ends of the titration curve. Taking into account the standard deviations, the results from the different analyses are in agreement. The slope method leads to a pK value of 8.850 which is well within the range of results from the other techniques and closer to the average pK obtained from the narrower p H range than it is to the average of the whole set. Of the six analyses in Table I, the last is unique in that it is completely independent of the assumption that the experimental titration data fit the Henderson-Hasselbalch equation. Furthermore, since the slope method is a direct analysis of the data not involving iterative procedures, it requires much less computer time than methods b, c, d , e in Table I. Another feature of this technique is that it requires data only in the region around pK and, for reliable analysis, a large number of points in this region is needed. I t may be concluded that under proper conditions this simple method of numerical analysis can be reliably applied to the determination of pK from experimental titration curves.
ACKNOWLEDGMENT We wish to thank Frank R. N. Gurd for kindly providing facilities for part of this work. LITERATURE CITED (1)C. Tanford. Adv. Protein Chem., 17, 70 (1962). (2)P. George and G. I. H. Hanania. Discuss. Faraday Soc., 20, 216 (1955). (3) J. G. Beetlestone and D. H. lrvine in "Probes of Structure and Function of Macromolecules and Membranes", Vol. 11, B. Chance et al.. Eds, Academic Press, New York, 1971,p 267. (4)G. I. H. Hanania and D. H. Irvine, J. Chem. SOC.A,, 2389 (1970). (5) S.J. Shire, G. I. H. Hanania. and F. R. N. Gurd, Biochemistry, 13, 2967, 2974 (1974). (6)P. George and G. i. H. Hanania, Biochem. J., 65, 756 (1957). (7)E. T. Nakhleh. P h D Thesis, American University of Beirut, Lebanon, 1971. (8) L. G. Sillen. Acta Chem. Scand., 16, 159 (1962). (9)M. J. D. Powell. Comput. J., 7 , 155 (1964). (10)R. C. Marshall, unpublished work. (1 1) A. Savitzky and M. J. E. Golay, Anal. Chem., 36, 1627 (1964).
RECEIVEDfor review May 2,1974. Accepted April 4,1975.
Preconcentration of Trace Metals Using Chelating Groups Immobilized via Silylation Donald E. Leyden and G. Howard Luttrell' Department of Chemistry, University of Georgia, Athens, Ga. 30602
Immobilized chelating functional groups were prepared by reacting silica gel with various silylating reagents. In this way immobilized ethylenediamine, its dithiocarbamate, and primary and secondary amines and their dithiocarbamate groups were prepared. These materials were easily prepared and have reasonable stability except for hydrolysis in concentrated acid or base. The pH, time, and concentration dependence of the extraction of metal ions by these reagents were studied systematically. Most of these studies were performed by direct determination of the metals on the silica gel substrate using X-ray fluorescence. These studies demonstrate that these materials have potential for the preconcentration of metal ions from solution.
Although there are many methods of determining trace metal ions, generally each ion must be treated individually or a t best treated in groups. Such procedures often result in tedious extractions and separations. Methods that offer multielement analyses with little sample preparation are much to be desired in the area of trace metal analysis. One such technique is X-ray fluorescence spectrometry. Unfortunately, X-ray fluorescence methods are inconvenient for direct determinations using liquid samples. Additionally, X-ray techniques are not intrinsically trace techniques and generally trace metals must be first preconcentrated in a form suitable for analysis. Various methods for preconcentration specifically for X-ray work have been reviewed ( I ) . Ion-exchange resin impregnated papers ( 2 ) , ion-exchange membranes ( 3 ) , and ion-exchange resin beads ( 4 ) have been used as preconcentration aids for X-ray analysis. There are, however, various 1612
ANALYTICAL CHEMISTRY, VOL. 47, NO. 9, AUGUST 1975
problems involved in each of these. The resin loaded papers suffer from problems of unequal distribution of the metal ions within the paper. The membranes are subject t o low capacity and very long equilibration times; periods of hours are not uncommon. The ion-exchange resins may also require extensive equilibration periods in addition to the deleterious exchange effects they undergo in the presence of large amounts of alkali and alkaline earth ions. The above materials are not easily prepared and are available commercially in only limited varieties. Recently, Weetall and coworkers ( 5 ) reported the immobilization of an 8-hydroxyquinoline chelate on controlled pore glass. This was done through the use of commercially available y-aminopropyltriethoxysilanethat was chemically attached to the glass via a silylation reaction. The amino functional group is converted to the quinoline through a series of chemical reactions. Similarly, Hercules (6) reported the use of an immobilized dithiocarbamate for trace metal analysis. This was accomplished via the silylation of a Fiberglas matrix with a silane containing an amino group which was easily converted to the dithiocarbamate. In each of the above cases, X-ray fluorescence was not used as the ultimate analytical tool. Additionally, in both of these studies, low capacities were reported for the materials investigated, but the implication for a preconcentration method for X-ray is clear. For this study, silica gel was selected as a matrix to which silanes with simple amino and diamino functional groups were attached. Since dithiocarbamates form very stable chelates with many metal ions of interest (7, 8) and are simple to make, these materials along with the amines were investigated to determine their potential as a preconcentration matrix for X-ray analysis.
