal hydroxide present should be maintained throughout the'entire methylation period or process at that quantity in excess of any amount of alkali metal hydroxiderequired stoichiometrically to-react with'an arsenious oxide present which provides an alkali metal hydroxide concentration of from about 1% to about by weight, based on the total quantity of methylated arsenite solution. This can be accomplished by addition of alkali metal hydroxide. The presence of excessively large amounts of alkali metal hydroxide should be avoided since at very high hydroxide concentrations the formation of methanol may become the predominant reaction. Preferably, the amount of alkali metal hydroxide present is free from about 2% to about 8%, based on methylated arsenite solution. The preferred alkali metal hydroxide is sodium hydroxide. The amount of sodium hydroxide present can be determined by titration of the reaction solution With a strong acid, such as hydrochloric acid, using an indicator which changes at a pH of about 5. After correction of values for arsenite present, hydroxide concentration is obtained.

In carrying out the process of my invention it was observed that in addition to the requirement of conducting the methylation in the presence of alkali metal hydroxide, certain process conditions should be employed if high yields of the desired alkali metal methyl arsonate herbicide product are to be obtained. It was observed that the initial concentration of alkali metal arsenite of the alkali metal arsenite solution, which is methylated by dimethyl sulphate, should be at least about 35, by weight, of the total solution. If the initial concentration is substantially less than 35% alkali metal arsenite, yields may fall off as much as 30%. The reason for the low arsonate yields at low initial arsenite concentrations is believed to be due, at least in part, to hydrolysis of the alkali metal arsenite to form alkali metal acid arsenites or the meta arsenite ion. Apparently the tendency of alkali metal arsenites to hydrolyze in this manner is substantially increased at lower arsenite concentrations, and is substantially retarded at higher arsenite concentrations. In the methylation process, a side reac tion between alkali metal hydroxide and dimethyl sulphate is continuously taking place with the formation of methanol. When the loss of alkali metal hydroxide by this side reaction has reached a point where its ratio to arsenious oxide is below that required for the formation of alkali metal arsenite, methylation with the formation of the desired arsonate product is substantially arrested, and the reaction favoring the formation of methanol becomes predominant. Thus, alkali metal arsenite solutions wherein the initial concentration of arsenite is not less than about 35 by weight of said solution, are employed in my process. Preferably, the initial concentration of arsenite in the arsenite solution is from about 60% to about 70%, based on arsenite solution. It will be observed that by carrying out the process in the presence of added alkali metal hydroxide, sufiicient hydroxide is present to satisfy the reaction forming methanol and the tendency of alkali metal arsenite to hydrolize is further decreased.

The alkali metal arsenite solution of the desired concentration may be formed by merely dissolving a suflicient quantity of alkali metal arsenite, preferably sodium arsenite, in a suitable liquid reaction medium which is a solvent for the reactants, for example aqueous medium, or the alkali metal arsenite may be formed in situ in liquid reaction medium by contacting alkali metal hydroxide with arsenious oxide in the reaction medium. Suitable liquid reaction media include aqueous medium such as water, alcohols such as ethyl alcohol, ethers such as dioxane, and ether-alcohols such as ethylene glycol mono-ethyl ether. The preferred solvent is water, and more particularly aqueous methylated arsenite solution from which alkali metal methyl arsonate and sodium sulphate crystals have been separated, i.e. recycled process solution. Recycle of methylated arsenite solution will become more clearly apparent further here inafter.

As was stated earlier, methylation of the arsenite solution is accomplished by means of dimethyl sulphate. Since the reaction between dimethyl sulphate and alkali metal arsenite, such as sodium arsenite, is exothermic, these reactants are brought together in a controlled manner. Preferably, the dimethyl sulphate is added in small controlled amounts to the alkali metal arsenite solution. The total quantity of dimethyl sulphate employed should be such that the molar ratio of sulphate to arsenite is from about 0.2:1 to about 0.8:1. At molar ratios outside this range, the elficiency of the process decreases rather sharply. On the other hand, a yield of sodium methyl arsonate of about 77%, based on dimethyl sulphate available, was obtained by methylating an aqueous solution of sodium arsenite having an initial concentration of about 40%, the molar ratio of arsenite to sulphate employed being about 0.411. Employing a sodium arsenite solution having an initial concentration of about 60%, and dimethyl sulphate in a molar ratio of sulphate to arsenite of 0411, there being sodium hydroxide present, a yield of sodium methyl arsonate of 89.5% was obtained. The preferred molar ratio of dimethyl sulphate to alkali metal arsenite is from about 0.4:1 to about 0.5: 1.

