JPH04218596A - Selective acquisition of highly unsaturated fatty acids - Google Patents

Abstract

PURPOSE:To purify the title fatty acids selectively under mild conditions by such a technique as to enable industrially continuous operation. CONSTITUTION:Silver complexes of the title fatty acids are formed taking advantage of their high unsaturation nature, and the complexes are separated from other lipids taking advantage of said complexes' hydrophilicity and subjected to dissociation means; and then, taking advantage of such fatty acids' hydrophobicity, the objective fatty acids of higher purity can be obtained. For the dissociation means, addition of a dissociating agent and silver reduction have been mentioned. For the dissociating agent, besides water, a number of compounds each capable of forming a poorly soluble bound product with silver have been disclosed.

Description

Detailed Description of the Invention [0001] [Industrial Application Field] The present invention provides highly pure unsaturation of 3
The above highly unsaturated fatty acids or their esters, acid amides, and highly unsaturated aliphatic compounds with an unsaturated degree of 3 or more, such as highly unsaturated alcohols with an unsaturated degree of 3 or more (hereinafter simply referred to as highly unsaturated fatty acids). This is the method of obtaining The present invention deals with highly unsaturated fatty acids such as eicosapentaenoic acid and docosahexaenoic acid contained in fish oils such as mackerel and sardines, or their derivatives, and highly unsaturated fatty acids such as arachidonic acid contained in animal oils such as pigs. Fatty acids and their derivatives, α-linolenic acid and γ-linolenic acid contained in vegetable oils
It is possible to extract, separate, and purify highly unsaturated fatty acids such as linolenic acid and their derivatives, as well as highly unsaturated fatty acids and their derivatives derived from algae and microorganisms, without causing denaturation or alteration, and on an industrial scale. Low cost, efficient and selective extraction and separation of highly unsaturated fatty acids with degree of unsaturation of 3 or more.
Can be used for purification methods. [0003] Known methods for extracting, separating and purifying highly unsaturated fatty acids include urea addition, molecular distillation, solvent fractionation, and chromatography. However, these methods produce highly unsaturated fatty acids and their esters in high purity without denaturing them.
Separation and purification in large quantities at low cost is extremely disadvantageous. That is, the urea addition method has the drawback that the obtained highly unsaturated fatty acids and their esters have low purity, and the molecular distillation method has the drawback that polymerization and isomerization occur easily, resulting in denaturation. [0006] Solvent fractionation and chromatography are unsuitable for large-scale separation and purification on an industrial scale. [0007] It is generally known that silver ions form complexes with unsaturated bonds of organic compounds. [0008] As one method of utilizing this property, in JP-A-63-208549, silver ions are immobilized on an adsorbent, and by utilizing the difference in affinity between silver and unsaturated fatty acids, eicosapentaenoic acid, etc. A method for purifying highly unsaturated fatty acids or derivatives thereof such as esters is disclosed. [Problems to be Solved by the Invention] An object of the present invention is to improve the above-mentioned prior art by using linolenic acid, arachidonic acid,
Highly unsaturated fatty acids with a degree of unsaturation of 3 or more, such as eicosapentaenoic acid and docosahexaenoic acid, can be selectively produced in large quantities at low cost without altering the target product under extremely mild conditions. It is an object of the present invention to provide a method that enables extraction, separation, and purification. [Means for Solving the Problem] Based on the above-mentioned prior art, the present inventor has developed a method that can form a complex with an unsaturated bond and a containing substance containing a highly unsaturated fatty acid having a degree of unsaturation of 3 or more. By stirring a solution of silver salt in an aqueous medium at a certain concentration or more, only highly unsaturated fatty acids with an unsaturation degree of 3 or more selectively form an aqueous medium-soluble silver salt-highly unsaturated fatty acid complex. After extracting into the aqueous medium phase and separating the aqueous medium phase,
