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Behavior of mercury(II) salts in electrothermal atomic absorption spectrometry
Analytica Chimica Acta, 167 (1985) 269-276 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
BEHAVIOR OF MERCURY(I1) SALTS IN ELECTROTHERMAL ATOMIC ABSORPTION SPECTROMETRY Application to the Determination of Thiosulfate
T. NOMURA* and I. KARASAWA Department
of Chemzstry,
Faculty
of Science,
Shinshu
University,
Asahi, Matsumoto
390
(Japan) (Received 19th April 1984)
SUMMARY Mercury(I1) salts have different decomposition temperatures in a graphite tube or tantalum coil used for electrothermal atomic absorption spectrometry. The nitrate, perchlorate and acetate were spontaneously reduced to mercury vapor at room temperature, but the thiosulfate, sulfide, cyanide and bromide were reduced only on heating. Chloride and thiocyanate in a graphite furnace and iodide in a tantalum coil did not give mercury absorbance on heating. Thiosulfate (l-10 x 10v6 M) was determined by addition to mercury(I1) nitrate in acetate buffer, removing the response from the excess mercury( II) nitrate by drying below 100°C in the graphite furnace, and measuring the mercury absorbance on heating, which was proportional to the thiosulfate concentration.
It has been difficult to determine directly trace amounts of mercury(I1) in solution by graphite-furnace atomic absorption spectrometry (a.a.s.) because of the volatility of mercury. For example, mercury(I1) nitrate vaporizes at ca. 60” C because of reduction by graphite [l] and it is necessary to convert the mercury(H) salt either to the oxide with hydrogen peroxide [ 21, or to a compound with potassium permanganate, which vaporizes at ca. 370°C [3]. When mercury(H) nitrate solution containing a small amount of iodide is heated with a ramp mode in a graphite tube, two mercury absorption peaks appear, one from mercury(I1) nitrate at room temperature and the other from mercury(I1) iodide after heating. The latter peak height is proportional to the iodide concentration [4] . In the present paper, the behavior of mercury(I1) salts in a graphite tube is compared with that in a tantalum atomizer, which has no reducing properties, and a method for the determination of thiosulfate with a graphite furnace atomizer is discussed. The a.a.s. methods used for the determination of anions may be classified as direct and indirect methods. These have recently been reviewed [5]. The indirect determination of thiosulfate by a.a.s. has scarcely been reported because this anion does not usually react stoichiometrically with metal ions. However, mercury(I1) nitrate solution containing thiosulfate gave a mercury absorption peak by graphite-furnace a.a.s. on heating and the peak height was proportional to the concentration of thiosulfate. 0003-2670/85/$03.30
0 1985 Elsevier Science Publishers B.V.
270 EXPERIMENTAL
Reagents Mercury(I1) nitrate stock solution (0.01 M) was prepared as described previously [4] . Mercury(B) acetate and perchlorate solutions were prepared by dissolving the analytical-grade salts (Wako Pure Chemical Industries) in water. Other mercury(I1) salt solutions were prepared by mixing the appropriate potassium salt (chloride, bromide, iodide, cyanide, thiocyanate) or sodium salt (thiosulfate, sulfide) solution with mercury(I1) nitrate solution, because the mercury(I1) salts were only slightly soluble in water. Thiosulfate stock solution (0.1 M) was prepared by dissolving 12.40 g of analytical-grade sodium thiosulfate in water, diluting to 500 ml with water, and standardizing by titration with potassium iodate (starch indicator) [6]. Other solutions were prepared by dissolving the analytical-grade chemicals in water directly. Appam tus Mercury absorbance was measured with the same instruments as described previously [4] . The tantalum atomizer was set up as Fig. 1, in which the tantalum coil heater (6 turns, 3 mm i.d.) was made with tantalum wire (0.4 mm o.d.). The quartz cell containing the heater and thermocouple (Becker PR-13 type) was fixed perpendicular to the optical axis, and the light passed through the center of the heater. Gradual heating of the tantalum coil was achieved with a power supply and a variable resistor operated by a motor. The operating rate of the motor was controlled to match the temperature ramp of the graphite furnace, which was heated at 1.0 A s-l for 60 s, and the temperature was measured with the thermocouple. Each tantalum heater could be used for ca. 30 measurements. Aliquots of sample solution
I
:
1 0
1
5
10
15
20
Tune (mm)
Fig. 1. Vertical section of the tantalum atomizer. Argon gas flowed from A to B; solutions injected onto tantalum coil C through B. Fig. 2. Mercury absorbance from mercury(I1) nitrate solution left for 19 min on the tantalum coil at room temperature, followed by ramp heating at 1.0 A s-* for 60 s.
