ANALYST, NOVEMBER 1987, VOL. 112 1623

Application of the Sulphide Podand 1,12-Di-2-thienyl-2,5,8,11- tetrathiadodecane to the Determination of Silver in Ores and Tailings by Solvent Extraction - Atomic Absorption Spectrometry

Elwira Lachowicz Department of Analytical Chemistry, Warsaw University of Technology, ul. Noakowskiego 3, 00-664 Warsaw, Po land

A method has been developed for the determination of silver in copper ores and tailings by AAS based on the solvent extraction of silver with Ir12-di-2-thienyl-2,5,8,1 1 -tetrathiadodecane, an acyclic neutral ligand with six sulphur donor atoms (sulphide podand), in IBMK. The separation was carried out on samples dissolved in 2 M nitric acid in the presence of boric acid. The accuracy of the method was examined using standard reference materials [a copper - zinc ore and a copper (pyrite) ore]. An r.s.d. of 2.8-4.9YOfor IO-4-lO-3% silver in ores was obtained. Keywords: Sulphide podand; silver determination; copper ore; solvent extraction; atomic absorption spectrometry

The determination of trace amounts of silver in complex inorganic and geological samples by atomic absorption spec- trometry (AAS) is often combined with separation and concentration of the silver by solvent extraction. A number of extraction - AAS procedures have been reported since the publication of a review' on this subject in 1981. Donaldson and co-workers determined silver in ores after extraction of the tribenzylarnine - silver bromide ion-association complex with CHC1&3 (followed by back-extraction and determina- tion in the aqueous phase) or with IBMK,4 and also after IBMK extraction of silver as iodides5 (also followed by determination in the aqueous phase). Zolotov and Vanifat- ova6 investigated various selective extractants for silver isopropyl N-alkylthiocarbamates and applied these to the analysis of geological samples. An AAS procedure7 was developed for the determination of silver after extraction from hydrochloric acid with di-2-ethylhexylphosphoric acid in toluene. Silver and gold were extracted from HC1 solution with a mixture of petroleum sulphides (m.p. 220-280 "C) and isopropyl N-methylthiocarbamate8 in toluene.

Neutral sulphur-containing ligands such as thioethers have an especially high extraction power with respect to silver and other elements belonging to the soft acid class as defined by Pearson.9 The extraction of silver by organic sulphides has been discussed in the reviews by MojskilO, Torgovll and Vanifatova et al. 12 A very efficient extraction of silver has been demonstrated using cyclic polythioethers.13-20

In a previous paper21 we reported the extraction ability of 1,12-di-2-thienyl-2,5,8,Il-tetrathiadodecane ('ITD) [C4H3S- (CH2SCH2)4C4H3S], an acylic neutral multidentate ligand with six sulphur donor atoms. This reagent belongs to a group of podands which, according to Weber and Vogtle's classifica- tion,22 form host - guest complexes. A good selectivity and high degree of extraction of silver (log D = 1.8-2.2) was obtained from solutions of pH 1-6 and from HN03 and HC104 solutions by ?TD in 1'2-dichloroethane or IBMK.

The purpose of the work reported in this paper was to apply this reagent to the determination of silver in geological samples and reference materials by extraction - AAS.

Experimental Reagents All reagents were of analytical-reagent grade unless stated otherwise.

TTD solution, 1 X 10-3 M. Prepared by dissolving 0.1017 g of the reagent in 250 ml of 4-methylpentan-2-one (IBMK). The solution may be stored for 1 week. A synthesis has been described previously.21

Silver stock solution, 1 mg ml-1. Prepared by dissolving 1.575 g of silver nitrate in water with 2 ml of concentrated nitric acid and diluting to 1 1.

Nitric acid. Spectroscopic grade. Boric acid, saturated solution. Recrystallised from water.

Apparatus

A Pye Unicam SP 90A Series 2 atomic absorption spec- trometer was used for the determination of silver at 328.1 nm. The air flow-rate was adjusted to 5 1 min-1 and the acetylene flow-rate to 0.6 1 min-1. A Teflon-lined bomb was used for sample decomposition under pressure.

Procedure Decomposition of tailings and ores The procedure given is based on the dissolution of copper ores reported previously.23

Weigh 1 g of sample into a Teflon-lined bomb, add 15 ml of concentrated nitric acid and 15 ml of concentrated hydro- fluoric acid. Seal the bomb and heat at 150°C for 4 h. After cooling transfer the solution into a PTFE beaker and evaporate to dryness. Next, evaporate the contents to dryness twice with 5-ml aliquots of nitric acid. Add 1.5 g of boric acid, ca. 5 ml of water and 5 ml of nitric acid and evaporate to dryness again. Add 10 ml of water and the appropriate amount of nitric acid to obtain a final solution 2-2.5 M in nitric acid, then heat until the precipitate is dissolved. Transfer the solution into a calibrated flask and dilute to the mark with water.