The present inventors have discovered that only highly unsaturated fatty acids can be selectively extracted, separated, and purified on an industrial scale by subjecting the complex in the aqueous medium phase to dissociation procedures, leading to the completion of the present invention. The highly unsaturated fatty acids in the present invention are defined in the header, and include ester type derivatives such as methyl ester, ethyl ester, triglyceride, diglyceride, monoglyceride, carboxylic acid type derivatives such as amide and methylamide, aliphatic Contains alcohol, etc. [0012] The term "highly unsaturated fatty acids" as used herein means fatty acids having a degree of unsaturation of 3 or more. To specifically explain the present invention, an aqueous medium solution of a silver salt capable of forming a complex with an unsaturated bond is added to a highly unsaturated fatty acid-containing material and stirred for 5 minutes to 4 hours to form an aqueous solution. A medium-soluble silver salt-highly unsaturated fatty acid complex is formed, and only the highly unsaturated fatty acids are selectively dissolved in the aqueous medium phase. [0014] Regarding the reaction temperature, the lower limit is as long as it is liquid and the upper limit is 100°C, but the reaction temperature depends on the stability of the highly unsaturated fatty acids, the solubility of the silver salt in water, the rate of formation of the complex, etc. Considering this, the reaction time is preferably around room temperature and 10
Preferably from minutes to 2 hours. [0015] In addition, the oxidative stability of highly unsaturated fatty acids,
Considering the stability of the silver salt, it is preferable that the present invention be carried out in an inert gas atmosphere, such as nitrogen, while shielding from light. There are no particular restrictions on the silver salt that can form a complex with an unsaturated bond, and silver compounds soluble in aqueous media such as silver nitrate, silver perchlorate, silver acetate, and silver tetrafluoroborate are generally used. used. [0017] The aqueous medium referred to in the present invention means, in addition to water, a compound having a hydroxyl group such as glycerin or ethylene glycol, and a mixture of these can also be used in the present invention. The molar ratio of highly unsaturated fatty acids and silver salt is 1:100
~100:1, the concentration of silver salt solution in aqueous medium is 0
．． The present invention can be carried out efficiently in the range of 1 mol/l to saturation. [0018] If the concentration is below this level, a complex will not be sufficiently formed and the highly unsaturated fatty acids will not be soluble in an aqueous medium. Considering the recovery rate of highly unsaturated fatty acids,
A molar ratio of 1:5 to 1:1 and a concentration of 1 to 20 mol/l are desirable. Since the complex formed by the above method exists in the aqueous medium phase, the aqueous medium phase and the fat/oil phase are mechanically separated, or the fat/oil phase is extracted using an organic solvent and other fatty acids that do not form complexes are removed. By removing the complex-containing portion, only the complex-containing portion can be separated from the reaction system. As the organic solvent in this case, it is preferable to use an organic solvent that can be separated from water, such as hexane or ether. The aqueous medium phase separated in this operation contains a silver salt-highly unsaturated fatty acid complex. By dissociating the complex in the aqueous medium phase, the highly unsaturated fatty acids can be liberated and recovered by reducing the solubility in the aqueous medium. Methods for dissociating the complex include a method of adding a complex dissociating agent to dissociate the complex, particularly a method of dissociating the complex by diluting it with water, or a method of dissociating the complex by reducing silver ions to silver. be. These means will be explained in detail below. [0025] In the dissociation of the complex by the complex dissociation agent, the highly unsaturated fatty acids forming a complex with silver are replaced by the complex dissociation agent, and the complex dissociation agent forms a bond with the silver ion, forming a silver compound with silver. This is achieved by forming a The complex-dissociating agent may be any compound that has a strong ability to dissociate the complex, but the following are listed by group: alcohols such as ethylene glycol, glycerin, diethylene glycol, 1,4-dioxane, tetrahydrofuran, An oxygen-based compound that has an oxygen atom in its molecule, including ethers such as crown ether, carbonyls such as acetone, etc., and that forms a bond with silver ions. Also, nitrogen containing amines such as ammonia, isobutylamine, ethylenediamine, diethylamine, triethylamine, pyridine, piperidine, amides such as dimethylformamide, N-methylpyrrolidone, nitriles such as acetonitrile, ammonium salts, amine salts, etc. A nitrogen-based compound that has atoms in its molecule and forms bonds with silver ions. Sulfur-based compounds having a sulfur atom in the molecule, including thiols such as 1,2-ethanedithiol, sulfides such as diphenyl sulfide and tetrahydrothiophene, sulfoxides such as dimethyl sulfoxide, and sulfones such as sulfolane; A compound that forms a bond with silver ions. In addition, trivalent phosphorus compounds such as triphenylphosphine, pentavalent phosphorus compounds such as phosphoric acids such as trimethyl phosphate and hexamethylphosphoric acid triamide, arsenic compounds such as triphenylarsine, phosphorus containing antimony compounds, etc. A compound of the phosphorus group that has a phosphorus atom in its molecule and forms a bond with silver ions. or ethylene, cyclohexene, 2-methyl-2-butene, which has an unsaturated bond capable of forming a complex;
