(12) McLafferty, F. W., A p p l . Spectroscopy 11,148 (1957). (13) hlclafferty, F. W., Gohlke, R. S., ASTM E-14 Conference on Mass
Spectrometry, San Francisco, Cali., RIay 1955. (14) hleyerson, S., A p p l . Spectroscopy 9, 120 (1955). (15) Percival, W. C., ANAL.CHEX 29, 20 (1957). (16) Phillips, C., “Gas Chromatography,”
Buttern-orths Scientific Publications, London, 1956.
17) Ray, N. H., J . A p p l . CWem. 4, 21 (1954). 18) Reed, T. M., 111, ANAL.CHEX.30, 221 (1958). 19,) Simmons, &I. C., Snyder, L. R., ”.,30,32 (1958). Jalcirqn, -J. - -D., - nletropoliton-Tiic~eis Gazette July 1Ybti. (21) White, J. U., Alpert, N. L., TT‘einer, S., Ward, W. PI., Gallaway, W. S.,
Pittsburgh Conference on Applied Spectroscopy and Analytical Chemistry, Pittsburgh, Pa., March 1958.
(22) Wiley, 1%. C., McLrtren, I. H., Rev. Sci. Insti.. 26,1150 (1955).
RECEIVED for review May 31, 1958. Accepted November 10, 1958. Presented in part before the Division of Analytical Chemistry, 132nd Meeting, ACS, New York, N. Y . , Ele tember 1957, Pittsburgh Conference on lnalytical Chemistry and Applied Speclxoscopy, 1958, and Symposium on Gas Chromatography, National Institutes of Lealth, Bethesda, Rld., 1958.
Identification of Pigments in Paint Products by Infrared Spectroscopy T. R. HARKINS, J. T. HARRIS,l and 0. D. SHREVE Marshall laborafory, E. 1. du Ponf de Nemoors & Co., Inc., Philadelphia, Pa.
b Infrared spectroscopy is a useful tool for the qualitative identification of pigments in paint products of unknown or questionable formulation, Positive identification can b e made of most of the inorganic pigments in common use, as well as of organic pigments. A generalized procedure for isolation of the pigment from the vehicle is given. The pigment can usually b e classified as inorganic and/or organic by the number and shape of the absorption bands observed in its infrared spectrum. More positive identification is then made b y consulting a compilation of reference spectra obtained on known materials. The reference spectra of 21 inorganic pigments and five typical organic pigments are presented.
A
is forniulated by dissolving one or more synthetic and/or natural resins, along with other minor ingredients, in an organic solyent mixture and dispersing therein one or more pigments. Modern paint formulators have a t their disposal a great variety of pigments, including both inorganic and organic materials. Several different pigments are often used in a formulation to achieve the desired color effects and other properties. Because of the inherent complexity of these formulations, the analytical identification of all components present by wet chemical procedures can be a time-consuming and difficult undertaking. The literature contains no extensive description of the application of infrared spectroscopy to the analysis of such pigment systems. Kendall (6) TYPICAL PXIXT PRODUCT
Present address, 489 Norwood St., Spartanburg, S. C.
