ANALYST, JUNE 1992, VOL. 117 1013

Oxidation Product of Ammonium Pyrrolidin-I-yldithioformate as a Coprecipitator for the Preconcentration of Vanadium, Cobalt, Zinc, Arsenic, Iron, Cadmium, Selenium and Mercury From Aqueous Solution

Magnuss Vircavs Institute of Physics of the Latvian Academy of Sciences, Miera 34, Salaspils, 229021, Latvia Agrida Pelne, Vallija Rone and Daina Vircava Institute of Inorganic Chemistry of the Latvian Academy of Sciences, Latvia

The use of ammonium pyrrolidin-I-yldithioformate [ammonium pyrrolidinedithiocarbamate (APDC)] as a complexing agent for V, Co, Fe, Cu, Zn, As, Cd, Se and Hg, and its oxidation product as a collector, for the preconcentration of these elements from aqueous solutions, has been studied. All these elements are simultaneously concentrated 1600-fold in a single precipitate of the collector. The coprecipitation depends on the concentration of APDC in aqueous solution and the pH. The recoveries are generally greater than 93% although some exceptions are noted. The method can be recommended for the preconcentration of the elements in the analysis of natural waters and for the purification of MnCI2 solution in the presence of Co, Fe, Cur Cd, Hg and other transition elements. In this instance, the recovery yields are greater than 92%. Keywords: Coprecipitation with disulfide; ammonium p yrrolidin- 1- yldithioformate; transition element; preconcentra tion

Dithiocarbamates are widely used organic chelating reagents. A large amount of information on these reagents is available in several text-books, monographs and special papers. A review on dithiocarbamates, in which their chelates with elements, their oxidation products, thiuram disulfides, and their applications in analytical chemistry are described, has been presented in the monograph by Birko.1 In most instances, sodium diethyldithiocarbamate (DDTC) and ammonium pyrrolidin-1-yldithioformate [ammonium pyrrol- idinedithiocarbamate (APDC)] are used in chemical enrich- ment of trace elements by liquid-liquid extraction prior to their determination, for example, in natural waters. Although coprecipitation with inorganic and organic collectors has advantages over concentration by extraction, such as a higher concentration coefficient and the fact that the concentrate of trace elements is obtained in the solid phase, this preconcen- tration technique is rarely applied. Coprecipitation, especially with organic collectors, is mainly suitable for the determina- tion of trace elements by neutron-activation and X-ray fluorescence methods.

It is considerably more efficient to use an organic reagent that forms chelates with trace elements for coprecipitation and also as a collector. In this connection, the coprecipitation behaviour of V, Fe, Co, Cu, Zn, As, Cd, Se and Hg with APDC, which was first described by Malissa and Schoff- mann,2 has been studied. The oxidation product of APDC, bis(pyrro1idin-1-ylthiocarbonyl) disulfide (DPTD), was used as the coprecipitator for the complex compounds of the elements. In an investigation of the possibility of coprecipitat- ing trace elements with the oxidation product of potassium ethyl dithiocarbonate (PEX) and hexahydroazepinium hexa- h ydroazepine- 1-di thiocarboxylate (HMA-HMDC) , these reagents were applied tentatively.

The purpose of this paper is to discuss the feasibility of the coprecipitation of trace elements with APDC as the chelating agent and with its oxidation product DPTD as the collector.

Experimental Instrumentation

The investigation was carried out by using the. method of radioactive tracers.

Radioactivity measurements were effected with a y-ray spectrometer equipped with an NaI(T1) crystal coupled to an LP-4900 multichannel analyser (Nokia, Finland).

The absorbance of DPTD in chloroform was measured with a Specord ultraviolethisible spectrophotometer (Carl Zeiss, Germany).

A precision pH meter (Type OP-205/1) (Radelkis, Hun- gary) was used.

All laboratory glassware was washed with 6 mol dm-3 nitric acid and rinsed with triply distilled water. Coprecipitation was carried out in composite filtration equipment constructed in Plexiglas. Nuclear filters of pore size 0.46 ym (Dubna, Russia) of filter-papers (Filtrak-90) were used.

Reagents

All solutions of acids and salts were prepared from triply distilled water and analytical-reagent grade chemicals, the classification of which was accepted in the USSR. Radio- nuclides of 48V, 54Mn, 57C0, 59Fe, W u , 65Zn, 74As, 75Se, 115Cd and 203Hg were used. Radionuclides of 48V, 54Mn, 57Co and 59Fe were used in the form of chlorides, radionuclides of W u , 65Zn, 115Cd and 203Hg in the form of nitrates, and radionuclides of 74As and 75Se in the form of arsenite and selenite, respectively.