Compounds that serve as π-bond ligands for alkenes such as isoprene, alkynes, aromatic compounds, carbon monoxide, isocyanides, nitrogen monoxide, etc. [0031] Also, sodium chloride, which is a compound and ion that forms a silver compound that is poorly soluble in silver ions and an aqueous medium, or a compound and its ion that has the effect of inhibiting the formation of a complex between silver ions and highly unsaturated fatty acids. , halogen salts such as sodium bromide, sulfates such as sodium sulfate, sulfites, nitrates, thiosulfates, carbonates such as sodium carbonate, carboxylates such as sodium acetate and sodium tartrate, thiocyanates such as ammonium thiocyanate. Salts, cyanide salts, azide salts such as sodium azide, permanganates and their acid forms, hydroxide compounds such as sodium hydroxide, and compounds that form bonds with silver ions. . A mixture of these compounds may be used, and the complex dissociating agent may have a plurality of sites for dissociating the above-mentioned complexes in one molecule. Dissociation of the complex by dilution with water occurs by diluting the aqueous medium phase by adding water, and highly unsaturated fatty acids are liberated. [0034] The amount of water to be diluted is generally required to be at least the same amount, although it is sufficient to cause dissociation of the complex, and preferably 10 to 40 times as much. Alternatively, a method for reducing silver ions to silver, sodium borohydride, formaldehyde, Fe(II)
The complex can also be dissociated by reduction using a reducing agent such as -EDTA, hypophosphorous acid, Cu, ascorbic acid, reduction by electrolysis, reduction by light, or the like. [0036] Considering the recycling of silver salts, the method of diluting with water is advantageous. The liberated highly unsaturated fatty acids are extracted with an aqueous medium such as hexane or ether and an organic solvent that can be separated.
Silver ions can be removed by washing the separated organic solvent phase with water and saturated saline. [0038] Furthermore, it is possible to recover and reuse the silver salt in the aqueous medium phase containing silver ions by distilling off the aqueous medium. Furthermore, in the present invention, it is possible to further increase the purity of highly unsaturated fatty acids by repeating the above separation and purification operations. As mentioned above, the present invention firstly utilizes the complex-forming ability of an aqueous medium-soluble silver salt capable of forming a complex with an unsaturated bond and highly unsaturated fatty acids, and secondly utilizes the ability of the present complex to form a complex with a highly unsaturated fatty acid. By utilizing the solubility of polyunsaturated fatty acids in aqueous media, it is possible to selectively separate the portion containing only highly unsaturated fatty acids, and thirdly, this is achieved by dissociating the complex and liberating the highly unsaturated fatty acids. This is a method that can selectively extract, separate, and purify highly unsaturated fatty acids from fatty acid-containing materials on an industrial scale. [0041] According to the method of the present invention, unlike conventional methods, only polyunsaturated fatty acids can be extracted from a fatty acid mixture at a large scale at almost room temperature, with simple operation and equipment, and at low cost. can be selectively extracted, separated, and purified. The method of the present invention can be applied to unrefined fish oil fatty acids and their esters, unrefined animal oil fatty acids and their esters, and can also be applied to fish oil fatty acids and their esters, animal oil fatty acids and their esters refined by conventional methods. By applying this method, highly unsaturated fatty acids and esters thereof with even higher purity can be obtained. [Example] The present invention will be explained in more detail with reference to Examples. In the Examples, the fatty acid composition was determined by gas chromatography after conversion to methyl ester, and the fatty acid ester composition was also determined by gas chromatography. [Example 1] 139.6 g of silver nitrate was added to 100 g of a fatty acid mixture containing 60% eicosapentaenoic acid in 80 ml of distilled water.
An aqueous solution dissolved in 1 ml was added under a nitrogen atmosphere while shielding from light. After stirring for 2 hours, the reaction solution was washed twice with 1 L of hexane. 2 L of distilled water was added to the obtained aqueous layer and stirred for 1 hour to liberate the fatty acid that had formed the complex. The released fatty acids were extracted twice with 1 L of hexane, and the obtained hexane layer was washed with distilled water and saturated saline.
