1
~-
1 - x
[I
...
- 1+z+z2+
(+ +) ($)+ . .I (4)
+ $ ($)”+
-
X
,
The corresponding chronoamperometric equation is (3):
The coefficients of the first-order terms are
d’ =
Bl
r1i22 =
= =li2
=
0 89 1 77
These are essentially identical to the corresponding experimental coefficients for the unshielded planar electrode. Therefore, it is numerically valid but philosophically misleading, to ‘Yransform” the chronopotentiometric and chronoamperometric constants for spherical electrodes into the correspond-
ing constants for unshielded disk electrodes by substituting the radius of the circular disk for the radius of the sphere. This is not to say the unshielded electrode has an “effective spherical radius” equal to its circular radius. What is true is that the constants obtained with an unshielded electrode have, to first order, the same functional dependence on the radius of the circular electrode as the constants obtained with a spherical electrode have on the radius of the spherical electrode. When using unshielded electrodes, the chronoamperometric technique is superior to the chronopotentiometric technique for analytical purposes and for the determination of n-values and diffusion coefficients. Chronoamperometry avoids the uncertainty associated with the measurement of the chronopotentiometric transition time and requires only a fraction as much time to obtain sufficient data to perform the extrapolation to t 1 ’ 2 = 0.
LITERATURE CITED
(1) Anson, F. C., ANAL.CHEM.3 6 , 520 (1964). ( 2 ) Bard, .4.J., Ibid., 3 3 , 11 (1961). (3) Delahay, Paul, “Sew Instrumental Methods in Electrochemistrv.” ChaD. 3 , Interscience, Xew York, i954. (4) Fieser, L. F., Fieser, &I., “Advanced Organic Chemistry,’’ p. 846, Reinhold, S e w York, 1961. (5) Kolthoff, I. M.,Orlemann, E. F., J . Am. Chem. Soc. 6 3 , 664 (1941). (6) Laitinen, H. A,, “Chemical Analysis,” Chap. 26, RlcGraw-Hill, New York, IF)^
ACKNOWLEDGMENT
(7)-Laitinen, H. A , Trans. Electrochem. SOC.82, 289 (1942). (8) Laitinen, H. .4.,Kolthoff, I. M.,J . Am. Chem. SOC61, 3344 (1939). (9) Lingane, J. J., J . Electroanal. Chem. 2, 46 (1961). (10) Xlacero, D. J., Rulfs, C. L., J . Am. C‘hem. Soc., 81, 2942 (1959). (11) Ibzd., 81, 2944 (1959). (12) llarnantov, Gleb, Delahay, Paul, Ibid.. 76. 5323 119543. (13) Peters, D. G , Lingane, J. J., J . Electroanal. Chem. 2 , 1 (1961). (14) Russell, C. D., Peterson, J. M., Ibid., 5 , 467 (1963). (15) Stackelberg, von, hf., Pilgram, M., Toome, V , , Z . Electrochem. 57, 342 (1953).
I t is a plreasure to thank Fred C. Anson and Robert A. Osteryoung for helpful discussions and Martin S. Itzkowitz for assistance in the leastsquares programming.
RECEIVED for review March 12, 1964. .4ccepted April 17, 1964. This investigation was supported in part by a Public Health Service fellowship, GPM-16, 81 1, Division of General Medical Sciences, held by the author
Characterization of Selected Heavy-Metal Salts as Adsorbents for Gas Chromatography ALAN G. ALTENAU’ and L. B. ROGERS Department o:f Chemistry, Purdue University, lafayette, Ind.
b Adsorbents were produced by careful elimination of water from crystals of CuSO4.5H20, 3CdS04.8Hz0, and MgC12.6H20; pyridine from Cu(Py)4(NOB).! and Cu(Py),SO4; and ammonia from C U ( N H ~ ) ~ ( N O &These . adsorbents had small specific surface areas that necessitated very small samples. A column of Cu(Py)2(N03)2gave very good separations of numerous aliphatic and aromatic hydrocarbons, alcohols, esters, ethers, and ketones with minimal tailing of peaks. It was particularly effective for separating 2- and 3aliphatic ketones. At the other extreme, C u S 0 4 . H z 0showed strong adsorption for aromatics and oxygencontaining compounds. Separations appeared to b e greatly influenced by interaction between the metal ion of the adsorbent and the 7r-electrons or nonbonded electrons of the adsorbate. Heats of adsorption and effects on the efficiency of particle size, column temperature, and flow rate of the carrier gas were determined.
