Solvent Extraction of Niobium-4-(2-Pyridylazo)resorcinol Complex and Spectrophotometric Determination of Niob um Oxalato Solutions Marija Siroki,' Ljlljana Maric, and Marko J. Herak Laboratory of Analytical Chemistry, faculty of Science, The University of Zagreb, Strossmayerov trg 14, 4 1000 Zagreb, Yugoslavia
Clrlla Djordjevic Department of Chemistry, College of William and Mary, Wllllamsburg, Va. 23 185
Solvent extraction of niobium In the form of the pyridylazo resorcinol complex has been studied. The strongly colored niobium complex which forms at pH 5.5 and contalns 4-(2pyr1dylaro)resorclnoI and oxalate as ligands can be extracted Into the chloroform by means of the tetraphenylarsonium and tetraphenyiphosphonlum chloride, respectively. The mechanism of the extraction is based on the formation of the ion-associated compounds that form between the hydrophobic organic catlon and the oxo(oxalato)( pyridylazoresorclnolo)niobate(V) anlon. The chloroform phase Is more stable than the aqueous solutions of the complex, and It follows Beer's law In a signltlcantly wider concentration range. A method has been developed for the spectrophotometric determlnatlon of niobium In the organic phase after the chloroform extraction. This method has certain advantages over the nlobium determlnatlon with 4 4 2-pyridylazo)resorcinol in the aqueous oxalate medium.
Tetraphenylarsonium and tetraphenylphosphonium chloride are well known reagents, which can yield ion-associated compounds with large anions of low solubility. In most cases, a hydrophobic ionic pair is formed, which can be extracted from the aqueous solutions into the organic solvent of suitable dielectric constant. In past years, prominent attention has been paid to extraction systems involving organophilic cations and anionic metal complexes. T h e extraction of some highly colored metal complexes with thiocyanates (1-5), catechol (6),and pyridylazoresorcinol (7-9), respectively, with tetraphenylarsonium or tetraphenylphosphonium ion has been described. Such methods increase the selectivity of corresponding sensitive, but less selective reagents for the spectrophotometric determination of metals. T h e 4-(2-pyridylazo)resorcinol complex of niobium, which is formed in the oxalate solutions has been shown t o be extractable by chloroform solutions as well by using the tetraphenylarsonium and phosphonium salts, respectively ( 1 0 ) . Under the same conditions, the pyridylazoresorcinol complexes of titanium and zirconium cannot be extracted, and the tantalum complex is extracted t o a very small extent. Since there are not many selective methods available for the niobium determination, and furthermore, in the majority of the separation procedures involved in the analytical processes, small amounts of tantalum, titanium, and zirconium are left with niobium ( I I ) , we have considered it worthwhile to make use of the extracting properties of the pyridylazoresorcinol complexes of these metals, and to extend the analytical application of 4-(2-pyridylazo)resorcino1 t o oxalatoniobium solutions. This sensitive reagent has been widely used for the spectrophotometric determin-
nation of small amounts of niobium (12-17). However, the determination is generally carried out in the tartarato aqueous solutions, and much less attention was given t o the oxalato systems (18).
