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type of grinder, and temperatures well over 100' in the impact crusher type. In the ball mill the possibility of sparks can be largely reduced, as suitable choice of the material of the mill itself and of the balls, copper mills with copper balls being almost free from risk of this sort. In the impact crusher type of mill, however, for example, where the dye is disintegrated by being submitted to blows from a revolving beater arm, the accidental intrusion of foreign particles can easily cause sparking through being struck by the rapidly revolving beater. Sparks due to static electricity must also be considered in grinding. The method for determining the effect of heat on the mass of dye in grinding is, of course, identical with that used for the corresponding factor in drying. It is obviously necessary, however, in grinding, to determine the effect of the additional factors introduced, the effect of temperature on the mixture of dust and air. I n order t o determine the stability of the dust and air mixture, there is employed the apparatus developed by the Bureau of Mines for the determination of the explosibility of coal dust and air mixtures. This consists essentially of a 1000 ec. bulb containing a small, hollow cylinder of platinum, heated from the interior by means of a platinum coil, the temperature of the platinum cylinder being accurately determined by means of a thermocouple. The dye under investigation is placed in a funnel a t the bottom of the bulb and the dust cloud is carried by blowing air upward through the funnel. This delivers the dust cloud around the heated platinum cylinder. The explosion point is taken as the temperature a t which a noticeable development of pressure takes place, which is registered ofi a gauge connected with the apparatus. At this temperature a visible flame is seen to develop and spread through the bulb. By a slight modification of this apparatus, the dust doud is subjected to the effect of sparks from an induction coil or from a small emery wheel. Little work has been done on this latter method of testing, however, but from the former, that is, the use of the platinum cylinder, results ranging from 400' to 1100' C. have been obtained as the explosion points of dusts with varying amounts of air, and it is possible to check these results within about 50'. The results obtained are in fair agreement with plant experience, as regards the relative safety of the different dyes so far investigated. This apparatus, however, has the weak point that the dust-air mixture used is not necessarily that most sensitive or most likely to explode. Somewhat more reliable results are therefore given by a modification of the apparatus for determining the decomposition temperature. This apparatus consists essentially of a large test tube immersed in an air bath with a tube for admitting air to the bottom of the test tube. The test tube is gradually heated up, while passing through it is a current of air sufficiently rapid to keep the tube completely filled with dust. In this manner dust clouds of widely varying densities, and therefore sf widely varying sensitivities, are secured, ranging from that at the bottom of the test tube, which consists almost entirely of dye, to that a t the extreme top of the tube, which contains only a few particles of dye, consisting chiefly of air. The temperature of spontaneous ignition is determined by inspection. Usually the decomposition is very noticeable, being detected by change in color and development of fumes. The increased sensitivity of this apparatus over the Bureau of%Minestype is shown by the fact that the temperatures at which explosions occur in the Bureau of Mines apparatus range from 400' to IIOO' C., while the temperature range for the same series of dyes in this apparatus is from 2 5 0 ° to 550' C. The data obtained from the two forms of apparatus, however, give a very clear idea of the relative stability or instability of the dye, as the Bureau of Mines apparatus gives the relative ease with which the different dyestuffs will propagate an explosion, once started, whereas the so-called spontaneous ignition temperature gives fairly definitely the order of stability of the different dyes
Vol.
