T E C H N I Q U E S FOR EVALUATION OF C H E M I C A L S A S MARINE ANTIFOULANTS SIGMUND M . MILLER M i a m i Marina Research, Inc., M i a m i Reach, Flu.
Porous test panels impregnated with candidate antifoulants b y techniques consistent with their properties are immersed in the sea. The open-cell structure permits active antifoulants to maintain a toxic concentration at the panel-sea water interface. Dense local populations of fouling organisms permit distinction between active and inactive chemicals after 4 to 8 weeks of testing in situ. A new class of biologically active organotin chemicals shows promise in antifouling coatings. Rates of consumption of two tributyltin chemicals from antifouling paints were estimated by means of tin analyses of nearly exhausted films, after 13 months' immersion. Derived values indicate that organotins can adequately control fouling when consumed at rates more economic than previously reported minimum critical leaching rates for copper-based antifoulants.
AVAILABLE records indicate
that cuprous oxide was first used successfully as a n antifoulant in paint in 1563. Although paints based upon cuprous oxide and other toxic heavy metals contribute to galvanic corrosion problems, accessory products a r d shipyard practices have been developed which make their use feasible, if not as convenient or economical as is desired. Nevertheless, a need exists for chemical antifoulants capable of imparting longer and more reliable antifouling protection at reasonable cost, which d o not cause the galvanic corrosion problems associated with the use of heavy metal antifoulants. In the search for new antifoulants, a variety of techniques have been employed for evaluating the ability of chemicals to affect the behavior of fouling organisms. Testing the performance of candidate antifoulants by incorporating them into organic coatings has proved to be of little value for preliminary screening. T h e mechanisms which regulate the supply of incorporated antifoulant to the surface of a coating immersed in sea water are influenced greatly by formulation parameters, so that it is improbable that a n active antifoulant will exhibit its capability in tests on one or a feiv formulations. Some investigators have attempted to use the dosage of chemical which is lethal or repellent to captive test organisms as a n index of the chemical's antifouling potential. Such tests have disclosed an impressive number of compounds which appear to be from ten to hundreds of times more active than copper ( 3 ) ,but in subsequent paint studies feiv of these have demonstrated efficiency even comparable with that of cuprous oxide. There are two logical reasons for this discrepancy. First, only one or a few species of organisms can be used in the laboratory screening tests, while hundreds or even thousands of species ma); be encountered in the sea. Second, the lowest concentration of chemical which is lethal or repellent in laboratory tests may be imposqible to maintain on a paint surface exposed to an unlimited volume of diluent in the sea. For commercial acceptance, a n antifoulant must be capable of controlling the entire spectrum of marine fouling organisms. These represent a \vide variety of behavioral and physiological characteristics among both plants and animals. T h e screening test lvhich this article describes utilizes the natural environment of Biscayne Bay. Florida: as a test medium, where a majority of the relevant organisms in attachment stages of development 226
l&EC PRODUCT RESEARCH A N D DEVELOPMEN1
are encountered throughout the year. In principle, the method involves in situ testing of a model of an "ideal" antifouling paint film, in which the candidate antifoulant is contained within the interstices of a permeable binder material, and is capable of being released from the pores at the bindersea water interface at a rate consistent with its inherent solubility in sea water. T h e solution technique for depositing candidate antifoulants within the pores of test specimens was employed by the author to screen several hundred chemicals at the Marine Laboratory, University of Miami, but the scope of that project and value of its results were limited to some extent by the inability to find appropriate solvents for a npmber of candidate chemicals. In some instances it was necessary to resort to solvents Lvhich may have drastically altered the composition of candidates. Also, when low-boiling solvents were required, profuse "blooming" of chemical on the surfaces of test specimens \vas often unavoidable, and led to quesrionable results. T h e alternate (dispersion) technique which is now employed to supplement the solution technique virtually eliminates these problems. The procedure followed for subsequent evaluation of chemicals which show promise in preliminary screening is also briefly described in the following sections. Screening Techniques and Materials