EXPERIMENTAL Apparatus. Spectrophotometric measurements were carried out on the Beckman spectrophotometer, Model DU-2, with a reagent blank as reference. The measurements of radioactivity (95Nb)were performed with a well-type gamma scintillation counter (NaI/Tl) from Ecco Electronic. For the extraction, a Griffin Flask Shaker with a time switch was used. Reagents. AnalaR grade reagents and chemicals were used 4(2-pyridy1azo)resorcinol was from Merck, tetraphenylarsonium and tetraphenylphosphonium chloride from Fluka, and niobium pentoxide from BDH, respectively. 95Nb was obtained from the Radiochemical Centre, Amersham, in the form of oxalato complexes in a 0.5% solution of oxalic acid. Only freshly prepared solutions of 4-(2-pyridylazo)resorcinolin deionized water were used. Tetraphenylarsonium and tetraphenylphosphonium chloride were dissolved in chloroform (AnalaR, containing 0.5% ethanol) and in water, respectively. The acetate buffer of pH 5.5 was prepared by mixing the solutions of sodium acetate (1M) and acetic acid (1M). The oxalate solution (0.01M) of pH about 4, needed for the dilution of niobium solutions, was prepared by mixing sodium oxalate solution (O.lM, 20 ml) with oxalic acid solution (O.lM, 5 ml), and diluting it to 250 ml with deionized water. Standard Solution of Niobium. Niobium solution (about 2 X 10-*M) in oxalate solution (0.2M) was used. Niobium pentoxide (0.3 g) was fused with potassium hydrosulfate (4 g) in a platinum crucible. The melt was treated with oxalic acid (5%,100 ml). To remove the sulfates, niobium was precipitated with ammonia, The precipitate was centrifuged and washed. Freshly precipitated niobium hydroxide was then dissolved in oxalic acid (0.4M, 50 ml) by digesting it on a water bath. The solution was filtered, diluted to 100 ml with deionized water, and standardized by precipitating the niobium with tannin (29). The solutions of lower concentrations were prepared by diluting the standard solution with 0.01M oxalate solution, or with deionized water, in order to reach a final 0.01M concentration of oxalate (pH about 4). Oxalate solutions of tantalum and titanium were prepared similarly by fusion of metal oxides with potassium hydrosulfate and dissolving the melt in oxalic acid. Determination of the Distribution Ratio. Distribution of niobium between aqueous and organic phase was followed radiometrically. The organic and aqueous solutions (usually 5 ml) were shaken in stoppered flasks using a mechanical shaker. All systems were shaken for 30 min, although the time necessary to reach equilibrium was 2 minutes. After separation of the layers, 1 ml of each phase was taken for radiochemical analysis. Distribution ratios were calculated from counts/100 sec of both phases. Spectrophotometric Determination of Niobium in Chloroform. T o an aliquot (5 ml) of aqueous oxalate solution (0.01M in oxalate, pH about 4) containing 5-50 wg of niobium, add a solution of PAR ( 2 X 10-3M, 2 ml) and acetate buffer pH 5.5 ( 1 M , 2 ml). Check the pH on the pH-meter. Transfer the mixture to a separatory funnel and extract twice with chloroform solution of tetraphenylarsonium or tetraphenylphosphonium chloride (5 X 10-3M, 5 ml at a time), shaking for 2 min. Collect the organic phase fracANALYTICAL CHEMISTRY, VOL. 48, NO. 1, JANUARY 1976
55
101
I
,
1
60
45
55
50
60
65
70
PH
Figure 1. Dependence of absorbance on pH 0
The absorbance was measured in (1) ( 0 )aqueous solution at , ,A = 550; (2) (0) and (3) ( 0 )organic phase at . ,A = 560 nm. The extractant was (CeH5)dAsCIor (C.&)4PCI
0,
I
-3
I
'"I
-2
LOG OXALATE CONCENTRATION ( M I
I
Flgure 3. Dependence of distribution ratio of niobium on oxalate con-
centration Extraction was carried out with 5 X 10-3M (C&15)4PCIin chloroform. Concentration of the reactants as follows: (1) (A)Nb = PAR = 2 X 10-5M (2) (B)Nb = PAR = 1 X 10-4M, (3) (A)Nb = 2 X 10-5M, PAR = 2 X 10-4M (4) (0) Nb = 5 X 10-5M, PAR = 5 X 10-4M (5)(0) Nb = 1 X 10-4M, PAR
=1
x
10-3~
, * 1 0002
OW4
0006
0008 001
002
004
006
008
010
OXALATE CONCENTRATiON (MI
Flgure 2. Influence of oxalate concentration on absorbance Concentration of reactants: Nb = 2 X 10-5M, PAR = 4 X lO-'M, extractant = 5 X 10-3M. The absorbance was measured in (1) (0) aqueous solution at Amax = 550 nm: (2) (0)and (3) (0)in organic phase at , ,A = 560 nm. The extractant was (C&).AsCI or (C&I&PCI
O x a l a t e concn,
tions in a 10-ml volumetric flask and measure the absorbance a t 560 nm, in a 1-cm cuvette vs. reagent blank.
