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ANALYTICAL CHEMISTRY, VOL. 51, NO. 3, MARCH 1979
Potentiometric Stripping Analysis in Non-Deaerated Samples Daniel Jagner Department of Analytical Chemistry, University of Goteborg, Fack, S-402 20 Goteborg, Sweden
Instrumentation. A semi-automaticPotentiometric Stripping Controller (Radiometer prototype) equipped with a three-electrode potentiostat and a timer/switcher unit was used in all experiments. The controller could be programmed to yield plating times between 1 and 64 min. Immediately before the commencement of the stripping, Le., disconnection of the potentiostat, the paper feed of the recorder (Radiometer REC 61 Servograph, amplifier REA 105/120) was automatically started and was terminated after a programmed fraction of the plating time. Electrodes. A platinum foil (Radiometer P 101) was used as counter electrode and a saturated calomel electrode (Radiometer K 4040) was used as reference electrode. The working electrode was a glassy carbon electrode (Radiometer F 3500). Electrochemical Cell. A titration assembly (Radiometer TTA 60) was used as electrochemical cell. The three electrodes could be screwed into the lid of the titration vessel. The sample was agitated by a constant rate mechanical stirrer (Radiometer TTA 60). Analytical Procedures. During the pre-concentration of metal analytes by means of potentiostatic reduction and amalgamation, the glassy carbon working electrode must be coated with a mercury film which is thick enough to dissolve all metals reduced. This can be achieved either in situ by adding mercury(I1) ions to the sample prior to analysis or by pre-coating the working electrode with mercury in a separate solution. If the latter procedure is used, as in the experiments below, mercury(I1) ions need not, of course, be added to the sample. The procedure is, however, more elaborate than in-situ mercury coating. In the in-situ procedure, the working electrode is first polished with 3 - ~ m diamond paste for 30-60 s. It is then cleaned carefully with acetone to remove polishing lubricant. The electrodes are screwed tightly into the lid of the sample vessel. After acidification of the sample with hydrochloric acid, 10-50 mg/L of mercury(I1) is added and sample stirring is started. The plating potential is adjusted to -0.95 V vs. SCE and the plating time to 1 min. A minimum of four plating/stripping cycles, each cycle comprising 60 s of plating and 15 s of stripping, are then performed automatically. Immediately after completion of the last plating/ stripping cycle, the plating potential and the plating time are adjusted to the values to be used during the subsequent analysis and the potentiometric stripping controller is initiated. I t is advisable to minimize the time during which the mercury-coated working electrode is exposed to the mercury(I1)-containingsample in the absence of a reducing potential applied t o the electrode. A couple of minutes of such exposure may result in the formation of calomel on the electrode surface, according to the reaction
Potentiometric stripping analysis is based on the potentiostatic pre-concentration of analytes in a mercury film and the subsequent registration on a xt-recorder of the resulting redox titration curve obtained when the potentlostatic circuitry is disconnected. I n the present study, it is shown that zinc, thallium, cadmium, lead, bismuth, and copper can be analyzed in acid solution using dissolved oxygen, in equilibrium with air, as oxidant. For analyte concentrations above approximately 25 pg/L potentiometric stripping analysis in non-deaerated samples is more rapid than analysis in deaerated samples using mercury(I1) as oxidant.
