US2948614A - Stabilized photographic silver halede - Google Patents

Description

Unite States Patent STABILIZED PHOTOGRAPHIC SILVER HALIDE EMULSIONS CONTAINING S-THIOCTIC ACID Charles Francis Hitchcock Allen, Bernard D. Illingsworth,

and John J. Sagura, Rochester, N .Y., assignors to Eastman Kodak Company, Rochester, N.Y., a corporation of New Jersey No Drawing. Filed July 11, 1957, Ser. No. 671,129

13 Claims. (21. 96-109) This invention relates to a fog-inhibiting agent or stabilizer for photographic silver halide emulsions, and a method of preventing scum in photographic silver halide emulsions containing certain types of sensitizers.

It is well known that photographic emulsions on storage tend to lose sensitivity and to become spontaneously developable without exposure to light. There is normally a detectable amount of the silver salt reduced during development in the areas where no exposure was given;

this is commonly called fog, and sometimes called N chemical fog where it is necessary to distinguish between it and the effects of accidental exposure to radiation; in this invention, we are not concerned with the latter.

Fog depends both on the emulsion and the conditions of development; for a given emulsion it increases with the degree of development. With constant development conditions, it tends to increase with. time, temperature and relative humidity of storage conditions; it is common practice to make accelerated tests of the stability of photographic emulsions by storage at increased temperature or humidity, or both. It is, of course, desirable to have V.

emulsions as stable as possible under the conditions of high temperature and humidity which may occur in tropii cal climates, for example. Fog usually appears over the whole area of the sensitive coating, but when severe, it frequently is non-uniform. Fog may also be caused by exposure to chemicals, for example, hydrogen sulfide and other reactive sulfur compounds, hydrogen peroxide vapor, and strongly reducing materials. While antifoggants and stabilizers may protect, to some extent, against such effects, it is normally understood that an antifoggant protects against spontaneous growth of fog during prolonged storage or storage at temperatures and humidities, or during development to maximum contrast and speed, or both.

When high-speed negative emulsions containing an alkylene oxide polymer as a sensitizer are developed in high-solvent developers, such as. Kodak Developer DK- 20, a brown image tone or a brown surface deposit commonly called scum is observed.

It is, therefore, an object of our invention to provide a method of stabilizing photographic silver halide emulsions. A further object is to maintain the sensitivity and fog of silver halide emulsions at or close to initial optimum values under keeping conditions of high temperature and humidity. Another object is to provide a means of reducing or removing scum which forms during development of high-speed negative emulsions containing alkylene oxide polymers as emulsion sensitizers. Still another object is to provide photographic silver halide emulsions containing a stabilizer or fog-inhibiting agent. Other objects will become apparent from a consideration of the following description and examples.

According to our invention, photographic silver halide emulsions can be stabilized against the formation of spontaneous fog by incorporating in the emulsions 5- thioctic acid. This compound can be illustrated by the following formula:

7 H2O S CHzCHzCHiCOOH C H H:

for this particular purpose, can be added either to the empil sion, to a pre-bath or to the developing solution itse For the purpose of inhibiting fog formation, the 5- thioctic acid can be added to the emulsion during the process of manufacture to avoid loss of sensitivity and to inhibit the growth of fog with passage of time under non-ideal conditions of storage.

A solution of the S-thioctic acid of the invention when added in suitable concentration, before coating, to unsensitized, chemically sensitized, or optically sensitized photographic emulsions does not appreciably alfect the sensitometric values for sensitivity and fog when measurements are made soon after coating. When sensitometric measurements are made at appreciable intervals of time, at elevated temperatures and dry or somewhat humid conditions, this compound does stabilize photographic speed and maintain fog at a low level.

The preparation of silver halide emulsions involves three separate operations: (1) the emulsification and digestion or ripening of the silverihalide, (2) the freeing of the emulsion from excess soluble salts, usually by washing, and (3) the second digestion or after-ripening to obtain increased sensitivity. '(Mees, The Theory of the Photographic Process (1942).). We prefer to add the fog-inhibiting agents after the final digestion or afterripening, although they can advantageously be added prior to digestion. I I

The photographic emulsions used in practicing our invention are generally of the developing-out type; also, it is to be understood that photographic emulsions of varying halide content can advantageously be used. The 5-thioctic acid used in our invention has been found particularly useful when employed in conjunction with gelatino-silver-bromiodide emulsions, although it can also be advantageously employed in other silver halide emulsions, such as gelatinosilver-chloride, bromide, chlorobromide, chlorobromiodide, etc.

