444
INDUSTRIAL AND ENGINEERING CHEMISTRY
surface (under ordinary conditions) is rigorously uniform, the diameter of the contact mark varies from point to point. If the average diameter is wrong by 1 per cent the error of the A3 ratio ; is 3 per cent. When the value of
A3
is measured or extrapolated, 8 may
be calculated using Table I, which gives the values of the function *(2 - 324 for 49 values of 0. For nonCOS e coss e)
+
tabulated values of
$ a linear interpolation is sufficient. A3
Table I also shows that a 3 per cent error in f causes an error of about 1 per cent (or less) in the value of 8. If the solid surface is very good, like those of built-up multilayers (g), can often be determined within *0.6 per cent and 8 therefore calculated within * 0.2". In a recent paper Benedetti-Pichler and Rachele (1) estimated the size of minute droplets of water on cellulose nitrate by measuring the diameter of the circle of contact. They found that the ratio was about 9; Table I shows then
Vol. 13, No. 6
that the contact angle between air, water, and cellulose nitrate was about 55". The method here described was used for drops of water on built-up multilayers of soaps ( d ) , on lacquered tin plate (S), and on glass plates. U'hen compared with the usual method of a direct measurement of 8 this method is indicated when determination of volume and length is simpler than that of angle. It has also another advantage. When the profde of a drop is observed, the angle measured is that for one point only; the same drop viewed from another side may have a different shape and form a different contact angle. The area of contact between drop and solid is a measure of an average of all the contact angles present along the circumference of the drop, and the shape of the mark shows how constant (or otherwise) is the contact angle along this circumference.
Literature Cited (1) Benedetti-Pichler, A. A., and Rachele, J. R., IND.ENQ.CHEM., Anal. Ed., 12, 233 (1940). (2) Bikerman, J. J., Tram. Paraday SOC.,36, 412 (1940). (3) Sumner, C . G., The Metal Box Go., Ltd.. London, W. 3., private communications. (4) Wark, I. W., J . Phys. Chem., 37, 623 (1933).
A Wet-Combustion Micromethod for Determination of Carbon and Hydrogen Iodic Acid as an Oxidant for Wet Combustion BERT E. CHRISTENSEN AND ROBERT WONG O r e g o n S t a t e College, Corvallis, Ore.
C
HRISTEKSEN and Facer (1) have recently published a wetcombustion method using iodate as the oxidant for the determination of carbon and hydrogen. Previous experience has convinced the writers that the use of iodate in this connection is best adapted to micro work. Since a method which would handle small samples (2 to 5 mg.) is highly desirable in many cases, the possibility of devising such a procedure was studied. In connection with these studies the method of Christensen and Facer was modified. Changes in the procedures for determining the evolved carbon dioxide and the method of treating the unreduced iodate, and a number of other modifications resulted in a considerable saving of time. Although these wet-combustion methods are simple, permit intermittent operation, and employ the minimum of apparatus, their success depends upon the use of iodic acid as an oxidant for the organic material. This reagent has given excellent results for many organic compounds, yet some compounds under the conditions specified for analysis cannot be quantitatively oxidized by iodic acid. It was the opinion of the authors that much more information must be obtained regarding the behavior of this reagent toward all kinds of organic compounds (volatile, insoluble, etc.) before a wet-combustion method involving iodic acid could be established. It was possible that further study of the limitations of iodic acid as an oxidant in wet combustions might afford a criterion for predicting the behavior of a given compound when treated with iodic acid. For these reasons the following investigation was undertaken.
Reagents Sulfuric acid, 96 per cent. It is important that the sulfuric acid be made as free as possible from organic matter. This was accomplished by addin 50 ml. of Superoxol dropwise t o 200 ml. of hot concentrated sulfwic acid. This mixture was just brought to boiling and the heating continued until the sulfuric acid attained approximately its original volume. Barium hydroxide, approximately 0.1 N , was prepared and protected by a soda-lime tube. Hydrochloric acid, approximately 0.05 N , was prepared and standardized with potassium iodate. Thymol blue indicator, 0.2 gram, was dissolved in 43 ml. of 0.01 N sodium hydroxide and diluted t o 500 ml. with distilled water. Potassium iodidebarium chloride solution, approximately 0.5 N with respect to each reagent. Potassium iodate, c. P. grade. Potassium permanganate, approximately 0.1 N . Acetone, acid-free.