Figure 1. Titanium dioxide, T i 0 2
Figure 2. Calcium sulfate (hydrous), Cas04 .xHzO
Figure 3. Diatomaceous earth (amorphous silica) SiOz
WAVE
has published partial spectra of several polymorphic forms of crystals of para red, copper phthalocyanine, and titanium dioxide. Tyler and Ehrhardt (8) presented the spectra of benzidine yellow and copper phthalocyanine in connection with the spectra of evaporated films. Only a few authors have treated the infrared spectra of inorganic materials extensively. Miller and Wilkins (6) presented a useful compilation of spectra of 159 inorganic compounds found in a chemical laboratory. Other work has dealt with the spectra of minerals, rocks, clays, and related inorganic compounds (2, 4). More recently, Tai and Underwood (7) described a quan-
L E N G T H (MICRONS)
titative infrared method for the determination of inorganic sulfates. Chemical methods of pigment analysis, in eluding methods of separating pigment from vehicle, have been recently ‘iutlined by Hanson ( I ) . EXPERIMENTAL
Generalized Procedure. T o prepare the ss,mple, the pigment is first separated from the paint product by high speed centrifugation. This is best accomplished with a supercentrifuge mpable of a t least 40,000 r.p.m. The separated pigment is isolated and washcd with a suitable solvent t o remo’re any traces of adhering resin and m y other vehicle components. \‘OL.31, NO. 4, APRIL 1 9 5 9
541
An acetone-toluene mixture is usually satisfactory for this purpose. The residual wash solvent is removed from the pigment by drying in an oven a t 110" C. for 1 hour. This drying usually leaves the pigment in a "caked" form. The pigment cake should be ground with a mortar and pestle so that the relatively small portion of the sample used for infrared analysis will be represent a t'we. Infrared spectra of the pigments are then obtained by more or less standard methods. As the materials are solids, they are either mulled in Nujol or ground into a potassium bromide disk. In either instance, a Wig-L-Bug vibrator (Crescent Dental Mfg. Co., Chicago, 111.) is used to provide a finely ground mixture. Mulling with Nujol is preferred because it is less t,ime consuming. The spectra presented here were all obtained as Nujol mulls. Instrumentation. The spectra were obtained on a Perkin-Elmer BIodel 21 double-beam spectrophotometer equipped with rock salt optics. Instrumens settings were those recommended by the inanufacturer for general qualitative analysis (slit program: 927; 2- to 15-micron region scaiiiied in 15 minutes). Wave length calibration was checked using a thin film of polystyrene. Reference Materials. All samples were of commercial origin.
'
-I'
"
I
"1
"
'
Figure 4. Crystalline silica, Si02
Figure 5. sium MgSiOs
Magnesilicate,
Figure 6. China clay (hydrated aluminum silicate)
I J ,
I
I
W,\'/E 1'
I
I
I
I
I
I
I
t
/ I
L E N G T H (MICRONS) I
Figure 7. Barytes (Bas04 from mineral deposifs)
DISCUSSION
As with all other qualitative applications of infrared spectroscopy, the optimum use of this technique for identification purposes first requires the accumulation of a library of spectra run on pigments of known identity. Unknowns are then identified by comparison of their spectra with the reference spectra. Figures 1 to 22 represent typical infrared reference spectra of pigments. The absorption bands marked by N are due to Nujol. Classification of Pigment Type. Figures 1 t o 21 represent the spectra of inorganic pigments-e.g., calcium carbonate, barium carbonate, and titanium dioxide. There are only a few and generally broad bands in the 2- t o 15-micron infrared region. This is characteristic of inorganic pigments and provides one with a general rule: If inorganic pigments have any infrared absorption a t all, the spectrum will generally be simple in nature. In contrast with these spectra are those of typical organic pigments, shown in Figure 22. The latter spectra are rich, having a number of sharp, discrete absorption bands. Most organic pigments have highly complex infrared spectra, in contrast t o the simple spectra observed for the inorganic materials. Thus, from a simple observation of an unknown spectrum, the type(s) of pigment present can usually be determined. 542
ANALYTICAL CHEMISTRY
Figure 8. Blanc fixe (chem. mfg. BaS04)
2
4
6 WAVE
8
IO
I2
14
L E N G T H (MICRONS)
Infrared Spectra. 1 . TITANIUM DIOXIDE. The spectriim of titanium dioxide consists of ciie broad, illdefined absorption band in the 14micron region. I n adzlition, absorption due to scattering ii evident a t the lower wave lengths. As titanium dioxide is one of the most common pigment materials found in paints, it is fortuitous that its infrrtred spectrum is sufficiently characteri ;tic for identification and yet reasonady free of absorption bands a t the r a v e lengths where most of the other pigments absorb.
