(2) J. L. Swartz and T. S. Light, Tappi, 53, (1). 90 (1970). (3) M. S. Frant and J. W. Ross, Jr., Tappi, 53, (9), 1753 (1970). (4) J. Papp, Cellul. Chem. Techno/..5 , 147 (1971). (5) J. Papp, Sven. Papperstidn.. 74, 310 (1971). (6)J. Papp, Cellul. Chem. Techno/.,7, 733 (1973). (7) B. L. Lenz and P. J. Magnell, Tappi, 56, (12). 195 (1973). (8)J. Revenda, Collect. Czech. Chem. Commun., 6, 543 (1934). (9) I. M. Kolthoffand C. S. Miller, J. Am. Chem. Soc., 63, 1405 (1941). (10) L. Julien and M. L. Bernard, Rev. Chim. Miner., 5, 521 (1968).
(11) P. Ahlgren, Sven. Papperstidn., 70, 730 (1967). (12) I. M. Kolthoff and J . J. Lingane in “Polarography”, Interscience, New York, 1952, Vol. 2, p 785. (13) L. Julien and M. L. Bernard, Nectrochim. Acta, 13, 149 (1968) (and ref-
erences therein).
RECEIVEDfor review December 27, 1974. Accepted March 28, 1975.
Cathodic Stripping Coulometry of Lead H. A. Laitinen’ and Noel H. Watkins2 Roger Adams Laboratory, Department of Chemistry, University of Illinois, Urbana, //I.
The anodic deposition of PbOp on conductive tin oxide electrodes, followed by cathodic stripping using linear potential sweep voltammetry has been studied to determine the importance of the nucleation, two-dimensional growth, and three-dimensional growth of the deposit in relation to mass transport control in defining conditions for quantitative deposition. Stripping peaks associated with crystalline PbOp and with fractional monolayers are clearly distinguishable. The role of specific and irreversible adsorption of lead(l1) and of iron(ll1) has been studied with the aid of radiochemical tracers. Phosphate has been found to inhibit crystal growth of PbOn, and at high acid concentrations, to solubilize lead( IV) species detectable at a rotating ring-disk electrode. Conditions have been defined for the quantitative deposition of lead at concentrations down to 2 X lO-*M, for avoiding “memory” effects due to adsorption of Pb( ll), for minimizing the interference phosphate, and for extending the sensitivity limit to lower concentrations. Iron should be removed because of its interference with nucleation and growth of PbO? and because of its adverse effect on the oxygen overpotential of tin oxide electrodes.
Although trace analysis for lead through cathodic stripping voltammetry of electrodeposited PbOs has been suggested as early as 1953 ( I ) and applied by several investigators (2-8), several shortcomings of past procedures can be identified. First, measurement of stripping peak current assumes a proportionality between this quantity and the amount of deposited PbOz which may be invalid because the peak shape can be highly variable, especially for small quantities of deposit. By measuring coulometric charge rather than peak current, this problem can be overcome, but due attention must be paid to stripping the last monolayer of deposited PbOz, a process that extends over a relatively wide range of potentials (8). Second, the fraction of lead in solution deposited as PbO2 is subject to large variations unless the deposition conditions are rigidly controlled. Not only are the usual variations in mass transport conditions during deposition of concern, but the rates of nucleation and growth of PbO2 on platinum are potential-dependent and subject to different growth laws (9-12). As the concentration of lead is del Present address, Department of Chemistry, University of Florida, Gainesville, Fla 32611 Present address, Monsanto Company, Pensacola, Fla. 32501.
1352
ANALYTICAL CHEMISTRY, VOL. 47, NO. 8, JULY 1975
creased, the nucleation rate becomes more important in determining the initial deposition rate, so that an induction period is observed for deposition. This problem can, in principle, be overcome by allowing sufficient time for quantitative deposition. Yoshimori and coworkers (2) used deposition times of four hours to determine lead at concentrations down to 2.5 X 10-6M. Third, the use of platinum or gold electrodes is limited, especially for the coulometric stripping of extremely small quantities of PbOz, by the surface oxidation and reduction processes occurring a t “noble” metal electrodes. Our work on tin oxide electrodes has indicated that, with properly prepared antimony-doped tin oxide layers on glass, there is virtually no surface redox chemistry in a potential range of interest to the deposition and stripping of PbOn. Moreover, the low double layer capacitance and high oxygen overpotential of the tin oxide electrode make it advantageous over other solid electrodes (13, 14). In a preliminary note ( I s ) ,we indicated an unusuallyshaped cathodic stripping peak for PbOP electrodeposited on tin oxide from “ 0 3 solutions. A sharp cathodic spike near the reversible Pb02-Pb2+ potential, followed by a rounded hump spanning some 600 mV of potential range was typically observed. The sharp spike was attributed t o crystalline PbO2, whereas the rounded hump was ascribed to fractional monolayers of PbOz attached to the tin oxide substrate. Similar curves have been recently reported by Kinard and Propst (8) in H2S04, and used for the determination of lead in concentrations down to 5 X 10-9M. The present study was designed to investigate the feasibility and limitations of trace analytical methods based on anodic deposition of PbOz on tin oxide, followed by cathodic stripping coulometry. Mechanistic studies of nucleation, growth, and stripping of the Pb02 films will be reported elsewhere.
