Beer's law is obeyed over the range 0.15 -1.5 µg of uranium/ ml. ... HNO3, HF (48%), HC104, and other reagents were of ... crystallized from petroleum ether (90 -100 °C). ... Add. 10 ml of aluminum nitrate solution and 10 ml of ethyl acetate. Shake the funnel and collect the organic ... another 10 ml portion of ethyl acetate.
ANALYTICAL
SCIENCES
Extraction Uranium
DECEMBER
1991, VOL.
7
Spectrophotometric in Geological
931
Method
Samples
2-(p-Methylphenylazo)pyridine
for the Determination
Using a Ternary and Thiocyanate
Complex
of of
Partha CHATTOPADHYAY*t, Tapan KUmar SINHA*and T. S. Basu BAUL** *Geological Survey of India, Chemical Laboratory, Nongrim Hills, Shillong-793 003, India **Regional Sophisticated Instrumentation Centre, North-Eastern Hill University, Shillong-793 003, India
An uranyl ion (UO22+)reacts with 2-(p-methylphenylazo)pyridine in thiocyanate system. The species is extractable quantitatively into methyl isobutyl ketone (MIBK) in the presence of nitrate ion, showing an absorption maximum at 370 nm. Beer's law is obeyed over the range 0.15 -1.5 µg of uranium/ ml. The molar absorptivity and Sandell's sensitivity are 0.166X1041mol-1 cm-1 (on the basis of uranium content) and 0.0014 µg/ cm2, respectively. Various international geochemical reference samples were chosen to check the validity of the method. The results obtained were in good agreement with the published data. Keywords
Spectrophotometry, uranium, 2-(p-methylphenylazo)pyridine, thiocyanate,
Triphenylmethane dyes containing salicylic acid donor groups are sensitive but unselective reagents for uranium in the presence of cationic and nonionic surfactants. Jancer et al. have recommended a spectrophotometric determination of UO22+with Eriochromazurol B (2",6"dichloro-3,3'-dimethyl-4,4'-hydroxyfuchsone-5,5'-dicarboxylic acid) and Chromazurol S in the presence of cationic surfactant Septonex. They have applied this method to the determination of UO22+ in drinking, surface and waste waters.l Kojima et al. have reported a good method for estimation of uranium in ores by spectrophotometry after extraction with trioctylphosphine oxide (TOPO) in a molten mixture of biphenyl and naphthalene.2 O'Leary and Meier proposed a method for determining uranium in rock, soil and stream sediment samples by ultraviolet fluorescence.3 Kanai et al. estimated uranium contents in 36 geological reference samples by fluorometry after ion-exchange separation, and also spectrophotometry using Arsenazo III after solvent extraction with TOPO.4'5 Although the spectrophotometric method is rapid, it is difficult to apply to the samples containing ultra-trace amounts of uranium such as dunite. The application of 2pyridylazo compounds to chemical analysis has been studied extensively.6 1-(2-Pyridylazo)-2-naphthol (PAN) and 4-(2-pyridylazo)resorcinol (PAR) have proved to be the most versatile reagents for metals. In addition to being excellent metallochromic indicators, they can serve as chromogenic agents in the determination or detection t To whom
all correspondence
should
be addressed.
address:
C. P. A. F. Division,
Regional
Research
Bhubaneswar-751013,
India.
Present Laboratory,
extraction
of over 30 elements.6 Both PAN and PAR have been widely used in trace analysis. Unfortunately, both PAN and PAR cannot be applied directly for estimation of uranium in various rocks and minerals. In a project to develop new methods for estimation of uranium contents in geological materials, 2-(p-methylphenylazo)pyridine has been used for spectrophotometric method in thiocyanate (SCN) system. The present method is fairly rapid and sensitive. Uranium contents of various international samples (issued by Geological Survey of Japan, British Geological Survey, U.S. Geological Survey and Analytisk Sporelement Komite (ASK), Norway) determined by the present method have been compared with the published data.
