Behaviour of mercury complexes in a graphite tube furnace for atomic absorption spectrometry

Analytica ChimicaActa, 226 (1989) 203-211 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

203

BEHAVIOUR OF MERCURY COMPLEXES IN A GRAPmTE TUBE FURNACE FOR ATOMIC ABSORPTION SPECTROMETRY

1. GRGIC* and V. HUDNIK Boris Kidric Institute of Chemistry, P.O. Box 30, 61115 Ljubljana (Yugoslavia) S.GOMISCEK

Department of Chemistry, Eduard Kardelj University, Ljubljana (Yugoslavia) (Received 10th April 1989)

SUMMARY The influence oftetramethylene dithiocarbamate (TMDTC) and tetraethylthiuram disulphide (TETD) on the thermal stability of mercury in the graphite furnace was investigated. To explain the atomization of mercury complexes , the behaviour of mercury TMDTC and mercury TETD in inert and oxidizing atmospheres at different temperatures was studied. Additionally, x-ray diffraction analysis of the compounds formed was performed. All studies confirmed the formation of HgS before atomization.

The application of electrothermal atomic absorption spectrometry (ETAAS) to the determination of mercury is very limited because of the significant losses of mercury during the preatomization stage in the graphite tube furnace. Therefore, the first condition for the successful determination of mercury by ETAAS is its conversion into compounds thermally more stable by the use of convenient stabilizers. The use of many stabilizing reagents, e.g., potassium dichromate [1,2], silver nitrate with potassium permanganate [3], ammonium sulphide [4], hydrochloric acid and hydrogen peroxide [5,6] and complexing agents with thiol groups [7] has been reported. Ni and Shan [8] reported the chemical stabilization of mercury with the aid of microgram amounts of gold, platinum or palladium. The aim of this work was to study the effect of some sulphur-containing reagents, i.e., pyrrolidinecarbodithioate (tetramethylenedithiocarbamate, TMDTC) tetraethylthiuram disulphide (TETD) and dithizone (diphenylthiocarbazone, H 2Dz) on the thermal stability of mercury and to ascertain the possibilities of their analytical application in ETAAS. 0003-2670/89/$03.50

© 1989 Elsevier Science Publishers B.V.

204

EXPERIMENTAL

Apparatus A Perkin-Elmer 300 S atomic absorption spectrometer equipped with an HGA 72 graphite furnace was used for all absorption measurements. Absorption signals obtained uner argon stop-flow conditions were recorded with a Hitachi Perkin-Elmer recorder (Model 56) and a storage oscilloscope (Tektronix 5115). A Westinghouse mercury hollow-cathode lamp (lamp current 4 mA; 253.7 nm; band pass of monochromator 0.7 nm) and a deuterium background corrector were applied. The furnace control settings were drying at 100°C for 30 s, charring at 150, 200, 250, 300 or 400°C for 30 s and atomization at 2000°C for 5 s. In the chemical identification studies, Hg(TMDTC)2 and HgTETD were decomposed in an electric resistance tube furnace, x-ray diffraction diagrams were obtained with a Guinier de Wolff (Nonius) powder camera (Cu Ka rays) and for differential thermal analysis (DTA) and thermogravimetric analysis (TGA) a Mettler TA 1 apparatus was used. Differential scanning calorimetric (DSC) curves were obtained with a Perkin-Elmer DSC-2 differential scanning dynamic calorimeter. Reagents All chemicals were of analytical-reagent grade. NH 4TMDTC was synthesized in our laboratory according to the procedure of Malissa and Schoffmann [9] . A 1 mg ml- 1 mercury stock solution was prepared by dissolving 0.6768 g of mercury (II) chloride in 500 ml of doubly distilled water. Hg(TMDTC)2 was prepared by precipitation of Hg(II) ions with NH 4TMDTC solution at pH 2. The white mercury complex was filtered, washed and dried at 60°C. A 0.3% TETD solution was prepared by dissolving 0.3 g of TETD in 100 ml ethanol. A 0.01% H 2Dz solution in CCl4 was prepared according to Marczenko [10]. Procedures HgTETD solution was prepared by the addition of 2-3 ml of 0.3% ethanolic TETD solution to 1 ml of Hg (II) standard solution (2,ug ml- 1 ). A 10-,u1 volume of organic solution was introduced into the graphite tube furnace and the absorption measurement was made at 253.7 nm. A 50-ml volume of Hg(II) solution was quantitatively transferred into a separating funnel, the pH was adjusted to 2-3 with dilute HCI and 1 ml of 2% (w/v) NH 4TMDTC solution was added. Hg(TMDTC)2 was extracted with 25 ml of chloroform (10+10+5-ml portions). Before the second and third extraction, 1 ml of 2% (w/v)NH 4TMDTC solution was added. All three extracts were collected and 10-,u1 aliquots of the solution were introduced into