EXPERIMENTAL Linear potential sweep experiments were performed with a conventional three-electrode polarograph in conjunction with a linear sweep generator (16) and a Hewlett-Packard Moseley X-Y recorder, Model 7001 AM. Double pulse potentiostatic experiments were performed with a Wenking fast rise-time potentiostat, Model 61 RS, programmed with a double pulse generator constructed from Tektronix pulse generators, Models 161 and 163 (171, measuring t h e resulting current-time curves with a Tektronix Model 503 oscilloscope equipped with a Tektronix Model C-12 camera. Experiments involving potentiostatic electrodeposition followed by linear voltammetric stripping required special switching to prevent loss of potential control during the connection and disconnection of t h e potentiostat ( 1 7 ) .
C
0
I
1
I 02
06
04 .E
VI
PbO,/Pb**
I
08
(VI
Figure 3. Voltammetric stripping of lead dioxide deposited at constant overpotential Figure 1. Construction details for the rotating cylindrical electrode and coulometric cell
0.25M Pb(NO& in 1M HN03, 7 = +0.902 V, v = 50 ( A ) 0.5, ( E ) 1, ( C )2.5, (0) 5 , (07 . 5 , ( A 10
electrode compartment, (C) Counter electrode compartment, (0) Porous Vycor junctions. (6Thermometer adapter, 10118, (6Mercury pool contact
mV/sec.Time (msec):
( A ) Cylindrical electrode, ( E ) Reference
'O
t
1
Figure 4. Variation of secondary peak charge density with deposition time at constant overpotential 0.25M Pb(NO& in 1 M " 0 3 ,
il
= +0.902 V
-C
Figure 2. Construction details for rotating disk electrode and coulometric cell ( A ) Rotating disk
electrode, ( E ) Reference electrode compartment. ( C )Coun-
ter electrode compartment
The four-electrode potentiostat used with the rotating ring-disk electrode was that of Miller (18),modified by the addition of a difference amplifier to permit the simultaneous measurement of ring and disk currents by means of x-y recorders as a function of the disk potential. An electronic integrator to permit integrating the current between two preselected potentials was constructed from operational amplifiers. Details are given in the thesis of Watkins (17).
Tin oxide electrodes were prepared by spraying a solution 3M in SnC14, 0.067513.1 in SbC13 in 1.5M HCI, prepared from anhydrous SnC14 and SbC13, onto a Pyrex glass substrate heated t o 550' (19), taking care to avoid excessive cooling of the surface during bursts of spraying. As many as twenty or more sprayings were required, depending upon the geometry of the electrode, t o build up the desired film thickness of 0.75 to l micrometer, as monitored by observing the sequence of interference colors during the building of the film, or by measuring the surface resistance and calculating the film thickness from the known resistivity (20). A cylindrical cell and electrode of large areas (15 cm2) and small solution volume ( 2 . 5 ml), which permitted rotation of the electrode at 600 rpm, is pictured in Figure 1. A similar cell, designed to use a rotating tin oxide disk electrode made according to Harrington and coworkers (21) of 2 cm2 area with a small solution volume (3 ml) is shown in Figure 2.
Details of radioactive tracer experiments using 212Pband jgFe are given in the thesis of Watkins (17). In both cases, activity was counted directly from the electrode surface.
