(3) Korff, S. A., “Electron and Nuclear Counters,” p. 15, Van Nostrand, New York, 1946. (4) Lovelock, J. E., J . Chromatog. 1, 35 (1958). (5) Lovelock, J. E., Lipsky, S. R., J . Am. Chem. SOC.82, 431 (1960). (6) Montgomery, C. G., Montgomery, D. D., J . Franklin Inst. 231, 447 (1941).
(7) Rochester, G. D., McCusker, C. B. A., Nature 156,366 (1945). (8) Ryce, S. A., Bryce, W. A., Ibid., 179,
(11) Weisz, P. B., Phys. Rev. 74, 1807 (1948).
(9iShahin, h l . M., Lipsky, S. R., ANAL. CHEM.35, 467 (1963). (10). Sugihara, T. T., Wolfgang, R. L., Libby, W. F., Rev. Sci. Instr. 24, 511
RECEIVEDfor review June 1, 1964. Resubmitted February 25, 1965. Accepted A ril 12, 1965. Division of Analytical 8hemistry, 144th Meeting ACS, Los Angeles, Calif., April 1963.
,541 11957).
(1953).
Adsorption of Methyl Orange and Ethyl Orange on Tailored Silica Gels GEORGE H. REED’ and
L. 8. ROGERS
Chemistry Department, Purdue University, lafayetie, Ind.
b To explore factors that might influence the selectivity of adsorption, a study has been made of the effects of small changes in the structure of the azo dye coprecipitated with silica gel on the ability of that gel to adsorb methyl orange and ethyl orange. The capacity of the gel was always increased (40 to 350%) by the coprecipitation step, even when the selectivity was reversed or erased, despite the fact that a visible amount of coprecipitated dye could not b e removed by washing. Absence of a sulfonate group in the coprecipitated dye led to greater ease of removal b y washing. In two cases, selectivity was eliminated, a result that might b e useful in proving that two or more adsorbates had certain structural features in common.
S
is an intriguing and promising aspect of adsorption for which a number of applications seem imminent. Dickey ( 8 , 9 ) was the first to report that silica gels formed in the presence of methyl orange (MO) or ethyl orange (EO) showed an increased capacity for adsorption of that particular dye. More important, he found that the “natural” selectivity of silica gel for 310 relative to EO could be reversed by forming the silica gel in the presence of EO. Those results have been confirmed by others (11, 16). Dickey ( 8 ) has also shown that propyl orange can, in the same way, be selectively adsorbed relative to MO, EO, and butyl orange. I n addition, Bernhard (4) prepared a gel using a dye that had a p’-sulfonamido group in place of the sulfonic acid on the XI0 structure. His result seemed to indicate that there was little or no charge effect involved in the general phmomenon of specific adsorption. I n similar studies, Frlenmeyer and FECIFICITY
Present address, Chemistry Department, I-niversity of Wisconsin, Madison,
47907
Bartels (10) have observed, using thin layer chromatography (TLC) , specific adsorption for N,N-dimethyl- and N,Ndiethyl-anilines on their respective gels, but those gels did not differentiate between MO and EO in thin layer chromatograms and barely so in adsorption isotherms. However, Morrison et al. (21) reported enhanced sorption of MO and EO on a gel tailored with sulfanilic acid or p,p’ diaminodiphenyl. Going one step further, “stereoselective” adsorbents have been reported for compounds such as quinine, quinindine, cinchonine, and cinchonidine by Beckett and Anderson (2, 3) and for camphorsulfonic acid by Curti and coworkers (6, 7 ) . Other work closely related to specific adsorption has been done by Basmadjian and coworkers (1) concerning the effect of dissolved polymers on pore size and surface area of silica and alumina gels. The exact reason for the increase in adsorption capacity that can sometimes reverse the selectivity is still a matter of debate. Some workers favor the “imprint” theory (9, 15). Waksmundzki et al. ( 1 7 ) attribute the effect to a change in pore structure (diameter). Others believe that the portion of coprecipitated dye that remains in the gel after
exhaustive extraction is responsible for the phenomenon (11). Snyder (14) has proposed an important role for site type and topographical distribution. The present study is concerned chiefly with adsorption of -MO and EO on silica gels prepared in the presence of similar azo dyes. The effects of coprecipitated dyes having no sulfonate group or having methyl and ethyl groups in different positions were examined by means of adsorption isotherms and thin layer chromatograms. EXPERIMENTAL
