Babko and Lisetskaya (14) in 1956 determined the solubility of stannic sulfide in hydrogen sulfide solution at different pH values and confirmed that the gram-molecular ratio of the components SnSz and NazSis 1 to 1. According to their work, the equilibrium constant of the formation reaction is 1.1 x lo5 in alkaline sulfide solution as measured by a colorimetric and light-scattering method. However, the direct measurement of the sulfide ion concentration in such equilibrium solution by means of potentiometric techniques has not previously been studied. We therefore carried out complex formation studies by measuring the equilibrium potentials with the sulfide ion-sensitive membrane electrode (us. SCE) and the pH values of several series of equilibrium suspensions containing tin(1V) and sulfide at a constant ionic strength of 0.10 M. Table I gives the results of experiments carried out at different total sodium sulfide concentrations and varying pH values. The first column represents the original concentrations of sodium sulfide added to the suspension of stannic sulfide. The second and third columns give the equilibrium potentials and the corresponding free sulfide ion concentrations derived from the potential ES. -log [S2-] calibration curve ( p = 0.1M). The fourth and fifth columns give the pH and the corresponding hydrogen ion concentrations calculated by using the activity coefficient of hydrogen ion, 0.76, at an ionic strength of 0.1M. Substituting these experimental values of the free (14) A. K. Babko and G. S. Lisetskaya, Zh. Neorg. Khim., 1, 969 (1956).
sulfide ion concentration, [S2-], the hydrogen ion concentration, [H+],and Kz = 3.635 x 10-15 ( p = 0.1M) in Equation 6, one obtains the unreacted total sulfide ion concentration, [S2-],, given in the sixth column. The concentrations of the thiostannate complex ion formed can then be calculated by subtracting [Sz-]i from the concentrations of sodium sulfide initially added ([SnS32-] = [NazS] - [S2-],. The values of the formation constant, K,, calculated via Equation 9 are listed in the last column in Table I--e.g.,
K,
=
[SnS32-]/[S2-]
(9)
The calculated values are fairly constant, with an average value of (2.062 f 0.098) X lo5at 25 "C and p = O.lM, which agrees well with a previously reported value of 1.1 X lo6 at 20 "C(14). Although stannic sulfide also dissolves in alkaline solution to form the hydroxystannate complex ion, SnSzOH-, the present method does not suffer from this complication because the electrode measures only the activity of the free sulfide ion. Thus, the use of sulfide ion-sensitive membrane electrode for the determination of equilibrium constants in complex formation systems offers considerable advantages over conventional methods and could be profitably applied to a variety of practical problems.
RECEIVED for review Feburary 8, 1968. Accepted March 18, 1968. The financial support of NSF Grant GP-6485, NIH Grant GM-14544, and the Alfred P. Sloan Foundation is gratefully acknowledged. One of the authors (T.M.H.) also acknowledges a Fulbright travel grant.
Capacitance Measurements on Platinum Electrodes for the Estimation of Organic Impurities in Water Leonard0 Formaro and Sergio Trasatti Laboratory of Electrochemistry and Metallurgy, University of Milan, Via Venezian 21, 20133 Milan, Italy
The measurement of the double-layer capacitance on platinum electrodes can be used as an experimental method for the determination of last traces of single organic substances in aqueous solutions as well as for the evaluation of industrial water quality. Principles of the method and experimental techniques needed to ensure a high sensitivity are described. Possibilities and limits of the method are discussed taking into account the nature of the organic impurities. Results obtained in the analysis of solutions containing traces of styrene, toluene, acrylonitrile, and amyl alcohol, and in the test of some samples of industrial water are shown. I n the latter case, although the sensitivity is lower than in the former, it was possible to detect carbon contents down to 0.03 & 0.005 ppm.
THERE ARE only a few experimental methods ensuring sufficient accuracy and satisfactory sensitivity for the control of organic impurity level in water. Titration with permanganate allows water quality evaluation down to 10 f 2 ppm (I). The determination by an infrared analyzer of the C02 formed in the oxidation with 0 2 of the carbonaceous material present in (1) R. B. Schaffer, C. E. Van Hall, G. N. McDerrnott, D. Barth, V. A. Stenger, S. J. Sebesta, and S. H. Griggs, J. Water Pollution ControI Federation, 37, 1545 (1965).
