EP0257523B1 - Process for manufacturing water soluble triphenylmethane dye compounds of foodstuff standard - Google Patents

Description

IP-OS 130481/1979 describes an electrochemical process for the production of triphenylmethane dyes by anodic oxidation of the corresponding leukotriphenylmethane compounds. In this process, the oxidation takes place in an electrolysis cell, which is divided into two by a semi-permeable membrane made of unglazed porcelain. A network of lead dioxide is used as the oxidation anode, whose oxygen overvoltage at a current density of - 0.05 KA / m ^{2 is} more than 700 mV. The electrolysis takes place in the pH range between 6 and 7, current and material yields of 50 to 65%, based on the leuco compounds used, being obtained. When the leuco compounds of the dye CI Acid Blue 9; CI No. 42090 gives this process technology a cloudy and greenish blue dye that can only be used to a limited extent.

Products which provide pure and brilliant colorations are obtained in high yield according to the prior art by chemical oxidation of the leuco compound of C.I. Acid Blue 9; C.I. No. 42090 with alkali dichromates in media containing oxalic acid. Difficulties here arise in separating the chromium compounds from the color, so that the color produced in this way is not suitable for the food sector.

Furthermore, it was proposed to produce water-soluble diamino-triphenylmethane dyes with 2 to 4 sulfonic acid groups by anodic oxidation of the leuco compounds in a two-part electrolysis cell, the leuco compounds being anodically at a pH s 4, optionally in the presence of oxygen-bearing mineral acid anions, with an oxygen overvoltage of ≤ 400 mV be oxidized. Dyes free of chromium ions are obtained, but the content of 2-, 3- and 4-sulfobenzaldehyde in the dry dye is above the concentration of 1.5% by weight permitted for foodstuffs.

The object of the present invention was to provide a reliable electrochemical process by which pure diamino-triphenyimethane dyes containing sulfonic acid groups, in particular CI No 42090, which meet the specifications for food dyes (CI Food Blue 24; CI No. 42090) can be produced. .

This object is achieved with the aid of the method of the invention. The invention relates to a process for the preparation of water-soluble 2 to 4 food-grade diamino-triphenylmethane dyes by anodic oxidation of the leuco compounds in a two-part electrolytic cell, which is characterized in that the leuco compounds are present in the presence of small amounts of C ₂ - to C ₄ - Alkanols, urea, urea derivatives or mixtures thereof anodically oxidized at potentials ε _h ≤ 1250 mV and temperatures ≤ +40 _° C.

The process according to the invention gives diamino-triphenylmethane dyes containing sulfonic acid groups, in particular CI No 42090, in food quality with a content of <1.5% by weight of 2-, 3- and 4-sulfobenzaldehyde.

The process according to the invention is generally carried out in such a way that the solution of the leuco dye and the alkanol and / or urea additives are introduced into the anode compartment of a two-part electrolytic cell which is equipped with an anode of low oxygen overvoltage. The cathode compartment contains a current-conducting solution, preferably a mineral acid. The electrolysis takes place at an oxygen overvoltage of s 100 mV.

Examples of oxidizable leuco compounds include those of the acidic dyes CI 42045, 42051, 42052, 42053, 42075, 42080, 42085, 42090, 42095, 42100, 42105, 42120, 42135, 42150, 42155 and 42165. The leuco compounds of the dyes CI 42090, 42045, 42051, 42052, 42053, 42080, 42105, 42135 and 42165 are preferred, in particular the leuco compound of the C.I. Acid Blue 9; CI 42090.

Suitable C ₂ -C ₄ -alkanols are n-butanol, iso-butanol, n-propanol, iso-propanol and ethanol. Instead of urea, urea derivatives such as guanidine, sarcosine, arginine, creatine and creatinine can also be used. Ethanol or urea is preferred as an additive to the leuco solution. The solution of the leuco dyes to be electrolyzed contains, based on the solution, 0.01 to 2% by weight, preferably 0.1 to 0.7% by weight, of these additives.

The content of sulfate ions in the leuco solution is ≦ 2% by weight, based on the solution. The potential of the anode during the electrochemical oxidation is ε _h ≦ 1250 mV, preferably 750 to 1000 mV. In the anolyte used, this corresponds to an oxygen overvoltage of 100 mV to 0 mV.

The temperature during electrolysis must not exceed +40 _° C to prevent thermally induced formation of sulfobenzaldehyde.

Suitable anode materials with low oxygen overvoltages for the leuco dye-containing electrolytes are valve metals such as titanium, tantalum and niobium with a low microroughness, the metals being surface-doped with electrically conductive, non-stoichiometric mixed oxides of valve metals and platinum group metals or platinum metal compounds. Electrodes made of titanium or niobium are preferred for this purpose, which have mixed oxides of titanium and ruthens, tantalum and iridium or ruthens and iridium on the surface. The surface of the valve metals should not be roughened by sand, corundum or iron gravel radiation.