Apparatus The ap aratus is similar in design to one described in a previous article (8r Two changes were made in the reaction vessel: The bottom was enlarged to form a bulb and the cup on the side was constructed from a male standard taper joint. The bulb made it easier t o dissolve many substances, while the joint afforded a better means of connectin the carbon dioxide-free air system to the apparatus and provifed better protection a ainst organic contamination. Since no carbon monoxide was formed during iodate oxidation, the slow combustion unit was deleted from the train. The modified apparatus is illustrated diagrammatically in Figure 1. For the protection and accurate measurement of the standard barium hydroxide solution the automatic pipet described by West ( 4 ) was employed.
ANALYTICAL EDITION
June 15, 1941
u mu
1 A
FIGURE 1. DIAGRAM OF MODIFIED APPARATUS
B
'Capillary tubing'
C DETERMINATIOS OF CARBON DIOXIDE. D is now removed and allowed to stand for 20 minutes, 5 ml. of acid-free acetone (to improve the end point) are added, and the excess barium hydroxide is titrated to the thymol blue end point. To prevent the diffusion of air into the flask durin the titration, a rubber sheet (such as is used for a dental dam) is loosely fitted over the mouth of the vessel by means of a rubber band. The tip of the buret is inserted through a small hole in the rubber. The amount of carbon dioxide evolved is then determined from the amount of standard acid required to neutralize the excess base and that required in the case of the blank run. DETERMINATION OF OXYGEN CONSUMED.The contents of reaction vessel A are diluted t o approximately 100 ml. and boiled until the disappearance of the iodine color. The solution is then cooled, 1 gram of potassium iodide is introduced, and it is titrated with 0.1 N thiosulfate using a 10-ml. microburet. This measures the excess iodate. VOLATILE COMPOUNDS.Compounds which are too volatile to weigh directly can be readily treated in the followin manner. Fifteen to 20 grams of carbon-free sulfuric acid are weig%ed out in a dropping bottle provided with a ground-glass pipet. Approximately 100 mg. of the volatile compound are then mixed and dissolved in the acid and the solution is weighed, giving the organic content per unit weight. Portions are then removed from the dropping bottle for analysis. I n addition t o handling volatile compounds this procedure permits the microdetermination of carbon and hydrogen without the use of a microbalance. BLANKDETERMINATION. It is important that the sulfuric acid be as free of organic matter as possible. Because the sulfuric acid may contain small amounts of oxidizable matter, it is necessary to run blanks on the acid in order to determine the amount of iodate (if any) consumed per milliliter of acid. This should be repeated with each new batch of acid. I n order to correct all possible sources of error (such as carbon dioxide from reagents, residual air, etc.) the measured charge of barium hydroxide (5 cc.) was compared to the standard acid after it had been subjected t o a blank determination. The blank value for the barium hydroxide should also be redetermined with each new batch of acid. I n series of blank runs the iodate consumption was 0.0 mg. per ml. of sulfuric acid. Five milliliters of barium hydroxide required 9.10 ml. of hydrochloric acid by direct titration, while on blank runs only 9.04, 9.06, and 9.06 ml. were required. CALCULATIONS. The milliliters of standard hydrochloric acid required for the blank determination, A , minus the milliliters of hydrochloric acid required to titrate the excess barium hydroxide, B, are equivalent to carbon dioxide evolved. Expressed as a n equation
?
The heating bath was made from ;t 150-ml. beaker, wound with fifteen turns of No. 22 Nichrome wire. The temperature was controlled by a variable transformer (Adjustavolt, Standard Electrical Products Co., St. Paul, hlinn.). Phosphoric acid was used as a bath medium. This arrangement permitted control of the temperature within k 5 " C. wit,h only an occasional adjustment.