In potentiometric stripping analysis ( I ) , analytes are pre-concentrated in a mercury film (plated) by potentiostatic reduction a n d amalgamation. T h e potentiostat is then disconnected a n d t h e reduced analytes are oxidized by a suitable oxidizing agent. T h e redox titration curve thus obtained is registered on a high input impedance recorder or on a p H meter. T h e time elapse between two consecutive equivalence points is taken as a measure of t h e sample concentration of the particular analyte oxidized in this interval, t h e analyte being identified by the redox potential. As in anodic stripping voltammetry, t h e technique is restricted t o elements forming reversible amalgams. T h e instrumentation is, however, much less complicated. In a previous paper ( I ) , t h e use of p p m concentrations of mercury(I1) as oxidant in potentiometric stripping analysis has been described. In order t o ensure t h a t mercury(I1) was the predominant oxidant in t h e sample, dissolved oxygen was removed by bubbling nitrogen gas through t h e sample for 20 min previous t o the commencement of the analysis. I n samples containing relatively high concentrations of metal analytes where short plating times could be used, the deaeration process was the most time-consuming part of the analysis. Consequently, it seemed advantageous to investigate whether or not dissolved oxygen could be used as an oxidant in potentiometric stripping analysis. T h e oxidation of dilute amalgams by dissolved oxygen has been studied in detail by Pelletier, Buffle, and Monnier using a hanging mercury drop electrode (2,3). Although the purpose of their work was t o investigate whether or not this reaction could be exploited for the analysis of traces of dissolved oxygen, some of their findings are applicable to the present study. T h e authors conclude that the rate of t h e reaction hl(Hg)
+ 2P
0,(1)
-
M ( n ) 1-
n
-
P
Hg
-
Hg,Cl,(s)
which will decrease the sensitivity. Separate pre-coating of mercury can be carried out in a solution containing approximately 40 mg/L of mercury(I1) and 0.5 M hydrochloric acid. After introducing this solution into the sample vessel, the stirring is initiated and the plating potential and time are adjusted to 1 min and -0.95 V vs. SCE,respectively. A minimum of four plating/stripping cycles is then performed automatically. After completion of the last cycle, the solution is removed and the working electrode is carefully cleaned with distilled water in order to remove mercury(I1) ions. If not removed, these ions may cause formation of calomel on the electrode surface. The pre-coated electrode, which can be used for the analysis of several samples, should be stored either dry or in distilled water. Irrespective of the mercury pre-coating method employed, the analytical results are evaluated either by standard addition by using a calibration curve or by the use of an internal standard.
O(-p)
where p = 1 (H202)or p = 2 (H,O), depending on t h e metal M, is controlled by the diffusion rate of dissolved oxygen or amalgamated metals. Consequently, t h e oxidation rate depends on the concentration of dissolved oxygen and amalgamated metal. EXPERIMENTAL Chemicals. All chemicals used except mercury(I1) nitrate were of Suprapure grade. 0003-2700/79/0351-0342$01. O O / O
+ Hg(I1) + 2C1-
C
1979 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 51, NO. 3, MARCH 1979
-i
8
set.
343
I
Figure 1. Potentiometric stripping analysis in a non-deaerated sample containing 0.005 M hydrochloric acid. Plating at -1.30 V for 16 min. Zinc, cadmium, and lead concentrations 200, 400, and 800 pgIL in curves I,11, and 111, respectively. The distance between points (a) and (b) on curve 111 is taken as the analytical signal for zinc
RESULTS AND DISCUSSION Determination of Zinc, Cadmium, and Lead. T o 40 mL of a sample containing 0.005 M hydrochloric acid, 200 pg/L of zinc(II), cadmium(II), and lead(I1) was added. The sample was plated for 16 min a t -1.30 V and the potentiometric stripping curve registered. T h e procedure was repeated for zinc(II), cadmium(II), and lead(I1) concentrations of 400 and 800 bg/L, respectively. T h e potentiometric stripping curves are shown in Figure 1 and the linear relationship between concentration and stripping signal is shown in Figure 2. Determination of Copper and Bismuth. A sample, 40 mL, containing 200 yg/L of copper and bismuth in 1 M hydrochloric acid was plated a t -0.75 V for 8 min. After the registration of the potentiometric stripping curve, the copper and bismuth concentrations were increased stepwise to 400, 800, and 1600 pg/L. T h e potentiometric stripping signals registered after each increment increased linearly with increasing concentration. T h e potentiometric stripping curves for 200 and 400 pg/L are shown in Figure 3. pH Dependence. During reductive pre-concentration of the trace analytes in oxygen-containing samples, a pH gradient is built up close t o the electrode. T h e p H a t the electrode surface is higher than t h a t in the bulk of the sample. This may cause hydrolysis of the metal ions close to the electrode surface and t h u s decrease the sensitivity. T h e extent of hydrolysis depends on the particular metal ion analyzed, on t h e sample pH, a n d , more important, on the buffering capacity. T o investigate the p H effect on potentiometric stripping analysis in non-deaerated samples, 40 mL of 0.1 M sodium chloride solution containing 100 pg/L of cadmium, copper, and lead was acidified by stepwise addition of hydrochloric acid to give total concentrations of 0.0005,0.005, 0.05, and 0.5 M. The potentiometric stripping curves registered after each increment of acid did not differ significantly. When the
Signal, sec. =
Zn
=
Cd
o = Pb
/
20
10
500
Figure 2. Calibration curve obtained by potentiometric stripping analysis in non-deaerated solution. Plating for 16 minutes at -1.30 V (cf. Figure 1)
experiment was repeated without the addition of acid, the resulting potentiometric stripping curve showed a pronounced decrease in sensitivity. Further experiments with p H variations revealed t h a t the optimum pH range for potentiometric stripping analysis in non-deaerated solution is 0-3.5 for cadmium, lead, thallium, and copper, 2-4 for zinc and gallium, and 0-0.5 for bismuth.