205

the graphite tube furnace. The sample was dried for 30 s at 100°C, ashed at 250°C for 30 s and atomized at 2000°C for 5 s. The absorbance was measured at 253.7 nm. A 50-ml volume of Hg(II) solution was quantitatively transferred into a separating funnel and H 2S04 was added to give a final acid concentration of ca. 1 moll-I. Mercury was extracted with 5 ml of 0.001% H 2Dz solution in CC14 (5 + 5-ml portions). The solution was shaken with CC14 (5 + 10 ml) before and after the second extraction. All extracts were collected, 10-,ulaliquots of the solution were introduced into the graphite tube furnace and the mercury was determined as above. RESULTS AND DISCUSSION

It is well known that the extreme volatility of mercury causes difficulties in its determination by ETAAS using the graphite tube furnace. In order to avoid these difficulties, matrix modification or stabilization should be applied. As has already been stated, the literature reports the use of many stabilizing reagents, e.g., oxidizing agents, sulphur-containing compounds and elements which readily form stable amalgams. In the preliminary stage of this investigation, different stabilizing reagents and the sensitivities achieved were compared. The results obtained are shown in Fig. 1. It can be seen that even the drying of mercury solutions in 1% HN0 3 results in losses of mercury and consequently the sensitivity obtained is very low. On the other hand, no detectable losses of mercury occurred in the solutions containing potassium dichromate, silver nitrate with potassium permanganate, sodium sulphide and H 202 , HN03 and HCl mixture at temperatures Abs.

40 30

20 10

o

oo

200

300

400 T loc]

Fig. 1. Effect of ashing temperature on 20 ng of mercury in the presence of various stabilizing reagents: (0) 0.05% w/vK2Cr207 in 1% w/vHN0 3 ; (1'.) 0.5% w/v AgN0 3+O.1% w/v KMn0 4 in 1% w/v HN0 3 ; (0) 2% HCl+2% H 202; (.) 2% HCl+2% H 202+2% HN0 3 ; (A) 10% HCI+2% H 202+2% HN0 3 ; (.I» 2% HF; (*) 5% HF+2% H 202; (e) 1% HN0 3 ; (X) 5% w/v Na2S in 1% HN0 3 ; (.6.) l.zzg ml " ! Pd.

206

up to 200 or 250°C during the ashing step. Comparing these reagents, the best sensitivity for mercury was obtained with the solutions containing 5% Na2S in 1% HN03 • Palladium showed the greatest enhancement effect and temperatures up to 450 °C can be used without causing significant losses of mercury. Even more evident is the influence of the above-mentioned compounds on the atomization of mercury from the oscilloscopic traces. Fig. 2 represents these dependences at ashing temperatures of 100,200 and 300°C. The atomization in the presence of all oxidants examined shows a similar course; it starts after 0.5 s and finishes in 2 s, The atomization process in the presence of palladium differs in some respects; the atomization profile is broader and its maximum is shifted toward higher temperatures, and the duration of atomization is longer. All these observations can be explained on the basis of the thermal stability of palladium amalgam. This is also in accordance with the observation that the absorption signal in the presence of palladium is the highest and remains unchanged also at 200 and 300°C. The last statement is true also ofthe atomization in the presence of K2Cr207, whereas the absorption signals in the presence ofKMn04 + AgN0 3 and HCl + H 202are lower at both temperatures. This confirms the former statement (Fig. 1) and indicates that the volatility losses of mercury occur at these temperatures during the ashing stage. The losses are more pronounced in the presence of nitric acid alone; the absorption signal at 100°C is low and disappears completely at higher ashing temperature.

t(s)

Fig. 2. Oscilloscopic traces of the absorption-time profile for 20 ng Hg per 10IIIin the presence of some inorganic stabilizers at different ashing temperatures: (a) 100°C; (b) 200°C; (c) 300°C, 1, 1% HN0 3 ; 2,0.05% w/v K2Cr207 in 1% HN0 3 ; 3, 0.5% w/v AgN0 3+O.1% w/v KMn0 4 in 1% HN0 3; 4, 2% HCl+2% H 202; 5, 1/lg ml- 1 Pd. Abscissa, 0.5 s per scale unit; ordinate, 0.2 V per scale unit.

