- Email: [email protected]
Substoichiometric radioisotope dilution analysis for traces of mercury by solvent extraction with silver diethyldithiocarbamate
Analytica
C&mica Acta, 93 (1977)
Q Zlsevier Scientific
301-306 Publishing Company, Amsterdam
-Printed
in The Netherlands
Short Communication
SUBSTOICIiIOMET.RIC RADIOISOTOPE DILUTION ANALYSIS FOR TRACES OF MERCURY BY SOLVENT EXTRACTION WITH SILVER DIETHY L!XTHIO6ARBAM_4TE
J. 31. LO, J. C. WEI. and S. J. YEH
institute (Received
of Sucfeor
Scierxce,
3rd January
Xationat
Tsing Iiuo Liniversity.
Hsinchu
300 (Taiwan)
1977)
hIany metal ions form complexes with the diethyldithiocarbamate anion, (CtHs)&CSz(DDC), and can be extracted into organic solvents. Extraction coefficients for metal ions with DDC under some conditions have been reported by several authors [l--6 J_ For analytical extractions, some complexes, Me(DDC),, in the organic solvent are often preferred as reagents to HDDC and NaDDC, which are less stable and easily decomposed in acidic solution [l, 2, ‘i-91 . Moreover, a certain Me(DDC), in a suitable organic solvent can be chosen to extract metal ions selectively, in accordance with the order of the extraction coefficients [ 11. For example, in 0-W _. _. M H,SO, media, the order suggested by Wyttenbach and Bajo [l] is Hg’*, Ag’, Ni’+, Cuz+, Bi’+, Sb3’, Ted*, hIo6’, Se”, I.n3+, _A,s3+,Cd2+, and Zn’+. The extraction coefficient for _4g’ is higher than those for other metals except HgZ+, hence none of these metal ions should be extracted if XgDDC is used as the reagent. The extraction coefficient for Ag’, lo*=, is 3 orders of magnitude less than that for Hg”+, 10n [Z, 63 _ ..4ccordingly, mercury in water should be readily and selectively extracted into a solution of AgDDC in an organic solvent. In this study, substoichiometric isotope dilution analysis, as developed by Suzuki and Kudo [lo. 111 and R”uZi&a and Stary [12], was applied for the determination of traces of mercury in aqueous solution, by solvent extraction of Hg” with AgDDC in chloroform. The same quantities of AgDDC in chloroform are used to extract the same quantities of mercury to be partly separated in the standard and test solutions. The standard solution contains only the mercury radioisotope and the test solution is a mixture of test sample and a known amount of standard solution. Theoretically, the radioactivity of each extract should be inversely proportional to the initial total mass of mercury. in both solutions; and the general formula for the calculation of substbichiomeiric isotope dilution analysis is Mx 2 M[(AL~‘)-l]
(1)
where A and A’ are the activities extracted from the standard and test solutions, respe&veIy, and..42 and MS are the masses of mercury in the standard and
sample, respectively.
30%
It is shown below that eqn. (1 j requires determination amounts of Ir,ercury (< 1 ccg) in solution.