The emulsions can also be chemically sensitized by any of the accepted procedures. The emulsions can be digested with naturally active gelatin, or sulfur compounds can be added such as those described in Sheppard US. Patent 1,574,944 and U.S. 1,623,499, and Sheppard and Brigham US. Patent 2,410,689.

The emulsions can also be treated with salts of the noble metals such as ruthenium, rhodium, palladium, iridium and platinum, all of which belong to group VIII of the periodic table of elements and have an atomic weight greater than 100. Representative compounds are ammonium chloro-palladate, potassium chloroplatinate and sodium chloropalladite, which are used for sensitizing in amounts below that which produces any substantial fog inhibition, as described in Smith and Trivelli US. Patent 2,448,060, and as antifoggants in higher amounts, as described in Trivelli and Smith US. Patents. 2,566,245 and 2,566,263. I

The emulsions can also be chemically sensitized with gold salts as described in Waller and Dodd US. Patent 2,399,083, or stabilized with gold salts as described in Patented Aug.-9, 1960 3 Damschroder U.S. Patent 2,597,856 and Yutzy and Leermakers U.S. Patent 2,597,915. Suitable compounds are (potassium chloroaurite, potassium aurithiocyanate, potassium chloroaurate, auric trichloride and 2-aurosulfobenzothiazole methochloride.

The emulsions can also be chemically sensitized with reducing agents such as stannous salts (Carroll U.S. Patent 2,487,850), polyamines such as diethylene triamine (Lowe and Jones U.S. Patent 2,518,698), polyarnines, such as spermine (Lowe and Allen U.S. Patent 2,521,925), or bis-(,B-aminoethyl) sulfide and its watersoluble salts (Lowe and Jones U.S. Patent 2,521,926).

The emulsions can also be stabilized with the mercury compounds of Allen, Byers and Murray U.S. Patent 2,728,663, Carroll and Murray U.S. Patent 2,728,664, and Leubner and Murray.U.S. Patent 2,728,665.

The S-thioctic acid is effective in the presence or absence of optical sensitizing dyes. Since optical sensitizing may affect stability of emulsions with respect to sensitivity, fog and latent image changes, the action of the S-thioctic acid of this invention is not completely independent of optical sensitizing or other emulsion variables. We have found, however, that both unsensitized emulsions and emulsions sensitized with cyanine and developed for 18 minutes at 68 F. in a developer (D-l9) having the following composition:

Water, about 125 F. (50 C.) cc 500 Kodak Elon developing agent gram 2.0 Kodak sodium sulfite, desiccated do 90.0 Kodak hydroquinone do 8.0 Kodak sodium carbonate, monohydrated -do 52.5 Kodak potassium bromide do 5.0

Cold water to make 1.0 liter.

Fresh tests and incubation tests (1 week at 120 F., constant humidity) were also run for the same emulsion batches with development for 5 minutes in a normal developer (DK-SO) (same as used in Table Ia below) for comparison with forced development (D-l9). The speed, gamma and fog for each of the emulsion coatings were measured. The speed figure shown in the following table is on a logarithmic scale, obtained by the formula l00(1l0g E), where E is the exposure in metercandle-seconds of sunlight quantity required to produce a density of 0.30 above fog. Consequently, a difference of about speed units represents a difference of about 100 percent in emulsion speed. The results are given in the following table:

Table I 5 min. Kodak Developer DK50 18 min. Kodak Developer D-19 Coating Number Addenda Fresh 1 Wk. 120 Inc. res

