Concentration and Separation of Trace Metal Cations by Complexation on Polyamine-Polyurea Resins Joseph Dingman, Jr., Sidney Siggia, Carlos Barton, and Kathryn B. Hiscock' Department of Chemistry, University of Massachusetts, Amherst, Mass. 01002 Polyamine-polyurea resins are used to complex heavy metal cations from dilute solutions. The resins are prepared by polymerizing and cross-linking various polyethyleneimines of known molecular weight with toluene-2,4-diisocyanate. The monomer polyamines studied are ethylenediamine, triethylenetetraamine, tetraethylenepentamine, and polyethyleneimines of average molecular weight 1200 and 1800. The effects of pH, equilibration time, resin cross-linking, and monomer units are considered in a batch equilibrium study on the chelation of Cu2+, Ni2+, Znz+, and Co2+. The metal analyses are accomplished by atomic absorption spectrophotometry. By using one resin in a column mode of operation, copper, cobalt, and nickel ions have been quantitatively concentrated, recovered, and determined from known aqueous solutions having concentrations as low as 4 parts in 10'0 with concentration factors as high as ~ 1 0 0 0 . High concentrations of alkali and alkaline earth metals, since they are not complexed by these resins, do not compete with the complexation of trace heavy metals. This is a desirable capability for trace metals analysis in natural aquatic systems both fresh and saline.
IN THIS INVESTIGATION a family of polyethyleneimines (PEI) are polymerized and cross-linked with toluene diisocyanate (TDI) to yield insoluble polyamine-polyurea resins which are utilized in their free amine form to complex heavy metal ions from dilute aqueous solutions. NH,(CH,CHzNH)xCH,CHzNHz CH&sHa(NCO)z
=
+
-[NH(CHzCH?NH)x X
CHzCHzNHCONHCH 3C6H sNHCO] - (1) Although the isocyanate-amine condensation reaction is known (1-.3), never before has a comprehensive study of the reaction of metal ions with a polyamine-polyurea resin of this type been made. Anion-exchange resins of PEI cross-linked with epichlorohydrin ( 4 ) , with allyl chloride (3, and with ethylene dibromide (6) have been synthesized. Nonagaki et al. ( 7 ) complexed copper and cobalt ions onto a polymer of PEI cross-linked with ethylene dichloride t o investigate anion exchange on the metal ion itself. Andelin and Davidson (8) studied the absorption of cupric and mercuric ions by Amberlite IR-4B, a weak-base anion exchange resin. Shepard and Present address, American Cyanamid, Stamford, Conn. (1) H. Ulrich, U.S. Patent 2,222,208, to General Aniline and Film Corporation (1941); Chem. Abstr., 35,1903. (2) P. Esselman, German Patent 701,003, to I. G. Farbenindustrie A.G. (1940); Cliem. Abstr., 35,7581. (3) G. Johnson, British Patent 509,334, to I. G. Farbenindustrie A.G. (1939); Chem. Abstr.,34,3848. (4) W. Hagge et al., Belgium Patent 622,716, to Farbenfabriken Bayer A.G. (1963); Clzem. Abstr., 58, 14242. (5) D. P. Sheets, U.S. Patent 3,134,740 to Dow Chemical Company (1964); Chem. Abstr., 61,7197. (6) J. Shepard and J. A. Kitchener, J . Chem. SOC.,1957,86. (7) S. Nonogaki, S. Makishima and Y. Yoneda, J . Phys. Chem., 62,601 (1958). (8) J. Andelin and N. Davidson, J. Amer. Cliem. SOC.,75, 5413 ( 1953).