207

Chelating agents, e.g., NH 4TMDTC and TETD, are often used for the determination of elements in complex matrices by ETAAS because they enable one to separate metal ions from interfering elements. In the determination of mercury in a graphite tube furnace for AAS they promise an improvement in the thermal stability of mercury because of the formation of slightly soluble metal complexes. Therefore, the study of the behaviour of Hg (TMDTC )2and HgTETD complexes in a graphite tube furnace seems useful. The preliminary experiments with Hg(TMDTC) 2and HgTETD dissolved in organic solvents gave encouraging results. These organic sulphur-containing ligands stabilized mercury in the graphite tube furnace . In Fig. 3 the results of the measurements for Hg(TMDTC)2 in chloroform and HgTETD in ethanol are shown. It can be seen that both reagents and H 2Dz shift the temperature of the thermal decomposition toward higher temperatures, making the thermal stability of corresponding compounds comparable to those obtained by the addition ofK2Cr207' Thus, temperatures up to 200°C for HgTETD and up to 300°C for Hg (TMDTC )2in an organic solvent can be used in preatomization steps without significant losses of mercury. Moreover, the sensitivity of mercury determination shows an improvement by these chelating reagents and it is higher than with K2Cr207' To obtain a more detailed knowledge of the phenomena taking place in the graphite tube furnace during the ashing and atomization steps and to explain the corresponding reactions, further experiments were performed. The first measurements of the absorption-time profile, which are presented in Figs. 4 and 5, already suggest the same reaction mechanisms for HgTETD Abs.

80 70

60

10

o

100

200

300

u»

T.IOC]

Fig. 3. Influence of ashing temperature on the absorption of mercury (20 ng per 10 ,Ill ). ( X) HgTETD in C2HsOH; (0 ) Hg(TMDTC) 2 in CHCI3 ; (0 ) Hg (HDz )2in CCI4 ; ( .6.) Hg+0.05% w Iv K2Cr207in 1% HN0 3 •

208

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Fig. 4. Oscilloscope traces of the absorption-time profile for 20 ng Hg per 10 III in the presence of some stabilizing reagents. 1, HgTETD in C2H60H; 2, Hg(TMDTC )2in CHCI3 ; 3, Hg(HDz )2 in CCl.; 4, Hg+K2Cr207 in HNO J • T 1=100 °C for 30 s; T 2=20Q oC for 30 s; T J=200Q oC for 5 s. Abscissa, 0.5 s per scale unit; ord inate, 0.2 V per scale unit. Fig. 5. Oscilloscopic trace of the absorption-time profile for 20 ng Hg per 10 III for some mercury dithiocarbamate. Solvent: CHCI 3 • 1, HgDDTC; 2, Hg(DDTC)2; 3, Hg (TMDTC) 2.For measuremen ts , see Fig. 4. Abscissa , 0.5 s per scale unit; ordinate, 0.2 V per scale unit.

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30 10

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100

100

300

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Fig. 6. Influen ce of ashing temperature on the absorption of 20 ng Hg per 10 Ill. (D) Hg(TMDTC) 2 in CHC lJ (Ar): ( X) Hg (TMDTC )2in CHCl 3 (02) .

and Hg(TMDTCh and also for other dithiocarbamates, i.e., HgN, N-DDTC and Hg(DDTCh This suggests that during the destruction of mercury organic complexes the same compound is formed. Also, the dependence of the absorbance signal on the ashing temperature and atmosphere shows no differences, as is shown in Fig. 6 for Hg(TMDTC) 2 in argon or oxygen. This can be explained by the similarity of the atomization process. The prerequisite for this similarity is the formation of the same compound in inert and oxidizing environments during the decomposition step. Because it is difficult to study and elucidate the reaction mechanisms and the chemical equilibria in the graphite tube furnace, the experiments were performed on a larger scale under conditions with destruction of Hg (TMDTC) 2 and HgTETD compounds in argon and oxygen at higher temperatures [11].

209

The freshly precipitated mercury compounds were heated at various temperatures in argon and oxygen atmospheres and the products obtained were identified by x-ray diffraction analysis. This was possible because the primary substances and their decomposition products were crystalline. The results of the x-ray diffraction study of the decomposition products of Hg(TMDTC hand HgTETD at temperatures from 80 to 280°C are given in Table 1. The results indicate that Hg(TMDTC hand HgTETD are decomposed in oxygen and argon to mercury (II) sulphide. Microanalysis of the decomposition products of the Hg(TMDTCh and HgTETD confirmed the results of the x-ray diffraction analysis. The results for mercury and sulphur corrected for the carbon present in the samples were very close to the theoretical values for mercury (II) sulphide. Additionally, the decomposition of Hg (TMDTC )2and HgTETD was investigated by differential thermal analysis (DTA), thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) . The results are given in Table 2. Hg(TMDTC)2 and HgTETD decompose according to the TG curves in the temperature intervals from 247 to 300°C and from 190 to 300°C, respectively, the melting point for Hg(TMDTC)2 being 277°C and decomposition temperature 291°C. The TG curves (Figs. 7 and 8) show that the results are in good TABLE 1

Products of decomposition of Hg(TMDTC) 2and HgTETD in argon and oxygen Compound

Temperature ( °C)

Hg (TMDTCh

80

160 280 HgTETD

100

250

Argon

Oxygen

Dithiocarbamate Dithiocarbamate HgS (hex. ) + HgS (cub.)

