ANALYTICAL
July 15, 1942
of 0.170 gram per 100 ml. of solution, a figure much lower than that of Allen (1). This also gave a specific rotation in ether of [aft +176. The residual cinchonine when dried gave a specific rotation of +224.0. This sample of the base was used for all optical activity determinations as described above under quinidine. The results for the alcohol-water curves of the free base, the sulfate (B2.H2SO4), and the dihydrochloride (B.2HC1) are shown in Table I, and data concerning neutralization with sulfuric and hydrochloric acids in Table II. Because of the more limited solubility of cinchonine base in water, the data for the former in Table I could not be extended to alcohol concentrations below 32 per cent. Even at this point only 0.02 gram of base per 100 ml. was possible. The error of the determination is therefore correspondingly large. =
Cinchonidine A commercial sample of the free base was recrystallized from solution in 95 per cent ethyl alcohol by addition of sufficient water to reduce the alcohol percentage to 25 per cent. The resulting precipitate was filtered, washed with 25 per cent ethyl alcohol, and dried. Three such treatments gave a product showing, in absolute ethyl alcohol, a constant optical —107.3. A value of —111.0 was reported activity of [a]™ by Rabe. Since further recrystallization did not alter the author’s figure ( 107.3), it was taken as representing the pure base under the conditions specified. Using this sample, the determinations were made as described above for quinidine. The results for the alcohol-water curves of the free base, the sulfate (B2.H2SO4), and the dihydrochloride (B. 2HC1) are shown in Table I, and data concerning neutralization of the free base with sulfuric and hydrochloric acids in Table II. =
EDITION
545
It is obvious that the curves of all of these alkaloids follow the same pattern. As in the case of quinine, the water-alcohol curves of optical activity of free base, sulfate, and dihydrochloride all rise to flat maxima at intermediate percentages of water and alcohol, giving lower values of specific rotation in both pure water and pure ethyl alcohol. The position of these maxima varies with the different cinchona derivatives examined, but it may be said that for the free bases, the maxima lie between 60 and 95 per cent alcohol by volume, for the sulfates between 50 and 75 per cent, and for the dihydrochlorides between 20 and 30 per cent. Using, in each case, the percentage of alcohol giving maximum rotation for that salt, the neutralization curves shown in Table II were run. These follow the course previously reported for quinine: a smooth curve with no breaks corresponding to any stoichiometric ratios. In some cases, particularly with the hydrochlorides, addition of excess acid causes a slight drop in the maximum rotation attained. Some examples of this will be noted in the table.
Acknowledgment The author wishes to acknowledge the assistance of the Samuel S. Fels Fund in providing means for carrying out this work.
Literature
Cited
—
(1) Allen, “Commercial Organic Analysis”, 5th ed., Vol. VII, Philadelphia, P. Blakiston’s Son & Co., 1929. (2) Andrews, J. C., and Webb, B. D., Ind. Enq. Chem., Anal. Ed., 13, 232 (1941). (3) Butler, C. L., and Cretcher, L. H., J. Am. Pharm. Assoc., 22, 414 (1933). (4) Henry, T. A., “The Plant Alkaloids”, 3rd ed., Philadelphia, P. Blakiston's Son & Co., 1939. (5) Rabe, P., Ann., 492, 242 (1932).
Oil Absorption of Pigments M. A. AZAM1, Industrial
Research Laboratory, Calcutta, India
At a certain point in oil absorption of pigments the paste in oil is very easily taken off on the palette knife. This marks the
and the indication is sharp within 1 drop of oil (0.05 cc.) from an ordinary standard buret. When pastes, fully or partly saturated
point of saturation
term “oil absorption” has been vaguely defined and
the method of determining its value has not been truly THE
standardized. In view of the circumstances, a rigorous study of oil absorption with strictly accurate and reproducible results has not yet been possible. Oil absorption has been defined in various ways: as nothing but the filling of a void (5) or the filling of interspaces of the pigment particles with oil, the wetting power of the liquid (S), or the maximum quantity of oil required with any pigment for an intimate mixture in liquid state with minimum tendency to flow. A point which does not appear to be appreciated and which adds to the difficulty of attaching 1
Present address, Plastics Industries Technical Institute, Los Angeles, Calif.
with oil according to the above standard, are immersed in a bath of oil stock used for absorption, saturated pastes absorb no more oil, while unsaturated pastes absorb oil almost equal to the calculated deficiency for saturation in one to two days (24 to 48 hours). significance to oil-absorption measurement is the problem of defining experimentally the condition of complete wetting of the pigment. The criterion usually adopted by specifications depends on a rather vague visual judgment of consistency at the end point, which although it may afford a fairly accurate basis for comparing samples of the same pigment, does not necessarily serve well for comparison between different pigments. Apart from such interpretations, there is in general neither a fixed method nor a sharp finishing point in the determination of oil absorption. The current methods of locating the end point have more or less the common flow of a rule-ofthumb experiment and, as such, the end point determined by these methods is scarcely reproducible with exact precision.
