Anal. Chem. 1989. 6 1 , 1272-1275
1272
Chromatographic Determination of Metal Ions after Complexation with Bis(quaternary ammonium hydrazones) of 2,6-Diacetylpyridine M a r k V. Main a n d J a m e s S. F r i t z * Ames Laboratory and Department of Chemistry, Iowa State University, Ames, Ioua 50011
Two new bis hydrazones of 2,bdiacetylpyrldine were prepared and characterized. Both of these contain a quaternary ammonium group that greatly enhances the water solubility of the reagent and its metal-Ion complexes. When the solid reagent was dissolved in water, an equlllbrlum was attained in which three geometric forms coexist. A rate constant was determined for this equlllbrlum. Several metal ions were determined quantltatlvely and selectively by complexation with one of the bis hydrazones and chromatographic separation of the metal Ion complexes. Interferences due to the presence of other Ions in solution were investigated.
The determination of metal ions by various chromatographic techniques has become increasingly popular owing to the speed, simplicity, and low cost of such methods. An attractive approach is to add an appropriate complexing reagent to the sample and separate the metal complexes by liquid chromatography. Several review articles have been published on this technique (1-6). Most reagents form complexes that are insoluble in water and require a solvent extraction step prior to the chromatographic separation. Although, this does allow some preconcentration of the metal complexes, it is more convenient to use a reagent that forms water-soluble complexes. Water-soluble derivatives of some popular reactants have been synthesized, as in the case of @-diketones( 7 ) and hydroxamic acids (8). Other reagents, such as lJ0-phenanthroline, form charged metal complexes thereby making them water soluble (9). However, no one class of complexing reagents works for all metals and typically only a few metals are determined in a single chromacographic separation. The low concentrations of solutes used in modern chromatography make it somewhat difficult to find complexes that will not fall apart to some extent during the chromatographic separation. Metal parts in the chromatographic system and ion-exchange properties of silica-based column packings contribute to the instability of some metal complexes. Addition of complexing reagent to the eluent (as well as to the sample) enhances the stability of the complexes due to an equilibrium effect, and permits re-formation of the complexes if dissociation tilkes place (7,8). However, adding complexing reagent to the eluent is unnecessary when the metal complexes are kinetically inert. Although the dissociation might be favored thermodynamically, the kinetics of the dissociation are slow and the metal complexes traverse the chromatographic system intact. Derivatives of 2,6-diacetylpyridine bis(aroy1hydrazone) constitute a class of chelating reagents that have received very little attention from analytical chemists. Researchers in Italy have studied several of the metal complexes in the solid state by X-ray and spectroscopic techniques (10-1 7 ) . Four chelate rings can be formed by one molecule of reagent, a feature that undoubtedly enhances the stability of the metal complexes. 0003-2700/89/0361-1272$01 5010
Casoli et al. published a short paper on the use of 2,6-diacetylpyridine bis(benzoylhydrazone) for the chromatographic separation of uranium(V1) and copper(I1) (18) and for the determination of uranium(V1) by spectrophotometry and by differential pulse polarography (19). Garcia-Vargas et al. used the same reagent to determine antimony in non-ferrous alloys (20). Bonilla-Alvarez et al. published a paper on the concentration and determination of uranium using two slightly different bis hydrazones (21). In the present work two new bis hydrazones were prepared which contain two quaternary ammonium groups. Both the reagent itself and the metal ion complexes had a far greater water solubility than previously prepared bis hydrazones of 2,6-diacetylpyridine. The new reagents were used for the chromatographic determination of several metal ions.
