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Internal electrolytic determination of silver in solution with a piezoelectric quartz crystal
Analytica Chimica Acta, 155 (1983) 231-234 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
Short Communication
INTERNAL ELECTROLYTIC DETERMINATION OF SILVER IN SOLUTION WITH A PIEZOELECTRIC QUARTZ CRYSTAL
T. NOMURA* and T. NAGAMUNE Department (Japan)
of Chemistry,
Faculty
of Science,
Shinshu
University,
Asahi, Matsumoto
390
(Received 20th May 1983)
Summary. The piezoelectric quartz crystal is connected to an oscillator constructed with an integrated circuit, which has insufficient potential between the electrodes to electrodeposit metal ions spontaneously from the solution. Silver (10-s-10‘6 M) in solution is determined by internal electrodeposition on the platinum electrodes of the crystal connected to a zinc rod immersed in the same solution.
A piezoelectric quartz crystal connected to a transistor-based oscillator oscillates in liquids [l] . The frequency shift depends on the specific conductivity and density of the solution [ 21. Metal ions, however, electrodeposit on one of the electrodes because there is a potential of about 1.5 V between the electrodes [ 21. One electrode on the crystal, therefore, should be covered to avoid electrodeposition of metal ions from the solution by electrolysis when the crystal is used. An oscillator constructed with an integrated circuit (i.c.) [3] also allowed the crystal to oscillate in liquids. It was found that no metal ion spontaneously electrodeposited on the electrodes, but the crystal otherwise behaved similarly to that connected to the transistor-based circuit. Internal electrolysis of silver in solution, therefore, was possible by using the crystal connected to the i.c. oscillator. Internal electrolysis [4] is now rarely used for the determination of metal ions because it is time-consuming, and cannot be used to determine minute amounts of metal ions. Shiobara [5] determined silver which is electrodeposited on a weighed platinum electrode against copper wire, using a potassium nitrate/agar bridge. In this communication, micromolar concentrations of silver are determined in an experiment lasting several minutes by using a zinc rod for the counter electrode. Experimental Apparatus and reagents. The piezoelectric quartz crystal was as described previously [6], and was set in a flow cell as shown in Fig. 1. A zinc rod, (2 mm diameter, 30 mm long) was used as a counter electrode to the platinum electrode of the crystal, 1 mm apart, and connected to the platinum electrode with a copper wire. The oscillator was built with an integrated circuit 0003-2670/83/$03.00
o 1983 Elsevier Science Publishers B.V.
232
Fig. 1. Vertical section of the flow cell: (A) cell; (B) crystal; (C)zinc (E) silicone tubing.
rod; (D) leading wire;
Fig. 2. Oscillator circuit [ 31.
as shown in Fig. 2 [3]. The digital counter, recorder, air bath and peristaltic pump were used as described previously [6]. A silver stock solution (0.1 M) was prepared by dissolving 1.6987 g of silver nitrate in water and, diluting to 100 ml with water; it was stored in an ambercoloured bottle. The pH was adjusted with 0.1 M tartaric acid and sodium tartrate solutions. Determination of silver (10-5-10” M). Transfer the sample or standard solution, which is 1 X 10m3M in EDTA and 5 X 10T3 M in tartrate buffer (pH 3.9), and the reagent blank solution to their respective containers. Pass the reagent blank solution through the cell at 10.4 ml min-‘. When the crystal frequency has become constant (F,), pass the sample solution for exactly 10 min (or 5 min for 10e5 M silver), and then pass the reagent blank solution again until the frequency is constant (F2). The frequency change, AF = F, - F2, is proportional to the concentration of silver, and a calibration graph may be constructed on this basis. After several experiments, the electrode with deposited silver is removed from the cell, dipped in 6 M nitric acid for 5 s, washed with water and acetone, and dried. The zinc rod is cleaned by the same method, but with 6 M hydrochloric acid instead of nitric acid. Results and discussion Behavior of the crystal immersed in aqueous solution. The frequency shift of the crystal connected to the i.c. oscillator when the whole crystal was immersed in aqueous solution was investigated by the same method as described previously [2]. The frequency of the crystal in water decreased from that in air by about 6400 Hz, depending on the position of the crystal with regard to the water surface. Frequency shifts resulting from the specific con-
233