Organic anions and cations have been used to separate the anode and cathode compartments proven membrane. Suitable ion exchangers are, for example, polymers and copolymers of styrene, styrene and divinylbenzene, styrene and maleic anhydride, acrylic esters and divinylbenzene, olefins, perfluorinated olefins, vinyl chloride and acrylonitrile, which carry sulfonic acid groups and / or primary, secondary, tertiary amino groups as secondary charge groups. Cation exchange membranes are particularly suitable.

When using cation exchange membranes to separate the cathode and anode compartments, aqueous mineral acids, salt solutions or alkalis can be used as the catholytes, especially aqueous sulfuric acid or solutions of alkali metal hydroxides. In the case of the use of alkali lyes, the lye corresponding to the alkali application of the leuco dye is used, so that 25 to 35% by weight aqueous alkali lye can be obtained as a by-product. When using anion exchange membranes, aqueous alkali solutions, ammonia or alkali metal or ammonium carbonate or bicarbonate solutions are suitable as catholytes. To avoid damage to the membranes is one of the alkalis advantageously primary, secondary or tertiary amines, for example those with C1 to _C4 alkyl groups at the nitrogen atom, the alkyl radicals optionally bearing a hydroxyl or alkoxy group as a substituent, in a concentration of 0.02 to 10% by weight, based on the alkali metal hydroxide solution.

Cation exchange membranes and dilute sulfuric acid are preferably used as the catholyte. The concentration of sulfuric acid is preferably 2 to 10% by weight. This combination has the advantage that the dye solutions are additionally cleaned of foreign metal cations during the oxidation and damage to the membrane is prevented.

The aqueous solutions of the leuco compounds are used as anolytes. The concentrations of leuco compounds in the anolyte are generally between 2 and 40, preferably 5 to 30,% by weight, based on the solution.

In order to avoid diffusion impediments, a good flow against the anode (turbulent flow and mean flow velocities ≥ 0.1 m / s) must be ensured in the anode area. The current densities in the electrolysis are 0.05 to 1 KA / m ² , preferably 0.1 to 0.6 KA / _M ² .

Based on the leuco compound used, electrolysis is preferably carried out up to a theoretical current supply of 90% with a current density of 0.5 to 0.6 KA / m 2 and then the current density is reduced to approximately 0.3 KA / mε.

Since oxygen can be formed as a side reaction, the effective power supply for the leuco connection should be more than 100%. Electricity offers of 101 to 110% are preferred, based on the leuco connection used.

The anodic oxidation of the leuco compounds takes place at temperatures ≦ +40 _° C, preferably at temperatures in the range from +5 to +30 _° C.

In order to avoid deposits on the anode surfaces and overoxidation of the dye, it has proven to be advantageous to switch the anodes briefly, ie 15 to 30 seconds, when the potential rises to ε _h = 1250 mV. In this procedure, it is expedient to use electrodes which can be reversed when using doped titanium anodes and which contain intermediate layers of titanium, tantalum or niobium suboxides or carbides, silicides or borides of these metals. Electrodes with titanium suboxide intermediate layers have proven to be particularly favorable here.

Furthermore, in order to avoid deposits on the anodes, it has proven advantageous to briefly (5 to 10 minutes) develop oxygen at the anode in aqueous, 5 to 10% by weight sulfuric acid before the electrolysis of the leuco solutions.

The method is to be additionally explained by the following exemplary embodiments. The parts and percentages relate to the weight.

example 1

a) A flat titanium profile electrode is used as the anode in an electrolytic cell divided by a cation exchange membrane, the surface of which is doped with a non-stoichiometric tantalum-iridium mixed oxide and has an intermediate titanium suboxide layer. A Haber-Luggin capillary is attached to the surface of this anode and is connected to a silver / silver chloride reference electrode via a current key. A copper electrode is used as the cathode. Aqueous, 5% by weight sulfuric acid is added as the catholyte into the cathode compartment. A 20% by weight aqueous solution of the leuco compound of CI Acid Blue 9; CINr. 42090, which contains 0.5% by weight of urea (the leuco dye was obtained by condensation from o-sulfobenzaldehyde and N-ethylsulfobenzylaniline). This solution contains - 0.9% free sulfate. The anolyte is pumped through the anode compartment at an average speed of - 0.5 m / s and electrolyzed at a current density of 0.6 KA / m ² up to a current supply of 90% based on the leuco compound used (ε _h = 960 mV).

The current density is then reduced to 0.3 KA / m ² and the electrolysis is ended after a current supply of 103%.

