The vanadium concentration can be correlated by preparing a plot similar to Figure 2. For example, Figure 4 shows the vanadium concentration of a function of absorbance for samples of Tiajuana resid which have been deasphalted under different conditions.
LITERATURE CITED (1) M. S. Patel, Anal. Chem., 46, 794 (1974).
RECEIVED for review December 27,1976. Accepted March 4, 1977.
Analytical Studies and Applications of Ferroin Type Chromogens Immobilized by Adsorption on a Styrene-Divinylbenzene Copolymer Joel L. Lundgren and Alfred A. Schllt * Department of Chemistry, Northern Illinois University, DeKalb, Illinois 60 1 15
Of four different representative types of ferroin chromogens tested, 3-( 2-pyridyi)-5,6-diphenyi-l,2,4-triazine (PDT) proved to be the most effectively adsorbed on Amberlite XAD-2. Isotherms for the adsorption of PDT and its iron(I1) chelate on the copolymer were measured and interpreted. Distribution coefficients and retention capacities of PDT coated XAD-2 for various transition metal ions were determined. Effects of pH, different anions, and flow rates on column operation were evaluated. Applications of PDT coated columns include purification of reagents, concentration of trace metal ions from dilute solutions, and group separation of metal ions prior to analysis. The analysis of seawater and of various reagent grade chemicals for trace quantities of iron, cobalt, nickel, copper, and zinc ions is described.
The remarkable ability of Amberlite XAD-2, and other similar high surface area styrene-divinylbenzene copolymers, to adsorb or bind various water-soluble organic substances was first recognized and investigated by Gustafson and coworkers (1).Their investigations convincingly demonstrated the promising utility of XAD-2 as a macroreticular adsorbent for separation and purification of water-soluble organic species. Further studies of significance and practical application were quick to follow. In 1969, based upon his studies of its uptake of nonpolar solvents, Pietrzyk recommended XAD-2 for use as an inert support for reversed-phase chromatography (2). In subsequent papers, Pietrzyk and co-workers reported measurement of the heats of immersion and swelling of XAD-2 (3) and application of XAD-2 to separation of nitro- and chlorophenols ( 4 ) and of organic bases (5) by liquid chromatography. Another early proponent for the use of XAD-2 in analysis, Fritz and co-workers applied it to the analysis of water for various trace organic contaminants (6-8) and also for the liquid-liquid chromatographic separation of gallium, indium, and thallium (9). Ionic resins of the polystyrene type, closely related to XAD-2, also exhibit pronounced binding properties toward uncharged organic compounds. Various studies, particularly those by Walton and co-workers (10-1.2), have revealed that adsorption by polystyrene resins primarily involves the “solvent” action of the polymer matrix for the organic solutes. Ionic groups on the polymer matrix are of secondary importance; their major influence is to promote solvation and 974
ANALYTICAL CHEMISTRY, VOL. 49, NO. 7. JUNE 1977
swelling of the resin. Walton has also emphasized that adsorption is especially pronounced in the case of aromatic hydrocarbons, which he suggests is almost certainly a consequence of a-electron overlap between styrene moieties and adsorbate molecules (13). Consideration of the foregoing led us to believe that XAD-2 should exert appreciable adsorptive affinities toward such highly aromatic compounds as 1,lO-phenanthroline and related chelation reagents and possibly also for their metal ion chelates as well. If true, the analytical possibilities are obvious. Most appealing among these are the following: (1)immobilization of chelation reagents on a solid matrix without recourse to chemical bonding which ordinarily restricts geometries of chelation, preventing formation of fully coordinated species and discouraging strong retention of metal ions, (2) removal of trace quantities of certain transition metal ions from concentrated solutions of electrolytes, possibly without serious adverse competition from other cations, and (3) preconcentration of metal ions from very dilute solutions to enhance the sensitivity of their determination. To explore the many possibilities, four different representative ferroin-type compounds were selected for study: 1,lO-phenanthroline, 2,2’-bipyridine, 2,4,6-tripyridyl-1,3,5-triazine (TPTZ), and 3-(2-pyridyl)-5,6-diphenyl-1,2,4-triazine (PDT). We present here our findings in these matters together with a number of promising applications that were developed and proved practical in our investigations.