Dithiocarbamate Dithiocarbamate Dithiocarbamate+ HgS (hex.)+HgS (cub.)

HgTETD HgS (hex.) + HgS (cub.)

HgTETD Hg (hex.)+HgS (cub.)

TABLE 2 Decomposition temperatures ( OC) of Hg(TMDTC )2and HgTETD in argon established by TGA, DTAandDSC Method

Hg(TMDTC) 2

HgTETD

TGA DTA DSC

247-300 276 2778 , 291b

190-300 113,22 5,241

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Sb

(%)

blank values (ng per 10 Ill)

Sb

(mm )

(%)

Detection limit (ng per 10 Ill)

K2Cr20 7 , HNO a

11.9±0.9

7

0.08±0.01

12

0.10

TETD, C2HsOH

20.9±2.1

6

0.35±0.02

5

0.40

NH 4TMDTC, CHCla

32.8±3.7

8

0.23 ± 0.02

9

0.30

Stabilizer

r

r

ao.5 ng Hg per 10 Ill. b n =10

agreement with theoretical values. For Hg (TMDTC )2the residue after decomposition represented 47% (theoretical value 47.2%) of the total mass and for HgTETD 45% (theoretical value 46.8%). This is additional proof that both mercury complexes decompose to mercury (II) sulphide. On the basis of the results of this investigation, the following mechanism of reactions taking place in the atomization of Hg(TMDTC)2 and HgTETD in a graphite furnace can be proposed: 100°C

277°C

291°C

Hg(TMDTC h(org.) ------> Hg(TMDTC )2(ROlid) ------> Hg(TMDTC )2(liq.)------> Ar/02

580°C (hex.)

HgS(solid)

2000°C

100°C

• HgS(gas) ~HgO HgTETD(org.)-HgTETD(solid)

446°C (cub.)

211 190-300°C

580°C (hex.)

~ HgS(solid) Ar/02

2000°C

• HgS(gas)~Ht

446°C (cub.)

Thus, the complexation of Hg (II) by TMDTC or TETD enables one to exploit it not only for improving the thermal stability of mercury in the graphite tube furnace but also for the separation of the sample from the matrix and for the preconcentration of traces of mercury. Hg (TMDTC ) 2 is readily soluble in organic solvents. The extraction of the complex with chloroform, isobutyl methyl ketone or ethyl acetate and the injection of the organic solution into the graphite tube furnace improves the ETAAS determination of mercury with respect to sensitivity, accuracy and precision. The drawback of the use of TMDTC and TETD is their usual contamination with mercury, which can be lowered by purifying the reagents. Table 3 shows all the analytical parameters for both reagents and also a comparison with the values for K 2Cr20 7 in HN0 3 • The financial support of the Research Community of Slovenia is gratefully acknowledged. REFERENCES 1 2 3 4 5 6 7 8 9 10 11

A. Rattonetti, Report No. 12, Instrumentation Laboratory, Wilmington, MA, 1980. G.F. Kirkbright, S. Hsiao-Chuan and R.D. Snook, At. Spectrosc., 1 (1980) 85. R.D. Ediger, At. Absorpt. Newsl., 14 (1975) 127. H.J. Issag and W.L. Zielinski, Anal. Chem., 46 (1974) 1436. J.W. Owens and KS. Gladney, Anal. Chem., 48 (1976) 787. J .F. Alder and D.A. Hickman, Anal. Chern., 49 (1977) 336. Z. Slovak and H. Docekalova, Anal. Chim. Acta, 115 (1980) 11l. Z.-M. Ni and X.-Q. Shan, Spectrochim. Acta, Part 8, 42 (1987) 937. H. Malissa and K Schoffman, Mikrochim. Acta, Part I, (1955) 187. Z. Marczenko, Spectrophotometric Determination of Elements, Horwood, Chichester, 1976, p.350. S. Gomiscek, Z. Lengar, J. Cernetic and V. Hudnik, Anal. Chim. Acta, 73 (1974) 97.