of very small
Experimen&al Reagents. AgDDC was prepared by precipitation of O-5 g of AgSO, in water with 2 g of NaDDC - 2H:O in ethanol. The precipitate was filtered, washed with water, dtied at 60-70°C. then dissolved in as little chloroform as possible and the same volume of ethanol was added. XgDDC was recrystallized by slow evaporation of chloroform at 70-8O’C. The filtered crystals were kxshed repeatedly with pure water until no silver(I) was detected then dried at 60-70°C for at least 10 h. AgDDC solutions in chloroform were obtained by a&u-rate weighing and dissolution in chloroform with appropriate dilution. Aqueous mercury( II) solutions were prepared by dissolution of HgO in cliWte perchloric acid, and suitable dilution. ‘97*Z3Hg and “om_!g were produced by irradiating pure HgO and izgK03 in the Tsing Hua Open-Pool Reactor at a thermal neutron flus of about 2 - 1OL2n cm-’ 5-l for 30 h and 90 h, respectively_ The chemicals used were SupEpur grade (E. Merck). The water used was distilled, passed through an ion-exchange column and further purified by the sub-boiling distillation method suggested by Kuehner et al. [ 131. Treatment of containers. _&ll containers were cleaned by immersing in (1 + 1) HNOJ for 1 d, washing with distilled water several times and then with the sub-boiled distilled water. Before each series of experiments, the cleaned vessels were washed with AgDDC--CHC13 solution. The last traces of AgDDC were removed with chloroform. Solvent extraction and activity measurements. Glass-stoppered Erlenmeyer flasks (50 ml) were used as extraction vessels. The solutions were mechanically shaken for 2 min at room temperature (amplitude, 2.5 cm; frequency, 3 s-l). Increased shaking times did not affect the extraction rates. Niquots of the organic and aqueous phases me-x taken from the extraction flasks, and the activity was measured with a well-type Sal crystal connected to a single-channel analyzer_ Results and disctlssion Stability of AzDDC in chloroform during storage. A 3.0 - 10-j 31 solution of AgDDC in chloroform was divided, and one part was stored at 5’C in the dark and the other at 20°C in daylight. X mercury(II) solution (5 ml) of pH 0.5 (HCIO.,) labelled with ‘%*=‘Hg was extracted with 5 ml of the initial AgDDC solution; 65% of the radioactivity was estracted. Similar t&s were carried out over a period of 3 weeks with the stored AgDDC solutions_ The radioactive He+ solution of pH 0.5 was proved to be stable for several weeks. The percentage extraction of the radioactivity by the XgDDC solution stored at 5°C in the dark remained constant (65%) for 3 weeks, whereas the estnction with the reagent, stored at 20°C in the light decreased gradually to ca. 55% during that time.
303
Stability Of AgDDC in chloroform during extraction with acidic solutions. AgDDC labelled with ‘*OrnAg was used for these tests. Solutions (10 ml) of HCI, HNOX, HCZOJ and H:SOJ at different concentrations were shaken with 10 ml of the labelled 5.0 -lO-’ M AgDDC solution in chloroform. The percen*age of AgDDC remaining in the organic phase was measured by its radioactivity after shaking. HCI and HXO, decomposed AgDDC rapidly as the acidity increased up to 2 M, whereas HCIOJ and H,SO., had no effect on the stability of AgDDC even at acidities of 5+5 31. The mercury(U) sample solutions must be adjusted with HCIOj or H,SOs to pH < 1, to ensure the presence of only the Hg’+ form in the solution (141, and to prevent losses of mercury by adsorption on the container [15] _ Extraction of metal ions other than mercwyjll). Metal ion solutions (10 ml, 50 p-p-m-) at pH O-5 (HCIOJ) were shaken with 10 ml of the labelled 5.0. lo-’ hl AgDDC solution in chloroform_ At these concentrations, silver ion would be released quantitatively into the aqueous phase, if the other metal ion were extracted. Miquots (2 ml) of aqueous phase were taken for ‘lom_\g radioactivity measurement, and the percentage of the activity in the aqueous phases after extraction was calculated. When Ba”, Xi’+, Bi3+, bIgi+, Sn”, Zni*, Fe”, hln”, T13’, Co2*, and PtJ*were tested, less than 0.2% of the ““mL2g activity appeared in the aqueous phase, with Cu*’ and Cd”; the values were about 1.5%. Thus none of these metal ions are extracted, which agrees with the theoreticai predictions Ii j . Unfortunately, _4u3* and Pd’ were extracted strongiy and seriously affected the substoichiometric d.etermination of mercury. When AL? and Pd” were tested, the percentages of ‘lomAg activity found in the aqueous phase were 94% and 90% respectively_ Clearly, the estraction coefficients for Au3’ and Pd’* are higher than that for Ag’. Substoichiometric radioisotope dilution analysis for mercury. The preceding experiments showed that for a successful analysis, the reagent solution must be stored at 5°C in the dark, the aqueous solution must be acidified with HCIO, to pH < 1, and Au3 ’ and Pd’ ’ must be absent. To investigate the appiicability of this method for the determination of traces of mercury in water, a series of mercury(il) solutions (20 ml) containing 12-40 p.p.b., were estracted with 5 ml of unlabelled 1.7 - lo-’ hi AgDDC in chloroform. The total mass of mercury In these solutions was in the range 0.24-0.80 fig, and each solution contained the same amount of ‘K*“3FIg activity. The same mass of mercury should be e_tiracted into the organic phase from each solution, The percentages of radioactive mercury extracted were between 20% and 60%. The reciprocal activity e-acted from each solution was plotted against the total initial mass of mercury in each solution as a straight line (Fig. 1). but. this line did not pass through the origin, which is at variance with the theory. As indicated geometrically in Fig. 1, (IV + M,)/M -+-ii/A’ so that eqn. (1) could not be used for the calculation of the present experimental msu&. These experiments were checked repeatedly and the same results were always obtaked. l
304
tI x 5
5 d
.n
L6
J
3 E
L!.Y:
___-__
___--
_-
-
ce
5
-u
Y’Y‘
--__-_-___
-
i
Fig. 1. Relationship between total mass of mercury in initial solution ‘*‘*““Hg aetisity extracted by AgDDC in chloroform.