Reagents. DYES. Except for azobenzene (Matheson Coleman & Bell) and methyl orange (Mallinckrodt), the organic reagents used to synthesize three of the dyes and the remaining dyes themselves were obtained from Eastman. Except where noted, chemicals were used as received. N , N - Dimethyl - p - phenylazorn-toluidine was prepared according to the literature (12). The melting points of 68” C. were identical. N - Ethyl - N - methyl - p - phenylazoaniline was prepared by coupling freshly distilled aniline and N-ethyl-N-methyl aniline, available as the hydrochloride (13); p - N , N - Diethylamino - K , N dimethyl - p - phenylazo-aniline was
Table I. Equilibrium Behavior of Gels toward Methyl Orange and Ethyl Orange’ -Adsorbate __
Tailored gel
I. Control 11. Methyl orange 111. Ethyl orange
MO
Ratio EO- MO/EO
7.0
IV. N ,A’-Dimethyl-p-phenylazo-aniline S‘. N,K-Dimethyl-p-phenylaso-m-toluidine . . _ . VI. Azobenzene S‘II. p‘-Al’,Al’-Diet hylamino-N,N-dimethyl-p-phenylazo-aniline VIII. .V-Et hyl-A’-methyl-p-phenylazo-aniline
14.8 24.5 15.0
11.0 13.5 22.0 20.0
1.50 1.59
0.82
1.93 1.73 1.04 1.05
a Concentration ratios (moles per Kg./moles per liter) taken from adsorption isotherms for methyl orange and for ethyl orange at an equilibrium solution concentration of 2 . 0 0 ~ M.
WE.
VOL. 37, NO. 7, JUNE 1965
861
prepared by coupling N,N-diethylagainst such a conclusion is the nearly aniline and N,N-dimethyl-p-phenyleneidentical capacities of gel IV and the diamine, available as the double hydroM O gel for EO. I n any case, it is bechloride ( I S ) . cause the capacity for EO did not GELS. The silica gels were prepared change that gel IV had a higher from sodium silicate solution (Fisher) selectivity toward MO than the MO gel. according to the hydrochloric acid The effect of adding a methyl group to method of Dickey (8). Five-tenths the aniline ring of the dye molecule can gram of dye was used in each preparabe seen by comparing gels IV and V. tion. After drying for 10 to 15 days, the gels were ground and extracted with Both the capacity and the selectivity of acidified methanol until no dye could be gel V for MO were somewhat less than detected in the extract. For extraction, of gel IV. The capacity was also the gels mere packed in a column, and significantly smaller than that of an methanol was circulated in a continuousMO gel, but the selectivities were the feed setup. Extraction periods ranged same, within experimental error. (Once from a few days to nearly three weeks. again, the coprecipitated agent was Procedures. ADSORPTIONIsofound to be extracted from the gel much THERMS. These were obtained a t 25’ faster than was N O from the MO gel.) + l o C. in a 5% acetic acid solvent Gels VI and VI1 were of special following a procedure similar to t h a t of Dickey (8). The equilibrium coninterest because the natural selectivity centration of dye was measured spectroof silica gel for M O had been destroyed photometrically at 510 mp using a in both cases. The absence of any inPerkin-Elmer Model 202. The recrease in selectivity seemed logical; comproducibility of points on an isotherm plete erasure of the natural selectivity was generally within 5%. THIN LAYER CHROMATOGRAXS.was surprising for gel VI. It should also be noted that the capacities of each gel These were obtained using dry adfor MO and EO were greater than those sorbent without binder (5) on glass of the control. Furthermore, since the plates, 18 X 10 em., propped up in large crystallizing dishes which served dye used for gel VI1 had both a dias developing chambers. T o increase methyl and a diethyl amine group the Rl values for 110 and EO and to whereas the azobenzene used for gel VI facilitate the location of pink spots on had neither, the presence of the alkyl pink adsorbent, Celite was sometimes groups must account for the signifiadded as an inert diluent. cantly greater capacity of gel VII. Results for gel VI11 show the effect of RESULTS having ethyl and methyl groups on the same nitrogen. I n contrast to gel VII, The adsorption isotherms for MO and gel VI11 showed the usual selectivity E O on a control gel, MO gel, and EO for 140 as well as the greatest capacity gel, respectively, were in good agreefor MO. Retent’ion of selectivity for ment with those of Dickey (9, 8 ) . M O comparable to that for the &IO gel, Individual isotherms are not shown in spite of an increase in the capacity here; instead, the value of the confor EO to a value close to that for an EO centration ratio for a gel in equilihrium gel, was particularly surprising in view