amount of the added EGTA is consumed by magnesium and the assumptions made in the derivation of the Gran plot are no longer fulfilled. The Gran plot based on the E values is also linear up to u = 9 ml as shown in Figure 2. However, on extrapolation to F = 0, an intercept on the u axis equal to 10.8 ml is obtained. This is, of course, due to the contribution to the membrane potential from interfering ions. As can be seen from Figure 2, it is, in this example, for the E values, advantageous to exploit the u value ( u = 9.8 ml) corresponding to the intersection of the two legs formed by the Gran plots before and after the equivalence point. This procedure is not, of course, suitable for the E' values (cf. Figure 3).
0.8
0.6
0.4
0.2
2
4
Figure 3. Gran plots for
6
The E values have been derived from Equation 6 and the Equation 7
F
= (100
+
v ml
10
8
the titration of calcium
u)10 exp(E/29.58)
a
ueq
E'
values from
- u
(u < u e q ) (8) When extrapolated to F = 0 the plot will intersect the u axis a t u = ueq, provided that the main reaction dominates completely and that no ions other than calcium contribute to the membrane potential. When exploiting E' values only, disturbing side reactions have to be taken into account. As seen from Figure 2, this Gran plot is linear u p to u = 9 ml, the linear part intersecting the u axis a t the theoretical equivalence volume, 10 ml, when it is extrapolated to zero. At greater u values, a considerable
ANALYTICAL PRECISION If the correct emf value in the equivalence point is determined separately, the relationship between the estimated uncertainty in the emf measurements and the analytical precision in titrations to a prechosen emf can be obtained from the calculated titration curve. This is illustrated in Figure 3 for the E us. u curve assuming an emf uncertainty of & l mV. The corresponding analytical precision is thus &2%, i.e., considerably less than the systematic error obtained by the second derivative method, -6.1%. An Algol version of the computer program Haltafall for the IBM 360/65 can be obtained as punched cards from the authors a t cost price. Received for review January 15, 1973. Accepted April 6, 1973.
Direct Determination of Sulfide by Rapid Direct Current Polarography D. R. Canterford Department of Physical Chemistry, University of Melbourne, Parkville 3052, Victoria, Australia
Because of the extreme toxicity of hydrogen sulfide, sensitive analytical procedures for the determination of sulfide ion are necessary. Most of the methods that have been developed for the determination of sulfide ion in aqueous solution are indirect. In many cases, interferences from other anions (particularly cyanide) severely limit the practical application of these techniques. A direct method for the determination of a number of common anions, including sulfide, is based on the anodic polarographic waves corresponding to the formation of soluble mercury complexes or insoluble mercury compounds ( I , 2 ) . I t has recently been shown that the so-called "rapid" polarographic technique, in which mercury drops are mechanically dislodged from a capillary a t short time intervals and fast scan rates of potential are used, offers advantages over conventional dc polarography for such anodic processes ( 3 ) . For example, analytically undesir-
able phenomena such as maxima and erratic drop behavior, which are often associated with the deposition of insoluble mercury compounds on the electrode, can be eliminated. Previously, natural short drop times ( 4 ) or a streaming mercury electrode ( 5 ) have been used to extend the range over which the current is a linear function of concentration for waves corresponding to mercury salt formation. However, the use of mechanically controlled short drop times is experimentally simpler and not prone to adverse streaming effects. The present work describes a detailed investigation of the application of the rapid technique to the direct determination of sulfide ion in aqueous solution. As well as comparing the sensitivity of the conventional and rapid methods, the possibility of interference from other anions and the effect of pH of the supporting electrolyte have been studied.
(1) I . M. Kolthoff and C. S. Miller, J. Arner. Chem. SOC., 63, 1405 (1941) . (2) J. Heyrovsky and J. Kuta, "Principles of Polarography," Academic Press, New York, N.Y., 1966. (3) D. R. Canterford. A. S. Buchanan, and A. M . Bond, Anal. Chern.. 45, 1327 (1973).
(4) P. Zuman, J. Koryta, and R. Kalvoda, Collect. Czech. Chem. Cornmun.. 18, 350 (1953). (5) J. Koryta and P. Zuman. Collect. Czech. Chern. Comrnun., 18, 197 (1953).
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ANALYTICAL CHEMISTRY, VOL. 45, NO. 14, DECEMBER 1973
(a)
EXPERIMENTAL Analytical reagent grade chemicals were used throughout without further purification. Concentrated stock solutions of sodium sulfide were prepared using de-oxygenated, triply distilled water and were stored under argon. Small aliquots of stock solution were transferred by microsyringe to a known volume of supporting electrolyte which had previously been thoroughly deaerated with oxygen-free argon and thermostated a t 25.0 "C. Argon was passed over the surface of the solution while polarographic measurements were being made. Polarograms were recorded with a PAR (Princeton Applied Research) Model 170 Electrochemistry System using a three-electrode cell. A short drop time of 0.16 sec was obtained with a Metrohm Polarographie Stand E354.All potentials reported are relative to a silver/silver chloride (1M NaC1) reference electrode, separated from the polarographic test solution by a salt bridge containing 1M NaC104. Platinum wire was used as the third (auxiliary) electrode.