EXPERIMENTAL Materials. The nonionic, macroporous, styrenedivinylbenzene copolymer Amberlite XAD-2 (Rohm and Haas Co., Philadelphia, Pa.) (20-50 mesh) was extracted with methanol for 12 h in a Soxhlet extractor,vacuum dried, and stored in a desiccator. Since the dry copolymer is not readily wetted by water, weighed quantities of the dry adsorbent taken for experimentation were first stirred with methanol for 10 min and then filtered by suction to remove non-imbibed methanol. Treated in this fashion, the samples of copolymer typically retained approximately 0.8 g of methanol per g of dry resin and were readily wetted by water. The 1,lO-phenanthroline, 2,2’-bipyridine, 2,4,6-tripyridyl-striazine (TPTZ), and 3-(2-pyridyl)-5,6-diphenyl-1,2,4-triazine (PDT) were obtained from the G. Frederick Smith Chemical Co., Columbus, Ohio. Impregnation or coating of the XAD-2 with PDT was accomplished by introducing a methanol slurry of a known weight of dry copolymer into a glass tubular column equipped with a stopcock to control solution flow rate and a plug of glass wool at the top. For each gram of adsorbent, 10 mL of a 0.024 M
solution of PDT in methanol was passed through the column at a flow rate of approximately 1 mL/min. The column was then washed with 5 bed-volumes of distilled water to remove the methanol. Determination of PDT content of the combined effluent and water wash typically indicated that 120 A 10 pmol of PDT was retained per gram of dry XAD-2. After removal from the column by back-washing with water, the PDT coated XAD-2 was collected by suction filtration and air dried. Reagent grade chemicals were employed in preparation of all solutions. Standard iron(I1) sulfate solution containing 0.5585 mg of iron(I1) per g of solution was prepared by dissolving a weighed amount of pure iron wire in sulfuric acid and dilution to known concentration with distilled water. The 10% hydroxylamine hydrochloride, used as a reductant, was prepared by dissolving 100 g of the salt in 900 mL of distilled water. Stock solutions of the other metal ions of interest were prepared from their chloride or acetate salts and standardized by the same method used for their determination in subsequent studies. Apparatus and Instruments. A Blue M Electric Co. Model MSB-1122A-1 constant temperature shaker bath was used to control temperature for adsorption isotherm studies. All pH measurements were made with a Corning Model 7 pH meter utilizing a saturated calomel-glass electrode system. Solution absorbance measurements were made with a Beckman Model DU spectrophotometer. Atomic absorption measurements were made using a Beckman Model DB-G spectrophotometer equipped with a Beckman Model 1301atomic absorption accessory and a laminar flow burner. Rates of Adsorption of Iron(I1) Complexes. Solutions,0.200 mM in iron(I1) complex, were prepared by introducing weighed amounts of appropriate organic ligand (3:l ligand to metal ion ratio) into 500-mL volumetric flasks each containing 5 mL of ethanol, 10.0 mL of 0,0100 M iron(I1) sulfate, and 2 mL of 10% hydroxylamine hydrochloride. After thorough mixing and complete dissolution, the contents were diluted to volume with distilled water. A 5.00-mL portion of each iron(I1) complex solution thus obtained was added to one of four absorption cells, each containing 0.163 g of XAD-2 (dry weight) of the same batch prewetted with methanol as described above. The mixtures were continuously agitated in identical fashion at room temperature, except for brief interruptions to measure solution absorbances (in the stoppered cells, after rapid settling of solid phase) to determine the concentration of iron(I1) complex remaining in solution. Adsorption Isotherms. Weighed amounts of the methanol wetted copolymer were introduced into a series of 60-mL screw-cap bottles, each containing exactly 20 mL of a solution of different known concentration of the solute under investigation. The bottles were tightly stoppered and mechanically agitated in a water bath at the appropriate temperature for 48 h to assure attainment of equilibrium, after which portions of the solutions were analyzed for solute concentrations. Solution concentrations of the iron(11)-PDT complex were determined by measurement of absorbance at 555 nm ( 1 4 ) . Concentrations of the methanolic PDT solutions were determined through formation of the iron(I1)-PDT complex by treatment of Bliquots with 5 mL of 0.0100 M iron(I1) sulfate, 1 mL of 10% hydroxylamine hydrochloride, and 4 mL of 1 M ammonium acetate, followed by dilution in a 25-mL volumetric flask to volume with distilled water and measurement of absorbance at 555 nm ( 1 4 ) . Distribution Coefficients. Weighed quantities of PDT coated XAD-2 were introduced into a series of 60-mL screw-cap glass bottles, and to each was added 20.0 mL of a different, appropriately buffered, 1.00 mM metal ion solution. The tightly capped bottles were mechanically agitated in a water bath for 48 h at 25 OC, after which time the concentration of metal ion remaining in solution was determined. Cadmium(I1) and lead(I1) were determined by formation of the dithizonate complexes, extraction into carbon tetrachloride, and measurement of the absorbance at 520 and 525 nm, respectively. Manganese(I1) was determined by oxidation to permanganate ion with periodate and measurement of the absorbance at 520 nm. Chromium(II1) was determined by oxidation with hydrogen peroxide in sodium hydroxide solution to chromate ion and absorbance measurement at 370 nm. Cobalt(II), nickel(II), copper(I), copper(II),and zinc(I1) concentrations were determined through formation of their re-