RESULTS AND DISCUSSION Preliminary Observations. Experiments using a rotating ring-disk electrode of tin oxide (21) were designed to detect any soluble oxidizing intermediates, such as Pb4+, P b 0 2 + , or Pb(III), formed during the anodic oxidation of Pb2+ to PbOz. No such intermediates could be detected. This finding is consistent with the mechanistic studies of Hampson, Jones, and Phillips (22-25), and Fleischmann and coworkers (9-12, 26), in which adsorbed but not soluble Pb(II1) and Pb(1V) species were postulated as intermediates. Mechanistic studies, to be published elsewhere, indicated t h a t the nucleation of PbOs on tin oxide, as on Pt ( 9 ) , occurs randomly on a limited number of sites. At high overpotential, the growth is non-preferred, so that the initial deposit consists of randomly sized hemispheres. At low overpotential, a preferred type of growth leads to well-developed crystals of PbOs. Stripping patterns of PbOz consist of a sharp spike of current near the reversible Pb02/Pb2+ potential, followed .by a rounded hump (Figure 3 ) . From the conformance of the area under the hump with the expected nucleation law, we concluded (27) that the rounded hump corresponds to t h e reduction of fractional monolayers of PbO2 directly a t tached t o the substrate. A similar conclusion was reached by Kinard and Propst (8) who reported t h a t under proANALYTICALCHEMISTRY, VOL. 47, NO. 8, JULY 1975
* 1353
------O
I
zr
1
L
Flgure 5. Lead dioxide deposition rate as function of pH
2
O 1
3
1.2 X 10-6M Pb(NO&. Nitrate concentration maintained at 1 M with NaN03.
V = 5 ml. 9 = 0.34 V.Rotating cylindrical electrode with coulometric cell. pl I
(Ai
Eel
LS.
f k , , Iiq2SOA
1.34 1.22 2 1.10 3 0.98 Theoretical mass transport control 0 1
(6') (C)
(D) (E)
'0
5
IO
re,
15 imrn
i
20
HN03. V = 5 ml. Rotating cylindrical elec-
5
6
7
8
with
solution number for a previously used
10-6M Pb(NO& in 10-2M "03. V = 2.5 ml. Eel = +1.6 V vs. Ag/AgCI. lO-*MHCI. tel = 10 min. Rotating cylindrical electrode with coulometric cell
1
tential 1.2 X 10-6M Pb(N03)z in trode with coulometric cell
Figure 7. Variation of Q and air dried electrode
25
Figure 6. Lead dioxide deposition rate as function of electrolysis po-
4
Solution Number
I 2
1 3 Electroi-
1
4 Nunber
1
1
5
6
Figure 8. Variation of Q with number of electrolyses on a single lead
solution 10-6M Pb(NO& in 10-2M"03. V = 2.5 ml. = +1.6 V vs. Ag/AgCI, 10-2MHCI. tel = 10 min. Rotating cylindrical electrode with coulometric cell
E C ]\ s . Hi, ' I h * S C d
('1)
(D) (E)
(F) for
7lQ
(17
0.24 1.1 0.34 1.2 0.44 1.3 0.54 1.4 0.64 The o r et ic a 1 mas s trans port con t r 0 1 1.0
(B) (C)
a
(VI
O v e r p o t e n t i a l calculated f r o m a n i n i t i a l equilibrium p o t e n t i a l
Pb02/Pb2+, 10-2MH'.
longed (16 min) controlled high overpotential deposition conditions, a limiting coverage was reached with increasing Pb2+ concentration. This coverage corresponded to 0.1 wg Pb/cm2, or 100 HC/cm2, or 33% coverage of available surface sites. Under the conditions depicted in Figure 3, namely, short pulses a t controlled and high overpotential and high Pb2+ concentration, a limiting coverage of only 8.3 wC/cm2 corresponding to 3% of monolayer coverage was observed (Figure 4), owing to the high growth rate under these conditions. Deposition Efficiency. At low Pb2+ concentrations, it can be anticipated t h a t nucleation rate, as well as mass transport rate, may be important in determining the rate of deposition of PbO2. T o determine the importance of pH a t constant overpotential, the electrolysis potential was varied with pH in accordance with the change of equilibrium potential with pH. The results for 1.2 x 10-6M Pb2+, shown 1354
ANALYTICAL CHEMISTRY, VOL. 47, NO. 8, JULY 1975
in Figure 5 , indicate a marked increase in nucleation rate with increasing pH, a t an overpotential of 0.34 V. A t higher overpotentials, a t constant nitric acid concentrations of 10-2M, the nucleation rate increases rapidly, so that the induction period has disappeared a t an overpotential of 0.44 V (Figure 6). To achieve full mass transport control a t 10-6M Pb2+,the pH should be 2 or higher and the overpotential 640 mV or higher. A higher pH limit or higher overpotential may be required for lower concentrations of lead. Chemisorption of Lead(I1) on Tin Oxide. Definite electrochemical evidence was noted for retention of lead by the tin oxide surface and for re-dissolution of the lead during later electrolyses. When an electrode was allowed to become air-dried and to stand overnight after use, the first five or six electrolyses the following day using a succession of fresh solution samples gave high results, as shown in Figure 7 , because lead is released from the surface. This history effect could be decreased considerably by soaking the electrode in 1M H N 0 3 prior to use each day. A second type of evidence for retention of lead was observed when a single solution was electrolyzed repeatedly (Figure 8). Progressively less lead was recovered after each of the first five or six cycles, after which the recovery leveled off to a constant but low value. The effect was most pronounced when using a high ratio of tin oxide electrode area to solution volume.