with a 2.00 x 10-~Jf solution of a dye of the results with gels VI and VII. is reported. The values in Table I TLC studies of most of the gels facilitate rapid comparisons of capacities yielded results which were in qualitative and selectivities. The results for gels I , agreement with the data from ad11, and I11 in the t’able are in accord sorption isotherms (Table 11). As with those of Dickey. The MO-EO expected, the R J values for MO and EO ratio for the EO gel, gel 111, shows not were smallest on their respective tailored only that the capacity for each dye gels. An apparent anomaly was the increased but that the capacity for EO smaller R I value for EO on the control was changed sufficiently to reverse the gel. This anomaly is presently under selectivity. (18) and it appears to be the result study The effects of removing the sulfonate of using dry gel in TLC studies as group from methyl orange were studied opposed to prewet‘2.l gels (570 acetic with gel IV. The first effect was acid) in the equilibrium studies. strikingly apparent during preparation of the gel: the time required to extract the coprecipitated dye was only 10 to Table II. Average R/ Values for Methyl 15% of that required for an M O gel. Orange and Ethyl Orange on Various An estraction period of only two days, Gels rather than two and a half weeks was Adsorbate sufficient. The second effect, shown in Tailored gel ?*IO EO the table, is the higher capacity of the gel for 310, a capacity greater than that Control 0.22 0.20 Methyl orange 0.07 0.11 of the R.10 gel itself. The greater ease Methyl orange (40% Celite) 0.17 0.24 of estraction may also be associated Ethyl orange (307,Celite) 0,Oi 0.02 with a more complete removal of the N,iV-Dimethyl-p-phenylazocoprecipitated dye which, in turn, might aniline (307,Celite) 0.05 0.07 N,h’-Dimet hyl-aniline 0.04 0.02 esplain its higher capacity for methyl orange. One point that may argue
862
ANALYTICAL CHEMISTRY
A gel prepared in the presence of N,N-dimethylaniline also gave somewhat anomalous data which need further study. As expected, the Rl values for both dyes were smaller than on the control gel, but again, as on the control gel, EO was more strongly held. This disagrees with Erlenmeyer and Bartels (IO) who found no measurable difference between MO and EO using TLC and only a very small selectivity for 110 using isotherms. Whether or not there was inversion, the lack of high selectivity toward MO in both studies was undoubtedly related to the fact that in the acidic medium of gel formation, the N,N-dimethylaniline was protonated and did not have a configuration similar to that for protonated MO. DISCUSSION
Cnlike earlier studies, the emphasis in the present study was to examine the effects on adsorpt’ion of 1x0 and E O using a series of structurally-related coprecipitated species in gel preparation. I n this way, it should be possible to evaluate the relative importance of different structural changes and, eventually, to learn how to optimize the selectivity for a particular pair of compounds. In the case of gel IV, both capacity and selectivity were greater for MO relative to EO than when XI0 itself was used to tailor the gel. This unexpected result opens a new avenue for exploration. I n contrast, gels VI and VI1 show that the “natural” selectivity of an adsorbent can also be destroyed. We had encountered this in earlier unpublished studies with cortisone acetate and prednisone acetate, two steroids whose structures differ only by one double bond in the “A-ring.” Instead of being enhanced by the prior coprecipit’ation with one compound or the other, the separation was destroyed regardless of which steroid had been coprecipitated. Such behavior can be of practical value if one wishes to test or demonstrate cert,ainstructural similarities between an unknown compound and one or more knowns, and it complements the idea of using sorption on tailored gels to demonstrate structural similarities (3). The steroid example, in particular, emphasizes the possibility of interpreting the data in terms of the probable posture of the adsorbate molecule with respect to the surface of the gel. I n general, dyes which had no sulfonate group were extracted significantly faster than EO which, in turn, was faster than 110. I t would be interesting to know if substitution of the sulfonamido group for the sulfonate (4) had a similar effect. If so, one might assign most of the difference to the charge on the sulfonate.