Experimental Apparatus A Beckman 10 mm quartz ments.
double beam spectrophotometer cell was used for absorbance
with a measure-
Reagents a) Stock uranium solution, 1000 µg/ ml. Dissolve exactly 0.211 g uranyl nitrate (U02(N03)26H20) in 100 ml of 4 M HNO3. Prepare 100, 10 and 1 µg/ ml standard uranium solutions by successive 10-fold dilutions of 1000 µg/ ml stock uranium solution with 4 M HNO3. b) A 25%(w/ v) KSCN solution was prepared by dissolving 25 g of KSCN in 100 ml of demineralized water.
932
ANALYTICAL
c) Aluminum nitrate solution: dissolve 1000 g aluminum nitrate nanohydrate (Al(N03)29H20) in 500 ml of hot water. Ethyl acetate, methyl isobutyl ketone (MIBK), HCI, HNO3, HF (48%), HC104, and other reagents were of analytical grade. 2-(p-Methylphenylazo)pyridine was prepared from p-nitrotoluene and 2-aminopyridine following the method described elsewhere.' The crude product was washed with water, air dried and recrystallized from petroleum ether (90 -100 °C). Red needles, m.p. 72 - 74 °C, C12H11N3. Found (%): C, 73.4; N, 21.7; H, 5.6. Calcd (%): C, 73.1; N, 21.3; H, 5.6. Analytical procedure General method. Transfer an aliquot containing 2 - 20 µg uranium to a 150 ml separatory funnel, add 1 ml of approx. 10% HNO3 and 2 ml of 25% KSCN, swirl to dissolve and make the volume to 10 ml with water. Now add 2 ml of 2-(p-methylphenylazo)pyridine (0.01% solution in acetone) and 5 ml of MIBK and shake for 3 min. Collect the organic extract in a 10-ml volumetric flask over anhydrous Na2SO4. Repeat the extraction with another 3 ml of MIBK and finally make up the volume with the solvent and measure the absorbance at 370 nm against the reagent blank reference. Method for geological samples. Weigh 1.0 - 5.0 g of powdered sample in a Teflon beaker. If the sample contains more than 0.5% of non-carbonate carbon, burn out the carbon in a platinum crucible beforehand. Add 3 ml HNO3, 5 ml HC104 and 10 ml HF to the sample and evaporate the solution slowly to dryness on a hot plate. This operation should be repeated at least twice. Dissolve the residue by heating with a minimum of 3 ml of 15% HNO3 (and the volume of the acid used should be proportional when more sample mass is taken) and transfer the solution into a separatory funnel. Add 10 ml of aluminum nitrate solution and 10 ml of ethyl acetate. Shake the funnel and collect the organic layer in a platinum crucible. Repeat the extraction with another 10 ml portion of ethyl acetate. The combined organic solvent was allowed to evaporate slowly. Dissolve the residue in the crucible with 4 M HNO3 and transfer to a separatory funnel and follow the general procedure as is described in general method Add 1 g NH4HF2 and about 2 g ammonium hydrogenphosphate prior to addition of reagent solution.
Results Absorption
and
Discussion spectra
The free ligand in MIBK and SCN shows a sharp absorption maximum at 355 nm, but U022+ forms a complex with the ligand in presence of SCN- and shows a clear and sharp maximum at 370 nm. This wavelength was chosen for absorbance measurements. The role of SCN- in the formation of ligand-UO22+ complex has not been studied in detail. But it is presumed that a ternary U02-ligand-SCN complex is formed, which is
SCIENCES
DECEMBER
1991, VOL.