1
I
I
I
t r
,/
16
1
1
i2
14
E
I%
1 10
,
08
AqlAgCI. I V r M HCI
"
1
1
06
04
02
i
i
I #>
Figure 9. Voltammetric stripping with a new and used electrode
3
10@M Pb(NO& in 10-'M HN03. V = 2.5 ml Eel = +1.6 V vs. AgIAgCI, HCI. tel = 10 min. v = 2 V/min. Rotating cylindrical electrode with coulometric cell. ( A ) Freshly prepared electrode. ( E ) Previously used electrode
lO-'M
I
1
lo-'
lo-'
I
10.'
10-4
I
1
10-1
[Pb'.]
T h e history effects were accompanied by changes in electrode behavior. not ably a decreased oxygen overpotential and increased double layer charging and cathodic residual currents. These changes are noticeable in Figure 9, comparing voltammetric curves with a new and used electrode. Radiochemical tracer experiments were carried out with 212Pb,using 2 ( i i ' li:-',II?b"+ in lO-?.C.l " 0 3 . Direct &ray counting from 212Biin secular equilibrium with z12Pb was used to monitor the distribution of lead between the solution and electrcde. When a given electrode was repeatedly anodized and cathodically stripped in an active lead solution, about 10% of the lead was retained after each sequence. After six such cycles, both electrnchemical and radiochemical data showed that 43% of the lead originally in solution was accumulated on the electrode and could not be stripped electrochemically. Moreover, three subsequent deposition and stripping cycles using inactive Pb2+ did not decrease the lead activity on the electrode, leading to the conclusion t h a t the accumulated lead was not exchangeable with solution lead under these normal operating conditions. However, after the electrode had been allowed to stand air-dried, overnight, 87% of the accumulated active lead was solubilized by simply soaking the electrode surface in 10-2M " 0 2 . This solubilized lead is the source of the excess lead observed in Figure 7. I t is believed t h a t a slow dehydration upon standing in an air-dried condition accounts for the release of lead. but the chemical form of the accumulated lead is still unknown. T h e amount of accumulated lead varied with the amount of time an electrode had been in use, presumably because an older electrode surface is more extensively hydrated and has more reaction sites for chemisorption of lead(I1). Adsorption from solution in the tracer experiments contributed only 3% loss of lead(I1) from solution, as compared with 43% after repeated electrolysis and stripping. An adsorption isotherm for lead(1J) in 10-2M H N 0 3 was determined using presoaked tin oxide surfaces. The isotherm is shown in Figure 10. Rate studies (Figure 11) indicated t h a t the accumulation of lead in successive deposition-stripping cycles could not be explained by assuming t h a t the local concentration of lead(I1) during the cathodic cycle becomes high enough to become adsorbed in accordance with the adsorption isotherm. Instead, it appears t h a t the initial PbO. monolayer is reduced, first to an adsorbed Pb(II) intermediate, and then to an adsorbed lead-
Figure IO. Adsorption isotherm for lead(1l) on tin oxide in Iap3M "03
Surface pre-soaked 8 hours in 10-'M Pb(NO& solution
I
I
"03.
1
then soaked 3 hours in active
I
-V I
Figure 11. Adsorption of lead(l1) on tin oxide as function of time 10-5M Pb(N03)~in 10-2M "OB.
