forms built up on nitcrliate half cycles cannot be dissipated so readily as in the diffusion prow TKOother possibilitics would be ration in an ohmic potential gradient or non-Fickiaii diffusion causecl by ti,(, in~lonlogcneous Of electric'il layer. However, neither of tlic latter processes nould be eq>cctcd t o contribut,e significantly at tilc frequel1(.ics current-anililitudcs e~nployedby I3aui.r and con-orkcrs. LITERATURE CITED
(1) Aten, A . U o c t o r d thesis, Free University- of A i n ~ t c r d n m (1959), through H. H. Bxiier, private communlcation (1960). (a) Barlcer, G , c,, ..t,aal. ~ / ~ id c, t~a 18, ~ , 118 (1958). (3) Barker, G . c., i l l Yeager, E., Ed., "Transactions of the Symposium 011 Electrode Processes," Philadelphia, May 1959, \T'iley, S e w \-ark> in press.
(4) Bauer, H. H., J . Electrounal. C'hem. 1, (lY5').
(5) Bauer, H. H.. Elving, P. J . , J . A m . them, sac, 82,2091 ( 1 9 ~ 0 ) . (6) Bauer, €1. H., Elving, P. J., U. S. Atoniic Energy Comm. Rept. Xo. 58, Contract .kT(11-1)-70, Project 8, July 1960; J . Ekefroanal. Cheui.in press. ( 7 ) 13nuerj H. H., Smith. 1). L., Elving, P. J., Ibid.,82, 2094 (1960). (8! Brrycr, B., Bnuer, H. H., Hacobian, b., A u s t r a l i a n J . Chem. 8 , 322 (1955). ( 9 ) Dclahay, P., "Sox Instrumental Methods in Electrochcmistr3," Chap. 7, Interscience, S e w York. 1954. (10) Doss, I Kandles, I., J. E. B., 1 rans. F a r a d q j Sac. 51, 54 (19%). (18) Matsuda, H., 2. Elektrochem. 61, 489 (1957); 6 2 , 977 (19581. (19) Randl(,s, J . E. B., Discussions F a m d a y Sac. 1, 11 (1947). (20) Ibid., 1, 47 (1947). (21) Ilnndles, J. E. B., in Yeager, E., Ed., "Transactions of the Symposium on
Electrode Processes,'' Philadelphia, May 1950. \\-iley, Kex York, in press. ( 2 2 ) Rangarajan, S. K., J . Electround. C'henz. 1, 396 (1960). ( 2 3 ) Senda, !VI.>Tachi, I., B~u l l . Cheuc.~ ~ ~ Sac. J a p a n 28, 632 (1955). (24) Tachi, I., Kambara, T., Ibid., 28, " (""). W. H. REINYUTH D. E. SIIITH Department of Chemistry Colunlbia University New York 2 7 , K. Y.
Determination of Surface Area by Gas Chromatography S i R : A method has bcen described ( 3 , 4) for t h e detelmin:tticn of surface area which involves the measurement of the uplake of nitrogen from a nitrogen-helium gas stream. A chromatographic column packed n ith the test material and cooled to -196" C. is used. T o obtain sufficient data for the calculation of a surface area several adsorption-desorption runs are necessary, each run giving one point on the adsorption isotherm. The object of this communication is t o suggest a similar method for determining the surface area but needing only one chromatographic experiment. The method resembles that described by Gregg and Stock ( 2 ) and the suggested procedure is outlined belon-. A small, weighed amount of the adsorbent is used to pack a short gas chromatographic column and is de-
gassed in a stream of helium as described by Kelsen and Eggertsen (3). While continuing the flow of helium the column is cooled to - 196" C. Neat the gas stream is changed to a heliumnitrogen mixture whose composition is such that the relative nitrogen pressure, p,, a t the temperature of the column is approximately 0.5-Le. about 30 to 40 em. of mercury. When the column has been completely saturated n i t h nitrogen at this pressure, as nill be shown by a constant deflection on the recorder used to indicate the composition of the effluent gas, the gas stream is changed again to pure helium. The adsorbed nitrogen nill be eluted and the complete isotherm up to the pressure p , can now be calculated from t h e elution curve. ProvidPd that the isotherm is not Type I11 or V in the B E T (Brunauer-Emmet-Trller)
TIME
Figure 1 . Chromatogram obtained by saturating the column with nitrogen at pressure pa and then eluting with pure helium
966
ANALYTICAL CHEMISTRY
classification, a record will be obtained similar to that shown in Figure 1. It is assumed that a differential type of detector cell is used-e.g., a thermal conductivity cell. The amount of nitrogen adsorbed a t the pressure p , corresponds to the area ABCD when this has been suitably corrected for the dead volume of the apparatus. The amount eluted corresponds to the area EFHJ and obviously ABCD 3 E F H J ; homeever, the pronounced tailing will make i t difficult t o measure the area under the curve H F where this approaches the baseline. To calculate a point on the isotherm
I PRESSURE Figure 2.