0.05 0.10 0.20 0.40
RESULTS AND DISCUSSION The strongly colored 442-pyridy1azo)resorcinol complex of Nb(V), which forms in oxalic solutions at pH 5 is readily extracted into chloroform with tetraphenylarsonium and tetraphenylphosphonium chloride, respectively. The chloroform phase shows the maximum absorbance at 560 nm, and the aqueous solution displays maximal absorbance at 550 nm. The maximum does not shift with the changing concentration of niobium, and it does not depend upon the concentration ratio of reactants, implying that only one niobium species is extracted. However, the equilibrium concentration of the metal complex is dependent highly upon pH and the other parameters described below. Optimal pH for the complex formation in oxalic solutions is 5.5. Figure 1 shows the dependence of absorbance of the organic phase upon pH, as compared to the absorbance dependence of the aqueous phase. This figure clearly illustrates that, for reproducible results, it is necessary to control pH, and keep it at 5.5 f 0.3. In adjusting the pH, the following fact has to be taken into consideration. Oxalato complexes of niobium become unstable at higher pH values and, by making the solutions more alkaline in a careless manner, local hydrolysis may occur, resulting in the formation of nonreactive metal species. In the case where the preparation of the solutions for analysis is carried out by digesting the bisulfate melt with ammonium oxalate instead of oxalic acid, it is usually possible to increase the pH of solutions to the required value by addition of buffer only. Acetate buffer proved to be very convenient, since the acetate ions do not interfere either with the formation of metal complex or with its extraction. Buffer was added in amounts, which allowed the final concentration of 56
ANALYTICAL CHEMISTRY, VOL. 48, NO. 1, JANUARY 1976
-___
Table I. Reextraction of Niobium from the Organic Phase with Oxalic Acid (A) and Ammonium Oxalate-Oxalic Acid Mixture (pH 4.5). (B) . . N b reextracted, %
M
A
B
0.01
94.88 98.72 98.91 99.11 99.21
52.60 95.70 98.25 99.31 99.41
-
acetates in aqueous solutions to be 0.2M, which simultaneously kept the ionic strength approximately constant. Influence of the Oxalate Concentration. The concentration of oxalate ions has a significant influence on the equilibrium of the reaction of niobium with 4-(2-pyridylazo)resorcinol and, thus, on the niobium determination in the aqueous solution and in the chloroform phase as well. The maximal and constant absorbance is achieved in the concentration range of oxalates 3 X to 1 X 10-2M (Figure 2). Systems are unstable at concentrations less than above. At low oxalate concentration the sum of radioactivity of organic and aqueous phase does not correspond to the total amount of radioactive niobium taken. Loss of niobium which occurs is probably due to invisible precipitation. A t oxalate concentrations higher than 1 X 10d2M, the concentration of niobium-PAR complex in the aqueous phase decreases sharply (Figure 2, curve l),and so does the distribution ratio of niobium (Figure 3), and the absorbance of the chloroform phase (see Figure 2, curves 2 a n d 3). Indeed, it is possible to reextract niobium quantitatively from the organic phase with still higher oxalate concentration (0.2M oxalate solution a t pH below 4.5) as shown in Table I. Colorless oxalato complexes of Nb(V), although present in aqueous solutions in an ionic form (20-22) are not extracted by chloroform solutions of tetraphenylarsonium and tetraphenylphosphonium chloride, respectively, under the experimental conditions used. Oxalate ions also are not extracted. Influence of the 4-(2-Pyridyl~zo)resorcinolConcentration. For the formation of the niobium-PAR complex in
I 1Q
-
-
I -
-05
2
, 1
-2
-3
-4
-
LOG PAR CONCENTRATION ( M I
Figure 4. Dependence of distribution ratio of niobium on 4-(2-pyri-
dy1azo)resorcinolconcentration Concentration of oxalate and tetraphenylphosphonium chloride was 5 X 10-3M. Niobium concentrations: (1) (A)Nb = 2 X lO+nn; (2) (0)Nb = 5 X 10-5M, and (3) (0)Nb = 1 X 10-4M
I O
,
I
I
2
3
4
LOG EXTRACTANT CONCENTRATION [MI
Figure 6. Dependence of distribution ratio of niobium on extractant
concentration Oxalate concentration was 5 X 10-3M. (CeH5)4XCIas extractants. Other concentration were as follows: (1) ( 0 )Nb = PAR = 5 X 10-5M, X = P: (2) ( 0 )Nb = PAR = 5 X 10-5M, X = As; (3) (A)Nb = 2 X 10-5M, PAR = 2 X 10-4M, X = P; (4) (0)Nb = 5 X 10e5M, PAR = 5 X 10-4M, X = P: (5)(0) Nb = 1 X 10-4M, PAR = 1 X 10-3M, X = P
-
--
7-
a
’
I
02-
z 0 +
3
,
I
,
I - %
2
4
6
~
1
1
1
0 10
20
30
40
50
05r
m
E vr B~
MOLE RATIO, [PARI/[Nbl 0
Figure 5. Influence of PAR concentration on the absorbance Concentration of niobium 2 X IO-’M, and of extractant 5 X 10-3M. The aqueous solution; (2) (e) measurement of absorbance was made in (1) (0) and (3) (0)in organic phase, with (CsH5)4AsCIor (C&)4PCI
-5
-45
-4
-3 5
LOG NIOBIUM CONCENTRATION (MI
oxalate solutions, a relatively large excess of the reagent is necessary, Under the conditions studied here, the maximal concentration of the metal complex is achieved at the Nb/ PAR molar ratio of 1/10. However, if this ratio is between 115 and 1/20, absorbance of the aqueous and chloroform solutions of the colored species does not change much. Only a t larger PAR concentrations, the distribution ratio (Figure 4) and absorbance of the organic phase (Figure 5) are decreased because the reagent itself is also extracted under the conditions studied. Influence of the Extractants Concentration. For the maximal extraction of the metal complex, a relatively large excess of extractants is necessary. Distribution ratio (Figure 6) and the absorbance of the organic phase increase sharply with the increase of the concentration of the extractants to about 1 x 10-3M. On a further concentration increase, to 0.1M, the distribution ratio increases only slightly, and the value of the absorbance of the organic phase becomes nearly constant. However, the extraction of niobium is not quantitative even under the optimal conditions. Maximum 95% of total niobium concentration is extracted, and identical results are obtained on using aqueous and chloroform solutions of extractants, respectively. Dependence of the Extraction upon the Niobium Concentration. Efficiency of the extraction depends upon the total concentration of niobium present in the system.
