GB2263067A - Herbicidal composition with reduced acute oral toxicity - Google Patents

Abstract

A herbicidal composition comprises an aqueous solution of a salt of paraquat or diquat, or a mixture thereof, in admixture with a gelling agent that will gel at the pH of human gastric juice (e.g. magnesium silicate), an emetic (e.g. a triazolopyrimidine emetic) and a purgative comprising sodium sulphate or a non-toxic water-soluble salt of magnesium other than magnesium sulphate. In the event of ingestion of the composition, these additives act synergistically to reduce absorption of bipyridylium compound from the gastrointestinal tract into the bloodstream, and thereby reduce the acute oral toxicity of the composition.

Description

HERBICIDAL COMPOSITION This invention relates to herbicidal compositions containing a herbicidal bipyridylium diquaternary salt as an active ingredient.

Various herbicidal bipyridylium diquaternary salts are described in United Kingdom Patent Specification No. 785732, 813531 and 813532. Certain of these compounds (e.g. salts of paraquat and diquat; the chemical identity of these compounds is given later in this specification) are in widespread use in agriculture, and are manufactured for sale in the form of concentrated aqueous solutions. When required for use, these concentrated solutions are diluted with water to form a solution which is then sprayed upon unwanted plants. When used with due care, and in accordance with recommended procedures, the concentrated aqueous solutions present no hazard. However, hazards may develop if adherence to the recommended handling practices is not maintained.Thus it may happen that an operator may decant some of the concentrate into a domestic container such as a beverage bottle for subsequent use in his own garden at home. The hazard associated with such a practice is of course that a child or incautious adult coming upon the bottle may swallow some of the contents, with possible serious consequences.

Various methods have been proposed to reduce the likelihood of accidental swallowing of concentrated solutions of bipyridylium diquaternary salts in the circumstances described above. These include the inclusion of an odourant in the concentrate as a warning (see UK Patents 1406881 and 1570981), the inclusion of a water-soluble blue dye, and the inclusion of a substance to convert the concentrate into a thixotropic gel (see UK Patent 1395502).

Another approach to reducing the possibility of harmful effects following the ingestion of a bipyridylium salt concentrate has been to include an emetic in the concentrate, so that the concentrate will be removed by vomiting shortly after its ingestion (see UK Patent 1507407). A further approach has been to formulate the bipyridylium diquaternary salt as a multiple emulsion, as proposed in published European Patent Application No. 276911. In addition to methods of formulating bipyridylium herbicides so as to reduce the likelihood of accidental ingestion, a variety of methods has been proposed for formulating bipyridylium herbicides so as to improve their suitability for particular applications.

Thus, it has been proposed to formulate an aqueous solution of paraquat as an invert emulsion in oil (see United States Patent 4115098) in order to reduce drift and evaporation of the droplets of spray when the paraquat is applied as a herbicide.

In our European Patent Application No. 91305458 (publication number 0467529) there is disclosed a herbicidal composition comprising a salt of paraquat or diquat, or a mixture thereof, in admixture with a gelling agent that will gel at the pH of human gastric juice, and further comprising (1) an emetic, or (2) a purgative, or (3) both an emetic and a purgative.

A preferred form of the compositions described in EPA 91305458 comprises an aqueous solution of a salt of paraquat or diquat, or a mixture thereof, in admixture with a gelling agent that will gel at the pH of human gastric juice, an emetic, and a purgative. The purgative specifically mentioned in the Examples included in EPA 91305458 is magnesium sulphate.

Other purgatives which may be used include sodium sulphate and non-toxic water-soluble salts of magnesium other than magnesium sulphate. In particular circumstances it may be desirable to use one or more of these alternative purgatives rather than magnesium sulphate.

According to the present invention, therefore, there is provided a herbicidal composition comprising an aqueous solution of a salt of paraquat or diquat, or a mixture thereof, in admixture with a gelling agent that will gel at the pH of human gastric juice, an emetic, and a purgative comprising sodium sulphate or a non-toxic water-soluble magnesium salt other than magnesium sulphate.