1060
ANALYTICAL CHEMISTRY
water shows a sensitivity of 1-2 ppm. The precision of the method is 2 0 z at these water pollution levels ( I ) . It provides a measure of all the carbonaceous material in a water sample, both organic and inorganic. However, if a measure of organic carbon alone is desired, the inorganic carbon content of the sample can be removed during sample preparation ( 2 ) . If no volatile carbonaceous materials are present in the sample, the solution can be concentrated and the practical sensitivity goes down to 0.1 pprn. However, this possibility is rather rare. These methods never provide the sensitivity required in the control of organic impurities in laboratory water in which the pollution level must be low enough for the most careful experimental work. In many cases, the sensitivity of these methods is poor, even for industrial purposes. An electrochemical method ensuring an accuracy of about 15% at carbon contents of 0.03 ppm will be described in this paper. The test of a water sample can be carried out in a few minutes and no concentration of the sample is required to increase the sensitivity. This method is unaffected by low contents in inorganic materials. (2) C. E. Van Hall and V. A. Stenger, ANAL.CHEM., 39,503 (1967).
PRINCIPLES OF THE METHOD
The capacitance of the electrical double layer at the metal/ solution interface is extremely sensitive to the state of the electrode surface. When an organic substance becomes adsorbed on the metal surface, the double-layer capacitance is depressed more or less markedly depending on the nature of the adsorbate as well as of the adsorbent (3). Platinum is a particularly active metal from this point of view; it shows very marked chemisorption phenomena due to its catalytic properties (4, 5 ) . Platinum electrodes are very sensitive to last traces of organic impurities present in laboratory solutions, and a content of 10-6-10-7 eq/liter is easily detected by capacitance measurements (6, 7). The adsorption of organic substances on an electrode can be regarded, as far as the effect on the double-layer capacitance is concerned, as the substitution of a dielectric in an electrical condenser (3). From a molecular point of view, adsorption can be considered as replacement reaction of water molecules on the electrode surface by organic molecules. The metal/ solution interface can be thus regarded to a first approximation as two electrical condensers in parallel (3), the former with capacitance CO determined by the physical properties of the solvent (water), the latter with a capacitance C1 determined by the physical properties of the adsorbate. Thus, the capacitance of the electrical double layer may be written as :
ce=
co(i -
e) + cl.e
(1)
where 0 is the coverage with organic substance. With Equation l, 0 can be evaluated directly from experimental measurements :
e=- co - ce co - c1 where CO - C1is the drop in capacitance observed on completely covered electrode. Generally, a definite relationship exists between 0 and the concentration of organic substance in solution, the form of which depends on the nature of the adsorption. Simple relationships have been found for several organic substances on platinum (4, 5 , 8). Some results (7) have suggested that a similar simple relationship also exists in the case of organic impurities in water. Hence, the measurement of AC = CO- C, alone at a suitable potential can provide informations about the amount of organic substance in solution, once the relationship existing between coverage and concentration is known. Since this relationship will be different depending on the nature of the organic substance, it will be obtained easily by direct electrochemical measurements only when the substance is known. If the nature of the organic impurity is unknown, the method cannot be absolute, but relies on the analysis by other methods of a water sample impure enough for the analysis to be fairly accurate; the calibration curve (relationship between 0 and concentration) can then be built (3) B. B. Damaskin and A. N. Frumkin, in “Modern Aspects of Electrochemistry,” Vol. 3, J. O’M. Bockris (eds.), Butterworths, London, 1964, p 149. (4) L. Formaro, and S. Trasatti, Chim. Znd. Milan, 48, 706 (1966). (5) S. Trasatti and L. Formaro, J . Electroanal. Chem., 17, in press (1968). (6) S. Trasatti, Electrochim. Metall., 1, 267 (1966). (7) L. Formaro and S . Trasatti, Electrochim. Acta, 12,1457 (1967). (8) V. S. Bagotzky and Yu. B. Vassiliev, Electrochim. Acta, 11, 1439 (1966).