As the ore MrA is fairly soluble, it was decomposed in a PTFE beaker in the same mixture of acids and then treated as described above. Extraction Pipette 20 ml (or less) of the solution into a separating funnel and shake with 5 ml of l o - 3 ~ TTD in IBMK for 5 min. Transfer the organic phase into a 5-ml calibrated flask, dilute to 5 ml with solvent and measure the absorbance of 328.1 nm.

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1624 ANALYST, NOVEMBER 1987, VOL. 112

Calibration To a series of 50-ml calibrated flasks transfer appropriate amounts of silver standard solutions to cover the range 0-0.2 pg m1-1,12.5 ml of saturated boric acid solution and 6.3 ml of concentrated nitric acid. Dilute to the mark with water. Pipette 20 ml of each solution into a separating funnel and proceed as given under Extraction.

Results and Discussion According to the results obtained previously21 with a 11OAgm tracer, the value of log D (log of the distribution coefficient) for 10-5-10-4 M silver separated from nitric acid solutions by 1 X 10-3 M TTD in IBMK is 1.8 in 2 - 6 ~ HN03. In this work the separation of silver was carried out from the solutions of a decomposed sample or synthetic solutions in 2-2.5 M nitric acid and silver was determined in the organic solvent phase by flame AAS.

Separation of Silver from Synthetic Solutions Preliminary experiments were carried out in order to investi- gate the separation of silver from synthetic solutions contain- ing relatively high concentrations of cations commonly present in inorganic materials. The recoveries of 0 . 1 4 pg ml-1 of silver from 0.5-8 mg ml-1 solutions of copper, nickel, lead or zinc in 2 M HN03 ranged from 95 to 102%.

Calibration The good stability with time of aqueous standard solutions of silver in nitric and boric acid mixtures reported previously23 and the similar stability of silver in solutions of decomposed samples are significant advantages of this medium. Silver standards in TTD - IBMK solution are stable for 4 d. The absorbance signal for 2 pg of Ag extracted from various volumes of aqueous phases by 5 ml of organic solution does not depend on the ratio of aqueous to organic solution when V,,/V,,, = 1-5; for V,,/V,,, = 10 it is ca. 2% smaller.

Sample Analysis Three samples of copper tailings and two ores were analysed according to the described procedure. Boric acid was added (as in the Bernas scheme for the analysis of silicate ro~ks24~25) to complex any excess fluoride ion and to dissolve insoluble fluorides. This fluoroborate system has been shown to eliminate interferences in the determination of major rock components by AAS.24 The results obtained are summarised in Table 1.

The good agreement of the results obtained here for the standard reference copper - zinc ore RUS-1 and the ore MrA with the certified or reported values indicates the accuracy of the proposed method. The addition of silver standards (1) to aliquots of the sample solution after decomposition and (2) to 1 g of solid sample in a Teflon-lined bomb before acid digestion was investigated, As far as the first instance is concerned, full recoveries were observed and therefore the calibration graph method of determination could be used. This is also an indication of the precision of the extraction - AAS procedure. For accurate results, it is necessary to wash the PTFE vessels and glassware with ammonia solution followed by distilled water.3

In order to investigate the effect of a preliminary ignition, tailing sample C was also analysed after a 15 min ignition in a quartz crucible before digestion in a Teflon-lined bomb. The results obtained were ca. 14% lower (X = 4.8 X n = 4).

Table 1. Results of determination of silver in tailings, ores and standard ores

Ag standard additions

After Before decompo- decompo-

sition sition

R.s.d., Agl Recov- Agl Recov- Sample x , % n YO pg ery,% pg ery,%

TailingA . . 2.85 x 10-4 8 3.2 2 100 - - TailingB . . 2.47 X 10-3 4 4.8 2 100 - -

4 102 - - TailingC . . 5.59 x 10-3 4 4.9 2 98 - -

4 103 - - Standard ore

RUS-1* . . 2.82 X 10-3 5 2.8 - - 30 96.5

OreMrA* 1.70X 10-4 9 3.4 - - 2 99.6 (n = 3)

(n = 2)

* Samples and certified values given were provided by the Geological Institute, Warsaw. RUS-1 standard reference material, copper - zinc ore, certified value (2.7 k 0.2) X10-3%, mean from 19 laboratories (probability level 95%). MrA, copper ore (79% pyrite), reported value 1.4 x lO-4% (mean value of 1.0 x and 1.8 X

from two laboratories).

Conclusion The proposed method for the separation and determination by AAS of lO-4-lO-3% of silver is applicable to copper - zinc and pyrite ores, tailings and matrices containing macro-amounts of copper, nickel, lead and zinc.

This work was supported by the research programme CPBP- 01.17.

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Paper A7124 Received January 26th, 1987

Accepted April 21st, 1987

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