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I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
determined but once for a given set of conditions. However, it is a simple operation to redetermine one or two points on the curve each time a determination on an unknown sample is made. LITERATURE CITED
(1) Berezova, M. K., Hig. d. Sanit. (U.S.S.R.), 1940, No. 10,31. (2) Block, R. J., and Bolling, D., “Amino Acid Composition of Proteins and Foods”, pp. 91-5, Springfield, Ill., Charles C. Thomas, 1945. (3) Brown, W. L.,J. Biot. Chem., 154, 57 (1944). (4) Feigl, F., “Qualitative Analysis by Spot Tests”, p. 308, New York, Nordemann Publishing Co.. 1937. (5) Ganassini, D., Boll. 8oc. med. chir., Pa& (May, 1912). (6) Humphreys, F.B.,J . Infectious Diseases, 35, 282 (1924).
Vol. 17, No. 4
(7) Komm, E.,2. physiol. C h m . , 156, 35 (1926). (8) Meth, C h . - Z t g . , 30,666 (1906); C h . Z&r., 1906,11, 822. ENQ.CH~M., ANAL.ED., 15, 107 (1943). (9) Moshier, R. W.,IND. (IO) Moureu, C., and Boismenu, E., ,J. pharm. Aim., 27, 49, 89 (1923). (11) Nierenstein, M.,Collegium, 1905, 158; C h m . Zentr., 1905, 11, 169. (12) Reindel, F., and Sichert. K., 2. Spiritmind., 63, 281 (1940). (13) Tsalapatanis, L.,Anales SOC.qufm. argenlina, 5, 244 (1917). (14) Ucdina, I. L., H i g . Truda Tekh. Bezopasnosti (U.S.S.R.), 15, 63 (1937). (15) Voisenet, E., Ann. inst. Pasteur, 32,476 (1918). (16) Zhitkova, A.S., and Kaplun, S. I., tr. by Ficklen, 3. B., “Some Methods for Detection and Estimation of PoisonousGases and Vapors in the Air”, pp. 133-6, Service to Industry, Box 133, West Hartford, Conn., 1936.
Microscopical Identification of Ultramarine Blue in Complex Pigment Mixtures I. M. BERNSTEIN, H. D. Roosen Company, Brooklyn, N. Y. A microscopical method at 1940 magnification has been developed b y which ultramarine blue in low concentrations can be positively and quickly identified. The ashing of a thin film of printing ink or paint in situ on a microscope slide for direct examination avoids obscuring the tinctorially weak ultramarine blue when opaque whites or colored ash pigment components are present. The presence or absence of the cobalt blues or violets must be determined, since these pigments, being also heat-resistant, interfere with the ultramarine blue identification. A microscopical procedure based on the differential retistance of the cobalt blues to cold dilute mineral acid has been developed. Bocsuse of the greater analytical difficulty of identifyiig ultramarine blue in the presence of the cobalt blues and violets three micromethods have been devised: a method for identifyiny small amounts of hydrogen sulfide using as reagent an acidified qel, a differential method for destruction of ultramarine blue, and a method for identifying ultramarine blue b y microscopic identification of cubic sodium chloride crystals.
I
T IS often of analytical interest to identify ultramarine blue in
pigment mixtures, which may be in the form of wet printing ink or paint, dry color, or even printed matter or dried coating. The identification of this pigment by heretofore used methods may be simple or of considerable difficulty, depending on the composition of the pigment mixture as well as on the percentage of ultramarine blue contained therein. What little there is in the literature (I, 6, 7, 8, 10) deals with simple cases, and is therefore of minor analytical value. Refractive index measurements (6)can be used, but the procedure is questionable when dealing with small percentages of ultramarine blue in complex pigment mixtures. Since it is formed as a result of calcination, ultramarine blue is heat-resistant. Therefore when present by itself or in substantial proportions in admixture with opaque whites, transparent extenders, or organic colors, ‘asimple ashing in a crucible with resulting blue coloration may be regarded as a presumptive test for ultramarine blue. The cobalt blues and violets, however, are also heat-resistant, and the appearance of a blue ash cannot be taken as proof of ultramarine blue unless it can be shown that the cobalt pigments are absent. The cobalt blues consist of Thenard’s blue (cobaltous aluminate), Coeruleum blue (cobaltous stannate), and smalt (potassium cobaltous silicate) ; and the cobalt violets, of cobaltous arsenate and phosphate. Other heat-resistant blue pigments are oil blue (cupric sulfide), Colour Index No. 1289, and Egyptian blue (calcium copper silicate), Colour Index KO.1284, but these latter are not commercially used and are not considered in this paper.