spective complexes with 1-(2-pyridylazo)-2-naphthol and measurement of solution absorbances according to procedures outlined by Shibata (15). Iron(I1) and iron(II1) concentrations were determined by formation of the iron(I1)-PDT complex and measurement of absorbance at 555 nm (14). From the equilibrium molar concentrations (C)thus found and the known initial concentrations and quantities taken, the moles of metal ion taken up per gram of PDT coated XAD-2 (Q)were calculated. Distribution coefficients (D)were calculated from the defined expression: D = QJC. Retention Capacities. Solutions of individual metal ions or complexes of known concentrations were continuously introduced onto essentially identical columns of either XAD-2 or PDT coated XAD-2 of measured bed dimensions at reproducibly controlled flow rates. The capacity of each column for the solute under test was considered attained at that point when the solute first could be detected in the effluent. In those instances when frontal analysis chromatograms were to be obtained, the effluent was collected continuously in 2-mL portions, and the metal ion concentration in each portion was determined spectrophotometrically as described above. Removal of Metal Ions from Reagents and Solutions. The optimum concentration of a reagent grade chemical to prepare for its purification treatment depends on its solubility and particular chemical nature. Ideally the pH of solutions prepared for treatment should be between 3 and 7 . The method is not applicable to strong acids or bases. The size of column and amount of PDT coated XAD-2 to employ are dictated by the quantity of reagent taken to be purified and the levels of trace metal ions present. Preliminary tests are necessary to establish optimum conditions. Purification of the reagent grade chemicals selected for the present study proved generally satisfactory using 2 M concentrations of the reagent (0.1 M if it was a heavy metal salt), a PDT coated XAD-2 column bed dimension of 100-cm length and 0.9-cm diameter, and flow rates approximating 1mL/min. Column capacities were not exceeded by passage of 250 mL of sample solution, even when spiked by addition of 2.5 mmol each of cobalt(II), nickel(II), copper(I), and iron(I1). Analysis of Reagent Grade Chemicals for Trace Metal Content. An accurately weighed sample was dissolved in 250 mL of distilled water to give the desired approximate concentration and passed through a column of PDT coated XAD-2 as described above. After washing with several bed-volumes of distilled water, the coated copolymer was transferred quantitatively to a Soxhlet extractor and continuously extracted with 60-70 mL of methanol for 5 h. The methanolic extract was transferred quantitatively to a 100-mLvolumetric flask and diluted to volume with methanol. The individual concentrations of the various metal ion-PDT complexes in the methanol extract were determined by atomic absorption spectrophotometry. Calibration curves for each metal ion were determined from standards prepared by introducing measured volumes of standard metal ion solutions into 25-mL volumetric flasks and diluting to volume with 0.02 M PDT in methanol. For samples low in nickel and cobalt, simultaneous determination of iron and copper can be accomplished by solution spectrophotometry after treatment of the methanolic extract with hydroxylamine hydrochloride to assure complete conversion to iron(I1) and copper(1) chelates of PDT. The method of standard additions was employed to check the recoveries and reliabilities of the analytical results. Subtraction of the known added amount of each metal ion from the total found should give the amount of that present in the original, in agreement with that found for the nonspiked sample. Analysis of Seawater. A known solution was prepared to approximate the composition of ordinary seawater; it contained 18000 ppm chloride, 12000 ppm sodium, 1300 ppm magnesium, and accurately known concentrations in ppb levels of iron(II), copper(II), cobalt(II), nickel(II), and zinc(I1). A 10.0-L sample was passed through a column of PDT coated XAD-2 (100 cm X 0.90 cm diameter) at a flow rate of 4-5 mL/min. After rinsing with 25 mL of distilled water, the copolymer was transferred to a Soxhlet extraction apparatus to extract the metal complexes into 25.0 mL of methanol. Portions of the methanolic extract were analyzed for cobalt, copper, nickel, and zinc content by atomic absorption spectrophotometry as described above; iron(I1) content was determined by measurement of solution absorbance ANALYTICAL CHEMISTRY, VOL. 49, NO. 7, JUNE 1977
975
Figure 2. Adsorption isotherm for the PDT complex of iron(I1)on XAD2 from aqueous solution of pH 3 at 25 OC
I I IC
1
I
I
20
30
40
I 5C
I 60
TIKE, m l n
Figure 1. Rates of adsorption of iron(I1)complexes by XAD-2 at pH 3: curves A, B, C, and D, respectively, are for 2.00 X M iron(I1) complexes of PDT, TPTZ, 1,lO-phenanthroline, and 2,2'-bipyridine
at 555 nm, after an accurate 25-fold dilution of the extract with more methanol.