Rutile and anatase titniiiuni dioxide cannot be differentiated by their 2- to 15-micron infrared spectra. 2. CALCIUMSULFATE (HYDROUS). All inorganic sulfates have a strong, broad absorption band a t about 8.85 to 9.26 microns and a sharp weaker band from 14.7 to 16.4 niicrons (6). Hydrous calcium sulfate has its maxima a t 8.9 and 14.9 microns, respectively. The remaining bands a t 2.S0, 2.93, 5.92, and 6.16 microns are characteristic of water of crystallization. 3. DIATOMACEOUS EARTH,AXORPHOUS SILICA. Silica pigments have
W
0
z a
I-
C
Figure 11. Basic lead carbonate
f
4
cc
ItZ W
0 K W
n
’
Figure 12. Strontium chromate, StCrO4 1
1
2
4
2
4
1
Figure 13. Basic lead chromate
Figure 14. Lead chromate, PbCrO4
Figure 15. Zinc chromate, ZnCr04
xH~O
.
an extremely broad, unsymmetrical band with niaximum absorption in the 9.1- to 9.4-micron range, which unequivocally distinguishes them from silicate pigments n-hose absorption maximum is near 10 microns. I n addition, diatomaceous earth has a weaker absorption band a t 12.6 microns. 4. CRYSTALLINE SILICA.The strong characteristic silica absorption band a t 9.1 t o 9.4 microns has been noted for diatoniaceous earth. Crystalline silica is differentiated from diatomaceous earth by its two sharp bands a t 12.55
region distinguishes it from siIica and magnesium silicate types. The infrared spectra of 15 silicate-type minerals have been prewnted ( 3 ) . 7 . BARYTEP, (Barium Sulfate from Mineral Deposits). Barium sulfatetype pigments can easily be distinguished from other sulfates by multiple absorption bands in the 8.3- to 10.3micron region. 8. BLANCFIXE (Chemically Manufactured Barium Sulfate). Blanc fixe yields a better defined spectrum than barytes, parbicularly in the 8.3- to 10.3-micron 1 egion, and this serves to differentiate the two types of barium sulfate. 9. BARIU\.I CARBONATE.Inorganic carbonates (Axhibit their strongest absorption a t 6.7 to 7.0 microns. A weaker, shaiper band at 11.70 microns is useful iri identifying barium carbonate. 10. CALCIUMCARBONATE.I n addition to its strongest absorption at 1 1 1 1 1 1 1 1 1 l 7.0 microns indicating carbonate, a 6 0 10 12 14 WAVE LENGTH (MICRONS) weaker, shE rper band a t 11.45 microns differentiates calcium carbonate from barium cai bonate. 11. BALICLEAD CARBONATE.The spectrum of basic lead carbonate has the strong; 7.0-micron carbonate absorption band, but does not have the weaker bonds a t 11.70 and 11.45 microns chaiacteristic of barium and calcium carbonate, respectively. Rather, its weaker absorption band is a t a much longer wave length, 14.70 microns. 12-16. CHROMATES. The chroniates exhibit strong characteristic absorption in the 10.5- to 12.5-micron region, the shape and position of which are governed by the particular cation. 17. EONE BLACK. Bone black is the only black pigment having characteristic infrared absorption. Carbon does not absorb in this region; the strong 1)and a t 9.6 microns is caused by calcilm phosphate. 18. ~IINERAL VIOLET. Manganese ammon uni phosphate has a relatively complex spectrum for an inorganic material and hence, teiids to be unique. 19. ANTIMONY OXIDE. The broad, symmetrical band centering a t 13.5 mi12 14 IO 6 0 WAVE LENGTH (MICRONS) crons and a much weaker band a t about 10.4 nticrons can be used to identify antimony oxide specifically. Zinc oxide does not absorb in the infrared region. and 12.85 microns and a m a k e r band 20. RAWSIENNA, LIGHT. Hydrous a t 14.45 microns. iron oxides exhibit two strong absorption 5. ~TAGNESIUV SILICATE,Magnebands a t about 11.0 and 12.6 microns sium silicate pigments, including talc, and a broad hydroxyl absorption a t 3.2 exhibit a strong, broad absorption band microns. Anhydrous iron oxide pigwith a maximum a t about 9.85 t o 9.90 mentri have essentially no absorption microns. A weaker but sharp band bandrg in the infrared and are, therefore, a t 2.70 to 2.74 microns (0-H), also indisiinguishable. found for aluminum silicate but not for The infrared spectra of natural crystalline silica and diatomaceous iron 3xides, siennas, ochres, and umbers earth, is evident. 6. HYDRATED ALUMINUM SILICATE, gene rally allow partial identification of the siliceous components of these pigCHINA CLAY. The absorption of aluments. Both silicon dioxide and aluminum silicate in t)he 9- to 11-micron VOL. 31, NO. 4, APRIL 1959
