The procédure found most satisfactory for rou
tine work and used, with slight modifications, in the experiments described in this paper, is as follows : Weigh 50 grams of sample into a 120-ml. (4-ounce) tared bottle (small neck, plastic top) and place this and the bottle of nitrating acid in a constant-tempera- ture bath at 68° F . Pipet 50 ml. of nitrating acid in the vacuum flask and
F i g u r e 1. E q u ip m e n t f o r R e a c t io n - H e a t T e s t place the flask on the 563
Fi g u r e 2. As s e m b l y o f Re a c t io n- He a t Ap p a r a t u s A. Motor stirrer, 1600 r. p. m.
B. Thermometer, Taylor *No.
21,520, length 18 inches, range 45° to 100° F.
0. Vacuum flask, 2 X 10 inch, silvered
D. Glass bearing E. Glass stir shaft
F. Level of mixture while stirring G. Level of acid while stirring //. Glass propeller, 1 X 0.25 inch,
approximately 40° pitoh 1, Felt-lined metal container
readings a minute or two apart. In addition, an error was introduced when the room temperature changed between any two readings, because both the stirrer and the ther
mometer, when removed, changed to room temperature and were then replaced in the vacuum flask. Then, too, each time one or the other was removed some of the solvent was lost.
It was discovered that the stirrer paddle, which consisted of a 7-mm. glass rod with 19 mm. of one end flattened to a width of 13 mm. and turning at 3000 r. p. m., svas not a very efficient stirring device for a two-
phase mixture, such as cyclohexane and nitrating acid. This poor mixing tended to lengthen the time of complete reactionbetween the benzene and the ni- 3 trating acid. Therefore, with a 10- to 20-minute period in which to meas
ure the temperature rise, any change in the room temperature and subsequent change in the rate of radia
tion of heat from the vacuum flask introduced a considerable error.
To overcome these diffi
culties, it was necessary to construct the apparatus shown in Figures 1 and 2, and to adjust the amount of nitrating acid used, to en
sure thorough mixing. The results which follow point out the distinct advantages of this redesigned reaction heat apparatus.
R e a g e n t s . Nitrating Acid. Mix 2 volumes of 60 to 70 per cent nitric acid (density 1.41) with 1 volume of 95.5 per cent sul
furic acid (density 1.82).
Cyclohexane, density at 20° C. 0.7781, freezing point 6.0° C.
Benzene, density at 20° C.
0.8790, freezingpointo.5°C.
564 I N D U S T R I A L Ä N D E N G I N E E R I N G C H E M I S T R Y Vol. 15, No. 9
1.5 2 0 2.5 3 0 3-5
P E R C E N T B E N Z E N E B Y W E I G H T
Fi g u r e 3. Re a c t i o n He a t De r i v e d p r o m Kn o w n Mi x t u r e s o f Be n z e n e AND C Y C L O H E X A N E
a t. Difference between initial temperature and temperature after reaction mixture
Nitrating acid, same as No. 2, Table IX
Ta b l e I. Re a c t i o n- He a t Te s t Da t a ( Fi g u r e 3) Weight
Percentage of Initial Room
Benzene Temperature AT Tempcrati
0 F. ° F. °
F
0.0 69.90 - 0 .6 0 74
0.5 71.01 2.20 77
1.5 71.25 7.80 76
2.0 70.32 10.80 76
2.5 70.72 13.50 76
3.0 70.75 16.40 76
4.0 71.22 22.20 76
5.0 71.10 27.00 76
lessened the precision of the results. On the other hand, any change which increased the mixing effect beyond that of the ap
paratus described in this paper, made almost no change in the results, except to decrease slightly the time of complete reaction. In fact, results were very satisfactory when 100 ml. of nitrating acid and 50 grams of sample were used in a larger vacuum flask (6.25 cm., 2.5 inches in inside diameter) than the one described in this paper.
E f f e c t o f S t r e n g t h o f N i t r a t i n g A c i d .
The effect of acid concentration was tested with acids of the composition shown in Table II. Figure 5 shows that N o . 2 acid mixture is very satisfactory. It also has the advantage of being the one most easily pre
pared from ordinary laboratory c . p. acids and has been chosen therefore for use in the general procedure. A slight variation in the acid strength will not cause any ap
preciable error in the results. Figure 6 illus
trates the variation in AT with the time of reaction for a 2 per cent benzene in cyclo- hexane mixture when nitrating acids of different strengths are used. A similar variation was found to hold true in 0.65, 3.0, and 4.0 per cent benzene-cyclohexane mixtures.
E f f e c t o f C e r t a i n I m p u r i t i e s i n t h e S a m p le . It was found necessary to determine benzene in cyclohexane which contained a small amount of acetone and methyl alcohol.