271
were injected through hole B with a micropipette. The outer gas flow rate for the graphite tube was 1.4 1 min-’ . Procedures PreZiminaSy experiments on mercury(II) salts. Inject 10 ~1 of various mercury(I1) salt solutions into the graphite tube or tantalum coil, set the inner argon gas flow at 120 ml min” for a fixed time, and atomize the salts after interruption of this gas flow. Record the absorbance of mercury at 253.7 nm throughout the procedure. Recommended procedure for the determination of thiosulfate. Place the sample solution containing 5.6-56 pg of thiosulfate and 5 ml of 0.1 M acetate buffer (pH 4.0) in a 50-ml volumetric flask, add 5 ml of 2 X lo4 M mercury(I1) nitrate solution, and dilute to volume with water. Transfer 10 ~1 into the graphite furnace. Measure the absorbance of mercury at 253.7 nm under the following conditions: rate of outer and inner argon flows, 1.4 1 min” and 120 ml min-' , respectively, with the inner flow stopped during ramp heating; current to the furnace, 20 A for 30 s, then a ramp of 4.0 A s-l for 30 s; spectral slit-width, 0.6 nm. Procedure for samples containing silver, copper(U), chloride, bromide, iodide, thiocyanate, cyanide and sulfide. Adjust the sample solution to pH 6.3 with 0.1 M sodium acetate, add 5 ml of ethanolic 2.5% (w/v) %quinolinol solution and heat in a water bath for 5 min. Filter off the precipitate and extract with two lo-ml portions of chloroform. Adjust the aqueous solution to pH 4.0 with 0.1 M acetic acid, heat for 10 min just below the boiling point and leave for 5 min after adding 5 ml of 1 X 10” M bismuth(I11) nitrate solution. Filter off the precipitate, add 5 ml of 2 X lo4 M mercury(II) nitrate solution, dilute the filtrate with water to 50 ml and follow the recommended procedure above. RESULTS
AND DISCUSSION
Behavior of mercury(II) salts in the graphite tube and the tantalum coil When 10 ~1 of 1 X lo4 M mercury(I1) nitrate solution was injected into a graphite tube or tantalum coil and left in argon gas at room temperature, mercury absorbance was observed after about 10 min (graphite) or 15 mm (tantalum) and there was little further absorbance when the furnaces were heated after 19 min. Mercury(I1) perchlorate or acetate solutions of the same concentration behaved in the same way. The absorbance change for mercury produced from its nitrate on tantalum is shown in Fig. 2. The absorbance present for the first 5 min is due to obstruction of the light beam by the solution. Mercury absorbance appeared after 15 min when the solvent (water) had evaporated. Mercury absorbance from a methanol solution of these salts appeared almost at the time of injection for graphite and about 2 min after for tantalum, because the solvent was more volatile. It was found that these mercury(I1) salts on the graphite or tantalum surfaces were spontaneously
272
reduced to mercury at room temperature, the mercury(I1) salts in the graphite tube being reduced more rapidly than on the tantalum coil. There are two reasons for the difference. First, mercury(I1) salts are reported to be reduced to mercury vapor by carbon at room temperature [ 71. Campbell and Ottaway [l] , however, reported that mercury(I1) nitrate solution in a graphite tube evaporates as mercury at ca. 60“ C and the process of reduction is I-W(s)
+ C(s) = Hg(g) + CC(g)
The equation indicates that mercury(I1) nitrate is converted first to mercury(II) oxide and then is reduced to mercury vapor by the carbon. This process, however, cannot explain the reduction of mercury(I1) nitrate in a tantalum coil and the fact that no mercury absorbance appeared when a fine powder of mercury(I1) oxide was placed in a graphite tube at room temperature. Secondly, evaporation of the solvent is promoted by the permeability of the graphite tube compared with a tantalum coil. Mercury(I1) nitrate was more easily reduced to produce mercury absorbance in a used graphite tube than in a new tube because the used tube had a rougher and more permeable surface. Mercury(I1) nitrate solutions containing bromide, iodide, cyanide or thiosulfate did not show mercury absorbance when their solutions were left in the graphite tube or tantalum coil at room temperature. Mercury absorbances from these solutions appeared when each atomizer was heated with a ramp mode of 1.0 A s-’ for 60 s. The excess of mercury(I1) nitrate used to prepare these solutions evaporated spontaneously at room temperature to mercury vapor as mentioned above, whereas mercury(I1) chloride or thiocyanate was atomized with the ramp mode in the tantalum coil but not in the graphite tube, and