EXPERIMENTAL SECTION Synthesis and Characterization of H2dapmp. T o a 500-mL round-bottom flask were added 0.012 mol of Girard’s Reagent P (Aldrich) and 0.006 mol of 2,6-diacetylpyridine (Aldrich) in approximately 250 mL of absolute ethanol. The mixture was refluxed for 3 h and the resulting white precipitate was collected. The product (2,6-diacetylpyridine bis(Nmethylenepyridiniohydrazone) dichloride (abbreviated H2dapmp)) was then recrystallized from a 85%/15% ethanol-water solvent. Products were allowed to precipitate overnight before collection. The product (2,6-diacetylpyridine bis(N-methylene-N,N,N-trimethylammoniohydrazone) dichloride (abbreviated H2dapmta)) was synthesized from Girard’s Reagent T (Aldrich) and 2,6-diacetylpyridine in a similar manner. The following procedures were used to characterize the H2dapmp. The infrared spectrum was taken on a IBM IR/98 (KBr pellet). The NMR spectrum was obtained on a Nicholet NT-300 using D20 (99.8%) as the solvent and chemical shift standard. The mass spectrum of the sample was taken on a Kratos MS-50 employing a fast atom bombardment (FAB) procedure. The matrix was a thioglycerol/water mixture and the background was subtracted out electronically. LC Studies. The chromatographic system consisted of a LKB 2150 HPLC pump, LKB 2152 HPLC controller, LKB 2040-204 low-pressure mixer, LKB 11300 Ultrograd mixer driver (mixing chamber volume of 0.5 mL), Rheodyne 7000 injector (10 pL injection loop), and a Kratos Spectroflow 783 UV-vis detector. The column was a PLRP-S 100-8, 5-pm polystyrene-divinylbenzene (15 cm X 4.6 mm) column from Polymer Laboratories. Eluents were prepared from Fisher HPLC grade acetonitrile and water purified with a Barnstead Nanopure I1 system on a percent volume basis. No corrections were made for nonadditive volumes. Acids, bases, and ion-pairing salts were all reagent grade or better. All eluents were prepared by diluting 0.5 M formate (pH 3.3) and 1.0 M NaC104 to yield 25 mM formate and 50 mM NaClO,. Possible changes in apparent pH were ignored. The eluent components were mixed and filtered with 0.2-pm Nylon 66 filters (Rainin) before use in C 1989 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 61, NO. 11, JUNE 1, 1989
1273
Table I. Characterization of Hzdapmp melting ("C)
cm-'
>300
3441 3132 3053 2976 1690 1636 1570 1489 1273 685
Hzdapmta : R = CH2
Hzdapmp: R = CHz
I
IR maxima,
I
I
{N+ n \
range
CI-
H C- 1 NLCH,
C i
Figure 1. Structure of H2dapmp and H2dapmta.
the chromatographic system. The flow rate was 1 mL/min. The detection wavelength was 340 nm. Preparation of Solutions. Metal ion stock solutions were prepared either from metal ion salts or from the metal. Uranium(VI), thorium(IV), and iron(II1) stock solutions were all made from the nitrate salts. Vanadium(1V) and -(V) stock solutions were made from vanadyl sulfate and vanadium pentaoxide in H2S04, respectively. Titanium(1V) solutions were prepared by dissolving titanium metal in a HN03/H,S0, mixture. Stock solutions for the interferent metals were all made from the chloride or nitrate metal salts. The UV-vis spectra of H,dapmp and titanium complex were taken a t a p H of 3.3 in a 20 mM formate buffer. Both spectra were blanked with this formate buffer and contained 40 pM Hzdapmp. Enough Ti(1V) was added to H2dapmp to yield 10 pM titanium complex. Chromatographic samples were prepared by taking an aliquot of stock metal solution and adding enough reagent to make the final sample 0.4-4 mM H,dapmp. All solutions were buffered a t pH 3.3 (20 mM formate). All adjustments in pH were done with NaOH. Samples also contained NaClO, (20 mM). All samples were allowed to stand 30 min before injection to ensure complete formation of the complexes. The samples for the interference studies were prepared as described except that the H2dapmp was held constant a t 2 mM, and no NaC10, was added. Metal complexes were formed by adding excess reagent to a somewhat acidic sample and then adjusting the pH to 3.3. Sample and interferent metals were both present before the addition of reagent. All interferent samples were allowed 1 h to equilibrate before injection.