Aqueous solutions of APDC (1.5% m/v), PEX (2% d v ) and HMA-HMDC (1.5% d v ) were prepared daily, following purification by crystallization. A 1% d v aqueous solution of hydrogen peroxide was also prepared daily. Aqueous solu- tions with defined pH were prepared using either hydrochloric acid, acetic acid, sodium acetate, sodium borate and sodium hydroxide.

Procedures

Determination of the p H range of DPTD solid phase To 50 cm3 of aqueous solution was added 1 cm3 of APDC solution, followed, after 15 min, by 1 cm3 of hydrogen peroxide solution. The reagent solution was left to stand for 30 min and was then filtered through a nuclear filter. The precipitate of DPTD on the filter was washed with approxi- maJely 10-15 cm3 of water solution, which had the same pH as

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ANALYST, JUNE 1992, VOL. 117

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the initial water solution. The filter, plus DPTD, was then transferred into a test-tube, 5 cm3 of chloroform were added and the tube was sealed with a stopper and shaken for approximately 5 min. When the precipitate of DPTD had dissolved, the absorbance of the chloroform extract and the pH of the aqueous phase were measured.

Determination of p H range and recovery yield of coprecipitation of pyrrolidinedithiocarbamates To 100 cm3 of aqueous solution was added 1 cm3 of aqueous solution containing radioactive tracer. After mixing, 1 cm3 of APDC solution was added, followed, after 15 min, by 1 cm3 of hydrogen peroxide solution. The solution was filtered after 30 min. The disulfide (DPTD) remained on the filter-paper, together with the coprecipitated complexes of any transition elements present. The walls of the glassware were rinsed with approximately 10-15 cm3 of aqueous solution, which had the same pH as the initial aqueous solution. The radioactivity of the precipitate, together with that of coprecipitated radio- tracers and radiotracer standards, was measured. The data obtained were used to calculate the recovery yield of coprecipitation. The pH of the aqueous phase was also measured.

Results and Discussion Essentials of the Coprecipitation With DPTD

The compound APDC forms four-membered rings of water- insoluble chelates with ions of p- and d-elements; it is more stable in acidic solution than are other, similar dithiocarba- mates. Apart from the complexing properties of APDC, the most important property of this reagent for coprecipitation is its capacity to oxidize by forming DPTD. Therefore, the mechanism of coprecipitation with APDC is, in general, as follows.

In this work, APDC performs two functions. Firstly, it reacts with ions of the elements, and their chelates are formed in aqueous solution. As the concentration of trace elements is at the pg dm-3 leve1,or even lower, they cannot form their own independent solid phase. Therefore, for their precipita- tion from aqueous solution, a solid phase must be formed. Secondly, the excess of APDC is oxidized to DPTD by several oxidizing agents according to the equation:

oxidant 2RSH - RS-SR + 2H+ + 2e-

where RSH is an organic reagent (its molecule contains an -SH group, such as in APDC, PEX or HMA-HMDC) and RS-SR is a disulfide, e.g., DPTD. Experience has shown that hydrogen peroxide is suitable in this instance. The oxidation product DPTD is a water-insoluble compound. During the formation of the solid phase of DPTD, it coprecipitates the chelates of the trace elements. The separation of the collector from coprecipitating complexes, by filtration, then follows. The concentrate of trace elements obtained is used for their determination.

In principle, this preconcentration technique can be applied with the use of other organic reagents, the molecules of which contain -SH functional groups. The above-mentioned copre- cipitation was first described for quinoline-8-thiol3-6 and DDTC.7

Formation of Solid Phase of Some Thiuram Disulfides

The formation of a solid phase of DPTD occurs over a wide range of acidity, from 3.5 mol dm-3 HCI to pH 6.3 (Fig. 1); DPTD is formed as tiny crystals. Use of 100 mg of APDC would yield 61 mg of DPTD. As is well known, DDTC decomposes in acidic solutions up to pH 4-5. Therefore, its oxidation product, tetraethylthiuram disulfide (TETD), is

HCVmol dm-3 6 4 2

" 0 2 4 6 8

PH

Fig. 1 Formation of the solid phases of the products from the oxidation of 1, APDC; and 2, PEX depending on the acidity of the aqueous solution

Table 1 Time for the formation of the solid phase of some disulfides

Disulfide derived from APDC DDTC* PEX Time/min 30 60 90

0.3 ml of HC1.7 * The TETD is formed in aqueous solution only in the presence of

able to form its solid phase in the pH range 4.3-6.7.7 In addition, DPTD does not decompose (further oxidation) in a 10-fold excess of hydrogen peroxide with respect to the reagent (APDC). For TETD, it is necessary to consider the ratio of DDTC to H202 = 2 : 1, this ratio can never be less than 2: 1.

