Heterocyclic dithiophosphates as analytical reagents

Talanta, Vol. 37, No. 5, pp. 527429, 1990 Printed in Great Britain. All rights reserved

0039-9140/90 $3.00 + 0.00 Copyright 0 1990 Pergamon Press plc

HETEROCYCLIC DITHIOPHOSPHATES ANALYTICAL REAGENTS INDIRECT EXTRACTION-SPECTROPHOTOMETRIC OF SILVER IN COPPER CONCENTRATES D. ATANASSOVA

AS

DETERMINATION AND ORES

and A. N. SHISHKOV

Plovdiv University “Paissii Hilendarski”, Department of Analytical Chemistry, 4000 Plovdiv, Bulgaria (Received 22 March 1989. Revised 29 July 1989. Accepted 14 November 1989)

Summary-The proposed method is based on the exchange reaction between the complex of copper(H) with 4,7-dimethyl-2-thiol-2-thion-l,3,2-dioxophosphorinan (DOPh,,,) in toluene and an aqueous solution of silver(I). The decrease in the absorbance of the Cu(DOPh,,,), solution is proportional to the silver(I) concentration for 5-90 pg of silver in 5 ml of toluene extract and S-30 pg in 10 ml of toluene extract. The method is applied to the determination of silver in copper concentrates and ores.

The heterocyclic formula

dithiophosphates

of general

R/“\p/s

‘0’ ‘SK where R is CH,CH,CH2 (DOPh,) CH(CH,) CH2CH2(DOPh,,) or CH(CH,)CH,CH,CH (CH,)(DOPh,,,) are reagents utilized for the direct spectrophotometric determination of copper’ and moybdenum.z A study of the interaction of copper(I1) with various dithiophosphates was reported in a previous paper’. These complexes are coloured, and less stable than the corresponding colourless complexes of silver(I).3 The present paper describes a new method for determination of silver, based on the exchange reaction 2Ag&, + Cu(DOPh,,, )z(org) =2Ag(DOPh,,,

)(orBj+ Cu:,+,, ( 1)

EXPERIMENTAL

Reagents DOPh,,, was synthesized as described elsewhere.4 Its purity was checked by thin-layer chromatography. Aqueous O.OlM solutions of the reagent were prepared. Standard silver solution, 1000 ,ugg/ml, was prepared from high-purity silver nitrate and diluted to give a 5 pg/ml working solution.

Toluene (pro analysi grade) was treated with concentrated sulphuric acid, washed with distilled water until acid-free (pH-paper), dried with anhydrous sodium sulphate for 24 hr, and finally distilled. The purified toluene was stored in dark glass bottles and was stable for some months. Copper(II)DOPh,,, solution in toluene was made as follows. Dissolve 0.9828 g of pure CuSO,. 5H,O in distilled water, add 10 ml of 6iU sulphuric acid and dilute accurately to 250 ml with distilled water; 1 ml contains 1000 pg of Cu(I1). Take 25 ml of this solution, acidify it with 10 ml of sulphuric acid (1 + 2) add 30 ml of 1 x 10e2M aqueous solution of DOPh,,, and leave it for 15 min. Extract with 150 ml of toluene. Leave the phases to separate then wash the organic layer with three portions of 100 ml of copper(I1) solution (1000 pg/ml) and three portions of 500 ml of distilled water. The toluene extract should not contain any excess of DOPh,,, . It is checked for traces of DOPh,,, as follows. Shake 5 ml of the toluene solution with 10 ml of aqueous phase (5 ml of 1OOO-pgg/ml copper solution + 1 ml of perchloric acid + 4 ml of water) for 3 min. The absorbances of the initial and treated toluene layers should not differ. Washing of the toluene extract is continued until it gives a negative reaction for free DOPh,,,, then the extract is filtered through a fine filter paper, dried with anhydrous sodium sulphate and stored in a dark bottle. The solution is stable for more than 5 months.

527

528

D. ATANASSOVA and A. N. SHISHKOV

All other reagents used were of analyticalreagent grade and were not purified further.

