'July 1949
INDUSTRIAL AND ENGINEERING CHEMISTRY CONCLUSIONS
1. The seed material can be readily developed from soil, and within a few days maximum efficiencies can be obtained. 2. The volume of seed material returned does not appear to be critical. A ratio of 2 volumes of waste to 1 volume of seed material has been used successfully. 3. The pH of the waste does not seem t o be critical. Raw waste with a p H of 6.4 gave as high reductions a s one adjusted to 7.2. The pH increases during aeration. 4. The percentage B.O.D. reduction decreases with wastes above 3000 p.p.m. of B.O.D., but the p.p.m. of B.O.D. removal increases. B.O.D. reductions of 90% have been obtained after 24-hour aeration with wastes of less than 1000 p.p.m. of B.O.D.; about; 80% may be expected with wastes up t o 3000 p.p.m. of B.O.D.; wastes with higher B.O.D. give lower percentage reductions. 5. 0 timum results have been obtained with an aeration rate of 2 cufic feet per gallon of waste per hour for 24 hours or of 3 cubic feet per gallon per hour for 12 hours for a waste with an initial B.O.D. of 3800 p,p.m.. Weaker wastes give equivalent percentage B.O.D. reductlons in shorter time and consequently require less air than sfronger ones. A wash water with
1415
an initial B.O.D. of 2160 p.p.m. required 1430 cubic feet of air per pound of B.O.D. removed. Spent broth with a B.O.D. of 3800 p. p. m. gave 70% reduction of B.O.D. with an air consumption of 2000 cubic feet per pound of B.O.D. removed. 6. Sludge may be formed through the sloughing off of side wall growths. Normally the quantity of sludge formed is small. 7. The effluent will have a higher turbidity than the raw waste. 8. The process may be used as a pretreatment unit ahead of other conventional biological units t o remove a substantial portion of the B.O.D. and to enable the secondary oxidation devices to operate without difficulty. LITERATURE CITED
(1) Am. Public Health Assoc., "Standard Methods for Examination of Watar and Sewage," 9th ed., 1946. RECEIVED May 10, 1948. Presented before the Division of Water, Sewage, and Sanitation Chemistry a t the 113th Meeting of the AMPXICAN CHEMICAL SOCIETY, Chicago, Ill. Journal Series Paper of New Jersey Agricultiiral Experiment Station.
Reactions of Ions in Aqueous Solution with Glass and Metal Surfaces STUDIES WITH RADIOACTIVE TRACERS JAMES W. HENSLEY', ARTHUR 0. LONG, AND JOHN E. WILLARD University of Wisconsin, Madison, Wis, Radioactive tracers are shown to afford a direct, sensitive and rapid means for studying the sorption of ions in solution on solid surfaces. Preliminary studies have been made of the effect of time of immersion, pH, temperature, and pretreatment of the surface on the sorption of sodium ions on soft glass. Tests have been made on sorption of sodium ions on fused silica, steel, aluminum, silver, and platinum, and of sorption of cesium and silver ions on soft glass. Determinations of apparent activation energy of sorption are used as a means of comparing sorption processes. Radioautographs produced by sorbed radioactive ions are suggested as a test for surface cleanliness.
A
GREAT deal of evidence indicates that glass surfaces are able t o take part in exchange and adsorption reactions with ions in aqueous solution ( 5 ) . Somewhat similar reactions may occur also a t metal surfaces. I n general, however, the methods by which evidence on such sorption processes has been obtained are indirect and tedious. (Sorption as used in this paper indicates any type of process by which ions from solutions may adhere to solid surfaces.) The authors have explored the possibility of using radiotracers as a tool to study such phenomena more effectively. The method consists simply of immersing small flat samples in a solution of thc radioactively tagged ion or molecule, removing, rinsing, and drying the samples and determining the intensity of the radioactivity on each with the aid of a GeigerM;iller counter. The pretreatment, of the sample, and the time, temperature, and pH of immersion were varied t o study the effects of these factors. Initial results, reported here, show that the method is direct, rapid, and sensitive, and is applicable to the study of processes which could not be studied by any other method, such as the exchange of sodium ion in solution with the 1 Present address, Research Department, Wyandotte Chemicals Corporation, Wyandotte, Mich.