Selecting Technique for Test Specimen Preparation. Depending upon their solubility characteristics. two possible techniques are available for depositing candidate chemicals within the pores of the test specimen material: solution in organic solvents and subsequent recrystallization Lvithin the pores, or mechanical dispersion of small particles. The suitability of the solution technique is first determined by attempting to dissolve approximately 50 nig. of the chemical in 1 ml. of xylol and methyl isobutyl ketone. If one of the solvents proves to be adequate, the chemical is processed by the solution technique. Otherwise, the dispersion technique is employed. Solution Technique. The chemical (2.5 grams) is dissolved in 50 ml. of solvent. A carbon test specimen (porous carbon, special fine grade, cut to 63-mm. square X 5 mm. from large blocks obtained from Filtros, Inc., Rochester, N. Y . )is \veighed, placed in a Petri dish bottom, and covered with 40 to 50 ml. of the solution. T h e dish is placed under vacuum (>26 inches
of Hg) for 5 minutes in a vacuum desiccator. T h e test specimen is then transferred to a drying rack in a freezer (ca. - S o C.) for 4 to 6 hours, then to a refrigerator for 16 to 24 hours. Final drying is a t room temperature, to constant weight. Dispersion Technique. T h e chemical must be triturated to micron particle size range before it can be impregnated by mechanical dispersion. To d o this, 2.5 grams a i the chcmical and 11 to 12 grams of 3-mm. borosilicate glass bcads are placed in a 35-ml. polyethylcne capsule (Figure 1). Six milliliters ol water and 0.4 ml. of dispersing agent (Tamol 731-25%, Rohm & Haas Co.) are added, and the capsule is agitated in a high-spced shaker (Toothmaster Hi-Speed Amalgamator, Toothmaster Co., Racine, Wis., modified to accommodate a 35-ml. polyethylene capsule) for 2 minutes. T h e dispersion is transferred to a Petri dish, using a small sieve to retain the beads. T h e capsule and beads are then rinsed with three 10ml. portions of water, and the rinsings transferred to the Petri dish. A porous carbon test panel is impregnated with water under vacuum in a Petri dish, then placed in an ultrasonic cleaner tank (Di-Sontegrator System Eightv, 120-watt ultrasonic cleaning apparatus, Ultrasonic Industries, Inc., Plainview, N. Y.) and vibrated for 15 minutes to remove loose carbon particles. T h e panel is then dried, weighed, and reimpregnated with water. I t is placed in the Petri dish containing the dispersion, vibrated again for 10 minutes, then turned over in the dish, and vibrated for an additional 10 minutes. I t is then rinsed under a running t a p and dried a t room temperature to constant weight. Some chemicals which d o not dissolve in xylol or methyl isobutyl ketone are readily dispersed by one or the other, without the use of a wetting agent. I n such cases it is preferable to use one of the organic liquids rather than water as a dispersing medium. Exposure Testing. Panels are immersed in the sea in Plexiglas racks (Figure 2) which suspend them vertically. T h e distance between test panel faces is approximately 2’11 inches, with Masonite spacer panels inserted between them to prevent the influence of effective antifoulants on adjacent panels. T h e racks are suspended from a pier, immersed about 1 foot helow mean low tide level. Inspections are made a t monthly intervals, consisting of actual counts of the various forms of organisms attached to test surfaces. A qualitative activity rating is assigned the chemical a t each insppection, indicating the types of organisms present on nontoxic control panels but not on the least heavily fouled face of the test panel. Typical contrast between a nontoxic control and a test panel impregnated with a n active antifoulant, after one month of immersion, is shown in Figure 3. Tests of erective compounds are repeated during different fouling seasons, if necessary, until all nine types of osganisms are encountered on nontoxic surfaces. Immersion testing of each chemical is continued until a t least 50% of both its panel faces are covered with fouling. For chemicals which successfully resist fouling attachment for a t least one month, an approximation of the antifoulant consumption rate is derived on the basis of original content of antifoulant in the test specimen and the duration to 50% fouling. A relatively low estimated ccnsumption rate is the criterion of selection for suhseqiient formulation studies.
Results of Screening Tests
When the solution technique was first used (7), the chemicals listed in Table I were effective for controlling algae, barnacles, and a t least three other types of fouling. Organometallic chemicals predominate, but some organic chemicals also are capable of broad-spectrum control. Until late in 1963, subsequent use of the antifoolilant screening techniques was limited largely to confidential tests of chemicals from sevcral commercial sources. Although a number of organic chemicals have demonstrated broadspectrum antifouling activity in these tests, information which might help to relate chemical structure to antifouling activity is not genrrally available. A wide variety of organotin
Figure 1. High-speed agitator used to grind and disperse solid chemicals
Figure 2. panels
Screening test exposure rack, holding ten test
Figure N o n t o r i ~control mid-winter
3.