543
minum silicate, for example, are evident in the 9- to 10-micron region of the spectrum of raw sienna. 21. FERRIC FERROCYANIDE. This blue pigment has a strong, characteristic absorption band a t 4.77 microns. 22. ORGANIC PIGMENTS. These spectra, Toluidine Yellow, Monastral Blue, an Arylide Maroon, Monastral Green, and Litho1 Rubine, are included for comparison to point out the distinct spectral difference between organic and inorganic pigments. The number of organic pigments in commercial use is such that they could not be adequately covered in a report of this type. As with the organic pigments, the number of inorganic pigment spectra has also been kept to a minimum. A certain material, for example, will exhibit subtle spectral variations depending on origin, type of manufacture, and so forth. One example of this, chemically manufactured and naturally occurring barium sulfate, has been included. Other similar situations arise and are best appreciated when an individual laboratory compiles its own file of reference spectra. Prussian Blue and Milori Blue are complex compositions of ferric ferrocyanide, Fec [Fe(CN)6]3, which have color differences produced by different conditions of manufacture. Their infrared spectra are practically identical in the 2- to 8-micron region, but differences are found in the 8- to 15-micron region. Pigments are often complex coprecipitated mixtures whose spectrum is a composite presentation of the spectra of the individual components. Certain chrome yellows and oranges are examples of this. Lead chromate is the main component, with minor components usually consisting of lead sulfate, lead carbonate, lead molybdate, lead oxide, etc.
Figure 16. Basic zinc chromate
Figure 17. Bone black (85% calcium phosphate, 15% carb on)
Figure 18. Mineral violet, manganese ammonium phosphate, MnNHdP04 WAVE
L E N G T H (MICRONS)
Figure 19. Antimony oxide, Sbz03
Figure 20. Raw sienna, light (hydrated yellow iron oxide)
ADVANTAGES
One of the advantages of the infrared method of analysis is speed. Most of the information comes from a 15minute spectral scan. Another distinct advantage of this type of analysis is that the infrared spectrum permits identification of specific compounds in a mixture, in contrast to the chemical identification of ions. A single spectrum, for example, could easily identify barium sulfate (Figure 8) and calcium carbonate (Figure 10) in a mixture as distinguished from barium carbonate (Figure 9) and calcium sulfate (Figure 2). Qualitative chemical analysis would have indicated barium, calcium, sulfate, and carbonate ions without regard to pairing of cations and anions. It is also often possible to identify minor amounts of specific organic pigments in the presence of major amounts 544
ANALYTICAL CHEMISTRY
Figure 21. Ferric ferrocyanide, Fe4 [Fe(CN)sla
of inorganic pigments without a prior separation of componerls. DISADVANTAGES
Fortunately, there a x only a few inorganic pigment mat Exrials that do not exhibit characterist c infrared absorption bands. Carbcii black, anhydrous iron oxide, ma qganese oxide, zinc oxide, lead oxide, cadmium sulfide, and cadmium selenide were the the only examples found to fall in this category. Chemical methods are a
needed supplement to the infrared in these instances. While clieniical tests are required to detect the presence of any of these components, discerning judgment on the part of the analyst can preclude the possibility of certain of these materials’ being present, so that a qualitative test is not always required. Almost all organic pigments can be detected by the infrared method, providing reference curves have been run. It is not desirable to try to identify the pigment components in a paint
TOLUIDINE YELLOW
product without first separating the pigment .'rom the vehicle. The strong overlappi 2g of absorption bands due to the resin and other ingredients of the finish, as8 well as scattering effects, minimizes the possibility of obtaining much quitlitative information without prior separation.