Kitchener (6) also noted the possibility of heavy metal ion chelation by cross-linked polyamine resins. More recently, Egawa and Saeki ( 9 ) have synthesized a chelating resin containing polyamine groups attached to a styrene-DVB co-polymer which was found to complex Aua+, Hgz+, and Cu2+ strongly. Muzzarelli and Tubertini (10) have studied the complexation of trace metal ions from aqueous and organic solutions by chitin, a naturally occurring polymer containing both glucosamine and N-acetylglucosamine groups, and by chitosan which is completely de-acetylated chitin. One of the advantages of the polyamine-polyurea resins discussed in this paper is that they can function in a compleximetric mode; hence they will tie up cations preferentially according to the stability constants of the cation complex formed with the resin. This difference in the stability constants is desirable since the metallic elements are strongly held on the resin but the alkali metals and alkaline earths are not. Hence one can isolate, concentrate, and measure small quantities of the metallic elements in the presence of large quantities of alkali and alkaline earth elements. This capability is of particular importance in the analysis for trace metals in natural waters (both sea and fresh water) since the heavy metal concentration is low but the alkali metals and alkaline earth elements are high. In addition to the aforementioned selectivity, the polyamine moiety will release complexed metals upon acidification. In fact, by manipulating the pH within a given range, one can control the stability constant. At low pH, the metals are essentially completely released. Another advantage of these resins is that they are easily prepared; they can be made in several modifications and to various degrees of cross-linking. Only the amino nitrogen is involved in the complexation. Resins made from TDI and ethylenediamine have only urea linkages. These resins exhibit no complexation. Also, resins made from PEI samples of higher molecular weight complex larger amounts of metal ions per weight of resin. In addition, the more cross-linked the resin, which occurs at the expense of available amino groups, the less is the capacity of the resultant resin. The monomers used in this study, ethylenediamine (EDA), triethylenetetramine (TETA), tetraethylenepentamine (TEPA), and polyethyleneimines (PEI) with molecular weights of 1200 (PEI 12) and 1800 (PEI 18), enter into condensation polymerization with TDI forming insoluble resins. The resins are used to sequester heavy metal ions from dilute aqueous solutions. The order of stabilities of the chelates formed on the resin is much the same as their respective soluble metal-ammonia complexes. In order to characterize the resin-metal ion relationship, absorption isotherms were obtained for a number of metals. In this case a batch type equilibrium procedure was employed. The amount of resin, the solution (9) H. Egawa and H. Saeki, Kogyo Kagaku Zasshi, 74, 772 (1971);-Chem. Abstr., 75, 64935. (10) R. Muzzarelli and 0. Tubertini, Talarira, 16, 1571 (1969). ~
ANALYTICAL CHEMISTRY, VOL. 44, NO. 8, JULY 1972
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ture of 80% of the 2,4 isomer and 20% of the 2,6 isomer. The polyamine monomers were obtained from a number of sources-EDA from Fisher Scientific Co., TEPA from Eastman Kodak, and TETA, PEI-12, and PEI-18 from Dow Chemical Co. The metal stock solutions were prepared by weighing the appropriate amount of dried metal acetate salt to give a n aqueous solution which was 1000 mg/liter (ppm) in the metal ion. The stock solutions as well as all other concentrations were stored in polyethylene containers. The lower concentrations of metals needed for standards and test solutions were prepared by making the appropriate dilutions of the stock solutions with de-ionized, distilled water. A CuUigan-mixed bed de-mineralizer was used t o de-ionize the distilled water. Procedure. RESIN SYNTHESIS. The following methods were utilized in the production of the tetraethylenepentamine (TEPA) resin. The remaining polyamine resins will be discussed later. Single Addition Method. One liter of 0.11M TDI in dried dioxane was added dropwise to a stirred one-liter dioxane solution of 0.10M TEPA at room temperature. The resin immediately precipitated from solution. Stirring was continued overnight after all the TDT was added. Continuous Flow Method. One liter each of O.llM TDI and 0.10M TEPA in dried dioxane were separately passed a t the same rate into the arms of a Y-tube. The reaction immediately took place a t the intersection of the tube and from there, the resinous product flowed into a receptacle where it was stirred overnight. Simultaneous Addition Method. One liter each of O.11M TDI and 0.10M TEPA solutions in dried dioxane were added dropwise and at the same rate from separate reservoirs into 1 liter of stirring dioxane. A precipitate formed immediately upon addition of only a few drops of the reactants. After the addition was completed, the product was stirred overnight. In all methods the polymer was washed with repeated charges of 95 ethanol and distilled water until the wash was clear of reactants and dioxane. This procedure entailed usually five 2-liter charges each. This was done by repeatedly decanting the previous wash, adding fresh wash, stirring, and decanting after the polymer had settled. The polymer was dried at room temperature over phosphorous pentoxide under vacuum. The dried resin was ground and passed through a series of sieves ranging from 20-80 mesh. The 60-80 mesh size was used in all the metal uptake studies presented. ANALYSIS FOR METALUPTAKE. A batch type equilibration procedure was used in the determination of the metal uptake from solution by the resin. Throughout this study, 50 mg of the 60-80 mesh size resin was placed in a 50-ml flask, to which was added 10 ml of the metal solutions whose concentrations ranged from 100-800 ppm. The two phases were shaken a t a constant rate on a mechanical shaker for 24 hours. The supernatant solution was then analyzed for the remaining metal not complexed by the resin. The resin samples weighed 50 mg =t0.5 mg. The volume of fhe metal solution was kept constant at 10 ml for all metals and concentrations investigated. All data was collected at room temperature. After shaking for 24 hours, the two phases were separated by centrifugation. The supernatent was then analyzed for the metal using atomic absorption spectrophotometry. A standard curve was made from known dilutions of the stock solution and dilutions of the experimental solutions were made to give a result within the linear portion of the standard curve. The optimum working ranges for the metals studied were the following (11):