sodium in a glass surface. Such studies may contribute to a fundamental understanding of surface phenomena and may be of practical importance. in connection with problems in such fields as those of detergent action, electroplating, mirror formation, glass electrode operation, and the catalytic effects of surfaces. Because the work of this paper is exploratory i t deals with the determination of the major effects of different variables rather than the quantitative evaluation of the effects. The results indicate that amounts of sodium ion ranging from approximately 0.01 monolayer to 10 monolayers are' picked up in the course of a few minutes or hours of exposure of soft glass, Pyrex, fused silica, steel, aluminum, or platinum to sodium nitrate or sodium carbonate solutions. [For the purposes of this paper a monolayer is arbitrarily defined as the number of ions required to cover the macro surface area of the sample if each ion covers an area equal t o the square of its ionic diameter (S).] The sorption of sodium ion on soft glass increases with time of immersion, with pH, and with temperature. It varies with the pretreatment of the glass but for two types of pretreatment, giving quite different rates of sorption, the rates show approximately the same temperature dependence, giving an apparent activation energy of about 10,000 calories per mole. The rate of loss of the radiosodium from the glass t o a water rinse solution is slow. The sorption on quartz decreases with increasing temperature in contrast to the effect with glass. Silver ion reacts with soft glass a t a rate similar to sodium ion, and reaction has been observed also with cesium ion. The sorption of carbonate ion by both the glass and metal surfaces tested is less than 0.01 monolayer. EXPERIMENTAL PROCEDURE
Radioactive Materials. Table 1 lists the radioactive isotopes which were used in this work, with their half lives, the chemical form in which they were used, the energy of the beta particle
I N D U S T R I A L A N D ENGINEERING CHEMISTRY
1416
about 3 mm. below the tube window, could be displaced about 3 mm. from the center position without appreciable counting loss, but that the counting rate fell off rapidly for greater displacements.
- the flame rather than to a chemical change of the surface by tlie flaming process. The data in support of these tentative conclusions are given in Figures 3 and 4 n-here the temperature dependence of the rate of sorption of sodium ions o n different types of glass surface is shown by plotting the logaritliiii of the rate of sorprion (given in monolayers per unit time) against the reciprocal of the absolute temperature. T h e rates of sorption a t different tcmperaturea are considered as proportional to the total sorption in equal times after the start of the ri'action, the times being on the nearly straight-line portion of the sorption against time-ofimmersion curves. The temperature dependence of the sorption reaction is indicated by the slope of t,hc curves of Figure 3 and 4 and by the apparent activation energies calculated from these slopes by the hrrhcnius equation. There is, of course, no evidence that the apparerit activation energies used here are true activation energies of any certain chemical reaction or reactions involved in the sorption. Among the difficulties involved in attempting to consider the data as representing a simple exchange type of reaction is the fact that the apparent equilibrium value for sodium sorbed is much higher at 90" t,han a t 25" C. The t,ent,ative conclusion that the rate determining step in the sorption reaction occurs a t or in the surface (as contrasted to alternatives such as diffusion or dehydration of ions) is suggested by the indieation that the apparent activation energy for sorption can be changed by pretreatment of the surface. This indication
Vd. 41, No. 7
is obtained b y comparison of the apparent activation energies (9200 to 10,000 calorics per mole) for the three series of samples in Figures 3 and 4, which were not presoaked, with that for the sample which was presoaked in 9 S hydrochloric acid (13,500 calories per mole). The second tentative conclusion, that the greater rate of sorption for flamed samples than for nonflamed samples is due to a further cleaning of the surface by the flame rather than to chemical change of the surface by the flaming process, is suggested by the data of Figure 4 which indicate that t,he apparent activation energy of sorption by flamed and nonflamed samples is the same although .the rate of sorption is much higher for th(, flamed samples. If the flaming process caused chemical changes in the surface such that the reaction Iyith sodium ion could t,alre place more easily, and if this rcaction was t,he rate