RESULTS AND DISCUSSION Adsorption Rates and Isotherms. Iron(I1) was selected as a model ion to test whether or not the metal ion chelates of 1,lO-phenanthroline, 2,2'-bipyridine, TPTZ, or PDT could be efficiently removed from solution by adsorption on XAD-2. All four iron(I1) chelates proved to be adsorbable, and a rate study to compare the four under similar conditions gave results shown in Figure 1. Pseudo-first-order rate constants, based -1.8 on data early in the study, are as follows: -2.3 X X -1.1 X and -0.9 X s-l respectively for the PDT, TPTZ, 1,lO-phenanthroline, and 2,2'-bipyridine complexes of iron(I1). Significantly, the presence of phenyl substituents in the chelate ligands enhances adsorption on XAD-2. The iron(I1) chelate of PDT, a tris chelate with a total of six phenyl groups, is most rapidly and completely adsorbed. This result is consistent with the a-electron overlap mechanism of adsorption suggested by Walton (13). Measurements taken 4 h and later, when equilibrium had been reached in the rate studies, indicated that extent of adsorption was 100% for the P D T and TPTZ complexes, 96% for the 1 , l O phenanthroline, and 88% for the 2,2'-bipyridine complex. On the basis of these results, PDT was selected as the most promising ferroin reagent for further study. Adsorption of the P D T complex of iron(I1) from aqueous solution on XAD-2 approaches saturation typical of a monolayer process. The adsorption isotherm at 25 "C, shown in Figure 2, conforms to the Langmuir equation. Graphical treatment of the data yielded the following Langmuir parameters: b = 145 pmol of complex per gram of copolymer and a value of 4.9 X lo5L/mol for K , the equilibrium binding constant. The Langmuir treatment demonstrated that K is invariant, indicating that the adsorbed entities do not influence each other appreciably and therefore are adsorbed in monolayer fashion and relatively distant from one another. Adsorption of PDT on XAD-2 was best investigated using methanolic solutions of PDT because of very limited solubility 976
ANALYTICAL CHEMISTRY, VOL. 49, NO. 7, JUNE 1977
0C
10G
ZC0
jCO
400
500
600 0
1/c, liters p e r m o l e
Figure 3. Langmuir treatment of the adsorption of PDT on XAD-2 from methanol solutions: curves A, B, C, and D, respectively, are for 38.5, 28.5, 20.0, and 4.0 O C
Table I. Thermodynamic Quantities for Adsorption of PDT from Methanol on XAD-2
T,"c
K , L/mol
AG*,
4.0 20.0 28.5 38.5
44.6 i 2.2 28.3 t 1.4 24.6 * 1 . 2 18.8* 0.9
-2.09 -1.95 -1.92 -1.82
kcal/mol t 2
f
i
0.01 0.01 0.01 0.01
AS*, e.ua
-7.7 -7.7 -7.6 -7.7
* i i i
0.4 0.4 0.4 0.4
a Based on the experimental value of - 4 . 2 i 0 . 2 kcal/ mol for the standard enthalpy of adsorption, AH', obtained from the temperature dependence of K .
in water. The copolymer is easily wetted by methanol and swells approximately 3% in volume. Swelling as a function of temperature change from 4 to 50 "C in methanol proved immeasurably small. Adsorption isotherms are omitted here, but the plots obtained from them by Langmuir treatment of the data are shown in Figure 3. The amount of adsorbed PDT corresponding to complete monolayer coverage, evaluated from the intercept for each plot and averaged, is 270 f 30 pmol of PDT per gram of copolymer. Under the experimental conditions used in obtaining the isotherms, with concentrations limited by solubilities, less than one half of the available adsorption sites were occupied on the copolymer, since approximately 100 pmol of P D T was adsorbed per gram from the most concentrated solution used. Values for K evaluated
Table 11. Effect of pH on Distribution Coefficients of Metal Ions Equilibrated with PDT Coated XAD-2 at 25 "C Distribution coefficient at pH Metal ion Fe(I1) Co( 11) Ni(I1) CU(I) ZnfII)
1 40
6 7 40 1.5
2
5
7
45 8 13 45 2
330 100 200
...
260 8
250 800 1150 25
Table 111. Distribution Coefficients of Metal Ions at pH 5 and 25 "C for Various Solutions Equilibrated with PDT Coated XAD-2
Metal ion Fe(I1) Co(I1) Ni( 11) CU(I) Zn(I1) Pb(I1) Cd( 11) Mn(I1) Cr(II1) Fe(II1) cum)
0.01 M NaOAc
Solution 1.0 M NaSCN
330 100
40
200
120 60 310 15 35
260 8 3 8 8
10 1010 750
90
... 28
... ...
1.0 M NaC10, 680 750 450 310 30
17
40 10 13
...
*..
from the slopes of the plots in Figure 3, at each of the four temperatures selected, are given in Table I. Standard free energy and entropy changes of adsorption are also included. Both enthalpy and entropy changes are negative for the adsorption of P D T on XAD-2 from methanol. The decrease in entropy is consistent with increased ordering of P D T molecules when adsorbed. Loss of order in solvent molecules and attendant increase in entropy associated with desolvation of P D T apparently is outweighed by the entropy contribution from the solute ordering accompanying adsorption. Distribution Coefficients. Distribution coefficients were measured for many of the transition metal ions that are known to form stable chelates with ferroin-type reagents. Determined under nearly identical conditions and with samples from the same batch of P D T coated XAD-2, the results compiled in Tables I1 and I11 show that both pH and nature of anion are determinative factors in the overall adsorption process. In general, retention of metal ions is favored by high pH and the presence of anions such as perchlorate and iodide which are known to form relatively insoluble salts with ferroin-type metal chelate cations. These same conditions promote favorable distribution ratios for liquid-liquid extractions of other ferroin-type complexes (16). An important observation is that the magnitude of the D values are such that column operation rather than a single or batch-wise treatment is mandated for efficient quantitative removal of the metal ions of interest from solution. Another observation is that advantage can be taken of the different anion effects to facilitate separations. Column Retention Properties. The effect of flow rate on retention was examined, and the basis for selecting between two different approaches was established. The question to be resolved was should the metal ions be complexed with PDT prior to passage through a column of XAD-2, or should a PDT coated XAD-2 column be employed so that preliminary complexation would be unnecessary? Experimental results, shown in Figure 4, indicate that the two approaches are comparable in efficiency for removal of iron(I1) a t flow rates between 1and 5 mL/min, rates that should be practical for most purposes. Clearly, the use of P D T coated columns is
0
1
2
3
4
5
6
7
8
9
FLCW RATE, ml/rnin
Figure 4. Effect of flow rate on breakthrough volume at pH 3 for the iron(I1)-PDT-XAD-2 system: curve A, continuous introduction of 2.35 X lo4 M ferrous ion onto PDT coated XAD-2 (120 Kmol WT/g XAD-2); M [Fe(PDT),]S04 onto curve B, continuous introduction of 2.35 X noncoated XAD-2. In both cases, column bed dimensions were 10.5 cm X 1.0 cm
Table IV. Iron(I1) Retention Capacity of PDT Coated XAD-2 Columns as a Function of Anion and Concentration Retention capacity, mol Fe/g resina
Anion added
Anion molarity
None Sulfate Sulfate Chloride Bromide Iodide Nitrate Perchlorate Perchlorate Oxalate Tartrate
...
