Table I.
(4) Delahay, P., Berzins, T., J . A m Chem. SOC.75, 2486 (1953). (5) Galus, Z., Adams, R. N., Ibid., 84, 2061 (1962). (6) Galus, Z., JThite, R. M., Rowland, F. S., Adams, R. N., I b i d . , 84, 2065
Ionization Rate Constants Calculated by Means of Equation 2
Compound p-Phenylenediamine
PH 1 . 0 and 0.7
kf1 eec.
K
1.59 x 10-3
ca.
N ,~?’-Dimethyl-p0.2 (1.59 x 10-3)b phenylenediamine Average of values obtained at the two pH’s. * Assumed equal to K for p-phenylenediamine; see text.
tion rate constant of the reaction given by Equation 1 (kb = ksb[H+])where k ” b is the second-order recombination rate constant), and K is the equilibrium constant, (kf/k”b),of the reaction given by Equation 1. I n Equation 2, C” is the total concentration of H2A++ and HA+ in solution; and n, F , and D have their usual electrochemical significance. The first term on the right hand side of Equation 2, n”W’CoD112/2 is the intercept of the straight.line plots and is equal to the diffusion controlled value of i+. The rate constants are calculated directly from slope of the straight-line plots which is equal to n112/2K(kr ka)1’2. The rate conqtants
+
k”b, litermole-’eec.-l
lo4”
ca. lo4
f 19621. \----I
ca. ca. lo7
calculated for the ionization reactions of the species studied are given in Table I. The fact that even in 12F H2S04N phenyl-p-phenylenediamine and diphenylamine do not exhibit the large behavior that potential shift or were observed for the other three compounds is no doubt because of their much weaker basicity. LITERATURE CITED
(1) Anson, F.
(19611.
c., ANAL. CHEM.33,
939
199, Interscience, Yew York,
(7) Gaylor, V. F., Conrad, A. L., Landed, J. H., ANAL. CHEM.27, 310 (1955). (8) Lee, H. Y., Adams, R. N., I b t d . , 34, 1587 (1962). (9) Lingane, J. J., “Electroanalytical Chemistrl-,” 2nd ed., Chap. S X I I , Interscience, New York, 1958. (10) Lingane, J. J., J . Electroanal. Chem. 4, 379 (1960). (11) Lord, S. S., Rogers, L. B., S s . 4 ~ . CHEM.26, 284 (1954). (12) Mizoquchi, T., Adams, R. S . , J . Am. Chem. SOC.84, 2058 (1962). (13) Parker, R. E., Adams, R. S., h s . 4 ~ . CHEM.28, 828 (1956). (14) Piette, L. H., Ludwig, P., Adams, R. N., Zbid., 34, 916 (1962). (15) Vanderbelt, J. M,, Henrich, C., Vandenberg, S.G., I b i d , 26,726 (1954). (16) Whitmore. F. C.. “Ornanic Chemistry,” 2nd ed., p. 618, Vin Sostrand Co., Kew York, 1951. RECEIVED for review December 31, 1962. ilccepted Xarch 1, 1963. Supported in part by the National Science Foundation, Contribution No. 2925 from the Gates and Crellin Laboratories of Chemistry, California Institute of Technology. Division of Analytical Chemistry, 144th Meeting, ilCS, Los Angeles, Calif., April 1963.
A Partial Condensation Variable Reflux Stillhead for the DistiIIation of Trace Components NORMAN ADLER Chemical Division, Merck & Co., Inc., Rahway,
b The design of a partial condensation variable reflux (PCVR) stillhead in comparison to various total condensation heads is discussed in terms of start-up behavior and of internal and external holdup. External holdup is divided into regular and random mixing effects, and the former is treated mathematically. The tendency of a PCVR still to approach a constant distillate composition still in behavior is described. The advantages of a PCVR head in the distillation of trace components-i.e., SOX-from macro mixtures are discussed. Performance is evaluated by use of the azeotropic, minimum boiling, acetone-isooctane system.
I
IS OFTEN DESIRABLE in many laboratory problems, and particulady in gas chromatographic analysis, to distill quantitatively trace components-i.e., 50X-from large quantities of less volatile liquids. The T
724
ANALYTICAL CHEMISTRY
N. J.
objective is usually to achieve a highly concentrated solution of the volatile impurities. The degree of fractionation required to achieve this goal for a particular system may be estimated from established distillation theory (11). I n practice, however, once a given fractionation system is selected, it is often the stillhead that is the limiting factor in the sharpness of separation attainable (10). Particularly for micro components, the holdup phenomena implicit in the design of many laboratory variable reflux stillheads may significantly reduce the over-all efficiency of fractionation. Commercially available stillheads that minimize these defects are often complex and costly. I n this work, the properties of the basic laboratory scale partial condensation variable reflux (PCVR) stillhead are compared to those of the more conventional total condensation variable reflux (TCVR) stillhead type, to show its advantages for the distillation of more volatile micro components
from macro mixtures. =Ilthough dills using a partial condensation head have been described previously ( I , 3 , 4, 8, 9 ) and have been recoinmended for selected special applications ( 2 , E ) , the intrinsic properties of this type of head do not appear to have been documented fully. DESIGN CONSIDERATIONS
The chief functions of a distillation column stillhead are to condense the vapors to the liquid phase, to direct the flow of vapor and condensate, and usually to regulate the reflux ratio. The TCVR type is considerably more popular than the PCVR type, presumably because a constant reflux ratio may be set and then maintained without subsequent attention ( 5 ) . Control of the reflux ratio is usually obtained by movement of some element to divert the flow of condensate in a proportional manner, either in terms of time or of volume. The movable element may be
Ir""'"
100
-
n
90 -
i
\ I I
b \
'!
\
'1
\
'\ 2
3
I) I
0.2
5 I
e
7 8 910 I I
12 I
0.4
n I h
O'.I4 0.0
-
Weight of distillate, gm.
Figure 2.
Distillation using PCVR stillhead
System: 0.4 wt. % acetone (41 5 mg.) in iso'dctane. Fraction numbers and sizes a r e shown above abscissa Ordinates: 0curve A: % acetone in each fraction e curve 6 : cumulative % recovery of acetone
Figure 1.
Variable reflux stillhead designs
A, 6, C, D : Conventional total condensation heads E: Partial condensation head
a rotatable cold finger condenser, (drawn to a point at its tip), a magnetically operated swinging funnel, a solenoid-operated plur ger valve, a stopcock or other device tcJ permit variable throttling of the d stillate, etc. (2, 5). TCVR Stillheads. Consideration of t h e design of several laboratory scale TCVR heads may serve t o draw attention t o several defects t h a t become important when dealing with t h e quantitative recovery of micro components-Le.,