0.6 r

Procedures Calibration graph. Pipette 1.0, 3.0, 5.0, 10.0 and 15.0 ml of the 5 pg/ml silver solution into lOO-ml separating funnels. To each add 5 ml of concentrated perchloric acid and dilute to 50 ml with distilled water, then add 5 ml of Cu(DOPh,,,), solution in toluene and shake the mixtures for 2 min. Centrifuge (or let stand for 45 min) to separate the layers. Filter the organic layer through a filter paper previously impregnated with toluene. Measure the absorbance at 420 nm in a l-cm cell against toluene. Plot the calibration graph. Analysis of copper concentrate. Weigh 1.0 g of the concentrate into a 150-ml conical flask. Add 10 ml of nitric acid (1 + I), cover the flask with a watch glass, and heat on a hot-plate until evolution of nitrogen oxides ceases. Remove the watch glass and evaporate almost to dryness. If the samples are to be analysed for gold, repeat the evaporation step, with 15 ml of aqua regia. Add 20 ml of concentrated perchloric acid and heat until the volume has decreased to 8-10 ml, cool, wash down the walls of the flask with water, and heat until fumes of perchloric acid are evolved. Cool, dilute with 2&30 ml of distilled water and filter through a double filter paper into a lOO-ml standard flask. Wash the conical flask and residue with 6 portions of distilled water. Dilute the filtrate and washings to volume and analyse 50 ml of the solution as described above for the calibration graph. Analysis of copper ore. Weigh 4 g of the ore into a beaker and add 16 ml of aqua regia. Evaporate on a hot-plate almost to dryness then add a few ml of concentrated hydrochloric acid and evaporate again (repeat this step twice more). Finally add 20 ml of perchloric acid and proceed as for analysis of copper concentrate, except that the filtrate and washings should be diluted to 50 ml, and the total volume extracted with Cu(DOPh,,,), solution. RESULTS AND DISCUSSION

Extraction of the silver complex

When an aqueous solution of silver and a toluene solution of Cu(DOPh,,,), (silver and copper in stoichiometric ratio) was shaken for 1 min, the toluene layer became colourless, indicating that silver completely replaced the

Acid concentration,

M

Fig. 1. Plot of absorbance vs. acid concentration: (1) HNO, , (2) H,SO.,, (3) HClO,.

copper in the complex [see reaction (l)]. In all further experiments the phases were shaken for 2 min. The effect of mineral acid concentration on the exchange reaction (1) was studied by acidifying 10 ml of 5 pg/ml silver solution, diluting to 20 ml with water, shaking the solution with 5 ml of Cu(DOPh,,,), solution for 2 min, then measuring the absorbance of the organic layer as described above. Plots of the absorbance vs. acid concentration (Fig. 1) show that the reaction occurs in 0. l-l .OM nitric acid, 0.14M sulphuric acid and 0.1-4M perchloric acid. At concentrations above lit4, nitric acid causes destruction of the Cu(DOPhi,,),. The exchange reaction can also be done in 0.8-1.5M hydrochloric acid, but in 2M hydrochloric acid it is strongly hindered, and does not occur when the acid concentration is above 3.5M. A study of the effect of changing V,_: Vas showed that at ratios of up to 1: 50 the replacement of Cu(I1) and Ag(1) was not affected, making preconcentration of silver possible. The plot of absorbance vs. silver concentration was linear in the range 5-90 pg of silver in 5 ml of toluene extract and 5-l 30 pg of Ag in 10 ml of extract. The effect of various ions that commonly occur with silver in natural samples was studied by analysing model solutions containing 50 pg of Ag. Up to 80 mg of iron(III), 100 mg of copper( magnesium(II), calcium(II), strontium(II), barium(II), zinc(II), cadmium(II), aluminium(III), chromium(III), nickel(II), molybdenum(VI), tungsten(VI), manganese(II), uranium(VI), cobalt(II), 70 mg of rhenium(VII), 40 mg of tin, 15 mg of lead(II), 5 mg of antimony(III), 1 mg of platinum(I1) and