12
0.5 1.0
13 13 19
1.0
1.0 1.0 1.0
21 26 22
0.5
24
1.0
25 8
0.5 0.5
6
As measured by continuous introduction of 1.0 x 10-3M iron(I1) at pH 3 and a flow rate of 1.0 mL/min to a 20 cm X 1cm column containing 125 pmol PDT per gram XAD-2 until iron(I1) was detected in the effluent, simpler; also it circumvents the serious complication that PDT has only very limited water solubility. Moreover, since practical applications would require that an excess of P D T be added to assure complete chelation of metal ions, uncoated columns would soon become coated with P D T through use. The retention capacity of P D T coated XAD-2 for iron(I1) under dynamic conditions of column operation is considerably influenced by anion type and concentration as evidenced by the results in Table IV. Halide and perchlorate ions increase the quantity retained per gram of stationary phase prior to the point of breakthrough of iron(I1). Large, easily polarized, monovalent anions enhance retention presumably by more effective charge neutralization of the P D T complex metal ANALYTICAL CHEMISTRY, VOL. 49, NO. 7, JUNE 1977
977
Table V. Metal Ion Retention Capacities of PDT Coated XAD-2 Columns and Efficiencies of PDT Utilization as a Function of pHa Retention capacity, pg metal/g resin Metal ion
pH 2
Zn(I1) Co(I1)
7.1
4.3 7.3
6.2 9.5
Ni(I1) . ,
13.8
19.4
24.3
Cu(1)
21.3
Fe(I1)
13.3
2.5
pH
5
pH 7
102.5 99.7 14.7
.. .
% PDT utilized based on assumed M:L ratiob
M:L 1:3 1:2 1:3 1:2 1:3 1:l 1:2 1:3
PH 2 6 11
12
17 22 33 17 34 32
31 47 82
PH 7 15 15 23 39 58 80
35
. ..
PH
5
10 18
>loo >loo
ccitir
c i t w E ~ T ,II.
Flow rate of 0.5 mL/min and metal ion feed concentration of 1.0 X l o F 3M. Based on 125 Mmol of PDT per gram of coated XAD-2 and assuming the complex formed has the metal to ligand mole ratio indicated.
Figure 5. Frontal analysis chromatograms or breakthrough curves for solutions of a single metal ion at pH 5, 0.5 mL/min flow rate, 1.0 X M metal ion feed concentration, and 20 cm X 0.55 cm columns of PDT coated XAD-2: curve A, zinc(I1);B, cobalt (11); C, iron(I1);D,
cations, promoting stronger retention by decreasing mutual coulombic repulsion between adsorbed cations as well as ionic attractions from anions in solution. Geometry and orientation of adsorbate species on the stationary phase may also be more favorable for metal chelate salts of monovalent anions. Oxalate and tartrate ions give rise to decreased retention of iron(I1). These undoubtedly compete with P D T for complexation of iron(II), thereby discouraging formation and adsorption of the PDT-iron(I1) complex on XAD-2. Retention capacities for various metal ions as a function of pH are compiled in Table V. These values were determined from frontal analysis chromatograms, some of which are reproduced in Figure 5 to illustrate the chromatographic behavior of the metal ions on P D T coated XAD-2. The retention capacity, Y, of a given metal ion was calculated from the expression Y = CoVT(AL/A~),where Co is the concentration of metal ion introduced to the column, VT is the total volume of solution passed, A T is the area of the chromatogram bounded by V = 0 to V = VT and C/Co = 0 to C/Co = 1,and AL is the area to the left of the curve bounded by C/Co = 0 to C/Co = 1. The % PDT utilized was calculated from Y and the amount of PDT per gram of copolymer, assuming a given metal to PDT ligand ratio complex was formed. The results show several interesting features. Retention capacities increase with increasing pH, reflecting the influence of pH on distribution coefficients previously noted. At pH 5 and 7, copper(1) retention is very efficient, and essentially all of the P D T on the coated XAD-2 is available for chelation of copper(1) ions. Apparently copper(1) is retained predominately in the form of its monochelate with PDT. Less efficient utilization of immobilized PDT by the other metal ions investigated may be due in part to the fact that they tend to form bis and tris chelates. Furthermore, much of the adsorbed P D T undoubtedly resides within inner regions of the macroporous polymer which are inaccessible to bulky solvated or complexed metal cations. Copper(1) ions, unlike the others