EXPERIMENTAL Reagents. Stock metal ion solutions (10-3M) were made from the perchlorate salts obtained from J. F. Smith Co. or from the nitrate salts from J. T . Baker Co. All other chemicals used were of reagent grade quality. Apparatus. All X-ray fluorescence data were obtained using a Philips model PW-1410 X-ray fluorescence spectrometer. In all cases, a LiF-200 analyzing crystal was used along with gas flow and scintillation detectors in tandem. P-10 gas (Selox, Inc.) was used in the flow detector. Either a Mo or W X-ray tube was used as a source of X-radiation and was operated a t 50 kV and 50 mA. The counting times varied, but were less than 40 seconds in all cases. Spectrometer parameters were optimized using the usual procedures of step scanning t o locate the peaks and background 2 8 angles and a PHA scope t o determine the pulse height energy window. Pellet Preparation. T h e silica gel was pelletized for use in the X-ray spectrometer. Because of the mechanical instability of pressed silica gel pellets, a binder of powdered cellulose was mixed with the silica gel before pressing. Equal weights of cellulose and silica gel were used. A blank pellet was prepared for each case and was used as a blank correction for reagents and impurities in the silica gel. Silylation Reaction. A 10% silylation solution of N-@-aminoethyl-y-aminopropyltrimethoxysilane (Daw-Corning 2-6020) was prepared according to the instructions of the manufacturer (9). Other solutions of silylation agents, namely N-methyl-y-aminopropyltrimethoxysilane (Dow-Corning XZ-2024) and y-aminopropyltriethoxysilane (Lnion Carbide A-1100) were made in the same way. Optimal loading of the silane onto the silica gel (type G, E.M. Reagents) was determined by adding 10 ml of a 10% silylation solution to various weights of silica gel. This mixture was allowed to stir for 15 minutes, then placed in an oven t o dry for 12 hours a t 80 OC. T h e silylated silica gel was water washed and allowed t o air dry. T h e amount of silane on the matrix in each case was followed by allowing 2 pmol of Cu(I1) in 25 ml of water to be mixed with 100mg portions of the silylated silica gel for 15 minutes, keeping the p H a t 8.0 f 0.2. T h e silylated silica along with the retained copper was filtered, mixed with 100 mg of powdered cellulose (Whatman C F - l l ) , and then pressed into a pellet at 10,000 psi. The extracted copper (determined by X-ray fluorescence) was used t o estimate the degree of silylation. Synthesis. The N,N-dialkyl dithiocarbamate of XZ-2024, the mono N-alkyl-dithiocarbamate of 2-6020, and the silyl xanthate of silica gel were made by stirring 30 g of each of these materials with a 100 ml of benzene, 20 ml of 2-propanol, 20 ml of CS2, and 5 ml of a 10% methanolic solution of tetramethylammonium hydroxide for 15 minutes. The bis-dithiocarbamate of 2-6020 was made by stirring for 15 minutes 20 g of the silylated silica gel with 100 ml water, 25 ml of 0.25N NaOH, 25 ml of 2-propanol, and 20 ml of CS2. In all cases, each of the above was removed from the reaction mixture by filtration. Each of the silylated matrices was washed with 2-propanol, allowed to air dry, and stored in a refrigerator. Determination of Capacity. Since the immobilized chelates may have different capacities in different chemical situations, these capacities were determined by several different methods. Potentiometric titrations were performed on the A-1100 and the XZ2024 silylated silica gels using 0.1N HC104 as the titrant. T h e capacities with respect t o Zn(I1) and Cu(I1) were determined for the silica 2-6020 diamine, XZ-2024 dithiocarbamate, and the 2-6020 bisdithiocarbamate as follows. In each case, 0.2 g of silica gel containing the immobilized chelating group was equilibrated with 50 ml of a 0.1.44 metal ion solution (copper or zinc) for 15 minutes. T h e solid extractant along with the metal ion were separated from the solution by filtration. The metal ions were eluted from the matrix with 4M HC1 and in the case of Zn2+ directly titrated with standard EDTA according to established procedures. T h e copper was determined indirectly by back titration of an added excess of EDTA with a standard Zn2+ solution. T h e capacities of the dithiocarbamates with respect to the -CSS- groups was also determined. Dithiocarbamates are changed by mild oxidation to thiuram sulfides and this reaction may serve as the basis of their determination. The amount of each immobilized dithiocarbamate was determined by adding an excess of 13- to 0.3 g of each type of silica gel. The capacity was then found indirectly by back titration of the unreacted iodine with standard thiosulfate solution.
