228
Anal. Chem. 1982, 54, 228-231
(14) Chrlswell, C. D.; Chang, R. C.; Fritz, J. S. Anal. Chem. 1975. 47. 1325. (15) Court, W. A. J . Chromatogr. 1977, 130, 287. (16) Jayaraman, S.; Vasundhara, T. S.; Parihar, D. B. Mlkrochim. Acta 1978. II.365. (17) Coates, J. P. J. Insf. Pet. 1971, 57, 209. (18) Schabron, J. F.; Fenska, L. E. Anal. Chem. 1980, 52, 1411. (19) Schabron, J. F.; Hurtubise, R. J.; Sliver, H. F. Anal. Chem. 1979, 51, 1426. (20) von Wandruszka, R. M. A.; Hurtubise, R. J. Anal. Chem. 1978, 48, 1784.
-.
(21) Ford. C. D.: Hurtubise. R. J. Anal. Chem. 107Q. 51. - , 659 --(22j Niday, G. J:; Seybold, P. G. Anal. Chem. 1978, 50, 1577. (23) Cline Love, L. J.; Skrilec, M. Am. Lab. (FalrfleM, Conn.) 1981, 13 (No. a), 103.
RECEIVED for review June 22,1981. Accepted November 9, 1981. Financial support for this project was provided by the Department of Energy, Division of Basic Energy Sciences, Contract No. DE-AC02-80ER10624.
Spectrophotometric, Potentiometric, and Gravimetric Determination of Lanthanides with perbDihydroxynaphthindenone Saad S. M. Hassan*l and W. H. Mahmoud Department of Chemistry, Faculty of Science, Ain Shams University, Cairo, Egypt
Sensltlve and reasonably selectlve methods are descrlbed for the spectrophotometrlc, potentlometrlc, and gravlmetrlc determfnatlon of lanthanldes using perl-dlhydroxynaphthlndenone as a novel chromogenlc and preclpltatlng reagent. The reagent forms a stable 1:2 (metakreagent) type of complex wlth light lanthanides at pH 2-7 In 1:l ethanol-water mlxture. Low metal concentratlons ( < I O pg/mL) develop 580-590 nm, emax (4-6) X lo4 L mol-' colored specles (A, cm-')whlch obey Beer's law. Quantltatlve preclpltatlonof the complexes from metal solutlons of concentratlons >100 pg/mL permlts both gravlmetrlc quantltatlon by lgnltlng the preclpltatesto the metal oxides and potentlometrlc tltratlon of the excess reagent. Results wlth an average recovery of 98% (standard devlatlon 0.7%) are obtainable for 0.1 kg to 200 mg of all light lanthanldes. Many forelgn Ions naturally occurrlng or frequently assoclated wlth lanthanldes do not Interfere or can be tolerated.
The worldwide interest in lanthanides as fission products and their uses for production of some modified alloys, ceramics, optical crystals, and semiconductors have necessitated the development of suitable methods for their accurate determination. Among the various instrumental methods, spectrophotometry has shown to be a simple and reliable technique applicable for quantitation of a wide range of concentrations (1-17). However, many of these methods are not sensitive enough to permit accurate determination of trace levels of these metals, involve a time-consuming extraction step, develop unstable colors, and require a long time for full color development (6,8,10,13,16).On the other hand, metals frequently associated or naturally occurring as contaminants with lanthanides (e.g., alkaline earths, zinc, and aluminum) seriously interfere with most of these reagents (3, 6, 12, 14, 16,17).
In previous study, we have demonstrated that a-amino acids, ascorbic acid, and hydrazines quantitatively reduce Present address: Department of Chemistry, University of Delaware, Newark, Delaware 19711. 0003-2700/82/0354-0228$01.25/0
peri-naphthindane trione into peri-dihydroxynaphthindenone (18-20). The similarity between the structure of peri-dihydroxynaphthindenone and some of the lanthanide chromogens stimulated us to investigate the usefulness of this compound, which has never been used in analytical chemistry, for lanthanide quantitation. In the present paper, we wish to point out the potential usefulness of peri-dihydroxynaphthindenone as a novel multipurpose reagent for lanthanides. The keto-ene-diol chelating group of this reagent proved to be suitable for both separation and determination of light lanthanides by spectrophotometric, potentiometric, and gravimetric techniques. The reagent offers clear advantages in terms of sensitivity, selectivity, and wide applicability over many of the available chromogenic and precipitating reagents.