o o
‘ O t , 0 100
200
300
400
500
600
700
800
900
ppm C u t +
Figure 1. Effect of TEPA resin synthesis method on absorption of cuz+ U Simultaneous addition method 0
Single addition method
0 Continuous ffow method
volume, the contact time with the resin, the temperature, and the amount of agitation were kept constant while the concentrations of the metal ions were varied. These parameters are discussed in more detail later. Alkaline earths and alkali metals are not complexed by these polyamine resins, as is expected. The p H level plays a n important role in the uptake of heavy metal ions by the resins; as well as, in reverse, desorption of metals from the resins. Consequently, the potential is available for the separation of metals by a gradient elution mode of operation by adjusting the p H of the eluent. Since the urea linkages in the resin d o not enter into metal chelation, there is a strong dependence on metal uptake by the resin t o the concentration ratio of the reactant species in the polymer synthesis. The greater the cross-linking of the resin (accomplished by increasing the TDI concentration in the reaction), the less is the capacity of the resulting resin for metal ions. Finally, in a column type operation using the TEPA resin, cobalt, copper, and nickel ions have been quantitatively concentrated, recovered and analyzed from known aqueous solutions having concentrations as low as 4 parts in loxoof the metal ion with a concentration factor as high as X 1000. The concentration of trace metals up t o the working range of existing methods of analysis is probably the most important role of the polyamine-polyurea resins which have the added advantage of being simple and inexpensive t o produce. Although interest lies predominantly in the metal ion-amine relationship in this study, one must realize that anions accompany the metal cations into the resins in order t o preserve electroneutrality. The nature of the anion could be very important t o the sequestration of metal ions, but, as yet, has not been fully exploited. PART I. ABSORPTION OF METALS ON POLYAMINEPOLYUREA RESINS EXPERIMENTAL
Apparatus. A Perkin-EImer Model 290 atomic absorption spectrophotometer, a single beam instrument, was used for the metal analyses in Part I. Reagents. The dioxane was stored over K O H pellets for drying purposes. Commercial grade TDI contained a mix1352
ANALYTICAL CHEMISTRY, VOL. 44, NO. 8, JULY 1972
(1 1) “Analytical Methods for Atomic Absorption Spectrophotometry,” Pub. no. 990-9461, Perkin-Elmer Corporation, Norwalk, Conn., 1966.
.
cu Zn Ni co Ca
2-20 ppm 0.2-3 ppm 2-25 ppm 4-40 ppm 1-10 ppm
The amount of metal in the supernatent subtracted from the amount initially added gave the amount of metal absorbed by 50 mg of resin. This was re-evaluated to give the more standard expression of milligram metal uptake per gram of resin.
Polymer Synthesis. The continuous flow method and the simultaneous addition method attempt to overcome a major drawback in the single addition method. When TDI is initially added to the total amount of TEPA, in the single addition method, its concentration is immediately reduced with respect to the latter. Thus, the polymer initially is less cross-linked than would be expected from a 1.1 :1.O, TDI : TEPA ratio. As polymer begins to precipitate from solution, the TEPA concentration in the reaction is diminished while the freshly added TDI becomes more concentrated relative to the TEPA. Toward the end of the TDI addition, whatever unreacted TEPA is left will be at a relatively low concentration. Of course, this premise is based on the belief that TDI reacts slowly with the insoluble resin. After stirring for 12 hours, any excess TDI will react with soluble, low crosslinked polymer. From the copper uptake data by these resins (see Figure l), it can be seen that the continuous flow method produced the resin with the highest capacity. The order of resin method and copper uptake was in agreement with apparent crosslinking and yields. The higher the cross-linking, the less was the copper uptake since the urea linkages do not complex metals (ethylenediamine-TDI resin did not complex metals). The simultaneous addition method gave the highest yield of resin (95 %), whereas the other two methods gave about 60 yield, due probably to the formation of soluble, low crosslinked polymers. The high yield resin did not swell nearly as much as the other two and it was successfully employed in a column without having flow stopped from swelling. The first two methods produced resins that were difficultto employ in a column since they swelled greatly in acid. Batch Equilibration. The contact time between the metal and the resin must be constant in order to correlate all the data. Sheperd and Kitchener (6) found that for a n aliphatic cross-linked polyethyleneimine resin, the time required for small inorganic ions such as hydrochloric acid to reach equilibrium was 10 days, and 14 days for sulfuric acid. However, they were using resin rods and diffusion was slow. Kunin and Meyers (12) thought a 48-hour equilibration period was sufficient for their work with anion exchange equilibria using Amberlite-IR-4B, an amine type resinous exchanger. A study of our own resin (TEPA resin) showed only a very slight increase of metal uptake between 24 and 48 hours. The 24-hour contact time was taken as a constant parameter in our investigation. Figure 2 shows a time study using zinc ion uptake as the criterion for equilibration time. Uptake of Some Metals by the TEPA Resin. The uptake of Ca2-, CoZT,Cu2-, Ni*+, and ZnZTwere investigated in the 100-800 ppm metal concentration range. Their absorption isotherms are given in Figure 3. The TEPA resin used in this particular study was made by the single addition method and (12) R. Kunin and R. Myers, J . Amer. Clitm. Sac., 69,2874 (1947).
