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Determination of traces of molybdenum in sea water by combined anion-exchange - graphite furnace atomic absorption spectrometry

R. Kuroda, N. Matsumoto, and K. Oguma Laboratory for Analytical Chemistry, Faculty of Engineering, University of Chiba, Yayoi-cho, Chiba, Japan

Bestimmung von Molybd~inspuren in Meereswasser mit Hilfe einer Kombination yon Anionenaustausch und elektrothermischer Atomabsorptionsspektrometrie

Summary. Traces of molybdenum in sea water have been preconcentrated by anion-exchange from acidified samples in the presence of sodium azide. Molybdenum adsorbs strongly on a column of Bio-Rad AG 1 (C1-) and can be stripped easily by elution with 2 tool/1 NH4C1 - 2 mol/1 NH4OH solution. Molybdenum in the effluent is determined by graphite furnace atomic absorption spectrometry. The combined method allows to determine traces of molyb- denum in sea water as well as non-saline water on a 100 ml sample basis. The method gives a relative standard deviation of better than 8% at a molybdenum level of 10 gg 1-1 of sea water.

Introduction

As a biologically active micronutrient element for growth in aquatic environment, many methods have been reported on the determination of molybdenum in sea water. However, the abundance of molybdenum in sea waters is low, so pre- liminary isolation methods have usually been employed in its determination. Although direct determination of molyb- denum in sea water by means of graphite furnace atomic absorption spectrometry [1] has been attempted with as- corbic acid addition to reduce matrix effects, the precision and sensitivity are not satisfactory. In many cases, reliable determinations of molybdenum in sea water require a pre- concentration step. Information up to mid-1983 has been reviewed in the previous paper [2], so it will not be repeated here. Since then, many papers have appeared, which present a variety of techniques for the preconcentration of molyb- denum. Solvent extraction of the molybdenum-8- quinolinate complex in diisobutylketone [3], of dithio- carbamates into chloroform [4], and of thiocyanate com- plexes with capriquat-benzene [5] have been used recently.

Trace coprecipitation of molybdenum on an iron(III) - indium (III) loaded cellulose [6], coprecipitation on cobalt - APDC chelate [7], column adsorption on fibers (polyacryl- nitrile) treated with polyepoxidamine [8], sorption on chelating ion-exchange resins containing sulfur ligands [9], sorption on chelating resin followed by elution and extrac- tion with 1,4-dihydroxyphthalimide dithiosemicarbazone in

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NN-dimethylformamide - isoamyl alcohol [10], ion flo- tation with cetylpyridinium chloride [11] and colloid flo- tation on hydrous iron(III) oxide [12] have also been re- ported, each used for the determination of molybdenum in sea water. In this work it has been found that molybdenum can be concentrated from sea water simply on a strongly basic anion-exchange resin by treating the water with sodium azide. The anion-exchange concentration, coupled with the graphite furnace atomic absorption spectrometry enables traces of molybdenum to be determined successfully.

Experimental

Reagents

A stock solution (1.00 mg/ml) of molybdenum for atomic absorption spectrometry was purchased from Kanto Chemi- cal Co. (Tokyo). The strongly basic anion-exchange resin Bio-Rad AG 1, X-8 (100-200 mesh) in the chloride form was used. A slurry of 2.0 g of the resin was poured into a conventional ion-exchange tube (1.2 cm diameter) to make a 3.2 cm bed.

Apparatus

A Shimadzu Model AA-646 atomic absorption spectrometer was fitted with a deuterium background corrector, a GFA-4 furnace atomizer, and a Model U-135 strip chart recorder. Background correction was performed for all measure- ments. The radiation source used was a Hamamatsu TV L233-42NB single element hollow cathode lamp for molyb- denum. The settings for the spectrophotometer were as fol- lows: lamp current 9 mA; wavelength 313.3 nm; bandwidth 0.38 nm; and read-out as peak height. The atomization pro- gramme is summarized in Table I. The protection gas used was argon. A Shimadzu pyrolytic graphite tube 200 - 54525 was employed.

Determ#tation of distribution coefficients"

The distribution coefficients of molybdenum(VI) were deter- mined by the batch equilibrium method. Weighed portions of dried resin (0.5 g each) were mixed with 40 ml portions of 0.5 tool/1 sodium chloride - 0.3 tool/1 hydrochloric acid solution containing various concentration of sodium azide and 10.4 ~tmol of molybdenum(VI). The mixtures were shaken for 5 h at 25~ and the weight distribution coef- ficients determined as described earlier [13].