LITERATURE CITED
(1) Basmadjian, D., Fulford, G. N., Par-
dons, B. I., Rlontgomery, D. s., J . Catalysis 1, 547 (1962). (2) Beckett,, A. IT., .4nderson, P., J . Pharm. Pharmacol. 12, 228T (1960). (3) Beckett, A. H., Anderson, P., Sature 179, 1074 (1957). (4) Bernhard, S. A , , J . Am. Chem. SOC.74, 4946 (1952). (5) Cerny, Y.)Jaska, J., Lablen, L., Collection Czech. Chem. Commun. 26, 1658 (1961). 16) Curti. R.. Colombo. L-., J . Am. Chem. ’ koc. 74; 3961 (1952).
(7) Curti, R., Colombo, V.,Clerici, F., Gazz. Chim. Ital. 82, 491 (1952). (8) Dickey, F. H., J . Phys. Chem. 59, 695 i 1 R.i.i’l. -\ -
( 9 ) Dickey, F. H., Proc. S a t . Acad. Scz.
C.S.35,227(1949).
(10) Erlenmeyer, H., Bartels, H., Hela. Cham. Acta 47. 46 (19641. (11) LIorrison, J, L.,‘Worlsey, &I.,Shaw, D. R., Hodgson, G. W., Can. J . Chem. 37, 1986 (1959). (12) Korthup, R l . L., Rogers, L. B., un-
published work, 1964. (13) Samelson, Be?. 33, 3479 (1900). (14) Snyder, L. R., J . Phys. Chem. 67, 2622 (1963). (15) Vysotskii, Z . Z., Divnich, L. F., Polyakov, XI. I-.$Colloid J . ( U S S R )23, 211
(1961) Eng. tr. (16) I-ysotskii, Z. Z., Divnich, L. F., Polyakov. hl. V., Proc. Acad. Sci. U S S R 39. 637 (1961) Eng. tr.; see also C . A . 56; 135773’(1962). (17) Waksmundzki, I. A., Oscik, J., Xlatusewicz, J., Nasuto, R., Rozylo, J., Przemysl Chem. 40, 387 (1961); C. A . 56, 20182 (1962). RECEIVEDfor review January 25, 1965. Accepted March 29, 1965. Supported in part, by the Kational Science Foundation’s Slimmer Research Program for Undergraduates and in part by a U. S. Atomic Energy Commission Contract AT( 11-1)1222.
