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accelerators. The activity of 1 per cent of hexamethylenetetramine, for example, was stated to be equivalent to that of 2.5 per cent of thiocarbanilide. It is interesting to note how he arrived a t these figures. Discarding without any experimental evidence the coefficient of vulcanization as a measure of state of cure, since it “presents considerable variations,” Endres uses the maximum physical properties as the criterion of correct cure, i. e., the maximum tensile strength (or tensile product) a t break. He states that he uses the stressstrain curves to obtain supplementary information. In the case cited above this latter was apparently overlooked, as the following figures for the equivalent cures will indicate:
1 Per cent Hexamethylenetetramine 50-Min. Cure a t
ELONGATIOIV 145” C. Load in Lbs./Sq. In. 100
185 333 556 964 1700 2800
200 300 400 500 600 675
...
(break)
Vol. 14, No. 10 banilide 2.5 Per cent BO-Min. ThiocarCure a t 1.15’ C. Load in Lbs./Ss. I n . ‘114 228 ’ 458 725 1140 1980 2660 (break)
Endres’ results are further invalidated by the fact that the hexamethylenetetramine stock (above) contains 6 per cent of sulfur (to the rubber), while the thiocarbanilide stock contains 8 per cent. It is difficult to understand how a logical comparison can be drawn under such circumstances. ~~
Quantitative Determination of A n t hraqui none’sz By 0.A. Nelson and C. E. Senseman COLORLABORATORY, BUREAUOF
A method has been deueloped by which anthraquinone may be determined quantitatively in mixtures containing also phenanthraquinone, anthracene, phenanthrene, phthalic anhydride, phthalic acid, or other oxidation products of anthracene or phenanthrene. The method consists essentially of reducing the anthraquinone to the red oxanthranol using zinc powder and 5 per cent solution of sodium hydroxide. The red solution is filtered in uacuum and titrated with standard potassium permanganate. A detailed outline for carrying out the analysis is given, together with seueral representative results showing the accuracy of the method.
N A PAPER from the Color Laboratory by H. F. Lewis3
I
a method is described by which anthraquinone may be determined quantitatively and with a fairly high degree of accuracy. While this method seems to be theoretically good, the authors have found it very troublesome because of the great length of time consumed in filtering and thoroughly washing the finely divided anthraquinone. Grabe and Liebermann4 found that by the reduction of anthraquinone with zinc dust and dilute sodium hydroxide, a red compound to which they gave the name of oxanthranol, or anthrahydroquinone, was formed. This compound, which is soluble in the solution of sodium hydroxide, is very readily reoxidized to anthraquinone. Since the reduction to oxanthranol and the subsequent oxidation back to anthraquinone are quantitative, these reactions formed the basis for Lewis’s method of determining anthraquinone. Lewis’s method as outlined consists essentially of first reducing the anthraquinone with zinc and a 5 per cent solution of sodium hydroxide to the red oxanthranol, then reoxidizing it by shaking in air and weighing the anthraquinone recovered. As already stated, the chief difficulty encountered in this process was the filtering of the recovered anthraquinone from the alkaline solution because the finely divided precipitate invariably clogged the pores in the asbestos mat of the Gooch crucible. In several of our experiments the time required for filtering alone was nearly a day. 1
Received May 31, 1922.
* Contribution No. 62 from the
Color Laboratory. permission of the Department of Agriculture. 3 THISJOURNAL, 10 (l918), 425. 4 Ann., 160 (1871), 126.
Published with the
CHEMISTRY,
WASHINGTON,D. C
In the modified method this filtering is eliminated and the red solution of oxanthranol is titrated with standard potassium permanganate, thus determining the anthraquinone volumetrically instead of gravimetrically as in the older methods. But as the oxanthranol solution is readily oxidized in air, the titration with potassium permanganate must be done in a vacuum or in the presence of an indifferent gas or vapor. It was therefore necessary to design a special apparatus for this purpose. The filtering tube, a, somewhat magnified diagram of the construction of which is shown in Fig. l ~was , approximately 3.5 em inside diameter and 24 cm. long, and drawn down to a diameter of about 5 mm. On the bottom of this tube were placed an ordinary Witte filter plate, A, ground down to fit inside the glass tube, and an asbestos mat, B, in. thick, which was held in place by a 100-mesh brass gauze, C. Several dents were made in the wall of the tube just above the brass gauze to keep it in place, when the stirrer was set in motion. As brass is affected very little by alkali, one gauze would last through a large number of runs. The reducing and filtering tube was made of heavy pyrex glass. The following procedure for making an analysis has given very satisfactory results, and it can be completed in about a half hour: Weigh out 0.5000 g. of a very finely ground sample and mix this with 3 or 4 g. of zinc dust. The finer the sample and the more thoroughly it is mixed with the zinc dust, the more readily reduction takes place. A good way is to mix the sample and the zinc dust in an agate mortar, and stir the mixture until no yellow particles or streaks of anthraquinone are visible. From the mortar, if one is used, transfer the dry mixture to a small beaker (150 to 200 cc.). Pour in about 100 cc. of boiling 5 per cent solution of sodium hydroxide and allow the solution to stand with occasional stirring for about 5 min. The ternperature of this solution should be a trifle below the boiling point of the mixture, and the heating should not be continued too long. Too long heating results in the formation of some anthranols which contains one oxygen atom less than does the oxanthranol. Run through the filter enough water that has previously been boiled and allowed to cool somewhat to form a layer about an inch deep in the bottom of the suction flask. Evacuate the suction flask until the air has been completely displaced by water vapor, The purpose of this is to sweep out all the air from the apparatus, thus obviating any oxidation of the red solution by oxygen other than that obtained from thepotassium permanganate. 6
Liebermann and Gimel, Ber., 20 (1887),1854.