RESULTS AND DISCUSSION Selection and Synthesis of Chelating Reagents. Bis(aroylhydrazones)of 2,6-diacetylpyridine are easily prepared and form very stable complexes with a number of metal cations. However, these reagents and their metal ion complexes are very insoluble in water and require a high proportion of an organic solvent to keep in solution. Girard's P reagent and T reagent both have a quaternary ammonium group in order to form water-soluble hydrazones after reaction with an aldehyde or ketone. Both the P reagent and T reagent reacted readily with 2,6-diacetylpyridine to form a product with good water solubility that formed strong complexes with a number of metal ions. The structural formula of these reagents is shown in Figure 1. Most of the chromatographic studies were performed with Hzdapmp although the H2dapmta appears to function in a similar manner. The physical characteristics for H2dapmp are given in Table I. Conditions for Detection. The metal complexes of H,dapmp and the free reagent both absorb strongly in the UV spectral region. Spectra for H2dapmp and titanium(1V) complex are shown in Figure 2. The other metal complexes studied had a yellow or yellow green color and gave spectra similar to that for the titanium complex, except for the iron(II1) complex, which was brown. Detection a t 340 nm
major mass spectral lines m f z (700)
proton shift of NMR
(integration)
466.2 (22.0) 430.2 (100) 352.2 (17.2) 337.1 (28.8) 280.1 (14.5) 254.1 (11.7) 215.6 (14.4) 181.0 (14.8)
8.80-8.59 8.59-8.44 8.15-7.67 6.03-5.84 5.64-5.53 5.53-5.24 2.63-2.55 2.42-2.36 2.30-2.22
(6.57) (3.00) (11.9) (2.45) (1.60) (1) (1.56) (2.75) (3.78)
1.20
0.96
0.72 W 0
z
8
wa (r
m
0.46
0.24
200
200
360
440
WAVELENGTH (nm) Figure 2. Absorbance spectra of H2dapmp(-
- -) and titanium com-
plex (-).
provided good sensitivity while minimizing the absorbance of the reagent. The molar absorptivities of the metal complexes studied were in the 3000-8000 (M.cm)-' range a t the detection wavelength. Chromatographic Conditions. It was decided to make the reagent more selective by working at an acidic pH. Earlier work (21) with 2,6-diacetylpyridine bis(furoy1hydrazone) showed that maximum formation of iron(III), titanium(IV), and uranium(V1) complexes occurred a t or below p H 3.0. Therefore, a p H of 3.3 was selected for the present work. Formate buffers were incorporated into the eluent and samples to maintain the pH a t 3.3. All chromatographic separations were carried out on a column packed with styrene-divinylbenzene polymeric beads. Previous work ( 7 ) had shown that the use of such columns avoids the partial decomposition of metal-organic complexes that often results when silica-based column packings are used. All separations of the metal complexes employed mixtures of water and acetonitrile in the eluent. Other authors have shown that acetonitrile is a better wetting agent than methanol for polymeric resins and yields better separations ( 7 ) . The two quaternary ammonium groups impart a 2+ charge to the complexing reagent, which will attract anions to form an ion pair. The chromatographic retention times of the reagent as well as the metal complexes are likely to be affected by the particular anion that is present. To avoid possible difficulty, sodium perchlorate was added to the eluent. Perchlorate was chosen because it is known as a good inorganic anion for ion pairing ( 7 ) .
1274
ANALYTICAL CHEMISTRY, VOL. 61, NO. 11, JUNE 1, 1989
I
-i
r ~ . T
0
4
~
8
1
2
Time(minules)
Figure 3. Separation of metal complexes of H,dapmp on a PLRP-S column. Eluent conditions: gradient elution; solvent A, acetonitrile/ water (10:90); solvent B, acetonitrile/water (40:60):linear gradient 70 % A, 30% B at 0.0 min to 100% B at 10 min.