EXPERIMENTAL SECTION Reagents. All the reagents used were of analytical reagent grade unless otherwise stated. Twice distilled organic solvents and deionized water were used throughout. peri-Naphthindane trione hydrate was prepared from naphthalic anhydride according to the method of Errera (21). A 0.5 M aqueous solution of the trione was treated at 70 "C with an excess of 0.5 M ascorbic acid to precipitate peri-dihydroxynaphthindenone.The precipitate (red crystals, mp 258 "C) was recrystallized out from 1:l acetic acid-water mixture. The precise molecular weight obtained by mass spectrometry and elemental analysis conformed with the molecular formula C13H803.The infrared and nuclear magnetic resonance spectra conformed with the presence of a keto-ene-diol group substituted at the 1:8 position of the naphthalene nucleus. Working solutions of the reagent (5 X 10-1 and 5 X 10-3M) were prepared in 96% ethanol and standardized by potentiometric titration with standard iodine solution using a platinum-calomel electrode system. Lanthanide oxides of purity not less than 99.9% were obtained from Sigma Chemical Co. (St. Louis, MO). Stock 1mg/mL lanthanide(II1) perchlorate solutions of pH 3-5 were prepared and standardized by visual titration with EDTA (22). Cerium(II1) perchlorate was prepared from Ce(OH)3obtained by reaction of cerium(II1) nitrate with sodium hydroxide. Apparatus. The spectrophotometric measurements were carried out with a UNICAM SP 1800 spectrophotometer at 25 A 0.5 "C using 1.00-cm quartz cuvettes. The potentiometric measurements were made with a PYE UNICAM M-290 pH meter using an Orion combined platinum-calomel electrode (Model 96-78) and an Orion iodide ion selective electrode (Model 94-53) 0 1982 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 54,NO. 2,FEBRUARY 1982 229
in conjunction with E.1.L platinum electrode. Spectrophotometric Determination of Lanthanides. Various volumes (0-4mL) of lanthanide(II1)perchlorate solution (10 pg/mL) were transferred to 10-mL volumetric flasks and a I-mL aliquot of 1% w/v aqueous gelatine solution was added M of the reagent. The solutions followed by 2 mL of 5 X were diluted to the mark with a 1:l ethanol-water mixture, shaken, and allowed to stand for 15 min for full color development, and the absorbance was measured at 580-590 nm in a 1.00-cm cuvette against a blank similarly prepared but without the metal ions. Solutions containing up to 1mg/mL of copper(I1) and zinc (11) ions were similarly determined after addition to 1mL of 2% w/v aqueous sodium thiosulfate solution just before the addition of the reagent. Lanthanide solutions containing up to 10 pg/mL of the metal and 1 mg/mL of Cd2+,Pb2+,Fe3+, BiS+,Th4+,and U4+ were determined after extraction of the interfering metal ions. A 5.0-mL aliquot of the solution was transferred to a 50-mL separatory funnel followed by 5 mL of 0.1 M sodium diethyldithiocarbamate (NaDDC) and 25 mL of a 1:l isoamyl alcohol-benzene mixture. The mixture was shaken for 5 min, the two phases were allowed to separate, the aqueous phase was removed, and extraction was repeated once again, and the aqueous phase washed with two 25-mL portions of a 1:l isoamyl alcohol-benzene mixture. The aqueous phase was then quantitatively transferred to a 25-mL volumetric flask, followed by 1 mL of 1% w/v aqueous gelatine and 5 mL of 5 X 109 M of the reagent. The volume was completed to the mark with 96% ethanol, the solution was shaken and allowed to stand for 15 min, and the absorbance was measured in a 1.00-cm quartz cuvette against a blank similarly prepared but without the metal ions. Potentiometric Determination of Lanthanides. A 0.2-1.5 mL aliquot of the lanthanide solution (1mg/mL) was transferred to a 20-mL graduated centrifuging tube and a 5.0 mL aliquot of M of the reagent added. The precipitate was allowed 5X to settle and the volume completed to 10.0 mL with a 1:l ethanol-water mixture. After centrifugation for 2 min at 3000 rpm, 5.0 mL of the supernatant was transferred to a 50-mL beaker and diluted with an