controlling step in the sorption, it might be expected that the apparcmt activat,ion energy of reaction would be loweied. I n the experiments of Figure 3 all samples were prepared by washing with water and vapor degreasing with carbon tetrachloride, following which those represented on the upper curve n-ere flamed. The original purpose of the experiments of Figure 4 was to test the hypothesis that a glass surface where the normal sodiuni content had been replaced by hydrogen by prolonged soaking iia acid n-ould react less readily with sodium ion than a normal sodium glass surface. This hypot.hesis n-as suggested by the observation that a decrea.se in the pH of a n immersion solution causes a 'decrease in sorption of sodium ion. The data of Figure 4 indicate that the rate of sorption by the samples presoaked i n acid is uniformly less a.t a given temperature than those presoaked in water or those which were not soaked although tlie points for the n-ater-soaked samples scatter badly. The data of Figures 3 and 4, taken a t pH 8.7 and 7.1, respectively, suggest that the apparent activation energy is no? sensitive to p H over this range. Similar test,s a t a p H of 11 indicate a lon-er apparent activation energy of 6750 calories per mole. Removal of Sodium Ions Which Have Been Sorbed on Soft Glass. For the purpose of obtaining reproducibility in this v,-ork it was important to know whether the sorption observed on a sample n-as altered by variations in the time and conditions of the rinsing process used. Such information was needed also for the evidence which it could give as to whether the sorption was a rapidl5- reversible adsorpt,ion equilibrium or involved a stronger chemical binding. Table IV shoT7-s the results of three series of tests in v\hich the specimens included in each test had been immersed in radioactive sodium carbonate solution under identical conditions for iclentical times. The specimens >?-eretransferred from the active solution to the rinse water a t the same time but were removed iron1 the rinse water a t diffcrent times; folloir.ing the rinse they were dried and counted. K t h these samples there was II(> rapid loss of activity during periods of rinsing from 15 j .minutes, or from 0.5 minute t o one hour, or from 0.5 hour to 10 hours. The fluctuat,ion in the values for the individual specimens froin each series of tests is no greater than that which viould
Test 1 (\Tit11 Agiiation)
T e s t I1 Test I11 (mit,hoiit Agitation) (>vitlioiitAgitation) _________Activity o n .Ictiyjty 011 dctivjty on Rinsing sy)eciinen. Rinsing specimen, Finsing specimen, time, see. count/min." time, min. count/min.n 1iinc1, lir. c o u n t / m n . " 197 15 378 112 623 ?'ll 20G 30 35i 2 680 140 60 392 8 I511 2 167 120 400 32 532 240 357 64 330 10 147 Corrected t o a standard rime a t vhich 610 count,s \\.ere equivalcnt to o n ? monolayer.
INDUSTRIAL AND ENGINEERING CHEMISTRY
July 1949
have been expected from variations in the sorption process, if the several samples had all been rinsed for the same length of time. Multiple samples were used for these tests to avoid possible effects of intermittent rinsing and drying. Other rinsing tests have shown a relatively rapid removal of about one third of the sorbed activity and suggest that the percentage of sorbed activity removed per unit time of rinsing may depend on the length of time which the sample was immersed in the metal ion solution. SORPTION OF SODIUM IONS BY FUSED SILICA
The sorption of sodium ions from solution by fused silica Samples was determined in a few tests and the results were compared 5-ith those for soft glass samples. I n both cases the pretreatment was that of washing with water, vapor degreasing with carbon tetrachloride, and flaming. The data of Table V show that, under the conditions of these tests, the sorption by fused silica was less than by glass at room temperature and that the temperature coefficient of the reaction on fused silica was negative in contrast t o the positive coefficient for glass. This fact indicates that the mechanism of sorption by fused silica is different from that by glass and suggests a surface adsorption in contrast to an ionic exchange or other reaction involving bond breaking in the silicate network. The possibility cannot be ruled out that there is actually no activity picked up by the fused silica itself and that the observed activity was picked up by residual surface impurities which were leached away more effectively by the immersion solutions at the higher temperatures than by those a t the lower temperatures. The observed sorption on fused silica is shown by the data of Table I1 to be insensitive to the concentration of sodium ion in the range from 0.0057 to 0.045 M a t a pH of 9.5.
TABLE1'11.
