Electrochemical Generation of Silver(1) through Silver Sulfide Membranes Application to Coulometric Titration of Chloride in Strong Oxidizing Media K. S. Fletcher 111 and R. F. Mannion Research Center, The Foxboro Company, Foxboro, Mass. 02035
THECOULOMETRIC technique utilizing passage of selected ions through permselective membranes under the influence of constant current was first reported by Hanselman and Rogers ( I ) . These workers employed synthetic ion exchange membranes and demonstrated feasibility for generation of halide, hydroxide, EDTA, hydrogen, and calcium ions. Grutsch applied this technique to generation of vanadyl, lithium, thorium, phosphate, and several other ions (2). In these studies, coulometric inefficiencies of 1 to 2 were noted and were attributed to less than perfect permselectivity of the membranes. Despite this disadvantage, the technique is of particular value when no electrode material is available for ion generation or when the titrate solution contains components which would interfere with classical coulometric procedures. In a recent report, Durst and Ross used a permselective membrane of europium-doped lanthanum fluoride for the first reported electrochemical generation of fluoride, and attained coulometric efficiency of 9 9 . 2 z (3). The present report utilizes membranes of Ag2S for the selective generation of silver ion. Though Ag(1) can, of course, be easily generated from a silver anode (4, this procedure can give difficulty when the titrate solution contains components which can oxidize the anode independent of the flow of current. Ag2S was chosen for this study because of its great inertness to chemical oxidation. The silver halides and silver chalcogenides have received considerable study in recent years because of their high electrical conductivity (5,6). This conductivity has been attributed to the facile movement of silver ion in the solid material and this property has been utilized in various solid state devices (6-8). Solid silver sulfide, in its p form (below 177 “C) can exhibit mixed electronic and ionic conductivity. Since 100% current efficiency crtn result only if the transference number of the silver ion is unity, the electronic contribution must be suppressed. Wagner has shown that the transference number for either charge transport mechanism can be made unity through choice of terminal connection to the silver sulfide (5, 9). Thus, metallic contact to the material will favor electronic
z
(1) R. B. Hanselman and L. B. Rogers, ANAL.CHEM.,32, 1240 (1960). (2) J. F. Grutsch. U.S. Patent 2,954,336(1960). (3) R.A. Durst and J. W. Ross, Jr., ANAL.CHEM.,40, 1343 (1968). (4) J. J. Lingane, ibid., 26, 622 (1954). (5) C. Wagner, Proc. 7th Meeting Intern. Cornm. Electrochem. Thermodynam. Kinetics, London, 1957, pp 361-77. (6) J. H. Kennedy, F. Chen, and A. Clifton, J. Electrochem. SOC., 115, 918 (1968). (7) J. W. Ross, Jr., “Solid-State and Liquid Ion-Exchange Membrane Electrodes,” National Bureau of Standards Symposium on Ion-Selective Electrodes, Washington, D.C., Jan. 3&31,1969, Proceedings, in Press. (8) Electronics, 40,No.7, 186 (1967). (9) M. H. Hebb, J. Chem. Phys., 20, 185 (1952).
conductivity and electrolytic contact will favor ionic conductivity. In the work reported here, a saturated solution of silver nitrate was placed in the generator compartment of the cell in contact with a silver wire anode and the Ag2S membrane. With this arrangement, passage of charge favors transfer of silver ion from this large reservoir through the membrane. This procedure allows coulometric generation of silver ions into solutions containing strong oxidants where use of a silver wire anode is not possible. EXPERIMENTAL
Reagents and Materials. A g S was used as received from K & K Laboratories (Plainview, N. Y.). An emission spectrographic analysis of this material showed no impurities above the 0.01% level; only Al, Fe, Pb, and Si in the 0.001 to 0.01% range; and Mg, Na, Ni, Ca, and Mn at less than 0.001 %. Standard KC1 solutions were prepared determinately from the dried salt and the titers were verified, within 0.2%, by coulometric generation of Ag+ from a silver rod (assayed at following the procedure of Marinenko and Taylor 99.96
z),
(10).
Preparation of Ag,S Pellets. Ag2S pellets (0.960-cm diameter by about 0.25-cm thickness) were pressed in a hardened steel die using 150,000 psi. Typical densities obtained ranged from 7.10 to 7.15 g/cm3, in good agreement with the theoretical density, 7.24 g/cm3(11). Apparatus. The cell was constructed of Lucite and consisted of three compartments. The anode compartment contained a solid silver wire dipping into 100 ml of saturated AgN03. The working compartment had a total capacity of 200 ml and contained an Orion Model 94-16 sulfide ion electrode and a bridge arm containing 1.OMKNOI from a remote Ag, AgCl, 4.OM KCl reference electrode (12). The cathode compartment contained a platinum wire electrode dipping into 100 ml of 1.OM KNO,. AgzS membranes were mounted in Lucite holders using Pyseal Wax (Fisher Scientific Company) and the holder was sealed into a lateral hole drilled near the bottom of the cell between the anode and working compartments using two O-ring seals. An agar bridge containing K N 0 3 connected the cathode and working compartments. The anode and working compartments of this cell were both stirred. Current-potential curves were obtained using a Tacussel Type PRT-40-1X potentiostat and a Tacussel Type GSTP2-A (10) G. Marinenko and J. K. Taylor, J. Res. Nut. Bur. Stand., 67A, 31 (1962). (11) “ASTM-Hanawalt Card-File of X-ray Diffraction Data,” American Society for Testing and Materials, Philadelphia, Pa. (12) T. S. Light in “Analysis Instrumentation,” Vol. 5. L. Fowler. R. G. Harmon, and D. K. Roe, Eds., Plenum Press,.New York; 1968, pp 73-87.