RESULTS AND DISCUSSION In a recent reinvestigation of the polarographic behavior of sulfide ion, four distinct dc waves were observed a t high concentrations with normal drop times (6). This behavior was explained by deposition on the electrode surface of three successive layers of HgS which inhibit the dissolution of mercury as HgS2*-. As previously shown ( 3 ) , polarograms are simplified considerably by using the rapid technique, with only two waves being observed for sulfide concentrations up to 2 x lO-3M. Figure 1 shows typical conventional and rapid dc polarograms for sulfide in 1M NaC104. It should be noted that in recording these polarograms, the same capillary with the same head of mercury was used, with only the drop time and the scan rate of potential being changed. Under both conventional and rapid conditions, the total limiting current varied linearly with sulfide concentration irrespective of the number of waves present ( 3 ) .At normal drop times, the linearity of this plot and the absence of maxima or erratic drop behavior is in contrast with the behavior observed for many other systems involving mercury compound deposition ,on the electrode surface. Thus, for sulfide analysis, the only apparent advantage of rapid polarography is the much shorter time required to record a polarogram. However, as shown in a subsequent section, a consideration of interferences has revealed another, previously unreported, advantage of the rapid technique. Detection Limit of Rapid Direct Current Polarography. Although the sensitivity of dc polarography decreases as the drop time is shortened, for reduction of copper(I1) this decrease is only marginal when the drop time is changed from a normal value (Le., 3 sec) to 0.16 sec (7). Similar results were found in the present work. The detection limit under rapid polarographic conditions was 4 X 10-6M sulfide, compared with 3 X 10-6M under conventional conditions. With the rapid technique, a plot. of limiting current us. concentration was linear down to 7 x 10-6M. Between 7 X 10-6M and the detection limit, the relatively high contribution of double-layer charging current to the total current made accurate evaluation of the limiting current difficult. Effect of Interfering Ions. Interference from common anions would appear to be a major problem with many of the recently developed methods of sulfide analysis. Table I lists some of these methods and the anions which interfere. Because many anions other than sulfide also give rise to anodic polarographic waves corresponding to mercury (6) D. R.
press.
(7) A . M.
Canterford and A. S. Buchanan, J. Electroanal. Chem., in Bond and D. R. Canterford.Anal. Chem., 44, 721 (1972).
wave 1 %
I
- 08 Figure 1.
-06 VOLT
-DL vs.
-02
Ag/AgCI
Conventional and rapid dc polarograrns of 1.5
X
10-3M sulfide in 1M NaCIO4 a. drop time, 2.9 sec; b. drop time, 0.16 sec
Table I . Anions Causing Interference with Some Recent Methods of Sulfide Analysis Method Interfering anion(s) Polarographic (indirect) (8) CN Direct titration ( 9 ) CN-, OHSpectrophotometric (70) CN-, I-, S ~ 0 3 ~ Spectrofluorirnetric ( 7 7) C N - , SCNSpectrofluorirnetric (72) CN Ion-selective electrode (73, 7 4 ) CN -
compound formation ( I , 2 ) , the presence of these anions could interfere with the determination of sulfide. Comparison of Figures l a and l b shows that the possibility of such interference will be less with the rapid technique. Under conventional conditions (Figure la), the start of anodic discharge and the beginning of the total limiting current plateau differ in potential by about 650 mV. Obviously, sulfide cannot be determined at this concentration (1.5 X lO-3M) in the presence of any other electroactive species which give rise to a wave over this extremely large potential range. On the corresponding rapid polarogram, however, the start of anodic discharge and the total limiting current plateau are only about 250 mV apart (Figure l b ) , which greatly reduces the possibility of interference from overlapping waves. The determination of sulfide in the presence of cyanide, which with many methods is not possible (see Table I), provides an excellent illustration of this advantage of the (8) L. C. Gruen, Anal. Chim. Acta, 52, 123 (1970). (9) S.A . Kiss, Talanta, 8, 726 (1961). (10) S. Komatsu, T. Nomura, and T. Ito, Nippon Kagaku Zasshi, 90, 171 (1969). (11) L. S. Bark and A. Rixon, Analyst (London),95, 786 (1970). (12) F. Vernon and P. Witham, Anal. Chim. Acta. 59, 155 (1972). (13) R. Naurnann and C. Weber, Fresenius' 2. Anal. Chem., 253, 111 (1971). (14) J . Vesely, 0. J. Jensen, and 8. Nicolaisen, Anal. Chim. Acta, 62, 1 (1972).