equal volume of a 1:lethanol-water mixture. The M solution was than potentiometrically titrated with 5 X iodine solution using either a platinum-calomel electrode system or an iodide ion selective electrode in conjunction with a platinum electrode. Visual titration was alternatively used by addition of 5 mL of 4% w/v potassium iodide, 1mL of a 1% w/v aqueous starch solution, and 2 mL of 4 M hydrochloric acid to a 5.0-mL M aliquot of the supernatant followed by titration with 5 X aqueous N-bromosuccinimide (NBS) until a blue color appears. Blank experiments were made in both cases under identical conditions and the metal content was calculated from the difference in the titers (2 mol of I2 1mol of NBS M3+). Gravimetric Determination of Lanthanides. Aliquots of the lanthanide solutions (1-5 mL) containing 50 mg/mL of the metal were transferred to a 100-mL covered beaker and diluted with 20 mL of deionized water, the mixture was heated on a water bath at -60 O C , and 10 mL of 5 X lo-' M of the reagent was gradually added. The precipitate was allowed to settle at room temperature for 30 min, filtered off in Whatman No. 40 ashless filter paper, and washed portionwise with 20 mL of 1:l ethanol-water. The precipitate was dried at 120 OC for 1h and ignited in a muffle furnance at 900 OC till a constant weight of the metal oxide (-2 h) was obtained. RESULTS AND DISCUSSION Reagent Characteristics. The absorption spectrum of peri-dihydroxynaphthindenone in 1:l ethanol-water shows two maxima at 342 nm (emar 11 X lo3 L mol-l cm-l) and 460 nm,e( 3.6 X lo3L mol-' cm-'). The latter absorption band is pH independent in the range of 2-7 and significantly affected by chelation with lanthanide metal ions. Above pH 7.5, this band is subject to a bathochromic shift to 510 nm with the appearance of an isobestic point a t 490 nm. The pH-absorption profile (Figure 1) was used for the measurement of the pK, of the reagent by plotting the absorption of the ionized and nonionized species at 510 and 460 nm, respectively, or the ratio of their logarithms as a function of the
I
440
480
I
I
520
560
Wavelength, [ n m )
Figure 1. Absorption spectra of peridihydroxynaphthindenone (1.75 X IO4 M) at various pH values: (-) 2-7; (0) 7.9;(0) 8.1; (A)8.4; (A)8.9; (0) 9.3; (m) 9.7.
440
480
520
560
600
640
Wavelengh, ( n m )
Figure 2. Absorption spectrum of lanthanum-peridihydroxynaphthindenone in 1:l ethanol-water at pH 5 using 1 pg/mL La. Table I. Molar Absorptivities and Formation Constants of Some Lanthanides-peri-Dihydroxynaphthindenone
Complexes
lanthanide La ce Pr Nd Sm Eu Gd
Amax,
nm
580 f 1 581 f 2 583f 1 583 f 2
585 f 2 586i 2 588 f 2
log K 4.3 x 4.1 x 5.0 x 4.4 x 4.8 x 5.2 x 5.4 x
104 104 104 104 104 104 104
9.04 9.38 9.53 9.90 9.96 10.04 9.64
pH (23). The average pK, value obtained by both methods is 8.7. Reaction with Lanthanides. peri-Dihydroxynaphthindenone reacts at pH 2-7 with all the lanthanide metal ions in 1:l ethanol-water to form stable-colored complexes. Metal concentrations lo0 pg/mL in the absence of gelatine. Complex Composition a n d Stability. The composition of lanthanide-peri-dihydroxynaphthindenonecomplexes was determined by the continuous variation, molar ratio and slope ratio methods (23). A 1:2 metal-reagent type of complex
230
ANALYTICAL CHEMISTRY, VOL. 54, NO. 2, FEBRUARY 1982
Table 11. Spectrophotometric, Potentiometric, and Gravimetric Determination of Some Lanthanides Using peri-Dihydroxynaphthindenone spectrophotometry potentiometry gravimetry amt added, % std dev, amt added, % std dev, amt added, % lanthanide fig recoverya % mg recoverya % mg recoveryaib La 0.1-2.5 98.2 0.7 0.2-1.0 98.8 0.5 50-100 98.1 Ce 0.4-3.0 97.7 0.6 0.3-1.5 97.9 0.4 70-150 97.1 Pr 0.1-4.0 99.5 0.9 0.2-1.5 99.1 0.4 50-200 98.6 Nd 0.2-3.5 97.8 0.8 0.2-1.2 98.5 0.3 40-180 98.4 Sm 0.1-3.8 98.5 0.6 0.3-1.5 98.7 0.5 50-150 97.9 Eu 0.1-4.0 98.7 0.7 0.2-1.3 99.5 0.3 80-200 98.0 Gd 0.1-3.2 98.4 0.7 0.2-1* 1 98.7 0.4 60-140 98.1 a Mean of five measurements. Ln,O, is the weighing form of all the metals except praseodymium (Pr6011).