SILVER ACTIVITYFROM SURFACE BY RINSING
REMOVAL O F
1419 S O F T GLASS
(Single soft glass specimen prepared by immersing in 3.9 X 10-6 'iA radiosilver nitrate, rinsed in distilled water a t room temperature) Activity on Rinsing Time, Hr. Specimen, Count/Min. After initial rinsing 233 169 20 (no agitation) 161 44 (no agitation) 149 72 (no agitation) 129 138 (no agitation) Additional 4 hr. In 1 N "Os 59
soaked in a similar hydrochloric acid solution without stannous chloride showed a lesser increase in pickup. As with sodium ion, the silver ion picked u p by soft glass was not rapidly removed by water. This is illustrated by the data of Table VI1 for a test sample which was subjected to successive rinses in water totaling 138 hours and an additional rinse in 1 N nitric acid. It is clear that the use of nitric acid causes a more rapid removal of the silver ion than does water but the rate of removal is still not rapid. SORPTION OF CESlUM ION BY SOFT GLASS
A limited number of tests made on the sorption of cesium ion on soft glass indicate that about 0.01 monolayer is sorbed in 5 hours immersion a t a p H of 5, room temperature, and 9.6 X IOp4 M CsNOa. The samples were prepared by wiping with dry filter paper and flaming. Adequate data are not yet available for comparison with the results on sodium ion but t h e work has indicated similarity in that the reaction with cesium ion has a positive temperature coefficient. The cesium activity which has been sorbed is removed more rapidly by rinsing with water than is sodium activity.
TABLE 5'. COMPARISOX OF SORPTION OF SODIVM IONS BY SOFT GLASS.4ND B Y FUSEDSILICAAS FUNCTION O F TEMPERATURE (oH. 8 6: Na Temperature, C. Soft glass, monolavers Fused silica, mon&yers
+. 0.0226 251; 30'
0 13 0.04
immersion time. 30 min.) 48' 69' 0.21 0.87 0.02 0.003
SORPTIOK OF SILVER ION BY SOFT GLASS
.A considerable number of tests made on the sorption of silver ion from water solutions by soft glass a t room temperature indicate that the reaction is similar to that of sodium ion. Observations made on the effect of pretreatment recorded in Table VI indicate that, as in the case of sodium ion, flaming greatly increases t h e reactivity of the surface. Heating in an oven at 200" C. produced an increase but to a lesser extent. Because i t is known t h a t soaking glass surfaces in a solution of stannous chloride and hydrochloric acid prior to silvering leads to the formation Qf improved mirrors, a test was made of the sorption of silver ion by glass surface which had been soaked in such a solution. I n 5 hours of immersion i t gave a sixfold higher pickup than untreated samples immersed for 27 hours. A comparison sample
TABLE VI.
SORPTIOX OF SILVLRIONFROM WATERSOLUTIOK BY SOFTGLASS
(.I 3 91 + X 10-6 , M; approx. neiitral solution:
room temperature; ail samples rinsed in liquid CCh before pretreatment) Length of Pretreatment Immersion, Hr. Monolayers 27.5 0.035 27.5 0.038 27.5 0 038 0.53 27.5 0.12 19.5 J
5
0.24 0.13
Figure
S. Radioautographs of Cesium Ions Sorbed on Soft Glass
Light areas indicate presence of radiocesium
Some glass specimens which were prepared by heating in an oven at 400" C. for 1 hour and then immersed for 3 hours in the radiocesium solution showed much higher sorption than had been previously observed; the sorption on one side of the specimen was sometimes tenfold as high as that on the other for no apparent reason. T o determine whether this increase was occurring as a homogeneous layer or in localized spots, radioautographs were taken which showed a pronounced nonuniformity of distribution of the activity on the surface; this suggested either that the surface is not thoroughly cleaned by the heating or does not remain clean, or that i t is not homogeneous in its composition or condition of strain. Examples of such radioautographs are shown in Figure 5. These were prepared by allowing glass squares with a sorbed activity of about 10,000 disintegrations per minute to stand in contact with Eastman Eo-Screen x-ray film for 24 hours. The reproduction in Figure 5 is a positive print in which the light areas indicate the presence of sorbed radiocesium. During the heat treatment of these samples in the furnace prior to immersion in the radiocesium solution, they were mounted on edge in a glass frame in a glass beaker in such a way that i t appeared that all prevailing
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
1420
conditions were uniform over the surface and were the same for both faces.