The process is best carried out at somewhat elevated temperatures above about 68 C. Temperatures up to the boiling point of the solution of reactants can be employed under atmospheric pressure. Higher temperatures such as temperatures up to about 180 C. can be employed if the process is carried out under superatmospheric pressure. Preferred temperatures are from about 68 C. to about C. Because of the exothermic nature of the methylation reaction, it is generally desirable to cool the reaction zone in order to control reaction temperature.

Because the process is carried out in the presence of free alkali metal hydroxide, the solution of reactants will be strongly alkaline. Preferably, the pH of the reaction medium is maintained at not less than about 14 throughout the reaction. By use of this strongly alkaline pH, the alkali metal methyl arsonate product can be crystallized from the reaction mixture or methylated arsenite solution, and the crystalline product is in a form which is easily separable from the reaction solution by filtration, centrifugation or like means. The employment of a less alkaline pH may lead to poor crystal formation resulting in difliculties in product separation. The pH of the reaction medium can be maintained at the desired strongly alkaline level by the addition of alkali metal hydroxide, such as flake sodium hydroxide.

After the rnethylation reaction has ceased, the reaction mixture or solution is cooled and crystals of alkali metal methyl arsonate and alkali sulphate form. The reaction mixture should be cooled to a temperature from about 5 to about 25 C. and preferably from about 15 to about 20 C. to obtain crystals which are easily separable from the reaction solution by the usual liquidsolid separation means, e.g. a basket type centrifuge. If the methylation of alkali metal arsenite solution is carried out according to the previously described reaction conditions, easily separable crystals are obtained merely by cooling. However, the specific gravity of the reaction solution or methylated arsenite solution should not exceed about 1.45 and preferably should be less than about 1.3. Adjustment to obtain a proper specific gravity can be made by adding additional reaction medium or solvent or other means.

Crystals of alkali metal methyl arsonate and alkali metal sulphate are separated from the reaction medium by filtration, centrifugation or the like. The separated crystals are then dried in a tray, rotary drum or other suitable dryer, and if necessary, the dried product blended with a suitable amount of diluent, such as an alkali metal sulphate, e.g. sodium sulphate, and packaged for shipment.

The reaction medium or methylated arsenite solution from which crystals of alkali metal methyl arsonate and alkali metal sulphate have been separated may be utilized as the reaction medium for methylation of additional arsenite. In this case alkali metal arsenite and/or alkali metal hydroxide and arsenious oxide are added to the methylated arsenite solution and the resulting solution is subjected to methylation with dimethyl sulphate in the same manner and under the same conditions as heretofore described. Thus, it is immediately apparent that recycle within the process of reaction medium represents a considerable cost advantage for unreacted reactants, such as alkali metal arsenite, which are discarded according to the prior processes, are not discarded but enter into a later conducted methylation reaction, and are thus ultimately consumed substantially 100%.

Reference is now made to the accompanying drawing where a preferred mode of carrying out the process of my invention is illustrated by means of a flow sheet. To simplify the discussion, the process described with relation to the flow diagram is that for the production of sodium methyl arsonate; however, a similar process can be employed for the production of potassium methyl arsonate.

Aqueous medium from a source hereinafter described is withdrawn from a holding tank 10 and transferred by linelZ to mixer 14 provided with suitable fluid agitating means such as a propeller. Arsenious oxide and flake sodium hydroxide, from storage zones 16 and 18, respectively, are added to the aqueous medium in mixer 14. The quantity of arsenious oxide and sodium hydroxide added to the aqueous medium should be sufiicient so that the resulting sodium arsenite solution has an initial concentration of about 60%, sodium arsenite, based on total solution. The sodium arsenite solution is conducted from mixer 14 to reaction zone or reactor 20 by means of line 22 wherein dimethyl sulphate from storage zone 24 is added to the arsenite solution via line 26.

As was stated previously, according to my process, the methylation of the sodium arsenite solution is conducted in the presence of free sodium hydroxide. Thus, preferably more sodium hydroxide is added to aqueous medium in mixer 14 than is required to react with arsenious oxide present to form sodium arsenite. The sodium hydroxide present as such should represent preferably from about 2% to about 8%, by weight, based on arsenite solution leaving mixer 14.

Dimethyl sulphate is added to the arsenite solution in controlled amounts, the total amount of dimethyl sulphate employed being added to the arsenite solution in small increments over the desired period of time for conducting the rnethylation. The total amount of dimethyl sulphate employed preferably should represent a molar ratio of sulphate to arsenite of about 0.4:1.

As the methylation progresses, sodium hydroxide present in the arsenite solution is consumed, thus it is necessary to add adidtional sodium hydroxide from time to time in order to maintain the desired concentration of sodium hydroxide. The additionally required sodium hydroxide preferably is added to aqueous medium in mixer 14 and the resulting hydroxide solution, which preferably is a highly concentrated solution, is conveyed to reactor 2% by line 22.