std dev, %
0.9 1.1 1.0 0.8
0.9 0.7 0.8
I
-
E 0.6-
In u
i 0.4 0
m
.
'? 67
2
0.2
+320-
- -380
+280-
- -420
Z +240-
- -460
-; --
0
a
-
0
2
8
4 [ La
I/(
La
8
+200-
- -500
+160-
- -540
I+ (R 1 1
Figure 3. Continuous variation plot for the stoichlometry of lanthanum-perfdlhydroxynaphthindenone complex (total molar concentration
H 90
Spectrophotometric Measurements. Beer's law is obeyed with metal concentrations in the range of 0.1-4 Fgg/mL with an average sensitivity of about 3 X Hg/mL (equivalent to log Zo/Z = 0.001). The color is stable for at least 6 h and changes of the pH in the range 2-7, temperature from 20 to 40 OC, ionic strength from 0.01 to 0.7 M KN03, and reagent concentrations up to 10-fold molar excess have no
1
2
1
1
,
1
1
1
6 mL
4
Iodine, ( 0 . 0 0 5 M )
5 x 10-4 MI.
(Figure 3) was confirmed with the light lanthanides (La, Ce, Pr, Nd, Sm, Eu, Gd). Heavy lanthanides (Er, Tm, Yb, Lu), however, showed no definite stoichiometry fluctuating from 1:l to 1:2 (metalxeagent). The overall formation constants of the light lanthanide complexes were calculated by using the continuous variation method (23). The data (Table I) show that these complexes exhibit remarkable high stability as compared to the lanthanide complexes of many known hydroxy and ketohydroxy ligands (24). The stability of these complexes increases uniformly with the increase of the atomic number from La to Eu, decreasing to Gd, a rather severe example of the "gadolinium break" which is apparently always observed with the rare earth chelates (24). Complex Structure. Since peri-dihydroxynaphthindenone reagent is a bidentate doner with two -OH groups available for chelation and the lanthanide metal ions are in the stable trivalent state, it is possible to assume that one -OH group from one molecule of the reagent partakes in the complexation with two -OHgroups of a second molecule (eq 1). Elemental analysis and infrared, and laser-Raman spectra of the solid complexes of the light lanthanides confirm with the general molecular formula 2L:M:2H20. Detailed structural characterization of these complexes will be published elsewhere.