Figure 7. Dependence of distribution ratio on niobium concentration Concentration of oxalate and tetraphenylphosphonium chloride was 5 X 10-3M, the molar ratio of Nb/PAR: (1) (e)111 and (2) (0) 1/10
At the optimal concentration of 4-(2-pyridylazo)resorcinol (Figure 7 , curve 2), the niobium extraction is increased to about 5% on changing the total niobium concentration from 2 X to 2 X 10-4M. The curves indicate clearly the tendency of gradual decrease of the distribution ratio on proceeding towards very low niobium concentrations. Such a dependence of the distribution ratio upon the metal concentration may be explained by polymerization of the complex in the organic phase, or invisible hydrolysis and precipitation in the aqueous phase. Despite this fact, a linear relationship has been obtained by measuring the absorbance of the organic phase as a function of niobium concentration, and the line extrapolates to zero. Yet, the results for niobium concentrations below 1 X 10-5M are less precise and tend towards low values (Figure 8, curve 2). The optimal concentration range for spectrophotometric to 5 measurements has been found to be between 5 X X 10-5M Nb, which corresponds to 0.5-5 ,ug/ml of niobium. The aqueous solutions show in this concentration range significant deviations from Beer’s law at higher concentrations (Figure 8, curve l),but the chloroform extracts follow the linearity even outside of this concentration range, ANALYTICAL CHEMISTRY, VOL. 48, NO. 1, JANUARY 1976
57
2.4
22 .o ’2
1
L
Ix
1
Figure 9. Determination of stoichiometric molar ratios of reactants
+
(a) Determination of Nb/PAR ratio: (1) (e)Nb PAR = 5 X 10-5M, concentration of oxalate was 8 X lo-% and tetraphenylphosphonium chloride 5 X IOT3M.(2) ( 0 )Nb PAR = 1 X concentration of oxalate and tetrap’henylarsonium chloride was 5 X l O @ M , (b) Determination of (Nb-PAR)/ (C&)4XCI = 2.5 X 10-4M, extractant ratio: (1) (0)and (2) (M) (Nb-PAR) X = As or P; (3) (e)and (4) (0)(Nb-PAR) (CsH5)4XCI= 5 X 10.-4M, X = As or P
+
+
NlOBlUM(~g/rn0
Figure 8. Calibration curves The absorbance measurements were conducted in (1) (e)aqueous soiutions: (2) ( 0 )organic phase
which has been determined by differential spectrophotometry. Larger concentration than 5 wg/ml of niobium can be determined with a satisfactory accuracy. The lower limit of the niobium determination can be decreased by taking a larger volume of the aqueous phase in relation to the chloroform volume. Satisfactory results have been obtained up to the volume ratio of Vorg/Vaq= 1/5. Influence of Foreign Ions. Because of the low solubility of niobium compounds in common solvents, the determination of niobium is generally carried out in the presence of larger amount of sulfate, introduced into the solution upon the fusion with bisulfate. For this reason, the influence of sulfate ions on the extraction and the determination of niobium was studied in the first place. Sulfate ions have no influence on the niobium determination in a large concentration interval of 1 x 10-3 to 1M. Large amounts of acetate, sodium, potassium, and ammonium ions, respectively, do not interfere either. In analytical systems, along with nio-
Table 11. Effect of Tantalum, Titanium, Zirconium and Uranium on the Determination of Niobium in the Aqueous Oxalate Solution and in the Chloroform Extract. Niobium taken: 18.6 ,ug
.,
.