I
I
RESULTS AND DISCUSSION
50
0
100
200
300
400
ppm Zn++
Figure 2. Effect of batch equilibration time on absorption of Zn by TEPA resin 0 One-hour contact time 0 24 hours 0 48
hours
7 6o
5 K m LI
50
t
-
40-
-
*m $ 30+ 1 0
PO-
E
10.
V I100
'0
I
I
I
200
300
400
I
I
I
I
I
500
600
700
800
900
I
M + + Conc.(pprn)
Figure 3.
Absorption of heavy metal ions on TEPA resin 0 CU2f 0 E
Ni2+
Zn2+ m C02' Ca2+ is not absorbed over this concentration range the analyses for the metals followed the above procedure. From this graph, it can be seen that copper was complexed more strongly than any of the other metals and that the order of decreasing uptake was in the same order as their respective aqueous ammonia stability constants, where Cu > Ni > Zn > Co >> Ca. As expected, calcium was not complexed by the polyamine resin. The Effect of Polyamine Monomer on Copper Complexation. Studied in this investigation was the complexation of four polyamine-polyurea resins with copper. The four different polyamine monomers were triethylenetetraamine (mol. wt,, 146.18), tetraethylenepentamine (mol. wt., 189.31), PEI-12 (mol. wt. 1200), and PEI-18 (mol. wt., 1800). The latter two polyethyleneimines are highly branched polyamines. They are prepared by acid catalyzed polymerization of the monomer ethyleneimine. The monomer units of the latter two PEI polymers have two carbons per nitrogen, and these units are randomly distributed in the approximate ratio of one primary amino nitrogen per two secondary amino nitrogens per one tertiary amino nitrogen (13). (13) Dow Chemical Company, Form No. 125-965-68, Midland, Mich., 1966. ANALYTICAL CHEMISTRY, VOL. 44, NO. 8, JULY 1972
1353
I
0.01
I
I
I
1.0
5000
1000 2000 3000 4000 C o n c e n t r a t i o n o f O r i p f n a l Copper Solution ( p p m l
0
I
I
I
I
I
' ' ! I
1.2 1.4 16 1.8 Mole Ratio o f Polymer
1
2
Figure 6. Basicity of TEPA resin given in milliequivalents/gram of dry resin as a function of the mole ratio of the polymer reactants, TD1:TEPA
Figure 4. Cu2+ uptake by four polyamine-polyurea resins. Resins are denoted by their polyamine monomer PEI-12 PEI-18 TEPA 0 TETA
I
30 70.0 I K D
;2 o
-
3 D
1
2 IO
0
IO
20
30
4.0
50
60
70
80
9.0
PH
001 IO
I
12
I
I
I
I
I
I
I
14 16 18 20 22 24 26 Mole R a t i o o f T D I : Tefraethylenepentamlne
I
28
I
30
Figure 5. Uptake of Cu2+/gram of dry resin from a 700 ppm Cu2+ solution as a function of the TEPA resin cross-linking. The greater the TD1:TEPA mole ratio, the greater is the expected cross-linkingof the resin formed
The polyamines were polymerized and cross-linked with TDI having a reactant molar ratio of 1.1 TD1:I.O polyamine and were prepared by the single addition method outlined above. The effect of the polyamine resin type on copper uptake is given in Figure 4. The concentration ranges were extended to 5000 ppm copper due to the increased uptake of the higher molecular weight polyamines. The procedure for analysis is described above, under the heading, Analysis for Metal Uptake. The order of complexation capacity of the resins, designated by the polyethyleneimine from which they were prepared was PEI-18 (at low concentrations), PEI-12, TEPA, TETA. Although the dependence of copper uptake on monomer length of the polyamine is graphically seen, there is no relationship between the copper uptake and the number of available amino nitrogens for complexation on each polymer, bearing in mind that the urea linkages do not complex metal ions. The TEPA resin is calculated to be the most efficient in copper uptake per amino content with the TETA resin next in line. The PEI-12 and PEI-18 resins are well below their expected capacities, compared to the lower molecular weight poly1354
ANALYTICAL C H E M I S T R Y , VOL. 44, NO. 8, J U L Y 1972
Figure 7. Dependence of heavy metal ion uptake by TEPA resin on the pH of 500 ppm metal ion solutions. The pH was adjusted with nitric acid only 0
Cu2+;0 NP+; 2 Zn2+; B Co2-
amines. It could be that the 24-hour equilibration period is not sufficient, and/or that a certain geometric relationship of the amino nitrogens on the higher branched polyamine resins necessary for complexation was not efficiently produced. Nonogaki et al. (7) have proposed that copper might complex with PEI resins to give a square planar, caged structure. If so, the complexation of copper might be limited to the number of these geometric arrangements on the polyamine resins investigated here. It could also be that the tertiary amines found only on the PEI-12 and PEI-18 monomers do not complex metal ions as well as the primary and secondary amines. A decrease in the uptake of Cut+ by the PEI-18 resin at higher Cu*' concentrations may be due to the presence of soluble, lower molecular weight polymers. Effect of Cross-Linking of TEPA Resin on the Uptake of Cu2+. This study was done initially in the program to find the molar ratio of reactants which would yield a cross-linked insoluble resin having the greatest capacity. The molar ratio of TD1:TEPA was varied from 1.O:l.O to 2.0:l.O using the continuous flow method in synthesizing the resins. Figure 5 shows the uptake of Cu2+ by the various cross-linked resins from a 700 ppm in Cu2+solution by following the same procedure as the previous metal uptake studies, while Figure