1
Figure 4. Typical potentiometric titration curves of pen'dihydroxyM) with iodine solution using ( 0 ) naphthlndenone (10 mL of 3 X platinum-Mi ion selective electrode system (negativepotential scale) platinum-calomel electrode system (positive potential scale). and (0) influence on the intensity and stability of the color. However, increasing the pH above 10 causes precipitation of both the metal hydroxide and gelatine turning the solution turbid, whereas decreasing the pH below 2 discharges the color due to complex disintegration. Determination of the light lanthanides in the range of 0.1-4 fig/mL give results with an average recovery of 98.4% and a mean standard deviation of 0.7% (Table 11). The coefficient of variation calculated from 10 repetitive measurements made on 1Hg/mL of lanthanum is 0.6%. Potentiometric Measurements. Lanthanides in concentrations >lo0 pg/mL are determined by precipitation with excess peri-dihydroxynaphthindenonefollowed by iodimetric measurements of the unreacted reagent, since the ene-diol group of the reagent is known to react quantitatively with iodine (25). The titration is potentiometrically monitored using either a platinum-calomel electrode system or an iodide ion selective electrode in conjunction with a platinum electrode. Figure 4, shows a sharp inflection break at the equivalence point by both monitoring systems. One mole of iodine is stoichiometricallyconsumed per mole of the reagent which is converted into the colorless precursor peri-naphthindane trione (eq 2). Similar results are also obtained by visual titration with N-bromosuccinimde. @ H + 1 2
__r
8\
/
00
+ 2HI
(2)
\ /
The results obtained for the determination of as little as 0.2 mg/mL of the light lanthanides (Table 11)show an average recovery of 98.8% and a mean standard deviation of 0.3%. No significant interferences are caused by the presence of 100-fold excess of Mg2+, Ca2+,Ba2+ and A13+ ions.
ANALYTICAL CHEMISTRY, VOL. 54, NO. 2, FEBRUARY 1982
Gravimetric Measurements. Table I1 shows the results obtained for the gravimetric determination of the light lanthanides by precipitation with peri-dihydroxynaphthindenone followed by ignition. The mean average recovery of the metal oxides is 98% and the mean standard deviation is 0.9%. The presence of a 100-fold excess of the alkaline earths does not interfere. These results were further compared with those obtained by precipitation of some lanthanides as oxalate derivatives followed by ignition (26),where 88 f 6% of the anticipated metal oxides was only recovered. Moreover, the oxalate method is highly influenced by the change of the reaction conditions and suffer from severe interferences by the alkaline earths. In contrast, a big host of metals do not form precipitates with peri-dihydroxynaphthindenoneand this permits selective separation of the lanthanides by precipitation from solutions containing other metals. Effect of Foreign Ions. Solutions containing 1 pg/mL of La and Pr admixed with 1 mg/mL of 30 different foreign ions were prepared and examined by both the spectrophotometric and iodiietric procedures. No significant interferences are noticed from Li+, Na+, K+, Cs+, Rb+, Mg2+,Ca2+,Ba2+, SP+,Mn2+,Co2+,Ni2+,Fe2+,A13+, C$+, NO5 mg/mL develop a faint red to violet color. This causes interferences with the spectrophotometric measurement but does not affect the results obtained by the iodimetric method as these soluble complexes along with the excess reagent in the supernatant are titrated with iodine. On the other hand, 100 pg/mL of Pb2+,Cu2+,Zn2+, Cd2+,Fe3+,Bi3+,Th4+,and U4+ions seriously interfere due to the formation of insoluble violet complexes. Direct determination of the lanthanides in the presence of these ions by using some masking agents as EDTA, CN-, S2-, and Fproved not to be suitable. In the presence of S2092-, however, copper and zinc in concentrations up to 1mg/mL are completely masked and the lanthanides are quantitatively recovered by the spectrophotometric procedure. It is fortunate that all the interfering metals can be removed from the lanthanide solutions quite readily by extraction over a wide pH range with an isoamyl alcohol-benzene mixture containing sodium diethyldithiocarbamate (27). Resulta with an average recovery of 97 f 3% are obtained for various lanthanides in their mixtures with a 100-fold excess of Cu2+,Cd2+,Pb2+,Bi3+, Th4+,and U4+by the spectrophotometric procedure after prior extraction of the interfering metals with NaDDC. Advantages. peri-Dihydroxynaphthindenonelike all the available reagenta for lanthanides, complexes the entire series equally well. However, it offers the advantages of wider applicability, higher sensitivity, and reasonable selectivity. It is a multipurpose reagent utilizable for separation and determination of these metals by spectrophotometric, poten-
231
tiometric and gravimetric procedures. The limit of detection is lower than many of the reagents previously reported and the useful range of the spectrophotometric measurement (0.1-4 ppm) can favorably be compared with those of laser emission spectrography (10-60 ppm) (B), X-ray fluorescence (0.2-2 ppm) (29),and atomic absorption spectrometry (0.1-10 ppm) (30). Moreover, alkaline earth, zinc, aluminum, and other ions which commonly interfere with many of the available spectrophotometric procedures (3,6,12-14,16,17) have no effect. Interferences by some other metals can be tolerated by precipitating the lanthanides or extracting the interfering ions.