Metal ion added
Molar ratio. metal/ nio biirm
... Ti(1V) Zr(1V)
1 5 10 1 5 10 1 5 10 1
5 10
58
-
Iuiobium found. pg in Aqueous soh
17.8 17.5 35.8
> 60 26.3
> 60 > 60 18.6 22.5 29.7 18.6 20.0 21.9
Chloroform
18.5 17.0 18.2 17.4 18.3 18.7 19.0 18.5 18.5 18.3 22.5 31.7 44.5
ANALYTICAL CHEMISTRY, VOL. 48, NO. 1, JANUARY 1976
+
bium, small concentrations of metal ions of elements similar to niobium are common: ions of tantalum, titanium, zirconium, or tungsten. Tungsten does not react with PAR, but the other ions mentioned above give colored complexes with PAR and interfere in the determination of niobium in aqueous oxalic systems. The most serious is the interference from tantalum and titanium, less so from zirconium and uranium. Upon chloroform extraction, titanium and zirconium do not interfere, tantalum interference is significantly reduced, and only uranium interference is increased. In Table 11, the influence of the presence of tantalum, titanium, zirconium, and uranium on the determination of niobium with 4-(2-pyridylazo)resorcinolin aqueous oxalato medium and in the organic phase, respectively, is shown. These ions were added to the niobium solutions in the form of oxalato metal ion solutions. Composition of the Complex and the Mechanism of the Extraction. Spectrophotometric determination of niobium with 4-(2-pyridylazo)resorcinol and the complex species present in different systems were studied by several authors. Regardless of the presence of various other reagents suitable for the formation of stable complexes with niobium (e.g., oxalates, tartarates, fluorides, and peroxides), a niobium-PAR complex is formed with a 1/1 ratio. In this paper, the composition of the niobium extractable complex was studied spectrophotometrically by Job’s method, and also radiometrically. I t was found by Job’s method that the ratio of Nb/PAR is equal 1/1,and that the most probably ratio of (CGHS),XCl/(Nb-PAR) is also 1/1 (Figure 9). The slopes of the curves, obtained by the radiometric studies for the dependence of the log D on the log concentration of oxalate, PAR, or tetraphenyl onium ion, respectively, also indicate the equimolar ratio of the reactants (Figures 3,4,and 6). The extracted complex composition corresponds therefore to a ratio of Nb/Oxalate/PAR/ onium ions = 1/1/1/1.Compounds of this composition have been separated from the organic phase and characterized as tetraphenylarsonium and tetraphenylphosphonium oxo(oxalato) (PAR)niobate(V)( I O ) . Therefore, it can be assumed that the mechanism of the extraction consists of the formation of an ion-associated complex according to the equation:
where X = As or P, and R refers to the divalent ion of 4(2-pyridy1azo)resorcinolabbreviated as H2R. The increase of the distribution ratio with the increase of niobium concentration may indicate polymerization of the extracted complex in the organic phase. CONCLUSION
The strongly colored complex of niobium with 4-(2-pyridylazo)resorcinol, which is formed in the oxalato solutions at pH 5.5 can be extracted from the aqueous solutions in chloroform by means of tetraphenylphosphonium and tetraphenylarsonium chlorides as extractants. The system is complex and too complicated for a quantitative description of extraction equilibria. However, distribution ratio dependence upon the reactants concentration, as well as comparison of the absorbance of aqueous solutions and absorbance of chloroform extracts could be explained by the following assumption. In aqueous oxalate solutions, there are several complex niobium species which display a different reactivity towards 4-(2-pyridylazo)resorcinol.Upon extraction, the equilibrium between the colored species that contains PAR and the colorless oxalato complexes is shifted towards the formation of larger equilibrium concentration of the colored niobium complex ion, since the later is the only one being extracted into the chloroform phase. The colored complex species is present in aqueous solution in the anionic form and can be extracted by hydrophobic organic cations as an ion-association complex. The latter has a tendency to polymerization in the organic phase. The highly colored chloroform extract is suitable for the spectrophotometric determination. Quantitative determi-