6 gives the milliequivalents of basic groups per gram of each cross-linked resin. As we found earlier from the EDA resin study, the urea linkages in the polymer matrix do not complex metal ions. Since the increase in cross-linking occurs at the expense of basic amine groups, the decrease in both Cu2+uptake and basicity of the more cross-linked resins is expected. The unexpected decrease in copper ion uptake by the 1.O:l.O resin (Figure 5 ) is probably due to the presence of soluble, low molecular weight polymer which keeps the Cu2+in solution to some extent but still contributes to the basicity of the resin as seen by the higher basicity of this resin (Figure 6). A comparison of the Cu2+ uptake and the basicity of the 1.O:l.O resin indicates that approximately 1 meq of Cu2+is complexed by 3 meq of basic amines. This is an average since a Cuz+ can be complexed by one, two, three, or four amines. Dependence of pH on Metal Uptake by TEPA Resin. As the pH of the metal acetate solution is shifted down by addi-
tion of unbuffered, strong acid ("Os), the equilibrium is shifted causing less metal uptake. This is graphically seen in Figure 7 for Cu2+,Zn2+, Niz+, and Co2+using the TEPA resin synthesized by the continuous flow method. The p H taken was the initial p H of the metal acetate solution at a metal concentration of 500 ppm. Unbuffered systems were utilized because it was found that buffers complex the metals somewhat, causing a shift in equilibrium other than that caused by the acid. The pH was taken initially, knowing that it would change upon addition of the polyamine resin. What was sought here was a p H at which the metals could be separated on a column of resin by gradient elution. For example, at a pH of 1.8, Cu would remain on the column while Zn, Ni, and Co would be eluted. PART 11. CONCENTRATION OF METAL IONS FROM DILUTE SOLUTIONS EXPERIMENTAL
Table I. Column Specifications and Eluent Concentrations
LGCa ____ Resin
TEPA
Column diameter Resin bed height Resinbedvolume Resin size (mesh) Eluent
2 . 5 cm 15 cm 74cm3 40-60 0 . 5 N HC1
~
SPEC(b)d TEPA on LPECb SPEC(ap porous TEPA TEPA glass beads ~
1 . 5 cm 10cm 18cm3 40-60 6N HC1
0 . 6 crn 5cm 1.4cm3 8G120 6N HCl
0 . 6 cm 5crn 1.4cm3 200-4C0 lNHC1
LGC = Large Glass Column. LPEC = Large Polyethylene Column. SPEC(aj = Small Polyethylene Column. SPEC(bj = Small Polyethylene Column using TEPA resin anchored to 200-400 mesh porous glass beads a b
Apparatus. Perkin-Elmer Models 290 and 403 atomic absorption spectrophotometers were used for metals analyses. The Model 403 is a double beam spectrophotometer. The column specifications are given in Table I. Reagents. Reagent grade HC1 was distilled to remove unwanted metals. Heavy metal-free NaOH was accomplished by passing a IN solution through a I-inch by 1-foot column of the TEPA resin. All metal ion solutions, whether used for the concentration study or for standards for the AA analyses, were prepared by appropriate dilutions of the stock solutiohs with de-ionized, distilled water. Procedure. COLUMN PARAMETERS. I n general, the TEPA resin synthesized by the simultaneous addition method was utilized in a column to concentrate transition metals from known dilute solutions. The resin was sieved through standard mesh screens to give the desired particle size racge and was rinsed with HCI to remove any metal contaminants, then
Table 11. Recovery of Trace Metals with TEPA Resin Initialb RepliFlow soln Soln concn, Trial Soln Ionic cate rate vol, Column No." analysis pH strengthc liters ml/min a/l. LGC 1 3 9.0 0.17 100 20 30 2 9.0 0.17 3 20 300 100 LPEC 50 3 ... ... 0 5 20 6.0 0.17 50 4 2 5 20 5 ... 5 ... 0 5 20 6 6.0 0.17 2 5 5 20 7 10.3 5 2 10 0 10 8 2 10.3 0 5 2 10 9 ... 4.0 0.17 10 5 10 10 ... 10 5 9.9 0.17 10 11 ... 10.3 0.17 10 5 10 ... 12 10 10.6 0.17 5 10 13 ... 2 4.0 0.17 5 10 14 2 9.9 0.17 5 10 15 ... 2 10.3 0.17 5 10 ... 16 10.6 0.17 2 5 10 SPEC (a) 17 ... 10.3 0.17 10 5 10 ... 18 2 10.3 0.17 5 10 2 , . . 19 10.3 0.17 5 10 20 0.4 ... 10.3 0.17 5 10 SPEC (b) 21 ... 10 10.3 0.17 5 10 2 2 22 10.3 0.17 5 10 ... 23 0.4 10.3 0.17 5 10 Trials 1 and 2 analyzed by P.E. Model 290 AA; Trials 3-23 analyzed by P.E. The concentration of each metal (if present). Ionic strength of 0.17 = 200 g of NaC1/20 1. de-ionized H?O. .