From the analytical viewpoint the identification of the cobalt blues in the presence of ultramarine blue can, in certain cases, be established on the basis of their high cobalt content (18 to 30%), but. more generally the identification rests on the differential resistance of the cobalt blues to cold, dilute mineral acids (9). The identification of ultramarine blue in the presence of cobalt blues is, however, a more difficult problem. It has been suggested (4, 6-8) that the destruction of the blue color of an ash by dilute mineral acid with accompanying evolution of hydrogen sulfide may be considered further proof of ultramarine blue. While the absence of hydrogen sulfide on acidification would be proof of the absence of ultramarine blue, the presence of hydrogen sulfide might be due to other sulfides in the pigment mixture (a), or to barium sulfide which could be formed during the ashing as a result of the reduction of barium sulfate. Destruction of the blue color of the ash on acidification would be proof of ultramarine blue, provided the initial blue color of the ash were of sufficient intensity to enable one to note its disappearance. This presumes relatively high percentages of ultramarine blue, the absence of the cobalt blues, and the absence of a differently colored ash from some other component of the pigment mixture. If iron blue were present, the reddish brown iron oxide resulting from the ashing of the iron blue would obscure the ultramarine blue. Other colored ash pigments include ochre and synthetic iron oxides, chromium oxide and tetrahydroxide, lead and zinc chromates, cadmium sulfide, selenium sulfide, etc., I n both noncolored and colored ash pigment mixture components in which ultramarine blue is present in such small amounts aa to be unobservable to the unaided eye, microscopic examination of the ash will, to some extent, show the blue particles. There is, however, considerable danger in transferring the ash from the crucible to the slide, that the tinctorially weak ultramarine blue particles will be coated by the finer opaque white or colored ash particles and thereby obscured (3). This difficulty suggested performing the ashing on the microscope slide itself, so that no disturbance of the ashed pigment particles and therefore no obscuration by coating could occur. This method worked very well and has been used by the writer over a period of years. ASHING THE FILM
PREPARATION OF FILMFOR ASHING. The film is prepared on the slide for subsequent ashing, in the case of a wet printing ink or paint, by tapping out with one’s finger a very thin film in the csnter of and covering about one fourth the area of the slide. The ink or paint must be soft in body, t o allow the trans-
April, 1945
263
ANALYTICAL EDITION
ference of a very thin uniform film. If the ink or paint is too heavy i t is reduced yith a thin-bodied linseed oil. A heavy film should be avoided because the ash in such cases is difficult or impossible to resolve under the microscope. A film approximately one ultramarine blue particle thick is desired. The finger tap-out procedure is commonly used by printing ink techniciaus in applying a thin film of ink to a surface. It consists oftransferring a minute amount of the ink to the tip of the middle finger by lightly touching the ink mass, and of uniformly applying and distributing this ink to the new surface by a series of tapping with the pad of finger. I n applying a thin film to the microscope slide, this method is superior t o that of spreading the diluted ink or paint with a “doctor” blade; not only can a thinner film be applied, hut the ink film because of its broken surface will not run or blister off during the heating. If the pigment mixture to be tested is not already in the form of a wet printing ink or paint, but rather as a paper printing or other surface coating, the test can still be made, but i t must first he converted to the wet form for slide film preparation. This is done by ashing a mnall piece of the printing or scraped off coating in a crucible and then, after cooling, transferring the ash t o a small glass plate. The ash is thoroughly dispersed in a few drops of thin-bodied linseed oil by vigorously rubbing with the flat blade of a 3-inch palette knife t o farm an ink mass of suitable soft consistency. The thorough rubbing redisperses any part of the ash which may have coated any ultramarine hlne particles present. The resulting ink is then used for the preparation of a elide film. The same procedure is followed if the pigment mixture is present as a dry powder. Paper stock itself sometimes contains ultramarine blue and, therefore, when testing a paper print it is also advisable to test a piece of the blank paper as a control. FILM-ASHINQPROCEDURE. The slide is then flamed in an ordinary Bunsen burner, using a suitable holder, until the film is thoroughly ashed. Usually 10 to 20 seconds of heating are sufficient, and one can readily judge by inspection when the ashing is completed. Excessively long heating is not only unnecessary but undesirable, since ultramarine blue has a slight tendency to decolorize on heating in air (9). Furthermore, i t is important to heat the slide with the film side upward, since otherwise total decolorisatian quickly occurs. If such decolorization is allowed t o take place, the blue color can be partially regenerated by exposing the slide t o vapors of actively boiling concentrated nitric, hydrochloric, or perchloric mid. This curious phenomenon is reversible and can be repeated many times. While i t is of considerable theoretical interest, it has no hearing on the problem at band.