and reciprocal of
Deviation of experimental results from theory is often inevitable_ For example, the mercury in the standard solution containing only radioactive mercury cou!d not be determined accurately because of the very smali amount of radioisotope available. A modification of eqn. (1) is suggested for the calculation of the practical results. As indicated in Fig. 1, another standard solution with mercury mass = M, put containing no radioisotope, was added to the standard solution with mass = AI containing only radioisotope. The amount of mercury, d?,, was known accurately by dilution from a large amount of mercury (e.g. IOG mg) to a trace mercury solution. The activity extracted from the mixture fM 4 M,) was determined as A”, and the activity extracted from (M + M,) was A’. From Fig. 1, the re!ationship found is
from which the mass of mercury in the sample (31,) can be detetnlined. It is to be noted that M, the mass of mercury in the standard solution containing only radioisotope, is not involved in eqn. (2). This modification is practical in analysis for traces of mercury in water. Trace amounts of mercury can be
determined wxrately through calculation by the modified formula (eqn- 2) instead of the conventional formula, (eqn. 1). as shown in Table 1.
306
TABLE 1 Substoichiometric isotope dilution analysis for mercury by solvent extraction with AgDDC M = not analyzed. A’
Ma=
1.48 rg, A = 33012. A” = 21345
:jf, (r-ig)
27610 24330 1952s
16110 13538 11963
Taken
Found
0.48 0.94 1.66
0.53 0.96 1.56
2.7s 3.71 4.63
We .axe indebted
2.84 3.83 4.75
to Dr. Vianney
K. C. Cheng for helpful advice.
REFERENCES 1 2 3 4 5 5 7 8 9 10 11 12 13 14 15
A. Wytrenbach and S. Bajo, Anal. Chem.. 47 (1955) 1813. A. Wyttenbnch and S. Baja, Anal. Chem.. 45 (19i5) 2. H. Bode and K. J. Tusche. 2. Anal. Chem.. 157 (1975) 414. R. Wickhold. 2. Anal. Chem.. 152 (1956) 259. G. Eckert. 2. Anal. Chem.. 135 (1957) 23. J. Stary and K. Kratzer. Anal. Chim. Acta, 10 (1968) 93. J. Kucera, Radiochem. Radioanal. Lett.. 24 (19f6) 215. P. A. Schubiger and 0. &fuller. Ffadiochem. Radioanal. Lert.. 24 (1976) 353. J. 1%‘.hfitchell, Radiochem. Radioanal. Lett.. 24 (1976) 123. Ii. Suzuki, Proc. 2nd Conf. Radioisotopes (Jawn), 1958. K. Suzuki and K. Kudo. Anal. Chim. Acta, 32 (1965) 456. J_ Raiicka and J. Stary’, Talanta. 10 (1963) 2Si_ E. C. Kuehner. R. 1Uvarez,P. J. Paulsen, and T. J. Jfurphy, Anal. Chem., -f-1 (1972) R. de Levic. J. Chem. Educ., 17 (1970) 187. J. M. Lo and C. bl. Wai. Anal. Chem., 47 (1975) 1869.