studied, apparently have the ability or a sufficiently small size to penetrate effectively into such regions. Removal of Trace Metal Ions from Reagents. The results compiled in Table VI demonstrate that trace amounts of iron(II), cobalt(II), nickel(II), and copper(1) ions can be quantitatively removed from very dilute aqueous solutions. Even though recovery of nickel may be low (according to the test of significance at the 95% confidence level), no evidence of any nickel remaining in any portion of the effluent was found. All of the metal ions were retained in a tight band (colored as a result of chelation with PDT) on the upper one-third portion of each column indicating that tailing or bleeding of metal ions should be essentially nil. Sensitive chemical tests confirmed the absence of any of the four metal ions in the effluents. These results confirmed the prediction based on distribution coefficients that the metal ions should be quantitatively removed on passage of their very dilute solutions through PDT coated XAD-2. The low recovery observed for nickel(I1) is probably due to incomplete extraction of its P D T complex from the column. A variety of different reagent chemicals (each spiked to assure the presence of trace amounts of known metal salts) were tested to learn if the method was applicable to their purification. Those which were successfully purified and appropriate concentrations to employ are listed in Table VII. No failures were found, but then none were purposely sought. The scope of applicability for the method appears broad indeed. Especially noteworthy, and in conformance with predictions based on D values, is the fact that trace amounts of iron(II), cobalt(II), nickel(II), and copper(1) can be removed successfully from 0.1 M solutions of zinc(II), lead(II), chromium(III), cadmium(II), or manganese(I1) nitrates. Even though cations of these reagents normally tend to be retained also, they do not adversely impair removal of the more strongly retained metal ions.
a
nickel(I1);and E, copper(1)
Table VI. Metal Ion Recoveries from Dilute Aqueous Solutions" S o h tions
Metal ion
Concn, ppm
Micrograms metal ion Taken Foundb
Error, %
RSD %
Fe(I1) 0.592 59.2 59.4 0.3 0.5 -1.5 0.6 Co(I1) 0.548 54.8 54.0 Ni(I1) 0.581 58.1 55.6 - 4.3 1.2 CU(I) 0.712 71.2 70.7 - 0.7 0.5 a Buffered at pH 5 and 0.01 M in total acetate (acetic acid and sodium acetate) concentration. i~ The mean of three separate determinations, each performed with a separate 20 X 0.55 cm column of PDT impregnated XAD-2 and a flow rate of 0.5 mL/min. 978
ANALYTICAL CHEMISTRY, VOL. 49, NO. 7, JUNE 1977
as 100 mL could have been taken because of the exceptionally high sensitivity of the iron determination.
Table VII. Some Reagent Solutions and Molar Concentrations from Which Trace Amounts of Iron, Cobalt, Nickel, and Copper Can Be Quantitatively Removed by Recommended Procedure Aluminum nitrate, 0.1 Ammonium acetate, 2 Ammonium chloride, 2 Ammonium nitrate, 2 Ammonium perchlorate, 2 Ammonium sulfate, 2 Cadmium nitrate, 0.1 Calcium nitrate, 2 Chloroacetic acid, 2 Chromium nitrate, 0.1 Hydrox ylammonium chloride, 2 Lead nitrate, 1
CONCLUSIONS
Lithium chloride, 2 Magnesium nitrate, 2 Manganous sulfate, 0.1 Potassium acid sulfate, 2 Potassium chloride, 2 Potassium thiocyanate, 2 Sodium acid sulfate, 2 Sodium nitrate, 2 Sodium perchlorate, 2 Sodium thiosulfate, 2 Sulfamic acid, 2 Zinc nitrate 0.1
Analysis of Reagent Solutions a n d of Seawater. Use of P D T coated XAD-2 for preliminary separation affords a simple and effective means for preconcentration prior to measurement that greatly enhances sensitivities of trace metal determinations. The results compiled in Tables VI11 and IX demonstrate that the technique is applicable to the anaysis of a variety of different substances. The method of standard additions was applied to test the reliability of results obtained for those samples for which true values were not known. As evidenced by the data in Table VIII, recoveries of standard additions were quantitative within the precision of measurements. Replicate determinations typically differ from one another or their mean by 0.01 and seldom more than f 0.02 unit of ppm. The large relative standard deviations exhibited by the results are reasonable considering the low levels of sought-for substances. Analytical results for a sample with composition approximating that of seawater and of known trace metal content are compiled in Table IX. Replicate determinations reveal that the method affords good precision and accuracy even for the extreme case when samples as large as 10 L are taken. Samples this large were necessary for all but the determination of iron. If only iron were to be determined, samples as small