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1.0
2.0
3.0
4.0
5.0
POTENTIAL, VOLTS Figure 1. Effect of concentration of Ag+ on current-potential behavior of AgzS membranes Curve 1. 1.00MAgN03 Curve 2. 0.100MAgN0, and 0.900iMKN03 Curve 3. 0.010MAgN03 and 0.990MKN03 Curve 4. 0.001MAgN03 and 0.999MKNOa Each curve obtained using a cathodic sweep rate of 10 mV/sec
Table I. Current Efficiency for Generation of Ag(1) Number of determinations 1 3 5 Current, mA 0.5014 2.006 9.663 Time, sec 6300 5000 1000.0 Ne, mmolea 0.03274 0.1040 0.1001 Vol. KCl to ep, ml* 3.03 9.58 9.24 f0.02 zto.01 N,, mmoleo 0.0329 0.1039 0.1002 f0.0002 zto.OOO1 C.E., Zd 100.4 99.9 100.1 f0.2 &O. 1 a N e , the number of mmoles of Ag+ generated using Faraday’s law with i and t values shown, n = 1 equiv./mole, and F = 96,487 coul/equiv. b Volume shown is the average and standard deviation of the volume of 0.01085N KC1 required to reach the end point for the number of determinations shown. N,, the number of mmoles of Ag+ generated as determined by titration with standard KCI. C x d Current efficiencies were calculated using C.E., = N N*
100% *
the curves rise linearly from the origin. Since charge flow per unit are2 is given by (14),
J ramp generator (Tacussel Electronique, Solea, France). The constant current source and the procedure used for its standardization have been described (13). The conductivities of several AgzS membranes were measured with a 1000-Hz Foxboro Resistance Dynalog Recorder. Conductivity measurements were accomplished by mounting the membranes in the cell and placing 1.OM AgNO, in contact with each face. Contact to each solution was made with silver electrodes and cell resistances obtained ranged from 280 to 300 ohms, corresponding to specific conductances in the range of 0.0013 t o 0.0014 (ohm,crn)-’.
Current-Potential Behavior of AgzS Membranes. The current-potential behavior of AgzS membranes was studied using two compartments of the cell described above. The counter, test, and reference electrodes were all 0.3-cm diameter silver rods. Sufficient lengths of rod were used for the test and counter electrodes so that the areas in contact with the solution were about ten times the area of the Ag2S membrane. The counter and reference electrodes were placed in one compartment in which the concentration of AgN03 was varied, and the test electrode was placed in the other compartment with the silver nitrate concentration fixed at 1.OM. The Ag2S membrane used for this work had a diameter of 0.960 cm and a thickness of 0.286 cm and was mounted between these compartments. Both solutions were stirred. With this arrangement, the initial potential difference between the reference and test electrodes was zero and thus no current flowed before initiation of the voltage scan. Current-potential curves were obtained by sweeping the test electrode potential cathodically at a rate of 10 mV per second. A typical set of current-potential curves is shown in Figure 1 for solutions 1.00M in AgN03, 0.100M in AgN03 and 0.900M in KNO,, 0.010M in A g N 0 3 and 0.990M in KNOI, and 0.001M in AgN03 and 0.999M in KNO,. In each case (13) K. S. Fletcher 111, ANAL.CHEM., 41, 377 (1969). 286
nZPE
(1)
where J = current density, A/cmZ n = number of charge carriers per unit volume, Car-
rier/cm3
z = charge per
carrier in coulombs per carrier mobility of the charge carrier under the influence of the potential field in cm2/V sec E = potential gradient in V/cm P =
-
then the specific conductance, K, which is the product of the concentration and the mobility of the charge carriers; i.e.,
K RESULTS AND DISCUSSION
=
=
nZ1
(2)
can be calculated from the linear portions of the current-potential curves. As the potential gradient is increased, the current increases according to Equation 1 as long as the number of charge carriers (Ag+ ions) present is sufficient. In Figure 1, inflections occur in the curves obtained using the 0.01 and 0.001M AgN03 solutions. This is attributed t o concentration polarization of Agf at the Ag?S surface. Further increase in potential results in a second inflection ai about -3 V. This is accompanied by decomposition of the Ag,S membrane. Thus, to avoid coulometric inefficiency, the Ag,S membrane must be operated in the linear region of its currentpotential curve. Thus, the concentration of silver ion in the generator Compartment must be kept sufficiently large and the current must be kept sufficiently small to avoid polarization and decomposition of the membrane. On the basis of the current-potential curves shown in Figure 1, the specific conductance calculated from the linear segment of the curves is about 0.0016 (ohm.cm)-l, which compares favorably with the specific conductance values determined earlier. Further, the maximum current density which should be used is about 21 mA/cm2. Coulometric Efficiency for Generation of Ag(1). The current efficiency for generation of Ag(1) was determined using several (14) N. F. Mott and R. W. Gurney, “Electron Processes in Ionic Crystals,” Clarendon Press, Oxford, 1940.