ANALYTICAL CHEMISTRY, VOL. 45, NO. 1 4 , DECEMBER 1973
2415
,
/
1
/"
1OuA
[S2-l x I d/M Figure 3. Total limiting current plotted vs. sulfide concentration for various supporting electrolytes X , 1M NaC104; 0, 0.5 M NaOH; 0 , 1M NaOH. Drop time
-08
VOLT
Figure
2.
-0 4
-0.6 vs
Ag/AgCl
Rapid and conventional dc polarograms of 1.5
X
10-3M sulfide in the presence of 1 0 - ' M cyanide. Supporting
electrolyte is 1M NaCIO4 a. drop time, 0.16 sec; b. drop time, 2.9 sec
Table II. Results of Interference Studiesa Anion added
Conventional polarographyb
Rapid polarographyb
I
NI
SCN-
NI
s2032-
I NI I
NI NI
CN-
5032CI Br I-
OH-
I I NI
NI NI NI NI NI
a 1.5 X 1 0 - 3 M sulfide determined in the presence of 10-*M potentially interfering anion. I = interference; N i = No Interference.
rapid technique. Figure 2 shows rapid and conventional polarograms of 1.5 X lO-3M sulfide in the presence of 10-2M cyanide. With the rapid technique, the total limiting current was not affected by the addition of cyanide because the cyanide wave did not appear until the sulfide limiting current plateau was well developed (compare Figures 2a and l b ) . However, with a normal drop time, the cyanide wave appeared before the limiting current plateau for sulfide was reached (compare Figures 2b and l a ) , and it was not possible to determine the sulfide concentration under these conditions. The effect of a number of other anions which could interfere with the polarographic determination of sulfide was investigated. In each case, polarograms of 1.5 X 10-3M sulfide were recorded in the presence and absence of 10-2M potentially interfering anion and the total limiting current compared. The results given in Table I1 show that cyanide is by no means the only anion for which intcrference a t normal drop times could be overcome by using the rapid technique. 2416
= 0.16 sec
With decreasing concentration, the number of sulfide waves decreases (6) and, therefore, the difference in potential between the start of anodic discharge and the limiting current plateau becomes smaller. Hence, the above advantage of the rapid technique becomes less important as the sulfide concentration is decreased. Below about 2 X lO-*M sulfide, the conventional and rapid techniques become equivalent with regard to interferences, since only one dc wave is observed in each case. However, it should be noted that anions which interfere with the determination of 1.5 X lO-3M sulfide under conventional conditions (see Table 11) will not interfere with the determination of sulfide concentrations below 2 x lO-4M. An obvious method of decreasing the number of sulfide waves observed a t normal drop times, and thus reducing the possibility of interference from other anions, is to dilute the sulfide solution being analyzed. However, because sulfide ion in aqueous solution is rapidly oxidized by dissolved oxygen (i5),it is recommended that this procedure be avoided if possible, unless the solution can be kept out of contact with the atmosphere during dilution. Although this may not seem an important point, it has been the author's experience that reproducible results for the determination of small sulfide concentrations can be achieved only if reasonably stringent precautions are taken to guard against oxidation in any manipulation of the solution prior to analysis. Effect of pH. The anodic hydroxide wave (1, 2 ) occurs a t a much more positive potential than the sulfide wave(s), and, therefore, hydroxide ion itself does not interfere with the determination of sulfide. Recently ( 6 ) , it has been shown that the pH of the supporting electrolyte significantly affects the polarographic behavior of sulfide. For example, the shape of the first wave varies with pH. However, it has been observed in this work that under both rapid and conventional conditions, the total limiting current is independent of pH, which is an advantage in analytical applications as it eliminates the need to buffer the supporting electrolyte prior to analysis. To illustrate this advantage, Figure 3 shows the total limiting current plotted against sulfide concentration for rapid polarograms recorded in 1M (15)
M .Avrahami
ANALYTICAL C H E M I S T R Y , VOL. 45, NO. 14, D E C E M B E R 1973
and R .
M.Golding, J . Chem. SOC.A .
1968,647
NaC104 (pH 9 to 10.3, depending on sulfide concentration), 0.5 M NaOH, and 1M NaOH. In 1M NaOH, a maximum was observed on rapid dc polarograms for sulfide concentrations in the narrow range of 2 X 10-4 to 6 X lO-4M. Since this maximum prevented accurate evaluation of the limiting current, a supporting electrolyte of lower p H ( e . g . , 0.5M NaOH) is recommended if sulfide concentrations are expected to fall in this range. Supporting electrolytes of pH less than about 8 or 9 are not recommended because of the possibility of loss of sulfide as hydrogen sulfide.