100
~
90 -
80 n 70k-
60IhJ
50-
k-
40w a
30 20 10-
I I
2
3
4
5
6
7
8
9
l o l l
1213
PH
Figure 1. Extraction onto untreated silica gel as a function of pH (0)Hg2+, ( 0 )Cu2+, (0) Zn’+, (B) Mn2+, (a) Ag+
Investigation of Metal Uptake with Time. This study involved several different silylated substrates with several different metal ions. In each case, 100 mg of the immobilizated complexing or chelating agent was placed in a borate buffer along with 2 micromoles of the cation perchlorate of interest. T h e p H in each case was 7.0 f 0.2 and the final volume was 27 ml. These solutions were allowed t o stir for various periods of time. T h e solid extractant was removed by filtration, rinsed with water, mixed with 100 mg of cellulose, pelletized, and the amount of metal taken u p determined by X-ray fluorescence. Percent Extraction as a Function of pH. This procedure was essentially the same as the above rate study except t h a t no buffer was used. The p H was adjusted by adding 0.1M HC104 or 0.1N NaOH. The ionic strength was maintained by the addition of suitable amounts of 0.1M NaC104. T h e solutions were stirred for ten minutes. The percent metal extracted was determined by comparing the net X-ray counts for each pellet with that pellet t h a t gave the maximum number of counts. I t was ascertained that the extraction was quantitative for the sample giving maximum counts by determination of the metal remaining in the filtrate using atomic absorption spectroscopy. Determination of Distribution Coefficients. The procedure for this study was much like t h a t for the percent extraction as a function of pH, except in this case the filtrate containing the unextracted metal was saved after filtration. T h e filtration was diluted to a known volume and the amount of cation not extracted was determined by atomic absorption using the standard curve method.
RESULTS AND DISCUSSION T h e purpose of t h i s investigation w a s to determine the applicability of immobilized complexing and c h e l a t i n g g r o u p s as p r e c o n c e n t r a t i o n a i d s f o r m u l t i e l e m e n t analysis by X-ray fluorescence. To b e effective, these a i d s had to be inexpensive, readily available o r easily s y n t h e s i z e d , h a v e a f o r m s u i t a b l e f o r X - r a y analysis, and finally h a v e a h i g h d i s t r i b u t i o n coefficient f o r the metal ions of i n t e r e s t . These c o n d i t i o n s a r e met using silanes c o n t a i n i n g a p p r o p r i a t e f u n c t i o n a l g r o u p s immobilized o n a silica gel m a t r i x . The investigation progressed i n a s e q u e n c e considering the untreated silica gel, the immobilized p r i m a r y and s e c o n d a r y a m i n e s , the d i a m i n e , and finally the d i t h i o c a r b a m a t e derivatives. F i g u r e 1 d e p i c t s the e x t r a c t a b i l i t y of H g ( I I ) , C u ( I I ) , Zn(II), Ag(I), Mn(I1) as a f u n c t i o n of pH u s i n g untreated silica gel. The silicic-OH g r o u p s are c a p a b l e of c o m p l e x i n g m a n y metal ions. However, b e c a u s e the pK, values of the silicic a c i d s i n silica gel a r e relatively high, problems w i t h p r e c i p i t a t i o n of metal h y d r o x i d e s w e r e i n evidence. Also, as reflected i n the curves f o r Cu2+ and Zn2+, s e p a r a t i o n s u s i n g silica gel as a m a t r i x w o u l d be difficult. B e c a u s e a m i n e s and d i a m i n e s offer more selectivity w i t h respect to s e p a r a t i o n s and r e a c t i o n s m a y be p r e d i c t e d b y a n a l o g y w i t h soluANALYTICAL CHEMISTRY, VOL. 47, NO. 9, AUGUST 1975
1613
tion phase chemistry, work shifted to the silylated silica gel containing amino functional groups. Silylation Reaction. The silylation reaction is both fast and simple. I t may be done in either aqueous dilute acetic acid or in nonaqueous solvents such as benzene or toluene. The reaction is shown below in Equation 1.