1726
ANALYTICAL CHEMISTRY
F
Y E A ~ Z S adsorbents have been used as column packings in gas chromatography. The most common adsorbents have been activated carbon, activated alumina, and silica gel (22-25). These adsorbents have relatively large specific surface areas with no definite pore size (4). They have been used quite successfully for the separation of small molecularweight substances such as CH,, C2H6,He, Oz. S p ICO, CO,, etc., but have not found much application in gas chromatography for the separation of larger compounds because strong adsorption and unsymmetrical peaks are usually obtained. However, renewed interest in gas solid chromatography has recently occurred from both the theoretical and practical aspects (18, 34). hnother type of high-capacity adsorbent is the molecular sieve. I t separates molecules on the basis of molecular size and polarity (4). Most other inorganic substances, particularly salts, are never thought of OR VANY
as adsorbents although the surface area of many salts and oxides has been determined by adsorption of nitrogen and adsorption isotherms have been obtained using a few gases and relatively volatile liquids. These compounds have not heen used as adsorbents because of their relatively small specific surface areas. However, if sufficiently small samples of volatile substances are injected into chromatographic columns packed with a salt, selective retention (10, f 1, 33) and symmetrical peaks will often be observed. As a result, a large number of potentially useful adsorbents are available. Most of the adsorbents used in the present study were produred by driving off water from a hydrate or an amine from a transition-metal complex. As shown below, separations of a larger number of organic sitbstances have been accomplished easily with 1 Present address. C S ,4rniv S a t i c k Lahoratories, Xatick, Mass
some of these adsorbents without any peak tailing, while other adsorbents have tenaciously retained many aromatic hydrocarbons and oxygen-containing compounds. In general, separations by the adsorbents reported below appear to be governed by a n interaction between the metal ion of the adsorbent and the electronegative part of the sample molecule. EXPERIMENTAL
Chromatographic Apparatus and Procedures. T h e gas chromatographic equipment, preparation and packing of the adsorbents, and procedure of sample injection used in this work were described previously ( 3 3 ) . Copper tubing of 'I8-inch 0.d. was used. The samples of volatile compounds used in this work were assumed to be sufficiently purle because each of them gave only one chromatographic peak. For a study of the effect of sample size on retention time, response of the detector was calibrated by measuring
Table I.
Aliphatic hydrocarbons Methane Ethane Ethylene Propane n-Bu tane Butene-2 n-Pentane Pentenp-1 Dichloromethane Methyl iodide n-Hexane n-Butvl chloride Cyclohexane Cyclohexene n-Heptane 2,2,4-Trimethyl pentane n-Octane Aromatic hydrocarbons Benzene Toluene Chlorobenzene Ethylbenzene p - X ylene m-X ylene o-X ylene Isopropylben-
9
the area of the peak obtained upon injecting a known amount of a solution of n-heptane or bromobenzene in carbon disulfide. [Benzene was used instead of bromobenzene for a column of Cu(Py)SO4 because of the extremely long retention time of bromobenzene.] Standard solutions were prepared by weighing out 1 1301, 0 7602, and 0 5010 gram of bromobenzene, benzene, and n-heptane, respectively, and diluting with carbon disulfide until 1 pl. of the solution contained 0 113 pg. of bromobenzene, 0 076 p g . of benzene, or 0 050 pg. of n-heptane. The detector was calibrated during the day on which the effect of sample size on retention time was studied. -4ttenuation of the amplifier was adjusted to keep the peaks close to full scale on the recorder. An area for an unknown amount of sample was multiplied by the attenuation and related to sample size by the calibration data. Methane was used to calculate the adjusted retention times and volumes because the flame ionization detector does not detect air. This substitution appeared to be valid because methane,
Retention Times, in Seconds, as a Function of Adsorbent and Column Temperature Adsorbent
13.5 13.5
12
15
9
13.5 15
15 16.5
9
22 30
21
36 27 37
28 22 31
30 50 96
27 34
13 18
105 300
72 148
11
115 435
10
14
19 20
321> zene Bromobenzene n-Prop ylben42 1) zene Mesitylene t-Butylbenzene p-Cymene o-Ihchlorobenzene Benzyl chloride o-Bromotoluene p-Bromo toluene Iodobenzene
ethane, and propane usually had the same retention time, indicating that methane was not retained significantly. The procedure used to arrive a t adjusted retention times was to divide the retention time of the sample molecule by the retenticn time of n-ethane and subtract one from the resulting quotient. Dead volumes before and after the column were neglected. The adjusted retention volumes were calculated by multiplying the adjusted re'enticn times by the flow rate of the carrier gas. X-RAYDIFFRACTION DATA. Norelco x-ray powder diffraction equipa ent was used. 1 1 1 the samples were exposed for 2 hours to tl-e x-rays which passed through a nickel filter from a copper target. X-ray powder diffractirn patterns were taken of the compounds before and after decomposition of the hydrates and amine complexes. .\I1 were crystalline except Cu(Py)S04 which appeared to be amorphous. Upcn comparing the spacings of the lines of each of the adsorbents with those of its starting material, it became evident that the crystal structure of the adsorbent
9
9 10
1020 1410 1460 1770
35 80 137 195 210 232 230
29 50 81 100
1160
267 210
393 265
180
340
515 755
210 282 380 434
570
1200 375
440
755 1182 612 783
293 444 265 330 298
11
9
11 11
11 11
12
9
11 14.7
11 13.5
12 14
9
22
19
19
10
51 58 40
34
33 28 33
33
30
26
12
16
555 112 195 870 156 216 625
14
103
85
81
35
705 1800
25
116 306
90 215
89 240
12
38 86 120 170 210 217 240
13 15 15.5 22 30 37 70 90
16 31 75 81 82
395 1280
13 23 31 47p 73q
90
74 155 290 327 345 353 387
180 140
456 537
969 57
191 195
575 692
125q
310 n.p.O
98 120 2 1 0 ~ 425 103 435 30P 1053 1575
180
38 96
42 145 145 300 510
85
187 292 315 406 293 381 890
750'
1007 927
140
175
263
515p
410 547 733
1056 1120
( Continued)
VOL. 36, NO. 9 , AUGUST 1964
1727
cu-
Table I. (Continued) Adsorbent Cu-
)4Cu("3)zcuso4. MgC1,. ( ~ 0 ~(NO8)z ) ~ Cu(Py)~iN0~)2 Cu(Py)SOa ~ H , O CuS04.HzO CdSOa 2H20 38" C,a 38" C.* 81" C.c 3 m F C T 38' C f 3 8 " C.g 97" C.* 38" (3%.38" C.3 135" C.k 38' c.' 125' C.m38" C."
