686
ANALYTICAL CHEMISTRY
portant that the floor of the trap should slope to ensure this. Careful blanks showed no carry-over of sodium hydroxide and no loss of ammonia during distillation. Flask C is used also for the digestion, so that no transfer is necessary before distillation. The ground-glass joint a t its neck is lubricated before each distillation with a drop of strong alkali to eliminate the possibility of ammonia's being trapped in traces of sulfuric acid remaining in it. The volume of liquid, including alkali, in the flask before distillation should not exceed about 3 ml.
for rinsing the condenser. The accuracy of the titration is thereby increased several fold as end-point sharpness is inversely related to volume of distillate. The titration is carried out with 0.003 N sodium hydroxide, and the useful range is 5 to 250 p of nitrogen. W O R Ksupported b y a grant from the Medical Research Committee of the University of California. Apparatus is available from Microchemical Specialties Co.. 1834 University Ave., Berkeley 3, Calif.
Device for Preparing Gas Mixtures for Calibration. .4scher Opler and E. S. Smith (present address, Cnion Oil Co.. Brea, Calif.), Research Department, Western Division, The Dow Chemical Co., Pittsb'urg, Calif. v equipment for use in the analysis of gasnnxtures, Iit is usually possible to work with each component separately. However, the analysis CALIBRATING
of most gases by infrared spectroscopy necessitates the use of calibrating mixtures, because pressurebroadening effects cause undesirable interactions. Coggeshall and Saier [ J . Chem. Phus., 15, 65-71 (1917)j discuss this phcnomenon and describe the procedure using known mixtures. Such mixtures are also useful in checking gas-analysis apparatus of many other types. After many trials, the apparatus shou n a as evolved and has proved accurate and reliable in more than 2 years' servire.
Figure 1.
Apparatus
One would expect the rate of distillation of ammonia to be proportional to the amount remaining when this is small, n-hich gives the relation y = 1 - e-kt
The hollow glass pistons 46 mm. in outside diameter are flee to slide in the gas bulbs 48 mm. inside diameter. The apparatus is fastened to a 6 x 30 inch board for protection against breakage. Procedure for Preparing Binary Gas Mixture. Evacuate bulbs A and B and line C a t H with stopcock D open and E and F in positions 1 and 2, respectively (as in the diagram). Close all three stopcocks. With stopcock E in position 1, fill bull) d to the desiied pressure. Close E. Repeat with bulb B and stopcock F
where y is the proportion distilled in time t, and k is a constant for the conditions used. When the logarithm of the proportion remaining undistilled is plotted against time, a straight line of slope - k should result, which makes i t possible to extrapolate the proportion of ammonia undistilled after any interval.
C
Open stopcock D, turn E and F to position 2, and rock the apparatus repeatedly, allowing the sliding pistons to move to each end of the equipment in turn. Rocking may be performed by hand or by means of a motor driven device. Calculate the composition of t,he mixturr as follows:
7
$
8
s2 Ba
Vol. %, '4
=
A - pressure iii ;I.x vol. inpressure pressure in A x v o ~in B X vol. in B A~
:l:
c=
8 8 8 4-
J Y
20
40
60 80 100 T I M E IN SECON@S
120
Figure 2. Rate of Distillation of Ammonia in Large (Upper Line) and Modified (Lower Line) Apparatus Time measured from passage of steam into condenser
Figure 2 shows the results of a standard micro-Kjeldahl unit and the modified unit plotted in this way. With the standard apparatus, 99.9% of the ammonia was distilled in 2 minutes 20 seconds, while in the modification the same proportion came over in 40 seconds. The rate of distillation of steam was 7 and 3.5 ml. per minute, respectively, so that to reduce the remaining nitrogen to a negligible amount, 8 to 10 times as much water has to be distilled in the standard apparatus. The technique used in the present apparatus is to distill for 1minute and allow 20 seconds
+
&
Where high accuracy is required, correction for gas imperfection should be made. T o prepare mixtures containing three or more components, follow the above procedure, close D and E , evacuat,e bulb B and line C, t,hen add the third gas to bulb B , and proceed as above, using t,he original mixture as one of the components. Suitable correction for the volume of stopcock D should be made. This process may be continued for more than three componente. To demonstrate the mixing action of this device, a series of 1 to 1 propylene-air mixtures was prepared with varying numbers of complete strokes. The contents of each bulb was measured a t 100-mm. pressure in the 2i-cm. cell of a Beckman IR-2 infrared
spectrometer with potassium bromide optics. The absorbance a t 17.35 microns (propylene absorption peak) after varying cycles of rocking is shown in the following table: Absorbance Strokes n
5 15 30 40 50
GO
70
Bulb A n - . s1!2 -.. 0.711 0,508
0.449 0.431 0.420 0.416 0.413
Bulb B 0.000
0.101 0,363 0.389 0.401 0.409 0.413 0.415