up using successive dilutions of the water sample. This aspect of the method will be discussed later. An alternative approach may be the following. It has been shown (6) that the adsorption of impurities is diffusion-controlled and a linear relationship must then exist between coverage and t1I2. Since 0 and capacitance are bound by the simple Equation 2, plots of capacitance as a function of t l i z will then give straight lines and a measure of water pollution may be obtained from the slopes of the straight lines. Obviously it is not a true quantitative measure, but it is, in any case, possible to know which water sample is more contaminated and which is less. EXPERIMENTAL
Capacitance measurements were carried out using the ac bridge previously described (6). The frequency used throughout the investigation was 5 KHz. The cell used was similar to that described elsewhere (6); the only difference was that stirring was provided by a magnetic stirrer. Solutions were not degassed to avoid removing volatile organic substances. CO, does not affect capacitance measurements in the potential range employed (E > 0.3 V(RHE)). Similarly, results are unaffected by the presence of O2if we consider the purpose of these measurements. All experiments were carried out at 25 & 0.1 “C using an air-thermostat. Electrode. The electrode was prepared by melting a 99.98Z pure platinum wire (0.5-mm i d . ) in a Hz-02 flame to a small sphere and then sealing the wire into a capillary glass tube. The gas absorbed in the metal was eliminated by keeping the electrode in a vacuum furnace for 3 hours at 500 “C. The real surface area was determined by cathodic charging curves (9) and found to be 0.0493 cm2. Solutions. Since the method consists in determining the concentration of an organic substance from the measure of AC = CO - Ce,its sensitivity will be greater, the higher the value of Co-Le., the higher the purity of the blank solution. In view of the electric nature of the method which requires conductive solutions, the blanks were 1 M in HCIOl and were prepared from analyzed reagent grade 60Z HCIOr (Erba) and triply distilled water, the last distillation of which was made from alkaline permanganate under nitrogen atmosphere. The blanks were further purified by pre-electrolysis between platinized platinum electrodes at an apparent current density of 5 mA/cm2 for 15-20 hours under an atmosphere of nitrogen filtered through columns filled with alumina and active charcoal. Experiments were carried out with solutions containing known amounts of toluene, styrene, acrylonitrile, and amylalcohol (analyzed regent-grade chemicals), and water samples coming from an industrial plant, denoted Al, B1, and C1. These samples were analyzed for carbon content by the titrimetric analysis of the COz formed by combustion. Samples A1 and B1 were products coming from two different demineralization beds of the same plant; the carbon contents were 2.5 ppm and 3 ppm, respectively. Sample C1 was boiler water; the carbon content was 5.2 ppm. Treatment of the Cell and Electrode. Before each run, the electrode and the cell were washed with concentrated HzS04, then rinsed, boiled, and washed with triply distilled water. The electrode activation, which will be extensively discussed later, was performed before each measurement with potential step sequences using a 557 AMEL potentiostat. Cathodic charging curves were carried out using a PG-2 pulse generator (Intercontinental Instruments, Inc.) and photographed from the screen of a RM 546 Tektronix oscilloscope. (9) S. Trasatti, Electrochim. Metall., 2, 12 (1967). VOL. 40, NO. 7, JUNE 1968
0
1061
A
1.5 V
30scc* 1.2v
20sec"
+
70 sec
+-.
0.4 V
B
1.8 V
t
' 5 1 I
%2C
*
-I
1.5 V 45 scc*
E 6Osec*
*
+
1 2 0sec
0.06 V
WITH STIRRING
IO
set*
Figure 1. Potential/time sequences to activate the electrode A-for B-for
cathodic charging curves capacitance measurements
RESULTS AND DISCUSSION Purity of the Blank Solutions. Both inorganic and organic impurities can be present in laboratory water. The general approach used in determining the inorganic material content is to measure the conductivity. 2.108Q-cm was the resistivity (in air) of the triply distilled water for this work. This is a good value for a conductivity water. Organic impurities are far more troublesome and difficult to completely remove, for their continuous source is the atmosphere over the solution (6). A first approach to determine organic impurity level is represented by cathodic charging curves (6, 9). After electrochemical pretreatment the organic impurities adsorb onto the electrode surface, at a rate which depends on their concentration, and occupy a fraction of the sites available for hydrogen adsorption during a cathodic charging curve. Measuring q H ,the charge expended in depositing a hydrogen