Determination Table 1. Determination

of silver in copper concentrates and ores

529

of silver in reference standard samples and copper concentrates (6 replicates, probability 0.95) Silver,* g/ton This work

Sample MDK-1 MDK-2 MDK-3 Cu concentrate 1 Cu concentrate 2 Cu concentrate 3 Cu concentrate 4

Certified value II 17 23 -

By fire-assay -method 21.2 22.6 & 2.7 20.9 f 2.5 18.9 f 3.8

BY AAS method

21.2 24.2 + 3.0 22.3 + 3.8 21.9 f 3.3

Lab. 1 ll.lkO.3 17.1 f 0.3 23.0 f 0.3 21.1 f 1.5 24.9 f 1.6 22.6 f 2.8 22.0 f 3.1

Lab. 2 -

26.4 f 25.6 f 29.3 f 26.2 f

2.0 2.0 3.1 0.8

*According to the Bulgarian Standard Method,6 the tolerable differences between parallel determinations are < 5 g/ton for the l&20 g/ton range, and $7 g/ton for the 20-40 g/ton range.

arsenic(III), 0.5 mg of selenium(V1) and tellurium(VI), 0.5 mg of selenium(IV) and tellurium(N), 0.25 mg of bismuth(III), palladium(I1) and gold(I1) and 100 mg of sulphate and 25 mg of nitrate will not interfere in the determination of silver by the proposed method. More than N 0.25 mg of bismuth(III), gold(III), palladium(I1) and 0.5 mg of selenium(IV), selenium(VI), tellurium(IV) and tellurium(V1) will cause a positive error. Fluoride, thiocyanate, thiosulphate and mercury(I1) interfere with the exchange reaction. The proposed reaction was applied to the analysis of reference standard samples of silver in metal matrices. The results are shown in Table 1. APPLICATIONS

Silver in copper concentrate is usually determined by fire assay or atomic-absorption spectrometry.6 The fire assay is slow and requires special equipment and a skilled operator. The precision is poor. The second method gives better precision but an expensive piece of equipment is needed. Of the spectrophotometric methods available, those based on dithizone and rhodanine’ are suitable for determination of traces of silver and compete favourably with the fire-assay method. However, the rhodanine method is inapplicable to copper concentrates5 and the dithizone method suffers from serious matrix interference5.

Four industrial samples were analysed by the proposed method. The results are shown in Table 1. They are compared with data from the fire-assay and atomic-absorption spectrometric determination.6 The results agree very well. The chi-square values (P = 0.95, n = 6) calculated for concentrates 2 and 3 are 2.41 and 0.958, respectively. The critical values for x’(P,f) are 7.81 (P = 0.95) and 11.3 (P = 0.99).’ Six industrial samples of copper ores were analysed by the proposed method. The results were in good agreement with those of semiquantitative spectral analyses. The proposed indirect extractionspectrophotometric method is advantageous for determination of traces of silver in copper concentrates and ores and competes favourably with the classical fire-assay analysis. REFERENCES D. Atanassova, A. N. Shishkov and N. D. Jordanov. Submitted to Talanta. A. N. Shishkov and D. Atanassova, Commun. Dept. Chem. Bulg. Acad. Sci., 1984, 17, 340. I&m, Ditiofosfati i tekhni disuifidi kato analitichni reagenfi, Bulletin No. 342/89, Central Scientific Tech-

nical Library, Sofia, pp. l-114. 4. D. Atanassova and A. N. Shishkov. Commun. Dept. Chem. Bulg. Acad. Sci., 1989, 22, 13 1.

5. G. Charlot, Melody analilicheskoi khimii, p. 1008. Khimia, Moskva, Leningrad, 1965. 6. Bulgarian Standard Method, 8792-83. 7. K. Doerffel, Statistik in der analyrischen Chemie, VEB Deutscher Verlag fib Grundstofllndustrie, Leipzig, 1984.