NOTES
tJune, I N i l sociation of the nitrogen complex are very rapid. The methyl peaks then undergo exchange broadening and yet produce very little nitrogen complex at equilibrium in s ~ l u t i o n . ~ At low temperatures the two methyl peaks sharpen and become well resolved. 4 similar effect is observed in aqueous acidic solution. This has been attributed to a decrease in the rate of the exchange protolysis in water2 and can be attributed to a decrease in the rate of reaction 2 in the DRIPL system. An alternate explanation for the merging of the -S(CHJ2 peaks as the iodine concentration is increased can be eliminated as a result of the low temperature study. This explanation would propose that in the addition compound, the iodine coordinated to oxygen would shield the S-methyl group pointing toward it, shifting the resonance to higher field and causing the two S-CH, peaks to coalesce. If this were the case, the equilibrium constant would be greater and the effect more noticeable at the lower temperature. Instead, the peaks were found to be sharper and more clearly resolved at the low temperature. Data on the chemical shift of the -N(CHJ2 group, as well as that of the methylene and methyl protons of the ethyl group are reported in Table I. Within experimental error of the measurement there is no change in the chemical shift. Even in the spectra obtained on solution I11 (see Fig. 1) where about half of the amide is present in the form of complex there is no appreciable change in the resonance of the X-(CH,), or methylene protons. This indicates that the addition compound, whose heat of formation is about 5 kcal./mole, is held together mainly by dipole-induced dipole and London dispersion interactions and has a small amount of covalency in the bond.
LINE POSITIOXS (c./s.) Molarity of soln.
Temp., OC.
TABLE I DRIP-I? SOLUTIOXS IS
FOR
-K-(CHz)n
-CHr
0,101 M DMP ,0036 M 1 2
27
150 156
183 191 197,206
,101 M DhlP .0373 M 1 2
27
148 154
182 189 197 204
,101 31 DMP ,0467 A 1 I*
27
144 149
176,183 192,199
,101 M DMP ,223 -41Tl
27
146 152
179,187 194,200
101 M DMP 0481 M 1 2 120 '11DMP 479 M I,
2
145 151 146 154
179,185 143,201 181,188 196,203
-5
cc43 -CHa
257 264 272 255 262 270 250 258 264 253 261 266 252 250 266 255 262 269
(3) The numerical value8 listed above were calculated from speotra obtained with a 60 m h o . probe. The data are reported in cycles p e r recond meanured from the external itanderd reionance for CHrClr.
-
$
1067
.
i
i'
I
_z
Jw
J
1-0.1029G. DMP IN 10 ML CC14, ROOM TEMP, E-0.1029G. DMP; 0.573 G. 12; IN 10 ML. CC14. .ROOM TEMP. UI-0.1232G. DMP; 0.479 G. 12;IN IO ML. CC14,-5'C. I
Fig. 1.-N.m.r.
-
&
-
.
I
O
spectra of the -N(CHa)2 doublet of DMP-12 solns.
Acknowledgment.-The authors wish to acknowledge the financial assistance of the Atomic Energy Commission through Contract No, AT 111-758.
A SIMPLE ABSOLUTE METHOD FOR THE MEASUREMENT OF SURFACE TENSIO?; B Y E. J. SLOWIKSKI, JR.,AND
w.L. M.4STERTON
Unzuersity of Connectzcut, Storrs, Connectzcut Recezved Noiember 21, 1960
Many years ago Wilhelmyl suggested that one could determine the surface tension of a liquid by measuring the force exerted by its surface on a vertically-immersed plate. Harkins and Anderson2 briefly investigated this approach in connection with their development of a film balance, and reported promising results. We have recently made some measurements of surface tension using a modified Wilhelmy method, with results that imply that the method warrants greater attention than it has received in the past. Essentially our experiments involved determining the difference between the apparent weight of a thin metal cylinder as measured in air and as measured when the cylinder is wet by a liquid in which it is vertically-immersed to zero depth. This difference in apparent weight is related to the surface tension of the liquid by the very simple equation 27d7-l
+
T*)Y
= gAW
where r1 and r2 are the outside and inside radii of the cylinder, g is the acceleration of gravity, and AW is the apparent change in weight. The equation is exact and is essentially the one quoted by Harkins and Anderson. Our experimental procedure was very straightforward,
.4 platinum cylinder about 6 mm. high by 1.5 cm. in diam-
eter was constructed from 0.15 mm. sheet, and was provided with a wire hanger. The cylinder diameter was measured to 0.1 mm. with a micrometer. The lower edge of the cylinder was made planar and the hanger adjusted so that the cylinder would hang with that edge in a horizontal
L. Wilhelmy, Ann. Physik, 119, 177 (18631. W.D. Harkinn and T, F. ARhnrson, J . Am. Chem. Boc., 69, PIBB (1937). (1)
(2)
Vol. 65
NOTES plane. The cylinder was cleaned in a Bunsen flame and hung from the arm of a panless Chainomatic balance. The liquid to be measured was poured into a 240-ml. beaker to a depth of about 3 cm. and the beaker placed on a small jack inside t,he balance under the cylinder. The system was given time to come to temperature e uilibrium, and the cylinder weighed dry to 0.1 mg. With &e balance arm held horizontal, the liquid level was raised slowly until the cylinder just touched its surface and was wet by it. A t that point the' liquid level dropped very slightly (about 0.1 mm.). By observing the liquid surface m t h a cathetometer during the raisingoperation we were able to return the surface to the height i t had a t the moment of wetting. The cylinder was then reweighed under these conditions of essentially zero immersion. Temperature was measured to 0.1" on a calibrated thermometer placed in the liquid after this weighing was made.