2050.
C&mica Acta, 93 (1977)
Q Zlsevier Scientific
301-306 Publishing Company, Amsterdam
-Printed
in The Netherlands
Short Communication
SUBSTOICIiIOMET.RIC RADIOISOTOPE DILUTION ANALYSIS FOR TRACES OF MERCURY BY SOLVENT EXTRACTION WITH SILVER DIETHY L!XTHIO6ARBAM_4TE
J. 31. LO, J. C. WEI. and S. J. YEH
institute (Received
of Sucfeor
Scierxce,
3rd January
Xationat
Tsing Iiuo Liniversity.
Hsinchu
300 (Taiwan)
1977)
hIany metal ions form complexes with the diethyldithiocarbamate anion, (CtHs)&CSz(DDC), and can be extracted into organic solvents. Extraction coefficients for metal ions with DDC under some conditions have been reported by several authors [l--6 J_ For analytical extractions, some complexes, Me(DDC),, in the organic solvent are often preferred as reagents to HDDC and NaDDC, which are less stable and easily decomposed in acidic solution [l, 2, ‘i-91 . Moreover, a certain Me(DDC), in a suitable organic solvent can be chosen to extract metal ions selectively, in accordance with the order of the extraction coefficients [ 11. For example, in 0-W _. _. M H,SO, media, the order suggested by Wyttenbach and Bajo [l] is Hg’*, Ag’, Ni’+, Cuz+, Bi’+, Sb3’, Ted*, hIo6’, Se”, I.n3+, _A,s3+,Cd2+, and Zn’+. The extraction coefficient for _4g’ is higher than those for other metals except HgZ+, hence none of these metal ions should be extracted if XgDDC is used as the reagent. The extraction coefficient for Ag’, lo*=, is 3 orders of magnitude less than that for Hg”+, 10n [Z, 63 _ ..4ccordingly, mercury in water should be readily and selectively extracted into a solution of AgDDC in an organic solvent. In this study, substoichiometric isotope dilution analysis, as developed by Suzuki and Kudo [lo. 111 and R”uZi&a and Stary [12], was applied for the determination of traces of mercury in aqueous solution, by solvent extraction of Hg” with AgDDC in chloroform. The same quantities of AgDDC in chloroform are used to extract the same quantities of mercury to be partly separated in the standard and test solutions. The standard solution contains only the mercury radioisotope and the test solution is a mixture of test sample and a known amount of standard solution. Theoretically, the radioactivity of each extract should be inversely proportional to the initial total mass of mercury. in both solutions; and the general formula for the calculation of substbichiomeiric isotope dilution analysis is Mx 2 M[(AL~‘)-l]
(1)
where A and A’ are the activities extracted from the standard and test solutions, respe&veIy, and..42 and MS are the masses of mercury in the standard and
sample, respectively.
30%
It is shown below that eqn. (1 j requires determination amounts of Ir,ercury (< 1 ccg) in solution.