ductance of the medium are shown in Table 1. The frequency decreased with increasing specific conductance whereas it increased when the transistorized oscillator was used [ 21. The changes were much greater than that of the latter [2]. On the other hand, the frequency decreased with increasing density, similarly to the transistorized circuit (Table 2). The frequency also changed with the water temperature; the change from 24°C to 25°C was 30 Hz (the transistorized circuit gave 75 Hz). Some metal ions spontaneously deposited on the electrode of the crystal connected to the transistorized oscillator when the whole crystal was immersed in solution because of the potential between the electrodes on the crystal. However, the frequency of the crystal connected to the i.c. oscillator did not change in solutions of silver, copper( cadmium, lead or nickel (which deposited on the crystal connected to the transistorized oscillator) in 0.01 M acetate buffer (pH 4.7). It was expected that the electrodes connected to the i.c. oscillator would have a smaller potential difference so that metal ions would not electrodeposit. Internal electrolytic determination of silver. In order to determine silver selectively by internal electrodeposition, it is necessary to use a counter metal electrode having a slightly larger oxidation potential than silver [5]. Determination of silver using the platinum electrode of the crystal was initially examined with similar equipment to that in the earlier paper [ 51. However, micromolar concentrations of silver did not electrodeposit on the crystal because the potential difference between the platinum electrode and the copper counter electrode was too small to overcome the junction TABLE 1 Frequency
shifts of the crystal when changing from water to potassium chloride solutions
KC1 cont. Specific (X 10’ M) conductance (X 1O-5 Q-l cm-‘) 1 1.5 2 3 5
1.48 2.23 2.96 4.44 7.39
Frequency shift (Hz) -13 -41 -70 -153 -352
KC1 cont. (X 10e4 M)
Specific conductance (X lo+ a-’ cm-l)
8 10 20 50
11.8 14.7 29.2 71.8
Frequency shift (Hz) -661 -850 -1540 Unstable
TABLE 2 Frequency shifts when changing from water to sucrose solutions sucrose cont. (% w/v)
Density difference (mg cmb3)
0.5 1 2
1.99 3.95 8.59
Frequency shift (Hz) -75 -161 -221
Sucrose cont. (% WV)
Density difference (mg cm-‘)
4 6 8
16.60 24.38 32.28
Frequency shift (Hz) -356 -423 -536
TABLE 3 Tolerance limits for ions in the determination of 1
X
10“ M silver
Tolerance limit (Ion :silver mole ratio)
Ions
10
Fe(III), Co(II), Ni(II), Pb(II), Cd(II), Zn( II) SO:-, NO;, CN-, CO:Hg(II), Cu(II) s,o;SCN‘, Cl-, Br-, I-, S*-
1 0.1
potential of the agar bridge. When the copper counter rod was connected to the platinum electrode in the same solution, silver deposited on both surfaces of the platinum electrode and the copper rod and changed the frequency of the crystal. However, silver could not be determined because the frequency change had a plateau resulting from covering the copper rod with silver. The frequency changes were small or poorly reproducible when lead, aluminium or tin was used as the counter rod. Zinc rod was finally chosen for this purpose. Dependence of the frequency change on pH and flow rate. Frequency changes depending on the electrodeposition of silver were constant in the range of pH 3.4-5.4 (tartrate buffers); above this pH, silver formed its hydroxide. Frequency changes were exactly proportional to the flow rate for 2.1-15.2 ml min-’ and for electrodeposition times in the range 0.5-10 min examined. Electrodeposition for 10 min was used for the determination of micromolar silver solutions, and 5 min for larger concentrations. Calibration and reproducibility. The calibration graph of frequency change (AF) against silver concentration for lo-min electrolysis was linear for 1 X 10*-l X 10e5 M and is described by the equation [Ag] = (AF/23.8) X 10” M, where AF is measured in Hz. The standard deviation was 11.9 Hz (5.0%) for 5 determinations of 1 X lo-’ M silver. Effect of other ions. The effect of various ions on the determination of 1 X lo-’ M silver was investigated; changes of frequency of more than *8% were considered to result from interferences. The tolerance limits found are shown in Table 3. Interferences of mercury(I1) and copper(I1) arose from electrodeposition on the electrode, thus giving positive errors. Interferences of anions, except for thiosulfate, which formed a soluble silver complex, arose from the formation of precipitates of silver, giving negative errors. These anions preventing electrodeposition of silver would be eliminated by the method described previously [ 61. REFERENCES 1 2 3 4 5 6
T. Nomura and M. Okuhara, Anal. Chim. Acta, 142 (1982) 281. T. Nomura and M. Maruyama, Anal. Chim. Acta, 147 (1983) 365. S. Nakajima, Transistor Gijutsu, 19 (1982) 348. See, e.g., A. J. Bard, Anal. Chem., 36 (1964) 70R. Y. Shiobara, Bunseki Kagaku, 10 (1961) 1290. T. Nomura and M. Iijima, Anal. Chim. Acta, 131 (1981) 97.