Non-toxic water-soluble salts of magnesium include magnesium acetate, magnesium citrate, and magnesium chloride; the latter is particularly preferred.

If a quantity of a composition according to the invention is ingested, the acidity of the gastric juice (which ranges from pH1.5 to pH3.5) will cause the composition to gel in the stomach. Increasing the viscosity of the gastric contents slows down the rate of gastric emptying. The bipyridylium herbicide will consequently be trapped in the gel, and its movement from the stomach and into the absorptive small intestine will be impeded. Ideally, the gelling agent is one which will also reduce gastric acidity and therefore reduce local irritation of the gastric mucosal lining.The emetic present in the composition is absorbed rapidly and will in a short time cause expulsion of the gel containing the bipyridylium herbicide by vomiting, thereby preventing the ingested herbicide from moving further down the gastrointestinal tract, where absorption of the bipyridylium compound would otherwise take place. The purgative present in the composition assists in removing any non-absorbed bipyridylium herbicide which has passed from the stomach into the small intestine despite the action of the emetic. In the event of a bipyridylium composition according to the invention being ingested, the combined effects of the gelling agent, the emetic, and the purgative will be to substantially reduce the absorption of the bipyridylium compound from the gastrointestinal tract into the bloodstream, and thereby to reduce the acute oral toxicity of the concentrate.

Paraquat is the common name of the 1,1'-dimethyl-4,4'-bipyridylium cation, having the structure (I) (see formula sheet).

Diquat is the common name of the 1,1'-ethylene-2,2'-bipyridylium cation, having the structure (II) (see formula sheet).

Salts of paraquat and diquat necessarily contain anions carrying sufficient negative charges to balance the two positive charges on the bipyridylium nucleus.

Since the characteristic herbicidal effect of a bipyridylium quaternary cation is independent of the nature of the associated anion, the choice of the anion is a matter of convenience, depending, for example, on cost. Preferably the anion is one which gives rise to a salt of convenient water solubility. Examples of anions, which may be mono- or poly-valent, include acetate, benzenesulphonate, benzoate, bromide, butyrate, chloride, citrate, fluorosilicate, fumarate, fluoroborate, iodide, lactate, malate, maleate, methylsulphate, nitrate, propionate, phosphate, salicylate, succinate, sulphate, thiocyanate, tartrate, and e-toluenesulphonate. The salt of the herbicidal bipyridylium cation may be formed from a number of similar anions or mixtures of different ones. A salt having any particular desired anion may be prepared either by direct synthesis from reactants which include the desired anion, or by exchanging the anion of a previously prepared salt of the preferred anion by methods well known in the art, for example by passage of a solution of the previously prepared salt through an ion-exchange resin. For reasons of convenience and economy, paraquat is normally manufactured and sold as paraquat chloride while diquat is manufactured and sold as diquat dibromide.

Since the characteristic herbicidal activity of a salt of a herbicidal bipyridylium quaternary cation resides in the cation only, it is customary to quote concentrations of active ingredient and rates of application in terms of the amount of bipyridylium quaternary cation used. Application rates and concentrations quoted in this specification therefore relate to the amount of bipyridylium quaternary cation unless otherwise stated.

Preferably, aqueous compositions according to the invention contain at least 50 grams per litre of paraquat or diquat or mixtures thereof. The compositions may contain 100 grams per litre or more of paraquat or diquat or mixtures thereof. Compositions containing 200 grams or more per litre, for example 300 grams per litre, may be prepared.

The gelling agent is preferably magnesium silicate, although other gelling agents may be used.