CONCLUSIONS As discussed by Bark and Rixon ( I I ) , most methods available for the determination of sulfide have one or more disadvantages. These include practical disadvantages, such as strict time control, as well as interference from other anions. The silver sulfide ion-selective electrode represents a potentially simple means of sulfide ion determination. However, this electrode also responds to
cyanide (13, 14). Furthermore, the potential of the electrode depends on the ionic strength and pH of the sulfide solution, so that if it is to be used for direct potentiometry these parameters must remain constant. Although the direct method proposed in this paper is not as sensitive as some of the previously established techniques, it appears superior to many of these methods because of its simplicity, freedom from interference, and the independence of the limiting current on pH. For the determination of sulfide ion in natural samples, it is recommended that the sulfide be fixed by reacting it with zinc acetate and subsequently recovered by distillation, unless the sample is analyzed immediately after collection (16). The rapid polarographic method should be suitable for analysis of samples treated in this way if the evolved hydrogen sulfide is collected in a sodium hydroxide solution. Received for review May 1,1973. Accepted July 9.1973. (16) "Standard Methods for the Examination of Water and Wastewater," APHA. AWWA, WPCF, New York, N.Y., 13th ed.. 1971, Part 157, p 336.
Pulsed-Mode Atomic Fluorescence Utilizing a Demountable Hollow Cathode Excitation Source J. 0 . Weide and M. L. Parsons1 Department of Chemistry, Arizona State University, Tempe, Ariz. 85281
There have been several reports concerning the use of pulsed hollow cathode systems in flame spectrometry. Dawson and Ellis ( I ) originally applied this technique to an atomic absorption system, finding an increase in resonance line intensity. The radiation output over the pulse width in their system was observed to be several hundred times the original dc level with little increase in line width or self-reversal. Mitchell and Johansson (2, 3 ) originally applied this technique to atomic fluorescence measurements while developing a simultaneous multielement analysis technique. Their system incorporated a rotating interference filter wheel for element selectivity and a phase sensitive detector for signal detection. Barnett and Kahn ( 4 ) made a comparison of atomic absorption and atomic fluorescence utilizing a pulsed excitation system with synchronous demodulation (lock-in amplifier). Cordos and Malmstadt (5-8) showed that the relative standard deviation for their dual channel synchronous integrator measurements was less than 0.1%. Their atomic fluorescence system was To w h o m a l l correspondence s h o u l d b e addressed. (1) J. B. Dawson and D. J. Ellis, Spectrochim. Acta, Part A, 23, 565 (1972). (2) D. G. Mitchell and A. Johansson, Spectrochim. Acta, Part B, 25, 175 (1970). (3) D. G. Mitchell and A. Johansson, Spectrochim. Acta., Part 6, 26, 677 (1971). (4) W. B. Barnett and H. L. Kahn, Ana/. Chem., 44, 935 (1972). (5) E. Cordos and H. V. Malmstadt, Anal. Chem., 44, 2277 (1972). (6) E. Cordos and H. V. Malmstadt, Anal. Chem., 44, 2407 (1972). (7) E. Cordos and H. V. Malmstadt, Anal. Chem., 45, 27 (1973). (8) H. V. Malmstadt and E. Cordos, Amer. Lab., Aug., 35 (1972).
composed of several hollow cathode excitation sources operating in sequential on-off modes. In a preliminary investigation, the authors (9) showed approximately three orders of magnitude improvement in the limit of detection for zinc when using the pulsed system as opposed to a conventional dc system. In this system, a pulse controlled dual-channel boxcar integrator was used to detect the pulsed atomic fluorescence signal. High-voltage pulses produce high intensity emissions from hollow cathode lamps. The resultant pulsed atomic fluorescence signals are converted by the photomultiplier detector and operational amplifier into voltage pulses. The signal and the background are integrated separately in the dual-channel boxcar integrator. The sampling gate of the boxcar integrator can be adjusted to receive maximum signal, whereas the reference gate is adjusted to monitor background between signal pulses. In both cases the noise level is decreased considerably when the signal is integrated by an RC-circuit, but is also dependent upon the time required to charge the capacitor. Thus, the integration time is directly related to analysis time. The final dc output of the boxcar is the difference between the integrated sample and background signals.
EXPERIMENTAL Apparatus. F i g u r e 1 shows a b l o c k d i a g r a m f o r t h e p u l s e d a t o m i c fluorescence system. A pulse generator ( T e k t r o n i x - 1 1 4 ) was used t o synchronize t h e e x c i t a t i o n source w i t h t h e d e t e c t i o n system. A c o n t i n u o u s - f l o w l o w pressure h e l i u m atmosphere was used t o s u s t a i n t h e discharge within t h e d e m o u n t a b l e h o l l o w (9) J, 0. Weideand M. L. Parsons, Anal. Left., 5 , 363 (1972)
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