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Pour the red solution, together with the zinc dust and unchanged anthraquinone, into the reducing and filtering tube (Fig. 1, C), which should be kept hot by means of the electric heater D. If a rheostat is placed in the circuit the temperature of the heater can be kept very constant a t about 90’ to 95 O C., which is about right for the best results.
C B
A
FIG.1 A-KMnOl B-BURET
BoTThE
1
TUBE C-REDUCINGA N D FILTERING
D-ELECTRICHGATER E-STIRRER F-SECTIONFLASK
In filtering, care must be taken that no air is drawn into the filter flask. This is best accomplished by always leaving a small quantity of the solution in the filter tube. In order that the surface of the solution might be seen while filtering, the electric heater was held in place by a clamp fastened to a ring stand, and could thus be raised or lowered a t will. If care is exercised during filtration, two layers will form in the suction flask. This is highly desirable, inasmuch as the water already in the flask, which forms the upper layer, has been boiled free from dissolved air, thus protecting the red solution from any action of atmospheric oxygen. Usually three or four reductions are sufficient, although unless the sample is very finely powdered six or eight may be necessary. The reduction must be continued until no more red solution is formed in the reducing tube after heating and stirring for about 5 min. When the reduction has completed, fill the reducing and filtering tube approximately half full with the hot sodium hydroxide solution. After each increment of the titrating solution has been drawn into the filter flask, turn the two-way stopcock so as to admit some of this alkaline solution t o wash out the potassium permanganate solution lodged in the tube between the stopcock and the solution in the flask.
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The potassium permanganate used was prepared by dissolving 3.8 g. per liter and allowing the solution to stand for several days. The standardization of this solution was accomplished by grinding 0.5000 g. of pure anthraquinone to an impalpable powder and thoroughly mixing it with zinc dust. The reduction was then carried out as previously described. The presence of phenanthraquinone with the anthraquinone does not interfere with the determination of the latter. Several runs were made in which anthraquinone and phenanthraquinone were mixed in different proportions, but in every case the results obtained were well within the limits of experimental error. It should be noted at this point that if the sample contains only pure anthraquinone, as when standardizing the potassium permanganate solution, the end-point is reached when the solution in the filter flask becomes perfectly colorless. Upon the addition of more titrating solution the color changes to a dark green, owing to the formation of potassium manganate (K2Mn04). If phenanthraquinone is present, the solution has a slightly bluish green tinge for the end-point, which upon addition of more potassium permanganate solution changes to a brownish green color, becoming darker. This color change probably may be explained by assuming that a derivative of phenanthraquinone analogous to oxanthranol is formed by the action of zinc and sodium hydroxide solution, and that this compound is not as readily oxidized back to phenanthraquinone as oxanthranol is to anthraquinone. From the results obtained, after having made scores of determinations of anthraquinone in mixtures containing also phenanthraquinone, anthracene, phenanthrene, phthalic anhydride, and phthalic acid, together with other oxidation products of anthracene and phenanthrene, it has been observed that there is a distinct change in color of the solution in the suction flask after sufficient titrating solution has been run in to oxidize all the oxanthranol. Table I, showing a few results obtained on mixtures of anthraquinone and phenanthraqninone, will give an idea of the degree of accuracy obtainable with this method.
Weight of Anthraquinone Grams 0.5000
Weight of Phenanthraqui none Grams 0,0000
0,4750 0,4800 0,4250 0,4000 0.4000 0,3750 0.3500 0.2000
0,0250 0.0500 0.0750 0.1000 0,1000
0.2000
0.1000
0.1260
0.1500 0.0000
TABLE I ProporA?thration of quinone Anthra- . KMnOi Deterquinone Solution mined Per cent Cc. Percent REMARKS 100.00 37.85 100.00 Blank. Standard ization 95.00 36.00 98.11 90.00 34.00 89.83 85.00 32.20 86.07 80.00 30.30 80.06 80.00 30.22 79.81 75.00 28.32 74.82 70.00 26.40 69.75 100.00 29.1 100.00 Blank. Different KMnO4 66.667 29.1 66.667
The observed and calculated percentages of anthraquinone agree very well. Equally good agreements were readily obtained on check runs of the same sample, thus making duplicate determinations almost unnecessary. In Lewis’s method the anthraquinone was dissolved in a small quantity of alcohol. This is not permissible in the author’s modification, for the reason that the alcohol would be oxidized to acetaldehyde in the titration with the potassium permanganate solution, thus causing the results to run high. If the precautions outlined are observed in detail the average analyst should experience no difficulty in obtaining concordant results by this method which will save much time.
