1322
ANALYTICAL CHEMISTRY
microns, in contrast with the suggestion of a band in the Nembutal spectrum. Subgroup A2. Each of the .42 members is relatively distinctive. Phanadorn shows only three strong bands in Section 111. I n Evipal, the 3.2- to 3.25-micron band is missing and, unlike the A1 compounds, the 3.3-micron absorption band is strong. Seconal and Sandoptal are characterized by two strong bands at 10.05 and 10.7 microns. Seconal has strong absorption a t 7.7-7.8 microns, while Sandoptal shows only weak absorption in this range. Unlike Sandoptal, Seconal lacks the band a t 7.62-7.65 microns. Seconal bas a greater absorbing ability than any of the other barbiturates, with 15 mg. per ml. showing greater extinction throughout the spectrum than 25 mg. per ml. of the others. Sandoptal differs from the other members of the A group in having two absorption bands between 6.9 and 7.1 microns. Subgroup B1. The characteristic of the B1 compounds is the presence of the three major bands between 7 and 8 microns 111 Section 111. Cyclopal has a weak band at 10.0-10.2 and a strong band a t 10.7-10.85 microns. Luminal s h o w fewer absorption bands than do the other barbiturates. The 8.0- to 8.15-micron band is not present and there is weak absorption at 6.6-6.7 microns, while the others in this subgroup show none at this n-ave length. In Delvinal, the third band in Section I11 is shifted from 7.62-7.65 t o 7.7 microns. Its melting point is widely separated from those of Cyclopal and Luminal. Subgroup B2. Dial and Alurate have ah ell defined bands a t 10.0-10.2 and a t 10.7-10.8 microns. Dial and Mebaral show absorption at 6.94.95 microns. Alurate, unlike the other members of this subgroup, has a weak band a t 7.25-7.33 microns. Mebaral s h o w striking differences from the other members of the B2 group; the most significant are the strong absorption
bands a t 9.55 and 8.3-8 4 microne. A characteristic difference between Barbital and the other memberq of the group is the very weak absorption a t 7.35-7.44 microns, which is well defined in the others. The weak 7.45- to 7.5-micron band present in the Barhital spectrum is missing in Dial, whereas Barbital does not show the illurate bands at 7.7-7.8 and 7.82-7.93 microns or the Mebaral bands a t 7.82-7.93,8.3-8.4, and 9.55 microns Subgroup C1. Pentothal s h o w the general barbiturate absorption pattern, but differs in showing very strong absorption at 6.6-6.7, 8.55-8.7, and 8.8-9.05 microns. Subgroup C2. This group is distinguished from the barbiturates by the presence of the strong band a t 2.83 microns and a band a t 6.38-6.4 microns. Sedorniid differs from Bromural by its strong absorption at 8.3-8.4 and 10.85 microns. Carbromal has two distinct bands a t 11.8 and 12.0 microns and. unlike any of the other compounds. theie i q no absorption band a t 3.1-3.18 microns L I T E R i T C R E CITED
Gettler, A. O., Cmberger, C. J., and Goldbaum, L., ANLL. C H E M . , 22, 600 (1950). (2) Goldbaum, L., unpublished paper presented at annual meeting of American Academy of Forensic Sciences, 1951. (3) Koppanyi, T., Murphy, W. S.,and Krop, S.,Proc. SOC.Exptl. BwZ. Med., 31, 373 (1933). (4)Cmberger, C. J., and Feldstein, M., unpublished paper presented at annual meeting of American Academy of Forensic Sciences, 1949. ( 5 ) Umberger, C. J., and Schwartz, H., Proc. Am. A d . Forensic Sci., 1 , 250 (1951). (6) Cmberger, C. J., and Stolman, A., Ann. Western Med. Surg., 5 , 945 (1951). (1)
RECEIT E D for review December 17, 1981.