Adsorbate Alcohols 12 n.p. 143 50 Methyl 39 15 Ethyl 350 46 37 20p Isopropyl 45 555 61 Allyl 23p 570b 93 65 t-Amyl 549 165 95 Isobutyl 202 110 38~ n-Butyl 152 529 291 2-Pentanol 192 86q 382 2- Methyl-2pentanol 590 1105 2-Hexanol 496 n-Amyl 144q 392 880 Esters Ethyl acetate 16 n.p. 735 66 44 Dimethyl carbonate 14 705 52 37 Isopropenyl acetate 127 70 Ethyl propionate 24 145 79 Ethyl butyrate 53 400 179 n-Butyl acetate 63 224 520 Isoamyl acetate 120q 464 1165 Ethers 10 Ethyl 375 57 20 21 Tetrahydrofuran 13 300 50 40 Isopropyl 12 99 38 32 Ethylene glycol dimethyl 105 A0 177 n-Propyl 14.5 -. . -. 40 Allyl 360 123 86 p-Dioxane 382 78 n-Butyl 52q 480' 376 170 Ketones Acetone 12 n.p. 450 38 25 2-Butanone 15 780 77 50 3-Pentanone 19.5 1290 159 88 2-Pentanone 22 210 104 Methyl isobutyl 36q 457 195 Cyclopen tanone 200 425 Mesityl oxide 700 295 4-Heptanone 960 1260 3-Hep tanone 91q 470 2-Heptanone 125q 1750 643 Cyclohexanone 1155 470 183-em. column; flow rate, 26.7 ml. per minute. b 183-em. column; flow rate, 26.0 ml. per minute. 183-cm. column; flow rate, 26.8 ml. per minute. 168-em. column; flow rate, 27.2 ml. per minute. e 168-em. column; flow rate, 25,9 ml. per minute. f 183-cm. column; flow rate, 25.0 ml. per minute. 183-em. column; flow rate, 27.2 ml. per minute. * 183-em. column; flow rate, 26.2 rnl. per minute. 153-cm. column; flow rate, 26.4 ml. per minute. j 153-cm. column; flow rate 26.0 ml. per minute.
(id:' 70 78 84
n.p. n.p.
2479 3979 4779
n.p. n.p.
n.p. n.p.
n.p. n.p.
~ 5 0 0 n.p. ~
1320
112
n.p.
n.p.
n.p.
-4005
n.p.
n.p.
n.p.
-1708
215
-310*
3407
200q
155 295 297 725 934 1910 14
160
37
68
17
450 605
78
180q
2100 1165
123 114 148
23 42 145 23 31 66 69
325 705 1860
137 113
1728
ANALYTICAL CHEMISTRY
n.p.
n.p.
-3008 n.p. ~-4.50~
398 537 819 911 633 b 153-em. column; flow rate, 27 4 ml. per minute. 1 183-cm. column; flow rate, 25 0 ml. per minute. 111 183-cm. column; flow rate, 26 1 ml. per minute. 183-em. column; flow rate, 25 0 ml. per minute. n.p. = no peak within 60 minutes. P Slight tailing. q Tailing. Very broad symmetrical peak. a Severe tailing.
5
differed from that of its starting material. Only in the cases of CuS04. 5H20 and CuSOI.H20 were x-ray diffraction data found, and agreement with the literature values (19) was good. Since diffraction patterns of the starting materials and adsorbents were very complex ( I ) , it would have been extremely difficult to determine their crystal structures from x-ray powder diffraction patterns alone. (-1singlecrystal study was considered to be beyond the scope of this work.) The important fact was that a crystalline product usually did resuit and that its structure was different from that of the starting material.