P D T and its metal ion chelates are strongly adsorbed in monolayer, Langmuir, fashion on XAD-2. The retention mechanism probably involves a-electron overlap between aromatic groups of PDT and XAD-2. Certain anions, notably iodide and perchlorate, exert favorable influences on the adsorption of metal-PDT complex cations, presumably by more effective neutralization of coulombic repulsion between adsorbed chelate cations. An expedient method for analytical utilization of PDT in conjunction with XAD-2 is to employ columns of P D T coated XAD-2 rather than prior complexation of metal ions with PDT followed by adsorption on XAD-2. Aqueous solutions passed through such columns are not contaminated by PDT, provided that the solution pH is 2 or greater. Chelation and concomitant adsorption of metal ions is sufficiently rapid and efficient to permit use of practical flow rates of 1-4 mL/min and column lengths of 20-100 cm. Quantitative recovery of metal ions taken up by PDT coated columns can be achieved through extraction with methanol. A variety of different substances can be treated to remove trace amounts of certain metal ions, notably iron(I1) and (111), cobalt(II), nickel(II), copper(1) and (11), and zinc(II), by simply passing their aqueous solutions through PDT coated XAD-2 columns. Other metal ions, some yet to be investigated fully, also yield to separation; these include manganese(II), chromium(III), lead(II), and cadmium(I1). Reagents treated thus are ideally suited for use in ultra-trace metal determinations by virtue of greatly diminishing the blanks. Contamination by P D T bleeding from the column is nil, evidenced by no detectable ultraviolet absorption. PDT coated XAD-2 columns are suitable for on-stream or continuous sampling of aqueous solutions for certain trace metal ion contents. Whenever large amounts of sample are available, trace amounts of metal ions can be concentrated onto the column and retained for later, more convenient and sensitive determinations. Application to the analysis of
Table VIII. Analysis of Reagent Solutions for Trace Metals and Recovery of Standard Additions Reagent Concn, M
2 0.1 0.01 2
2
Iron, ppm Present ' Found P P t 025 P= P P + 025 P= P P + 025 P= P P t 1,36 P= P
0.05 0.58 0.04 0.03 0.62 0.05
1.70 2.23 1.69 0.65 2.03 0.66 0.78 0.85 0.65 P t 0.31 1.09 1.16 1.09 1.31 P + 0.63 1.37 1.37 P + 1.25 1.96 1.99 2.04 P = 0.76 RSD= 7.1%
Cobalt, ppm Present Found P P t 027 P= P P t 0,37 P= P P + 0,37 P= P P + 023 P= P
0.00
0.38 0.01
0.00
0.37 0.00
0.03 0.44 0.05 0.07 1.00 0.07 0.04 0.05 0.04 P + 0.30 0.34 0.35 0.35 P + 0.61 0.65 0.64 0.65 P + 1.21 1.25 1.26 1.26 P = 0.04 RSD= 4.1%
Nickel, ppm Present Found P 0.00 P + 0,53 0.53 P = 0.00 P 0.00 P + 0,53 0.48 P = 0.00 P 0.18 P t 023 0.76 P = 0.20 P 0.10 P+ 132 1.40 P = 0.09 P 0.05 0.07 0.07 P t 0.28 0.34 0.33 0.33 P t 0.55 0.61 0.62 0.62 P + 1.10 1.16 1.16 1.16 P = 0.06 RSD= 11.3%
Copper, PPm Present Found P P + 034
0.54 1.19 P = 0.54
P 0.00 P + Os4 0.68 P = 0.02 P 0.26 P + 0,64 0.95 P = 0.28 P 1.49 P + 129 3.02 P = 1.46 P 0.09 0.08 P
+
0.08
0.32
P + 0.63 P
0.40 0.38 0.39 0.71 0.10
0.71 1.34 1.35 1.34 P = 0.08 RSD = 9.5%
+
1.26
ANALYTICAL CHEMISTRY, VOL. 49, NO. 7, JUNE 1977
979
LITERATURE CITED
Table IX. Analysis of Synthetic Seawater Concentration, ppb
Metal ion Iron(I1) Copper(I1) Cobalt(I1) Nickel(I1) Zinc(I1) a
Present Founda Error, % RSD, % 22.7
23.5
18.8
18.8 11.7 10.3
12.0 10.7 12.6
3.5 0
-2.5 -3.7
12.7
0.8
0.8
5.3 1.7 1.9 5.5
Mean of three separate determinations.
seawater, for example, has proved practical. Although application to chromatographic analysis by conventional or HPLC techniques appears to be impractical, group separations of metal ions are feasible. For example, magnesium and calcium ions can be determined without interference from iron, copper, cobalt, nickel, and zinc ions by passing the sample solution through PDT coated XAD-2 prior to addition of buffer and titration with standard EDTA solution.