Accepted M a y 22, 1952
Solubility and Freezing Point Depression of Nitrous Oxide in Liquid Nitrogen Dioxide ARTHUR W. ROCKER Eastern Laboratory, Explosices Department, E . 1. du Pont de Nemours & Co., l n c . , Gibbstown, N . J .
A simple, rapid method for the determination of the solubility and freezing point depression of gases in liquid nitrogen dioxide was required for w-ork with liquid rocket oxidizers. A suitable apparatus was assembled from standard laboratory equipment. The solubility by weight of nitrous oxide in liquid nitrogen dioxide was found to vary inversely with temperature from 1.470 at 263" K. to 0 . 6 q ~at 283' K. The molal freezing point depression constant was 4000 and compound formation did not take place. The method proved useful for determining gas solubilities in highly corrosive liquids.
T
H E conventional gas-solubility apparatus of Bunsen and that of Ostwald are not suitable for work with compounds which react readily with mercury. Wright and Maass (3) modified the Ostwald-type apparatus for work with hydrogen sulfide by substituting a manometer with a glass diaphragm. Excellent results are claimed for this type of apparatus. I n the present investigation, a modification was desired which would require only equipment generally available in the laboratory. Emphasis was placed on speed, simplicity, and sufficient accuracy for exploratory work on gas solubility and on the determination of compound formation. A modified Ostwald gassolubility apparatus was designed, and the solubility of nitrous oxide (NzO) in liquid nitrogen dioxide (NO2) was determined a t several temperatures over the range 263" to 283" K. This range was chosen because the freezing point of nitrogen dioxide set a
lower limit, and exploratory work showed that the solubility was negligible a t higher temperatures. The freezing point depression was determined a t 0.51, 0.70, 0.81, 1.21, and 1.52% nitrous oxide concentration, and the molal freezing point depression constant was calculated. The standard deviation for the results of the solubility determination is 0.07. The deviation increases rapidly as the nitrogen dioxide freezing point is approached. This deviation is due to the increased difficulty in maintaining the desired temperature during the saturation of the nitrogen dioxide. A constanttemperature bath would increase the precision but would overly complicate the design. The standard deviation of the freezing point depression constant is 4%. A deviation of this magnitude ensures sufficient accuracy to justify the conclusion that no compound is formed.
V O L U M E 24, NO. 8, A U G U S T 1 9 5 2
1323
EXPERIMENTAL
Materials. The commercial nitrous oxide used had a, purity of 99.2% or better. The commercial nitrogen dioxide was distilled through a 36-inch column for immediate use. The purity .ranged from 97.8 to 99.0%. As there was no apparent effect of nitrogen dioxide purity on nitrous oxide solubility within the range of error of the determinations, distilled commercial nitrogen dioxide was considered acceptable. Apparatus. The modified Ostwald apparatus shoTvn in Figure 1 was used. The thermometers, total-immersion type, graduated in 0.1" C., had previously been calibrated against a certified National Bureau of St,andards thermometer. All temperatures were corrected for emergent mercury column. The gas buret was a Fisher Precision Model, 100 ml., graduated in 0.1 ml. The gas volume Tvas corrected to standard conditions. The absorption tube was fabricated from a standard spherical glass joint and graduated to 50 ml. in 1-ml. intervals. The freezing point attachment, D F , is illustrated in Figure 1. All connections %-ere of plastic t,ubing, and the ground-glass joints were lubricated with a fluorogrease to minimize the contamination which nlight result from reaction between the nitrogen dioxide and the lubricant.