Table I. Fraction of Surface Covered with Impurities After a Given Time t at 0.4 V (RHE) &H I , sec
4, (crC/cm2) q H , a - q H
223 223 223 223 223 222 219 218 212
5 10 20 30 45 60 90 120 180 ~
~~
1062
= 411.8
~
ANALYTICAL CHEMISTRY
... 0 0 0 0 1 4
5 11
&HhR,r
... 0 0 0 0
0.5 2.0 2.3 5.0
'
100
atom on every free site at different times after pretreatment, the impurity level can be approximately expressed as concentration in eq/liter (making some assumptions about the diffusion coefficient) or, better still, directly as fraction of electrode surface covered after a given time. A platinum electrode in contact with the atmosphere or kept at a not too anodic potential in solution is partly covered with impurities. If a clean reproducible surface is wanted, adsorbed impurities must be removed from the surface before each measurement with a suitable pretreatment. Potential/time sequence A shown in Figure l was used to control the water quality of the blanks. At 1.5 V (RHE), impurities are removed from the surface by oxidation; molecular oxygen and oxidation products are swept away by stirring. At 1.2 V, the oxygen layer on the surface is retained and protects the electrode from poisoning, while the solution is still stirred to restore concentrations of dissolved species at the interface to bulk values. After 20 sec, stirring is stopped to allow the solution to become quiescent to obtain controlled and reproducible conditions on the solution side too. The electrode is then brought to 0.4 V (RHE) where the oxygen layer is completely and rapidly reduced. At this point the surface is free of impurities but soon it becomes poisoned again at a rate depending on the impurity concentration. Table I shows results obtained with the above procedure in the case of the blanks employed in this work. It can be seen in Table I that the electrode remains free of impurities for about 45 sec after pretreatment. Referring to t = 2 min, the impurity level in the blank solutions can be conventionally defined as about 2 . 3 z . It has been shown (6) that the adsorption of impurities is diffusion-controlled. Employing the well-known equations i d = FDc/G and i d . 2 = AqH, where F is the Faraday constant, and assuming the
I
h
I
I
0.3
0.5
0.7
1
0.9
a-OM;
b-Z.lO-?M;
~-2.10-~M; d-2.10-5M
reasonable values of 10-5 cmZ/sec for the diffusion coefficient D and 0.01 cm for the diffusion-layer thickness 6 in quiescent solution, the impurity concentration c may be estimated to be about 6.10-7 eq/liter. It is practically impossible to express the concentration in mole/liter, because different species are probably present in the solutions and for each of them it would be necessary to know the exact number of covered sites per adsorbed molecule. It will be seen later that this impurity level has little effect on the sensitivity of the method in the analysis of the majority of common organic substances; the method can still detect lo-? moleiliter and even less. The problem is a little more complicated in the case of the analysis of water containing large but hardly adsorbed substances such as humic acids, which may be present in well water for boiler feeding. This aspect also will be discussed later. Estimation of Common Organic Substances. Experiments were carried out using solutions at known concentrations of toluene, styrene, acrylonitrile, and amyl alcohol. The concentration will be expressed as ppm of organic substance assuming the density of the solution to be 1 . Sequence B of Figure 1 was used to pretreat the electrode. This sequence was a little different from sequence A because of the different purposes of the measurements and the oxidation resistance of the substances employed. At 1.8 V (RHE), any adsorbed species is removed from the electrode surface by direct oxidation or mechanically repelled by oxygen evolution. The solution is kept stirred to sweep into the bulk of the solution any desorbed species as well as molecular oxygen. The step is only 5 sec long to avoid excess of oxygen and any hardly reducible oxide which may modify surface properties of platinum. The successive step is at 1.5 V for 45 sec to allow slowly oxidizable or desorbable species to be swept away under less drastic anodic conditions for the electrode. The potential is then brought to 0.06 V where the surface is reduced within the first milliseconds. The cathodic step is continued for 10 sec to reduce oxides formed at the most anodic potential and to desorb any absorbed oxygen (derrnasorp-
1
0.5 0.7 0.9 ELECTRODE POTENTIAL,Volts (RHE)
ELECTRODE POTENTIAL, Volts (RHE)
Figure 2. Capacity/potential curves for several toluene concentrations
I
0.3