mercury(I1) iodide was only atomized in the graphite tube in the ramp mode. It is supposed that the iodide, which sublimes above its boiling point (351” C) [ 81, when in the non-reducing tantalum coil would sublime prior to atomization, in the ramp mode. The mercury absorbances produced from mercury(I1) nitrate solutions containing various anions, when the ramp mode was used with either atomizer, were proportional to the concentrations of the anions, as shown in Fig. 3. From these experiments, it was supposed that mercury absorbances, obtained in the ramp mode, would arise from decomposition of the mercury(II) salts of each anion, as shown in Table 1. The compositions of the mercury(I1) salts were decided from the mole ratio method [9]. Mercury(I1) nitrate containing sulfate, phosphate or carbonate did not produce any mercury absorbance when either atomizer was used in the ramp mode. Mercury absorbances from the various other mercury(I1) salts could not be separated from each other in the ramp mode because the decomposition temperatures of the various salts were very close to each other, and the slowest ramp mode available (1 A s-l) was too fast to separate the mercury absorbances from the different salts.
273
Fig. 3. Mercury absorbance from 2 x 10 -4 M mercury(I1) nitrate solutions containing: (a) thiosulfate; (b) sulfide; (c) iodide; (d) cyanide; (e) bromide; (f) thiocyanate; (g) chloride. (A) Graphite tube;(B) tantalum coil. Solutions were left for 20 min at room temperature and then heated at 1.0 A s-’ for 60 s in the graphite tube or with similar temperatures in the tantalum coil. TABLE 1 Mercury(I1) salts atomized in the graphite tube or tantalum coil or both Atomizer
Salts (composition)
Both Tantalum Graphite
HgBr,, Bg(CN),, HgS,G,, HgS I-W&, Hg(SfW, HgI,
Measurement conditions for the determination of thiosulfate in the graphite furnace Mercury gave two peaks when a mercury(I1) nitrate solution containing thiosulfate was gradually heated (4.0 A s-l) without prior drying. The behavior on drying and during the ramp atomizing mode were similar to that of an iodide-mercury(I1) nitrate mixture [4]. The measurement conditions for the graphite furnace were optimized. Prior heating to drying the sample separated the mercury absorbance from the nitrate and that from the thiosulfate; a drying current of >25 A resulted in the decomposition of mercury(II) thiosulfate. Drying, therefore, was done at 20 A for 30 s. The rate of heating for atomization was fixed at 4.0 A s-l for 30 s; rates slower than 2.3 A s-l resulted in lower sensitivity and rates faster than 4.7 A s-’ gave poor reproducibility. These conditions for drying and atomization separated clearly the mercury absorbance into two peaks, one during drying and the other during ramp heating.
274
The effect of inner gas flow rate is shown in Fig. 4. The absorbance was almost constant in the range W-120 ml min-‘. The gas-stop mode operating throughout the drying and atomization steps gave increased sensitivity, but the graphite tube under such conditions was easily oxidized and had a short life-time. Under the recommended conditions, consistent results were obtained for about 200 determinations per tube. Such measurements were done with gas-stop conditions operating only during atomization, to give increased sensitivity. Effect of pH and mercury(II) nitrate concentration It was assumed that the stability of mercury(I1) thiosulfate would change with pH. The effect of pH on the mercury absorbance was investigated in acetate, phosphate, and borate/sodium hydroxide buffer solutions and in nitric acid. The optimum pH range was found to be 1.5-5.5; the blank absorbance was 0.003 in this region. A large error was obtained above pH 6.0 because of the formation of mercury(I1) oxide which contributed to the mercury absorbance in the ramp mode. Acetate buffers in the range 0.055 X lo5 M gave the same results. The determination of thiosulfate was therefore done at pH 4.0 in 0.01 M acetate buffer. The absorbance was affected by the concentration of mercury(I1) nitrate solution if it exceeded 5 X lo9 M because the excess of mercury(I1) nitrate was not evaporated completely on drying. A small excess of mercury(I1) nitrate over thiosulfate was suitable because the excess of mercury(I1) would form mercury(I1) compounds with other anions. Mercury(I1) nitrate (1 X lo4 M) gave quantitative mercury absorbances for thiosulfate in the range 1 X lo”--8 X lo-’ M; 2 X 10” M and 1 X lo4 M mercury(I1) nitrate when used to determine small amounts of thiosulfate gave the same sensitivity and reproducibility.