Short Communication
INTERNAL ELECTROLYTIC DETERMINATION OF SILVER IN SOLUTION WITH A PIEZOELECTRIC QUARTZ CRYSTAL
T. NOMURA* and T. NAGAMUNE Department (Japan)
of Chemistry,
Faculty
of Science,
Shinshu
University,
Asahi, Matsumoto
390
(Received 20th May 1983)
Summary. The piezoelectric quartz crystal is connected to an oscillator constructed with an integrated circuit, which has insufficient potential between the electrodes to electrodeposit metal ions spontaneously from the solution. Silver (10-s-10‘6 M) in solution is determined by internal electrodeposition on the platinum electrodes of the crystal connected to a zinc rod immersed in the same solution.
A piezoelectric quartz crystal connected to a transistor-based oscillator oscillates in liquids [l] . The frequency shift depends on the specific conductivity and density of the solution [ 21. Metal ions, however, electrodeposit on one of the electrodes because there is a potential of about 1.5 V between the electrodes [ 21. One electrode on the crystal, therefore, should be covered to avoid electrodeposition of metal ions from the solution by electrolysis when the crystal is used. An oscillator constructed with an integrated circuit (i.c.) [3] also allowed the crystal to oscillate in liquids. It was found that no metal ion spontaneously electrodeposited on the electrodes, but the crystal otherwise behaved similarly to that connected to the transistor-based circuit. Internal electrolysis of silver in solution, therefore, was possible by using the crystal connected to the i.c. oscillator. Internal electrolysis [4] is now rarely used for the determination of metal ions because it is time-consuming, and cannot be used to determine minute amounts of metal ions. Shiobara [5] determined silver which is electrodeposited on a weighed platinum electrode against copper wire, using a potassium nitrate/agar bridge. In this communication, micromolar concentrations of silver are determined in an experiment lasting several minutes by using a zinc rod for the counter electrode. Experimental Apparatus and reagents. The piezoelectric quartz crystal was as described previously [6], and was set in a flow cell as shown in Fig. 1. A zinc rod, (2 mm diameter, 30 mm long) was used as a counter electrode to the platinum electrode of the crystal, 1 mm apart, and connected to the platinum electrode with a copper wire. The oscillator was built with an integrated circuit 0003-2670/83/$03.00
o 1983 Elsevier Science Publishers B.V.
232
Fig. 1. Vertical section of the flow cell: (A) cell; (B) crystal; (C)zinc (E) silicone tubing.
rod; (D) leading wire;
Fig. 2. Oscillator circuit [ 31.
as shown in Fig. 2 [3]. The digital counter, recorder, air bath and peristaltic pump were used as described previously [6]. A silver stock solution (0.1 M) was prepared by dissolving 1.6987 g of silver nitrate in water and, diluting to 100 ml with water; it was stored in an ambercoloured bottle. The pH was adjusted with 0.1 M tartaric acid and sodium tartrate solutions. Determination of silver (10-5-10” M). Transfer the sample or standard solution, which is 1 X 10m3M in EDTA and 5 X 10T3 M in tartrate buffer (pH 3.9), and the reagent blank solution to their respective containers. Pass the reagent blank solution through the cell at 10.4 ml min-‘. When the crystal frequency has become constant (F,), pass the sample solution for exactly 10 min (or 5 min for 10e5 M silver), and then pass the reagent blank solution again until the frequency is constant (F2). The frequency change, AF = F, - F2, is proportional to the concentration of silver, and a calibration graph may be constructed on this basis. After several experiments, the electrode with deposited silver is removed from the cell, dipped in 6 M nitric acid for 5 s, washed with water and acetone, and dried. The zinc rod is cleaned by the same method, but with 6 M hydrochloric acid instead of nitric acid. Results and discussion Behavior of the crystal immersed in aqueous solution. The frequency shift of the crystal connected to the i.c. oscillator when the whole crystal was immersed in aqueous solution was investigated by the same method as described previously [2]. The frequency of the crystal in water decreased from that in air by about 6400 Hz, depending on the position of the crystal with regard to the water surface. Frequency shifts resulting from the specific con-
233
