I
1.4 1.2
Ir
x< 1.0
2
c
W
m
a
.8
9
.6
s:
I
a! .40
50CC/MIN. AIR FLOW I1 I, 0 100 CC/MIN. I #I A 450 CC/MIN.
4
.20
.4
o
.2 1
2
I
p H 7.3, Ni+2CONC. l.4'X 10-4m p H 1 2 , Ni+2 CONC. 4.2 X
3
4
5
6
7
6
9
1
I
20 0
NiClz solution, the complex has a molar absorptivity of 11,000 l./mol-cm at 267 nm, the wavelength of maximum absorption. The triple absorption peaks, 267, 284, and 310 nm with an intensity ratio of 16 :6:1, respectively, allow a qualitative identification of the simple cyanide ion in raw water in the absence of appreciable amounts of interfering substances. The intensity ratio may vary with interferences, but the absorption peak wavelengths remain unchanged. In the absence of interferences, the test has a detection limit of approximately 0.5 mg/l. of simple cyanide ion in raw water samples. In the early phases of this work, we were concerned that the presence of a limited amount of cyanide ion would result in incomplete formation of tetracyanonickelate(I1) anion and establish an equilibrium mixture containing lower cyanonickel complexes. The gram-atom ratio plot of CN-':NiZ+ ions (Figure 2 ) shows this is not the case. The sharp inflection at 4:1 gram-atom ratio of CN1-:Ni2+ ion and the straight line relationship passing through the origin for the lower ratios indicate that the tetracyano complex is formed almost exclusively over the pH range 7 to 12.
I
Figure 3. Distillation rate of KCN solution
M. MOL. CN-/M.MOL. Ni+' Figure 2. Gram-atom ratio of CNl-:NiZ+ complex in ammoniacal and neutral solution
t
40 60 TIME, MIN.
The cyanide distillation rate is independent of air flow between 50 and 150 cc/minute (Figure 3) from solutions 5 x and 1.7M with respect to KCN and H2S04,respectively. Under these conditions the distillation is quantitative in 30 minutes. HCN recovery in the absorber is also quantitative; a second absorber in series with the first did not show a detectable amount of cyanide ion. Ludzack et al. ( 2 ) have pointed out, however, that samples containing complexes such as Fe(CN)62-, Fe(CN)63-, c o ( c N ) ~ ~ -and , Pb(CN)42decompose and distil at a slower rate than simple cyanides. For this reason a one-hour distillation period is recommended. In spite of this, the method is relatively rapid compared to standard procedures since the absorber solution is ready for ultraviolet measurement at the end of the distillation period without further treatment. ,
ACKNOWLEDGMENT
The author wishes to thank John W. Miller for helpful discussions and L. c.Swander and B. R. Hamilton for assistance in the experimental work. RECEIVED for review November 18,1971. Accepted March 7, 1972.
Obstruction Factors Are Flow Sensitive Stephen J. Hawkes Oregon State University, Corvallis, Ore.
INA PREVIOUS PAPER (I) a value of 0.73 + 0.02 was given for the obstruction factor to diffusion in a chromatographic column of 60/70 mesh nonporous glass beads and contrasted with the value 0.60 f 0.02 obtained by Knox and McLaren (2) using a static method. Accordingly, the flow dependence of the obstruction factor was investigated. EXPERIMENTAL
The gas chromatograph consisted of a Carle microkatharometer with the column outlet fitting shortened to reduce dead volume and a highly efficient gas sampling system similar (1) S . J. Hawkes and P. S. Steed, J. Chromatogr. Sci., 8, 556 (1970). (2) J. H. Knox and L. McLaren, ANAL.CHEM., 36, 1477 (1964).
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ANALYTICAL CHEMISTRY, VOL. 44, NO. 7, JUNE 1972
to that described by Hawkes and Nyberg (3). The column was a 730-cm, 1/4-inchoutside diameter, coiled copper tube packed with 50/60 mesh Ballotini glass beads with a void fraction of 0.39. Helium carrier gas was used at 50 "C. The total dead volume of the system was less than 50 microliters (compared to a column void volume of 16 ml) which is an order of magnitude better than was needed for this project. Peaks were recorded on an Esterline-Angus El 101s recorder with a full-scale deflection time of 0.2 sec: as all the peaks had widths greater than 1.0 second, this was more than adequate. At the lowest velocities the peaks were significantly skewed: as the samples (nitrogen gas) were unretained this could not be the result of nonlinear isotherms, and it could not be the (3) S . J. Hawkes and D. G. Nyberg, ANAL.CHEM., 41,1613 (1969).