.
I
Elution vol, ml
Concentration factor
250 X 80 250 X 80 50 x 400 50 x 400 50 x 400 50 X 400 50 x 200 50 x 200 50 x 200 50 x 200 50 x 200 x 200 50 50 x 200 50 x 200 50 x 200 50 x 200 25 x 400 25 x 400 10 x 1000 10 x 1000 x 1000 10 10 x 1000 10 x 1000 Model 403 AA.
cuz+ 95 =k 5 76 i 6 40 44 =t2 9 31 i 2 38 i 0
o i o
45 52 62 50 50 37 0 48 82 88 88 125 91 90 i 7 110
Recovery, coz+
NiZ+
...
... ...
...
4 30 i 10 8 23 i 10 48 i 4 29 i 6 15 40 48 55 22 25 25 45 69 75 56 42 63 35 It 1 25
. . I
94 i 1
... 100 i 10 75 i 5 78 + 4 69 85 76 83 93 80 55 78 76 90 85 88 83 79 i 6 75
ANALYTICAL CHEMISTRY, VOL. 44, NO. 8, JULY 1972
1355
regenerated with NaOH to place the resin in its free amine state. The analyte solutions were contained in 20-liter capacity polyethylene carboys and were fed into the resin columns by gravity pressure and a t room temperature. The flow rate through the columns was controlled by metered stopcocks placed immediately below the columns. A blank containing everything that was added t o the analyte solutions except the “known” amounts of metals was run simultaneously with the “known” solutions. The resin used for the “known” and blank recoveries was from the same batch and was treated equally for both. Once the analyte solutions had passed through the columns, the sequestered metals were eluted with dilute HCl (see Table I) to a much smaller volume than the analyte solutions, hence resulting in concentration of the metal ions. The effluents were analyzed for the metals by atomic absorption spectrophotometry. The metal ion concentration found in the “blank” effluent was then subtracted from that found in the “known” solution effluent. This final result is then calculated as a percentage of that considered to be a complete recovery of the metals (Table 11). TEPA RESINCOATED GLASS BEADS. Since the TEPA resin swelled upon addition of the HCl eluent, the smaller the resin particle size used in the column, the greater was the tendency to plug. To alleviate this problem, a method was developed in which the TEPA resin was anchored t o the surface of small, porous glass beads (200-400 mesh). The polymer could be seen on the beads under a light microscope at 400X magnification. Flow rates with acids, bases, and neutral salts were uninhibited by the swelling of the resin in a column of resin coated glass beads. KINETICSOF METAL ABSORPTION.One gram of 6&80 mesh TEPA resin was stirred into 100 ml of a 10-ppm metal ion solution. The solution diffused into a n immersed glass fritted tube and was subsequently aspirated into a n atomic absorption spectrophotometer. A stripchart recorder was hooked up to the AAS and plotted the desired readout in solution concentration per time. Copper, cobalt, and nickel ions were tested separately in the above manner. The results are discussed below.
RESULTS AND DISCUSSION
The effects of pH, ionic strength, flow rate, and concentration of the trace metals in the analyte solutions, the column dimensions, and resin particle sizes (Table I) on the recoveries of copper, cobalt, and nickel ions are given in Table 11. The increased recovery of cupric and cobalt ions (Table 11) utilizing the small polyethylene column (SPEC) over that of the large polyethylene column (LPEC), poses a n interesting question. Applying the rule of thumb that the effective plate height of a n ion exchanger is 5-10 times the mean resin particle diameter ( I d ) , this would yield the identical number of effective plates for both columns, hence there is no difference between the SPEC and LPEC methods in this regard. The answer is found in the increased rate of film diffusion of the metal ions in the SPEC study. The rate of exchange for film diffusion is directly proportional t o the diffusion coefficient and the concentration of the metal ion in solution and is inversely proportional t o the resin bead radius, film thickness, and the total concentration of the species in the resin (15). A halving of the resin bead radius effectively doubles the rate of film diffusion of the metal ions. When the metal ion concentration of the solution is low, as in this case, film diffusion J. D ~ “Chemical ~ ~ Separation , Methods,” Van Nostrand Reinhold Company, New York, N.Y., 1969, p 101. (15) R. Patterson, “An Introduction to Ion Exchange,” Heydon and Son Ltd., London, 1970, pp 4C41.