In order to determine the pH range of formation of the oxidation product of PEX, the same procedure was used. Fig. 1 shows that PEX and its oxidation product are not convenient in practice for coprecipitation because of the narrow pH range and the long formation time of the solid phase of the precipitate. A comparison of the times for the formation of solid phases of disulfides, which are obtained by oxidizing APDC, DDTC and PEX under similar conditions, is shown in Table 1. This parameter includes the period of incubation of the crystals, the oxidizing time of the reagents and the formation time of the solid phase. In all instances, the filtration can be caried out after the designated times, which can never be less than those listed.

Coprecipitation of Pyrrolidinedithiocarbamates

In general, the coprecipitation depends mainly on many factors such as the pH ranges for the formation of the solid phase of the collector, the oxidation state of the concentrated elements and the temperature. The important factor for the coprecipitation with disulfides, which are formed directly in aqueous solution, is the stability of the chelates and disulfides in the presence of an oxidizing agent, e.g., hydrogen peroxide. Further, these factors must be realized simultaneously. The pH range for the coprecipitation and recovery yields of the coprecipitated elements are the main parameters used for comparing the effectiveness of various collectors.

The curves for the coprecipitation of pyrrolidinedithiocar- bamates of V, Fe, Co, Cu, Zn, As, Cd, Se and Hg with DPTD are shown in Figs. 2-6. They characterize the effect of pH on the recovery yields of coprecipitated complexes. For compari- son, the pH ranges and complete recovery yields of V, Mn, Fe, Co, Cu, Zn, As, Se, Cd and Hg with DPTD and TETD are reported in Table 2. The data obtained show that pyrroli-

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ANALYST, JUNE 1992, VOL. 117 1015

HCl/mol dm-3 6 4 2

I I I

PH

Fig. 2 Effect of pH on the coprecipitation of 1, Hg; 2, Cu; and 3, Zn with DPTD

HCl/mol dm-3 4 3 2

100 -

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Fig. 3 Effect of pH on the preconcentration of 1, Fe; 2, V; and 3, Co

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Fig. 4 Effect of pH on the coprecipitation of Cd with DPTD. Amount of APDC taken, 30 mg

dinedithiocarbamates are coprecipitated with DPTD over a wider pH range than that of diethyldithiocarbamates of the same elements with TETD. The maxima of the pH ranges for coprecipitation of pyrrolidinedithiocarbamates occur in acidic media, which is essentially different from the coprecipitation of diethyldithiocarbamates. This fact is connected with the stability of APDC and DDTC in acidic and weakly acidic solutions. The coprecipi ta tion of die th yldi thiocarbamates with TETD is limited by decomposition of the collector in the presence of excess of hydrogen peroxide. Therefore, 0.25 63733

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Fig. 5 Effect of pH on the coprecipitation of SelV with DPTD. Hydrogen peroxide taken: 1.0.5 cm3 of a 1% solution: and 2,l.O cm3 of a 1% solution

HCl/mol dm-3 3 2

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Fig. 6 Effect of pH on the coprecipitation of As"' with DPTD. Hydrogen peroxide taken: 1,0.5 cm3 of a 1% solution; and 2, l.Ocm3 of a 1% solution

of a 1% solution of oxidant was used.' Reproducible results for the coprecipitation of these chelates were obtained by using a concentration of DDTC of 3.5 x 10-3 mol dm-3. The difference in the recovery yields and incomplete formation of some chelates (those of V, Fe, Mn and partially Cd) coprecipitated with DPTD can be explained by these observa- tions.

Coprecipitation of VIV, As"' and Serv

Arsenic, selenium and vanadium are elements having differ- ent oxidation states. In aqueous solution, they exist in the form of 0x0-cations (VO2+ and V03+) and oxy-anions (e .g . , As03- and As043-), which are highly stable over the wide pH range over which their hydrolysis occurs. These factors are the cause of the incomplete coprecipitation of V'" pyrrolidine- dithiocarbamate and of the narrow pH range of the coprecipi- tation of As1" and Se'" diethyldithiocarbamates.