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ANALYTICAL CHEMISTRY, VOL. 51, NO. 3, MARCH 1979
E vs SCE
- Q8-
- 0.6-
-0.4-
8 sec.
I
-0.2-
-
a0
Figure 3. Potentiometric stripping analysis in a non-deaerated sample containing 1 M hydrochloric acid and 200 (curve I) or 400 (curve 11) Hg/L of copper and bismuth. Plating for 8 min at -0.75 V
The p H is preferably adjusted by means of hydrochloric acid and sodium acetate. C o m p a r i s o n b e t w e e n Dissolved Oxygen a n d M e r c u ry(I1) as O x i d a n t . T o compare the oxidation rate in a saniple saturated with dissolved oxygen and the oxidation rate in a deaerated sample containing small concentrations of mercury(I1) as the major oxidant, 40 mL of a 0.01 M hydrochloric acid solution containing 400 Wg/L of cadmium(II), lead(II), and copper(I1) was plated at 4 . 9 5 V for 4 min. The potentiometric stripping curve was registered and the same plating/stripping cycle was repeated twice. Two ppm of mercury(I1) nitrate was then added to the sample and a stream of nitrogen was passed above the sample surface. The potentiometric stripping controller was programmed to register automatically t h e potentiometric stripping curves obtained during consecutive 4-min plating/stripping cycles. The increase in potentiometric stripping signal due to the decrease in oxygen concentration is shown in Figure 4. As seen from Figure 4, deaeration by passing nitrogen above the sample surface is rather a slow process. After approximately 50 min of nitrogen purge, mercury(I1) ions are the predominant oxidants in the sample and a steady signal is obtained. It is also apparent from Figure 4 that the oxidation rates for lead and cadmium are approximately 25 times higher in a solution saturated with dissolved oxygen than in a deaerated sample containing 2 ppm of mercury(I1). For copper, the oxidation rate is only approximately 1 2 times higher. Oxidation of lead and cadmium by oxygen is a two-electron process while oxidation of copper is a one-electron process. T e m p e r a t u r e Dependence. Temperature variations affect the diffusion rates of metal ions, amalgamated metals, and dissolved oxygen, and the saturation concentration of dissolved oxygen. Consequently, temperature changes influence both the mass transport rate during plating and the rate of oxidative stripping. T o determine the effect of temperature variations
on the potentiometric stripping signals in non-deaerated samples, 40 mL of a 0.1 M hydrochloric acid solution containing 400 gg/L of cadmium(II), lead(II), and copper(I1) was thermostated a t 15 "C for 2 h. The temperature was then increased stepwise up t o 35 "C and a 8-min plating/stripping cycle was initiated after each increase in temperature. T h e relative changes with temperature in the potentiometric stripping signals were measured. The cadmium signal proved to be more sensitive to changes in temperature than the copper and lead signals. The relative increase in the cadmium signal was 0.04 O C - ' in the temperature range investigated, the corresponding values for lead and copper being 0.015 "C-' and -0.005 O C - ' , respectively. Accuracy. The accuracy was tested by means of calibration plots. Linear relationships through the origin were obtained for zinc, cadmium, thallium, lead, copper, and bismuth in the concentration range tested, 50-1000 gg/L. Precision. The precision was estimated from 15 repetitive plating/stripping cycles in a sample containing 200 ,ug/L of zinc, cadmium, and lead in 0.05 