ductance of the medium are shown in Table 1. The frequency decreased with increasing specific conductance whereas it increased when the transistorized oscillator was used [ 21. The changes were much greater than that of the latter [2]. On the other hand, the frequency decreased with increasing density, similarly to the transistorized circuit (Table 2). The frequency also changed with the water temperature; the change from 24°C to 25°C was 30 Hz (the transistorized circuit gave 75 Hz). Some metal ions spontaneously deposited on the electrode of the crystal connected to the transistorized oscillator when the whole crystal was immersed in solution because of the potential between the electrodes on the crystal. However, the frequency of the crystal connected to the i.c. oscillator did not change in solutions of silver, copper( cadmium, lead or nickel (which deposited on the crystal connected to the transistorized oscillator) in 0.01 M acetate buffer (pH 4.7). It was expected that the electrodes connected to the i.c. oscillator would have a smaller potential difference so that metal ions would not electrodeposit. Internal electrolytic determination of silver. In order to determine silver selectively by internal electrodeposition, it is necessary to use a counter metal electrode having a slightly larger oxidation potential than silver [5]. Determination of silver using the platinum electrode of the crystal was initially examined with similar equipment to that in the earlier paper [ 51. However, micromolar concentrations of silver did not electrodeposit on the crystal because the potential difference between the platinum electrode and the copper counter electrode was too small to overcome the junction TABLE 1 Frequency
shifts of the crystal when changing from water to potassium chloride solutions
KC1 cont. Specific (X 10’ M) conductance (X 1O-5 Q-l cm-‘) 1 1.5 2 3 5
1.48 2.23 2.96 4.44 7.39
Frequency shift (Hz) -13 -41 -70 -153 -352
KC1 cont. (X 10e4 M)
Specific conductance (X lo+ a-’ cm-l)
8 10 20 50
11.8 14.7 29.2 71.8
Frequency shift (Hz) -661 -850 -1540 Unstable
TABLE 2 Frequency shifts when changing from water to sucrose solutions sucrose cont. (% w/v)
Density difference (mg cmb3)
0.5 1 2
1.99 3.95 8.59
Frequency shift (Hz) -75 -161 -221
Sucrose cont. (% WV)
Density difference (mg cm-‘)
4 6 8
16.60 24.38 32.28
Frequency shift (Hz) -356 -423 -536
TABLE 3 Tolerance limits for ions in the determination of 1
X
10“ M silver
Tolerance limit (Ion :silver mole ratio)
Ions
10
Fe(III), Co(II), Ni(II), Pb(II), Cd(II), Zn( II) SO:-, NO;, CN-, CO:Hg(II), Cu(II) s,o;SCN‘, Cl-, Br-, I-, S*-
1 0.1
potential of the agar bridge. When the copper counter rod was connected to the platinum electrode in the same solution, silver deposited on both surfaces of the platinum electrode and the copper rod and changed the frequency of the crystal. However, silver could not be determined because the frequency change had a plateau resulting from covering the copper rod with silver. The frequency changes were small or poorly reproducible when lead, aluminium or tin was used as the counter rod. Zinc rod was finally chosen for this purpose. Dependence of the frequency change on pH and flow rate. Frequency changes depending on the electrodeposition of silver were constant in the range of pH 3.4-5.4 (tartrate buffers); above this pH, silver formed its hydroxide. Frequency changes were exactly proportional to the flow rate for 2.1-15.2 ml min-’ and for electrodeposition times in the range 0.5-10 min examined. Electrodeposition for 10 min was used for the determination of micromolar silver solutions, and 5 min for larger concentrations. Calibration and reproducibility. The calibration graph of frequency change (AF) against silver concentration for lo-min electrolysis was linear for 1 X 10*-l X 10e5 M and is described by the equation [Ag] = (AF/23.8) X 10” M, where AF is measured in Hz. The standard deviation was 11.9 Hz (5.0%) for 5 determinations of 1 X lo-’ M silver. Effect of other ions. The effect of various ions on the determination of 1 X lo-’ M silver was investigated; changes of frequency of more than *8% were considered to result from interferences. The tolerance limits found are shown in Table 3. Interferences of mercury(I1) and copper(I1) arose from electrodeposition on the electrode, thus giving positive errors. Interferences of anions, except for thiosulfate, which formed a soluble silver complex, arose from the formation of precipitates of silver, giving negative errors. These anions preventing electrodeposition of silver would be eliminated by the method described previously [ 61. REFERENCES 1 2 3 4 5 6
T. Nomura and M. Okuhara, Anal. Chim. Acta, 142 (1982) 281. T. Nomura and M. Maruyama, Anal. Chim. Acta, 147 (1983) 365. S. Nakajima, Transistor Gijutsu, 19 (1982) 348. See, e.g., A. J. Bard, Anal. Chem., 36 (1964) 70R. Y. Shiobara, Bunseki Kagaku, 10 (1961) 1290. T. Nomura and M. Iijima, Anal. Chim. Acta, 131 (1981) 97.