The coprecipitation of As"' and Sel" with APDC depends on the excess of hydrogen peroxide with respect to the amount of

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1016 ANALYST, JUNE 1992, VOL. 117

Table 2 pH ranges and maximum recovery yields of dithiocarbamate complexes with DPTD and TETD

Pyrrolidinedithiocarbamate Diethyldithiocarbamate Element and added

amount of radiotracerlyg

V1" (0.006) Mn" (0.002) Fell1 (0.5) CO"' (0 .Ool ) CU"' (0.1) Zn" (0.01) Cd"$ (1 .O)

As"'T[ (0.2) Se1"7 (0.2) Hg" (0.4)

pH range 1.3-4.0

NCS 1 mol dm-3 HCl-pH 5.6

2.0-5.6 1.8-7.3

3.2-5.6 0.5-2.7 3.2-5.7

1.5 rnol dm-3 HCI-pH 3.5

4.5 mol dm-3 HCI-pH 6.2 1.5-5.2

Recovery yield (%)* 85 f 3

79 f 4 96 & 3 94 * 5 98 & 4 93 1- 4 85 f 4 98 f 3 95+3 95 f 5

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pH range NSt

6.1-7.1 5.0-7.6 4.7-7.3 4.4-7.4 5.3-7.5 5.1-7.1 -

4.8-5.5 5.1-6.1 4.6-7.2

Recovery yield (YO)*

85 98 98 98 97 98

98 95 95

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* Recovery is given as the mean for five replicates (+ standard deviation of the mean at a significance level of 0.05). t NS = Not studied. $ NC = No coprecipitation. 0 APDC solution (2 cm3) was used. fi Hydrogen peroxide solution (0.5 cm3) was used.

Table 3 Dependence of the coprecipitation of the pyrrolidinedithio- carbamate of As"l on the excess of hydrogen peroxide

Excess of Recovery hydrogen pH of yield* (YO) peroxide coprecipitation 40f5 9.7-fold 3.6 70 + 4 6.4-fold 3.3 98f4 3.2-fold 2.8 98 * 5 1.6-fold 3.0 98 f 5 1.3-fold 3.0

deviation of the mean at a significance level of 0.05). * Recovery is given as the mean for four replicates (f standard

Table 4 Recovery yields of coprecipitation of some pyrrolidinedithio- carbamates with DPTD from 3 mol drn-3 MnCI2 solution

Element and added amount of Recovery* pH of radiotracedpg yield (%) coprecipitation

Co (0.5) 94 f 7 5.0-5.1

Cd (1 .O) 94 f 5 5.0-5.1

Fe (0.5) 92 f 5 1.7-5.5

Cu (1.5) 96 + 7 5.1-5.3

Hg (2.0) 98 f 5 1.7-5.5 * Recovery yield is given as the mean for five replicates (+ standard

deviation of the mean at a significance level of 0.05).

reagent. The data presented in Table 3 show that complete coprecipitation begins with a 3.2-fold excess of hydrogen peroxide. The same concentration of oxidant is necessary to coprecipitate SeIV chelates with DPTD.

Coprecipitation of Mn"

For the preconcentration of Mn" with APDC, contradictory data have been obtained. Hence, Malissa and Gomisceks reported that Mn" chelates cannot be extracted from acidic media or aqueous solutions up to pH 3. Yanagisawa et al.9 obtained results for the extraction of the Mn" complex at pH 7-9 with isobutyl methyl ketone or with pentyl acetate. Krishnamurty and Reddylo observed no coprecipitation of Mn" with Co pyrrolidinedithiocarbamate as a collector, and Tisue et al. ,* I with Cd pyrrolidinedithiocarbamate as a coprecipitant, obtained the same results, although Eckert et al. 12 coprecipitated Mn" with Co pyrrolidinedithiocarbamate. These discrepancies are obviously connected with the differ- ent conditions of preconcentration and with the fact that Mn" can be oxidized to MnlI1. These factors can, therefore, influence the recovery yield for the coprecipitation of the Mn" complex.

The preconcentration of Mn" pyrrolidinedithiocarbamate by coprecipitation with DPTD, in the pH range 1.0-7.0, was also studied. The data obtained show that the Mn" complex does not coprecipitate with DPTD. The coprecipitation at pH >7.0 is not feasible because the collector, DPTD, is not formed in sufficient amounts at pH >7. Therefore, the use of coprecipitation with DPTD for the separation of Mn" salts from other transition metals is suggested. For this purpose, the coprecipitation of Fe, Co, Cu, Cd and Hg from 3 mol dm-3 aqueous MnClz solution was studied. The procedure used was

the same as that described previously, but with the following exception: 100 mg of APDC in triply distilled water (100 cm3) were used. The data obtained (Table 4) show that the above-mentioned coprecipitation technique with DPTD can be used for the separation of MnCI2 from impurities such as the transition metals investigated. Hence, it is also possible to purify other water-soluble Mn" salts.