78 142 232 25 1
0
C U ( P ~ ) ~ ( S Osuccessfully ~)~ separated alcohols, esters, ethers, and ketones up to Retention Data. Blank runs were boiling points of approximately 160' C., made using as packing the material? alkylated and halogenated benzenes from which the volatile substances up to boiling points of around 185' C., were t o be removed t o produce the and aliphatic hydrocarbons starting adsorbents. Some compounds were from n-butane up t o those having boiling retained but separations of homologs points of about 210" C. (In Table I, were generally incomplete. KO d a t a could be obtained for C U ( P ~ ) ~ ( N O ~ )the Z compounds of a given class are listed in order of increasing boiling point.) because it lost pyridine readily a t room ,411 of the above classes of organic temperature. However, the data shown in Table I for C U ( ~ Y H ~ ) ~ ( N and O ~ ) ~compounds gave synimetrical peaks, with virtually no tailing. Figure 1 C U ( P Y ) ~ Sare O ~representative. shows the separation of some alcohols CO(Py),(NO&.. At a temperature of 38" C., a 168-cm. column packed with and ketones and also demonstrates that RESULTS
tip of the detector. This greenish subthe peaks were quite symmetrical. stance probably resulted from volatilizaBesides separating a great many comtion or decomposition (16)of the Cupounds, a column of Cu(Py),(NO& (SH3)2(x03)2, If the column temperaexhibited some outstanding complete ture remained below 105" C., no subseparations: isomeric ketones (2- and stance appeared on the quartz tip. 3-pentanone and 2-, 3- and 4-heptanone), Ethyl ether was the only oxygenn-amyl alcohol and 2-hesanol, and ethyl containing compound that came off a butyrate and n-butyl acetate. Smith 183-cm. column of C U ( S H ~ ) ~ ( a~t O and Johnson (36) showed complete 38' C. in 60 minutes. At 81" C . , the separation of 2- and 3-pentanone on a number of osygen-containing com4-meter column of 5% ethylene glycol pounds that came off the column was on firebrick. They also pointed out greatly increased; the retention times that no satisfactory gas chromatowere further decreased at 139' C. No graphic separation of these compounds tailing was observed for any of the had previously been achieved despite oxygen-containing compounds. several reported attempts to do so. S o amines (diethylamine, triethylaThe complete separation of 2- and 3mine, n-propylamine, n-butylamine) nor pentanone on a 1.68-meter colunin of acids (acetic acid) came off a column of C U ( P ~ ) , ( N O , )was ~ slightly better and CU(SH3)2 (IT03)2. was accomplished about eight times Comparison of the retention data on faster than on a column of 570 ethylene columns of CU(P~)~(XCTO~)~ and Cuglycol on firebrick. Various amines (xH3)2(x03)2, showed that Cu(NH&(diethylamine, triethylamine, n(r\'O3)2 exhibited much stronger adpropylamine, n-butylamine) and organic sorption than C U ( P ~ ) ~ ( N O ~The )~. acids (acetic acid, propionic acid) were type of group coordinated around the injected but did not elute within an copper(I1) ion had a very pron inent hour. effect on the adsorption characteristics. Upon increasing the column temperaCu(Py)S04. T o test the effect of a ture, normal decreases in retention change. in the number of coordinated times were observed.. Even at a groups, Cu(Py)&304 was first prepared temperature of 82' C., the previously and from it, Cu(Py)S04. A monomentioned amines and acids did not pyridine nitrate could not be formed bo, come off C U ( P ~ ) , ( X O ~ )The ~ . base line unfortunately, the substitution of sulbecame noisy near a temperature of fate ion for nitrate added another 90' C. and made difficukthe detection variable. of very small amounts of sample. The On a 183-em. column of Cu(Py)S04 noise was probably caused by the loss of a t 38" C.,methane and ethane were pyridine from Cu(Py)z[X03)z. partially separated for the first time; C U ( ~ \ " ) ~ ( S O ~ )By ~ . comparing the ethane appeared as a shoulder on the retention Characteristics of. Cu(NH&methane peak. %-Butane and methane (NOa)% and cu(Py)z(n'(&)~,some'knowlhad retention times differing by 17 edge of the effect of #different groups seconds whereas the greatest difference coordinated around the copper(I1) ion could be obtained. Furthermore, bepreviously observed was 3 seconds. cause the starting material, CU(ITH~)~- Although alcohols did not elute within 60 minutes, several aromatic hydro(x03)2,was sufficiently stable- to be carbons and ethers, along with acetone, used as a column packing near room methyl ethyl ketone, and ethyl acetate temperature, one could also observe the came off this column at 38" C. At effect on retention time of a change in 97"C., the number of eluted compounds the number of groups coordinated to the greatly increased. metal ion. Comparison of the firsb two S o amines (diethylamine, triethylcolumns of Table I ;gives a + typical amine, n-propylamine) or acids (acetic picture of the drastic changes that ocacid) came off a column of Cu(Py)S04. curred with all pairs of