Several metal ion complexes of H,dapmp can be separated by isocratic elution using acetonitrilewater eluents containing 18% to 28% acetonitrile. However, better separations were obtained by using gradients in which the percentage of acetonitrile in the eluent was increased. Figure 3 shows a separation of titanium(IV), iron(III), vanadium(V), and uranium(V1) complexes under gradient conditions. The concentrations of the metals in order of elution were 5 X 1X 1X and 5 X M. The four metal-complex peaks were separated nicely with little tailing. The complexes were sufficiently stable that it was not necessary to add reagent to the eluent. Characteristics of t h e Reagents. The chromatogram in Figure 3 shows three peaks from the excess Hzdapmp reagent. A t first, it was thought that the extra peaks were impurities in the reagent. Several attempts were made to purify the reagent further, but three reagent peaks remained in the chromatograms. It was finally ascertained that the multiple reagent peaks were due to (E,E)-Hzdapmp,(E,Z)-H,dapmp, and (Z,Z)-Hzdapmp. Other researchers have also reported E / Z geometrical isomers in similar hydrazone reagents (18, 22). The earliest eluting reagent peak in Figure 3 appears to be the chelating geometric isomer because it decreased significantly if an additional amount of metal ion was added to the sample solution. However, when an excess of metal ion (titanium(1V)) was added to the sample and the solution was chromatographed several days later, all three reagent peaks were absent. This indicates that an equilibrium exists between the three geometrical isomers. This equilibrium has been studied by other researchers on N-acyl- and N-aroylhydrazones in organic solvents (22). In the synthesis and purification it appears that the hydrazone reagents crystallize as the E,E isomer. When solid H,dapmp was brought into solution as quickly as possible and a portion of the solution injected onto the LC column, the first reagent peak was quite large and the second and third peaks were almost missing. Subsequent injections of the reagent solution after various time periods show that the first peak was decreasing with time and the second and thrid peaks were increasing. The E,E isomer was apparently being converted into the E,Z and Z,Z forms so that an equilibrium was reached
0
i
I 4
8
12
16
Time (minutes)
Figure 4. Separation of metal complexes of H,dapmp on a PLRP-S column. Eluent conditions: gradient elution: solvent A and B same as Figure 3: linear gradient 80% A, 20% B at 0.0 min to 100% B at 16 min.
in which all three geometric isomers are in solution. From chromatographic data a first-order rate constant of 8 X min-' was calculated for the conversion of (E,E)-Hzdapmpinto the other isomers at pH 3.3. This rate constant is only an approximation because no strict control of temperature was undertaken. The existence of three geometric forms of the reagent causes no real problem in the separation of metal ion complexes because none of the metal complexes studied had a retention that coincided with one of the reagent isomers. However, the existence of three isomers means that a larger excess of the reagent will be needed to completely complex the metal ions in the sample. Possibly a smaller excess of the reagent could still be used if a sufficient time is provided for equilibration. Presumably only the E,E isomer reacts with a metal ion. Chromatographic Separation of Metal-Hydrazone Complexes. At an approximately neutral pH a fairly large number of metal ions were found to form colored complexes with Hzdapmp and H,dapmta. However, the hydrazone reagents become more selective as the sample was made more acidic. At pH 3.3 we were able to obtain chromatographic peaks for the following metal ion complexes: titanium(IV), iron(III), vanadium(1V) and -(V), thorium(IV), and uranium(VI). Figure 4 shows that most of these complexes can be completely resolved by using gradient elution. In Figure 4 it will be noted that only the complexes of the very heavy metals thorium(1V) and uranium(V1) elute after the first reagent peak. It probably is not meaningful to speculate on the reasons for this. It is known that crystallized metal-hydrazone complexes can be prepared that are neutral or positively charged (10). The nature of the metal complexes formed has not been established. In Figure 4 it will be noted that peaks for both vanadium(IV) and -(V) complexes with H,dapmp were obtained. This indicates that speciation of different oxidation states of vanadium should be possible. However, complexation and chromatographing a sample containing only vanadium(1V) gave peaks for both vanadium(1V) and -(V) complexes. This happened even when a freshly prepared solution of vanadium(1V) was used and the complexation was carried out under an inert gas. Apparently partial oxidation occurred during complexation. If the sample was prepared in 10 mM ascorbic
ANALYTICAL CHEMISTRY, VOL. 61, NO. 11, JUNE 1, 1989
Table 11. Comparison of Retention Times for Hzdapmp and H,dapmta Metal Complexes" t,, min Ti02+ Fe3+ V02+ V02+ Tho2+ UO+ : Hzdapmp H2dapmta
3.4 2.8
5.5 4.6
5.8 4.9
8.2 7.1
10.0
13.3 12.0
"Eluent conditions: gradient elution; solvent A and solvent B same as Figure 3; linear gradient 80% A, 20% B a t 0.0 min to 10% A, 90% B a t 16 min. Flow rate = 1.0 mL/min.