nation in the organic phase has certain advantages over the determination of niobium in aqueous solutions. T h e sensitivity and selectivity of the determination is increased, and Beer's law is obeyed in a significantly wider concentration range. ACKNOWLEDGMENT
The authors are grateful to Mrs. G. Lalovie and D. Krhatlic for the technical assistance. LITERATURE CITED ( 1 ) H. E. Affsprung and J. L. Robinson, Anal. Chim. Acta, 37, 81 (1967). (2) H. E. Affsprung, N. A. Barnes, and H. A. Potratz, Anal. Chem., 23, 1680 (1951). (3) R. J. Magee and M. A. Khattak, Microchem. J., 8, 285 (1964). (4) J. W. Murphy and H. E. Affsprung, Anal. Chim. Acta, 30, 501 (1964). (5) M. Shinogawa, H. Matsuo, and R. Kohara, Jpn. Anal., 5, 29 (1956). (6) M. pchlabsky and L. Sommer, Talanta, 15, 887 (1968). (7) M. Siroki and C. Djordjevic, Anal. Chim. Acta, 57, 301 (1971). (8) M. Siroki, Lj. Maric, 2 . Stefanac, and M. J. Herak, Anal. Chim. Acta, 75, 101 (1975). (9) Lj. Maric, M. Siroki. and M. J. Herak, J. lnorg. Nucl. Chem., in press. (10) M.Siroki and C. Djordjevic, Anal. Chem., 43, 1375 (1971). (11) R. W. Moshier, "Analytical Chemistry of Niobium and Tantalum", Pergamon Press, London, 1964. (12) T. Belcher, T. V. Ramakrishna, and T. S. West, Talanta, I O , 1013 (1963). (13) i.P. Aiirnarin and Han Si-i, Zh.Anal. Khim., 18, 182 (1963). (14) S.V. Elinson and L. I. Pobedina, Zh. Anal. Khim., 18, 189 (1963). (15) D. F. Woodand J. T. Jones, Analyst(London), 93, 131 (1968). (16) P. Pakains, Anal. Chim. Acta, 41, 283 (1968). (17) C. L. Luke, Anal. Chim. Acta, 34, 165(1966). (18) S. V. Elinson, L. I. Pobedina, and A. T. Rezova, Zh.Anal. Khim.. 20, 676 (1965). (19) H. J. Bhattacharya. J. Indian Chem. Soc., 29, 871 (1952). (20) A. K. Babko, Zh.Neorg. Khim., 13, 718 (1968). (21) B. I. Nabivanec, Zh. Neorg. Khim., 11, 2732 (1966). (22) N. Kheddar and B. Spinner, Bull. Soc. Chim. Fr., 2, 502 (1972).
RECEIVEDfor review June 18, 1975. Accepted August 25, 1975.
Extraction of Colored Complexes with Amberlite XAD-2 Raymond B. Willis" and Darrel Sangster Department of Chemistry, Kentucky State University, Frankfort, Ky. 4060 7
iron was analyzed by forming the 1,lO-phenanthroline complex and extractlng the complex from aqueous solution using the adsorbent Amberlite XAD-2. This has several advantages. The iron complex can be extracted using the adsorbent under conditions that would not work for an organic solvent, the iron can be concentrated by a factor greater than 200; and the interference of any ion is no greater when the adsorbent is used and, in the case of chromium(lii), the interference Is totally eliminated.
The usefulness of a colorimetric analytical procedure can be enhanced by extracting the colored species into a nonaqueous solvent. In so doing, it is possible to concentrate the colored species by factors approaching 10 or 20. Also, it is often possible to eliminate interferences that would otherwise make the method useless. Unfortunately, there are times when a nonaqueous solvent cannot be found into which the colored species can be extracted. There are also times when it is necessary to concentrate the species being
measured by factors greater than 10 or 20. In cases like this, a simple solution is to use a solid adsorbent that has a high affinity for organic species and a very low affinity for ionic species. Using a solid adsorbent, it is possible to extract colored complexes that cannot be extracted using a nonaqueous solvent. The complex can be concentrated by much greater factors than can be achieved with a nonaqueous solvent, and some interferences can be eliminated in the process. Alternatively, it is possible to achieve large factors of concentration by using any one of a number of methods of preconcentration and then taking the desired species, form a colored complex and measure the absorbance. An example of this is a method for phenol by Vinson and co-workers ( I ) . In this case, it is necessary that the sample undergo treatment both before and after being concentrated on a column. In the example used in this study, nothing needs to be done to the sample after being concentrated on the column, other than to measure the absorbance of the eluted solution. ANALYTICAL CHEMISTRY, VOL. 48, NO. 1, JANUARY 1976
59