CR 7 Fog C/R 'y Fog C/R 7 F08 Speed Speed Speed 311 1. 25 15 205 1. 25 .37 332 1. 07 .33 (b) 5-Th10ct 1c 2 1d (I) 3.0 gJmole g 313 1, 34 12 291 0, 3s .11 333 1.10 .18 (c) D -6-th1oct1c ae1d I) 0.03 21 6 AgX 311 1. 24 14 300 1. 2s .15 332 1. 25 .20 DL-6-th10ct1c ac d (II) 0.3 gJmole AgX 301 1,15 10 292 1. 07 .13 330 1. 32 .23 (e DL6-th1oct1c and (II) -0 g-/m01e AgX 297 11 237 1. 13 13 321 1. 50 .1?- PEG 1500 of oleyl alcohol 0- -l e g 335 1.0a 13 321 0. s0 .41 354 1.05 .41 0 -lmole plus (I) 3 s-l s 32s 1. 04 15 315 0. s0 .22 315 1.12 .25 -l ole I) 0.03 27 A 325 1.05 13 320 1. 05 .21 341 1. 11 .36 (I 0- g-/mo1e I) 0.3 al ol 12 316 1.00 14 305 1. 04 .15 340 1.12 .25 -lmoie (I 3.0 e/ AgX 304 0. 95 11 286 1. 10 .21 325 1. 31 15 or merocyanine dyes, or both, can be treated with 5- thioctic acid according to our invention.

The S-thioctic acid useful in practicing our invention can be prepared as described in Jour. Am. Chem. Soc., vol. 76 (1954), page 1828.

It has been previously proposed to sensitize photographic silver halide emulsions with 6-thioctic acid. See U.S. Patent 2,728,668, issued December 27, 1955. This acid can be represented by the following formula:

However, 6-thioctic acid does not have the desirable stabilizing properties of 5-thioctic acid, particularly when emulsions containing the same are subjected to forced development. A comparison of the properties of the two acids in this respect is given in Table I below. These data were obtained using an ordinary high speed bromiodide emulsion chemically sensitized with sulfur and gold compounds, and spectrally sensitized with a cyanine dye. A separate series of tests was run for an identical batch of emulsion which had been chemically sensitized with the oleyl ether of polyethylene glycol (PEG) (molecular weight about 1500) (0.75 g./mol. AgX). This particular chemical sensitizer provides useful speed increases, but also produces an increase in fog. Each series of emulsions which had been coated on a cellulose acetate support were exposed on a Kodak Ib sensitomer The same type of emulsion used in arriving at the data in Table I above was treated with S-thioctic acid in the amounts shown in the following table, and the treated emulsions compared with a similar batch containing no S-thioctic acid, both where the emulsions contained no chemical sensitizer (the same oleylether of polyethylene glycol used in Table l) and where the emulsions contained this sensitizer in the same amounts used in Table I. A portion of each emulsion was coated on a cellulose acetate support and incubated for one week at a temperature of F. and constant relative humidity (obtained by placing the coatings in closed containers, the ambient temperature being about 70 F. and relative humidity about 55% prior to sealing the containers).

The efficiency of the S-thioctic acid was determined by measuring the speed, gamma and fog of the incubated emulsions containing 5-thioctic acid and comparing these measurements with those of the same batch of emulsion before incubation. Similar measurements were made with a photographic emulsion containing no S-thioctic acid both before'and after incubation. The coatings were then exposed on an intensity scale sensitometer (Eastman Type Ib) and developed for 5 minutes in a developer having the following composition:

Grams N-methyl-p-aminophenol sulfate 2.5 Hydroquinone 2.5 Sodium sulfite (desiccated) 30.0 Sodium metaborate 10.0 Potassium bromide 0.5

Water to make 1 liter.

alkylene oxides with aliphatic acids, e.g., lauric acid and glycine, condensation products of alkylene oxides with aliphatic amines or amides, e.g., glycine and lauryl amide, and condensation products of alkylene oxides with phe nols, e.g., phenol. The preparation of these condensation products is described in U.S. Patent 1,970,578, Condensation products of alkylene oxides with hexitol ring Table Ia 5-Thioctic Oleyl Fresh Tests Incubation, 1 Week at cid, Ether, 120 F. Coating g./mol g./mol.

AgX AgX Speed Gamma Fog Speed Gamma Fog none none 305 1. 24 16 292 1. 34 3. 0 none 307 1. 21 12 296 1.01 11 none 0. 75 328 l. 02 18 306 81 48 3. 0. 75 323 97 14 310 83 21 The S-thioctic acid useful in practicing our invention dehydration products as described in U.S. Patent can be used in various kinds of photographic emulsions. In addition to being useful in ordinary non-sensitized emulsions, it can also be used in orthochromatic, panchromatic and X-ray emulsions. If used with sensitizing dyes, it can be added to the emulsion before or after the dyes are added. Suitable dispersing agents for the silver halide emulsions stabilized according to our invention comprise gelatin, or other colloids, such as collodion, albumen, cellulose organic derivatives, synthetic resins, etc.