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Table 1. Analytical conditions

Dry Ash Atomize Clean

Temperature/~ 150 1800 2800 2900 Time/s 30 20 4 3 Mode Ramp Step Step Step Gas flow rate/1 min 1 1.5 1.5 0 1.5

Table 2. Distribution coefficients (Kd) of Mo(VI) on Bio-Rad AG 1, X-8 (C1- form) in 0.5 tool/1 NaC1 - xmol/1 NaN3 - 0.3 mol/1 HC1

NaN3 (mol/1) Kd

0.10 26 0.20 35 0.30 48 0.40 452 0.50 538

Procedure

Filter the sea water sample through a membrane filter (0.45 gm, Millipore). Take a 100-200 ml fraction of the filtrate, add sodium azide to give 0.5 tool/1 in azide and adjust to 0.3 tool/1 in hydrochloric acid. Pass the mixture through the ion-exchange column at a flow rate of 5 ml rain- 5. Wash the column with 20 ml of distilled water. Strip the molybdenum by elution with 2 tool/1 NH4C1 - 2 tool/1 NH4OH solution. Collect the early 12 ml fraction of the effluent into a 25 ml capacity volumetric flask. Add 5 ml of 20% L-ascorbic acid solution and dilute to mark with dis- tilled water. Inject 10 gl of the solution into the furnace and proceed according to the programme given in Table 1.

Results and discussion

Hydrazoic acid is a weak acid (pKa = 4.77 at 25~ resem- bling thiocyanic acid in all aspects. Molybdenum(VI) complexes with azide, are sorbed on the strongly basic anion- exchange resin from hydrazoic acid solutions [14]. The adsorption behaviour of molybdenum from sodium chlo- ride - azide solutions was examined, because of lack of information about the feasibility of the recovery of molyb- denum from sea water. The distribution coefficients of molybdenum(VI) on Bio-Rad AG 1 (chloride form) in acidi- fied 0.5 mol/1 sodium chloride (0.3 tool/1 in hydrochloric acid) are listed in Table 2 as a function of azide concen- tration. The distribution coefficient increases rapidly with increasing concentration of azide, reaching about 500 at 0.5 mol/l concentration. The presence of sodium chloride lowers the sorption of molybdenum to a significant extent as compared with the sorption from hydrazoic acid alone; molybdenum exhibits a distribution coefficient of more than 104 over the hydrazoic acid concentration range of 0.3 - 0.5 tool/1. However, it is easy to strip the sorbed molyb- denum azide complex out of the column. This can be ac- complished by elution with as little as 12 ml of 2 mol/1 NH4C1 - 2 mol/1 NH4OH solution, which may be further removed by evaporation and subsequent gentle heating, if necessary.

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Table 3. Determination of molybdenum in sea and lake waters (100 ml samples)

Sample collected Mo Mo location addedAtg found/gg

Inage, Tokyo Bay 0 0.85, 0.82, 0.83 av. 0.83 _+ 0.02

Kominato, Chiba 0 1.00, 1.00, 0.95 Pacific Ocean av. 0.98 _+ 0.03

1.00 2.11, 2.11, 1.84 av. 2.02 _+ 0.16

Lake Inba-numa a 0 0.69, 0.71, 0.64 av. 0.68 + 0.04

1.20 1.89, 1.99, 1.80 av. 1.89 +_ 0.10

a 200 ml taken

To test the application of the anion-exchange azide sys- tem for the determination of molybdenum in sea water samples, an artificial sea water (28.0 g of NaC1, 5.5 g of MgCI2 - 6 HzO, 6.9 g ofMgSO4 - 7 H20, and 1.5 g of CaCI2 per litre) was prepared, spiked with 2.0 gg of molybdenum per 200 ml, and analyzed according to the procedure above. Results of three runs yielded 2.01, 2.13, and 1.97 gg molyb- denum per 200 ml, averaging 2.04 gg + 0.08 gg Mo per 200 ml.

Sorption of molybdenum on AG 1 from hydrazoic acid is comparatively selective, allowing V(V), Fe(III), Cu(II), Zn, In(III), W(VI), Re(VII), platinum metals, Au(III), and Hg(II) to be retained more or less firmly. However, assuming that ultra traces of these metals in sea water behave similarly as molybdenum in the anion-exchange concentration and elution steps, they do not interfere with the atomic absorp- tion spectrometry of molybdenum by graphite furnace. The results of repeated determination of molybdenum in actual sea water samples collected at two stations are given in Table 3 together with those for some spiking tests. The over- all recoveries and precision are satisfactory.

The procedure can also be applied to natural water samples. Results are quoted on the lake water analysis for molybdenum in Table 3. The water sample is filtered through a filter paper and then 0.45 gm membrane filter, acidified to 0.3 tool/1 with hydrochloric acid, and analyzed, because the lake tested is highly polluted by human and aquatic plant activities. The results of addition tests give quantitative recoveries. However, the same lake water shows somewhat lower results for molybdenum when acidifying the sample immediately after collection, filtering and analyzing for molybdenum; 0.54, 0.53, 0.49 gg Mo/200 ml (av. 0.52 +0.03) were obtained. Scavenging the molyb- denum with fumic acids, lignins, suspended matters, etc. in acid media may be responsible for the lower results.

References

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Received August 5, 1987

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