Coprecipitation of Cd" and Fell'

It has been observed that coprecipitation of the cadmium complex with DPTD yields more stable results when the concentration of APDC is increased. It is essentially seen at pH >2.7 where the formation of hydroxo-complexes begins to play a significant role. The data in Table 2 show that the composition of the aqueous solution and the concentration of APDC are important factors in the coprecipitation of Cd1* and Fell1 chelates with DPTD. The influence of the composition of the aqueous solution can be explained as follows. The high concentration of inorganic salts (not reacting with APDC), acting as electrolytes, increases the rate of coagulation of DPTD. The disulfide is formed as fine crystals; therefore, they have a large surface area and many active sites. Hence, the collector has greater adsorption power, and this results in complete coprecipitation.

Coprecipitation of Hg"

Previous investigations of the copreciptation of Hg", in the form of its quinoline-8-thiolate4 and diethyldithiocarbamate,7 have shown that coprecipitation in acidic medium is incom- plete. This is presumably because of the high stability of

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DPTD, APDC and Hg" chelates in acidic solution. Therefore, it is possible to concentrate the Hg" complex selectively.

Coprecipitation of Zn", Cu" and Co"'

The data obtained by other workers show that the preconcen- tration of the pyrrolidinedithiocarbamates of Zn" (refs. 11 and 12), Cu" (refs. 11 and 12) and Co"' (ref. 12) occurs quantitatively by extraction. Analogous results have been obtained by coprecipitating these complexes with DPTD. In this instance, a 15 mg amount of APDC was sufficient.

Tao et aZ.13 have developed an extraction system with a combination of chelating agents, APDC-HMA-HMDC and HMA-HMDC alone, for the rapid, simultaneous multi- element determination of trace elements in natural water.

Under the conditions previously described, the coprecipita- tion of the complex of zinc with HMA-HMDC was investi- gated. It was found that the complex of zinc coprecipitates with the thiuram disulfide of HMDC in the pH range 4.8-5.5, with a recovery yield of 75%. The oxidation product of this reagent forms a solid phase in the shape of very tiny crystals that hinder the filtration. Therefore, further studies were not carried out.

The coprecipitation procedure , after optimization with respect to the concentrations of APDC and hydrogen perox- ide, and composition of the aqueous solutions, afforded satisfactory results for complexes of some elements (complete recovery). The method involving the use of APDC as a complexing agent and as a collector (after oxidizing) demon- strates its suitability for preconcentration of the transition elements from aqueous solutions. Concentration factors of 1600 are readily obtained.

8

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References Birko, V. M., in Dithiocarbamates, ed. Usatenko, Yu. I., Izd. 'Nauka', Moscow, 1984, pp. 77-211. Malissa, H., and Schoffmann, E., Mikrochim. Acta, 1955. 1, 187. Vircavs, M. V., Thesis, Institute of Inorganic Chemistry of the Latvian Academy of Sciences, Riga, 1980. Bankovsky, Yu. A., Vircavs, M. V., Veveris, 0. E., Pelne, A. R., and Vircava, D. K., Talanta, 1987, 34, 179. Vircavs, M. V., Pelne, A. R., and Djomina, L. L., Geokhimiya, 1988, 1037. Vircavs, M. V., and Fazlullin, S. M., Vulkanol. Seismol., 1990, 101. Vircavs, M. V., Pelne, A. R., Vircava, D. K., Rone, V. F., and Veveris, 0. E., Transactions of the 3rd Meeting of the USSR, Tomsk, May 21-23, 1985, Gidro-Meteoizdat, Leningrad, 1987,

Malissa, H., and Gomiscek, S., Fresenius' 2. Anal. Chem.. 1959, B169,401. Yanagisawa, M., Suzuki, M., and Takeuchi, T. , Anal. Chim. Acta, 1968, 43, 500. Krishnamurty, K. V., and Reddy, M. M., Anal. Chem., 1977,

Tisue, Th., Seils. C . , and Keel, R. Th., Anal. Chem., 1985,57, 82. Eckert, J . M., Leggett, K. E. A., Keene, J. B., and, Williams, K. L., Anal. Chim. Acta, 1989, 222, 169. Tao, H., Miyazaki. A., Bansho, K., and Umezaki. Y., Anal. Chim. Acta, 1984, 156, 159.

pp. 151-157.

49,222;

Paper 1 /02940A Received June 17, 1991

Accepted November 4, 1991

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