fl I
n
E
maintained on the nitrous oxide by the mercury leveling bulb to prevent nitrogen dioxide from backing up into the buret. The entire apparatus was shaken to effect solution; the rate of solution was accelerated by rapid vibration. The temperature of the solution was lowered to about half a degree below the desired point. and the solution was saturated with nitrous oxide. Care --as taken to keep the pressure, indicated on manometer E, as low as possible. The temperature of the saturated solution m-as next raised carefully to the desired value. The volume of gas given off was indicated by the increased manometer pressure. This pressure was recorded, together with the volume of solution in the absorption tube. From the previously determined volume of D A , the volume of the gas over the solution was obtained by difference and corrected for temperature and pressure. The tube as removed from the apparatus and a second top \!as added as illustrated in DF. The freezing oint was determined in the usual manner. The solution was %en weighed Calculations. The vapoi pressure of nitrogen dioxide a t various temperatures was taken from the work of Giauque and Kemp ( 1 ) . From these values and from the measured total pressure of the gas over the solution, the partial pressure of the nitrous oxide was calculated The nitrous oxide fraction and volume R ere calculated, and the volume of nitrous oxide in solution was obtained by difference The weight of nitrous oxide was calculated, assuming 22.4 liters as the molar volume. The per rent nitrous oxide in the solution n-as given by
N,O = calculated weight S20x 100 weight of solution
I
The molal freezing point depression constant, k , n-as calculated from the formula
F
The gas constant, R, nac taken as 1.9865, and the absolute freezing point of nitrogen dioxide was taken as 261.90" K. Ala is the heat of fusion in calories per gram, tn is the molecular m-eigh of the solvent, W is the actual weight of the solvent (grams) w is the n-eight of the solute (grams), and At is the measured Ion-ering of the freezing point (' K,). The results of t h e solubility determinations are given in Table I and Figure 2. In preliminary work, the nitrogen dioxide x a s saturated 11 ith nitrous oxide bi- gradually lowering the temperature to the desired point. Thew rt-ults compared favorably w t h
-G
D
'
9
'
Figure 1. Schematic Diagram of Apparatus Procedure. The nitrous oxide from cylinder A mas passed into jacketed buret B , where it was measured a t atmospheric pressure by balancing manometer C against the compensating tube, G . The temperature was recorded. After the freezing point of the nitrogen dioxide had been determined in the absorption tube with thermometer attachment as shown in DF, the absorption tube was connected to the absorption apparatus as shown in D A . The tube was graduated in 1-ml. intervals to 50 ml. The nitrogen dioxide was heated to its boiling point and allowed to boil to the atmosphere for several minutes through stopcock H to remove the air from the absorption tube. Stopcock H was then closed and the nitrogen dioxide was cooled slowly to the desired temperature by means of the bath, F. This bath contained a 1 to 1 mixture of carbon tetrachloride and chloroform, cooled by the addition of dry ice. The mercury leveling bulb was raised to force small amounts 01 nitrous oxide from B into the absorption tube, D A . After each addition, s t o cock I was turned to connect tube D A with manometer E. T i e progress of absorption was indicated by a tiecrease in the pressure. -4 small positive pressure was always
SOLVENT
TEMPERATURE
OK
Figure 2. Solubility of Nitrous Oxide i n Liquia Nitrogen Dioxide
1324
A N A L Y T I C A L CHEMISTRY
Table I.
Solubility of Nitrous Oxide in Nitrogen Dioxide
Solution Temp., K . 263.0 263.1 263.1 268.0 268.0 273.2 273.0 278.2 278.4 283.0 283.1 283.1
Partial Pressure of K20t hlrn. 620 630 630 570 570 510 510 440 430 370 350 340
Purity of
so*.