result of apparatus imperfections since these would be more apparent at high velocities. Probably they were the result of the non-idealities discussed by Wicar et al. (4). The skewing, whatever its cause, invalidated the usual method of measuring zone dispersion from the width at half the height, so it was measured by adding the second moments of a series of horizontal strips of the peak in the manner described by Sternberg ( 5 ) : measuring the width at half the height and calculating in the usual way gave higher values of y, but they showed the same general pattern as those obtained rigorously.
o.80r 0.70
CALCULATIONS
The obstruction factor, y,was calculated from H using the equation
w was evaluted at 1.3 by the following considerations. Too low a value of w would lead to values of y greater than unity at high velocities and too high a value would lead to y < 0. Since both of these are absurd, there is a limited range of w that is possible and for our experimental data this was 1.3 =t 0.2. Since the range of 1 0 . 2 in the estimate of w causes the calculated value of y to vary from zero to one at the highest velocities where the second term of the equation is high, data taken at high velocities could not be used to compute y. Accordingly, no data taken at velocities high enough to cause an uncertainty greater than 0.02 in y because of the uncertainty in w are shown in Figure 1.
RESULTS AND DISCUSSION
The results are shown in Figure 1. The obstruction factor is clearly flow dependent and the line can legitimately be extended to y = 0.60 at zero velocity in agreement with the static determinations of Knox and McLaren ( 2 ) , Fluendy and Horne (6), and Bennett and Bolch (7). A possible explanation for this dependence lies in the fact that the lowest flow resistance is offered by gaps or wide paths in the packing structure, especially at the walls, where y is greatest. Accordingly more sample flows through regions of high y,and the value of y is weighted by this effect. At low or zero flow this is offset by diffusion between domains of low and high y. The effect of such equilibration can be qualitatively computed by considering that the maximum value of y will be obtained when escape from a domain by flow is faster than escape by diffusion. This may be roughly estiof the material at a point mated as the velocity such that source at the center of a domain diffuses out of it before the source has moved of the distance to the domain boundary. This is true when od D
4
->-
3
where d is the diameter of a domain. If the domain is about the size of a single constriction (i.e,, about d,) as would be the case if flow-dependence were due to the offsetting of the constriction effect by flow through the constriction as predicted by Giddings (8),then the necessary U would be about 40
l i P 0.600 2 4 6 8 10 cm /sec Figure 1. Variation of obstruction factor with velocity, G
cmjsec: as the experimental flattening point is about 5 cm/ sec, it is more likely that a long-range effect is the cause. If the domain is the column cross-section, i.e., d = 3/116inch, then the velocity would be 5 cmjsec. This agrees well with the experimental data ; moreover the assigned value of w (1.3) is of the order predicted by Giddings (9) for trans-column and long-range interchannel effects. Accordingly, the most probable explanation of the flow dependence seems to be a flow-averaging effect over domains separated by a large fraction of the column diameter. ISOTROPY
Since there is no flow across a column diameter, it follows that radial diffusion has the static obstruction factor of 0.60 as has usually been assumed (8, IO, 11) although the usual value along the column axis will be 0.75. SYMBOLS USED d = diameter of a local domain d, = particle diameter D = diffusivity of sample in mobile fluid Do0 = diffusivity of sample in carrier gas at outlet pressure f = Giddings compressibility factor, j2(pt2jpo2 1) j = James-Martin compressibility factor, 3(pt2/p021)/2(pi3/pO3- 1) p i = inlet pressure po = outlet pressure = apparent linear flow velocity
+
ACKNOWLEDGMENT
The experimental work on this project was carried out by Ann Y.Maynard.
RECEIVED for review December 3, 1971. Accepted February 11,1972.
(4) S. Wicar, J. Novak, and N. Ruseva-Rakshieva, ibid., 44, in press (1972). (5) J. C. Sternberg, Advan. Chromatogr., 6,215 (1966). (6) M. A. D. Fluendy and D. S . Horne, Separ. Sci., 3, 203 (1968). (7) E. R. Bennett and W. E. Bolch, ANAL.CHEM., 43, 5 5 (1971). (8) J. C. Giddings, “Dynamics of Chromatography,” Marcel
Dekker, New York, N.Y., 1965, p 247.
(9) Ibid., p 56. (10) D. S. Horne, J. H. Knox, and L. McLaren, Separ. Sci., 1, 531 (1966). (11) A. B. Littlewood, “Gas Chromatography 1964,” A . Goldup, Ed., Institute of Petroleum, London, 1965, p 77.
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