Figure 3. Capacity/potential curves for different styrene concentrations a-OM; b-lO-?M; c-10-6M; d-1Ob5M; e-10-4M tion) (IO). At 0.06 V, the surface is covered with a complete layer of hydrogen (11) which can actually dissolve into platinum (12) but the duration of the step is too short to affect capacitance measurements at the working potential E. At this potential, adsorbed hydrogen is rapidly ionized (at least for E > 0.4 V) and the platinum surface is thus clean and in a reproducible state. The duration of this step is 3 min. The potentiostat is disconnected by switching to a potentiometer set at the potential E, which allows the ac of the bridge to be superimposed to the dc flowing through the cell. Since very low concentrations of organic substances were employed in these experiments, solutions were kept stirred during the first minute to make the mass transfer fast. During the last two minutes, stirring is stopped to obtain a better control of experimental conditions and therefore a higher reproducibility of results. Particular experiments showed that the capacitance read was practically the steady-state value. Figure 2 shows the effect of addition of small amounts of toluene upon the capacity of platinum. In the figure, the capacity curve obtained with the blank solution alone under the same experimental conditions is also shown. The presence of toluene depresses the capacity of platinum; this is a feature of adsorption phenomena (3, 4). The decrease in capacity is greater the higher the concentration of toluene. However, a dependence of the drop in capacity upon potential can be seen; the greatest decrease occurs at potentials ranging between 0.3 V and 0.5 V (RHE). A marked drop in capacity can still be seen for 2.lO-7M toluene solution (about 0.02 ppm). In Figure 3, capacity curves for styrene solutions are presented. The greatest drop in capacity can be seen for 0.3 c E < 0.5 V, in this case as well. The amount of organic substance which can be detected is very small (lO+M= 0.0104 (IO) T. B. Warner and S. Schuldiner, J. Phys. Chem., 69, 4048 (1965). (11) M. Breiter, H. Karnrnermaier, and C . A. Knorr, 2.Elektrochem., 60, 37 (1956). (12) s. &huldiner and T. B, warner, ElecrrOchi,,,. A ~ 11,~ 307~
(1966). VOL. 40, NO. 7, JUNE 1968
1063
,
I
I
22 CI NE
5 20 t
b
E 0
3
Y
I
18 16
a
0 J 14
2
a
12
W
lJ. !L
5 I
I 0.5
0.3
0.7
0.9
ELECTRODE POTENTIAL, Volts (RHE)
Figure 4. Capacity/potential curves for several acrylonitrile concentrations a-OM;
b-10-7M;
c-~O-~M;
d-10-6M
ppm). With respect to the previous figure, the curves are further apart-i.e., the platinum electrode seems to have a better “sensitivity.” It must be noted that the electrode shows phenomena of “saturation” for higher concentrations than 10-6M. In this concentration range the curves are markedly closer to each other. Results for acrylonitrile are given in Figure 4. A concentration of lO-7M (0.005 ppm) can be detected in this case as well, but the favorable potential range is narrower, since AC values decrease noticeably for E > 0.4 V. In Figure 5 , capacity curves for amyl alcohol are shown. This substance was chosen as an example of a molecule without aromatic nucleus, as well as double and triple bonds. The lowest detectable concentration was 10-6M (0.09 ppm) and the sensitivity of the electrode for this substance was lower than in the previous cases-Le., the curves are much closer to each other. A definite regularity in the relationship between AC and concentration can be seen in this case as well. The above results qualitatively evidence the possibility of using capacitance measurements on platinum to detect organic substances in a solution. The quantitative aspect of the phenomenon is shown in Figure 6, where AC is plotted against log c . AC is the difference between the capacity value at 0.4 V for the bare electrode (CO) and the capacity value at the same potential when the coverage of the electrode is 0 (Co). To a first approximation (the problem is rather complicated by the presence of aromatic nuclei), the value of AC can be considered as proportional to the coverage 0 ( 4 , 5 ) and the relationship between AC and concentration assumes, at least formally, the meaning of adsorption isotherm and its regular shape can be thus explained. Apart from any further theoretical consideration, we only want to stress the fact that from a practical point of view the relationship between AC and concentration can play the very useful role of calibration curue. The reproducibility of points determined in the same run was about 0.5 and the reproducibility from one run to another was better than 4-5 which provides satisfactory accuracy. The accuracy may be improved if the impurity level of the blank solution can be actually kept constant.