MONASTRAL BLUE
ACKNOWLEDGMENT
The authors gratefully acknowledge the assistance of R. M. McNamara and J. J. Wenlce in obtaining the data, AN ARYLIDE
MAROON
LITERATURE CITED
(1) +son,
N. W., J . Oil & Colour Chem-
ists Asssc. 41, 203 (1958).
MONASTRAL GREEN
LITHOL RUBINE
(2) Hunt, J. M., Turner, D. S.,ANAL. CHEM.25, 1169 (1953). (3) Hunt, J. M., Wisherd, M. P., Bonham, L. C., Ibid., 22 (1950). (4) Keller, W. D., Pickett, E. E., Am. J. Sci. 248, 264 (1950). (5) Kenda 1, D. N., ANAL. CHEM.25, 382 (1953). (6) Miller, F. A,, Wilkins, C. H., Ibid., 24,1253 (1952). (7) Tai, H,, Underwood, A. L., Ibid., 29, 1430 (1957). (8) Tyler, J. E., Ehrhardt, S. A., Ibid., 25,390 (1 953).
RECEIVED for review June 30, 1958.
Figure 22.
Infrared spectra of typical organic pigments
Accepted September 20, 1958. Presented in part, 2nd Annual Delaware Valley Regional Meeting, ACS, Philadelphia, Pa ., February 1958.
Dissolution of Uranium Metal and Its Alloys ROBERT P. LARSEN Chemical Engineering Division, Argonne Nafional laboratory,
b The most useful methods for the dissolution of uranium metal and its alloys are reviewed, with particular emphasis on the preparation of solutions for analysis. The behavior of the metal and its alloys in the common acids, ethyl acetate solutions of bromine and hydrogen chloride,and sodium hydroxide-peroxide mixtures is described. Recommendations for dissolving each of a wide variety of uranium alloys are summarized in tabular form.
T
HE search for reactor fuels which have superior corrosion resistance, irradiation stability, and more desirable nuclear properties has made it necessary to determine over-all chemical composition on a wide variety of uranium base alloys. Most of the other metallic elements have a t some time been added singly or in combinations as
P. 0. Box 299, lemont, Ill.
alloying constituents (2, 9). Fuel processing methods which do not afford significant removals of certain fission elements may result in further unavoidable alloying. The uranium content of operating and proposed reactors varies from pure uranium to bismuth alloys containing 0.1% uranium. Rodden (8) and Katz and Rabinowitch (6) have briefly reviewed the methods used for dissolving uranium and high uranium alloys, and special methods are scattered throughout the atomic energy project literature. However, no collection of detailed information is known to exist. This paper has been prepared to meet this need by presenting somewhat detailed discussions of those dissolution methods which in practice have been found to be most useful. No attempt has been made to give credit for the development of the procedures described, because, for the
most part, they have evolved over a period of years. Reference is made only when more thorough discussions exist. The discussions are confined to those alloys in which uranium is the principal constituent. The reactor fuels which contain only small percentages of uranium-st ainless steels, zirconium, aluminum, bismuth, etc.-are usually solubilized by those methods which would be used on the unalloyed matrix metal. Unless otherwise stated, the use of each method described effects complete solution of the metal sample. GENERAL DI~iSOLUTIONCHARACTERISTICS
Elemental uranium is very reactive, comparable io magnesium in many of its dissolution characteristics. Bulk metal reacts vigorously with 3N hydrochloric acid and 12N nitric acid, and is rapidly irurface oxidized by air. VOL. 31,
NO. 4,
APRIL 1959
545