M hydrochloric acid. The temperature was held a t 23.2 k 0.4 "C. The relative standard deviations were 0.04 for cadmium and 0.02 for lead and zinc. The same experiment was repeated in a 1M hydrochloric acid solution containing 200 kg/L of copper and bismuth. T h e relative standard deviations were 0.03 and 0.02 for copper and bismuth, respectively. Detection Limit. The detection limits in potentiometric stripping analysis cannot be defined uniquely since they decrease linearly with plating time. In practice, plating times of more than 1 h are seldom employed. To estimate the detection limits for such a plating time, a 0.01 M hydrochloric acid solution a t 4 . 9 5 V for 64 min prior to stripping was used. T h e potentiometric stripping curve registered is shown in Figure 5. Assuming a resolution of the xt-recorder of 0.1 s, the detection limits for cadmium, lead, and copper, plated for 64
ANALYTICAL CHEMISTRY, VOL. 51, NO. 3, MARCH 1979
345
Signal, sec.
A
=
cu
= Cd
40
=
Pb
30 Nitrogen purge
started
20
10
t
-
=
Figure 4. Increase in potentiometric stripping signal during sample deaeration with a stream of nitrogen above the sample surface. Repetitive plating for 4 min at -0.95 V prior to each stripping. Sample contains 0.01 M hydrochloric acid and 400 pg/L of cadmium(II),lead(I1).and copper(I1); 2 ppm of mercury(I1) was added immediately before the commencement of nitrogen purging E vs SCE
- 1.0
- a0
- a6 -OA
3 sec. ~
a2
cu
ac
Figure 5. Potentiometric stripping analysis in a nondeaerated sample of 0.01 M hydrochloric acid containing 40 pg/L of cadmium, lead, and copper. Plating for 64 min at -0.95 V
min, were estimated t o be 2.5, 2.5, and 1.0 pg/L, respectively. The lower detection limit for copper is due to the one-electron process in the oxidation with dissolved oxygen.
CONCLUSION I t has been shown t h a t dissolved oxygen can be used as
oxidizing agent in acid samples for all elements normally analyzed by potentiometric stripping analysis. T h e major advantage of this technique, compared to oxidation with mercury(II), is t h a t time-consuming deaeration is not necessary. The risk of contamination by the inert gas or by mercury ions added to the sample is also eliminated. Oxidation by dissolved oxygen can be particularly advantageous when performing potentiometric stripping analysis in a continuous mode. T h e main drawback of oxygen oxidation is the high concentration of oxygen present in a sample in equilibrium with air. This means t h a t the oxidation rate is very rapid and the potentiometric stripping plateaus may be difficult to detect on a normal xt-recorder if the sample concentration of analytes is low or if short plating times are exploited. For analyte concentrations above 20-50 pg/L, depending on the elements analyzed, potentiometric stripping analysis is more rapid and accurate in non-deaerated samples than in deaerated samples containing 2-4 m g / L of mercury(I1). The precision of potentiometric stripping analysis in oxygen-containing solutions is slightly less than that in deaerated samples. This is due t o the higher temperature sensitivity. T h e precision can be improved by avoiding sample temperature variations of more than *0.5 "C during analysis.
LITERATURE CITED (1) D. Jagner, Anal. Chem., 50, 1924 (1978). (2) M. Pelletier, J. Buffle, and D . Monnier, Chimica, 25, 61 (1971). (3) J. Buffle, M. Pelletier, and D. Monnier, J . f/ectroana/. Chern.. 43, 185 ( 1973).
RECEIL-ED for review September 19,1978. Accepted November 17, 19'78.