As commercially manufactured, (e.g. by reaction of sodium silicate with magnesium sulphate), magnesium silicate has a somewhat variable composition; thus, the ratio of silica (SiO2) to magnesium oxide (MgO) in commercial technical grades of this material may range for example from about 3.3 to 1.5. Typical chemical analyses for two commercial technical grades of magnesium silicate (designated PDM15 and PDM 33) are given below.

PDM 33 PDM 15 MgO 12-13% 22.6-24.6% SiO2 61-65% 49.5-53.5 Molecular ratio SiO2/MgO - 3.3 - 1.5 Fe 0.03% 0.03% SO4 2.5% 1.5X Loss in weight on ignition at 9000C 18-22% 20-30% In preparing herbicidal compositions according to the present invention, it is preferred to use a grade of magnesium silicate which has a relatively high magnesium oxide content, for example magnesium silicate approximating to the composition 2MgO.3SiO2. Magnesium silicate of this composition is often designated as magnesium trisilicate, and is so designated throughout this specification. Suitably purified and sterilised grades of this material are used pharmaceutically as an antacid.

Magnesium trisilicate is also known as magnesium mesotrisilicate. It is described in the Merck Index (Ninth Edition, published in 1976 by Merck and Co., Inc, Rahway, New Jersey, USA; see entry No. 5514 on pages 738-739) as an odourless, tasteless, slightly hygroscopic powder, having the chemical formula Mg2Si308. It is usually hydrated to some extent.

According to the British Pharmacopoeia, 1988, volume 1, page 348 (published by H M Stationery Office, London), magnesium trisilicate is a hydrated magnesium silicate of the approximate composition 2MgO.3Si02 containing water of crystallisation. The pharmaceutical grade contains not less than the equivalent of 29.0% of MgO, and not less than the equivalent of 65.0% of SiO2, both calculated with reference to the substance after ignition at 9000C. The use of magnesium trisilicate as a gelling agent in the compositions of the present invention has the advantage that it reduces gastric acidity, and thereby, as mentioned above, reduces local irritation of the gastric mucosal lining. Magnesium trisilicate and the silicon dioxide gel produced in the stomach both have adsorbent properties.This reduces the bioavailability of bipyridylium salts when they are in concentrated solution. The magnesium trisilicate is preferably present in a concentration of from 10 to 400 grams per litre of the composition, and more preferably in a concentration of from 10 - 100 grams per litre. Since magnesium trisilicate is insoluble in water, it may tend to settle to the bottom of the container in which the composition is stored. Liquid compositions of the invention containing magnesium trisilicate therefore preferably also comprise a suspending agent, to assist in keeping the magnesium trisilicate dispersed homogeneously throughout the composition.

Examples of suitable suspending agents include bacterial polysaccharides, for example the material known as xanthan gum, and in particular the xanthan gum sold under the trade name "KELZAN". Other suitable suspending agents include alginates. The suspending agents may be present in a concentration of for example from 0.01 gram to 100 grams per litre of the composition.

A variety of known emetics may be used in the compositions of the invention. However, preferred emetics are those compounds disclosed in UK Patent No. 1507407 for use in formulations of bipyridylium herbicides, and a particularly preferred emetic is 2-amino-6-methyl-5- oxo-4-n-propyl-4, 5-dihydro-5-triazolo[1,5-a]-pyrimidine, having the structural formula (A) (see formula sheet).

The amount of emetic used in the composition will vary depending upon the particular type of emetic used, but when an emetic of the class disclosed in UK Patent No. 1507407 is used, the concentration of emetic is preferably from 0.1 to 5 grams per litre of the composition.

For a composition containing 200 grams per litre of bipyridylium compound, a concentration of 1.5 to 2.0 grams per litre of emetic is preferred.

The concentration of the purgative is preferably from 10 to 400 grams per litre and more preferably from 50 to 100 grams per litre.

The compositions of the invention may also contain further ingredients for example one or more surface-active agents.

Surface-active agents may be cationic, non-ionic or anionic.