The optimum amount of S-thioctic acid as a fog-inhibiting agent can be determined by making the customary tests employed in emulsion making. Of course, the optimum amount for a given emulsion will vary depending on the presence of emulsion addenda, such as chemical sensitizers, optical sensitizers, etc. In general, we have found that from 0.25 to 4.0 g. of S-thioctic acid'per mole of silver halide is sufficient for the purposes of our invention.

As indicated above, the S-thioctic acid of our invention is particularly useful in inhibiting or preventing the formation of scum during the development of high-speed negative emulsions which have been sensitized with an alkylene oxide polymer.

The alkylene oxide polymers used .to sensitize the emulsions may be of various types. from which the polymers are derived contain from 2 to 4 carbon atoms, e.g., ethylene oxide, propylene oxide and butylene oxide. The preparation of polymers from these compounds is described in Ellis, The Chemistry of Synthetic Resins (1935), pages 990 to 994. These compounds are also referred to as polyalkylene glycols and their use as sensitizers for silver halide emulsions is described in U.S. Patents 2,423,549 and 2,441,389. The alkylene oxide derivatives may be prepared by condensing an organic compound containing an active hydrogen atom with an alkylene oxide polymer, or by condensing the active hydrogen compound with the alkylene oxide during the polymerization of the latter material.

Various derivatives of alkylene oxides may also be used to sensitize the emulsions, e.g., condensation products of alkylene oxide with organic compounds containing an active hydrogen atomf Examples of active hydrogen organic compounds, i.e., compounds in which a hydrogen atom may be replaced by reaction of the compound with metallic sodium, methylmagnesium iodide, etc., include alcohols, amines, mercaptans, acids, amides, hydrocarbons, such as acetylene, and compounds having The alkylene oxides the active hydrogen in a methylene group such as dibenzoylmethane. More specifically, We may employ condensation products of alkylene oxide with glycols, such as those having from 8 to 18 carbon atoms as described in U.S. Patent 2,240,472 and British Patent 443,559 as well as condensation products of alkylene oxides with aliphatic alcohols, condensation products of 2,400,532 may also be employed.

In each case the polyalkylene oxide or derivative of alkylene oxide should have :a molecular Weight of at least 300. Condensation products of ethylene oxide with long chain alcohols, acids, amines or amides should have a molecular weight of about 700. In general, the best results are obtained with the condensation products of ethylene oxide with long chain compounds having a chain length of 12 or more carbon atoms and with ethylene oxide polymers having a molecular weight of 1500 to 4000 or more.

The alkylene oxide derivatives used to sensitize the emulsions may be illustrated by .the following specific examples, although our invention is in no way limited to the use of these specific compounds.

HOCH CH O(CH CI-I O) CH CH OH Polyethylene glycol 0 (OH2OH2O)]JCH2CHQOH (CHQm O(OH2OHZO),,CH1OH OH Di-(polyethylene-glycoxy)-decane V n i HOCl-I CH O(CH CH O),,CH CH OC ';H V

Polyethylene glycol oleyl ether HOCH CH O (CH CH O) CH CH O-'CO--C -;H

Polyethylene glycol oleic ester where n=an integer greater than about 10.

Polyethylene glycol nonyl phenol ether N-methyl myristylamino polyethylene oxide N-methy1-n-dodecylamino polyethylene oxide The anti-soumming effect of the S-thioctic acid was observed by preparing a high-speed gelatino-bromiodide emulsion containing the usual addenda, such as hardener, cyanine dyes, and stabilizers. The emulsion was then divided into several batches and treated with the addenda indicated in Table II. The coatings are identified as a-d in the table. Separate sections of the coatings were exposed in an Eastman Type Ib sensitometer as indicated above with respect to Tables I and la, and the sections developed in a normal developer (DK-SO') and another section in a high-solvent developer (BK-20). The speed, gamma and fog measurements were measured in exactly the same fashion described above. Developer DK-50 has the same composition as the developer used in arriving at the data in Table Ia, while Developer DK-20 has the following composition:

Water to make 1 liter.