of very small
Experimen&al Reagents. AgDDC was prepared by precipitation of O-5 g of AgSO, in water with 2 g of NaDDC - 2H:O in ethanol. The precipitate was filtered, washed with water, dtied at 60-70°C. then dissolved in as little chloroform as possible and the same volume of ethanol was added. XgDDC was recrystallized by slow evaporation of chloroform at 70-8O’C. The filtered crystals were kxshed repeatedly with pure water until no silver(I) was detected then dried at 60-70°C for at least 10 h. AgDDC solutions in chloroform were obtained by a&u-rate weighing and dissolution in chloroform with appropriate dilution. Aqueous mercury( II) solutions were prepared by dissolution of HgO in cliWte perchloric acid, and suitable dilution. ‘97*Z3Hg and “om_!g were produced by irradiating pure HgO and izgK03 in the Tsing Hua Open-Pool Reactor at a thermal neutron flus of about 2 - 1OL2n cm-’ 5-l for 30 h and 90 h, respectively_ The chemicals used were SupEpur grade (E. Merck). The water used was distilled, passed through an ion-exchange column and further purified by the sub-boiling distillation method suggested by Kuehner et al. [ 131. Treatment of containers. _&ll containers were cleaned by immersing in (1 + 1) HNOJ for 1 d, washing with distilled water several times and then with the sub-boiled distilled water. Before each series of experiments, the cleaned vessels were washed with AgDDC--CHC13 solution. The last traces of AgDDC were removed with chloroform. Solvent extraction and activity measurements. Glass-stoppered Erlenmeyer flasks (50 ml) were used as extraction vessels. The solutions were mechanically shaken for 2 min at room temperature (amplitude, 2.5 cm; frequency, 3 s-l). Increased shaking times did not affect the extraction rates. Niquots of the organic and aqueous phases me-x taken from the extraction flasks, and the activity was measured with a well-type Sal crystal connected to a single-channel analyzer_ Results and disctlssion Stability of AzDDC in chloroform during storage. A 3.0 - 10-j 31 solution of AgDDC in chloroform was divided, and one part was stored at 5’C in the dark and the other at 20°C in daylight. X mercury(II) solution (5 ml) of pH 0.5 (HCIO.,) labelled with ‘%*=‘Hg was extracted with 5 ml of the initial AgDDC solution; 65% of the radioactivity was estracted. Similar t&s were carried out over a period of 3 weeks with the stored AgDDC solutions_ The radioactive He+ solution of pH 0.5 was proved to be stable for several weeks. The percentage extraction of the radioactivity by the XgDDC solution stored at 5°C in the dark remained constant (65%) for 3 weeks, whereas the estnction with the reagent, stored at 20°C in the light decreased gradually to ca. 55% during that time.
303
Stability Of AgDDC in chloroform during extraction with acidic solutions. AgDDC labelled with ‘*OrnAg was used for these tests. Solutions (10 ml) of HCI, HNOX, HCZOJ and H:SOJ at different concentrations were shaken with 10 ml of the labelled 5.0 -lO-’ M AgDDC solution in chloroform. The percen*age of AgDDC remaining in the organic phase was measured by its radioactivity after shaking. HCI and HXO, decomposed AgDDC rapidly as the acidity increased up to 2 M, whereas HCIOJ and H,SO., had no effect on the stability of AgDDC even at acidities of 5+5 31. The mercury(U) sample solutions must be adjusted with HCIOj or H,SOs to pH < 1, to ensure the presence of only the Hg’+ form in the solution (141, and to prevent losses of mercury by adsorption on the container [15] _ Extraction of metal ions other than mercwyjll). Metal ion solutions (10 ml, 50 p-p-m-) at pH O-5 (HCIOJ) were shaken with 10 ml of the labelled 5.0. lo-’ hl AgDDC solution in chloroform_ At these concentrations, silver ion would be released quantitatively into the aqueous phase, if the other metal ion were extracted. Miquots (2 ml) of aqueous phase were taken for ‘lom_\g radioactivity measurement, and the percentage of the activity in the aqueous phases after extraction was calculated. When Ba”, Xi’+, Bi3+, bIgi+, Sn”, Zni*, Fe”, hln”, T13’, Co2*, and PtJ*were tested, less than 0.2% of the ““mL2g activity appeared in the aqueous phase, with Cu*’ and Cd”; the values were about 1.5%. Thus none of these metal ions are extracted, which agrees with the theoreticai predictions Ii j . Unfortunately, _4u3* and Pd’ were extracted strongiy and seriously affected the substoichiometric d.etermination of mercury. When AL? and Pd” were tested, the percentages of ‘lomAg activity found in the aqueous phase were 94% and 90% respectively_ Clearly, the estraction coefficients for Au3’ and Pd’* are higher than that for Ag’. Substoichiometric radioisotope dilution analysis for mercury. The preceding experiments showed that for a successful analysis, the reagent solution must be stored at 5°C in the dark, the aqueous solution must be acidified with HCIO, to pH < 1, and Au3 ’ and Pd’ ’ must be absent. To investigate the appiicability of this method for the determination of traces of mercury in water, a series of mercury(il) solutions (20 ml) containing 12-40 p.p.b., were estracted with 5 ml of unlabelled 1.7 - lo-’ hi AgDDC in chloroform. The total mass of mercury In these solutions was in the range 0.24-0.80 fig, and each solution contained the same amount of ‘K*“3FIg activity. The same mass of mercury should be e_tiracted into the organic phase from each solution, The percentages of radioactive mercury extracted were between 20% and 60%. The reciprocal activity e-acted from each solution was plotted against the total initial mass of mercury in each solution as a straight line (Fig. 1). but. this line did not pass through the origin, which is at variance with the theory. As indicated geometrically in Fig. 1, (IV + M,)/M -+-ii/A’ so that eqn. (1) could not be used for the calculation of the present experimental msu&. These experiments were checked repeatedly and the same results were always obtaked. l
304
tI x 5
5 d
.n
L6
J
3 E
L!.Y:
___-__
___--
_-
-
ce
5
-u
Y’Y‘
--__-_-___
-
i
Fig. 1. Relationship between total mass of mercury in initial solution ‘*‘*““Hg aetisity extracted by AgDDC in chloroform.