(1) R. L. Gustafson, R. L. Albright, J. Heisler. J. A. Lirio, and 0. T. Reid, Jr., Ind. Eng. Chem., Prod. Res. Dev., 7, 107 (1968). (2) D. J. Pietrzyk, Talanta, 16, 169 (1969). (3) A. D. Wilks and D. J. Pietrzyk, Anal. Chem., 44, 676 (1972). (4) M. D. Grieser and D. J. Pietrzyk. Anal. Chem., 45, 1348 (1973). (5) C. Chu and D. J. Pietrzyk, Anal. Chem., 46, 330 (1974). (6) A. K. Burnham, G. V. Calder, J. S. Fritz, G. A. Junk, H. J. Svec, and R . Willis, Anal. Chem., 44, 139 (1972).
(7) A. K. Burnham, G. V. Calder, J. S . Fritz, G. A. Junk, H. J. Svec, and R . Vick, J . Am. Water Works Assoc.. 65,722 (1973). (8) G. A. Junk, J. J. Richard, M. D. Grieser, D. Witiak, J. L. Witlak, M. D.
Arguello, R. Vick, H. J. Svec, J. S. Fritz, and G. V. Cakler, J . Chromatogr.,
99,745 (1974). (9) J. S.Fritz, R. T. Frazee, and G. L. Latwesen, Talanta, 17, 857 (1970). (IO) P. Larson, E. Murgia, T. J. Hsu, and H. F. Wafton, Anal. Chem., 45,2306 (1973). (11) E. Murgia, P. Richards, and H. F. Watton, J. Chromafogr., 87,523 (1973). (12) H. F. Walton, J . Chromatogr., 102,57 (1974). (13) D. M. Ordemann and H. F. Walton, Anal. Chem., 48, 1728 (1976). (14) A. A. Schilt and P. J. Taylor, Anal. Chem., 42, 220 (1970). (15) S.Shibata, “Chehtes in Analytical Chemistry”, Vol. 4,H. A. Flashka and A. J. Barnard. Ed., Marcel Dekker, New York, N.Y., 1972. (16) A. A. Schilt, R. L. Abraham, and J. E. Martin, Anal. Chem., 45, 1808 (1973).
RECEIVED for review January 21,1977. Accepted March 21, 1977.
Variable-Time Kinetic Determination of 2-Propanol and Other Alcohols by Means of Mutual Induction in Oxidations by Chromium(V I) Slndor Veres and Llszlb J. Cslnyi” Reaction Kinetics Research Group of the Hungarian Academy of Sciences, Department of Inorganic and Analytical Chemistry, Attila J6zsef University, Szeged, Hungary
A spectrophotometric-kinetic method is proposed for the determination of Isopropanol and other primary and secondary alcohols. The chromic acid oxidation of the alcohols Is accelerated by the addition of oxalic acid, as a consequence of a mutual induction. The method can be used to measure 1 X 10-3-4 X lo-* M 2-propanol with an accuracy better than 3%.
It has been recently described by Hasan and Rocek (1)that under conditions where either oxalic acid or 2-propanol are oxidized at only a moderate rate by chromic acid, the oxidation of a mixture of these two substances proceeds practically instantaneously. Setting out from this observation, and from the extensive kinetic findings of Rocek et al. ( 2 , 3 ) ,we propose a method for the determination of alcohols. The essence of the procedure is that the reduction of the chromate in a reaction mixture containing chromate, alcohol, and oxalic acid is followed in time, and the concentration of the alcohol is determined by reference to calibration curves obtained by applying the variable-time method. The reduction of chromic acid in the above-mentioned ternary system can be described by the following rate equation: - d‘Cr(V1)’ = k,[HCr04-][ROH][H+]2 dt t k , [ HCrO,-l[ROH][( COOH),] t k , [HCrO;][ (COOH),]z 980
ANALYTICAL CHEMISTRY, VOL. 49, NO. 7, JUNE 1977
where ROH is 2-propanol, and (COOH)2 is undissociated oxalic acid. The values of the constants are: kl = 1.26 X lo-’, kz = 8.78 and k3 = 0.242. From the values of the rate constants, it emerges that in the joint presence of the substrates, the combined oxidation predominates, in the course of which C02 and acetone are formed from the 2-propanol. On the use of much alcohol and only a little oxalic acid (see below), or in the presence of radical reagents (e.g., acrylonitrile), the ratio of the products is 1:1, this shows that the partners form a 1:l:l complex with chromate, and that the redox transformation of this leads to the products without the formation of a chromium species in an intermediate oxidation state (one-step, three-electron oxidation). This latter fact is responsible for the dramatic acceleration of the reaction. In this complex, the alcohol takes part with 2 equivalents, and the oxalic acid with 1equivalent, in the reduction of the chromate. Two overall equations may be given for the description of the joint oxidation. At 2propano1:oxalic acid ratio higher than 20, Equation A holds: 2 C r 0 , + B(CH,),CHOH t (COOH), = Cr,O, + 2(CH,),C=O + 2C0, + 3H,O (A) When the ratio of concentrations is less than 0.5, the ratio of the numbers of oxalic acid and alcohol moles oxidized will not be 1 2 , but 21, and Equation B holds: 2Cr0,
+
2(COOH), + (CH,),CHOH + (CH,),C=O t 4C0, t 3H,O
= Cr,O,
(B)
Depending on the proportions of the partners, every possible