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is the rate controlling step in the absorption of metal ions. A doubling of this rate is significant, hence the increased recovery of cupric and cobalt ions in the SPEC study. The rate of film diffusion is also the key factor in the greater recovery of nickel ions in the LPEC study. The SPEC study yields the expected order of recovery of Cu2+> Ni2+ > Co2+, in accord with their respective metal-amine complex stabilities, but not so in the LPEC study. Increased nickel ion recovery using the LPEC is explained in a batch-type absorption kinetics study (see above). The results have shown that the nickel ions were complexed by the polyamine resin in a shorter period of time than cupric or cobalt ions. The nickel ions took 4 minutes before reaction with the resin began, while copper and cobalt ions took 7 and 10 ri~inutes,respectively, before reacting with the polyamine resin. By decreasing the particle size of the added TEPA resin in the above kinetics study, the initial reaction time between the metal ions and the resin was decreased substantially while the order remained the same. This dependence of reaction rate on the particle size of the resin indicates that film diffusion is the rate controlling step, which is in accord with Turse and Rieman (16). Film diffusion control of chelate formation is again the main reason for the lower recoveries of copper and cobalt ions in the LPEC study. Trials 9-12 show the effect of p H on the recoveries of the metal ions a t a n initial solution concentration of 10 pg/l. Cobalt ions were most affected by the increase of p H t o 10.6 and from this trend, even greater recoveries might be expected by increasing the p H further. (Because of the extremely low concentration of metal ions, no metal hydroxide precipitates should occur.) Copper and nickel ions showed a n increase in recovery from pH 4 to p H 9.9 and then seemed to level off with a normal scattering of results. The decrease in recovery of cobalt ions in the resin coated on glass beads study (Trials 22 and 23) compared with the TEPA resin study (Trials 18 and 19), indicates that probably the surfaces of the glass beads are not efficiently coated with resin. A comparison of Trials 3 US. 4, 5 cs. 6, and 7 cs. 11 and 12 shows a general increase in the absorption of the metal ions by adding NaCl to the analyte solutions. This enhancement could be caused by an ionic strength effect which would cause a n increase in the metal-amine formation constant on the resin, or may be due t o a promotion of the absorption of metal-chloride ion pairs which are more readily formed under excess chloride ion conditions. In the analysis of trace metals in natural waters by this method, sea water would have a high enough ionic strength and chloride ion concentration. Fresh water, on the other hand, would usually have to be enriched with NaCl, as done in this study, in order to enhance the metal recoveries. As can be seen in Trials 19-23, the concentration factor is ~ 1 0 0 0 . If 20-liter samples were analyzed, this factor could be increased by a factor of two. However, this means a doubling of the analysis time and the blank recovery. An elution volume of 10 ml is sufficient for the analysis of three metals simultaneously by atomic absorption spectrophotometry, but no more. A sacrifice of the analysis time or the concentration factor would have t o be made for the analysis of more than three metals simultaneously. The analysis of the 0.4 ppb metal solution yielded a result whose value was twice that of the blank result. Below this concentration, the errors of analysis would cause a substan(16) R. Turse and W. Rieman 111, J . Phys. Cliem., 65, 1821 (1961).
tial decrease in the accuracy of this method of analysis. However, the incorporation of standard isotopic dilution analysis would greatly increase the accuracy of this method.
helpful guidance in its use. Thanks goes to Tony DeRoo a t Dow Chemical Company for donating the polyethyleneimine monomers and to Richard Hagstrom at Olin Corporation for the toluenediisocyanate.
ACKNOWLEDGMENT
Much appreciation goes to Tom Zajicek and Chuck Meade for the use of their atomic absorption spectrophotometer and
RECEVIED for review January 3, 1972. Accepted March 17, 1972.