x
1064
IO
x,
ANALYTICAL CHEMISTRY
I 0.3
0.5
0.7
0.9
ELECTRODE POTENTIAL, votts (RHE)
Figure 5. Capacity/potential curves for different concentrations of amyl alcohol a 4 M ; b-10-8M;
C-~O-~M; d-10-4M;
e-10-3M
Eventually, the very short time required for measurements (4 min. and even less, electrode pretreatment included) is one more aspect which makes the method promising, Sensitivity and Limits of the Method. Sensitivity and limits of the method are in some respects related because both depend on the nature of the organic substances. If concentration and any other factor (interactions, effect of the electric field in the double layer, dielectric constant, etc.) are identical for two organic substances, a higher AC will be observed for the organic substance which occupies, on adsorption, a greater number of sites per molecule. This is the reason why styrene, acrylonitrile, and toluene produce a higher drop in capacity than amyl alcohol. The presence of double and triple bonds as well as of aromatic nuclei makes adsorption more marked and consequently the method gains in sensitivity. A measure of sensitivity can be obtained from the slope of the calibration curve. However, an increase in sensitivity leads to restriction of the concentration range within which the method is applicable. Figure 6 shows that with styrene, the platinum electrode exhibits saturation at higher concentrations than 10 ppm. Nevertheless, this is the very advantage of the method which allows quantitative measurements to be made in the range of concentration where any other method as yet developed fails. As for the lower limits of the method (the lower limit for an organic substance can be assumed to be the value of concentration for which AC = 0) they are determined by the impurity level in blank solutions. These solutions are in equilibrium with the atmosphere and then contain organic impurities present in it (6). It has been shown that the concentration of impurities may be estimated by cathodic charging curves to be about 6.10-7 eqlliter. This value is confirmed by results obtained using amyl alcohol; this suggests that the residual impurities are light organic compounds. It is thus reasonable to expect that the smaller the organic substance added to a solution, the higher the minimum concentration which may be detected by a platinum electrode, because the drop in capacity per molecule of impurity may be
14
I BLANK"
WATER Bf
1
WAfER C (
0.01
0.1
1
IO
I
ORGANIC SUBSTANCE CONCENTRATION (ppm)
Figure 6. Relationship between drop in capacity at 0.4 V (RHE) upon addition of organic substances and concentration expressed as ppm
comparable with the drop in capacity per molecule of added substance. In spite of these restrictions, the method allows solutions containing organic compounds at lower levels than to be analyzed without any 0.01 ppm (about lO-7-lO-8M) tedious sample preparation. Actually, the value of Co is of prime importance and may represent an experimental problem. However, if the apparatus for the preparation of blank solutions is not changed, the oscillation in CO,mostly due to oscillation in the impurity level, never exceeds a few per cent and then well below the accuracy of the method. The results with common organic substances were obtained with the same reference value of Co. The results with industrial water were obtained a few months later and the reference value of Cowas to be changed because the pre-electrolysis apparatus had been modified a little. Much higher sensitivity may be obtained with this method if both blanks and sample preparation are carried out under high purity atmosphere. This is particularly clear if we observe that the impurities present in the blank solutions used in this work gave a capacity value at 0.4 V about 35 % lower than that measured on clean electrode surface (7). Since under our experimental conditions the drop in capacity upon addition of foreign substances was about 35 % at the maximum coverage (electrode saturation), it is evident that the maximum sensitivity of the method can be exactly doubled by working under high purity atmosphere. Practical Applications. Industrial Water Quality Evaluation. In the previous section, the possibility of using the method for analysis of water containing only one simple organic compound has been shown. From this point of view, the method can be employed for the test of water quality as well as for determination of organic substances in water. Serious problems of water quality also exist in industrial plants. We have applied our method to such determinations and the results have been promising. Organic impurities of unknown nature are present in boiler feed-water. In this case it is not possible to build up an independent calibration curve, but it is necessary to perform successive dilutions of the water relying on the analysis of the sample carried out with a different method. No doubt this is a restriction because our method will be affected by the accuracy of the preceding analysis, but this does not change its
0.3
0.5
0.7
0.9
ELECTRODE POTENTIAL, Volts (RHE)
Figure 7. Capacity/potential curves for different water samples Water A l : 2.5 ppm; B1: 3 ppm; C1: 5.2 ppm
Table 11. Characteristics of Examined Water Samples Carbon content, Type P P ~ Color Source AI 2.5 colorless Demineral. bed X B1 3.0 colorless Demineral. bed Y c1 5.2 slightly yellow Boiler