Dehydrated with magnesium sulfate. [0047] Concentration under reduced pressure yielded 39.78 g of fatty acid. [0048] As a result of examining the fatty acid composition, the purity of eicosapentaenoic acid was 96.0%. The obtained eicosapentaenoic acid with a purity of 96.0% was further purified in the same manner as above, and as a result, eicosapentaenoic acid with a purity of 98.5% was obtained. Example 2 An aqueous solution of 130.9 g of silver nitrate dissolved in 80 ml of distilled water was added to 101.1 g of a fatty acid mixture containing 54.0% docosahexaenoic acid under a nitrogen atmosphere while shielding from light, and the mixture was stirred for 2 hours. It was treated in the same manner as in Example 1 to obtain 43.52 g of fatty acid. [0051] As a result of examining the fatty acid composition, the purity of docosahexaenoic acid was 92.3%. Further, the obtained docosahexaenoic acid with a purity of 92.3% was purified in the same manner as above, and as a result, the purity was 92.3%.
6.4% docosahexanoic acid was obtained. [Example 3] Fatty acid mixture 1 containing 53% arachidonic acid
An aqueous solution prepared by dissolving 142.1 g of silver nitrate in 80 ml of distilled water was added to 01 g under a nitrogen atmosphere while shielding from light, and the mixture was stirred for 2 hours. It was treated in the same manner as in Example 1 to obtain 30.05 g of fatty acid. [0054] As a result of examining the fatty acid composition, the purity of arachidonic acid was 95.2%. Further, the obtained arachidonic acid with a purity of 95.2% was purified in the same manner as above, and as a result, the purity was 97.0%.
% arachidonic acid was obtained. [Example 4] Eicosapentaenoic acid ethyl ester
1.012g of fatty acid ethyl ester mixture containing 2.1%
An aqueous solution of 1.688 g of silver nitrate dissolved in 0.5 ml of distilled water was added to the mixture under a nitrogen atmosphere while shielding from light, and the mixture was stirred for 2 hours. The reaction solution was washed twice with 20 ml of hexane. 20 ml of distilled water was added to the resulting aqueous layer and stirred for 1 hour to liberate the fatty acid ethyl ester that had formed the complex. The liberated fatty acid ethyl ester was extracted twice with 20 ml of hexane, and the obtained hexane layer was extracted with distilled water,
It was washed with saturated saline and dehydrated with magnesium sulfate. Concentration under reduced pressure yielded 209.7 mg of fatty acid ethyl ester. As a result of examining the composition of fatty acid ethyl ester, the purity of eicosapentaenoic acid ethyl ester was 98.
It was 0%. [Example 5] Docosahexaenoic acid ethyl ester was added to 59
．． 713.5mg of fatty acid ethyl ester mixture containing 0%
To this, an aqueous solution of 1.6873 g of silver nitrate dissolved in 0.5 ml of distilled water was added under a nitrogen atmosphere while shielding from light, and the mixture was stirred for 2 hours. Processed in the same manner as in Example 4, 251
．． 4 mg of fatty acid ethyl ester was obtained. [0064] As a result of examining the composition of the obtained fatty acid ethyl ester, the purity of docosahexaenoic acid ethyl ester was 97.0%. [Example 6] Arachidonic acid ethyl ester 52.5%
An aqueous solution of 1.6781 g of silver nitrate dissolved in 0.5 ml of distilled water was added to 630.2 mg of the fatty acid ethyl ester mixture containing the mixture under nitrogen atmosphere while shielding from light, and the mixture was stirred for 2 hours. [0066] Processing was carried out in the same manner as in Example 4, and 180
．． 3 mg of purified fatty acid ethyl ester mixture was obtained. As a result of examining the composition of this mixed fatty acid ethyl ester, the purity of arachidonic acid ethyl ester was 96.
It had been raised to 3%. Example 7 An aqueous solution prepared by dissolving 1.0703 g of silver perchlorate in 0.5 ml of distilled water was added to 670 mg of a fatty acid mixture containing 54.0% docosahexaenoic acid under a nitrogen atmosphere while shielding from light. After stirring for 2 hours, add 20 ml of hexane to the reaction solution.