Table IV. Concentration (M) of Interferent Ions Tolerated in Determination of Sample Ions" interferent ion Na+ Ca2+ co2+ Mn2+ Zn2+ Cu2+ ~ 1 3 +
Table 111. Eluent Conditions for Calibration of Various Metals by HPLC Using Hzdapmp as the Complexing Agenta metal ion Ti(1V) U(V1) Fe(II1) V(V)
elution conditions 18% CH3CN 82% H20 2870 CH,CN 72% HZO 25% C H X N 75% Hz6 19% CH3CNb 81% HZO to 40% CHBCN 60% H20
linear calibration curve range, M 9
X
detection limit, M
to 1 X
8
X
1X
lo4
to 3
X
2
X
4
lo4
to 7
X
4
X
X
2 x 104to 1 x 10-3
low8
3 x 10-7
"For all eluents: Flow rate = 1.0 mL/min; UV detection, h = 340 nm, injection volume = 10 ILL,25 mM formate (pH 3.3), 50 mM NaC104. *Gradient elution: 10 min linear gradient.
acid, then only the peak from the vanadium(1V) complex was observed in the chromatogram. However, if vanadium(V) was also present, in the sample containing the ascorbic acid, it was reduced to vanadium(1V). Vanadium(V) can be complexed with Hpdapmp and chromatographed to give only the vanadium(V) peak. Comparison of Hzdapmpand Hzdapmta. Although most of the work was done with H,dapmp, the other reagent (Hzdapmta) gave very similar results. Table I1 shows that H,dapmta gives very similar retention times of the metal complexes when compared with H,dapmp. The order of elution was identical for both reagents, the only difference being that Hzdapmta was more hydrophilic than H2dapmp, as were its metal complexes. Calibration and Quantitation. Calibration curves were prepared for titanium(IV), uranium(VI),and iron(II1) under isocratic elution conditions and for vanadium(V) under gradient conditions in order to test whether quantitative determinations are feasible. The elution conditions, working ranges, and limits of detection are given in Table 111. All metal complexes eluted between 3 and 8 min under the conditions used. Good linear calibration curves were obtained over a concentration range of at least 2 orders of magnitude for the first three metal ions and over almost 3 orders of magnitude for vanadium(V). Possibly longer working ranges could be obtained by using a larger amount of reagent during the complexation step. These results indicate that good quantitative results should be obtained with this chromatographic method. Of course, quantitative results in any gradient method require that the conditions for the gradient are accurately reproduced from one run to the next.