ultramarine blue is positive when the percentage is 0.25% (Figure 1). When using the oil-immersion system it is necessary with some microscopes to use the extra thin oil-immersion slides instead of the ordinary slides for ashing the film, and greater care must therefore be exercised in the flaming so as not unduly
MICROSCOPICAL E X A M I N A T I O N OF ASHED FILM
The slide, after ashing, is first viewed in the microscope a t 200 magnification, using B 16-mm. objective and a 2OX Hyperplane ocular. Hlumination is by a substage illuminator equipped with B blue filter. This is preferahle to daylight illumination, since it enhances the color of the tinctorially weak ultramarine blue particles. Even in a mixture of ultramarine blue and iron blue, high in iron hlue content, the ultramarine blue particles can be distinctly seen among the reddish brown iron oxide particles, although with the naked eye only the color of the iron oxide is apparent in the ashed film. Good resolution can be achieved a t 200 msgnification when the ultramafine hlue in the pigment mixture is as low as 10%. With some experience in adjusting the illumination even 3% can be identified, although some eyestrain is experienced. Therefore in cases where the ultramarine blue percentage is less than 10, higher magnification and greater illumination are usu~lly neeessery. This is best accomplished by the use of the 1.8-mm. oil-imrnersion objective together with the 2OX Hyperplane ocular; giving a mkgnifiestion of 1940. A t this mzgnification the recognition of
Fi-uro 1 .
Photamicromanhr at 1940X
264
INDUSTRIAL A N D ENGINEERING CHEMISTRY Table I. Identilicetion 01 Ultramarine Blue
Ulfm">*,i"e
Blue in Pigmen, Mixture
70 10 1
1 0.25 10
I 1 0.25 10
Other Pigments
Xagnifieation
Used
Visibility of UiLramarine Blue
% 90 99
99 99.75 90 99 99 99.75
?O 20 20 PO 10
1
99
1
99
0.25
99.75
Iron blue Iron blue Iron blve Ironblue Titanium dioxide Titsnium dioxide Titanium dioxide Titaniumdioxide \Ionastral blve Iron blue Chromium oxide Cadmium sulfide Chrome vellor l b o v e pigments in same ratio .kbavs pigments insame ratio Above pigments insanlerafto
aoo
200
1940 1940 200 200 1940 1940
Good POCX
Ereellent Excellent
Good
P0.X Excellent Ereellent
200
Good
2w
P0.X
1940
EIeellent
1940
Excellent
to soften the glass and affect the planeness of the slide. A quartz slide of suitable thickness, if available, is ideal. The usual procedure oi placing B drop of oil of cedar (for immersion) on the Ahhe condenser in contact with the slide as Fell as on top of the slide should he fallowed. It is generally good practice to make a preliminary examination using the 16mm. objective at 2W magnification, as it enables one to estimate roughly the percentage of ultramarine blue because of the larger field which can he viewed. This can likenise be done n-ith the oil-immersion objective if one uses a 5x ocular giring 3 msgnification of 485. The final examinstion, horrever. should he at the 200 oil-immersion magnification. Table I summaizes results on tests performed rrith varying percentages of ultrmnrine blue in colored and noncolored ashpigment misture components. However, the method so far presented for the identification of ultramarine blue presumes the absence of the rohnlt blues and violets.
Vd. 17, No. 4
In the absence of sulfides, blue ash particles observed microscopically tihieh persist after the 4 N hydrochloric acid treatment obviously mean the absence of ultramarine blue and the presence of cobalt blue. Here a negative test for sulfides has value hut a positive t a t is without meaning, since it may be due to other sulfides. The presence or absence of small amounts of sulfides can he determined with considerable accuracy by the following procedure, based on the idea of generating hydrogen sulfide and testing far it in situ. Prepare a 20% aqueous gelatin solution containing 2% concentrated hydrochloric acid. While still warm place B drop on each of several S o . 2 circular 18-mm. microscope cover glasses. The drop will gel in a few minutes. Prepare several and set aside. Next dip the previously ashed microscope slide in a lo'% solution of lead acetate and allon- to dry. S o w firmly press the cover glass onto the slide with the acidified gelatin layer in contact with the ashed a m , and observe under the microscope at 200 magnification. If sulfides are present even in very small amounts, the acidified gelatin %-illgenerate hydrogen sulfide in situ, which will in turn cause the locslized formation of lead sulfide, readily observable under the microscope as dark patches. In the event of a positive sulfide test the two following additional tests have proved useful in arriving at a conclusion as to the presence or absence of ultramarine blue in conjunction with the cobalt blues.
MICROSCOPICAL IDENnFICAnON
C O B ~ LBTL ~ EASD S VIOLETSM PRESESCE OF CLTR.~.
