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 L., Rogers, L. B., unpublished work, 1964.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
863
and plastic containers, though fairly extensive investigations have been done on yt’trium sorpt’ion. At p H values above 5 , yttrium forms colloids in lowconcentration solutions and as such adsorbs readily on glass surfaces. Klein, Harten, and Kaufinan (6) have found Y9l contamination of the order of 40% of the dissolved ions when solutions with p H above 5.0 were contacted with polyethylene. They observed that adsorption on polyethylene was significantly higher than on borosilicate glass. EXPERIMENTAL
The radioactivit’y of both solution samples and glass samples was determined by means of a low-background beta-counting system (Tracerlab Omni/ Guard system). For each of the isotopes studied. counter corrections were al)i)lied for the different, geometries presented by the slight differences in shape, t,hickness, and surface areas of the slides, resin, and evaporated samples. Count rates were high enough in most tests to assure statistical errors of less than 5y0. Materials Tested. T h e following laboratory materials were tested : microscope cover glasses, noncorrosive, 22 mm. square, 0.13-0.16 m m . thick (Fisher KO. 12-524) made from Corning Optical glass KO.0211, a zinc borosilicate glass with titanium added, as drawn, without polishing; Pyrex laboratory glass beakers, various sizes, made from Corning glass No. 1000, as received; and polypropylene laborat,ory beakers, molded without filler or release agent’ by the Yalge Co., Rochester, S . Y., from Bakelite 4500 polypropylene resin.
Table 1.
Soluf ion I)ist,illed water Ilistilled water Distilled water I)ist,illed water Simulated water Simulated water Simulated water Simulated water Simulated water
Initial 7.0 7.0 3.6
One-inch square sections were cut from recent high-luster transparent plast,ic and glass 1-liter beakers to serve as adsorption specimens. Throughout this paper reference will be made to the use of a “simulated natural water solution.” This composition for a synthetic hard water sample was taken from an averaged value for Ca+2, Mg+2, Na+, and K + ions as calculated from the complete analysis of 20 water samples ( 9 ) varying in total hardness from 15 to 400 p.p.m. as CaC03. The compounds used balanced up the anions between SO,-*, NO3-, C03-2, and C1-. Details of the composition of the solution are given elsewhere ( 3 ) ; it is equivalent, to 363 p.p.m. total hardness as CaC03 and represents a very hard water. The t,erm “carrier-free” radioisot’ope in all cases implies tracer concentrat,ions below 10-l1S; most of the radioisot,opes were obtained from Oak Ridge Kat,ional Laborat’ories. Procedure. T h e slides were cleaned with chromic acid, 0.1.Y oxalic, acid, “Alconox” detergent, alcoholic X a O H solution, or 20-30y0 HC1 for 30 minutes. After thorough rinsing in demineralized, distilled water they were mounted in clean Plesiglass holders designed to position a number of slides, spaced parallel, for immersion in the radioactive solution. The radioactive tracer was added to 500 ml. of hard or distilled water and stirred for 30 minutes. Initial pH and activity were determined, before introducing the slide samples, by taking 1ml. samples with a new disposable polystyrene pipet and evaporating to dryness in a 2-inch planchet. Even spreading over the bottom of the planchet was facilitated by adding a
Adsorption of Strontium-Yttrium-90 % Adsorution Der sa. cm
PH
Final 7.1 7.1
3.6
... ...
7.4 7.4 7 4 7.4 7. 4
7.4 7.8 7.8 7.9 7.9
Table II.
Contact time, min. 15 15 15 15 10 20 40 60 180
a U
h C
d d
864
Solution Dist. water ]list. water Ilist. water Ilist. water Sirnul. water Simul. water
ANALYTICAL CHEMISTRY
Glass (Pyrex) . . . .. ... . . .
0.02 0.03 0.03 0.02 0.02
Polypropylene 0.09 0.09 0.002 0,001 0.02 0.05 0,09 0.10 0.09
.
Glass (cover slides) 0.05 0.06 0.001 0.001 . . ...
...
... ...
Adsorption of Yttrium-91
PH Sample
.
Initial
Final
5 2
, . .
4.3
. .
n.d.
n.d. 7.6 7.6
4.4 7.5 7.5
...