and reciprocal of
Deviation of experimental results from theory is often inevitable_ For example, the mercury in the standard solution containing only radioactive mercury cou!d not be determined accurately because of the very smali amount of radioisotope available. A modification of eqn. (1) is suggested for the calculation of the practical results. As indicated in Fig. 1, another standard solution with mercury mass = M, put containing no radioisotope, was added to the standard solution with mass = AI containing only radioisotope. The amount of mercury, d?,, was known accurately by dilution from a large amount of mercury (e.g. IOG mg) to a trace mercury solution. The activity extracted from the mixture fM 4 M,) was determined as A”, and the activity extracted from (M + M,) was A’. From Fig. 1, the re!ationship found is
from which the mass of mercury in the sample (31,) can be detetnlined. It is to be noted that M, the mass of mercury in the standard solution containing only radioisotope, is not involved in eqn. (2). This modification is practical in analysis for traces of mercury in water. Trace amounts of mercury can be
determined wxrately through calculation by the modified formula (eqn- 2) instead of the conventional formula, (eqn. 1). as shown in Table 1.
306
TABLE 1 Substoichiometric isotope dilution analysis for mercury by solvent extraction with AgDDC M = not analyzed. A’
Ma=
1.48 rg, A = 33012. A” = 21345
:jf, (r-ig)
27610 24330 1952s
16110 13538 11963
Taken
Found
0.48 0.94 1.66
0.53 0.96 1.56
2.7s 3.71 4.63
We .axe indebted
2.84 3.83 4.75
to Dr. Vianney
K. C. Cheng for helpful advice.
REFERENCES 1 2 3 4 5 5 7 8 9 10 11 12 13 14 15
A. Wytrenbach and S. Bajo, Anal. Chem.. 47 (1955) 1813. A. Wyttenbnch and S. Baja, Anal. Chem.. 45 (19i5) 2. H. Bode and K. J. Tusche. 2. Anal. Chem.. 157 (1975) 414. R. Wickhold. 2. Anal. Chem.. 152 (1956) 259. G. Eckert. 2. Anal. Chem.. 135 (1957) 23. J. Stary and K. Kratzer. Anal. Chim. Acta, 10 (1968) 93. J. Kucera, Radiochem. Radioanal. Lett.. 24 (19f6) 215. P. A. Schubiger and 0. &fuller. Ffadiochem. Radioanal. Lert.. 24 (1976) 353. J. 1%‘.hfitchell, Radiochem. Radioanal. Lett.. 24 (1976) 123. Ii. Suzuki, Proc. 2nd Conf. Radioisotopes (Jawn), 1958. K. Suzuki and K. Kudo. Anal. Chim. Acta, 32 (1965) 456. J_ Raiicka and J. Stary’, Talanta. 10 (1963) 2Si_ E. C. Kuehner. R. 1Uvarez,P. J. Paulsen, and T. J. Jfurphy, Anal. Chem., -f-1 (1972) R. de Levic. J. Chem. Educ., 17 (1970) 187. J. M. Lo and C. bl. Wai. Anal. Chem., 47 (1975) 1869.
2050.