The electrolysis takes place at a cell voltage of - 3.5 V and at a temperature of +25 _° C. The anode potential at the beginning of the electrolysis is approximately ε _h = 900 mV. After a theoretical power supply of 103%, based on the leuco connection used, the potential rises to about ε _h = 980 mV. The oxygen overvoltage is 0 mV. At this point the electrolysis is stopped and the electrolyte is worked up.

• λ_max.: 631 nm in 0,02 m Ammoniumacetatlösung.

Based on the leuco dye used, 99.9% pure CI Acid Blue 9; CI No. 42090 received with the following data:

• λ _max .: 631 nm in 0.02 m ammonium acetate solution.

The 2-, 3- and 4-sulfobenzaldehyde content is <0.1%, based on the dry dye.

Similar results (content of sulfobenzaldehyde <0.1%) were obtained using titanium anodes doped with titanium-ruthenium mixed oxide or ruthenium-iridium compounds.

b) If the electrolysis is carried out under otherwise identical conditions in the absence of urea, after a theoretical current supply of 103%, based on the leuco compound used, a dye is obtained which, based on the dry dye, contains 0.85% by weight Contains 2-, 3- and 4-sulfobenzaldehyde.

If electrodes are used whose supports have been roughened by sanding before doping with mixed oxides or iridium-ruthenium compounds, a dye is obtained under otherwise identical electrolysis conditions, which contains 1.6% by weight, based on dry dye, of 2-, 3- and Contains 4-sulfobenzaldehyde.

Example 2

The procedure was as in Example 1 a), but the power supply - based on the leuco compound - was varied between 60 and 110% and the content of the formylbenzenesulfonic acids was determined.

For comparison, the process was carried out as described in Example 1b) in the absence of urea.

Example 3

The procedure was as in Example 1, but a titanium flat profile anode doped with titanium-ruthenium mixed oxide was used. The solution of the leuco dye given in Example 1a) and, for comparison, the solution given in Example 1b) were once electrolysed at different currents and thus at different anode potentials up to a current supply of 103%. The dependency found in these tests on the anode potentials and oxygen overvoltages and contents of 2-, 3- and 4-sulfobenzaldehyde in the process product is summarized in Table 1.

Example 4

The electrolysis is carried out as indicated in Example 1a, but at different temperatures Electrolyzed up to a power supply of 103%. Depending on the temperature, the stated sulfobenzaldehyde contents were found:

Example 5

The electrolysis was carried out as in Example 1 a), but 0.8% by weight of ethanol was added to the anolyte instead of urea. It was electrolyzed with a current density of 0.5 KA / m ² up to a current supply of 90%. The current density was then reduced to 0.3 KA / m ² and the electrolysis ended after a current supply of 105%.

• A_max.: 631 nm in 0,02 m Ammoniumacetatlösung

Based on the leuco dye used, 99.8% pure Acid Blue 9; CINr. 42090, received with the following data:

• A _max .: 631 nm in 0.02 m ammonium acetate solution

2-, 3- and 4-sulfobenzaldehyde content: 0.2%.

Examples 6 to 9

The oxidation was carried out as described in Example 5, but the alcohols listed in the table were used. The corresponding dyes were obtained in high yields, good purity and good coloristic properties. The levels of 2-, 3- and 4-sulfobenzaldehyde are given in the last column.

Claims (7)

1. A process for the preparation of a water-soluble food-grade diaminotriphenylmethane color containing from 2 to 4 sulfo groups by anodic oxidation of the corresponding leuco compound in an electrolysis cell divided into two, wherein the leuco compound is anodically oxidized in the presence of a small amount of a C₂-C₄-alkanol, urea, a urea derivative or a mixture of these, at potentials ε_h ≤ 1250 mV and at +40_°C.

2. A process as claimed in claim 1, wherein the anodic oxidation is carried out at potentials εh of from 750 to 1000 mV.

3. A process as claimed in claim 1 or 2, wherein the leuco compound of the dye C.I. Acid blue 9 (C.I. No. 42 090) is oxidized.

4. A process as claimed in claim 1 or 2 or 3, wherein the solution of the leuco dye contains from 0.01 to 2% by weight, based on the solution, of an alkanol, urea, a urea derivative or a mixture of these.

5. A process as claimed in claim 1 or 2 or 3 or 4, wherein the leuco solution contains ≤ 2% by weight, based on the solution, of sulfate.

6. A process as claimed in claim 1 or 2 or 3 or 4 or 5, wherein titanium or niobium having low microrough- ness is used as the anode, which contains, on the surface, electrically conductive oxide compounds of titanium and ruthenium, of tantalum and iridium or of ruthenium and iridium.

7. A process as claimed in claim 1 or 2 or 3 or 4 or 5 or 6, wherein the anode space and cathode space are separated by a cation exchanger membrane.