In coating d of the following table, the S-thioctic acid was added to a gelatin layer which was coated over the emulsion layer containing the oleyl ether of polyethylene glycol indicated in the table. The amount used in the separate coating d equalled the coverage in part c. Development for the coatings with DK-SO was for minutes, while development time was 17 minutes for DK-20. The results obtained were as follows:

velopers are used with emulsions sensitized with polyethylene glycols. Polyvinyl pyrrolidone may be incorpo rated in such emulsions in amounts up to about grams per mole of silver or in a gelatin overcoating for the emul.

sion in amounts of from 1 gram to 15 grams per mole, of silver in the underlying emulsion. Polyvinyl pyrrolidone may be used in the developing solution in amounts of from about 0.05 gram to 0.5 gram per liter of developer.

Table II Emulsion Addenda Developer DK-50 Developer DK- (g. mol. AgX) Coating Number Oleyl 5-Thioetic Speed Gamma Fog Speed Gamma Fog Scum Ether Acid None None 325 95 .05 313 1. 12 17 None.

0.75 None 338 92 06 311 89 .20 Heavy. 0.75 0. 75 334 90 .04 289 1.05 13 Very Slight. 0. 75 None 335 93 05 292 1.03 .15 Do.

In exactly the manner described above with respect to the data of Table II, a number of alkylene oxide polymers were used to sensitize separate portions of the same high-speed negative bromiodide emulsion. These coatings were developed for 17 minutes in DK-ZO developer after exposure in the usual manner in a continuous step wedge. In the following table, sensitizer 1 is the oleyl ether of polyethylene glycol having a molecular weight of about 1500, sensitizer 2 is polyethylene glycol having a molecular weight of about 1540, sensitizer 3 is polyethylene glycol-bis-carboglutamic acid sodium salt, and sensitizer 4 is 3,6,9,12,l5,18,2l,24-octaoxahexacosane-bispyridinium perchlorate. The speed, gamma and fog for At the lower concentrations the polyvinyl pyrrolidone not only lowers fog but changes the image tone from brown to black.

What we claim as our invention and desire secured by Letters Patent of the United States is: l. A photographic silver halide emulsion containing (a) an alkylene oxide polymer selected from the group consisting of polyalkylene glycols and condensation products of alkylene oxide containing from 2 to 4 carbon atoms with an organic compound containing an active hydrogen atom selected from the class consisting of alc0-' hols, phenols, amines, mercaptans, acids, amides, and hydrocarbons, said alkylene oxide polymer having a molecthe coatings are indicated in the table, these data being 5 ular weight of at least 300, and (b) S-thioctic acid.

obtained in the same fashion described above.

2. A photographic silver halide emulsion containing Table III Sensitizer No 5-Thioctic Acid 5-Thioctic Acid (.75 g./mol. AgX) Coating (gm/moi. Number AgX) Speed Gamma Fog Scum Speed Gamma Fog Scum 280 .82 10 None 272 91 10 None. 270 66 12 Heavy 265 .86 10 Little. 283 .79 11 Little-.- 273 90 10 Very Little. 281 .83 .12 do 270 .89 .10 None. 279 90 14 do 267 88 .13 Do.

Alkylene oxide polymers containing anionic and cationic groups can also be used in our invention, including quaternary ammonium, quaternary sulfonium and quaternary phosphoniurn derivatives. Sensitizer 3 in Table III is an example of an anionic alkylene oxide derivative. Sensitizer 4 in Table III is an example of a quaternary ammonium alkylene oxide polymer derivative. Other examples of anionic and cationic derivatives of alkylene oxide polymers which can be employed in our invention can be prepared as described in Carroll, Sagal and Beavers US. application Ser. No. 627,135, filed December 10, 1956. Such derivatives generally have a molecular weight of about 200 to 1500. Other examples of such polymers and their derivatives are shown in Carroll, Graham, Elins and Wilson US. application Serial No. 627,136, filed December 10, 1956.

It has also been found that fog may be reduced and that scum on the surface of the film may be suppressed, by incorporating polyvinyl pyrrolidone in an emulsion which has been sensitized with polyethylene glycols as described above. Similar reduction of fog and suppression of scum may be accomplished in emulsions so sensitized by incorporating the polyvinyl pyrrolidone in a protective gelatin overcoat on the emulsion, or in the developer in which the emulsion is developed.

