July, 1923
I N D U S T R I A L A N D ENGINEERING CHEMISTRY
725
Limitations of the Obscuring Power Method of Determining the Particle Size of Pigments' By Ellwood B. Spear and Herbert A. Endres THEGOODYEAR TIRE
AND
RUBBER Co., AKRON,OHIO
0.1 E/. in diameter will not A critical study of the obscuring power method for determining affect even the shortest ing or diffusing the relative aoerage particle size of pigments has been made. light waves, and the obscurpower test is a turThe method is applicabkfor comparisons of particle size of the ing power methodwouldnot bimetric method of detersame pigment in the same medium where the average diameter of be expected to give accurmining the average particle the particles is greater than one-half the waoe length of light. I t ate quantitative resultswith size of pigments, and hap serves as a convenient and sapid check on the physical condition of particlesless than 0.175 p in been previously described two shipments of the same material produced by the same process diameter. To prove this in the l i t e r a t ~ r e . ~The ~~ euen when the average diameter of the particles is somewhat less than definitely the following exfundamental principle of one-half the wave length of light. periments were carried out: the method is the extincThe method, howeoer, is not applicable for the comparison of tion, partly by absorption, COLLOIDAL GOLD-A solutwo diflerent pigments if a n appreciable diflerence exists between tion of gold chloride was prepartly by scattering, due their respectioe refractive indices, or other optical properties, such pared containing 0.07 g. of both to refraction and'reas light absorption and reflection. For instance, zinc oxide and gold per liter. To 250 cc. of flection, of the direct light gas black will not give the same obscuring power oalues, eoen if the this solution a 0.1 per cent rays from the incandescent aoerage diameter of the particles is the same in the two cases. I t tannin solution was added drop by drop, thoroughly filaments of an electric lamp is not applicable for the determination of particle size in systems agitating and warming to caused by the dispersed pigwhere the diameter of the particles is less than onezfourth the waoe about 80' C. after each adment particles in the tube length of the light employed-i. e., in colloidal systems. dition until 1 cc. hadbeen through which the lamp is added and the colloidal gold thus Droduced had wassed viewed. The chief factors involved are the size of the particles, the differenceof refractive through the cherry-red and into the ireen. No obscuring power reading could be obtained a t this point, the solution indices of the dispersed material and the disperse medium, being optically clear. Additional quantities of tannin were then the absorption of light by the particles or by the medium, added. with the following results: and also reflection from particles, depending upon the angle Obscuring Power Total Tannin Sq. Cm./Cc. c c. presented to any ray. The importance of the difference of 1 Kone refractive indices of the dispersed phase and dispersing 2 i9,OOo 3 25,000 medium should be stressed, because if both phases have the 4 29,000 same refractive index there will be neither refraction nor 5 31,000 10 37,000 reflection. and the system will appear homogeneous and enAdded 5 drops 1 5 per cent BaClz., , , . . . 46,000 31,000 Added 2 cc. 15 per cent BaClz. . . , . . . . . tirely transparent. Obviously, in this case an obscuring 22,000 Heated t o boiling., , . , , . . . . . . . . . . . . . . power reading could not be obtained. 22,000 After standing 2 days,,. , , . . . . . . . . . . . . The apparatus employed is essentially a Sargent colorimThe significant fact borne out by these experiments is eter. Briefly, the obscuring power in square centimeters that the obscuring power increases with the particle size per gram is obtained by dividing the reciprocal of the con- until a maximum value is reached, after which it decreases centration of the suspension in grams per cubic centimeter by with increasing particle size. After the barium chloride the height of the column in centimeters. Multiplying the was added the particles became so large that they settled value thus obtained by the specific gravity of the suspended out on standing over night and the mixture had to be shaken material gives the obscuring power in square centimeters up before the final readings were taken. The final value of per cubic centimeter. 22,000 is not very different from that after 3 cc. of tannin The obscuring power method depends upon the property have been added, but the particle size is many times greater. which the particles have of cutting off light rays. It is APPLICATIONS TO GAS BLACK obviously impossible for particles to do this completely unless they can form a true geometric image of themselves. The dispersions of this material obtained by previous It is clear, therefore, that the obscuring power method cannot method^^?^ are not complete, as shown by microscopic give quantitative results where the diameter of the particles examination and the fact that they settle out readily on is less than that necessary to form a true geometric image. standing. In an effort to improve upon the degree of disFor this reason the obscuring power method is totally in- persion various protective colloids were employed, and tannin applicable for the determination of particle size in colloidal was found to give very satisfactory results. The method systems. The same argument obtains in the case of the best developed is as follows: pidments, such as very fine zinc oxide and gas black. Mix 50 mg. of the material into a few drops of glycerol in an Visible light is composed of light waves varying in length agate mortar. After the pigment is thoroughly wet add 0.1 g. from 4000 to 7000 Angstrom units, or 0.4 p to 0.7 p . It of solid tannin and continue trituration for 5 min. Use solid has been found that, for any given wave length, particles tannin instead of a solution, because it is essential to keep the less than one-fourth ,wave length in diameter do not appre- original trituration mixture as concentrated in pigment as is posfor complete dispersion. Then add water, 2 cc. a t a time ciably affect the wave front. Therefore, particles less than sible with titration after each addition, until 10 cc. have been added.
HE so-called obscur-
T
1 Presented before t h e Division of Rubber Chemistry a t the 64th Meeting of the American Chemical Society, Pittsburgh, Pa., September 4 t o 8 , 1922. 2 T H I JOURNAL, ~ 12 (1920), 890 8 I n d i a Rubber W o v l d , 66 (1922), 347.