Porous corhon test panels
and active ontifoulont afte. 1 -month immersion during
chcrnicals have also been screened, with the following gcneral resid ts
All arganotin chemicals tested thus far with the characterktic R 8 n X structure (where R is either a n alkyl or aryl radical) have cxhihited broad-spectrum activity. ‘The antifoulant consumption rate is influenced by the molecular wcight of the alkyl or aryl radical: thz higher the molecular weight, the more slowly the chemical is consumed. T h e nature of the anion can airect the consumption rate as grcally as the inolecular weight af the organic radical. T h e practical aspects of this variable are not as clearly defined as that of molecular weight, and are still being investigated. In the fall of 1963 a screening program utilizing the techniques discussed above was initiated, under sponsorship of VOL. 3
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Table 1.
Chemicals Effective for Fouling Control
Bis(tri-n-butyltin)oxide* Phenyl mrrcury oleate Copper pentachlorophenate Phcnyl mercury salicylate 2.2 '-Met hylenebisf 3.4.6-trichlorophenol) (Hexachlorouhenr Pl&n>l rnercui y naphthenate Bis(tri-n-butyitin )oxideh 2.2 '-Thiobist 4.6-dinitrophenol) (Bithionol) Allethrin 2,3,5,6-Tetrachloro-1.4-benzoquinone (Chloranil) 1.2.3-l'rictiloro-4.6-dinitrobcnzene Trihutyltin acetate 2-C~-clohcxyl-4.h-dinitrophenol . ~ - i ~ I ' r i c t ~ l o l o m r t h y l t )-4-cyclohexene-2-dicarboxhio irnidr (Captali)
hlethylmei cury pentachlorophcnatc €~esachlo~u-2.5-cyclohexaciien-l -onr
p-l)iisot)utylphei~~~l (Hesterol) Caprylphrnol
X
X
x
x
X X
X X
X X
X
x x x
x
X
X X X
x x
X X X X
x x
x
X
X
X
X X
x x
X
x
X
X
x
X
X X
X
x x
x
X
x
X X
X
X
X
x
X X
X X X
X X
x x x
X X
X X
X
X X X
X X
X
X X X X X
X
x
the L.. S. S a v y Department, Bureau of Ships. Samples of organotin. organolead. and organic chemicals are being contrihiired from a number of commercial sources; several have been modrrately succedul Ivith privately sponsored screening tests in the past. About 200 candidate antifoulants are to be tested during 1964. 'The results should a t least indicate whether relationships betbveen structure and antifouling activity can be established. Paint Formula Evaluation
Attempts have been made to formulate effective antifouling paints with several of the chemicals which have demonstrated outstanding activity in screening tests. Detailed results of studies of individual chemicals have no place in this article, but factors which have proved to be generally applicable for investigations of this sort may be Xvorth mentioning. For example, solid organic antifoulants Lvhich dissolve too readily in common paint solvmts may be difficult to formulate into paints. Their presence in colution is likely to affect the solubility and other properties of paint binder constituents, making it difficult to produce films of good physical qualit,-. This categor!- of antifoulant may also be difficult to evaluate in formulations if it recrystallizes during solvent evaporation, because it then tends to distribute itielf unevenly throughout the paint film. Attempts to develop ship-bottom paints with organic anrifoulants are basically studies of the effects of varying permeability of the paint films: as these influence the antifoulant leaching rate. LVhen the solubility of the antifoulant in paint solvents makes it difficult to accomplish this \vith conventional paint systems, resorting to latex (water-based) systems has usually produced more favorable results. The antifoulant then behaves essentially a5 an inert pigment. and has little or no effect on the behavior of binder constituents which are used to regulate film permeability. Xluch of the effort to forniulate successful antifouling paints has cmtered around RsSnX-type organotin compounds, because chemicalc representing a \vide range of physical properties may be readi1,- selected from this family of antifoulants. Exrensive empirical tests demonstrating that paints capable of satisfactory performance can actually be produced u i t h these chemicals havr been reporred (7, 3) or are in press (5). The l & E C PRODUCT RESEARCH A N D DEVELOPMENT
x
x x
x x
X X
Bmn. bar norips; E.B.: encmstirig bryozoans; .AI.. alqaf; By?., Rii,qula; H y d . , hydr0id.r; iiinicatcs: S i i m ~ nirrroorganisni , s/imefilm. Saniples o b h i n p d f r o m different sources.