Electrode presoaked 8 hours in 10-ZM
"03
(11) species. A quantitative consideration of the amount of lead(1V) in the directly bonded form, as measured by the area of the secondary stripping peak (SO%), in relation to the amount of lead retained after stripping (lo%),leads to the conclusion t h a t about one fifth of the initial monolayer leads to a non-exchangeable species. T h a t this species can be removed by allowing the surface to dehydrate is import a n t in preventing cross-contamination in trace analysis. Minimizing the electrode area relative to solution volume is clearly also favorable. Chemisorption of Iron(II1) on Tin Oxide. It was found t h a t iron(II1) interferes with quantitative deposition of lead by decreasing the nucleation and growth rates of PbOa and by causing a decrease in oxygen overpotential. T h e decreased oxygen overpotential persisted even after the electrode and all had been thoroughly rinsed with IOp2MHN03 in an effort to remove iron(III). Radiochemical tracer techniques, monitoring 59Fe y r a diation directly from the surface, were used to study the ANALYTICALCHEMISTRY, VOL. 47, NO. 8, JULY 1975
1355
Figure 12. Adsorption of iron(lll) on tin oxide a s a function of time (A) Newly prepared surface. (6) Surface pre10-6M Fe(lll) in lO-*M "03. soaked in 10-2MHN03 for 3 days
I
I
I
I
, t15
+I0
,
,
,
,
1
,
,
,
t05 E v s A g / A g U , IO-* ,y H a (v)
Figure 14. Effect of phosphate on lead dioxide deposition and voltammetric stripping I
10-6MPb(N03)2 in 10-2MHN03. V = 2.5 ml. v = 2 V/min. Eel= +1.6 V vs. Ag/AgCI, 10-'M HCI. Rotating cylindrical electrode with coulometric cell. Curve A. No H3P04. Curve 6. 10-4MH3P04. Curve C. 10-3MH3P04
Table I. Effect of Nitric Acid Concentration on Adsorption of Fe(II1) on SnOz Surfacea Concentration
Fnq,
(w
1
7
4
3
10-1 10-2 10" a Fe(II1) concentration, 10 4M
A dsorprton
c o x i r a q r , ( m o l 'm")
2.5 x lo-''
4 . 2 x lo-'' 3.0 x lo-" 8.0 x lo-"
- L o g [Fe:q
Figure 13. Adsorption of iron(ll1) on tin oxide as function of iron(ll1) concentration 10-*M HN03. Surface pre-soaked for 3 days in 10-2M HN03,then soaked in active solution for 24 hours
adsorption of iron(II1). As shown in Figure 12, a newly prepared surface adsorbed iron(II1) less rapidly and less completely than one t h a t had been hydrated by pre-soaking in 10-2M H N 0 3 for three days. The adsorption as a function of concentration is shown in Figure 13. The coverage levels off a t monolayer coverage a t concentrations adsorption higher than 10-3M. I t must be emphasized that Figure 13 is not a true isotherm because part of the adsorbed iron(111) is not in equilibrium with the solution. At a constant iron(II1) concentration of 10-4M, the amount of adsorption fell off with increasing H N 0 3 concentration, especially in the range of to 10-lM H N 0 3 (Table I). Calculation of the concentrations of various hydrolyzed species of iron(III), namely FeOH2+, Fe (OH)2+, and Fez (OH)24+,from equilibrium constant data leads to the conclusion t h a t the concentrations of these species decrease rapidly with decreasing pH, and that the decrease begins to occur a t pH values considerably higher than 2. The simplest interpretation is that the aquated Fe3+ ion is the species undergoing adsorption, but that the adsorption process involves a release of hydrogen ions from the surface. Two possible surface reactions of this type may be visualized. The first is a reversible type of adsorption involving the loose hydration of the tin oxide surface by formation of hydrogen bonds from water to oxygen atoms a t adjacent Sn0-Sn sites, followed by the formation of a bridge-bonded 1356
ANALYTICAL CHEMISTRY, VOL. 47, NO. 8, JULY 1975
iron(II1)-OH species a t the surface, where hydrogen bonding constitutes the binding mechanism. The other mechanism involves, as a first step, a more deep-seated surface hydration to form surface Sn-OH groups as postulated by Kirkov (28) and, as a second step, the displacement of protons with the formation of Sn-0-Fe bonds. This type of adsorption would be slow and irreversible in character. Attempts to prevent the adsorption of iron(II1) by complexation proved ineffective. For example, the addition of excess oxalate to 10-4M Fe(II1) in 0.01M HN03 decreased the extent of adsorption only by a factor of four, although the activity of free Fe(II1) was decreased to 5.5 X 10-9M. Effect of Phosphate. Carr and Hampson (29),using an ac impedance method, showed that phosphate is specifically adsorbed on lead dioxide, in contrast to the behavior of nitrate (30) which is not adsorbed, and sulfate, bisulfate, and perchlorate (31, 32) which undergo non-specific adsorption a t potentials positive to the point of zero charge. Phosphate was found to inhibit