Washed twice with 20 ml of distilled water was added to the resulting aqueous layer and stirred for 1 hour to liberate the fatty acid that had formed the complex. The released fatty acids were extracted twice with 20 ml of hexane, and the resulting hexane layer was washed with distilled water and saturated saline, and then dehydrated with magnesium sulfate. [0072] Concentration under reduced pressure yielded 211.9 mg of fatty acid. [0073] As a result of examining the fatty acid composition, the purity of docosahexaenoic acid was 93.1%. [Example 8] Pig liver oil obtained from pig liver by a conventional method was saponified to obtain free fatty acids of pig liver oil. It contained 14.3% arachidonic acid. [0076] 1.058g of this fatty acid was mixed with 0.5g of hexane.
An aqueous solution of 1.128 g of silver perchlorate dissolved in 0.5 ml of distilled water was added to the solution in a nitrogen atmosphere while shielding from light. After stirring for 1 hour, add 20 ml of hexane to the reaction solution.
Washed twice with 20 ml of distilled water was added to the obtained aqueous phase and stirred for 1 hour to liberate the fatty acid that had formed the complex. The released fatty acids were extracted twice with 20 ml of hexane, and the obtained hexane layer was washed with distilled water and saturated saline, and then dehydrated with magnesium sulfate. [0080] Concentration under reduced pressure yielded 70.8 mg of fatty acid. As a result of examining the fatty acid composition, the purity of arachidonic acid was 76.4%. Example 9 Linseed oil was saponified by a conventional method to obtain linseed oil free fatty acids. [0082] It contained 55.8% α-linolenic acid. [0083] 1.013g of this fatty acid was mixed with 0.5g of hexane.
An aqueous solution of 920 mg of silver perchlorate dissolved in 0.5 ml of distilled water was added to the solution in a nitrogen atmosphere while shielding from light. After stirring for 1 hour, add 20 ml of hexane to the reaction solution.
Washed twice with 20 ml of distilled water was added to the obtained aqueous phase and stirred for 1 hour to liberate the fatty acids that had formed the complex. The released fatty acids were extracted twice with 20 ml of hexane, and the obtained hexane layer was washed with distilled water and saturated saline, and then dehydrated with magnesium sulfate. [0087] Concentration under reduced pressure yielded 150 mg of fatty acid. [0088] As a result of examining the fatty acid composition, the purity of α-linolenic acid was 98.2%. Example 10 Borage oil was saponified by a conventional method to obtain borage oil free fatty acids. [0090] 23.1% of γ-linolenic acid was contained. [0091] 1.037g of this fatty acid was mixed with 0.5g of hexane.
An aqueous solution of 809 mg of silver perchlorate dissolved in 0.5 ml of distilled water was added to the solution in a nitrogen atmosphere while shielding from light. After stirring for 1 hour, add 20 ml of hexane to the reaction solution.
Washed twice with [0093] 20 ml of distilled water was added to the obtained aqueous phase and stirred for 1 hour to liberate the complexed fatty acids. The released fatty acids were extracted twice with 20 ml of hexane, and the obtained hexane layer was washed with distilled water and saturated saline, and dehydrated with magnesium sulfate. Concentration under reduced pressure yielded 59.7 mg of fatty acid. [0096] As a result of examining the fatty acid composition, the purity of γ-linolenic acid was 93.4%. [Example 11] Refined fish oil (triglyceride type, eicosapentaenoic acid 16.7%, docosahexanoic acid 11%).
An aqueous solution prepared by dissolving 853 mg of silver nitrate in 0.5 ml of distilled water was added under a nitrogen atmosphere while shielding from light. After stirring for 1 hour, add 20 ml of hexane to the reaction solution.
Washed twice with 20 ml of distilled water was added to the obtained aqueous phase and stirred for 1 hour to liberate the triglyceride that had formed the complex. [0100] The liberated triglyceride was dissolved in 20 m hexane.
The hexane layer obtained was washed with distilled water and saturated saline, and dehydrated with magnesium sulfate. Concentration under reduced pressure yielded 85.8 mg of fatty acid. [0102] As a result of examining the fatty acid composition, eicosapentaenoic acid is 38.5% and docosahexaenoic acid is 24.0%.
purity. [Example 12] 852 mg of silver nitrate was added to 0.5 ml of distilled water in a solution of 1.005 g of an aliphatic alcohol mixture containing 45% docosahexaenol dissolved in 0.5 ml of hexane.
An aqueous solution dissolved in was added under a nitrogen atmosphere while shielding from light. After stirring for 1 hour, add 20 ml of hexane to the reaction solution.