Contact time, min. 10
20
10
10 10 20
% Adsorption per sq. cm. PolyGlass propylene 0 04 0 12 0 02 0 01 0 003 0 01 0 002 0 001 0 04 0 12 0 09 0 15
few drops of “;1erosol OT” wetting agent. All planchets and glass slides were dried on a hot plat’e at’ 100’ C. At various intervals after immersion, slides were removed, rinsed with distilled water, and dried for counting. Cross checks were run on the final activity of the solution and on the slides, and final pH readings were taken. Some additional experiments were done in the presence of mixed cation and anion exchange resins, which were also dried and counted in a planchet. RESULTS
Strontium and Yttrium. These two elements, though chemically highly dissimilar, are grouped bogether here because t’he major strontium tracer isotope, strontium-90, is accompanied by a short-lived daughter, yttrium-90, which is responsible for t h e main beta-ray activity detected from Sr9@containing solutions. T o separate the adsorption effects of these two elements, a distinct series of tests was run employing the yttrium isotope, yttrium-91. The effect of contact time and pH on the adsorption of Sr-Yg@on glass and polypropylene slides was determined with carrier-free distilled water and in simulated hard water. Table I presents duplicate results for distilled water a t two p H levels for tracer concentrations of Srg0-Yg0of the order of 10-1@-10-12LV, and also some similar results following t’he course of adsorption from simulated water a t a neutral p H on two materials. Adsorption on polypropylene was higher than on glass a t neutral pH; raising the content of carrier ions reduced the adsorption to some extent, but lowering the pH with HC1 was much more effect,ive. The slight initial peak in t,he glass adsorption curve is typical for many of the adsorption tests observed in hard water solutions and probably represents a gradual shift in equilibrium conditions. Yttrium-91 has been studied in more detail since it was evident’ that yttrium presents more of a problem in t’erms of adsorption losses than strontium. Table I1 shows some results on Ygl solutions (-10-l25) in distilled and hard water. The first three results (samples a and b) represent duplicate runs in fresh beakers and distilled water. Sample c was obtained in beakers that had been pretreated with part of the test solution for 30 minutes. Sample d shows comparative figures in hard water in pretreated beakers. Adsorption on the plastic beakers was consistently higher than for glass at the higher pH levels and kept rising even after extended exposure. When the beakers had been presoaked in the solution, some equilibrium was reached on the glass surfaces between the sorbed yttrium and the ions in solution after about 20 minutes, whereas adsorption continued on the plastic, presumably in multilayers rather than
% 0.03
4
, -
1
Figure 1 . Ygl adsorption on glass from distilled water, effect of pretreatment
0.05 N ,
€ 0.02
D: W
n
g+ 0.01 n
0.005
9 , hta
CONTACT TIME
0.Wl' 0.001
by diffusion, and such pretreatment for yttrium solutions does not seem to be practical in the time periods of interest. I n distilled water, less adsorption was observed, which suggested the formation of pseudocolloids in the hard water with some of the dissolved hardness ions acting as agglomeration nuclei. Figure 1 shows the results of some tests done to determine the potential advantages of reducing the adsorption on the beaker walls by exposing them to a solution of trivalent ions of comparable or higher concentration than t'he sample solution. Glass slides were soaked in inactive 0.01S Y(X03)3 and Ce(K03)3 solutions for prolonged periods, before being immersed in dilute Ygl solutions, a t a p H around 5 . The results were rather erratic, but, as Figure 1 shows, 30y0 and 60% reduct,ions in Y9l sorption were observed in the two cases plott,ed; however, on prolonged contact (24.5 hours) the pretreated slides act,ually adsorbed more yttrium from solution than the untreated control group. However, another group of glass slides run under essentially the same conditions after 18 hours of pretreatment in a 0.03S Ce(xOa)s solution gave a maximum adsorption value of Y91 of o . 0 7 5 ~ 0 in 30 minutes, about four times that of the untreated slides. Cesium. When a glass cover slide was brought into contact with dist,illed, demineralized water, containing Cs137 tracer, for several successive exposures, removed between exposures and count,ed, maximum adsorption was reached at, 2 hours and additional exposure failed to alter it. R'hen six cover slides were exposed in distilled wat'er with added Cs13' tracer, a very rapid initial adsorption was observed. One slide was removed and counted a t various intervals and the adsorption decreased aft'er 10 minutes and t'hen st'arted to increase again after an hour. I n a parallel test, five polypropylene slides were cleaned in alcoholic S a O H solution and exposed in distilled water containing Cs13' tracer. A slide was removed periodically and counted. The results showed very low, constant adsorption.
Carrier Concentration. The effects which various concentrations of a common carrier ion would have on Cs137 adsorption on glass were studied using CaClz solution with added The concentration of the CaC12 was varied from 0.0025,V to 0.045 and it was found (Figure 2) that increasing the carrier concentration reduced the adsorption. The experiment was repeated using lower activity and the carrier effect was essentially the same. As expected, with less activity the per cent adsorption increased. I n an additional test a slide was exposed in CsCl solution with added to see if use of the identical carrier ion would reduce adsorption. The results yielded insignificant contamination of the slide. Because much of this work was directed toward adsorption effects in analysis of waters of varying hardness, the simulated hard water was diluted to
Table 111.
Glass Tracer useda Cs'37 cs
Cs'37
CS'S'
Ce14'
Ce144 Ba140 La140
Zr/NbS6
cs cs
cs
Pyr
Waterb H H D
0 OlN CaCh 0 01s CaCL 0 Oln. CsCl D D H
Pyr
H D D H
Contact time, pH min. 6.8 15 3 3 15 4.1 5 4.6 10 4.7 30 4 6 120
a
Effect of
centration on
'
'
4
0.04N.
'
cac12carrier con.
adsorption
~ ~ 1 3 7
0 High activity 0
l o w activity
give hardnesses varying between about 5 and 350 p.p.m. Five Pyrex slides were placed in each solution, with Csl37 added, and exposed for 30 minutes. The slides were counted and, as seen from Figure 3 , as the hardness increased, the adsorption decreased. pH Dependence. Table I11 shows the effect of p H on the adsorption of Cs137on cover slides and polypropylene, in duplicate runs for 15-minute exposures in hard water. At pH 6.8 the glass became more contaminated than the plastic. When the p H was lowered to 3.3 and four more slides were intro-
H
5%
Adsorption
per sq. em. Glass Plastic 0 002 0.0006 0 001 0 0003
o o
004 004 0 003 0 003
6 5
30
0 004
2 0
30
0 0007
10