LITERATURE CITED Ryabchikov, D. I.; Ryabukhin, V. A. "Anaiytlcal Chemistry of Yttrium and the Lanthanlde Elements"; Ann Arbor-Humphrey: . . Ann Arbor, MI, 1970; Chapter V. Fritz, J. S.; Richard, M. J.; Lane, W. J. Anal. Chem. 1058, 3 0 , 1776-1 779. Sawin, S. B. Talanta 1081, 8 , 673-685. Perlslc-Janjlc, N. U.; Muk, A. A.; Canic, V. D. Anal. Chem. 1073, 45 798-801. Budesinsky, B.; Hass, K. 2.Anal. Chem. 1065, 210, 263-270. Herrington, J.; Steed, K. C. Anal. Chim. Acta 1980, 22, 180-184. Rinehart, R. W. Anal. Chem. 1054, 2 6 , 1820-1822. Taketatsu, T.; Kareko, M.;Kono, N. Talenta 1974, 27, 67-91. Chung-Gin, H.; Chao-Sheng, H.; XI-Plng, J.; Jlao-Mal, P. Anal. Chim. Acta 1081, 124, 177-183. Shlbata, S. Anal. Chlm. Acta 1083, 28, 388-392. Munshl, K. N.; Dey, A. K. Mlcrochem. J. 1084, 8 , 152-159. Idrlss, K.; Seleim, M.; Awad, A.; Abu-Bakr, M. Anal. Chim. Acta 1080, 116, 413-416. Murthy, M.; Satyanarayana, D. Mlkrochlm. Acta 1080, 97-105. DeWet, W. J.; Behrens, 0. B. Anal. Chem. 1088, 4 0 , 200-202. Vekhande, C.; Munshi, K. N. Mlcrochem. J. 1078, 23, 28-41. Brittain, H. G. Anal. Chem. 1077, 4 9 , 969-972. Brittaln, H. 0. Anal. Chlm. Acta 1070, 106, 401-403. Hassan, S. S. M. Mlkrochim. Acta 1074, 51-57. Hassan, S. S. M.; El-Fattah, M.; Zaki, M. 2.Anal. Chem. 1975, 277, 369-371. Awad, W. I.; Nashld, S.; Hassan, S. S. M.; Zakharl, R. J. Chem. Soc., Perkn Trans. 2 1078, 128-133. Errera, 0. Qazz. Chlm. Ita/. 1013, 4 3 , 593-599. Vogel, A. "A Text Book of Quantitative Inorganic Analysis", 3rd ed.; Longmans: London, 1961; Chapter IV. Inczedy, J.; Tyson, J. "Anaiytlcai Appllcatlons of Complex Equillbrla"; Horwood: Chlchester, 1976. Moeller, T.; Martin, D. F.; Thompson, L. C.; Ferrus. R.; Feistel, G. R.; Randall, W. J. Chem. Rev. 1065, 65, 1-50. Verma, K.; Gulati, A. Anal. Chem. 1080, 52, 2336-2338. Clinch, J.; Simpson, E. A. Analyst (London) 1057, 82. 258-269. De, A. K., Khopkar, S. M.; Chalmers, R. A. "Solvent Extraction of Metals"; Van Nostrand-Reinhold: London, 1970; Chapter 8. Ishlzuka, T. Anal. Chem. 1073, 45, 538-541. D'Silva, A. P.; Fassel, V. A. Anal. Chem. 1073, 45, 542-547. Vanloon, J. C.; Gaibralth, J. H.; Aarden, H. M. Analyst (London) 1971, 96, 47-50.
RECEIVED for review July 29,1981. Accepted September 30, 1981. This work was presented at the Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Atlantic City, NJ, March 1981.