228
x
x
z14011.~ rnoliusks;
x X x
x
X X
X T . IV., tube worms;
Tun.,
test procedure described in the follo~vingsection is suggested as a possible method for evaluating the efficiency of antifoulants in paint formulations Experimental. Paints are prepared based on the formula variables of interest, covering ranges consistent \vith sensible maIiufacturing procedures I n the example cited here, 38 acrylic formulations were prepared which included two levels each of bis(tri-n-buty1tin)oxide and bis(tri-n-buty1tin)sulfide. Both of these antifoulants are colorless liquids which conveniently blend with the other ingredients employed. Two inert (extender) pigments were incorporated, each a t 0, 10: 20: 30,. and 40YG by volume of total solids. The paints \\.ere applied with a doctor blade to poly(vinylch1oride) panels through paper templates to fix the applied film area at 7 . 5 sq. inches (Figure 4 ) . T h e panels were Iveighed before application and after drying of the paint to determine film \\.eights. Calculated dry film thicknesses \\.ere all less than 2 mils. The panels ivere exposed in the sea and inspected at monthly intervals to ascess fouling resistance. T\vo of rhe panels were removed from test for residual tin analysis after 8' 2 months of testing, before they shelved any signs of failure. Four additional panels were removed for anal!-sis after 1 3 months, after their surfaces had partially fouled. Formulas for these six paints are shown in Table 11.
Table I I .
Formulas of Paints Used to Evaluate Efficiency of Organotin Chemicals Paint .Vo, - ~ _ _ _ ~~~
Ingredients, Ib. i 700 gal. Acrylic resin solu-
7
2
3
'
1
5
6
661.4 465.6 720.6 552 0 418.4 467.1 67.5 .. . 67.4 120 9 . . . , . . 8613 . , , .., , 86.1 Bis(tri-n-butyltinjoxide 50.0 100.0 . . . Bis(tri-n-buty1tin)sulfide .. 30.0 SO:O 50.0 1OO:o Antisettle compoundh 1 . 7 1 .8 . . . 1 7 3.0 1. 8 Mineral spirits 123 4 163.4 . . . 123 3 221.1 163.2 Organotin, ;c (solids basis) 1 4 . 8 26 8 14 8 14 8 14 8 26 8 Pigment x-olumr content. PVC 10 10 0 10 20 10 Acrjloid F-70 resin solution: 30'~ .solids in mineral thinnei iRohni 3 Troykjd Anti-Settle Special ( T r o ) Chernicai Co.). H a a s Co.). tion"
Diatomaceous silica Talc
'I
Discussion
Figure 4. Application of experiment01 paint to Mylar test panels Circvlor hole in pope, lernplote h e r pqinted orea at 7.5
19. inches
Performance of the paints during the test indicated that characteristics of the pigment and its volume in proportion to that ofothcr constituents have more of an eflect than organotin content, a t least a t the antifoulant level tested. For example, the performance of panel 4 was nearly identical with that of panel 6 throughout the test period, even though the lattcr contained nearly twice as much organotin. Throughout the series, on the other hand, films containing either no pigment or pigment a t the 10% PVC level consistently performed better than corresponding paints with more pigment. There appeared to be no significant, consistent difference in performance of corresponding paints prepared from different organotins. For the purpose of tin analyses, measured areas of paint film which were entirely free of fouling and adherent debris were cut from the panels and rinsed with fresh water. T h e samples were analyzed for their residual tin content, and the results used to calculate the data shown in Table 111. These data demonstrate t h a t : These arganotins are, in fact, slowly removed from the paint: by the action of sea water. Bis(tri-n-huty1tin)oxide is probably depleted a t a slightly greater rate than his(tri-n-butyltin)sulfid~ (from comparable formulations). T h e lowest rate of organotin consumption (greatest efficiency) is from films containing only a small volume of pigment or none a t all. Bis(tri-n-hutyltin)oxide will prevent fouling attachment while being consumed a t rates as low as 0.035 g r a m l q f t . / month. and bis(tri-n-hut\.ltinisulfide a t rates as low as 0.024 gramlsq. ft./month ~~
Table 111.