1
0
I
I
40
I
I
I
80
J 120
Inner-gas flow rate (ml mm-l)
Fig. 4. Effect of argon flow rate on mercury absorbance. 2 x lo+ M Mercury(I1) nitrate containing 5 X lo* M thiosulfate heated at 4.0 A s-l for 30 s after drying at 20 A for 30 s.
215
Calibration curve, reproducibility and interferences The calibration graph for thiosulphate was linear over the range 1 X lo”1 X 10” M (0.11-1.1 mg 1”) and was described by the equation [E&O:-] (mg 1-l) = 10.4(x - 0.003), where 0.003 is the absorbance of the reagent blank and 3c is the sample peak absorbance. The relative standard deviation was 5.3% for 8 determinations of 5.0 X lOA M thiosulfate. The effect of foreign ions on the determination of 5 X 10” M thiosulfate was measured. Deviations greater than twice the standard deviation were considered to be interferences. The results are shown in Table 2. Copper(I1) formed a complex with thiosulfate and decreased the thiosulfate response. Silver increased the response because silver can form an amalgam with mercury(I1) and emit mercury during ramp heating. Hundred-fold amount of these cations were eliminated by chloroform extraction of their 8quinolinol complexes at pH 6.3. It was expected that chloride and thiocyanate would not interfere because their mercury(I1) salts were spontaneously decomposed to mercury at room temperature, as described before. However, they did increase the thiosulfate response, as shown in Table 2. Other interfering ions such as bromide, iodide, cyanide and sulfide increased the thiosulfate response with the formation of mercury(I1) salts which gave mercury absorbance at the atomizing. The interferences of chloride, bromide and thiocyanate (each loo-fold by mole) were eliminated by the addition of excess of Lascorbic acid, which reduced the mercury( II) salts to the element. Cyanide (lOO-fold) was easily eliminated by heating the solution, pH 4.0, for 10 min just below 100°C. Iodide and sulfide, which could not be eliminated by these methods, were eliminated by precipitation with bismuth( III) nitrate and filtration.
TABLE 2 Absorbance change (%) caused by the presence of other ions in the determination thiosulfate (5 X 10” M) Ions
Mole ratio to thiosulfatea 100
Cu( II) Ag(I) ClBrISCNCNS-
-23 _b
43 35 343 98 233 _b
10
1 0
0
215 0 0 342 47 233
43 0 0 75 0 70 75
_b
aA 0.1 mole ratio was without effect in all instances. bAbsorbance greatly increased.
of
276 REFERENCES 1 W. C. Campbell and J. M. Ottaway, Talanta, 21(1974) 837. 2 H. J. Issaq and W. L. Zielenski Jr., Anal. Chem., 46 (1977) 1436. 3 K. Fujiwara, K. Sato and K. Fuwa, Bunseki Kagaku, 26 (1977) 772. 4 T. Nomura and I. Karasawa, Anal. Chim. Acta, 126 (1981) 241. 5 M. Garcia-Vargas, M. Milla and J. A. Perez-Bustamante, Analyst (London), 108 (1983) 1417. 6 W. J. Williams, Handbook of Anion Determination,Butterworths, London, 1969, p. 606. 7 I. Kunert, J. Homarek and L. Sommer, Anal. Chim. Acta, 106 (1979) 285. 8 P. G. Stecher (Ed.), The Merck Index, 8th edn., Merck, Rahway, NJ, 1968, p, 660. 9 J. H. Yoe and A. L. Jones, Ind. Eng. Chem., Anal. Ed., 16 (1944) 111.