Generally speaking cationic and non-ionic surface-agents are preferred to anionic surface-active agents for use in the compositions of the invention, since the latter may interact undesirably with the bipyridylium quaternary salt in the compositions, but anionic surface-active agents may be used where such interactions do not occur. Examples of non-ionic surface-active agents for use in the compositions of the invention include the condensation products of ethylene oxide with alkyl phenols such as octylphenol, nonylphenol and octylcresol for example the condensation product of -nonylphenol with 8 molar proportions with of ethylene oxide sold under the trade name of Synperonic NP8.Other non-ionic agents are the partial esters derived from long chain fatty acids and hexitol anhydrides, for example sorbitan monolaurate; the condensation products of the said partial esters with ethylene oxide; and the lecithins. Examples of cationic surface-active agents include quaternary salts and condensates of ethylene oxide with amines, for example the substances sold under the Trade Mark "Ethomeen", "Ethoduomeen", "Duoquad" and "Arquad". A particular example comprises the condensate of a C13 to C15 mixture of synthetic primary amines with 15 molar proportions of ethylene oxide sold under the trade name "Synprolam" 35x15. The proportion of surface-active agent present in the aqueous compositions of the invention is not highly critical and a suitable proportion may readily be decided upon by a person skilled in the formulation art without excessive experimentation. However by way of illustration an amount of 50 to 100 grams per litre of the composition is usually suitable; although more or less may be used depending upon the particular composition. The composition of the invention may also contain an odourant, such as a pyridine derivative, as described in UK Patent No.

1406881, or n-valeric acid. The compositions may also comprise a pigment or a dye to give them a distinctive colour.

In greenhouse tests, compositions according to the invention containing paraquat have been found to be equivalent in herbicidal effect to commercial aqueous formulations of paraquat, the two types of composition being applied at the same rate of application of active ingredient.

Aqueous compositions according to the invention are readily prepared by adding the various constituents of the formulation to an aqueous solution of a salt of paraquat or diquat or a mixture thereof, with suitable agitation.

The following Examples illustrate the invention.

EXAMPLE 1 This Example illustrates an aqueous composition according to the invention.

Constituent Concentration(grams/litre) Paraquat (as di chloride) 100 Magnesium trisilicate 10 Magnesium chloride 100 Kelzan 1.0 Emetic (Compound A) 1.2 Water to 1 litre EXAMPLE 2 This Example illustrates an aqueous composition according to the invention.

Constituent Concentration(grams/litre) Paraquat (as di chloride) 200 Magnesium trisilicate 100 Magnesium chloride 100 Kelzan 3.0 Emetic (Compound A) 2.5 Water to 1 litre EXAMPLE 3 This Example illustrates an aqueous composition according to the invention.

Constituent Concentration (grams/litre) Paraquat (as dichloride) 200.00 Synperonic NP8 35.00 Nansa 1169PS 117.00 Kelzan 3.00 Magnesium chloride 100.00 Magnesium trisilicate 10.00 Compound A (see page 7) 1.65 Pyridine base 10.00 Sulfacide Blue 5.00 Silcolapse 5020 0.25 Acetic acid 80% solution to pH 7.0+ 0.5 Water to 1 litre The above composition was prepared by simple admixture of the constituents or aqueous solutions thereof.

The compositions of the constituents indicated by the various trade names are as follows: Synperonic NP8 A surface-active agent comprising a condensate of p-nonylphenol with eight molar proportions of ethylene oxide.

Nansa 1169 PS A surface-active agent comprising an aqueous solution of sodium dodecylbenzenesulphonate in a concentration of 30%.

Kelzan Xanthan gum, comprising a bacterial polysaccharide, present as a suspending agent.

Pyridine base An odourant comprising a mixture of alkyl pyridines, typically comprising methyl ethyl pyridines (40-60%), propylpyridine (20-30%), ethylpyridine (2.5%)picolines (less than 5%) and other alkyl pyridines, most of the mixture distilling in the range 180-2200C.