Paint
NO. 1
flrgonotinm A
Performance Efficiency of Orgonotin Chemicals in Acrylic Paints Pigrnenf Immerrion Orgonofin Content, ~. GrornrlSg. Ft. Volume Period, 7nitiol Residual Contmt, % Monfbr 0 6027 0 1462 10 13
0 1183 0 3384
A . Bis(ei-n-bulylfin)oxidc. B .
Descriptions of the screening techniques have been presented here to how that arch methods can be of value for finding new antifoulants which arc c a p a b k of broad-spectrum activity. In order to predict the manner in which an antifoulant might perform if incorporated into permeahle or sparingly soluble paint matrices, the techniques are also employed to estimate the averagc rate a t which effective antifoulants are consumed from porous test panels by the action of sea water. T h e author recognizcs, of course, that the t:fficicncy of a n antifoulant depends upon its ability to be released from a coating system a t a rate just sufficient to prevent fording attachment, and that the leaching bchavior can be conti-olled, to some extmt, by the aver-all composition of the coating formulation. However, the lzaching behavior of chemicals which dirsolvr too readily in sea water cannot be adequately controlled by means of adjusting the coating composition. Therrforc, the over-all purpose of the screening techniques is to eliminate from further consideration any candidate antifoulants which are incapahle of controlling all types of fouling or demonstrate hroad-spect r i m antifouling activity, but are likrly to be difficult to formulate into coatings because of excessive solubility in sea water. T h e lattcr insufficicncy might bc more easily assessed by means of direct solubility measurements, except that the relationship betwcen the leaching behavior of antifoulants from paints and thrir solubility in sea water has not yet h e m adequately explored. T h e basic techniques employed here, if refined sufficiently to establish uniformity of antifoulant distribution within much thinner porous test specimens, would serve as a convenient means far determining such relationships for a variety of antifoulants. T h e paint test procedure is prescnted as a simplified example of one method which may he used to estimate the efficiency of active antifoulants. Conclusive information a n antifoulant efficiency and optimum formulation parameters can he obtained only after much more comprehensive studies than the one cited. Also, information is lacking which would permit valid extrapolation of test results of this type to practical conditions of paint application and service. However, if the pracedure can he used to demonstrate that a new antifoulant controls fouling a t consumption rates more economic than those of proprietary antifoulants, it a t least establishes a goal to seek when formulating more “practical” coatings with the new antifoulant. .The minimum rate a t which copper must leach from a paint film in order to control fouling has hcen established a t 10 *g./sq cm./day (0.28 gramlsq. ft.lmonth) (3). T h e test has shown that the organatins are active when consumed a t ahout one tenth that rate, and these d o not necessarily reprrSent minimum critical leaching rates. T h e cost of these chemicals is appreciahly less than ten timcs that of copper. Even disregarding other favorable properties of the new antifoulants, therefore, the test results indicate that their continuf.d
10 0
13 8 5
A”. Conrumplion Rntr of flrgonolin, Grorn/Sg. Ft./Month 0 0351 0 0714 0 0250
Bri(tri-n-hulylfin)rul~~/jrle.
VOL. 3
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229
development is warranted on the basis of economic considerations. Acknowledgment
?‘he author thanks the research and development staff of
M & ’I’ Chemicals, Inc., for conducting the tin analyses and for
(2) Summerson, T., Page; H., Zedler, R., Miller, S.; .Wafer. Protection 3, 62-71 (1964). (3) \Voods Hole Oceanographic Institution, “Marine Fouling and Its Prevention,” up. .. 250-2. 256. U. S. S a v a l Inst.. Annapolis. Md., 1952. (4) Zedler, R. J., Am. Paint J . 45, 78 (1961). (5) Zedler, R. J., “Proc. du CongrPs International de la Corrosion Marine et des Salissures,” Cannes, France: in press.
its helpful suggestions.
RECEIVED for review January 31: 1964 ACCEPTED July 8, 1964
Literature Cited ( I ) Mar inr
Laboratory: Uniwrsity of hliami, “Antifouling Potentials of Pesticidal hiatrrials.” rrport to Bureau of Naval LVeapons, March 1961 (unpublished).
Division of Organic Coatings and Plastic Chemistry, 147th Meeting, XCS, Philadelphia, Pa., :\pril 1964. Portions of the work supported by the Bureau of Naval \Yeapoiis, Bureau of Ships. and M RC ?‘ Chemicals, Inc.