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No. I I
or intermediates when heated in the presence of air. Little difficulty has been experienced in obtaining concordant results with this apparatus, checking within IO' to 15'. STANDARDIZING
In the standardization of dyes, explosion risks are very much the same as in grinding, although, of course, to a much lesser extent, as the temperatures encountered are much lower and the risks of striking sparks in the mixture are, of course, much less than the risks of striking sparks in the grinding, whether of the ball mill or impact type. An additional factor is introduced, however, in that standardizing reagents are usually added to bring the mixture to the proper strength. Stability of these reagents, and the possibility of a reaction between the reagent used and the dye or intermediate has to be considered; for instance, the accidental addition of soda ash to Victoria green would probably result in a vigorous reaction between the oxalic acid of the Victoria green and the soda ash, with the possibility of raising the temperature to the decomposition point of Victoria green. This investigation is still in the preliminary stages and the methods used are tentative. They are being used, however, pending the development of more satisfactory methods for covering these points, or other points which may be brought up. They are believed to be of considerable value, however, as each method has been developed to answer some definite problem known to have occurred, and only after having given a satisfactory answer to an original problem have they been applied to the solution of similar problems for other materials. One difficulty which has been encountered is the lack of satisfactory standards with which to compare the laboratory results. Certain materials, as for example alizarine yellow, are known to be unsafe, while certain others are believed t o be safe, and in default of more satisfactory data, these materials have been temporarily taken as standards in carrying out this work. It is realized, of course, that the mere fact that the material has been safe t o date does not prove that i t will remain safe indefinitely, and that some of the dyes now passed as perfectly safe may eventually be discovered to be dangerous. However, it is believed that by continuing this investigation along these lines, it will be possible t o establish fairly definite limits as regards temperature and the proper type of mill for grinding each material, and in cases where the element of risk warrants it, t o state which of these materials should be segregated or handled in small lots, or in extreme cases handled in the paste condition. An incidental advantage which has already been gained to some extent has been the selection of the proper temperatures in drying, so as to give the maximum speed of drying with the minimum of danger, thereby increasing the capacity of the drying units. We hope later to communicate more fully on this subject. JACKSON L A B O R A T O R Y
E. I.
NEMOURS 82 COMPANY WILMINGTON, DELAWARE
DU P O N T DE
SOME PROBLEMS IN THE IDENTIFICATTON OF DYES By
E. F. HITCH AND I. E. KNAPP
At the present stage in the development of the dye industry in the United States, the American manufacturers must necessarily follow the Germans. No matter how much we may dislike to be followers and not pioneers, we must, in the first few years, confine our efforts in this field largely to the manufacture of colors that have already been produced by foreign manufacturers. No matter how optimistic we may be, or how confident we are in the ability of the American chemist to produce results, we cannot overlook the fact that we are several decades behind the German chemists in our knowledge of the preparation of dyes. This handicap can be overcome only by an immense amount of very diligent work on the part of the American chemists and manufacturers.
Nov., 1919
T H E J O U R N A L OF I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y
Before any distinctively new and valuable dyes can be developed, we must first gain experience in the manufacture of some of the more important dyes, the constitution of which is well known. During this first period of development of our dye industry our efforts will be confined largely to a study of processes for the manufacture of the well-known staple dyes. I n the meantime, we must obtain as much information as possible on the more important unclassified colors that were imported into this country just prior to the war. These unclassified colors represent the results of comparatively recent investigations in color manufacture, and, in general, command a higher price. The average invoice price per pound of all the more important classified azo colors imported in 1913-14,as shown by Dr. Norton’s report on dyestuffs used in the United States, is 15.7 cents, whereas the average price of all the more important unclassified azo colors is 28 cents per pound. There is obviously a distinct advantage to be gained in producing the unclassified colors, and they are, in general, the better grade of colors. Most of these dyes are without doubt covered by patents already issued, and these patents contain a description of their method of preparation. The first problem for the American chemist is t o identify these important unclassified dyes and t o connect them up with the patents. The second problem is the synthesis of the dyes following the lines indicated by the identification investigation. It is an extremely difficult matter to locate in the patent literature a description of the process of manufacture of any one of the unclassified dyes. Let us suppose that a manufacturer of dyes in