Vol. 41, No. 7
O F ACTIVITY TABLE Ix. REMOVAL
F R O M STEEL
SORPTION OF SODIUiM ION BY METALS
Although i t did not seem probable that sodium ions would react with or adhere to the surface of less active metals immersed in solutions of sodium carbonate, a series of exploratory experiments was made to test this possibility. The results obtained with steel, platinum, silver, and aluminum are indicated by typical data given in Table VIII, with values for soft glass specimens included for comparison. All four of the metals, ranging from the electropositive aluminum, which reacted with the immersion solution, to the noble metals, silver and platinum, showed sorption. Results of tests with metals have varied considerably but all have been in general agreement with those of Table VIII. No visible effect on the surface occurred in the case of the steel, silver, or platinum. Much more information is needed before these somewhat surprising results can be adequately interpreted, but the present data are sufficient to suggest that effects which are not generally considered occur a t metal surfaces in solutions of ions.
TABLE 1-111.
SORPTIOK O F S O D I U X I O K OK
METALsa
(0.028 M solutions of sodium carbonate) Sodium Pickup, Immersion Monolayers a t O C. Time, Material Hr. 25 90 Glass (soft) 1 0.54 2.5 4 0.82 5 0 12 1. o 6.2 1 0.22 0.45 Steel 12 0.33 0.54 4 0.15 Platinum 0.14 12 0.3 ... 1 Silver 7.1 ... 20 min. Aluminum 1.5 5 min. 0:7Q a Values for sodium ion sorption were obtained by resolution of decay curves, as necessitated by t h e presence of radioactive impurity.
...
A considerable amount of data on the concentration and temperature relations in the sorption of sodium ion on metals was made valueless by the discovery that a radioactive impurity (discussed in a later section) present in tracer concentration in the sodium carbonate solution was being picked up preferentially by metals. Thus, the amount of sodium ion picked up was uncertain and large errors were made in decay corrections because of the difference in half life of the impurity and the Study of the sorption of sodium on metals under these conditions necessitates a determination of the decay curve for each specimen. Removal tests made on the platinum showed that an appreciable portion of the activity remained after rubbing with filter paper, immersing for 2 hours in fuming nitric acid, and flaming to redness. It was removed with the aid of aqua regia (Trhich attacked the platinum) or by polishing with abrasive. Tests were made on the removal of the activity from steel samples by rinsing in water. Several samples which had been prepared by identical cleaning and immersion in radiosodium carbonate solution were immersed in the rinse solutions a t the same time and withdrawn after varying time intervals. The counting rates of these samples, given in Table IX, indicate that the activity is slowly removed by the rinse solution. Dipping such samples in 10% sulfuric acid for 10 seconds at 40" C. (producing visible attack) removed about 95% of the activity. Immersion of such a specimen in boiling water for 2 minutes removed a negligible portion of the activity. About 10% of the activity could be removed by wiping vigorously with filter paper. The activity retained by the steel may be held by the impuiities such as sulfides which normally occur in a steel surface. I t appears, however, that it is not associated with carbonate from the sodium carbonate immersion solution as tests with radiocarbonate ion seem to indicate that steel sorbs carbonate ion much less readily than sodium ions.
Test I (with Agitation) Rinsing time, Activity, sec. count/min. 15 162 30 163 60 952 120 114 240 115
Test I1 (without Agitation) Rinsing time, Activity, mln. count /min. 0 , .i 118 2 103 8 49 32
64
41 43
SORPTION O F AVIOh-S
It is assumed generally that negative ions do not react with a clean glass surface as readily as positive ions and this fact was confirmed by a few tests made on the sorption of radiocarbonate in 0.004 M sodium carbonate solution by soft glass and of radiobromine ion in 0.003 M potassium bromide solution by soft glass and Pyrex samples. The maximum sorption of carbonate by soft glass amounted to less than 0.01 monolayer after 1-hour immersion a t 90" C. or at room temperature. KOsorption of bromide ion was found after immersion for 12 hours at 25 O C. and 1.5 hours a t 90" C., though as little as 4 X 10-4 monolayer could have been detected. Tests were made also with steel specimens immersed in the 0.004 M radiocarbonate solution for 1 hour a t room temperature and a t 90' C. The maximum sorption found at room temperature amounted to about 0.01 monolayer and a t 90" C. about 0.04 monolayer. Difficulty was experienced in these tests because of the tendency for steel to rust in the dilute sodium carbonate solution (rusting does not occur in the more concentrated solution used in radiosodium tests) and the rust tends to carry carbonate activity. Consequently the lorn activities found on steel specimens may have been carried in rust films. even though specimens counted appeared bright to the eye. SORPTION O F RADIOkCTIVE IRIPLRITY