BEHAVIOR OF MERCURY(I1) SALTS IN ELECTROTHERMAL ATOMIC ABSORPTION SPECTROMETRY Application to the Determination of Thiosulfate
T. NOMURA* and I. KARASAWA Department
of Chemzstry,
Faculty
of Science,
Shinshu
University,
Asahi, Matsumoto
390
(Japan) (Received 19th April 1984)
SUMMARY Mercury(I1) salts have different decomposition temperatures in a graphite tube or tantalum coil used for electrothermal atomic absorption spectrometry. The nitrate, perchlorate and acetate were spontaneously reduced to mercury vapor at room temperature, but the thiosulfate, sulfide, cyanide and bromide were reduced only on heating. Chloride and thiocyanate in a graphite furnace and iodide in a tantalum coil did not give mercury absorbance on heating. Thiosulfate (l-10 x 10v6 M) was determined by addition to mercury(I1) nitrate in acetate buffer, removing the response from the excess mercury( II) nitrate by drying below 100°C in the graphite furnace, and measuring the mercury absorbance on heating, which was proportional to the thiosulfate concentration.
It has been difficult to determine directly trace amounts of mercury(I1) in solution by graphite-furnace atomic absorption spectrometry (a.a.s.) because of the volatility of mercury. For example, mercury(I1) nitrate vaporizes at ca. 60” C because of reduction by graphite [l] and it is necessary to convert the mercury(H) salt either to the oxide with hydrogen peroxide [ 21, or to a compound with potassium permanganate, which vaporizes at ca. 370°C [3]. When mercury(H) nitrate solution containing a small amount of iodide is heated with a ramp mode in a graphite tube, two mercury absorption peaks appear, one from mercury(I1) nitrate at room temperature and the other from mercury(I1) iodide after heating. The latter peak height is proportional to the iodide concentration [4] . In the present paper, the behavior of mercury(I1) salts in a graphite tube is compared with that in a tantalum atomizer, which has no reducing properties, and a method for the determination of thiosulfate with a graphite furnace atomizer is discussed. The a.a.s. methods used for the determination of anions may be classified as direct and indirect methods. These have recently been reviewed [5]. The indirect determination of thiosulfate by a.a.s. has scarcely been reported because this anion does not usually react stoichiometrically with metal ions. However, mercury(I1) nitrate solution containing thiosulfate gave a mercury absorption peak by graphite-furnace a.a.s. on heating and the peak height was proportional to the concentration of thiosulfate. 0003-2670/85/$03.30
0 1985 Elsevier Science Publishers B.V.
270 EXPERIMENTAL
Reagents Mercury(I1) nitrate stock solution (0.01 M) was prepared as described previously [4] . Mercury(B) acetate and perchlorate solutions were prepared by dissolving the analytical-grade salts (Wako Pure Chemical Industries) in water. Other mercury(I1) salt solutions were prepared by mixing the appropriate potassium salt (chloride, bromide, iodide, cyanide, thiocyanate) or sodium salt (thiosulfate, sulfide) solution with mercury(I1) nitrate solution, because the mercury(I1) salts were only slightly soluble in water. Thiosulfate stock solution (0.1 M) was prepared by dissolving 12.40 g of analytical-grade sodium thiosulfate in water, diluting to 500 ml with water, and standardizing by titration with potassium iodate (starch indicator) [6]. Other solutions were prepared by dissolving the analytical-grade chemicals in water directly. Appam tus Mercury absorbance was measured with the same instruments as described previously [4] . The tantalum atomizer was set up as Fig. 1, in which the tantalum coil heater (6 turns, 3 mm i.d.) was made with tantalum wire (0.4 mm o.d.). The quartz cell containing the heater and thermocouple (Becker PR-13 type) was fixed perpendicular to the optical axis, and the light passed through the center of the heater. Gradual heating of the tantalum coil was achieved with a power supply and a variable resistor operated by a motor. The operating rate of the motor was controlled to match the temperature ramp of the graphite furnace, which was heated at 1.0 A s-l for 60 s, and the temperature was measured with the thermocouple. Each tantalum heater could be used for ca. 30 measurements. Aliquots of sample solution
I
:
1 0
1
5
10
15
20
Tune (mm)
Fig. 1. Vertical section of the tantalum atomizer. Argon gas flowed from A to B; solutions injected onto tantalum coil C through B. Fig. 2. Mercury absorbance from mercury(I1) nitrate solution left for 19 min on the tantalum coil at room temperature, followed by ramp heating at 1.0 A s-* for 60 s.