this country desires t o produce a direct brown which he believes to be the best direct brown used in the United States prior t o the war. The dyeing properties of this color are well known; the commercial name and the name of the German manufacturer are known; but nothing is known concerning the intermediates necessary t o produce the dye, or the process used. It is known that this color was placed on the market by the Bayer Company in 1900,and it is probably covered by one or more patents, but i t will be an extremely difficult matter t o locate the patent or patents corresponding with this dye, because the description of the dye as given in the patent is not always accurate. It may also be found that several patents were applied for by the Bayer Company about that time, each of which may represent this dye, and each may make claims for from twenty t o thirty diEerent combinations of intermediates or modifications of procedure. This method of attack without the aid of an identification investigation of a sample of the dye is an almost hopeless task. If a sample of this dye can be obtained, and the intermediates used in its preparation definitely determined by analytical methods, the problem of locating the corresponding patent is very much simplified. It is the methods of identification of the finished dyes that are of most interest t o us a t the present time. No specific rules can be laid down which will be applicable t o the solution of all identification problems, but we shall endeavor t o review the various methods that have been proposed, including those that we have actually used. In the literature on this subject, unquestionably the best book for general use is A. G. Green’s “The Analysis of Dyestuffs” (1916). This is not in any sense a catalogue of the properties of individual dyes, but it does offer an excellent scheme for the classification of unknown dyes according to both their chemical and their dyeing properties. The methods described are of general application and can be used, therefore, in identifying new dyes which are not listed in any of the handbooks. The chapter on the determination of the constitution of azo dyes is especially valuable. Volume 5 of Allen’s “Commercial Organic Analysis” contains an excellent description of a large number of the methods that have beep employed in the identification of dyes, and is useful in the solution of special problems. Cain and
Thorpel review briefly several of the older schemes that have been proposed, and include Green’s tables (in abridged form) for the identification of dyes on the fiber. Mulliken* describes group tests and specific tests for some fourteen hundred of the best-known dyes. The scheme is undoubtedly useful in identifying these particular brands of dyes, but the methods appear to have a somewhat limited application. The treatises of Formanek and Grandmougin3 describe in detail the investigation and identification of dyes by means of the spectroscope. Mathewson4 describes general methods for the separation of mixtures of dyes by the use of immiscible solvents. The separation of mixtures by absorption was proposed by Suida,G and elaborated by Dreaperc and Chapman and Seibold.7 Rota’s’ scheme for the classification of dyes depends upon the reduction with stannous chloride, and can often be employed to advantage. The problems that are likely to be met in the identification of dyes fall naturally into two groups: First, the identification of dyes in substance, and, second, the identification of dyes on the fiber. It is evident that the methods used in solving the problems in the first group will not in general be applicable to the solution of problems in the second group. The problems in Group I may be subdivided into three classes: (a)-Comparison of two or more dyes, one of which is of known composition. (b)-Determination of the constitution of an unknown dye. (c)-Separation and identification of the components of a mixture of dyes. The methods employed for the solution of problems in Class (a) are usually simple chemical tests, such as are described in the handbooks, e. g., Schultz, Heumann, and Knecht, Rawson and Lowenthal. The most common reagent for these tests is concentrated sulfuric acid. A very large number of dyes show characteristic colorations when dissolved in this reagent, and there is often a sharp color change on dilution with water. Hydrochloric acid, caustic soda, and other reagents are also used. We prefer to carry out some of the reactions as spottests on a filter paper; and we also include a reduction with zinc dust and ammonia, followed by an oxidation in air; a reduction with stannous chloride, and, finally, a rough determination of the solubility in several representative organic solvents. We find that with careful manipulation, the amount of material required for all of these tests need not exceed 50 mg. It may happen in comparing samples of the same dye made by different manufacturers, or in comparing different marks of the same dye, that there will be no noticeable differencein any of the foregoing chemical tests. I n such a case, recourse must be had t o an actual dyeing, and a comparison of the shades produced on the fiber. This is the final test in proving or disproving the identity of two or more dyes. However, this test alone is not sufficient, for mixtures can easily be made up t o produce almost any given shade on the fiber. The determination of the constitution of an unknown dye is often a rather difficult problem. Ordinarily, the &st step is t o determine t o what chemical class the dye belongs. In order to do this, reactions must be used which are not specific for any particular dye, but which are generally applicable t o a given class of dyes. For this purpose we have found the scheme of Green t o be unequalled, not only for its reliability, but also “The Synthetic Dyestuffs” (1918). Commercial Dyestuffs” (1910). 8 “Spectralanalytischer Nachweis KIlnsticher Organischer Farbstoffe” (1911). See also E. R. Watson, “Color in Relation to Chemical Constitution” (1918). and Dobbie, Baly and Stewart, Report of the British Association fou the Adoancement of Science, 1916, 131-186. 4 “The Separation and Identification of Food Coloring Substances,” U. S. Dept. of Agr., Bulletin 448. 6 Monatsh., 26 (1904). 1107. 6 J . SOC.Chem. I n d . , 28 (1909), 700. 7 Analyst, 87 (1912), 339. 8 Chem.-Ztg., 1898, 437. 1