sensitivity and its possibility of application. Further this criticism holds only from a quantitative point of view. From a qualitative point of view the method can always show with a high degree of sensitivity greater-or-less pure water. In this case no previous analysis is needed. Three different samples of water denoted with A l , B1, and C1 were tested. Table I1 summarizes their characteristics. All of these samples came from the same plant. The carbon content was evaluated from the titrimetric analysis of the Conformed by combustion. Figure 7 shows capacity curves for the three samples as well as for the blank. The capacity curves classify the samples in the same order as Table 11. However, a direct comparison of capacity values obtained with different organic substances cannot be made. This is certainly the case of water C1 to which some special organic compounds are intentionally added. Water B1 was therefore chosen as an example and several dilutions of the sample were carried out down to 0.03 ppm with laboratory water (blank). Some capacity curves for different carbon contents are shown in Figure 8. Over all the examined potential range, the capacity is more and more depressed with increasing carbon content. The calibration curve of Figure 9 was built up by plotting AC = Co - CS at 0.4 V as a function of the carbon content. It can be noticed that small AC values were observed despite the high carbon content of the sample (Table 11). With a change from 0.03 ppm to 3 ppm, a AC of only 18 was observed. This may be caused by the fact that the organic molecules contained in the VOL. 40, NO. 7, JUNE 1968
1065
I
In laboratory water quality tests, it is obviously not possible to dilute the samples to obtain a calibration curve, since the dilution can only be made with water having the same impurity level. Thus the method can be qualitative only, but it is still extremely sensitive. Figure 10 shows plots of C as a function of t 1 / 2 for two samples of laboratory water differently treated. Sample B2 was not pre-electrolyzed, sample A2 was pre-electrolyzed for about 15 hours. The linear plots show that adsorption of impurities is diffusion controlled in agreement with previous findings (6). These plots cannot represent a quantitative test, but they may be used as a very sensitive criterion of water quality. No doubt water A2 is purer than water B2. Plots of C against t l ' z are therefore more than sufficient to check the impurity level of the water sample for which the absolute carbon content is not required. In this case the method is even more sensitive, because it is possible to go below the limiting value of 0.03 ppm. There is no practical limitation to the method used in this way. The limit is a completely pure water for which the capacitance does not change with time. Finally, three other classes of substances must be considered as representative of organic impurities in water :amines, organic phosphates, and surfactants. Even though no quantitative data are available, a qualitative behavior similar to that of the majority of organic substances (3) is to be expected. In other words, this method can be considered, in principle, as applicable even with such impurities.
1 I
t
03
0.5
0.9
0.7
ELECTRODE POTENTIAL ,Volts (RHE) Figure 8. Capacity/potential curves for different dilutions of an industrial water sample containing 3 ppm of carbon
sample are of big size but they are hardly adsorbed on platinum. According to this hypothesis, each organic molecule occupies a number of sites comparable with that obscured by one molecule of impurity contained in the blank solution, but the former has a much greater number of carbon atoms than the latter. Thus the sample of water B1 (3 ppm) gives the same AC as a solution containing only 0.03 ppm of styrene. Therefore, greater sensitivity can be attained only by working out of contact with room atmosphere. In spite of these restrictions the results are encouraging. Figure 9 shows that it is possible to detect 0.03 ppm, a carbon content 100 times lower than the maximum sensitivity of other methods ( I , 2). Furthermore, the precision is generally better than 15 even at the lowest carbon contents and this residual inaccuracy is not due to the method, since the reproducibility in the same run is within 0.5 %, but it is probably due to error in diluting samples as well as to variations in impurity content of blanks because of solution handling. In our opinion, the average error of the method must fall to negligible values if the effect of impurities coming from the atmosphere can be avoided.
I
4
CONCLUSIONS
The determination of the capacitance of platinum electrodes can be employed as an experimental method to evaluate water quality as well as to determine last traces of single organic substances in water. Advantages. Average accuracy better than 15 %; sensitivity down to 0.03 ppm; insensitivity to carbon of inorganic source; and rapidity of measurement. Limitations. Restriction to substances adsorbable on platinum.
34 h
"E
32
i5 5 30
'
t
k 0
28
2
9
WATER 81
51
6.2
26
-I
F 24
za
22
k 20 I
4
9
I6
TIME 0.03
0.1 CARBON
0.3
I
3
CONTENT (pprn)
Figure 9. Relationship between drop in capacity at 0.4 V (RHE) and carbon content of an industrial water sample diluted several times 1066
ANALYTICAL CHEMISTRY
Figure 10. Capacitance as a function of time at 0.4 V (RHE) after pretreatment A2: solution pre-electrolyzed for 15 hours; B2: not preelectrolyzed solution
The method is absolute in the determination of single organic substances of known nature, but it is not absolute in the determination of the carbon content of water containing unknown impurities. However, in this case, it may be a very sensitive comparison method. The sensitivity and the precision may be affected by residual organic impurities in blank solutions. This limitation, also affecting the determination of a simple organic substance, can be overcome in industrial water quality evaluation if the Cali-
bration curve can be obtained with water at different carbon content but coming from the same plant. In this case, the nature of pollution is always the same and no dilution is needed to prepare samples of different carbon content.
RECEIVED for review November 16,1967. Accepted February 9,1968. Work was supported by the Consiglio Nazionale delle Ricerche, Rome, Italy.