compounds that After heating a column of Cu(Py)S04 could be measured. to 156" C. for 50 hours, the flow rate of On a 183-cm. column of Cu(XH3)Zcarrier gas increased from 27 to 41 nil. (XO3)z a t a temperature of 38". C., per minute. Relatively little pyridine aliphatic hydrocarbonE starting with was lost from Cu(Py)S04 during the propane were separated but methane, heating period (Before heating: Found ethane, and propane were not separated from one another. No tailingwas found % Cu 27.30, Theo. % Cu 26.63; after for any of these compo-unds or'for aroheating: Found yG Cu 27.93). It appears that the increase in flow rate matic hydrocarbons, but relatively long must have been caused by the crumbling retention times were observed for aromatic compounds such as the sylenes. of some of the Cu(Py)S04 to produce a channel in the column. At a temperature of 81" C., a larger number of aromatic h:ydrocarbons apThe retention data for columns of peared without any tailing, while a t C U ( P ~ ) ~ ( X OCu(Py)S04, ~)~, and Cu( P y ) S 0 4 , show that the number of 139" C. the retention times were further decreased. After using (h(SH3)2(xO& groups coordinated around the copper (11) ion had a major effect on the for two days at 139" C . a greenish subretention characteristics of the adstance deposited on the rim of the quartz '
~)~
W v)
z
g v)
W
a: W
D
I
a
0 0 W
a
o
i
2
i
i
S
S
RETENTION TIME (min.) Figure 1 . Chromatograms of certain alcohols and ketones on a 168-cm. column of Cu(Py)z(NOa)* Operating conditions: Sample size around 0.20 pg., 38' C., 27.5 ml. per minute nitrogen Row
sorbent. The more groups coordinated around the copper(I1) ion the less the retention of sample molecules. In comparing a sulfate with a nitrate salt, one assumes that the anion of the adsorbent has no major effert on the retention characteristics of the adsorbent. This assumption is certainly open to question but, as yet, it has not been completely evaluated. There are, however, indications in other studies (IO, 28) that some sulfates exhibit greater retention than the c~orreslionding nitrates. CuS04.H20. Copl~ersulfate pentahydrate is easily dehydrated to CuSO,. HzO. Since the remaining water in CuS04'HzO is hydrogen-bonded to the sulfate ion and not coordinated around the copper(I1) ion, the differences in the retention characteristics and heats of adsorption of CuS04.H20 compared to Cu(Py)SO4 may be used as a crude measure of the effect of removing a pyridine molecule from the copper(T1) ion. (The monohydrate collapses to a fine powder as the last molecule of water is removed.) VOL. 36, NO. 9, AUGUST 1964
1729
The blank runs on CuSOl.5H20, shown in Table I were striking because alcohols and esters were not eluted. but aliphatic and aromatic compounds, together with some ethers, were eluted. On going to a 153-cm. column of CuS04. H 2 0 only aliphatic hydrocarbons and benzene were eluted at 38" C . At 135" C., halogenated aromatic hydrocarbons showed no tailing, but alkylated aromatic hydrocarbons exhibited a tremendous tailing. Furthermore, halogenated aromatic hydrocarbons had considerably smaller retention times than alkylated aromatic hydrocarbons of about the same boiling point. Oxygen-containing compounds did not come off this column until the temperature had reached approximately 150" C.. Peaks for the few oxygencontaining compounds (ethyl ether, isopropyl ether, acetone, ethyl acetate) that came off a t 150" C. showed a tremendous amount of tailing. CuS04.H 2 0 exhibited greater retention for sample molecules (aromatic hydrocarbons and oxygen-containing compounds) than Cu(Py)SOa. This behavior agrees with the postulat'e that, as the number of groups coordinated by
Table II.
ALIPHATIC:
1.6
2 .c
E'THERS
z
0 $ I-
1.2
I-
w
[L:
0.8C W
I-
m
2
n- PRO PY L
TETRAHYDROFURAN
3
0.4(
0
0.80
a (3
2 0.0 50
70
90
110
Figure 6.
70
150
130
BOILING
POINT
90
110
150
130
("C)
Effect of groups directly attached to the aromatic ring on the retention time on a 183-cm. column of CdSOl Operating conditions: 125' C., 25.2 ml. per minute nitrogen flow
tion for aromatic hydrocarbons and oxygen-containing compounds than does CuSOI.H20. This may be explained on the basis that, although copper and cadmium each carry two positive charges, cadmium has 19 more electrons shielding its charge. lhrthermore, the 4d-orbitals of cadmiurn(I1) are completely filled, while those of copper(I1) in CuS04 H 2 0 are vacant. A column of MgC12.2H20 held aromatic hydrocarbons and oxygencontaining cornpounds less strongly than C u S 0 4 . H z 0 and CdSOd. The presence of water in hlgCl2.2H20 complicates any interpretation based on charge, ionic radius, and availability of orbitals. A column of C U ( S H , ) ~ ( X Oshowed ~)~ stronger adsorption for oxygen-containing compounds than either Cu(Py),or Cu(Py)S04. The most probable reason for this is the added effect of hydrogen bonding between the hydrogen of the ammonia molecule and the nonbonded pair of electrons on oxygen (20, SO). Hydrogen-bonding, as evidenced by tailing, was severe when the CU(?;H~)~(NYIC)~)~ was used as an adsorbent. The most severe tailing was encountered using ICUSO~ 5H20 as an adsorbent. For the last two column materials, even some aromatics tailed badly. This may be the result of hydrogen bonding with the aromatic ring (5,5, S I ) . The stronger retention of aromatic hydrocarbons and oxygen-containing 9