* “The Identification of
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for its simplicity. I t is unnecessary to set forth the details of this scheme here, for the book may readily be consulted. The composition of azo dyes can often be determined by reduction, followed by the isolation and identification of the reduction products. Either acid reduction (stannous chloride) or alkaline reduction (zinc dust and ammonia, or sodium hydrosulfite) may be used, depending on the nature of the dye. The common bases, ordinarily used as first components, can be separated quite easily, either by steam distillation, or by extraction with ether. Their properties and reactions are usually well known, and hence they can be identified easily. On the other hand, the amino derivatives of second components are difficult to isolate, and stJl more difficult to identify. Very little has been published regarding their properties and reactions. Green describes a few of the amino derivatives of the more common naphthalene sulfonic acids, but the list is by no means complete. In order to obtain the desired information, it may be necessary to reduce certain known dyes, develop methods for separating the reduction products, and finally determine the properties and characteristic reaction of the amino derivatives. This may result in a long and tedious investigation. The separation and identification of the components of a mixture may be rather difficult. For the detection of mixtures, it is usually sufficient to blow off a few milligrams of the dye on a moistened filter paper, or on concentrated sulfuric acid. In some special cases where two or more dyes are precipitated together, these simple tests may not suffice to detect the mixture. Under these conditions, a variety of other methods are available. The components of a mixture will rarely have the same affinity for the fiber, and, therefore, fractional dyeing may be useful. Small skeins are introduced into the dye bath, one after another, until the bath is exhausted. Comparison of the shades of these skeins will often give a clue to the components of the mixture. Examination under the microscope may tend to show the lack of homogeneity of the product. Such a test is especially valuable for detecting admixed inorganic material, such as sodium chloride or sodium sulfate. One component of a mixture may be soluble in alcohol and the other insoluble. Or one may be readily soluble in cold water, and the other soluble only in boiling water. Thus the varying solubility provides another means for the separation of mixtures. The extraction of aqueous solutions of dye mixtures with immiscible solvents offers still another convenient means for separating the components. This method has been applied to food colors, but it appears to be of general application. Ether, amyl alcohol, dichlorhydrin, and mixtures of amyl alcohol with petroleum ether are the solvents ordinarily used. Certain dyes can be fractionated by washing the amyl alcohol extract with hydrochloric acid of varying normality, and with acetic acid, sodium chloride solution, and dilute caustic soda. We have found the method especially valuable in detecting and separating mixtures when only a very small percentage of one component is present. A milligram of dye is usually sufficient to impart a strong color to 50 cc. of amyl alcohol. In some cases mixtures can be separated by adsorption on such material as kaolin, talc, and kieselguhr. Certain dyes are adsorbed and cannot be removed by washing with either water or alcohol. Others can be removed by washing with alcohol, while still other dyes are not adsorbed a t all. After the components of a mixture have been separated by some one of the foregoing methods, they can be identified by specific tests, such as have already been described or by means of the spectroscope. The identification of dyes on the fiber is usually a rather difficult problem because of the prevalence of mixtures. For straight
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dyeings we have found nothing so satisfactory as Green’s table. By following his scheme, the chemical class and dyeing class of an unknown dye can usually be determined. This information, together with the shade on the fiber, is sufficient to reduce the question to a choice between a few closely related dyes. Specific tests with the ordinary chemical reagents may then be employed, and the results compared with those published in the handbooks. As a final confirmation, the same group tests and specific tests should be made on fiber dyed with a known sample of theindicated dye, and these results compared with those obtained on the unknown. For the separation and identification of mixtures on the fiber, a variety of methods may be employed, depending on the nature of the problem. Among the separation methods are fractional reduction, and extraction with various solvents such as alcohol, acetic acid, aniline and acetic acid, pyridine, or cresylic acid and xylol. Specific tests are