Adsorption of Ions in Dilute Aqueous Solutions on Glass and Plastic Surfaces G. G. EICHHOLZ, ANN E. NAGEL, and R. B. HUGHES Engineering Experiment Station, Georgia Institute o f Technology, Atlanta, Ga.
b The relative adsorption, on glass and plastic surfaces, of some ions in highly dilute solution has been studied to permit an estimate of the error introduced into the analysis of natural waters by pipet and beaker contamination. Radioactive isotopes, including GI3’,SrgO, Ygl, Ce144,Ba-LaI4O, Zr95, 1131, and Ru-Rhlo6 were employed in these measurements. The surfaces compared included borosilicate glass and polypropylene beaker material and glass microscope slides. The effects of pH level and carrier concentration, particularly in hard waters, were investigated in detail, as well as several procedures for pretreating or coating the surfaces. The results indicate that, for most of the elements studied, it is preferable to use borosilicate glassware rather than polypropylene. Cesium, ruthenium, and zirconium in solution show less contamination in polypropylene beakers. Th,e total adsorption losses are small, except for yttrium and the rare earths, where some correction for contam ination losses may have to b e made in trace analyses.
A
OF IONS in tracer concentration on laboratory glassware has long been a well known problem in radiochemistry ( I , 6, 8, 1012 ) . Radiochemical procedures have been published that advise on the prevention of contamination of laboratory ware by the use of carriers, but many analytical procedures for lowlevel concentrations of inorganic ions in * aqueous solutions seem to ignore this factor coinl)letely, and it is difficult to find any quantitative information on the magnitude of such adsorption losses. DSORPTION
The present investigation arose from a requirement to determine the magnitude of the error introduced into the analysis of trace elements in natural waters by ion adsorption and to select a preferred container material both for water sampling and for subsequent preconcentration of the ions of interest ( 2 ) . The adborption of fission products on glass has been measured in some specific cases (6, 8, 1 1 , 1 2 ) , and there have been a few measurements, notably on chromate ions ( 7 ) ,to establish the efficacy of cleaning procedures. The most complete series of experiments on the adsorption of ions on laboratory ware, mainly from nitric acid solutions, is that of Starik and his collaborators (13). The mechanism involved has not so far found any satisfactory explanation. It is generally ascribed to the formation of the so-called “radiocolloids” ( I , 10); Haissinsky ( 4 ) , had to conclude that there is insufficient evidence to support any satisfactory theory for their formation a t the moment. Many of the difficulties encountered and reported elsewhere stem clearly from the difficulty of preparing consistently a clean and reproducible glass or plastic surface. At the concentrations under discussion, most cleaning agents will leave a small, though perceptible, film on the surface of sufficient extent to affect subsequent surface phenomena in a disconcerting fashion. I n addition, diffusion effects occur in the surface layers and affect the nature of the adsorption properties profoundly. For these reasons contamination measurements are usually considered to be valid only for th particular solutions involved and the particular material and surfaces, when cleaned in a sliecified manner.
The choice of experimental conditions was largely governed by the immediate requirements for an elucidation of those factors which are of importance in the analysis of highly dilute aqueous solutions in distilled and natural waters. All tests were done a t room temperature. The choice of test vessels poses a major problem since transfer losses to pipets and wall losses to beakers during the experiment may interfere seriously with test conditions and with evaluation of results run with supposedly standardized solutions. Polyethylene, polypropylene, Teflon, stainless steel, borosilicate glass, quartz, “soft glass,” and silicone-coated surfaces all adsorb appreciable quantities of certain ions. The cleaning procedure adopted often alters the surfaces of most materials and varies the degree of adsorption. New glass and plastic containers, often from the same batch or supplier, show great differences in adsorption properties even when handled and pretreated in an identical manner. These problems have been discussed in some detail by Yoe and Koch (14) and by Starik (13). The present work, of necessity, was confined to a more limited scope, Adsorption tests were run under two sets of conditions, low-level tests in distilled water to compare the relative adsorption rates on glass and polypropylene surfaces; and comparative tests with borosilicate (Pyrex) glass and polypropylene surfaces in hard water to observe the effect of high carrier ion concentrations on the adsorption characteristics. The determination of trace concentrations of strontium in milk and water is of widespread interest, but little information has been published on the adsorption of strontium on glass VOL. 37, NO. 7, JUNE 1965
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