COLUMN = C d S 0 4 2
/
-I
> z
pz
BENZENE IODO-
,p-BROMOTOLUENE
0
o-XYLENE
8
TOLUENE
1.s
w
p-XYLENE
a O-DICHLORO-
BENZENE
t-
w
a n w t-
UJ 1.0 3 '3
CHLOROBENZENE
n
U
0 0 1
100
120
140
160
I60
BOILING POINT ("C) Figure 7. Logarithm adjusted retention volume vs. boiling point of aliphatic ethers and ketones on a 168-cm. column of Cu(Py)z(NO& Operating Conditions:
38' C., 27.2 ml. p e r minute nitrogen Row
compounds on Cu(Py)S04 compared to C U ( P ~ ) ~ ( N Omay ~ ) Zbe rationalized on the basis that the copper(I1) ion in Cu(Py)S04 has one more bonding orbital available. There may also be some effect of a change in steric factors and of the different anion. A plot of the logarithm of the adjusted retention volume us. boiling point
of various aliphatic ethers (Figure 7 ) on a column of C U ( P ~ ) ~ ( N Oshows & that ethers fall above the line formed by homologous aliphatic ethers if they have oxygen atoms which are more exposed or less sterically hindered (tetrahydrofuran), if they contain double bonds (allyl ether), or if they contain two ether linkages (ethylene VOL. 36, NO. 9, AUGUST 1964
e
1733
glycol dimethyl ether). Similar decrease in retention time with inbehavior was exhibited by these comcreasing sample size for aromatic pounds (and p-dioxane) on C U ( X H ~ ) ~ - hydrocarbons. Also, on this basis. one ( S 0 3 ) ~ and Cu(Py)S04. One would would espect the difference between the expect such a deviation of the less percentage increase in retention time sterically hindered ethers if hydrogen for aromatic and aliphatic hydrocarbons bonding or a copper(I1) ion interaction to increase as the reactivity of the between the adsorbent and the oxygen adsorbent increases. This latter predicof the ether molecule were a primary tion was found to be correct because the factor governing the separation. largest difference between bromoAs shown in Table I, a column of benzene and n-heptane was found for C U ( P ~ ) ~ ( Sseparated O ~ ) ~ some isomeric the most reactive adsorbent, CuSO'H20; ketones (2- and 3-pentanone and 2-. the smallest difference was found for the 3-, and 4-heptanone). These separations least reactive adsorbent, Cu(Py),may be explained on the basis of an (Yoa)~. interaction between the Cu(I1) ion and H e a t s of Adsorption. Heats of the oxygen atom of the carbonyl group. adsorption for benzene, n-heptane, The ethyl ketones (3-pentanone, 3and ethyl ether usually increase in heptanone) have smaller retention times the same order as the relative adjusted compared to their respective methyl retention volumes and adjusted reketones (Figure 7 ) . I n the latter, the tention volumes (Table VI). The oxygen atom is more exposed and can exception, once again, vias C u ( P y ) S 0 4 . interact more strongly with the copper I t s position on the benzene list moved (11) ion. I n the case of 4-heptanone, so t'hat it fell below CdS04 and was the oxygen atom is even less exposed very close to C u S 0 4 . H z 0 . Possibly than in the case of 3-heptanone in the differences in the heats of adsorpaddition to which 4-heptanone has about tion of aliphatic hydrocarbons on the a 6' C. lower boiling point. various adsorbents reflect differences d L I P H A T I C HYDRoC.4RBOKS. ionin the surface areas. induced dipole interaction between t'he With regard to the heats of adsorption for oxygen-containing coinpounds ions of the adsorbent and the aliphatic and aromatic hydrocarbons on Cuhydrocarbons is probably a primary contributing factor in the separations (SHs)2 (NO& and Cu (Py) (303)2 , the observed with aliphatic hydrocarbons. difference may be caused in part by a of hydrogen bonding. As Table VI shows C U ( P ~ ) ~ ( S O S ) Zcontribution , Cu(NH3)z (KO3)2! IlgC12.2H20, CdSOI, and CuS04 H90 exhibit about FUTURE STUDIES the same affinity for aliphatic hydroThe present study obviously includes carbons while Cu(Py)S04 has a much only a very small fraction of the possible greater affinity. This might be because number of useful adsorbents. Severamorphous Cu(Py)S04 has a larger theless, a wide range of selectivity has surface area. Also, it may be functionbeen demonstrated which should be useing, in part, as a molecular sieve, since ful in analyzing a complex sample by it was able to separate partially methane selectively removing one or more types from ethane. Hopefully, a better underof organic compounds. standing will evolve from future studies. The use of solids may also lead to Effect of Sample Size. Sample improved separations of certain isomeric molecules are adsorbed on the sites mixtures similar to those reported for with the greatest activities, but when the methyl- and ethyl-aliphatic