9 s mentioned in a previous section, i t has been observed in the case of some of the samples which had been immersed in radiosodium carbonate solutions that a long-lived radioactive species remained after the short-lived S a 2 4(14.8-hour half life) had decayed. This impurity activity was shown to contribute only about 10-5 of the total activity of the radiosodium shipment when it was received in Madison 2 days after bombardment. I t a as found, however, that nith both glass and metal immersion samples, the activity picked up from the solution contained a higher ratio of impuritv to sodium than did the solution itself-that is, the sorption reactions showed some preference for the impurity and concentrated it relative to the sodium. Tablc S summarizes the data which shox this concentration effect. The highest preference was shown by steel for which the ratio of impurity to sodium was increased by a factor of 1.8 X lo5 on the surface as compared to the solution Although there nas a measurable amount of the radioactive impurity sorbed on glass specimens, the ratio of impurity to sodium activity on the glass was generally so low that the impurity activity caused no appreciable error in the measurement and correction of sodium activity for decay. I N SOLUTION TABLE s. REL.4TII-E C O U N T I S G RATEO F IMPURITY ASD ox IMVCRSION SPECIMENS
(Two days aftei neutron bombardment) Relative Counting Rate, Impurity/Sodium 0.00001 I n solution 0.01 On glass 0.5 On platinum 0.14 On aluminum 1.8 On steel 0.06 On Eilver
July 1949
INDUSTRIAL AND ENGINEERING CHEMISTRY.
To determine whether impurity might be present as a centrifugable radiocolloid an aged sample of solution of the sodium carbonate in which the radiosodium had decayed to background was centrifuged in a 15-ml. cone for 2 hours a t the top speed of an International clinical centrifuge. Aliquots were then carefully pipetted from the top, middle, and bottom fractions of the solution and prepared for counting. No evidence was found of any increased concentration in one layer as aresult of the centrifugation. Identification of the impurity adsorbed on specimens has not been completed. Decay and absorption studies made with thick samples of sodium carbonate in which the sodium activity had decayed to a negligible amount, have indicated a species with a half life of the order of 100 days with two beta components, one of about 0.18 mev. and the other of about 1.2 M.E.V. or greater, as estimated from the visual ranges in aluminum ( 2 ) . The softer component is in excess of the harder one, the ratio being of the order. of 50 to 1. Gamma activity is absent or very low. Decay curves have indicated another active species of shorter half life, which has not been further investigated. DISCUSSION
‘
It has long been known from many types of evidence that glass surfaces may be attacked by both atid and alkaline solutions and also that base exchange types of reaction may occur with various metal ions in aqueous solutions to which the glass is exposed. A clean glass surface is assumed to consist of a silica network in which the units are held together by Si-0-Si bonds and in which there are also Si-OH, Si-ONa or similar groups. It is generally believed that the effect of acid on glass is to convert groups of the type Si-ONa to hydrated Si-OH groups of the silica gel type. Under carefully controlled conditions this process is used to leach nearly all the alkali from glass as a step in the formation of high silica glasses. Following removal of the alkali the glass is heated and shrinkage occurs, probably due to conversion of Si-OH bonds to Si-0-Si bonds with loss of water. I n contrast to acid attackon bonds with glass the attack of alkali is believed to break Si-0-Si consequent formation of soluble silicates and an eating away of the glass surface. Among the sources of evidence that metal ions foreign to the glass network may be incorporated in the network by base exchange reactions in aqueous solution is the fact that test tubes which have contained solutions of silver nitrate or other heavy metal ions show bactericidal action on cultures placed in them, even after thorough rinsing (6). Another indication of such exchange is the fact that glass surfaces soaked in solutions of certain metal ions attain improved characteristics for the deposition of silver films, or become less catalytic in the decomposition of unstable reagents which may be stored in them. Radiochemists have long recognized that glassware which has been used for solutions of high activity retains activity even after thorough washing and that this activity may slowly come out of the glass and contaminate fresh solutions which are placed in the container. There is evidence that sodium ions in a glass silicate network may actually migrate through the network under the influence of an electric field in a process