Since the reaction between dimethyl sulphate and so dium arsenite is exothermic, reaction zone 20 may be in indirect heat exchange relationship with a heat transfer medium by which some of the heat of reaction may be removed. A water-jacketed reactor may be employed to advantage.

After the required quantity of dimethyl sulphate has been added to the arsenite solution, the mixture of reactants or methylated arsenite solution is transferred from reaction zone 20 to a crystallizer 28 via line 39. In crystallizer St the reaction mixture is cooled, as for example by being. in indirect heat exchange relation with a suitable refrigerant, to form crystals of sodium methyl arsonate and sodium sulphate. The resulting slurry of crystals in aqueous media is passed to a basket type centrifuge 32 by line 34 where the crystals are separated from the methylated reaction medium. The separated crystals are removed from centrifuge 32 and dried in a drier 36, which may take the form of a rotary drum drier.

Crystallizer mother liquor from centrifuge 32 is. passed by line 38 to holding tank 10 where the mother liquor is held until used in a later cycle of operation. Additional aqueous medium, such as water, can be added to or withdrawn from the system by line 49 or line 42, which may be equipped with suitable valve means. Methanol formed as a by-product of the reaction can be removed from the system by being vented into the atmosphere through a suitable vent line not shown.

Although the process of my invention produces a product which is in a form suitable for use as a herbicide, it may be desirable to further purify the alkali metal methyl arsonate to a rather high degree of purity, e.g. that purity required of a product for pharmaceutical use, by one of several rather inexpensive and relatively simple purification procedures.

According to one purification procedure, a mixture of moist alkali metal sulphate and alkali metal methyl arsonate crystals from the centrifuge or filter, where mother liquor is separated from the crystals, is diluted with water, the weight of water employed being approximately one half the weight of the moist crystals. The mixture of crystals and water are heated to a temperature at least about 85 C., where upon the alkali metal methyl arsonate dissolves together with a portion of the alkali metal sulphate. In this manner a very concentrated solution, for example a 40% to 70% solution of alkali metal methyl arsonate is obtained which on cooling to a temperature below about 35 C. deposits alkali metal methyl arsonate crystals but not alkali metal sulphate. Separation of the deposited arsonate crystals from the solution can then be efiected in the usual manner. Arsonate crystals of very high purity can be obtained by this process, particularly if these purification steps are repeated.

A clean cut, substantially complete separation of alkali metal sulphate from alkali metal methyl arsonate can be achieved by the use of formamide. By this meth- 0d, a mixture of sulphate and arsonate crystals are placed in about twice their volume of formamide, and the resulting mixture is heated to temperatures somewhat above ambient temperature. A solution of alkali metal methyl arsonate results, leaving the sulphate substantially undissolved. The arsonate solution can be used as such, diluted with water, or from the solution, the arsonate can be precipitated by dilution with isopropanol or a similar solvent.

The following examples provide illustrations of the practice of the invention, but are in no manner to be considered as limiting the scope of this invention.

Example I 635 lbs. of As O and 1000 lbs. of technical grade caustic soda were added in small amounts over a period of 6 hours to 2400 lbs. recycled mother liquor or methylated arsenite solution, from which sodium methyl arsonate and sodium sulphate crystals were separated, containing 43.3% trisodium arsenite and 4% sodium hydroxide. The resulting arsenite solution containing about by weight, of tn'sodium arsenite was methylated with 416 lbs. of dimethyl sulphate which was added to the arsenite solution as a continuous stream over a period of about 6 hours, the reaction solution being continuously agitated during addition of the dimethyl sulphate. Thereafter 200 lbs. of additional caustic soda and 800 lbs of recycled mother liquor containing 43.3% sodium arsenite was added to the reaction solution in small increments over a period of about 6 hours in order to retain the free sodium hydroxide content in the reaction solution throughout the reaction above about 2%, by weight, based on total reaction solution. Simultaneously with the addition of the last-mentioned mother liquor containing added sodium hydroxide, 416 lbs. of dimethyl sulphate were introduced to the reaction mixture over the same period. During the methylation by means of dimethyl sulphate addition, the reactor temperature was maintained from about 72 to about 78 C. At the completion of the entire methylation reaction, which took approximately 12 hours, and wherein the molar ratio of dimethyl sulphate to arsenite employed was about 0.49: 1, the reaction solution was cooled to about 20 C. in a crystallizer to crystallize sodium methyl arsonate and sodium sulphate and the resulting slurry of arsonate and sulphate crystals was passed through a centrifuge where mother liquor was separated from the crystals. The crystalline product was then dried. The product analyzed 55.7% disodium methyl arsonate, representing a yield of approximately 80% based upon dimethyl sulphate employed.