7
extractable in M [BK. A similar situation was observed in case of Sb(III) which does not react with PAN in water, but a red compound was formed in 0.25 - 0.5 M H2504 in presence of KI. In that case a ternary antimony-PANiodide complex (extractable to benzene) might be formed and this phenomenon can be used to the determination of trace antimony in ores.8 Molar absorptivity of U02-ligand-SCN was found to be 0.166X1041mol-1 cm 1 (on the basis of uranium content), and Sandell's sensitivity is 0.0014 µg/ cm2. Effect of diverse ions The effect of diverse ions on the determination of 5 µg of uranium was investigated by adding known amounts of each ion such as Li, Na, Cs, Rb, Be, Mg, Ca, Sr, Ba, La, Ti, Zr, vanadyl, Nb, chromate, molybdate, tungstate, Mn(II, VII), Fe(II, III), Ru, Co(II, III), Rh, Ni, Pt, Cu, Ag, Au, Zn, Cd, Hg, Al, Tl, Sn(II), Pb, As, Sb, Bi, Se and Te. Interferences from Al (up to 15 mg) and Ti (2 mg) were removed by the addition of 1 g of NH4HF2. Interference from La can be eliminated by the addition of excess phosphate. Each of Zn, Cd and Hg (up to 5 mg) did not interfere. A considerable amount of Se, Te and Ta (250 mg of each ion) caused no interference, but serious Interferences were found from only 10 mg of As, Sb and Bi. Copper(II) was removed from UO22+by extraction with SCN- to MIBK (extraction should be repeated several times until the organic layer fails to produce any color). Positive Interferences from Ni(II) and Co(II) (more than 6 mg) were observed; in cases of higher concentration of Ni(II) and Co(II), another method was used to remove these ions from the mixture.9"° About 15 mg of Fe(III) could be masked successfully by the addition of 1 g of NH4HF2. Chromium(VI), V(V), W(VI) and Mo(VI) produced serious interferences by producing colored products. There are no interferences from Be(II), Th(IV), Ce(III, IV), Re(III) and Zr(II). Recovery of uranium The recovery of uranium from ores was also examined. Known amounts of standard uranium solution (3 -10 µg) were added to standard rocks granodiorite, basalt and gabbro which were taken through the entire analytical procedure, and a recovery value of 95 -105% was found. Analytical results of this study and other published data for ten international geochemical reference samples are listed in Table 1. Agreement between the results of this study and these data is generally good. Authors Director
would General,
like
to
express
Geological
their
Survey
thanks
of India,
to
Deputy
Shillong
for
giving permission to carry out the work. Special thanks are also due to Dr. A. Ando, GSJ, Dr. B. Lister, BGS, Dr. F. J. Flanagan,
USGS,
GSC
who kindly
and
relevant
colleagues
Dr. 0. H. J. Christie, sent
papers.
who extended
sufficient Extra
amounts special
very pleasant
ASK
and Dr. S. Abbey,
of standard thanks
are
cooperation.
samples due
to
all
ANALYTICAL
Table
SCIENCES
1
a. Average
Uranium
DECEMBER
contents
1991, VOL.
in standard
7
geological
933
reference
samples
of 5 determinations.
References
1. L. Jancer, B. Slezackova and L. Sommer, Talanta, 36, 549 (1989). 2. T. Kojima and T. Shigetomi, Talanta, 36, 603 (1989). 3. R. M. O'Leary and A. L. Meier, "U. S. Geological Survey Circular", No. 948, 100 (1984). 4. Y. Kanai, N. Imai and S. Terashima, Bunseki Kagaku, 34, 199 (1985). 5. Y. Kanai, N. Imai and S. Terashima, Geostand Newsl.,10, 73 (1986). 6. R.Pirbil, "Analytical Applications of EDTA and Related Compounds", Vol. 52, pp. 267, 270, 340, 355, Pergamon Press, Oxford, 1972. 7. R. W. Fessinger and E. V. Brown, J. Am. Chem. Soc., 73,
4608 (1951). 8. H. A. Flaschka and A. J. Barnard, "Chelates in Analytical Chemistry", Vol. 4, p. 54, Marcel Dekker, New York, 1972. 9. P. Chattopadhyay and S. K. Majumdar, Mikrochim. Acta [Wien],1983 III, 213. 10. P. Chattopadhyay and S. K. Majumdar, J. Indian Chem. Soc., LIX, 621 (1984). 11. A. Ando, N. Mita and S. Terashima, Geostand. Newsl.,11, 159 (1987). 12. K. Govindaraju, Geostand. Newsl., Special Issue, 8, Appendix-1(1984). (Received May 14, 1991) (Accepted August 14, 1991)