PREPARATION OF DIMETHYL-2,6NAPHTHA LEN ED ICARBOXY LATE H E N R Y J. P E T E R S O N , A R C H I B A L D P. S T U A R T , A N D W I L L I A M D . V A N D E R W E R F F Rpsearch and Dmelopment Diiiszon, Sun Ozl Co., ..Marcus Hook, Pa.
With the advent of petroleum-derived naphthalene, large quantities of dimethylnaphthalene are available. An improved method of oxidation of 2,6-dimethylnaphthalene to 2,6-naphthalenedicarboxylic acid i s presented. The method consists of oxidizing the hydrocarbon, b y means of nitrogen dioxide and selenium, to an approximately equimolar mixture of 2,6-naphthalenedicarboxylic acid and 6-formyl-2-naphthoic acid; separation of the two components during esterification; and oxidation of the 6-formyl-2-naphthoic acid. The effect of the reaction variables is described. The over-all yield of polymer grade dimethyl-2,6-naphthalenedicarboxylate i s 85 to 90 mole
70.
TETE
of aromatic carboxylic acids- and especially aromatic dicarboxylic acids by the oxidation of alkylaromatic hydrocarbons- has bcen pursued vigorously in recent y-ears, the products finding extensive application in synthetic resins, fibers: and films. T h e ease of oxidation as described in elementary texts is an oversimplification as is vividly illustrated by the effort expended in developing commercial procewes for the oxidation of p-xylene to terephthalic acid. This laboratory has been concerned primarily with the oxidation of 2.6-dimethylnaphthalene to 2.6-naphthalenedicarboxylic acid, a procesq further complicated by the increased reactivity of the naphthalene nucleus ovcr the benzene ring. 2.6-9aphthalenedicarboxylic arid has been prepared by fusing dipotarcium-2,6-naphthalenedisulfonate \vith potassium cyanide to give the corregponding dinitrile, Lvhich is hydrolyzed ( 3 ); by a combination of diazotization and potassium c>-anide fusion on 2-naphthylamine-6-sulfonate to >-ield 2.6-dicyanonaphthalene. \vhich is h>-drolyzed ( I ) ; by oxidation of 2methyl-6-acerylnaphthalene \vith dilute nitric acid a t 200’ C . ( 9 ): by thermal disproportionation of potassium naphthoates a t 430” C . ( 6 i ; by thermal tearrangement of dipotassium 1,8naphthalenedicarboxylate a t 425’ C . (7. 8) ; and by oxidation of 2.6-dimeth~-lnaphthalrnrby nitrogen dioxide in trichlorobenzene solution using a selenium catalyst (9). The sclcniuni-nitrogeri dioxide process. ivhich has been licensed by Carbogen Corp. and TVilmot and Cassidy. Inc., to Sun Oil Co.. proved to be rhe most convenient and improved procrdure and greatly simplified and shortened the purification 5tei)s. PRoDuc’rIos
230
I&EC PRODUCT RESEARCH A N D DEVELOPMENT
Experimental
Materials. All of the reagents and chemicals used in these studies were available commercially. and were used without purification. T h e nitrogen dioxide \vas obtained from hlatheson Coleman & Bell; the 2.6-dimethylnaphthalene from Rutgerswerke A.G. ; the trichlorobenzene, from the Hooker Chemical C o . ; and the selenium, from Baker and ;2damron. Procedure. Figure 1 diagrams the apparatus found to be most effective for conducting the oxidation. T h e reactor is a 3-liter resin flask. and efficient stirring \vas accoinplished by using a high-speed stirrer equipped with triple turbine blades capable of 4500 to 5500 r.p.m. under reaction conditions. A further aid to stirring \vas the incorporation of stainless steel baffles. Two general modes of the reaction were used: a ”batch” process and an “incremental” process: the names referring to the manner of addition of the substrate. In both processes, the solvent (approximately 2000 ml.) and selenium were heated to the reaction temperature (usually 195’ C . ) ! and then the selenium \vas oxidized to selenium dioxide with nitrogen dioxide. In the batch process, all of the 2,6-dimethylnaphthalene was added (in hot solvent) after the oxidation of the sclenium. tlien nitrogen dioxide \vas passed through the mixture until the exit gases, originally colorless (nitric oxide), sho\ved a tinge of bronm coloration olving to unreacted nitrogen dioxide; the reaction \vas then stopped by replacing the nitrogen dioxide flow Iiith nitrogen. and cooling to room temperature. I n the incremental process, the 2,6-dimethyl-