ketones. the amount of sample is so large t h a t A similar steric effect has recently been the most ,active sites can absorb only reported ( 1 7 ) for the lutidines. In conpart of the sample, the remaining nection with that study Hannernan (18) sample is adsorbed on sites of progres-. recently reported that no change in sively lower activity ( 6 , 3 7 ) . These slope of the line for the logarithm of adsorbents exhibited a smaller decrease retention time 2;s. the reciprocal of the for aliphatic hydrocarbons than for absolute temperature occurred when the aromatic hydrocarbons. This fact may melt solidified. His observation tobe rationalized on the basis that arogether with an earlier one involving matic hydrocarbons, being polarizable silver salts (IO) indicate that some and often somewhat polar, interact separations effected by fused salts may more strongly with the adsorbents than be largely the result of surface phenodo the aliphatic hydrocarbons which are udies should be made to nonpolar and relatively nonpolarizable. test the selectivity factors of surfaces of As the sample size increases, sample salts, including the generally early molecules adsorb on the less active sites elution of cyclic compounds such as of the adsorbent thereby producing cyclohexane and cyclohesanone comsmaller retention times. Because the pared to the corresponding straightstrength of the interaction of the chain compounds. sample molecule and Fites of different The roles of the cation and the anion activity does not vary as much with should be more clearly elucidated. aliphatic hydrocarbons as with Although a recent survey ( 1 1 ) on inaroinatir, one would espect a greater 1734
ANALYTICAL CHEMISTRY
organic sorbents reported no apparent correlation when separating isomeric terphenyls, there are indications that correlations do exist ( I , 2, 10, 16, 28). ACKNOWLEDGMENT
The authors are indebted to K. S. Vorres for the use of the x-ray powder diffraction equipment; R. A. Lasoski and N7. A. Dippel of E. I. du Pont de Semours and Co., Inc., Gibbstown, S. J., and D. A. Keyworth of the Universal Oil Products Co., Des Plaines, Ill., for. meaburements of surface area and pore volume; and to C. S. Yeh for carbon, hydrogen, and nitrogen analyses. LITERATURE CITED
(1) .4ltenau, -4.G., Ph.D. Thesis, Purdue Cniversity, Lafayette, Ind., 1964. ( 2 ) altenau, -4.G., Rogers, L. B., Purdue Cniversity, Lafayette, Ind., unpublished data. (3) Basila, 31. R., J . Chem. Phys. 35, 1151 (1961). (4) Berl, W.G., ed., "Physical Xethods in Chemical Analysis," \.ol, 4 , p. 45, Academic Press, New York, 1961. ( 5 ) Brown, H. C., Brad,, J. D., J . .4m. Chem. Soc. 74, 3570 (i952). ( 6 ) Brunauer, S., "Physical Adsorption," Vol. 1, p. 246, Princeton University Press, Princeton, N. J., 1943. (7) Ibid., p. 298. (8) Brunauer, S., Emmett, P. H., Teller, E., J . Am. Chem. Soc. 60, 309 (1938). (9) Dal Nogare, S., Juvet, R. S., Jr., "Gas-Liquid Chromatography," p. 171, Wiley, Sew York, 1962. (10) Duffield, J. J., Rogers, L. B., AKAL. CHEM.34, 1193 (1962). (11) Favre, J . A . j Kallenbach, L. R., Ibid., 3 6 , 63 (1964). (12) Fieser, L. F., Fieser, AI., "Introduction to Organic Chemistry," p. 346, D. C. Heath., Boston, Mass., 1957. (13) Ibid., p. 345. (14) Greene, S. A., Roy, H. E., AIAL. CHEX 2 9 , 569 (1957). (15) Guran, B. T., Rogers, L. B., Purdue University, Lafayette, Ind., unpublished data. (16) Hanneman, FV. R., Informal remarks, Session of Gas Chromatography, 145th AIeeting, ACS, New York, S . Y., September 1963; See also Hannernan, \V. W., J . Gas Chromatog. l(12) 18 (1963). (17) Hanneman, W. W.,Spencer, C. F., Johnson, J. F., AKAL. CHEM.32, 1386 (1960). (18) Huber, J . F. K., Keulenians, A . I. 11.) "Gas Chromatography 1962," 31. van Swaay, ed., Butterworths, Washington, 1962. (19) Index to the S-Ray Powder Data File, -4STlI Special Technical Publication 48-1,p. 48, Philadelphia, Pa., 1960. (20) James, A. T., Biochem. J . 5 2 , 242 (19.32).
(21) Janak, J , Ann .Y Y A c a d Scz 72, 606 (1959) ( 2 2 ) Knapman, C E H ed "Gas Chromatography Abstracts 1958," Butteruorths Scientific Publications, London, 1960. (23) I b i d . , 1959. (publd. 1960). (24) I b i d . , 1960. (publd. 1961). (25) Ibzrl., 1961. (publd. 1962). ~
(26) La Villa, R. E., Bauer, S. H., J . Am. Chem. SOC.85, 3597 (1963). (27) Mellor, J. W., “A Comprehensive
Treatise on Inorganic and Theoretical Chemistry,” Vol. 4, p 303, Longmans, Green, London, 1923. (28) Moffat, A. J., Solomon, P. W., U. S. At. Energy Comm., Res. and Development Dept., IDO-16732, 1961. (29) Nelson, F. M., Eggertsen, R. T., ANAL.CHEM.30, 1387 (1958). (30) Pimentel, G. C., MpClellan, A. L., “The Hydrogen Bond p. 255, W. H.
Freeman, San Francisco and London, 1960. (31) Zbid., p. 202. Y . Acad. (32) Purnell, J. H., Ann. Sci. 72,598 (1959). (33) Rogers, L. B., Altenau, A. G., ANAL. CHEM.35, 915 (1963). (34) Scott, C. G., “Gas Chromatography 1962,” M. van Swaay, ed., p. 36, Butterworths, Washington, 1862. (35) Smith, E. D., Johnson, J. L., ANAL. CHEM.35, 1204 (1963).