Scum on the surface of the film is particularly promoted by developers containing the thiocyanate ion such as Kodak Developer DK 20, especially when such de- (a) an ethylene oxide polymer selected from the group consisting of polyethylene glycols and condensation products of ethylene oxide with organic compounds containing an active hydrogen atom selected from the class consisting of alcohols, phenols, amines, mercaptans, acids, amides, and hydrocarbons, said ethylene oxide polymer having a molecular weight of at least 300, and (b) 5- thioctic acid.

3. A photographic silver halide emulsion according to claim 2 wherein the silver halide is silver bromiodide.

4. A photographic gelatino-silver-halide developing-out emulsion containing (a) a condensation product of ethylene oxide and a monohydric aliphatic alcohol containing at least 12 carbon atoms, said condensation product having a molecular weight of at least 1500 and (b) 5 thioctic acid.

5. A photographic gelatino-silver-halide developing-out emulsion containing (a) a condensation product of ethylene oxide with an organic compound containing an active hydrogen atom selected from the group consisting of alcohols, phenols, amines, mercaptans, acids, amides, and hydrocarbons, said condensation product having a molecular weight of at least 300, and (b) S-thioctic acid.

6. A photographic silver halide emulsion containing (a) the oleyl ether of polyethylene glycol having a molecular weight of at least 700 and (b) 5-thioctic acid.

7. A photographic silver halide developing-out emulsion containing (a) a polyethylene glycol having a.

9 molecular Weight of at least 300 and (b) S-thioctic acid.

8. A photographic silver halide emulsion containing (a) polyethylene glycol-bis-carboglutamic acid alkali metal salt having a molecular Weight of at least 300 and (b) S-thioctic acid.

9. A photographic silver halide developing-out emulsion containing (a) 3,6,9,12,15,18,21,24-octaoxahexacosane-bis-pyridinium perchlorate and (b) S-thioctic acid.

10. A photographic silver halide emulsion stabilized by the addition of at least 0.25 g. per mole of silver halide of S-thioctic acid.

11. A photographic silver bromiodide emulsion stabilized by the addition of at least 0.25 g. per mole of silver bromiodide of S-thioctic acid.

12. A method of preventing scum during the development of an exposed photographic silver halide emulsion containing a sensitizer selected from the group consisting of polyalkylene glycols and condensation products of an alkylene oxide containing from 2 to 4 carbon atoms with an organic compound containing an active hydrogen atom selected from the class consisting of alcohols, phenols, amines, mercaptans, acids, amides, and hydrocarbons, said alkylene oxide polymer having a molecular weight of at least 300, comprising developing said ex- 10 posed photographic silver halide emulsion in the presence of S-thioctic acid.

13. A photographic element comprising a support and coated on said support at least 2 gelatin layers, at least one of said gelatin layers containing light-sensitive silver halide, sensitized with an alkylene oxide polymer selected from the group consisting of polyalkylene glycols and condensation products of an alkylene oxide containing from 2 to 4 carbon atoms with an organic compound containing a reactive hydrogen atom selected from the class consisting of alcohols, phenols, amines, mercaptans, acids, amides, and hydrocarbons, said alkylene oxide polymer having a molecular Weight of at least 300, and at least one of said gelatin layers containing S-thioctic acid.

References Cited in the file of this patent UNITED STATES PATENTS Jones May 10, 1955 Mochel Dec. 27, 1955 OTHER REFERENCES Bullock et al.: J. Am. Chem. Soc., 74, 3455 (1952).

Claims (1)

1. A PHOTOGRAPHIC SILVER HALIDE EMULSION CONTAINING (A) AN ALKYLENE OXIDE POLYMER SELECTED FROM THE GROUP CONSISTING OF POLYALKYLENE GLYCOLS AND CONDENSATION PRODUCTS OF ALKYENE OXIDE CONTAINING FROM 2 TO 4 CARBON ATOMS WITH AN ORGANIC COMPOUND CONTAINING AN ACTIVE HYDROGEN ATOM SELECTED FROM THE CLASS CONSISTING OF ALCOHOLS, PHENOLS, AMINES, MERCAPTANS, ACIDS, AMIDES, AND HYDROCARBONS, SAID ALKYLENE OXIDE POLYMER HAVING A MOLECULAR WEIGHT OF AT LEAST 300, AND (B) 5-THIOCTIC ACID.