then made on both the extracted dye and the dye remaining on the fiber; or the solutions may be subjected to spectroscopic examination. Spectroscopic methods for the identification of dyes deserve special mention. It is a well-known fact that many chrornophoric groupings in the molecule produce absorption bands. Furthermore, the position of these bands, i. e., the wave length of maximum absorption, and their degree of symmetry are characteristic of any given dye. The results obtained by the use of the ordinary spectroscope are for the most part only qualitative. The edge of an absorption band, as determined with this instrument, is the point a t which absorption has attained some appreciable value, perhaps 5 or 15 or 30 per cent, depending on the sensitiveness of the observer’s eye and on the instrument. Furthermore, only a rough approximation of the degree of symmetry of the band can be obtained. Unfortunately, Formanek and Grandmougin’s comprehensive and systematic study of the absorption spectra of dyes was made in this qualitative manner. Their tables are useful, however, in showing the position of the maxima of absorption in dilute solutions in various solvents. The results obtained with the ordinary spectroscope are of value in determining the chemical class to which an unknown dye belongs; also, for determining the purity of standards, for the detection of mixtures, and, in general, as a check on the results obtained by other methods. By means of the spectrophotometer, exact quantitative data can be obtained. Curves can be plotted to show the relation between the percentage of light absorbed and the wave length for a given concentration of solution. By plotting several of these curves for different concentrations, a thickness-wavelength curve can he constructed. Such a curve shows a t a glance the degree of symmetry of the absorption band, together with the wave length of the maximum absorption. As has been stated, this is sufficient to identify the dye. Before this method can be applied generally, a vast amount of painstaking research must be carried out, using a carefully calibrated spectrophotometer, in order to determine the exact nature of the absorption bands for a large number of especially pure dyes. In some cases it may be necessary or desirable to extend this study to the infra red and the ultra-violet. Until such a comprehensive quantitative study has been made, it is evident that the application of spectroscopic methods in the identification of dyes will necessarily be limited. The major portion of this work could very well be undertaken by the universities and colleges in cooperation with the industries. The universities could also assist the industries in a $ompilation of a catalogue of intermediates. Chemists working on the identification of dyes are seriously handicapped because they do not have ready access to full and complete information on all known intermediates. A catalogue of intermediates should be prepared which would show, for each intermediate, its formula,
SOV.,
1919
T f I E JOURNAL OF INDUSTRIAL A N D ENGINEERING CHEMISTRY
a brief outline of the methods of preparation, the ordinary physical properties, such as melting point and solubility, references to the literature and to patents, and a list of the dyes and the other intermediates that are made from it. This list of dyes derived from each intermediate should show, for each dye, the Schultz number or the patent reference, the chemical classification (e. g., azo, xanthone, oxazine, etc.), and the other intermediates that are used in its preparation. The compilation and publication of such a catalogue of intermediates would appear to offer an excellent opportunity for the universities to cooperate with the industries. The completed work would be a real contribution to our chemical literature, and it would be very valuable, not only m the identification of dyes, but also as an aid to the dye manufacturers in directing their research work. Another compilation which would be very useful in identification work is a classification of all the dyes listed in Schultz’s “Farbstofftabellen” (1914),according to their color and their dyeing properties. There would probably be eight main color divisions (red, orange, yellow, green, blue, violet, brown, and black) each of which might be subdivided into acid, basic, direct, mordant, vat, lake, spirit, etc. It might also be advisable to include the chemical classification of each dye. This list could be supp1t:mented from time to time by the addition of new dyes from the patent literature as soon as their classification is determined. Still another way in which the university laboratories could assist the dye industry would be to determine the properties of the amino derivatives of the second components of azo dyes, that is, the compounds that are formed when azo dyes are reduced. The separation of these amino derivatives, the determination of their properties, and the development of methods for their detection and identification, would appear to offer ideal subjects for one-year investigations, such as are conducted by candidates for the master’s degree and by seniors specializing in chemistry. Such an investigation on a single amino derivative would not be so difficult or so highly technical that it would require a long period of study in preparation for the actual work; it would not be so broad as to require more than one college year of work; and the results sought would have sufficient concrete value to maintain the investigator’s enthusiasm. There are so few of these amino derivatives described in the literature that the publication of such data would be a real service to the dye industry. JACKSON