INDUSTRIAL
546
AND
ENGINEERING
CHEMISTRY
Vol. 14, No. 7
The variation in the results did not exceed 1 drop of oil from a standard buret or 1 per cent in the value of oil absorption. There is, however, another aspect of the end-point determination by this method. The results have supplied a newer Calculated definition of oil absorption of pigments. The smooth pigDeficiency Degree Oil Absorbed of Oil of ment pastes which absorbed oil up to the sharp end point 24 36 48 for Quantity SaturaTaken hours hours Saturation Pigment located by the author did not absorb more oil when immersed Cc. Cc Cc. Grams Cc. do in a bath of the oil stock used for absorption, whereas pastes 0.8 0.8 0.8 50 1.0 with lesser oil did actually absorb more oil almost equal to 10 Red oxide of iron 0.4 0.4 0.4 0.45 75 the calculated deficiency of saturation. A graduated cylin1.2 1.2 50 1.2 1.3 der, used as an oil bath, was filled up to a certain point with 20 White lead the paste carefully scraped into, and totally immersed in, oil. 0.2 90 0.2 0.2 0.26 The depression in the level of oil in the cylinder was noted 0.7 50 0.7 0.7 0.75 10 after 24, 36, and 48 hours (Table I). Lithopone 0.05 0.09 0.09 90 0.15 Thus comes a new interpretation of oil absorption, which might now be defined as the minimum quantity Table II. Percentage of Oil Absorption by Common Pigments of oil required to saturate a pigDoublement by grinding. The end point Boiled Linseed Linseed Castor Tung as located is not only supported on Oil Oil Oil Oil logical and scientific grounds, but 0.930 Specific gravity Quantity 0.931 0.953 0.936 also throws a flood of light on the Acid value Taken for 2.7 6 7 6.6 Each 192 198 Saponification value 186 200 true approach to this obscure problem Iodine (Hanus) value 170 145 83 165 of a very well-known practice. ment°, Grams Mean Oil Absorption Values Pigments Table II gives oil-absorption 20 White lead 12.15 12.32 12.12 12,17 values for some common pigments 10 White zinc 17.89 16.81 17.63 16.85 10 14.18 13.02 Lithopone 13.34 13.1 with different oils, as determined 20 19.65 19.29 Whiting 19.77 19.07 20 11.16 Barytes 10.48 10.18 10.35 by the author at the Paint and 10 Yellow ochre 27.9 23.83 23.4 Varnish Section, Industrial Research 10 Red ochre 22.39 18 ¡83 20.1 18.25 10 Ultramarine blue 33.48 33.48 35.26 31.82 Laboratory in Calcutta, Bengal, Prussian blue 56.27 53.37 10 Red oxide India. Each result represents a 16.74 14 ¡88 16.20 15 68 20 Red lead 7.44 7.44 7.15 7.02 mean of 70 duplications, mostly 6 According to recommendations of I. S. D. specifications. concurrent. In the few cases where variations were noted, the difference did not exceed 1 per cent. A steel spatula (Sheffield, London) The Gardner-Coleman method which determines the “oil with a 8.75 X 2.5 cm. (3.5 X 1 inch) blade and a marble slab 30.0 cm. (1 foot) square -were used for grinding. The absorption factor” is accurate within 0.2 cc. (of oil added). The elimination of grinding and incorporation of certain pretemperature ranged from 23.89° to 29.44° C. (75° to 98° F.) cautions make it easier to duplicate the results within that and humidity varied between 52 and 67 per cent. limit of accuracy. But, unfortunately, the oil-absorption The author’s attention has lately been drawn to a recent factor does not work out a paste of the conventional consistU. S. Navy Department Specification, 52-Z-7 (int.), September 1, 1941, Bureau of Ships ad interim specification, advoency approved by painters; and the method, apart from its theoretical importance, has not much to recommend itself to cating a method identical with the author’s. Incidentally, the paint maker in his everyday practice. Obviously, the the experiments by the author were conducted towards the standard rub-out method and the Gardner-Coleman method end of 1940. This happy coincidence is in itself a very give results wide apart, one using much less oil than the other strong argument in favor of the method for location of the end point. (*). The author of the present communication, after a prolonged Conclusion search, has observed definite indications locating a sharp end point. The results provide a dependable basis for a comIn view of the fact that pastes produced by various pigparative study of oil absorption of pigments. ments with oil show a considerable difference in characterIt has been consistently observed by the author that at a istics, the end point is better located at the point of saturation certain point, on gradual addition of oil to the pigment with as described. The saturated paste is invariably a coherent, for constant rubbing thorough incorporation of the oil with stiff, puttylike mass which does not break or separate and the pigment, the paste sticks to the palette knife with very which lends itself conveniently for further operation in the little effort. The difference in the coherence of the mass is preparation of mixed paint. clearly distinguishable before and after this definite stage is reached. The results are reproducible with very close coin-
Table I.
Absorption
of
Oil by Unsaturated Pigment Pastes
_
¡
Literature
cidence.
Following the A. S. T. M. procedure (1), using a buret and taking about 20 minutes for each experiment for comparable extent and pressure of grinding, oil absorption of pigments was determined using raw linseed oil of known specifications. The quantity of pigment varied according to its nature from 5 grams in the case of Prussian blue to 20 grams in the case of white lead, following the recommendations of the I. S. D. specifications (4). This limits the amount of oil added, in most cases to approximately 40 drops (1 drop 0.05 cc.). =
Cited
(1) Am. Soc. Testing Materials, Standards on Paint, Varnish, Laoquer, and Related Products, Designation D281-31 (adopted
1931). (2) Gardner, .
A., “Physical and Chemical Examination of Paints, Varnishes, Lacquers, and Colors”, 9th ed., Chap. 16, Washington, D. C., Institute of Paint and Varnish Research, 1939. (3) Grosse, Farbe u. Lack, 1930, 418. (4) I. S. D. Specifications for Pigments, Oil Pastes, Paints, Varnishes, etc., 7th ed., p. 12, Calcutta, Superintendent of Printing, Government of India. (5) Klumpp, E., and Meir, H., Farben-Ztg., 35, 1712 (1930).