The results of surface tension measurements on five liquids of reagent grade are given in Table I. The values obtained show a mean deviation from interpolated values taken from the International Critical Tables of about 0.6%. If one uses the literature value for water for calibration of the rather poorly-known cylinder diameter, the mean deviation of the calculated values is about 0.45%, caused almost completely by a poor agreement in the results for carbon tetrachloride. The precision of this method as determined bv measurements on these liqu:ids on successive " days averaged *0.05%. TABLE I" RESULTS OF
SIJRFACE TENSIONMEASUREMENTSBY MODIFIEDWILHELMYMETHOD
Water 27.0 0.6869 Benzene 26.6 .2687 Toluene 27.4 .2645 Acetone 26.1 .2187 Carbon tetrachloride 26.9 ,2513 T ; = 0.752 cm.; r2 = 0.737 y = gAW/2n(TiS- 12).
72.00 28.17 27.72 22.92
71.72 28.01 27.59 22.92
... 28.06 27.61 22.83
26.34 25.91 26.24 cm.; g = 980.3 cm./sec.*;
We believe the modified Wilhelmy method described has tihe distinct advantages of relatively high speed coupled with potentially very high absolute, or relative, accuracy. If one ignores the correction for fall of the liquid level at the moment of wetting, which affects the calculated value of surface tension by about 0.2%, one can easily and quickly make riurface tension measurements with better than 1%accuracy on a du Nouy tensiometer modified by uciing the cylinder and method described. The authors are grateful to the National Science Foundation for a grant in support of this work.
nitrides exhibit interesting magnetic properties. Three distinct types of iron nitrides are known. Fe4N, which is cubic, has a narrow compositional range and a moment of 2.25 Bohr magnetons per metal atom. The hexagonal Fe,N has a compositional range of 2 5 z 5 3. Orthorhombic FezN has a small moment of -0.19 Bohr magneton per metal atom. However, hexagonal compounds, with compositions approximately Fe2N, possess moments up t o ten times that of the orthorhombic samples. It was decided to undertake a further study of the hexagonal compounds in order to determine whether the change in moment was gradual or showed a marked decrease when the crystal structure changed. Orthorhombic Fe2N was used as the starting material for the preparation of the hexagonal nitrides. It was prepared by first heating finely divided spectroscopic grade iron powder in a stream of hydrogen a t 500" for one hour to remove traces of oxide and then heating in a purified ammonia atmosphere at 350" for four hours. The hexagonal nitrides were then prepared by the thermal decomposition of Fe2N at 450 to 500" under varying ammonia to hydrogen ratios. A range of composition of hexagonal Fe,N was obtained where 2.8 2 x 2 2. TABLE I PROPERTIES OF HEXAQONAL IRON NITRIDES Compound
F e N (orth) F~.MN
F&.ioN Fez.sN Fe2.d Fen.asN Fe2.44N Fe.siN Fe.epN
Fe.loN
Cell volume
88.8 87.4 86.9 85.8 85.7 85.2 85.0 84.6 84.1 83.7 83.2
9 (Bohr magnetondmeta atom)
0.19 0.69 1.15 1.47 1.60 1.64 1.68 1.92 1.97 1.99 2.02
The per cent. nitrogen was determined by heating the nitrides in a stream of hydrogen a t 800" and obtaining the weight loss. Iron was determined by standard analytical procedures. Saturation moments were obtained by extrapolating to infinite fields magnetic measurements made from 4100 to 10,800 oe. with a vibrating coil magnetometer. The results of chemical analysis and of crystallographic and magnetic measurements are summarized in Table I. The chemical compositions are plotted against the saturation moments in Fig. 1. It can be seen that between the compositions Fe2.06Nand approximately ??e2.6Nthe moment varies linearly with the chemical composition. HEXAGONAL IRON NITRIDES Below Fe2.05N there is a sharp drop off in the BY AARON WOLD, IXONALD J. ARNOTTAND NORMAN MENYUK moment and above Fe2.bN saturation is reached. Lincoln Laboratory,' Moasachusetta Institute of Technology, Lexington 73, The cell volumes (not plotted) vary linearly Mossachusetta with the moment above the composition F ~ w ; N . Received November $6, 1960 Jack4 has pointed out that the hexagonal-orthoIt has been reported2sapreviously that the iron rhombic transition is sharp and takes place close to the composition Fe2N. The iron atoms retain (1) Operated with iupport from t h e U. 9. Army, Navy and Air Force.
B. Goodenough, A. Wold End R. J. Arnott, J. SUPP., so. 343s (igsal). (2) J.
A p p l . Phya..
(3) R. J. Arnott and A. Wold, J . Chem. P h y a . Solids 15, No. 152-156 (1960). (4) I(. H. Jack, Acto Ciyst., 6 , 404 (1952).
I/Y,