271
were injected through hole B with a micropipette. The outer gas flow rate for the graphite tube was 1.4 1 min-’ . Procedures PreZiminaSy experiments on mercury(II) salts. Inject 10 ~1 of various mercury(I1) salt solutions into the graphite tube or tantalum coil, set the inner argon gas flow at 120 ml min” for a fixed time, and atomize the salts after interruption of this gas flow. Record the absorbance of mercury at 253.7 nm throughout the procedure. Recommended procedure for the determination of thiosulfate. Place the sample solution containing 5.6-56 pg of thiosulfate and 5 ml of 0.1 M acetate buffer (pH 4.0) in a 50-ml volumetric flask, add 5 ml of 2 X lo4 M mercury(I1) nitrate solution, and dilute to volume with water. Transfer 10 ~1 into the graphite furnace. Measure the absorbance of mercury at 253.7 nm under the following conditions: rate of outer and inner argon flows, 1.4 1 min” and 120 ml min-' , respectively, with the inner flow stopped during ramp heating; current to the furnace, 20 A for 30 s, then a ramp of 4.0 A s-l for 30 s; spectral slit-width, 0.6 nm. Procedure for samples containing silver, copper(U), chloride, bromide, iodide, thiocyanate, cyanide and sulfide. Adjust the sample solution to pH 6.3 with 0.1 M sodium acetate, add 5 ml of ethanolic 2.5% (w/v) %quinolinol solution and heat in a water bath for 5 min. Filter off the precipitate and extract with two lo-ml portions of chloroform. Adjust the aqueous solution to pH 4.0 with 0.1 M acetic acid, heat for 10 min just below the boiling point and leave for 5 min after adding 5 ml of 1 X 10” M bismuth(I11) nitrate solution. Filter off the precipitate, add 5 ml of 2 X lo4 M mercury(II) nitrate solution, dilute the filtrate with water to 50 ml and follow the recommended procedure above. RESULTS
AND DISCUSSION
Behavior of mercury(II) salts in the graphite tube and the tantalum coil When 10 ~1 of 1 X lo4 M mercury(I1) nitrate solution was injected into a graphite tube or tantalum coil and left in argon gas at room temperature, mercury absorbance was observed after about 10 min (graphite) or 15 mm (tantalum) and there was little further absorbance when the furnaces were heated after 19 min. Mercury(I1) perchlorate or acetate solutions of the same concentration behaved in the same way. The absorbance change for mercury produced from its nitrate on tantalum is shown in Fig. 2. The absorbance present for the first 5 min is due to obstruction of the light beam by the solution. Mercury absorbance appeared after 15 min when the solvent (water) had evaporated. Mercury absorbance from a methanol solution of these salts appeared almost at the time of injection for graphite and about 2 min after for tantalum, because the solvent was more volatile. It was found that these mercury(I1) salts on the graphite or tantalum surfaces were spontaneously
272
reduced to mercury at room temperature, the mercury(I1) salts in the graphite tube being reduced more rapidly than on the tantalum coil. There are two reasons for the difference. First, mercury(I1) salts are reported to be reduced to mercury vapor by carbon at room temperature [ 71. Campbell and Ottaway [l] , however, reported that mercury(I1) nitrate solution in a graphite tube evaporates as mercury at ca. 60“ C and the process of reduction is I-W(s)
+ C(s) = Hg(g) + CC(g)
The equation indicates that mercury(I1) nitrate is converted first to mercury(II) oxide and then is reduced to mercury vapor by the carbon. This process, however, cannot explain the reduction of mercury(I1) nitrate in a tantalum coil and the fact that no mercury absorbance appeared when a fine powder of mercury(I1) oxide was placed in a graphite tube at room temperature. Secondly, evaporation of the solvent is promoted by the permeability of the graphite tube compared with a tantalum coil. Mercury(I1) nitrate was more easily reduced to produce mercury absorbance in a used graphite tube than in a new tube because the used tube had a rougher and more permeable surface. Mercury(I1) nitrate solutions containing bromide, iodide, cyanide or thiosulfate did not show mercury absorbance when their solutions were left in the graphite tube or tantalum coil at room temperature. Mercury absorbances from these solutions appeared when each atomizer was heated with a ramp mode of 1.0 A s-’ for 60 s. The excess of mercury(I1) nitrate used to prepare these solutions evaporated spontaneously at room temperature to mercury vapor as mentioned above, whereas mercury(I1) chloride or thiocyanate was atomized with the ramp mode in the tantalum coil but not in the graphite tube, and