Determination of Nitrous Oxide KELSO B. MORRIS AND ETHEL M. DAVIDSON Howard University, Washington, D . C.
1" T H E course of oxidation studiesunder an OfficeofNsvalResearch contract, it became necessary to determine nitrous oxide, a product of the reaction. The authors elected to use two wellknown methods-slow combustion and catalytic reduction-in obtaining data on nitrous oxide and nitrous oxide-nitrogen mixtures. The chemical equation that represents the reaction in both methods is
in which the total contraction is equal to the volume of nitrous oxide. Kobe and MacDonald (1) employed with success a commercial silica gel catalyst containing 0.125% of platinum and reported that nitrous oxide may be reduced o v ~ rthe catalyst by a limited excess of hydrogen a t 515' C. A special Burrell Build-Up gas analysis unit containing slowcombustion and catalytic oxidation (or reduotion) assemblies was used in the work. The latter assembly provides for heating of the catalyst tube by means of a Perme. Therm heater (Burrell Teohnicd Supply Co., Pittsburgh, Pa.). The manufacturer states that the heater is adjusted to operate at any point between 500' and ,525' C. and the maximum variation over a range of 105 to 125 volts is *2.5- C. The manufacturer, though not willing to re-
lease data a n the catalyst, asserts that its activity differs from that of Kobe and other previous catalysts. The volume of hydrogen used was from 2 to 2.5 times the volume of nitrous oxide in the sample. One double pass a t the rate of 4 to 5 ml. per minute was the procedure adopted for the slowcombustion pipet. For the catalyst tube, the method involved three double passes a t 25 ml. per minute. Data. show that the commercially available crttdyst tube and heater are convenient and satisfactory for the determination of nitrous oxide, and indicate practically the same order of accuracy for the two methods. One and three double passes, respectively, a t the flow rates indicated, are satisfactory for the slow-combustion technique and the catalyst tube. For the same volume of gas passed, the catalyst tube is faster. Thus, the minimum handling time for 50 ml. of gas would he 20 to 25 minutes in the slow-oombustion analysis a6 compared with 12 minutes with the catalyst tube. Traces of ammonia were noticed oocasionally where the slow-combustion pipet had been used, but never where use had been made of the catalyst tube. LITERATURE CITED
( 1 ) Kobe, K. A,, and MacDonald, R. A.. IND. ENG.CHEM..ANAL. RPOElY
ptobenzot
y ! ,,yf2a C
P
solution. The commercial samples usually contain considerahle ammoniu m chloride, which should be r,emoved by extraction using vt?ry slightly alkaline solutions.
~
/
~ - I Y I ~ ~ - l i ~ ~ ~ " D e nISe O very ~ n mmeui& , ~ ~ ~ , ~GO
crystaiiize ana no orystds except from the melt apparently ever show reproducible and definite interfacial angles. The best crystals obtained for this study were obtained hy slow evaporation of a chloroform or
aI
\
CRYSTAL MoRPHoLoGY ( d e termined by W. C. Mc100
Crone). Crystal System. Monoclinic. Fqrm- and Habit. Crystals obt,ained from chloroform were long flat rods showing basal inacoid [ 001 1 ; orthopinaooid 100); andprisms (210). The interfacial angles vary by as much as 10" from crystal t o crystal and even on the same crystal; many feces a m curved. A x i a l R a t i o . a:h:e = 2.655: 1: 1.335. Interfacial Angles (Polar). 210 A 210 = 77". Bets. .4ngle. 109'. Cleavage. 010, 100.