The reaction between these impurities and the nitrating acid caused a considerable rise in temperature. This difficulty was overcome by extracting the sample with water, and drying with calcium chloride. Any appreciable amount of water in the sample would no doubt cause a rise in tempera
ture. However, a mixture of cyclohexane and benzene contains only 0.01 per cent of moisture when saturated at room temperature.
3 minutes of stirring
stirring device as shown in Figure 2. After 3 minutes of standing, start the motor, and after approximately 1 minute record the “initial”
temperature which will be about 70° F. Stop the motor. Pour the sample from the 4- ounee bottle into the vacuum flask and start the motor. Take readings after 1, 2, 3, and 5 minutes of stirring. The difference between the temperature at 3 minutes and the initial temperature is a measure of the benzene con
tent of the sample which is determined from Figure 3.
D is c u s s io n
E f f e c t o f S t i r r i n g P e r i o d . The ni
tration is complete after 2 minutes of stirring, as shown in Figure 4. Any slope to the curve after the 2-minute period is a result of radiation from the vacuum flask, since the initial temperature of 70° F. is below the room temperature, which is 74° to 76° F.
E f f e c t o f I m p r o p e r M i x i n g . The most important point in this discussion is the efficiency in the mixing of the reacting sub
stances. It was found that any change in the design of the apparatus, the quantity of reacting substances, or the speed of stirring, which tended to lessen the mixing effect, greatly increased the reaction time and
Fi g u r e 4 .
12 15
IN M IN U T E S
Va r i a t i o n i n AT w i t h Ti m e o f Re a c t i o n f o r Va r i o u s Mi x t u r e s o f Be n z e n e i n Cy c l o h e x a n e
Initial temperature, approximately 70° F.
Nitrating acid, same as No. 2, Table I I Stirring motor was stopped after first 10-minute period
September 15, 1943 565
Fi g u r e 5. Re a c t i o n- He a t Da t at o r Kn o w n Mi x t u r e s o f Be n z e n e i n Cy c l o h e x a n e Us i n g Ac i d s f r o m
Ta b l e I I
AT. Difference between initial temperature and temperature after 3 minutes of stirring reaction mixture
R e s u lts
The accuracy of the results obtained by the described method is indicated in Table II I. These results were ob
tained on knowns made from mixtures of cyclohexane from
Ta b l e II. Co m p o s i t i o n o f Ni t r a t i n g Ac i d s ( Fi g u r e s 5 a n d 6)
Acid No. HNOa H N O i HtSO« HîO
% % % %
1 42.94 0.16 38.35 18.55
o 41.32 0.01 38.06 20.61
3 40.58 0.01 37.28 22.13
4 39.77 0.01 36.54 23.68
Ta b l e III . An a l y s i s o f Kn o w n s AT, 3-Minute minus
Initial Tem Benzene Benzene
perature Present Found Difference
° F. IF ¿Jlrl U{/C*
7.82 1.50 1.50
0.00
7.75 1.50 1.47 -0 .03
10.63 2.00 2.00
0.00
10.87 2.00 2.04 +0.04
10.48 2.00 1.97 -0 .03
10.40 2.00 1.95 -0 .0 5
10.82 2.00 2.02 +0.02
10.98 2.00 2.05 +0.05
10.79 2.00 2.02 +0.02
16.30 3.00 3.00
0.00
16.35 3.00 3.00
0.00
21.78 4.00 3.95 -0 .0 5
22.08 4.00 4.02 +0.02
Fi g u r e 6 . Va r i a t i o ni n AT w i t h Ti m e o f Re a c t i o n f o r Fo u r St r e n g t h s o f Ni t r a t i n g Ac i d
various sources, and benzene. Different batches of nitrating acid were used in the course of the experimentation. In each case the acids were made up to approximately the same strength in accordance with the method outlined under reagents.
S u m m a r y
The method described in this paper was found to be very workable and rapid, giving a high degree of precision and accuracy. A similar method has been developed to determine benzene in ethylene dichloride and in propylene dichloride, following the same general procedure and using the same strength of acid. However, it was necessary to draw a separate curve of AT against per cent benzene for each sol
vent, as the heat of the nitration reaction in the presence of different types of solvents was not the same.
A c k n o w le d g m e n t
The authors are indebted to F. J. Hopkinson of the In
dustrial Laboratory for many helpful suggestions and the equipment used in this work, and to S. C. Pool and the staff of the Chemical Plant Laboratory of the Eastman Kodak Company for the analyses of the nitrating acids.
L ite r a tu r e C ite d
( 1 ) Corson, B. B., a p d Brady, L . J . , I n d . E n g . C h e m . , A n a l . E d . , 1 4 , 5 3 1 - 3 ( 1 9 4 2 ) .