(36) Trapnell, B. >I “Chemisorption,” ., p. 143, Butterworths Scientific Publications, London, 1955. (37) Zbid., p. 6.
RECEIVED for review January 27, 1964. Accepted June 8, 1964. Supported in part by the U. S. Atomic Energy Commission, under Contract AT( 11-1)-1222. Presented a t 2nd International Symposium on Advances in Gas Chromatography, University of Houston, Houston, Texas, March 23-26, jY64.
Piezoelectric Sorption Detector WILLIAM H. KING, Jr. Analytical Research Division, Esso Research and Engineering Co., linden, N. 1.
b Piezoelectric quartz crystals have long been used as frequency and time standards accurate to 1 part in lo* or better. These stable elements become selective gas detectors when coated with various materials. Crystals coated with gas chromatographic substrates produce gas chromatographic detectors which have several advantages: Sensitivity increases with solute boiling point, detectors can be made selective to compound type and respond in 0.05 second, and the output signal is a frequency which simplifies integration of peak areas and digital presentation of the datu. The crystals used in this work were quartz plates ‘/z inch in diameter, 7.3 mils thick, that vibrate at 9 Mc. A readily measured signal of 1 C.P.S. corresponds to a weight increase of about gram. Coated-crystal moisture detectors sensitive to 0.1 p.p.m. are now commercially available. Hydrocarbon detectors sensing as little as 1 p.p.rn. of xylene have been tested.
P
quartz crystals are used in great numbers for controlling frequency in communication equipment, and are widely used as selective filters in electrical networks. Special quartz crystals, are available which can control frequencies to 1 part in log and very accurate clocks can be run from this signal. Other less familiar uses include the generation of ultrasonic waves ( 7 ) and the measurement of temperature (S), thickness of evaporated metal films (X), dew point of gases ( 2 ) , and adsorption of gases on quartz (10). In the latter three uses advantage is taken of the very high sensitivity of a vibrating crystal to the presence of foreign material on its surface. Sauerbrey (9) developed a relationship between the weight of metal films deposited on quartz crystals and the change in frequency (Equation 1). IEZOELECTRIC
AF = 0.38 X IO6 X
F -
T
X
AW -
A
(1)
For common crystals this reduces to
can be made specific for certain vapors and are now employed on a commercially available A ater vapor detector (Gilbert and f3arker Manufacturing Co., West Springfield, Mass.) (6). SORPTION DETECTOR CRYSTAL
where AF = frequency change due to metal coating, c.p.s. F = frequency of quartz plate, Mc. T = thickness of quartz plate, cm. AW = weight of deposited film, gram A = area of quartz plate or electrode, sq. cm.
Equation 2 predicts that commercially available 1 5 3 I c . crystals having electrodes 5 mm. in diameter will have a mass sensitivity of 2600 c.p.s. per pg. It is therefore apparent that the vibrating quartz crystal can be an extremely sensitive weight indicator. The detection limit is estimated to be about 10-l2 gram. I n the manufacture of certain crystals, metals are often evaporated directly on the quartz plate to serve as electrodes. The amount of metal deposited is adjusted to bring the frequency to a desired point. Metals and many other solids do not greatly affect t h e crystal’s ability to vibrate. However, when liquids are deposited on the quartz surface, the ability to vibrate is often impaired, probably because the vibrating crystal surface is dissipating energy in the liquid. If a gas is allowed to absorb into the liquid coating, the amplitude of vibration is again further reduced. I n this way the amplitude of vibration can be used to detect gas composition. This paper presents information on the use of coated piezoelectric quartz crystals to detect and measure the composition of vapors and gases. Both the amplitude change method and the frequency change method were employed. These sorption detectors
Many sizes, shapes, and frequencies of quartz crystals can function effectively as sorption detectors. The most convenient type to use are thin disks vibrating in the thickness shear mode ( 7 ) . The sensitivity of detection is inversely proportional to the square of the electrode diameter and thickness of the crystal. Thus, the most sensitive disk would be infinitely small. Obviously, these parameters cannot be taken to their extremes and a compromise must be sought. We have found that a frequency of around 9 Mc. is convenient. This choice is a compromise of cost, sensitivity, and ruggedness. A typical 9-Mc. crystal has a sensitivity of about .500 C.P.S. per pg. Sorption detectors have been ma-de from two types of commercidly available crystal mountings, plated electrode and pressure mounts. The plated metal electrode crystals rnost often used are designated as plated electrode, 9 Mc., AT cut, H C 6 i U holder. The quart,z plate is about 12 mm. in diameter and 0.15 mm. thick. Support wires provide electrical connection to the electrodes and maintain the quartz plate in the center of the holder. The volume of this housing is about 2 cc. Gas conduits are soldered to the brass can cover of this housing. Figure 1 is a drawing of the other type of mounting (FT-243), where the quartz plate is retained between two metal electrodes with tabs on each corner. The gas space between the electrode and the quartz plate is the order of 0 . 0 2 cc. Thus. this type of mounting provides the detector with a very small volume. Both types of crystal detector housings have been used with success. The only change VOL. 36, NO. 9, AUGUST 1964
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