the growth of reversibly reduced lead dioxide and to promote the growth of the irreversibly reduced deposit, as shown in Figure 14. I t appears that adsorption on the lead dioxide monolayer face preferentially inhibits crystal growth and thus enhances the twodimensional growth of nuclei. It was found ( 1 7 ) that this growth process could be approximately described by an equation derived by Fleischmann and Thirsk ( 3 3 ) and Armstrong and Harrison ( 3 4 ) for the potentiostatic current-time relationship expected for the two-dimensional growth of randomly placed nucleus centers as cylinders one unit cell high. A second effect of phosphate is to solubilize lead(1V) as a phosphate species. Such species have been reported in
Table 11. Powder X-ray Diffraction Pattern of Deposit Formed a t +0.6 V vs. P b 0 2 / P b 2 + from 0 . 2 3 4 P b ( N 0 3 ) a~n d 8OmM H 3 P 0 4 in 1M "03 d (A)
Io
1 1
10 5 1 1 1
100 100 2 1 5 1 2 2
8.66 4.72 2.829 2.402 2.178 2.130 2.061 2.014 1.962 1.832 1.745 1.723 1.238 0.862 0.835
Ass, qnm enf
P-Pb02
$-PbOo 13-Pb02
3-Pb02
Figure 15. Voltammetric stripping of multiple layers of lead dioxide from tin oxide 0.25M Pb(NO& and 80 mM H3P04 in 1M " 0 3 . sec. Quiescent solution
q = +0.60 V. v = 50 mV/
l~~Llml\st5
phosphoric acid and mixed sulfuric-phosphoric acids (3537). Compounds of lead(1V) and phosphate have been isolated (26, 35) with Pb:P ratios ranging from 1:l to 1:6. In the present research, a ring-disk electrode gave evidence for a soluble, reducible lead species formed during the oxi0.25M P b (N03)2, 0.08M dation of lead(l1) in 1M "03, &PO4. No such intermediate was observed during the reduction of the lead(1V) deposit from the disk, nor was it observed on the anodic cycle when the concentration of "03 was decreased to 0.01M. When larger amounts of deposit were accumulated for chemical analysis, the solution acquired a distinct yellow-brown tint which faded over a period of several days as a brown precipitate formed. T h e precipitate was not chemically characterized, but the X-ray powder diffraction pattern distinctly showed the presence of P-Pb02 Chemical analysis of the anodically formed deposit confirmed t h a t the lead is present as Pb(IV) and indicated a molar raiio of 3.16 for lead to phosphate. T h e X-ray powder pattern showed faint lines for b-Pb02, but predominantly the previous!y unreported pattern listed in Table XI. The cathodic stripping pattern of multiple layers of PbOJ deposited in the presence of phosphate showed multiple peaks as shown in Figure 15. Even in deposits with several molecular layers (up to 3000 fiC/cm2),no phosphate could be detected by ESCA, suggesting t h a t the abnormal activity of PhOz on tin oxide persists well beyond the first molecular layer, as has been observed for copper, silver, and bismuth on platinum (38-43). T h e cathodic stripping process may also he complicated by the formation of chemisorbed Pb(I1) as a reduction product, in analogy to the formation of adsorbed Hi(II1) during the oxidation of Bio from platinum, as suggested by Cadle and Bruckenstein ( 4 3 ) . Moreover, chemisorbed Pb(II1) is a likely precursor to chemisorbed Pb(I1) during the stripping. In summary, the effects of phosphate are threefold: t o inhibit crystal growth, to solubilize lead(1V) phosphate a t high HNO j concentrations, and to incorporate phosphate into the deposit a t low " 0 3 concentrations. In the classical gravimetric determination of lead as PbOa, phosphate is listed as an interference leading to low results ( 4 4 ) , which can be attributed to the predominance of the first two effects. Effect of Chloride. Chloride can cause two types of problems. A t relatively high concentrations (>O.lM) anodic evolution of chlorine limits the useful range of the tin
Cun e
l i l h L , iii
1-9 10 11 12 13 14
0.140 100 200 300 500 1000
0,bC
%.:,!2
10-66 88 172 266 600 200
oxide anode. A more subtle interference, even a t lower concentrations, is caused by "chemical stripping" by reducing agents, including chloride, T o accentuate the effect, a high concentration of hydrogen ion and chloride ion were used in a series of experiments in which a known amount (corresponding to 900 fiC) of PbOs was deposited from IM "03 onto the cylindrical electrode shown in Figure I . The electrode was potentiostated and voltammetrically st,ripped after the addition of chloride in amounts to adjust the concentration to a final value of lo-', lo-", and 10-2M. T h e amounts of PhOz determined were 890, 460, 74, and 2 fiC, respectively. T h e theoretical magnitude of the effect can be estimated from the equilibrium constant and mass transport rate, using the equilibrium constant for the reaction PbOe + 2C14H+ C12 ( a s ) -t Pb+' 2H20, K = 1.07 X 