Determination of Atmospheric Sulfur Dioxide by Differential Pulse Polarography Robert W. Garberl and Claude E. Wilson* Department of Chemistry, Unicersity of Pittsburgh, Pittsburgh, Pa. 15213 The procedure outlined describes a simple, fast, and sensitive method for the determination of atmospheric sulfur dioxide down to 0.1 ppm sulfur dioxide in air. This procedure has been applied to samples of air and nitrogen containing sulfur dioxide at low concentrations. The sulfur dioxide is absorbed into solution by bubbling the air sample through dimethyl sulfoxide containing a supporting electrolyte, e.g., lithium chloride, at a concentration of 0.1M. After deaeration, a differential pulse polarogram is run cathodically on the solution. A peak, between -0.7 and -0.8 V VI. the silver-silver chloride reference electrode, results from the electroreduction of sulfur dioxide. The height of the peak is a linear function of the concentration of sulfur dioxide in the solution. This method is free from interference from sulfides and sulfates; however, oxidants such as nitrogen dioxide have been found to interfere, probably through oxidation of the sulfur dioxide.
RAPIDANALYSIS of atmospheric samples for sulfur dioxide (SOs) is a subject of considerable interest in analytical chemistry and related fields (1-3). Although electrochemical methods have been suggested for such analysis (2, 4, existing methods are not well suited to the purpose. Polarography is not sufficiently sensitive and conductivity is not sufficiently selective for the analysis of typical air samples. The method described below for the analysis of atmospheric sulfur dioxide is both selective and very sensitive. The improved sensitivity of the methods results primarily from the use of pulse polarographic methods which are inherently more sensitive than other voltammetric methods (5, 6). In addition, the use of dimethyl sulfoxide (DMSO) as collecting agent and solvent permits almost full exploitation of the sensitivity of the electrochemical Present address, Tennessee Valley Authority, Sheffield, Ala. 35660. 2 To whom correspondence should be directed. Present address, Department of Chemistry, Indiana-Purdue University at Indianapolis, Indianapolis. Ind. 46205. (1) A. P. Altshuller, ANAL.CHEM.,41, 3R (1969). (2) Arthur C. Stern, Ed., “Air Pollution,” 2nd ed., Vol. 11, Academic Press: New York, N.Y., 1968, pp 55-75. (3) Morris B. Jacobs, “The Chemical Analysis of Air Pollutants,” Interscience, New York, N.Y., 1960, pp 17G9. (4) H. Dehn, H. Kirch, V. Gutmann, and G. Schober, Monafsh. Chem., 93, 1348 (1962). (5) E. P. Parry and R. A. Osteryoung, ANAL.CHEM.,36, 1366 (1964). (6) Ibid., 37, 1635 (1965).
technique. Measurements of the solubility of sulfur dioxide in dimethyl sulfoxide carried out by Smedslund (7) indicate that quantitative collection of sulfur dioxide from other gases by scrubbing with the solvent is possible. Essentially quantitative removal of sulfur dioxide from the carrier gas was found even at low sulfur dioxide concentrations and relatively short carrier gas to dimethyl sulfoxide contact times. The pulse polarographic methods have been described in recent literature (8,9). EXPERIMENTAL
Purification of Dimethyl Sulfoxide. Commercial reagent grade dimethyl sulfoxide (e.g., Fisher Certified, Catalog No. D-128) occasionally contains sulfur dioxide as a trace impurity. Therefore, it is recommended that the solvent be further purified by the user. Vacuum distillation at a pressure of 1.0 Torr or below and at a temperature of 30 OC has produced a solvent with no detectable sulfur dioxide. The water in the condenser should be kept at 20 “C and not be permitted to go below 18.5 “C as dimethyl sulfoxide freezes at about 18 “C. Other Chemicals. Lithium chloride, Fisher Certified Reagent, was recrystallized from ethanol and dried at 100 “C in vacuo. Sulfur dioxide anhydrous, Matheson Company, was used directly from the lecture bottle. Nitrogen, prepurified,Airco,99.97z pure containing 0.001 % oxygen and 0.0012 water. Apparatus. A PAR (Princeton Applied Research) Model 170 Electrochemistry System equipped with PAR Model 172 mercury drop timer was utilized. The electrolysis vessel was a Brinkmann titration vessel, water-jacketed Model EA875-5 having a capacity of ca. 10 ml equipped with penton upper portion Brinkmann Model EA874. The counter electrode was a platinum square Sargent Model S-30515D. The reference electrode (AgR) was a silver-silver chloride electrode in a dimethyl sulfoxide solution of 0.1M lithium chloride. The junction between the titration vessel and the reference electrode was an asbestos fiber sealed in a glass tube. The electrical resistance of the fiber tip was about 0.5 Mohm. Calibration. A standard solution of about 1 M sulfur dioxide in dimethyl sulfoxide is prepared by passing anhydrous (7) T. H. Smedslund, Finska Kemistamfundets Medd., 59, 40 (1950). (8) G. C. Barker and A. W. Gardner, 2. A m / . Chem., 177, 79 (1960). (9) E. P. Pairy and R. A. Osteryoung, ANAL.CHEM.,37, 1634 (1965). ANALYTICAL CHEMISTRY, VOL. 44, NO. 8, JULY 1972
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