mercury(I1) iodide was only atomized in the graphite tube in the ramp mode. It is supposed that the iodide, which sublimes above its boiling point (351” C) [ 81, when in the non-reducing tantalum coil would sublime prior to atomization, in the ramp mode. The mercury absorbances produced from mercury(I1) nitrate solutions containing various anions, when the ramp mode was used with either atomizer, were proportional to the concentrations of the anions, as shown in Fig. 3. From these experiments, it was supposed that mercury absorbances, obtained in the ramp mode, would arise from decomposition of the mercury(II) salts of each anion, as shown in Table 1. The compositions of the mercury(I1) salts were decided from the mole ratio method [9]. Mercury(I1) nitrate containing sulfate, phosphate or carbonate did not produce any mercury absorbance when either atomizer was used in the ramp mode. Mercury absorbances from the various other mercury(I1) salts could not be separated from each other in the ramp mode because the decomposition temperatures of the various salts were very close to each other, and the slowest ramp mode available (1 A s-l) was too fast to separate the mercury absorbances from the different salts.
273
Fig. 3. Mercury absorbance from 2 x 10 -4 M mercury(I1) nitrate solutions containing: (a) thiosulfate; (b) sulfide; (c) iodide; (d) cyanide; (e) bromide; (f) thiocyanate; (g) chloride. (A) Graphite tube;(B) tantalum coil. Solutions were left for 20 min at room temperature and then heated at 1.0 A s-’ for 60 s in the graphite tube or with similar temperatures in the tantalum coil. TABLE 1 Mercury(I1) salts atomized in the graphite tube or tantalum coil or both Atomizer
Salts (composition)
Both Tantalum Graphite
HgBr,, Bg(CN),, HgS,G,, HgS I-W&, Hg(SfW, HgI,
Measurement conditions for the determination of thiosulfate in the graphite furnace Mercury gave two peaks when a mercury(I1) nitrate solution containing thiosulfate was gradually heated (4.0 A s-l) without prior drying. The behavior on drying and during the ramp atomizing mode were similar to that of an iodide-mercury(I1) nitrate mixture [4]. The measurement conditions for the graphite furnace were optimized. Prior heating to drying the sample separated the mercury absorbance from the nitrate and that from the thiosulfate; a drying current of >25 A resulted in the decomposition of mercury(II) thiosulfate. Drying, therefore, was done at 20 A for 30 s. The rate of heating for atomization was fixed at 4.0 A s-l for 30 s; rates slower than 2.3 A s-l resulted in lower sensitivity and rates faster than 4.7 A s-’ gave poor reproducibility. These conditions for drying and atomization separated clearly the mercury absorbance into two peaks, one during drying and the other during ramp heating.
274
The effect of inner gas flow rate is shown in Fig. 4. The absorbance was almost constant in the range W-120 ml min-‘. The gas-stop mode operating throughout the drying and atomization steps gave increased sensitivity, but the graphite tube under such conditions was easily oxidized and had a short life-time. Under the recommended conditions, consistent results were obtained for about 200 determinations per tube. Such measurements were done with gas-stop conditions operating only during atomization, to give increased sensitivity. Effect of pH and mercury(II) nitrate concentration It was assumed that the stability of mercury(I1) thiosulfate would change with pH. The effect of pH on the mercury absorbance was investigated in acetate, phosphate, and borate/sodium hydroxide buffer solutions and in nitric acid. The optimum pH range was found to be 1.5-5.5; the blank absorbance was 0.003 in this region. A large error was obtained above pH 6.0 because of the formation of mercury(I1) oxide which contributed to the mercury absorbance in the ramp mode. Acetate buffers in the range 0.055 X lo5 M gave the same results. The determination of thiosulfate was therefore done at pH 4.0 in 0.01 M acetate buffer. The absorbance was affected by the concentration of mercury(I1) nitrate solution if it exceeded 5 X lo9 M because the excess of mercury(I1) nitrate was not evaporated completely on drying. A small excess of mercury(I1) nitrate over thiosulfate was suitable because the excess of mercury(I1) would form mercury(I1) compounds with other anions. Mercury(I1) nitrate (1 X lo4 M) gave quantitative mercury absorbances for thiosulfate in the range 1 X lo”--8 X lo-’ M; 2 X 10” M and 1 X lo4 M mercury(I1) nitrate when used to determine small amounts of thiosulfate gave the same sensitivity and reproducibility.