10.
+
OEt I
3 E t O H (1)
The curing by heating is to ensure complete bond formation between the silica gel matrix and the silane as implied in Equation 1. No problems were encountered in executing the reaction. The silylation solution should be used within several hours of preparation because polymerization will occur upon standing (9). Optimal loading of the 2-6020 diamine onto the silica gel was found to occur when 10 ml of 10% aqueous silane was mixed with 5 g of silica gel. With smaller amounts of silica gel, a silanol polymer formed which could be rinsed away with water. Using larger amounts of silica gel did not improve the ability of the material to remove Cu2+ from solution. Synthesis of Dithiocarbamates and Xanthate. The three dithiocarbamates investigated were the N,N-dialkyldithiocarbamate(1) from XZ-2024, the N-alkyl(II), and bisdithiocarbamate of 2-6020 diamine(II1). The silyl xanthate(1V) was made from the surface-OH silicic acid groups on the silica gel. L O
-o-s~-(cH,),-N-cH, \ .
/
ro
s-
\ c=s
(1)
\S-
'S (IV)
The reaction for dithiocarbamate formation is shown in Equation 2.
Mono- and bisdithiocarbamates may be made from diamines depending to a large extent upon the choice of solvent for the reaction. In the case of the mono-dithiocarbamate of 2-6020 diamine, the reaction was carried out using benzene as the solvent. The bis form was made in essentially an aqueous media. Stability. The stability of these immobilized complexing 1614
a
ANALYTICAL CHEMISTRY, VOL. 47, NO. 9, AUGUST 1975
and chelating agents is a bifurcate problem. The first of these being that the silanol itself (including its functional group complexed or uncomplexed) is subject to both acid and base hydrolysis. At very low and a t very high p H values the silanol may be completely removed from the silica gel matrix via hydrolysis. However, within the scope of the studies of this paper the pH values ranged from about 1.5 to 10.5 for time periods of from one minute to several hours and visual examination for the filtrate of coiored complexes indicated no appreciable hydrolysis from the silica gel matrix. The other problem is one of chemical stability of the functional group. In the case of the primary, secondary, and diamine groups, there was no evidence of chemical degradation except when the materials were heated for over an hour a t a temperature of 150 "C. Under these conditions, the amino resins become yellow indicating oxidation and probable formation of some type of bound nitrogen oxide. Oxidation problems could also occur if the resins were placed in solutions containing appreciable amounts of very strong oxidizing agents. The immobilized amine resins are air stable and may be stored a t room temperature. Unlike the amines, the immobilized dithiocarbamates are intrinsically unstable under a variety of experimental conditions, especially in acidic solution. The solutions containing the dithiocarbamates would become cloudy after about 15 minutes a t pH values below 2.5. This cloudiness was most apparent with the mono- and bis-dithiocarbamates of 2-6020 diamine. This cloudiness was probably due to release of carbon disulfide through the acid catalyzed breaking of the nitrogen carbon bond (10) as shown below in Equation 3.
The N,N-dialkyldithiocarbamate of XZ-2024 seemed to be the most stable toward this type of degradation. For the dithiocarbamates containing N-monoalkyl groups, other pathways of acid degradation result in the formation of H2S and isothiocyanates ( 1 1 ) . However, no evidence of formation of these compounds was observed. Additionally, dithiocarbamates are easily oxidized by air to yellow thiuram disulfides and, in the case of the bis-dithiocarbamate(III), to a cyclic thiuram monosulfide (12). No evidence was observed for these oxidative pathways by air. However, in the case of the 2-6020 mono- and bis-dithiocarbamates, possible oxidation by copper(I1) (discussed in a later section) may have occurred. In general, all of the immobilized complexing and chelating groups investigated along with the various silica bound silanes are stable toward hydrolysis over a broad range of p H values and toward the various pathways of degradation by oxidation. This is compatible with their potential use as preconcentration agents for cations in natural waters. Capacity. Since the capacities of these materials may be different under various experimental conditions, they were determined in several ways. The results are tabulated in Table I. The capacities range from about 0.5 to 1.0 mmol per gram of silylated silica gel. While these capacities are not as high as many commercially available ion-exchange resins, they are adequate for many applications. This is especially true when one considers that both the amines and dithiocarbamates have little, if any, affinity for alkali or alkaline earth ions and, therefore, little competition by these ions for the functional sites ( 1 3 ) .Examples of distribution coefficients are given in Table 11. Rate. Studies of the rate of metal ion uptake were un-
Table I. Results of Capacity Studies (mmol/gram) 12 Titration
A-1100 1" amine Z -6020 diamine XZ-2024 dithiocarba-
PH
2T
c u2-
Titration
Zn
. .. ...