E. I.
LABORATORY
DE NEMOURS & COMPANY WILMINGTON, DELAWARE
DU P O N T
OBSERVATIONS ON THE ESTIMATION OF THE STRENGTH OF DYESTUFFS By W. H. W A T K ~ N S
In distinction from most buyers of chemicals, the buyer of dyes is not primarily interested in the chemical content of the product he buys. He buys a dye on the assumption that a definite amount of this dye will give him a definite shade when used under definite conditions on the material which he is interested in coloring. No scheme of chemical analysis in the ordinary sense has yet been devised which will effectively reveal the quality of the dyestuff. It is necessary, thercfore, to estimate the strength and quality of dyestuffs by color comparison. The colorimeter, properly handled, is very useful and may afford a valuable check on dye trials. The latter method is the method most generally used, and having in mind what the: buyer is actually purchasing, it is generally the most useful and reliable. It is obvious that a chemical analysis might show a satisfactory content of dyestuff and yet the dyestuff might contain sufficient impurity of one kind or another to make i t worthless to Ihe buyer. It is conceivable also that a colorimetric
I079
reading might indicate the desired strength and hue, and yet such a dyestuff might not work a t all the same way as the standard. It might not be, for instance, sufficiently level dyeing. The object of this paper is to point out some of the causes that lead to disagreement between seller and buyer, even when both parties are perfectly sincere in their work and findings. It ought to be unnecessary to call attention to the necessity of definite agreement on standards. I have found, however, that difference in standards is the most prolific cause of disagreement between buyer and seller. This was true before the war and is much more so now after a period of considerable lack of uniformity in deliveries. During the past five years the consumer’s great demand has been for dyestuff. He has not cared particularly whether or not it was exactly in accord with previous standards. Under pressure, the manufacturer has frequently delivered goods differing in one way or another from standard, because it was better for the buyer to have his dyestuff even if it were not exactly like the standard, rather than to have no dyestuff a t all. Somewhere along the line the buyer finds a delivery that particularly suits him, and adopts this as his standard. The next time he buys, the delivery may be exactly in accord with the manufacturer’s standard atid yet the buyer may feel that he has a legitimate ground for a claim. In the case of complaints that do not appear to be justified on a test against the manufacturer’s standard, the manufacturer should fully investigate the buyer’s standard and if necessary explain and adjust the whole situation. Having considered the standard, some attention is also due the sample. The importance df careful sampling is recognized in the case of most materials subject to analysis. It seems to be assumed, however, that as dyestuffs come on the market as powders or crystals, a small portion taken from a part of a lot will accurately represent the whole. As a rule, this will undoubtedly be the case, and in fact, it should be the case. It is well known, however, that all commercial dyestuffs contain inert material legitimately added for the purpose of bringing the product to standard strength. This applies to dyes in crystal form as well as to powdered products. Now, if, as is sometimes the case, the mixing is not thoroughly done, there is little chance that a small sample drawn from one part of the whole mixture will represent tbe entire mixture accurately. Before entering upon an ardent controversy this possibility should be investigated. In any event the sample as drawn should be finely ground, as otherwise the small amount weighed off may not be properly representative. The manner in which the water used for dyeing may affect the apparent strength is often overlooked. As a rule, basic dyes should always be dyed with the addition of a little weak acid, such as acetic. Occasionally, however, a buyer will be found who insists that he does not have to use acid and that there is no reason why each delivery should not be precisely like every other delivery when dyed under the same conditions in the same water. Now, with some basic dyes it is almost impossible to keep the amount of acid constant and in such a case, unless acid is added, a delivery that is practically neutral will not dye up as strong as a sample that is distinctly acid. I recollect a series of controversies on this point with a buyer in New York City over deliveries of a certain basic dye. Having located the source of the difficulty it was simply necessary to be sure that all deliveries to this buyer were distinctly acid. Some direct colors are especially sensitive to hard water and the way in which this may lead to a disagreement between buyer and seller should be considered. For instance, the dye itself may be partly in the form of its lime salt and in order to correct this and the effect of hard water in dyeing, a little soda may be mixed with it. Now, if in testing this the seller boils it up in fairly concentrated solution, he may convert the lime salt into the sodium salt and standardize the dye on this basis.