such as that used for introducing pure sodium into an evacuated container by electrolysis of molten sodium through a glass vacuum wall ( 1 , k ) . Many of the chemical effects which occur a t glass surfaces take place in such a thin layer of material (they might be called twodimensional reactions) that the number of moles of material reacted cannot be determined chemically, or if so only with great difficulty. Consequently much of the available information with regard to these reactions has been obtained from indirect measurements such as contact angles of droplets on the surface, refractive indexes of films and electrical methods ( 5 ) . Such methods are often tedious and their interpretation in terms of the chemical reaction which has occurred is difficult. The use of radioactive tracers offersa direct, rapid, and quantitative chemical method for the study of the adsorption and exchange of ions a t glass surfaces,
1421
Until more extensive data and a better understanding of the causes of random variations in the data are obtained it is premature to do more than speculate as to the implications of the present results. These results include the facts that several monolayers of the surface may undergo exchange with sodium ion in solution (Figure 1)during only a few hours of immersion and that the number of monolayers readily available to enter into exchange equilibrium with the solution increases rapidly with increasing temperature (Figure 1) but demeases slowly as a function of increasing hydrogen ion concentration in the solution (Figure 2) or in the glass surface (Figure 4). The data also suggest the possibility that the activation energy for reaction of sodium ion with a hydrogen glass surface is greater than for the reaction with the sodium glass surface and that the greater reactivity observed for glass samples which have been flamed is due, not to changes in the surface which allow sodium ion to react with a lower activation energy, but rather to an increase in the number of groups which are exposed in such a way that they can react. It is not improbable that one or several processes other than the base exchange reaction itself may play a role in governing the observed pickup of sodium ion. Such processes include the diffusion of the hydrated sodium ion through the film of liquid on the surface of the glass, the dehydration of the ion as a function of temperature, the changes in hydration and bonding structure of the glass which may occur as a function of temperature and of pH, and the process of diffusion of ions into and out of the interior of the glass. APPLICATIONS
The work reported here has been designed to give basic information about the properties of glass and metal surfaces. I n addition to contributing to this end the results obtained suggest the possibility of working out practical tests for the cleanliness and uniformity of surfaces based on their pickup of ions from aqueous solution and on the distribution of the retained activity as shown by radioautographs. SUMMARY
It has been shown with the aid of radioactive tracers that the
reaction of sodium ion with soft glass surfaces occurs to a n increasing degree with increase in time of immersion, pH, and temperature. Under some conditions the amount sorbed is several times the amount which would be necessary to cover the macro surface area of the test Specimen if each ion covered a n area equal to the square of its ionic diameter. The amount is in part determined by the pretreatment of the specimen and in particular is increased by flaming the sample. The removal of activity from the specimens by rinse solutions is a slow process. Sorption and removal of silver ions and cesium ions a t glass surfaces have been observed also. Sodium ion has been found to be picked up by aluminum, iron, silver, and platinum specimens immersed in sodium carbonate solutions, and a radioactive impurity present in the sodium carbonate solutions is concentrated on the surface relative t o the sodium. The radioautograph technique allows a study of the distribution of activity on the surface and may be the basis of a useful test for cleanliness and uniformity of surfaces of a variety of materials. ACKNOWLEDGMENT
This work has been supported by Wyandotte Chemicals Corporation and AMERICAN CHEMICAL SOCIETY fellowships and by Wisconsin Alumni Research Foundation funds allotted by the University Research Committee. LITERATURE CITED
(1) B u t , R.C.,J.Optical SOC.A m . , 11,87 (1925). (2) Glendenin, L.E., Nucleonics, 2,26 (1948). (3) Pauling, L.,“Nature of the Chemical Bond,” pp. 343-50,Ithaca, N. Y., Cornel1Univ. Press, 1945. (4) Strong, J., “Procedures in Experimental Physics,” p. 536, New York, Prentice-Hall, Inc., 1945. (5) Weyl, W. A.,Glass Ind , 28,231,300,349,408 (1947). RECEIVED August 23, 1948. This paper is based, in large part, on a paper by the same authors presented before the Division of Physical and Inorganic CHEXICALSOCIBTT, Chemistry a t the 113th Meeting of the AVERICAN Chicago, Ill.