0.50
.. .
...
0.54
...
0.52 0.50
0.53
mate
.,.
...
0.49
...
Z -6020 bisdithio -
1.00
...
0.94
0.97
carbamate Silyl xanthate
.,.
1.00
* ..
...
mate
2-6020 diamine
XZ-2024 dithiocarbamate
0.47
0.58
2 - 6 0 2 0 dithiocarba-
Table 11. Determination of Distribution Coefficients for Copper a n d Zinc
PH D(g/ml) log D ~
dertaken using Cu2+,Mn2+,Cr3+,Ag+, and Hg2+ with both the 2-6020 diamine and the XZ-2024 dithiocarbamate in a batch process as shown in Table 111. These particular metal ions were selected because they represent groups with varying affinities for the two chelating groups. As expected with a 1,2-diamine, copper(I1) and mercury(I1) were extracted very quickly, 90% in less than three minutes. The silver(1) preferably forms binuclear "en" complexes instead of chelates, the manganese(I1) complexes have very small formation constants with diamines, and chromium(II1) has notoriously slow ligand exchange rates even from simple aqueous perchlorate solutions. Therefore, these ions would be expected to be extracted more slowly. However, even they were 90% extracted between 17 and 22 minutes on the diamine matrix. Using the dithiocarbamate matrix, all the ions investigated except for Cr(1II) were over 90% extracted in less than four minutes. These rates indicate an obvious advantage over preconcentration with conventional ion exchange resins, even the chelating ones where equilibration times may be hours. Additionally, rate studies were undertaken using copper with the various silylated substrates as seen in Table 111. As expected, the XZ-2024 dithiocarbamate and 2-6020 diamine resins exhibited the highest rate of extraction for copper, namely, 90% in less than two minutes. The primary amine A-1100 and secondary amine XZ-2024 required respectively ten and fifteen minutes for 90% extraction. It should be remembered that in order to obtain a relative view of the rate of uptake of the metal ions investigated, the pH of the solutions was kept at 7.1 f 0.2. This pH, of course, was not optimal for each metal ion with each ligand. The rates would be faster for each metal a t its optimum pH for extraction. These experiments were performed, however, with analytical applications for natural waters in mind. pH Dependence. Complex and chelation formation is among other things a function of pH. The effect of pH on the extractability of various cations was investigated to find the conditions necessary for quantitative removal from solution. The data from these experiments are presented in Figures 2 and 3. The general extraction pattern of the percent extraction vs. pH curves was that expected based on the formation constants of the various metal ions with ethylenediamine in solution. Indeed, there is in most cases a very high level of correlation between the p H values a t 50% extraction (pHl/2) and the corresponding literature values of p2 for the ethylenediamine-metal complexes in solution indicating the M(en)Z2+form of the chelate on the matrix. The extraction may for convenience be placed into three groups. The first of these contains Hg(II), Cu(II), and Fe(II1) all of whose pH112 is below 5 . The second group contains Zn(II), Ag(I), Ni(II), Pb(II), and Cd(I1) and have pH112 values between 5 and 7. The third group containing Cr(III), Eu(III), Co(I1) and Mn(I1) have pH112 values from
7.5 92 3105 3.49
%E
_
_
_
_
~~
~
~
~
8.4 98 13230 4.12
6.5 98 13230 4.12
8.1 96 6480 3.81
~~~~~
Table 111.Time Required for 50% a n d 90% Extraction of Various Metal Ions at pH 7.0 T i m e required, ?-in Iniinohili7ed
iunctional qroup
Cntio11
50
90.;
Ethylenediamine Ethylenediamine Ethylenediamine Ethylenediamine Ethylenediamine .Y, .Y -diaky ldi thio carbamate .Y, S -d iak y Id it hi0 carbamate .Y,AY-diaky ldi th io carbamate .Y, -Y -di ak y Id i t hio carbamate ,Y,.Y-diakyldithiocarbamate Secondary amine Primary amine
Hg?'
,-IO.5
Figure 2. Extraction as
Hg2'