Adsorption isotherm
~
for any pressure between zero and p1 the equation of Glueckauf ( 1 ) can be used, viz:
where q = f(p) represents adsorption isotherm u = volume of pure helium passed betueen time nitrogen is shut off and pressure p of nitrogen is measured in elution curve 'I;= amount of nitrogen remaining on column when pressure p is measured 2: = weight of solid constituting column To illustrate t h e use of the equation, buppose that it is required to find the volume of nitrogen adsorbed a t a pressure p , which corresponds to the point G in Figure 1, then u p ( = up,) is given by the area ELGK and P; ( = pPg) is given by t h e area LFG. Thus the amount adsorbed a t pressure p , is given by t h e sum of t h e areas ELGK nnd LFG. The difficulty of measuring area LFG accurately can easily be overLFG) = come because area (ELGK (ABCD - G H J K ) and both ABCD
+
and G H J K lend themselves more readily to measurement. Any number of other points can be calculated in a similar way and used to obtain the usual B E T plot. Some workers prefer to use the Point B method for determining the monolayer capacity, V,, of the adsorbent. This involves estimating the volume adsorbed a t the point \There the isotherm becomes approximately linear (Figure 2). The point on the elution curve which corresponds to the Point B is also the point a t which the curve becomes approsimately linear; this is again G in Figure 1. Only one calculation is now required to determine T.,i A number of possible objections to t h e above methods may be mentioned: Surface areas measured in the n a y described will almost certainly differ from those obtained by static techniques because i t is assumed that equilibrium is attained during the measurement of t h e elution curve and this assumption is probably unjustified. However, if the results obtained by Gregg and Stock ( 2 ) using hydrocarbons as the adsorbates are any guide, discrepancies will not be large. Very low flow rates (10 cc. per minute) are recommended; diffusion effects will not be great provided a small column is used.
Strictly speaking, flution curves measure only the desorption isotherm; hence, i t is important that measurements are made only over those parts of the isotherm which are completely reversible. During measurement of the elution curve the flow rate of the helium-nitrogen mixture will be variable because of desorption of nitrogen. It will therefore be necessary to apply a correction to the elution part of the chromatogram to take this effect into account. The method will not be suitable for very fine powders because these v ill impede the gas f l o .~ It is hoped to test the method in the near future. LITERATURE CITED
(1) Glueckauf, E., J . Chem. SOC. 1947,
1302; Nature 156,748 (1945); 160, 301 (1947). (2) Gregg, S. J., Stp,ck, R., "Gas Chromatography 1958,. D. H. Desty, ed., p. 90, Butterworths, London, 1958. (3) Nelsen, F. &I., Eggertsen, F. T., ANAL.CHEV.30, 1387 (1958). (4) Roth, J. F., Elln-ood, R. J., Ibid., 31, 1738 (1959). RALPH STOCK Department of Chemistry, College of Technology, Byrom Street, Liverpool 3, England
Boron Contamination in Furnace Dry Ashing of Plant Material SIR: The accepted method of preparing plant material for boron analysis is by dry ignition in a muffle furnace (1, 4). Ignition is usually made on untreated plant niaterial or on material treated with a base such as Ca(0H)z to prevent loss of boron which takes place under acidic conditions. Vessels suggested for ashing are porcelain, quartz, or platinum dishes. Samples used vary from 0.25 gram to 2 grams. Ignition temperatures should not exceed 550" C. nith ignition time u p to 4 hours or as required to give a grey-white to nhite ash. A large number of barley plants, grown in culture solutions, were dried, ground, and analyzed for boron, using d r y combustion as a means of ashing and the colorimetric method of analysis developed by Dible, Truog, and Berger (2) for boron determination. This method uses cureuminoxalic acid solution as a reagent for developing t h e red boron complex. The samples were ashed in porcelain evaporating dishes in :in electric furnace for varying periods of time at 550" C. Some of the boron values obtained are reported in quadruplicate in t h e first section of Table 1. T'ariation in boron content ranged from 200 t o 800% of the lowest value for a
increased in each dish directly with teniperature in the muffle, time in the muffle, size of the dish, and area covered by the alkaline ash. Coors No. 2 high-form porcelain crucibles eliminated contamination from boron vapors. The height and narrow mouth of the crucible prevented the boron vapors from contacting the alkaline ash. The second section of Table I gives boron data obtained when high-form crucibles were used. Table I1 s h o w a comparison of values
given sample. Similar variation in boron values was noted n i t h four different furnaces. This large variation in boron content of duplicate samples of plant material was found t o be due to a contamination of the plant ash by boron coming from the furnace. The boron volatilized from t h e furnace walls was swept over the dishes,' condensing in the alkaline plant ash. Covering t h e evaporating dishes lessened the amount of contamination but did not stop it. Boron
Table 1.
Plant Sample
I
5-0 6-0 7-0
8;
8-0
50
9-0
10-0 5-x
6-N 7-N
&N
9-iY 10-N
Boron in Barley Leaves
P.P.M. Boron in Dry Plant Material No. 2 Coors Porcelain Evaooratine Dishes Hieh-Form Crucibles 85 61 26 38 17 35 28 60 42 68
28 45 24 60 39 48 18 21 68 36 34 29
55
23 30 100
67 71 30 8 16 25 25 15
Y
52 67 67 86 60 59
... 46
50
61 61 23
9.4 10.6 16.6 50
31.8 37.4 4.4 5.4
8.6 18.4 20.6 12.6
VOL. 33, NO. 7, JUNE 1961
9.8 11.4 16.0 50.8 31.2 38.0 5.4 6.0 9.8 19 4 22 13
967
