Analytica Chimica Acta, 134 (1982) 375-378

Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands

Short Communication

DETERMINATION OF TRACES OF ZIRCONIUM IN SILICATE ROCKS BY INDUCIWELY-COUPLED PLASMA EMISSION SPECTROMETRY

HIROSHI UCHIDA*, KIYOSHI IWASAKI and KATSU TANAKA

industrial Research Institute of Kanagawa Prefecture 3173, Showu-machi. Kanazawa-ku, Yokohama 236 (Japan)

CHUZO IIDA

Laboratory of Anclyticat Chemistry, Xagoya Institute of Technology, Gokiso-cho, Showa-ku, h7agoya 466 (Japan)

(Received 5th August 19Sl)

Summary. The sample is fused with a mixture of sodium and potassium carbonates. Zirconium is separated from the large amounts of sodium and potassium by precipitation of hydrated oxides before nebulization. The detection limit is 0.32 pg Zr 6.‘. Results for seven standard rocks are in accord with recommended values.

Traces of zirconium in silicate rocks have been determined by d.c. emission spectrography [l] , x-ray fluorescence spectrometry [ 23 and neutron acti- vation analysis [3] , without any chemical treatment. These methods are simple and useful, but the results reported for zirconium are erratic, probably because of matrix effects. Spectrophotometric methods are also used but generally require complicated separation procedures_ For example, zirconium must be isolated by ion-exchange separation prior to colour development with arsenazo-III [4] or >;ylenol orange [ 51.

Inductivelycoupled plasma (i-c-p.) is the best excitation source for emis- sion spectrochemical analysis because of its high sensitivity, wide dynamic range and comparative freedom from interferences_ In previous work [6] , minor and trace elements in various silicate rocks were determined by i.c.p. emission spectrometry after decomposition of the sample with a mixture of hydrofluoric acid and aqua regia in a sealed teflon vessel. This method gener- ally proved effective, but for the determination of chromium and zirconium in some standard rocks, the results were significantly low probably because of incomplete sample decomposition.

The present communication deals with the determination of zirconium in silicate rocks by i.c.p. emission spectrometry; five methods for the decom- position of the sample were investigated. Fusion with alkali metal carbonates is recommended for complete decomposition of a variety of silicate rocks, although zirconium must be separated from the large amounts of alkali metals prior to the i_c.p. measurement.

0003-2670/82/0000-0000/$02_75 0 1982 Elsevier Scientific Publishing Company

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Experimental Instrumentation and operating parameters_ An i.c.p. source (Model ICAP-1,

Nippon Jarrell-Ash Co.) was used at 1.4 kW r.f_ power, with argon flows of 14, 1.0 and 0.85 1 min-’ as the coolant, plasma and carrier gases, respectively_ The monochromator (Model JE-50, Nippon Jarrell-Ash Co.) was an Ebert- type (focal length O-5 m) with a grating (1200 grooves mm-‘). The entrance and exit slit-widths were both 10 pm. The analytical line was Zr II 343.823 nm, and the observation height was 15.0 mm above the induction coil.

Sample preparation. In a platinum crucible, 0.5 g of silicate sample is fused with 2 g each of sodium carbonate and potassium carbonate for 30 min. After cooling, the melt is leached with 15 ml of 6 M hydrochloric acid, and diIuted to 150 ml with water. Sodium hydroxide is added (with phenolph- thalein as indicator) to precipitate zirconium as its hydrated oxide together with iron, magnesium, etc. The precipitates are filtered off, washed with water, transferred to a platinum vessel and dissolved in 10 ml of 47% hydrofluoric acid and 4 ml of 60% perchloric acid. The solution is heated gently under an i-r. lamp till acid fumes cease. The residue is dissolved in 10 ml of 6 M hydro- chloric acid and diluted to 50 ml with water_

Stock solutions of zirconium and matrix elements were prepared as des- cribed previously [6, 7]_ The standard solutions of zirconium used were O-10 I_rg ml-’ in 1 M HCl. All contained aluminum (600 pg ml-‘), iron (600 pg ml-‘), calcium (500 fig ml-‘), magnesium (400 clg ml-‘), sodium (200 fig ml-‘) and potassium (200 pg ml-‘) as matrix elements_

Results and discussion Zirconium shows great sensitivity in i.c.p. emission spectrometry [S] ;

three analytical lines, Zr II 339.198 nm [9], Zr II 343.823 nm [lo] and Zr II 349.621 nm [S], have been generally used. For the determination of zirconium in silicate rocks, the 343.823-nm line is recommended because the other lines overlap with Th II 339.304 nm and Y II 349-608 nm, respect- ively, and thorium and yttrium are often present in silicate rocks at similar levels to zirconium. The effect of the viscosity of the sample solution could be decreased by preparing standard solutions of suitable matrix composition. The detection limit for zirconium (the concentration which gives an intensity equivalent to twice the standard deviation of the background from a blank matrix solution) was 0.32 fig g-* in silicate rocks.