1275
Hg2+ Zr02+ Ho3+
sample ions Fe3+ v02+
Ti02+ 0.1 0.1 0.001 0.001 (-) 0.001 (-) 0.001 (-) 0.01 0.001 0.0001 (-1 0.01
0.1
0.1 0.0001 (+) 0.01 0.01 0.001 (-1 0.01 0.001 0.001 0.01
0.1 0.1 0.0001 (+) 0.01 0.01 0.0001 (+) 0.01 0.001 0.001 0.001 (+)
UO?+ 0.1 0.1 0.001 0.001 (-) 0.001 (-) 0.0001 (-) 0.01 0.001 0.001 0.01
"Eluent conditions: same as Figure 3. The concentrations of Ti(IV), Fe(III), V(V), and U(V1) were 2 X 1X 1X and 3 x M, respectively. See text for explanation of values cited.
Interferences. The effect of possible interfering ions was studied by adding varying amounts of selected foreign ions to samples of metal ions to be determined. A foreign ion was considered to interfere when the peak height of a metal complex was changed by more than *470. Results are given in Table IV. For convenience, the concentrations of foreign or - in parentheses ions were varied by decades. The indicates the direction of the error. However the actual interference begins somewhere between the concentration listed and a 10-fold higher concentration.
+
LITERATURE CITED Willeford, 9.; Veening, H. J . Chromatogr. 1982, 251, 61. Veening, H.; Willeford, 9 . Adv. Chromatogr. 1983, 22, 117. O'Laughlin, J. J . Liq. Chromatogr. 1984, 7(S-1), 127. Nickless, G. J . Chromatogr. 1985, 313, 129. Steinbrech, 9. J . Liq. Chromatog. 1987. 10, 1. Tirnberbaev, A. R.; Petrukhin, 0. M; Zelotov, Yu. A. Z .Anal. Chem. 1987, 327,87. Palrnieri, M. D.; Fritz, J. S.Anal. Chem. 1987, 59,2226. Palrnieri, M. D.; Fritz. J. S.Anal. Chem. 1988, 60,2244. O'Laughlin, J. W. Anal. Chem. 1982, 54, 178. Paoiucci, G.; Marangoni, G.; Bandoli, G.; Clernente, D. A. J . Chem. SOC.,Dalton Trans. 1980, 1304. Pelizzi, C.; Pelizzi, G. J . Chem. SOC.,Dalton Trans. 1980, 1970. Pelizzi, C.; Pelizzi, G.; Predieri, G.; Resola. S.J . Chem. Soc.. Dalton Trans. 1982, 1349. Lorenzini, C.; Pelizzi, C.; Pelizzi, G.; Predieri, G. J . Chem. Soc., Dalton Trans. 1983, 721. Pelizzi, C.; Pelizzi, G.; Predieri, G. J . Organomet. Chem. 1984, 263, 9. Pelizzi, C.; Pelizzi. G.; Predieri. G.; Vitali, F. J . Chem. Soc., Dalton Trans. 1985, 2387. Pelizzi, C.; Pelizzi, G.; Vitali, F. Transition Met. Chem. 1986, 1 1 , 401. Pelizzi, C.; Pelizzi, G.; Vitaii. F. J . Chem. Soc., Dalton Trans. 1987, 177. Casoli. A.; Mangia. A,; Predieri, G. Anal. Chem. 1985. 57, 561. Casoli. A.; Mangia, A.; Mori, G.; Predieri, G. Anal. Chim. Acta 1988, 186, 283. Garcia-Vargas, M.; Belizon, C.; Milla, M.; Perez-Bustarnante, J. A. Analyst 1985,-110, 51. Bonilla-Aivarez. M.; Palrnieri. M.; Davis, D.; Fritz, J. S. Talanfa 1987, 473. Palla, G.; Predieri, G.; Dorniano, P.; Vignali. C.; Turner, W. Tetrahedron 1986, 42,3649.
RECEIVED for review December 2, 1988. Accepted March 10, 1989. Ames Laboratory is operated for the U.S. Department of Energy under Contract No. W-7405-Eng-82. This work was supported by the Director of Energy Research, Office of Basic Energy Sciences.
