in the laboratory and 8.5" C. inside the standard cabinet. Temperature inside the latter cabinet followed very closely that of the laboratory.
-
w 300a a 290-
2 27O -
ACKNOWLEDGMENT
(3
The authors express appreciation to Glen Grapp for valuable technical assistance.
V,
E [L
LITERATURE CITED
TEMPERANRE CHANGESINTHE LABM1ATORYlMAX VARAllON 9TC.j
28O
26' 25O
-
T E M P E R A N R E IN THERMOSTATIC CABINET [MAX VARIATION 0 32.C)
x
.......*
TEMPERATURE CHANGES IN COMMERClAltY AVAILABLE CABINET [ M h X VAR 05'CJ
-
24O
23O
n 220
(1) Dent, C. E., Biochem. J . 43, 169 (1948). (2) Jirgensons, B., University of Texas, Publ. 5109, 56 (1951). (3) Logothetis,J., NeuroZogy8,299 (1958).
210
200 I
2 3 4 5 6 7 8 9 IO It 12 1314 15
DAYS OF EXPERIMENT
WORK supported by grant B-1183 from the National Institute of Neurological Diseases and Blindness, National Institutes of Health.
Figure 3. Temperature fluctuation at various laboratory temperatures
Sodium Azide as an Internal Standard for Quantitative Infrared Analysis R. T. M. Fraser, Dominion Laboratory, Department of Scientific & Industrial Research, Wellington, New Zealand
A
to infrared quantitative analysis is the low solubility of some compounds in suitable organic solvents. The technique of mixing the sample with a known weight of an internal standard and incorporating the mixture in a mull or potassium bromide disk is designed to overcome this difficulty. Naphthalene (d), DL-alanine (f), lead thiocyanate (7), and calcium carbonate (3) have been suggested as internal standards, but the spectra of naphthalene and m-alanine are much too complex to be generally employed (6), and although calcium carbonate was added only in a 1 to 100 ratio, the carbonate peaks (1430,877, and 715 cm.-l) might interfere with the desired analytical peak. From the published spectra of inorganic compounds (5) potassium ferricyanide and then potassium thiocyanate were recommended as the internal standards for potassium bromide disks. Potassium thiocyanate was unsatisfactory because with the high proportion of standard to sample required (78: 100 to 540: loo), the 2020-cm.-l thiocyanate peak was very broad. These mulls showed a very high background, and water absorption was a problem. With quantitative running conditions (4, it is no longer necessary that the standard possess a reference peak close to that of the sample, and it is a distinct advantage if this peak is in the 2000- to 2800-cm.-l region where most organic compounds have little absorption. Sodium azide with a very strong peak a t 2140 crn.-' (Figure I) and a LIMITATION
1602
ANALYTICAL CHEMISTRY
weak peak a t 1309 cm.-' fulfills this requirement. In the determination of sodium fluoroacetate in dried residues from soil dispersion, sodium azide was used as an internal standard, but because' of the intense absorption of both the C-F peak (1338 cm.-l) and the azide peak it was necessary to use extremely thin mulls containing a large proportion of
Nujol. By adding a nonabsorbing compound such as potassium bromide as diluent, however, mulls of medium absorbance could be prepared in the usual way. The use of potassium bromide not only decreases the amount of Nujol required, thus increasing the transmission of the mulls, but also facilitates the grinding of the solids before mulling. Ratios of azide-so-
FREOUENCY Cm-1 4000
3000
2000
Y I
v)
f
E Figure 1. Infrared absorption spectrum of a Nujol mull of sodium azide 3 grams of ozide to i00 grams of potassium bromide CI
3.23 W Y P
33-0 P 1
Figure 2. Effect of grinding time on stand-ard to sample 'absorbance ratio
Z
Absorption of azide compared with absorption of free acid in a sodium salt
g2.75
2
8 2.5 CT
a
8 2.25
0 F
5
0
5
IO
15
MINUTES H A N D GRINDING
0
5
BALL GRINDINS
IO
djum fluoroacetate-potassium bromide ranged from 2.6:14:100 to 6.6:19:100 in standard mixtures, and ratios of fluoroacetate-potassium bromide of 1.2 to 100 have been employed for trace amountsin the presence of highly absorbing clays, Under these conditions there is a linear relationship between the ratio of the absorbances of azide and fluoroacetate ICTF) peaks and the ratio of standard to sample concentrations. A further advantage of sodium azide is the short grinding time required to
produce constant standard-sample peak height ratios for identical mixtures. No change in this ratio Occurs after hand grinding for 10 minutes or ball milling for 5 (Figure 2); milling for a longer time has a slight effect on the width of half peak height and decreases the scatter background a little, but has no effect on the height of the azide peak itself. The only disadvantage is the desirability of avoiding large amounts of water and or organic solvents which otherwise might be added to facilitate mixing of the sample and potassium bromide.
LITERATURE CITED
~R.B.,~ G ~~R. ~ e., ~ Williams, ~ ~, , E. F., Linsley, s. G., Peterson, E. M., IND.ENG.CHEM.,ANAL. ED. 19, 620
(1) B
(1947). ( 2 ) Childers, E., Struthers, G. W., ANAL. CHEM.27, 737 (1955). ( 3 ) Kuentzel, L. Ibid.i *’I 301 (l955). (4) McDonald, 1. R. C., Watson, c. Zbid., 29, 339 (1957). ( 5 ) Miller, F. A., Wilkins, C. H., Ibid., 24, 1253 (1952). (6) Wiberley, S. E., Sprague, J. W., Camp bell, J. E,, zbid,, 2 9 , 2 1 0 (1957). ( 7 ) jt’right, Norman, A p p i . Spectroscopy 9 , 105 (1955).
Wave length Calibration of Spectrophotometers Ian K. Walker and Harry J. Todd, Dominion laboratory, Department of Scientific and Industrial Research, Wellington, New Zealand
generally experience S’“no difficulty in wave length calibra