Five procedures for rock decomposition were first investigated for the standard silicate rock JG-1 (granodiorite; Geological Survey of Japan). The results for zirconium in each case are summarized in Table 1, together with an outline of the procedure_ Decomposition with an acid mixture (method I) gave only about 30% recovery, though it was effective for most minor and trace elements [6] _ The separation and fusion of the insoluble residue in method I with potassium hydrogensulfate (method II) was not successful. Most of the residue was dissolved after fusion with a mixture of sodium and potassium carbonates (method III) but some remained. Complete

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TABLE 1

Decomposition procedures and results for zirconium in JG-1”

Methodb

(I) Decomposition with acids in sealed t&on vesseld

1 ml HF/3 ml HNO, 36 9

(II) Fusion of insoluble matter in (I)e 0.5 g KHSO, 34 - (III) Fusion of insoluble matter in (I)e 0.25 g Na,C0,/0.25 g K,CO, 91 (IV) Initial fusionf 2 g Na,C0,/2 g K,CO, 123 3 (V) Fusion of the final residue in (IV)” 5 g KHSO, 12s -

Reagents Zr R.s.dsC found (70)

(pg g-’ )

=Recommended value and r.s.d. are 111 pg g-’ and 23.9%, respectively [ 111. b 0.5-g sample was decomposed and diluted finally to 50 ml with 1 M HCl. =Five determinations. dThe method reported earlier [6] was modified by using nitric acid instead of aqua regia, and evaporating under an ix. lamp. CInsoluble matter was filtered off and fused. The melt

was dissolved in a few ml of 6 M HCI and combined with the primary filtrate. ‘As in experimental. fiThe melt was dissolved with 1 M HCI, and zirconium was separated with cupferron from the large amounts of potassium_

decomposition occurred on fusion of the original sample with this flux for 30 min (method IV)_ However, it has been stated that after fusion with alkali metal carbonates, any remaining insoluble matter must be fused with potassium hydrogensulfate (method V) [ 121. However, there was no sig- nificant difference between the results of methods IV and V in the present experiments_ On the basis of the above results, method IV is recommended. Zirconium could easily be separated from the large amounts of sodium and potassium by precipitation as its hydrated oxide, so that nebulization into the plasma became reproducible_ The remaining small amounts of sodium and potassium did not cause ionization interferences.

TABLE 2

Results for zirconium in silicate rocks (pg g-‘)

Sample Found” Average R.s.d. Recommended

I IV V reported (S) value [ 161

(fig g-’ )

JB-1 (Basalt) 147 152 - 173 [ll] 38.2 153b MRG-1 (Gabbro) 99 113 - 112 [14] 26.6 105c AGV-1 (Andesite) 229 23’7 - 227 [13] 13.9 225 BCR-1 (Basalt) 194 197 - 165 [13] 19.2 190 G-2 (Granite) 57 334 320 316 [13] 14.1 300 W-l (Diabase) 7s 115 109 10s [15] 22.6 105 GSP-1 (Granodiorite) 30 603 604 544 [13] 19.8 500

aBy methods I, IV and V. bRef. [ 11 ] _ CRef. [ 14 1.

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Zirconium in other typical standard silicate rocks was determined by methods I and IV; method V was also used for samples that gave different results by methods I and IV. The results are listed in Table 2, and are com- pared with other reported values. &Tethod I was simplest; it could be success- fully applied to basalt, gabbro and andesite, but not to granite, diabase and granodiorite. The results of method IV compare reasonably well with the recommended values for all of the standard silicate rocks, though the pro- cedure was more complicated_ The secondary fusion with potassium hydrogen- sulfate (method V) seems to be unnecessary.

The authors express their gratitude to Dr. Tetsuo Uchida for his useful suggestions in this study.

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