Transfer the mixture to a 250-cc. volumetric flask, add about 100 cc. water and shake thoroughly, after which fill the flask to the mark and shake again. Should the mixture foam badly it can be settled by adding a few drops of ether. Transfer 25 cc. of this dispersion to a 100-cc. volumetric flask and fill to the
I N D U S T R I A L A N D ENGINEERING CHEMISTRY
726
mark with water. After thoroughly shaking this mixture obtain the obscuring power reading in the usual way.
Microscopic and ultramicroscopic examination and the extremely slow rate of settling show dispersions obtained in this way to be quite complete. Four samples of gas black were run by this method. According to ultramicroscopic examination the particle sixe of these increased in the order named. The following are the results of the obscuring power method: Sample 1
Obscuring Power Sq. Cm./Cc.
9200
2
1000
3
5000 10,000
4
There is every reason to believe that Sample 2 is graphitic, which may account in part for the low obscuring power value, for reasons to be given later. However, the results plainly show that the method is of no value when applied to gas blacks, which usually contain a considerable proportion of particles less than 0.10 p in size. As previously stated, particles less than one-fourth wave length (0.10 p ) in size will not appreciably affect even the shortest light waves. The reason for the inconsistent results on gas black is therefore perfectly obvious. The foregoing method gives satisfactory results, however, when applied to coarser carbon pigments, such as lampblack and graphite. APPLICATIONS TO ZINC OXIDE To improve upon the previous methods by obtaining a better dispersion of this material, protective colloids were again tried and gum arabic was found to give the best results, both from the standpoint of dispersion and manipulative difficulties due to foaming on dilution and mixing. The following method was adopted: Mix 0.5 g . of the material thoroughly with a few drops of glycerol in an agate mortar. To this add 10 cc. of a 1 per cent solution of gum arabic in water and continue trituration for about 5 min. Transfer the mixture to a 250-cc. volumetric flask and treat as in the case of gas black.
Three samples of zinc oxide were examined by this method. Microscopic examination and particle-size measurements, as well as obscuring power values would place them in the following order as t o particle size: Sample 1 2
3
1. Franklin Inst.. 192 (1921). 637.
Obscuring Power Sq. Cm./Cc.
Crystallized gel: F r e s h . . .................................. 2200 After standing 2 mo. in paste form . . . . . . . . . 3200 Precipitated: BaCIa (satd.) t HzS04 (1: 1) . . . . . . . . . . . . . . . . . .3750 BaCla (satd.) HzS04 (1 : 2).. . . . . . . . . . . . . . . . . 3450 Special precipitated.. ..................................... 2150 Regular ground 800
+
........................................
These results again point out the invalidity of the method when applied to particles which lie in the colloidal realm. The freshly crystallized gel, although many times finer than the special precipitated material, is not very different in obscuring power. Also the two-month-old sample of the crystallizedgel has a higher obscuring power than the same material when freshly prepared, in spite of the fact that the crystals grew considerably during this period. The method is not even qualitative for the first two samples, but is fairly quantitative for the others. The method has been tried out with several other pigments, and in general it can be applied with fairly quantitative results to pigments which do not contain particles smaller than 0.1 p in diameter. It is therefore applicable to all the presentday commercial pigments with the exception of gas black and some grades of antimony pentasulfide, and iron and zinc oxides. With pigments containing particles less than 0.1 p in size, the method is not quantitative and becomes less so as the proportion of these increases. Owing to the great influence of the refractive index of the pigment, as well as that of the suspending medium, on the obscuring power, the values obtained for the same pigment in different media and also different pigments in the same media are not c6mparable. The value of the method lies in determining the relative average particle size of different samples of pigments of the same composition and crystalline structure. As previously pointed out, amorphous carbon cannot be compared with graphitic carbon because of differences in optical properties-coefficient of refraction, reflecting power. RANGEOF APPLICABILITY The following diagram, illustrating the range of applicability of the obscuring power method, is based on information obtained from the experiments above described.
Obscuring Power Sq. Cm./Cc. 4000 0000 6750
Particle-size measurements by the method of Green4 have shown the particles of Sample 1 to average about 0.4 p in diameter, while those of Sample 3 average about 0.2 p. Sample 3 contains a considerable proportion of particles which are less than 0.1 p and this accounts for the low obscuring power value, which should be twice that of Sample 1, or 8000. Zinc oxides are usually much coarser than Sample 3, this being a specially prepared product. The obscuring power method is therefore generally applicable to the usual grades of this material. APPLICATIONS TO OTHERPIGMENTS BARYTES-A barium sulfate gel was prepared by the interaction of saturated solutions of barium iodide and manganous sulfate. Agitating this gel caused it to crystallize into a very finely divided barium sulfate. Obscuring power determinations were made on this material by the gum arabic method, as described above, and also on samples prepared by other precipitation methods and regular ground barytes. These are listed below, with their corresponding obscuring powers, in the order of increasing particle size as determined by microscopic examination and settling tests. 4
. MATERIAL
Vol. 15, No. 7
Jocrezrhg parfch s/ie
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The obscuring power-particle size curves are different for different pigments, and all show maxima in the colloidal realm. The values for obscuring power and particle size a t the peak depend upon the optical properties of the shbstance and are different for different substances. Obviously, the method is not applicable to pigments made up of particles which lie to the left of point (a) on the curve. This is the region of colloidal dispersions and in i t the obscuring power increases with increase in particle size. Gas black contains a considerable proportion of particles which lie in this region. Most of the other pigments, however, are made up of particles which for the most part lie to the right of point (a), and to them the method can be applied with fairly accurate results.