(y 1 : -
'4
\,
I
~:
7'\toe-
Figure% Orthographic Pmjeetion of Typical Crystal of Z-Mersaptobenzothiazole
Figure 1.. 2-Meroaptoben.othiazole Left. C w s t d s from chloroform and xylene Right. Fusion preperstion ahowing charaoteristio shrinkege
157
ANALYTICAL CHEMISTRY
?58 X-RAY DIFFRACTION DATA(determined by J. Whitney and I. Corvin). Cell Dimensions. a = 15.899; b = 5.989; c = 7.995. Formula Weights per Cell. 4. Formula Weight. 167.24. Density. 1.42 (buoyancy; x-ray). Principal Lines
d
I/Il
7.61 6.51 5.57 4.75 4.42 4.26 3.97 3.87 3.78 3.67 3.34 3.21 3.14 2.981 2,815 2.719
1.00 0.40 0.26 0.46 0.39 0.39 0.62 o.fi2
0.97 0.74 0.27 0 70 0.86 Very weak 0.48 Very weak
d 2,579 2,509 2.418 2.398 2.336 2.204 2,172 2.038 1.983 1.942 1.902 1,837 1.769 1.739 1.697 1.667
1/11 Very weak 0.19 0.28 0.26 Very weak 0.20 0.16 0.16 0.24
Very weak 0.26 0.18 0.31 Very weak Very neak Very weak
OPTICAL PROPERTIES (determined by 55’. C. %Crone). Refractive Indexes (5893A.: 25” C.). a: = 1.665 * 0.005; 1.667 (1). p = 1.96 += 0.01: 1.965 (1). y = 2.04 (calcd.): 2.06 (1). CY’ (projection on 001) = 1.688. (This refractive index
Polarograms by an “Undamped” Polarograph SIR: In a recent paper by Lingane (1) the following statement is made concerning the 25% sensitivity increase obtained by Schulman, Battey, and Jelatis m-ith “undamped” operation over that obtained by conventional damped operation of their new polarograph ( 2 ): “The alleged increase in sensitivity that results from measurement of the maximum rather than the average of the undamped recorder oscillations is more or less illusory, because the residual current in terms of maximum oscillation is also larger.” Lingane’s statement implles that the sensitivity is determined by the ratios
-or __ which do not change with change
I d i f fusion Iresidual
Iresidual’
in damping. Xeither these ratios nor the residual current are involved, however, in the expression for the sensitivity. This quantity is defined as K in the equation: I d , f f u s l o n = KC, where I d , f f L s i o n is the diffusion current and C is the concentration of electro-oxidizable or electroreducible substance. “Cndamped” operation increases both the residual current and the total current diffusion currentby 25%. Therefore their difference-the is likewise increased by %%, and sensitivity K is increased by this amount. The data of Schulman, Battey, and Jelatis unequivocally demonstrate the reality of this increase in sensitivity. It is granted that a sensitivity gain of 50 or 100%-had this been obtainable by changing from damped to undamped operation-would be of greater practical value than the observed 25% gain. I t appears to the writer that a sensitivity increase of this magnitude is nevertheless worth has6ng. Lingane has correctly stated that the principal advantage of the undamped instrument lies in the speed with which polarograms can be run m ithout distortion of half-wave potentials. LITERATURE CITED
(1)
Lingane, J. J.,AIEAL. CHEM.,21, 45 (1949)
is not correct, but is the Cargille liquid which, when s a k a t e d with mercaptobenzothiazole, causes the crystal to disappear.) Optic .$xial Angles (5893 A.; 25’ C.). 2 T’ = 52‘; 50’ (1). 2H = 70 ; 67” (1). Dispersion. Inclined T > 1’. (Sote. Conoscopic observation parallel to B X a shows both brushes in the field with K.A. = 1.25; one optic axis shows v > T , and the other, T > L,.) Sign of Double Refraction. Kegative. .icute Bisectri:. a:. Extinction. j la. Molecular Refraction ( R ) (5893 8.;25” C,), 4 37 = 1.881. R (calcd.) = 49.4. R (obsd.) = 53.9. FTXIOS DATA(determined by W.C. JIcCrone). 2-Nercaptobenzothiazole melts at 181O C. and solidifies spontaneously on cooling. There is a slight tendency for sublimation but usually only droplets or long needles (elongated parallel to b ) are obtained. The crystals g r o x rapidly from a small number of nuclei as spherites made up of large rods and needles groiving elongated parallel to b. Thcse cryst,alsshom all possible orientations normal to the b axis. Some views using oil immersion show both optic ares vithin the firld with 2H = 70”. The interference figure is unusual in that one optic axis shows strong dispersion, r > z’, and the other shows less dispersion, > r. The sign of double refraction is negative. LITERATURE CITED
(1)
RIitchell, ASAL. CHEM.,21, 448 (1949).
( 2 ) Schulman, J. H.. Battey, H. B., and Jelatis, D. C., Rev. S e i . Instruments, 18, 226 (1947).
JAMES H.
SCHL-LX1.4X
Naval Research Laboratory Washington, D. C.
SIR: Schulman is correct in his criticism of the reason which I stated for my opinion that the increased sensitivity Fyhich results from measurement of undamped maximum polarographic recorder oscillations is more or less illusory-viz., “because the residual current in terms of maximum oscillations is also larger.” I am glad to accept correction on this point. The thought in mind, which I did not express correctly, is that increasing the magnitude of a measured quantity by only 25% does not produce any very significant increase in the sensitivity or precision of the measurement. JAMES J . LISGASE Harvard University Cambridge, Mass.
Polarographic Method for Copper, - - _ lead, and Iron SIR: We have read with great interest the paper entitled “Polarographic Method for Copper, Lead, and Iron” [ANAL. CHEaf., 21, 176 (1949)l by Reynolds and Rogers. However, we feel that two topics of considerable importance have been unduly slighted in their discussion: 1. Directions are given for the preparation of the solution for analysis, but the optimum pH is not stated. This matter is likely to be of vital interest to a user of the method. 2. The reported effect of gelatin on the lead wave is strikingly similar to the effects we have found with a number of micelleforming agents in other systems. From a study of the effects of gelatin on several copper tartrate systems, we have concluded that the critical concentration for micelle formation of gelatin is