Washed twice with 20 ml of distilled water was added to the resulting aqueous phase and stirred for 1 hour to liberate the aliphatic alcohol that had formed the complex. [0106] The liberated aliphatic alcohol was dissolved in hexane 20
The hexane layer obtained was washed with distilled water and saturated saline, and dehydrated with magnesium sulfate. Concentration under reduced pressure yielded 145 mg of aliphatic alcohol. [0108] As a result of examining the composition, the purity of docosahexaenol was 98.5%. [Example 13] An aqueous solution of 847.2 mg of silver nitrate dissolved in 0.5 ml of distilled water was added to a solution of 1.003 g of a fatty acid mixture containing 60% eicosapentaenoic acid dissolved in 0.5 ml of hexane under a nitrogen atmosphere, shielded from light. and added. After stirring for 2 hours, add 20 ml of hexane to the reaction solution.
Washed twice with To the obtained aqueous phase, 20 ml of ethylene glycol was added as a complex dissociating agent and stirred for 1 hour to liberate the complexed fatty acids. The liberated fatty acids were extracted twice with 20 ml of hexane, and the resulting hexane phase was washed with distilled water and saturated saline, and dehydrated with magnesium sulfate. Concentration under reduced pressure yielded 133.8 mg of fatty acid. [0114] As a result of examining the fatty acid composition, the purity of eicosapentaenoic acid was 94.4%. [Example 14] An aqueous solution of 845.2 mg of silver nitrate dissolved in 0.5 ml of distilled water was added to a solution of 1.010 g of a fatty acid mixture containing 60% eicosapentaenoic acid dissolved in 0.5 ml of hexane under a nitrogen atmosphere, shielded from light. and added. After stirring for 2 hours, add 20 ml of hexane to the reaction solution.
Washed twice with To the obtained aqueous phase, 1.05 g of sodium chloride was added as a complex dissociating agent and stirred for 1 hour to liberate the complexed fatty acids. The liberated fatty acids were extracted twice with 20 ml of hexane, and the resulting hexane phase was washed with distilled water and saturated saline, and dehydrated with magnesium sulfate. Concentration under reduced pressure yielded 150.6 mg of fatty acid. [0120] As a result of examining the fatty acid composition, the purity of eicosapentaenoic acid was 93.1%. [Examples 15 to 49] As a complex dissociation agent, various compounds that form a bond with silver that is poorly soluble in water were tested, and the results showed that the purity of the highly unsaturated fatty acids ranked first. Shown in the table. All experimental conditions were as follows: 1 g of eicosapentaenoic acid from Example 1 or 1 g of eicosapentaenoic acid ethyl ester from Example 4, and using the dissociating agent shown in Table 1. In the case of the ethyl ester thereof, substantially the same operations as in Example 4 were carried out. [Table 1]

Claims (8)

[Claims] Claim 1: A complex of highly unsaturated fatty acids and silver is formed by contacting a substance containing highly unsaturated fatty acids with an aqueous medium containing a silver salt, and then an oil and fat phase is removed from the aqueous phase, and then this A method for selectively obtaining highly unsaturated fatty acids, which comprises obtaining highly unsaturated fatty acids by dissociating the complex in an aqueous phase. Claim 2: The concentration of silver salt in the aqueous medium is 0.1 mo.
2. The method of claim 1, wherein the ratio is greater than or equal to l/l. 3. The method of claim 2, wherein the means for dissociating the complex is the addition of a complex dissociating agent. 4. The method of claim 2, wherein the means for dissociating the complex is dilution by adding water. 5. The complex dissociating agent contains oxygen compounds including alcohols, ethers, carbonyls, etc., nitrogen compounds including amines, amides, nitriles, etc., or thiols, sulfides, etc. sulfur compounds,
Phosphorus compounds including trivalent or pentavalent phosphorus compounds, arsenic compounds, antimony compounds, etc. that form bonds with silver, or compounds having complex functional groups of these compounds, and mixtures thereof. Characteristic claim 3
the method of. 6. The method according to claim 3, wherein the complex dissociating agent is a compound or ion that produces silver ions and a silver compound that is sparingly soluble in an aqueous medium. [Claim 7] Claim 3, wherein the complex-dissociating agent is a compound having an unsaturated bond that has complex-forming ability.
the method of. 8. The method of claim 2, wherein the means for dissociating the complex is a means for reducing silver ions to silver.