Sulfacide Blue A water-soluble blue dye.

Silcolapse 5020 A silicone-based antifoaming agent EXAMPLE 4 This Example illustrates an aqueous composition according to the invention.

Constituent Concentration (grams/litre) Paraquat (as dichloride) 200.00 Synperonic NP8 35.00 Nansa 1169PS 117.00 Kelzan 3.00 Sodium sulphate 100.00 Magnesium trisilicate 100.00 Compound A 1.65 Sulfacide Blue 5J liquid 5.00 Silcolapse 5020 0.25 Acetic acid 80% solution to pH 7.0 + 0.5 Water to 1 litre EXAMPLE 5 This Example illustrates an aqueous composition according to the invention.

Constituent Concentration (grams/litre) Paraquat (as dichloride) 200.00 Synperonic NP8 35.00 Nansa 1169PS 117.00 Kelzan 3.00 Magnesium chloride 100.00 Magnesium trisilicate 100.00 Compound A 1.65 Pyridine base 10.00 Sulfacide Blue 5J liquid 5.00 Silcolapse 5020 0.25 Acetic acid 80% solution to pH 7.0 + 0.5 Water to 1 litre EXAMPLE 6 This Example illustrates an aqueous composition according to the invention.

Constituent Concentration (grams/litre) Paraquat (as dichloride) 50.00 Diquat (as dibromide) 50.00 Synperonic NP8 35.00 Nansa 1169PS 117.00 Kelzan 3.00 Sodium sulphate 100.00 Magnesium trisilicate 100.00 Compound A 1.65 Pyridine bases 10.00 Sulfacide Blue 5J liquid 5.00 Silcolapse 5020 0.25 Acetic acid 80% solution to pH 7.0 + 0.5 Water to 1 litre EXAMPLE 7 This Example illustrates an aqueous composition according to the invention.

Constituent Concentration (grams/litre) Paraquat (as dichloride) 200.00 Synperonic NP8 35.00 Nansa 1169PS 117.00 Kelzan 3.00 Magnesium chloride 100.00 Magnesium trisilicate 50.00 Compound A 1.65 Pyridine base 10.00 Sulfacide Blue 5J liquid 5.00 Silcolapse 5020 0.25 Acetic acid 80% solution to pH 7.0 + 0.5 Water to 1 litre CHEMICAL FORMULAE (in description)

Claims (10)

1. CLAIMS 1. A herbicidal composition, comprising an aqueous solution of a salt of paraquat or diquat, or a mixture thereof, in admixture with a gelling agent that will gel at the pH of human gastric juice, an emetic, and a purgative comprising sodium sulphate or a non-toxic water-soluble salt of magnesium other than magnesium sulphate.
2. 2. A composition as claimed in claim 1 wherein the purgative comprises magnesium acetate, magnesium citrate or magnesium chloride.
3. 3. A composition as claimed in claim 2 wherein the purgative comprises magnesium chloride.
4. 4. A composition as claimed in any one of the preceeding claims comprising 50 grams or more per litre of paraquat or diquat or a mixture thereof.
5. 5. A composition as claimed in claim 4 comprising 100 grams or more per litre of paraquat or diquat or a mixture thereof.
6. 6. A composition as claimed in claim 5 comprising 200 grams or more per litre of paraquat or diquat or a mixture thereof.
7. 7. A composition as claimed in any one of claims 1 to 6 wherein the gelling agent comprises magnesium silicate.
8. 8. A composition as claimed in any one of claims 1 to 7 wherein the gelling agent comprises magnesium trisilicate.
9. 9. A composition as claimed in any one of claims 1 to 8 wherein the emetic comprises 2-amino-6-methyl-5-oxo-4-n-propyl-4,5-dihydro-5-triazolo [1,5-a]-pyrimidine.
10. 10. A composition as claimed in any one of the preceding claims which further comprises a suspending agent.