10'to calculate the surface Concentration of chloride, and using the known mass transport rate constant of the electrode to calculate flux of chloride a t the electrode surface, as was done by Bruckenstein and Bixler ( 4 5 ) for the chemical stripping of silver by oxidants in solution. Such an est,imate is useful only as a rough guide for practical work because, in the voltammetric experiment, the electrode potential is potentiostatically controlled and the surface concentratiou of P b 2 + is determined not only by the stripping reaction b u t by the voltammetric conditions. Under more practical conditions of acidity, which are needed for high deposition efficiency, the stripping effect of chloride is much less pronounced. For example, it is possible to deposit and strip lead quantitatively from 10-.6A4 Pb(I1) and 1OP2M"0.1 in the presence of a t least 10--3M chloride. Higher chloride ion concentrations have not been investigated. Based on the calculated rate of chemical stripping a t this p H , a concentration as high as 10-'M could be tolerated, but interference due t o chlorine evolution during the deposition step would intervene. Effect of Hydrogen a n d Water. During the anodic preelectrolysis step, hydrogen is produced a t the counter elec-
+
ANALYTICALCHEMISTRY,
+
VOL. 47, NO. 8, JULY 1975
1357
1
I2
IO E
08 YI
06
Ag/AaCl,
24
02
I
D ' M HW (vi
Figure 16. Voltammetric stripping curves
At higher acid concentrations, phosphate tends to give low results due to the formation of soluble lead(1V)-phosphate complexes. Iron should be removed prior to lead determination, primarily because it is irreversibly adsorbed and lowers the oxygen overpotential of the tin oxide electrode, and secondarily because it interferes with nucleation and growth of lead dioxide. As in the classical gravimetric determination of lead, manganese and bismuth interfere by deposition as higher oxides (46, 47). I t has been reported ( 4 8 ) that copper ions prevent the co-deposition of manganese divide and that complexation with fluoride prevents the coprecipitation of bismuth, antimony, and tin.
l o @ to lO-'M lead. 10-*M "03. V = 2.5 ml. E., = 4-1.6 V vs. AgIAgCI, 10-2MHCI. tel = 10 min. Rotating cylindrical electrode with coulometric cell
trode. To prevent chemical stripping of PbO2 by this hydrogen, it is necessary to isolate the counter electrode. Although PbOa is thermodynamically unstable with respect to oxidation of water, it was found t h a t the electrode for ten mincould be left on open circuit in 10-2M " 0 3 utes without measurable loss of the deposit. Sensitivity. Using the cylindrical electrode and cell (Figure l), the stripping curves shown in Figure 16 were observed for concentrations of 2, 4, 6, 8, and 10 X 10-7M Pb2+. A working curve of coulombic charge, Q,vs. concentration had a slope of 483 microcoulombs per micromole as compared with a theoretical slope of 488. An intercept of about 100 microcoulombs due to residual lead impurity was observed. The detection limit for this electrode was about lOP7M, a t which concentration the charge from Pb02 reduction is twice the standard deviation in the residual charge. Using the rotating disk electrode and cell (Figure 2), the detection limit was about 2 X 10-sM. Higher sensitivity could be achieved, but a t the expense of increased pre-electrolysis time. For example, by decreasing the disk area by a factor of ten, the detection limit could be pushed to 2 X 10-9M by increasing the deposition time tenfold.
CONCLUSIONS Owing to the complication of distinct nucleation and growth steps for PbO2, a successful analytical method should be based on quantitative deposition of lead, rather than a reproducible fraction. Likewise, the cathodic step should be coulometric rather than voltammetric. The most favorable sensitivity is achieved not by using a large electrode area in a small volume of solution, but by increasing pre-electrolysis time and using efficient stirring with a small electrode to minimize the charge due to double layer charging. The tin oxide electrode is distinctly advantageous over noble 'metal electrodes because of lower blanks and higher oxygen overpotential. The "memory" effect of retention of chemisorbed lead(11) may be minimized considerably by soaking the elecprior to use each day. Use of small electrode in 1M "03 trodes is also favorable. The optimum acidity for quantitative deposition is 0.01M HN03. Chloride should be limited to to 10-3M. Phosphate interference is minimal a t 0.01M "03 concentration. Even though the gravimetric result for bulk deposits would be high due to incorporation of phosphate into the deposit and even though the peak shapes would be altered, the coulometric stripping result would be unaffected.