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constant currents for timed intervals. In each case, the working compartment of the cell contained 40 ml of 1.OM K N 0 3 and the generator compartment contained 40 ml of saturated AgN0,. After each electrolysis, the generated Ag(1) was determined by potentiometric titration with standard KCl; the midpoint of the potential break was taken as the equivalence point. The current efficiencies were calculated in the usual manner. The data are shown in Table I. Since the current efficiencies obtained are ideal, AgzS is perfectly permselective to Ag+. The two modes of current inefficiency predicted earlier on the basis of the current-potential curves were demonstrated. Use of 0.01M AgN03 in the generator compartment and a constant current of 9.663 mA for 1000 sec lead to a current efficiency of 97 %, and use of a constant current of 50 mA for 200 sec with saturated AgN03 in the generator compartment lead to a current efficiency of 89W. In both cases, decomposition of the Ag,S membrane was apparent. Use of larger currents would require membranes of larger area for ideal efficiency. Coulometric Titrations of Chloride. The use of AgzS as an ideally permselective membrane for coulometric titrations with electrogenerated Ag+ was tested using the titration of chloride in three media, 1.OM K N 0 3 , 1.OM HNO,, and 0.1M Fe3+ in 1.OM "0,. In each case, Agf was generated, prior to addition of the chloride sample, for a precalculated time sufficient to react with 9 5 z of the chloride sample to be added (10). This improved the response time of the indicator electrode since it avoided buildup of a coating of AgCl on the Ag,S membrane. Chloride samples in the range 3.5 to 0.35 mg (100 to 10 p moles) were added to 40 ml of the respective supporting electrolyte and the titration was continued with frequent interruption of the current for reading the silver/sulfide indicator electrode potential. The equivalence point was taken at the point of maximum potential change. The data and results are shown in Table 11. The data shown for each determination are the average result and standard deviation of several trials. Precision and accuracy for these determinations are excellent. Titrations of 0.035-mg samples (1.0 p mole) of chloride in 40 ml of each of the three solutions were accomplished using a pretitration technique (15). For this work, a large, known quantity of chloride (3.5 mg) in 40 ml of supporting electrolyte was (15) J. J. Lingane, "Electroanalytical Chemistry," 2nd Ed., Interscience, New York, 1958, pp 532-535.
Table 11. Coulometric Titrations of C1- in Various Media Titrations of C1- in 40 ml of 1.OM KN03 Number of determinations 4 2 5 5 pmoles of C1- taken 108.5 54.3 10.7 1.08 Current, mA 9.657 9.657 9.657 0.5014 Time, sec" 1086.6 542.1 106.7 208.4 f1.0 11.1 f0.6 f2.6 pmoles of C1- found 108.7 54.3 10.7 1.08 fO.1 fO.l fO.l f.13 Relative error +0.2% =kO.O% 1 0 . 0 % f O . O % Titrations of C1- in 40 ml of 1.OM HN03 Number of determinations 5 6 4 pmoles of C1- taken 54.3 10.8 1.07 Current, mA 9.666 9.666 0.5005 Time, sec" 543.0 108.2 204.3 kl.8 f0.7 f3.6 pmoles of C1- found 54.4 10.8 1.06 k0.2 kO.1 lt.02 Relative error +0.2% f O . O % -1.0% Titrations of C1- in 40 ml of 1.OM "03, 0.1M Fe(NO& Number of determinations 4 4 5 4 pmoles of C1- taken 108.5 54.3 10.8 1.07 Current, mA 9.657 9.657 9.657 0.5004 Time. seca 1083.0 541.1 108.0 204.2 f0.6 fl.3 ztO.3 f0.9 pmoles of C1- found 108.4 54.2 10.8 1.06 10.6 k0.1 f0 & .01 Relative error -0.1% -0.2% f O . 0 Z -1.0% o Times shown are average and standard deviations of times required to reach the end point for the number of determinations shown.
titrated to the equivalence point potential. The chloride sample (0.035 mg) was then added and the time required to return the indicator electrode potential back to the equivalence point was noted. The entire potential change noted for this microtitration was about 10 mV, corresponding to about 0.1 mV per 1 % titrated. The response stability of the indicating and reference electrode used for this work was *O.l mV and therefore this titration procedure is expected to be precise and accurate to about & 1 %. This expectation was borne out by the precision and accuracy data shown for these titrations in Table 11. RECEIVED for review September 18, 1969. Accepted November 17, 1969.
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