Role of Contamination in Trace Element Analysis of Sea Water David E. Robertson Battelle Memorial Institute, Pacifc Northwest Laboratory, Richland, Wash. 99352
A wide variety of solvents, reagents, and other materials normally encountered in the trace element analysis of sea water have been analyzed for trace element impurities by neutron activation analysis and multidimensional gamma-ray spectrometry. The concentrations of up to 10 trace elements, including Sc, Cr, Fe, Co, Cu, Zn, Ag, Sb, Cs, and Hf, were measured in these materials. Many of these substances contained extremely high impurity levels of various elements. These analyses provide an indication of which materials may contribute to the contamination of sea water samples which come in contact with these substances. The contamination of sea water during several typical chemical separations has been estimated. Suggestions for minimizing the sources of contamination are given. SEAWATER is a complex solution containing, either in a dissolved or particulate state, all of the elements of the earth’s crust. Yet, only 13 of these elements exist at concentrations greater than 1 ppm, while the majority are present at less than 1 ppb. The sampling and analytical problems involved in measurements at these low concentrations have limited studies of trace element behavior in the oceans. For many of the oceans’ trace elements the literature contains a wide range of reported concentrations, some of which vary by several orders of magnitude ( I ) . For the most part, these discrepancies seem to have resulted from contamination of the samples, either during collection and storage or subsequent analysis. Cooper (2) has discussed some of the difficulties inherent in sea water sampling. These include contamination of a sample by material leached from the sampling apparatus itself, which may contain metallic components, rubber washers, stoppers, and tubing. and these each contain significant amounts of various metal impurities. Contamination may also arise because of leaching of materials from the bottle in which the sample is stored. Contamination may result from the rust and corrosion products which accumulate on the hydrowires and become sloughed off during the movement of messengers down the wire. Samples are subject to contamination from smoke and ash from ship’s smokestacks, from sewage and waste discharged from the ship, or from corrosion products and paint leached from the ship itself. The soiled hands of an (1) J. P. Riley, “Chemical Oceanography,” Vol. 2, J. P. Riley
and G. Skirrow, Eds., Academic Press, London and New York, 1965. (2) L. H. N. Cooper, J. Marine Res., 17, 128-32 (1958).
operator who is taking the sample may also be a source of contamination. Most methods which have been reported for the analyses for trace elements in sea water require chemical preseparations for concentration or removal of interfering elements before an analysis can be performed. These procedures greatly increase the risk of contaminating the sea water sample. Contamination may come from reagents, vessels, filters, other apparatus used for the separations, or even from airborne particulate material in the laboratory. These sources are briefly discussed by several investigators ( I , 3 , 4 ) . In our initial trace element analyses of sea water by neutron activation analysis and multidimensional y-ray spectrometry (5, 6 ) , it was observed that serious contamination of several elements could occur unless rather extreme precautionary measures were enforced. Contaminants originated mainly from impurities present in irradiation containers, from filters, and from airborne particulate material in the laboratory. Because this analytical method was purely instrumental and required only a minimum of sample handling and no chemical separations, the magnitude of the contamination problem in the trace element analyses of sea water became quite apparent. This present study was conducted to determine the potential sources and levels of contamination one might encounter in the trace element analysis of sea water. Samples of container materials, reagents, solvents, and materials used in the direct construction of sea water samplers and in related material were analyzed for trace element impurities using neutron activation analysis in conjunction with multidimensional ?-ray spectrometry. This method provided the required sensitivity and selectivity for the direct instrumental measurement of up to 10 trace elements in these various materials. (3) A. Mizuike, “Trace Analysis-Physical Methods,” G. H. Morrison, Ed., Interscience, New York, 1965, pp 105-15. (4) J. F. Slowey, D. Hedges, and D. W. Hood, Progress Report
of Research Conducted through the Texas A&M Research Foundation for the Atomic Energy Commission, Second Progress Report, Aug. 1, 1961 to Nov. 1, 1962, TID-22660, Texas A & M University, College Station, Texas, Nov. 1962. (5) R. W. Perkins, D. E. Robertson, and H. G. Rieck, Pacific Northwest Laboratory Annual Report for 1965 in the Physical Sciences, Vol 2: Radiological Sciences, BNWL-235 2, pp 108-15, Pacific Northwest Laboratory, Richland, Wash., May 1966. (6) D. E. Robertson and R. W. Perkins, “Symposium on Trace Characterization-Chemical and Physical,” National Bureau of Standards, Gaithersburg, Md., Oct 2-7, 1966, paper No. 57. VOL. 40, NO. 7, JUNE 1968
1067