1358
ANALYTICAL CHEMISTRY, VOL. 47, NO. 8, JULY 1975
LITERATURE CITED (1) (2) (3) (4) (5)
(6) (7) (8) (9) (10) (11) (12) (13) (14) (15) (16) (17) (18) (19) (20) (21) (22) (23) (24) (25) (26) (27) (28) (29) (30) (31) (32) (33) (34) (35) (35) (36) (37) (38) (39) (40) (41) (42) (43) (44) (45) (46) (47) (48)
I. M. Kolthoff, J. Jordan, and A. Heyndrick, Anal. Chem., 25, 884 (1953). T. Yoshimori, S. Kori, and T. Takenchi, Jpn Analyst, 13, 309 (1964). L. G. Ruthberg and N. G. Chovnyk, lnd. Lab., 34, 931 (1968). H. Monien and K. Zinke. Fresenius'Z. Anal. Chem., 240, 32 (1968). H. Monien and K. Zinke. Fresenius'Z. Anal. Chem., 250, 178 (1970). E. Hrabankova, J. Dolezal, and P. Beran, J. Nectroanal. Chem., 22, 203 (1969). T. Miwa. H. Motosugi, and A. Mizuike. BunsekiKagaku, 20, 846 (1971). J. T. Kinard and R. C. Propst, Anal. Chem., 46, 1106 (1974). M. Fleischmann and M. Lilier. Trans. Faraday Soc., 54, 1370 (1958). M. Fleischmann and H. R. Thirsk, Nectrochim. Acta. I,146 (1959). M. Fleischmann and H. R. Thirsk, "Advances in Analytical Chemistry and Chemical Engineering", Voi. 3, P. Delahay, Ed., Interscience. New York, 1963, pp 123-210. M. Fleischmann and H. R. Thirsk. Electrochim. Acta, 2, 22 (1960). D. Elliott, D. L. Zellmer, and H. A. Laitinen. J. Electrochem. SOC., 117, 1343 (1970). H. Kim and H. A. Laitinen, J. Electrochem. SOC.,122, 51 (1975). H. A. Laitinen and N. H. Watkins. J. Electrochem. SOC., 119, 113C (1972). R. Bezman and P. S. McKinney, Anal. Chem., 41, 1560 (1969). N. H. Watkins. Ph.D. Thesis, University of Illinois, 1973. B. Miller, J. Electrochem. Soc., 116, 1117 (1969). J. M. Mochel, US. Patent 2564707 (1951). H. Kim and H. A. Laitinen, J. Am. Ceram. SOC.,58, 23 (1975). G. W. Harrington, H. A. Laitinen, and V. Trendafilov, Anal. Chem., 45, 433 (1973). N. A. Hampson. P. C. Jones, and R. F. Phillips, Can. J. Chem., 46, 1325 (1968). /bid., 45, 2039 (1967). /bid., 45, 2045 (1967). /bid.,47, 2171 (1969). M. Fleischmann, J. R. Mansfield, H. R. Thirsk, H. G. E. Wilson, and L. WynneJones, Electrochim. Acta, 12, 967 (1967). H. A. Laitinen and N. H.Watkins, Unpublished work, University of Illinois, 1973. P. Kirkov, Electrochim. Acta, 17, 519 (1972). J. P. Carr and N. Hampson, J. Electroanal. Chem., 28, 65 (1970). J. P. Carr, N. A. Hampson. and R. Taylor, J. Electroanal. Chem., 27, 109 (1970). lbid., 27, 201 (1970). /bid.,27, 468 (1970). M. Fleischmann and H. R. Thirsk. J. Electrochem. SOC., 110, 688 ( 1963). R. D. Armstrong and J. A. Harrison, J. flectrochem. SOC., 116, 328 (1929). V. F. Huber and M. S. A. El-Meligy. Z. Anorg. Allg. Chem., 367, 154 (1969). R. F. Amlie and T. A. Berger, J. Electroanal. Chem., 36, 154 (1969). R. F. Amlie and T. A. Berger, J. Electroanal. Chem., 36,427 (1972). H. Bode and E. Voss. Electrochim. Acta, 6, 11 (1962). G. W. Tindall and S. Bruckenstein, Electrochim. Acta, 16, 245 (1971). G. W. Tindall and S. Bruckenstein, Anal. Chem., 40, 1051 (1968). lbid., 40, 1637 (1968). S.H. Cadle and S. Bruckenstein. Anal. Chem., 43, 932 (1971). lbid., p 1858. S. H. Cadle and S. Bruckenstein. Anal. Chem., 44, 1993 (1972). W. T. Schrenk and P. H. Delano, lnd. Eng. Chem., Anal. Ed., 3, 27 (1931). S. Bruckenstein and J. W. Bixler, Anal. Chem., 37, 786 (1965). I. M. Kolthoff, E. B. Sandell, E. J. Meehan, and S.Bruckenstein. "Quantitative Chemical Analysis", 4th ed., The Macmillan Company, New York, 1969. E. Hrabankova. J. Dolezal. and P. Beran, J. Electroanai. Chem., 22, 203 (1969). P. A. Tucker, Analyst(London), 71, 319 (1946).
RECEIVEDfor review January 31, 1975. Accepted April 14, 1975.