1
0
I
I
40
I
I
I
80
J 120
Inner-gas flow rate (ml mm-l)
Fig. 4. Effect of argon flow rate on mercury absorbance. 2 x lo+ M Mercury(I1) nitrate containing 5 X lo* M thiosulfate heated at 4.0 A s-l for 30 s after drying at 20 A for 30 s.
215
Calibration curve, reproducibility and interferences The calibration graph for thiosulphate was linear over the range 1 X lo”1 X 10” M (0.11-1.1 mg 1”) and was described by the equation [E&O:-] (mg 1-l) = 10.4(x - 0.003), where 0.003 is the absorbance of the reagent blank and 3c is the sample peak absorbance. The relative standard deviation was 5.3% for 8 determinations of 5.0 X lOA M thiosulfate. The effect of foreign ions on the determination of 5 X 10” M thiosulfate was measured. Deviations greater than twice the standard deviation were considered to be interferences. The results are shown in Table 2. Copper(I1) formed a complex with thiosulfate and decreased the thiosulfate response. Silver increased the response because silver can form an amalgam with mercury(I1) and emit mercury during ramp heating. Hundred-fold amount of these cations were eliminated by chloroform extraction of their 8quinolinol complexes at pH 6.3. It was expected that chloride and thiocyanate would not interfere because their mercury(I1) salts were spontaneously decomposed to mercury at room temperature, as described before. However, they did increase the thiosulfate response, as shown in Table 2. Other interfering ions such as bromide, iodide, cyanide and sulfide increased the thiosulfate response with the formation of mercury(I1) salts which gave mercury absorbance at the atomizing. The interferences of chloride, bromide and thiocyanate (each loo-fold by mole) were eliminated by the addition of excess of Lascorbic acid, which reduced the mercury( II) salts to the element. Cyanide (lOO-fold) was easily eliminated by heating the solution, pH 4.0, for 10 min just below 100°C. Iodide and sulfide, which could not be eliminated by these methods, were eliminated by precipitation with bismuth( III) nitrate and filtration.
TABLE 2 Absorbance change (%) caused by the presence of other ions in the determination thiosulfate (5 X 10” M) Ions
Mole ratio to thiosulfatea 100
Cu( II) Ag(I) ClBrISCNCNS-
-23 _b
43 35 343 98 233 _b
10
1 0
0
215 0 0 342 47 233
43 0 0 75 0 70 75
_b
aA 0.1 mole ratio was without effect in all instances. bAbsorbance greatly increased.
of
276 REFERENCES 1 W. C. Campbell and J. M. Ottaway, Talanta, 21(1974) 837. 2 H. J. Issaq and W. L. Zielenski Jr., Anal. Chem., 46 (1977) 1436. 3 K. Fujiwara, K. Sato and K. Fuwa, Bunseki Kagaku, 26 (1977) 772. 4 T. Nomura and I. Karasawa, Anal. Chim. Acta, 126 (1981) 241. 5 M. Garcia-Vargas, M. Milla and J. A. Perez-Bustamante, Analyst (London), 108 (1983) 1417. 6 W. J. Williams, Handbook of Anion Determination,Butterworths, London, 1969, p. 606. 7 I. Kunert, J. Homarek and L. Sommer, Anal. Chim. Acta, 106 (1979) 285. 8 P. G. Stecher (Ed.), The Merck Index, 8th edn., Merck, Rahway, NJ, 1968, p, 660. 9 J. H. Yoe and A. L. Jones, Ind. Eng. Chem., Anal. Ed., 16 (1944) 111.