6:
98 3 99 0 97 8 98 0 97.8 98 4 97 8 98 0 97 8 97.8 97.9 98.0
% Solubility by Weight 1.28 1.33 1.52 1.21 1.27 1.11 1,04 0.81 0.82 0.65 0.59 0.63
Table 11. Freezing-Point Depressions % NzO 0.51 0.70 0.81 1.21 1.52
k
Af Observed 0.45 0.66
0.74 1.12 1.31 Calculated from Heat of Fusion
3900 4200 4000 4100 3800
k Max. 0.50 0.80 1.20
0.48 0.77 1.15
0.50 0.80 1.20
0.42 0.67 1 .oo
4230 4230 4230 k iMin. 3660 3660 3660
results obtained by saturation a t a slightly lower temperature and subsequent heating to the desired temperature. If any supersaturation occurred, the amount of nitrous oxide dissolved in escess of that required for saturation was less than the experimental error. Consequently, the more rapid method as desciibed was adopted. Table I1 gives the freezing point depression data. From the published values for the heat of fusion of nitrogen dioxide (Z), which ranged from 32.2 to 37.2 cal., the spread of values for the molal freezing point depression constant v a s calculated (Table 11). The resulting curves of freezing point depression are plotted in Figure 3 for compaiison with the curve obtained in this investigation. As the experimental curve falls betmeen the curves calculated from the heat of fusion, it may be assumed that
O M
Figure 3.
ow AI
-
om
I W
I20
1.0
I(*
FREEZING POINT DEPRESSION OK
Freezing Point Depression for Nitrous Oxide in Nitrogen Dioxide
Broken lines show freezing point depression charges calculated from highest and lowest heat of fusion values reported in literature
no compound formation takes place. The average value for the molal freezing point depression constant is 4000. LITERATURE CITED (1)
Oiauque, W. F., and Kemp, J. D., J . Chem. P h y s . , 6, 40-52
(2)
Mellor, J. K., “Comprehensive Treatise on Inorganic and Theoretical Chemistry,” London, Longmans, Green & Co.,
(3)
Wright. R. H., and Rlsass, O., Can. J . Research, 6,
(1938).
1928. 94-102
(1 932).
RECEIVED for review February 9. 1952 Accepted June 9 , 1952. Presented at a Symposium on the Practical Factors Affecting the Application of Kitric Acid and Mixed Oxides of Kitrogen as Liquid Rocket Oxidizers, at the Pentagon, Washington. D. C., October 10, 1951.
Colorimetric Method for Estimation of Digitoxin E. L. PRATT Kinthrop-Steurns Inc., Rensselaer, iV. Y.
F
OR many years pharmacologists used the biologic assay as a means of determining the potency of digitalis preparations. MIth the extended use of crystalline digitoxin, demands have arisen for a chemical method to supersede the older assays. One of the careful studies of the subject has been macle by Canback ( 8 ) ,who used the Raymond ( 1 2 ) reaction involving ail alcohol solution of rn-dinitrobenzene and sodium hydroxide. A variation of this method u a s described by Anderson and Cheii (1). h similar method wing sodium dinitrobenzoate %as suggested by Kedde ( 7 , Y). Other methods were proposed by \\ urren, Hornland, and Green (IO),McChesney and colleagues (IO),and finally by Bell and Krantz ( S X ) , who modified the Baljet (9)reaction. This last assay has been included in the XIVth revision of the E. S. Pharmacopeia (15). Various laboratories which have used the C.S.P. XIV (15) assay for digitoxin have found difficulty with the reproducibility (13) and nonspecificitj- ( 1 7 ) of the method, the instability ( 1 7 ) of the reagent, and
the proximity of the maxima of the picrate reagent and the picrate-digitoxin complex ( 7 ) . A method that includes action a t room temperature, simplicity, reproducibility by different operators, and specificity for digitoxin in the presence of similar compounds, especially gitoxin, might well be considered as approaching the ideal. The method described herein satisfies the first three of the above desiderata and produces approximately 50% as much color for gitoxin as i 3 produced by an equimolar amount of digitoxin. About 85% as much color is given by digoxin, but the latter is not usually found with digitoxin. This method, chosen after the experiments described below, is based on the combination of 3,5-dinitrobenzoic acid and benzyltrimethylammonium hydroxide, reagents which were recently proposed by Tansey and Cross ( 1 4 ) for the assajof certain ketosteroids. With digitoxin the resultant color iz a bluish-red u-ith a maximum at 5?0 mp. Canback (8) has discussed a reaction mechanism for the