Analytical Procedure The absorption vessel, D (Figure l), is evacuated by means of
a water pump, and filled with air drawn through a soda-lime
tower. It is then charged with 5 ml. of 0.1 N barium hydroxide accurately measured by an automatic pipet (5),and re-evacuated. The ground-glass joints are well lubricated with glycerol to prevent sticking. When charging the .vessel with barium hydroxide care must be taken not to leave it open longer than necessary. One milliliter of the solution of potassium iodide-barium chloride is placed in U-tube B to remove iodine vapors. To ensure the removal of any hydriodic acid which may be formed, 1 ml. of 0.1 N potassium permanganate is placed in U-tube E. Two t o 5 mg. of organic matter and 50 to 70 m of potassium iodate are weighed in a glass boat ;and introduce3 into vessel A which is then connected (as shown in Figure 1) to the gas reservoir, C . Sufficient carbon-free sulfuric acid t o cover the boat is introduced by means of stopcock a, which is then closed. At this time the charge in the boat, resting in the bulb of the reaction vessel, is agitated in order t o dissolve the charge as much as possible before heating. The reaction vessel, even though connected to the system, has sufficient freedom of motion to permit considerable agitation. The bulb of the reaction vessel is now immersed directly in the heated bath. The temgerature of the reaction chamber is kept at approximately 210 =t5" C. for the completion of the reaction by means of a phosphoric acid bath. This u$suallyrequires from 40 t o 60 minutes. The gases which are generated during the reaction expand into gasholder C. After 20 minutes the Ascarite tower is connected to the apparatus through stopcock a and by operating stopcocks a, 6, and c this gas is flushed into the absorption flask with approximately 50 ml. of carbon dioxide-free air. After 40 minutes, if the reaction is still incomplete (as evidenced by the continued evolution of small bubbles of gas) this flushing is repeated. At the end of the heating operation stopcock a is again opened and 1 to 2 ml. of distilled water are introduced in order t o decrease the free air space. The gas which has accumulated in C is transferred t o D. Sufficient carbon dioxide-free air is then flushed through the system to sweep all the carbon dioxide into the evacuated flask, D, bringing it to atmospheric pressure. The reaction vessel, A , is removed and its contents are transferred to a 500-ml. Erlenmeyer flask. While the reaction vessel is being dried in an oven, U-tube B is disconnected, cleaned, and recharged for the next run. It is important that the iodine be removed after each run, as the accumulation of too much iodine crystallizing in the male joint of A interferes with the recovery of carbon dioxide.
.
\
( A - B , ( N Of HC1) x mg. of sample
x
loo=
% carbon in sample
The per cent of hydrogen is calculated by means of Williams' equation (5)
grams of sample
INDUSTRIAL AND ENGINEERING CHEMISTRY
446 TABLE I. Samde
C O h f P O C X D s OXIDIZED IN O N E
Carbon Found Theoretical
% 68.6 69.1 63.5 Vanillin 63.4 42.2 Sucrose 42.1 42.3 65.3 Hydroquinone 65.3 40.7 Succinic acid 40.6 40.5 49.4 Adipic acid 49.4 49.3 59.5 Sebacic acid 59.5 59.6 62.7 Anisic acid 63.3 63.2 85.9 Benaophenone 85.7 33.9 o-Icdobenzoie ai:id 34 1 Cinnamic acid 72.5 72 6 38.0 Methanol" 38.0 38.0 61.8 Acetone 62.1 61.8 64.8 Ether 64.7 64.3 Acetophenone 80.3 80.2 0 Contained 0.5 per eent of ethanol.
Benzoic acid
7c
68 8 63.2 42.1 65.4 40.7 49.4 59.3 63.1 85.8
33 9 72.9 37.6 62.1 64.9
80.0
HOUROR LESS Hydrogen Found Theoretical
%
%
4.82 4.81 5.10 5.26 6.45 6.43 6.38 5.49 5.29 5.05 5.13 5.14 6.40 6.78 6.78 8.88
4.95
...
8.87 5.34 5.27 5.36 5.42 5.48
...
5.69 5.63 12.5 12.5 12.3 10.63 10.5 10.2 13.6 13.8 13.8 6.67 6.56
5.30 6.48 5.30 5.12 6.90 8.93 5.30 5.53
5.44 12.5 10.35 13.1 6.77
Results The results using the micromethod are tabulated in Tables I and 11. In Table I are listed the compounds which were completely oxidized in one hour or less. In Table I1 are listed other compounds which were investigated. Some of these substances were completely oxidized but only after heating for a considerable length of time.