Example 11 The method of this example was carried out in the same manner as the method of Example I, employing the following conditions:

Initial concentration of sodium arsenite, wt. percent 50 Mol ratio, dimethyl sulphate to sodium arsenite-- 0.66:1 Free hydroxide present, wt. percent reaction solution 5.3 Temperature of reaction 0.-.... 72-82 Yield, percent based on dimethyl sulphate used 85 Example Ill The method of this example was carried out in the same manner as the method of Example I, employing the following conditions:

Initial concentration of sodium arsenite, wt. percent 60 Mol ratio, dimethyl sulphate to sodium arsenite 0.4:1 Free hydroxide present, wt. percent reaction solution 5.5 Temperature of reaction C 70-80 Yield, percent based on dimethyl sulphate used 89.5

the group consisting of sodium and potassium, by the addition to said solution of dimethyl sulphate, the initial concentration of arsenite in said solution comprising not less than about 35%, by weight of said solution, the molar ratio of sulphate to arsenite comprising from about 0.2:1 to about 0.8:1, and the alkali metal hydroxide present throughout the reaction comprising from about 1% to 10% of methylated reaction mixture, said percentage amount of hydroxide being in excess of that amount required to react stoichiometrically with any arsenious oxide present to form alkali metal arsenite.

2. The method of claim 1 in which the alkali metal arsenite is sodium arsenite and the alkali metal hydroxide is sodium hydroxide.

3. The method of claim 1 in which the alkali metal arsenite is potassium arsenite and the alkali metal hydroxide is potassium hydroxide.

4. The method of claim 1 in which the solution is an aqueous solution.

5. The method of claim 4 in which the concentration of alkali metal hydroxide is from about 2% to about 8%.

6. The method of claim 5 in which the molar ratio of sulphate to arsenite is from about 0.4:1 to about 0.5 :1.

7. The method of claim 6 in which the initial concentration of arsenite is from about to about 8. The method of claim 7 in which the pH of the solution is at least about 14.

9. The method of claim 8 in which the methylation is conducted at a temperature from about 68 to about C.

10. The method of claim 9 in which the alkali metal arsenite is sodium arsenite and the alkali metal hydroxide is sodium hydroxide.

11. The method of claim 9 in which the alkali metal arsenite is potassium arsenite and the alkali metal hydroxide is potassium hydroxide.

12. The method of claim 10 in which the sodium arsenite solution is formed by contacting sodium hydroxide with arsenious oxide in aqueous solution.

13. The method of claim 12 in which said methylated reaction mixture is cooled to a temperature from about 5 to about 25 C. to form crystals of disodium methyl arsonate and sodium sulphate, said crystals are separated from said methylated reaction mixture, arsenious oxide and sodium hydroxide are added to said methylated reaction mixture from which said crystals have been separated, and the resulting solution is subjected to methylation with dimethyl sulphate.

14. The method of claim 13 in which the separated crystals of di-sodium methyl arsonate and sodium sulphate are treated at elevated temperatures with formamide to form a formamide solution of arsonate, and sodium sulphate crystals are separated from the formamide solution.

References Cited in the file of this patent Treflier: Chemical Industries, June 1944, pp. 854-855.

Claims (1)

1. A METHOD FOR MAKING A DI-ALKALI METAL MONOMETHYL ARSONATE, IN WHICH THE ALKALI METAL IS SELECTED FROM THE GROUP CONSISITING OF SODIUM AND POTASSIUM, WHICH COMPRISES METHYLATING A SOLUTION OF AN ALKALI METAL ARSENITE, IN WHICH THE ALKALI METAL IS SELECTED FROM THE GROUP CONSISTING OF SODIUM AND POTASSIUM, AT A TEMPERATURE ABOVE ABOUT 68*C. AND IN THE PRESENCE OF AN ALKALI METAL HYDROXIDE, IN WHICH THE ALKALI METAL IS SELECTED FROM THE GROUP CONSISTING OF SODIUM AND POTASSIUM, BY THE ADDITION TO SAID SOLUTION OF DIMETHLYL SULPHATE, THE INITIAL CONCENTRATION OF ARSENITE IN SAID SOLUTION COMPRISING NOT LESS THAN ABOUT 35%, BY WEIGHT OF SAID SOLUTION, THE MOLAR RATIO OF SULPHATE TO ARSENITE COMPRISING FROM ABOUT 0.2:1 TO ABOUT 0.8:1, AND THE ALKALI METAL HYDROXIDE PRESENT THROUGHOUT THE REACTION COMPRISING FROM ABOUT 1% TO 10% OF METHYLATED REACTION MIXTURE, SAID PERCENTAGE AMOUNT OF HYDROXIDE BEING IN EXCESSOF THAT AMOUNT REQUIRED TO REACT STOICHIOMETRICALLY WITH ANY ARSENIOUS OXIDE PRESENT TO FORM ALKALI METAL ARSENITE.

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