Discussion The only advantage of the micromethod is its ability to handle very small samples. The modifications which were made in this method-namely, the method of determining the carbon dioxide and unreduced iodate-are just as adaptable to the semimicroprocedure. Although numerous compounds are readily oxidized under these conditions it is evident from Tables I and I1 that the method does not have general applicability to all types of compounds. On the basis of these studies it is the opinion of the authors that solubility and volatility are the dominant factors in a wet combustion. The effect of volatility is closely linked with the question of solubility. It is the soluble nonvolatile substances such as carbohydrate materials and aliphatic acids which are best adapted to wet-combustion procedures. However, highly volatile but soluble compounds such as ether, acetone, and methanol have been successfully treated by this procedure. This can probably be attributed to the formation of nonvolatile molecular compounds with the sulfuric acid. Insoluble nonvolatile substances do not present much of a problem other than the question of time. Strebinger (3) reports the complete combustion with iodic acid of such insoluble materials as mineral oil. The ease with which these compounds are consumed depends for the most part on their resistance to oxidation. This may vary considerably between different compounds; o-iodobenzoic acid, for instance, is readily oxidized, but triphenylbenzene only after a considerable lapse of time.
Vol. 13, No. 6
Iodic acid in a more dilute sulfuric acid medium (40 per cent) has been shown to be specific in its behavior. Williams (7) has made a study of its specificity as an aid towards the determination of structure. Even in concentrated sulfuric acid medium this specificity still plays an important role. The compounds which present the greatest difficulties are those which are relatively insoluble and tend to sublime easily. These materials are usually found among the aromatic compounds, particularly the halogen derivatives. Even such substances as benzoic and cinnamic acids may give considerable trouble if care is not taken to ensure their solution (as much as possible) before heating begins. In other cases, where the volatile compound is not so soluble, resistance to oxidation becomes an important factor and it then becomes a question of quantitative oxidation before some of the material escapes from the reaction mixture. This behavior is clearly demonstrated in some experiments with dibromodimethoxybenzene. In certain runs this compound was quantitatively oxidized. However, in some of the experiments the material would frequently sublime (in considerable amounts) out of the reaction mixture. Both experiments were carried out under like conditions. From these studies it appears that any organic substance can be completely Oxidizing, provided the material can be retained in the oxidizing medium for a sufficient length of time. The problem of successful analysis depends upon this point. It is the opinion of the authors that wet combustion by iodic acid is best adapted to materials known to be readily oxidized by this reagent. For routine work with such materials or other compounds where combustibility by iodic acid can be previously tested (6), it offers a good tool for determination of carbon and hydrogen.
TABLE 11. COMPOUNDS OXIDIZEDAFTER ONE HOUROR MORE Sample
Time Hours
Phthalic acid Butylphthalate Heptylphthalate Methylcinnamate Triphenylcarbinol Triphenylbenaene Dibromodimethoxybenzene Anthraquinone p-Phenyl phenacyl bromide Anthracene Biphenyl Naphthalene
Carbon TheoFound retical
%
57 8 68 9 69 2 73 0 74 2 86 0 87 8 87 7 93 3 32 4 32 2 29.1 3 80.0 3 59.1 1 58.7 2 (2350) 9 4 . 4 2 (2350) 9 4 . 3 1 89.7 2 (185') 6 2 . 8
$
%
%
%
57.8 69.1
3.64 7.98 7.95 9.41 6.05
3.62 7.92
7i:9 74 1 87.8
..
94: 1 32.4
6:04 6.27
..
9:40 6.18 6.16
..
..
..
.. ..
80:7 61.2
3:i4
3:85
.. ..
.... .. .. ..
94:3
..
..
93:s
....
9318
..
11.4
(180') 7 5 . 0 1 (2100) 8 1 . 4
Hydro en $h,eoFound retical
..
.. ..
Literature Cited (1) Christensen, B. E., and Facer, J. F., J . Am. Chem. SOC.,61, 3001
(1939).
Christensen, B. E., Wong, R., and Facer, J. F., IND. ENG.CHEM., Anal. Ed., 12, 364 (1940). (3) Strebinger, Z. anal. Chem., 58, 97 (1919). (4) West, E. S., Christensen, B. E., and Rinehart, R., J. Biol. Chem., (2)
132, 681 (1940). (5) Williams, R. J., J . Am. Chem. Soc., 59, 288 (1937). (6) . . Williams, R. J.. Rohrman, E., and Christensen, B. E., Ibid., 59,
291 (1937). (7) Williams, R. J., and Wood, A., Zbid., 59, 1408 (1937).
PUBLISHED with t h e approval of t h e Monographs Publication Committee, Oregon State College, a s Research Paper No. 45, School of Science, Department of Chemistry.
