Of course, if short enough drop times were used, only one wave would be observed over the entire concentration range. Examination of current-time curves for mercury drops of natural life illustrates clearly why the rapid technique provides considerably simpler polarographic behavior. Figure 9 shows some current-time curves measured on the sulfide system. When the overall electrode process is diffusion controlled the current is proportional to t l l 6 (Figure 9a), but in potential regions where films of the reaction product inhibit the electrode process the curves deviate markedly from this shape (Figures 9 b and 9c). However, it can be clearly seen t h a t very early in the drop life, where rapid polarographic measurements are effectively made, almost normal shape is observed (i.e., i 0: t 1 I 6 ) irrespective of the behavior later in the drop life.
CONCLI.k3IONS This investigation has demonstrated the advantages of employing short controlled drop times for the polarographic analysis of systems in which the reaction product, a compound of mercury, is formed on the electrode surface. Abnormal behavior associated with such systems, which a t conventional drop times is often severe enough to prevent collection of analytically useful data, can be simply eliminated or minimized by using short drop times. This is more convenient than the usual, rather empirical, approach of adding gelatin or some other surfactant. I t has been established that a drop time of 0.16 sec, readily achieved with a commercially available mechanical drop timer, is sufficiently short to overcome many problems arising from mercury compound formation in the systems studied. Although a second wave was observed a t higher concentrations under these conditions (i.e , t = 0.16 sec), this is not considered a serious inconvenience to the analyst, as the total limiting current plateau was always very well defined and the total limiting current was linearly dependent on concentration. Since the sensitivity of dc polarography decreases as the drop
time Figure 9. Current-time curves for 1 X 10-3M sulfide in 1 M
NaCIO4 at various applied potentials Drop time = 2.0 sec. ( a ) -0.250; ( b ) -0.625; ( c ) -0.675 ( v s . Ag/ ASCI)
time is shortened, there appears little advantage from the point of view of analytical application of the rapid technique, in using drop times shorter than about 0.1 sec. The added advantage of the much shorter time required to record a polarogram permits us to conclude t h a t the rapid technique is considerably superior to conventional dc polarography for routine analysis of systems involving formation of mercury compound reaction products on the electrode surface.
ACKNOWLEDGMENT The authors acknowledge the assistance of J. W. Phillips for providing the sample of dithiothreitol. Received for review August 21, 1972. Accepted December 18, 1972.
Ultrapurification of Water for Electrochemical and Surface Chemical Work by Catalytic Pyrodistillation B. E. Conway, H. Angerstein-Kozlowska, and W. 6. A. Sharp Department of Chemistry, University of Ottawa, Ottawa, Ontario, Canada
E. E. Criddle Defence Research Establishment, Defence Research Board, Ottawa, Ontario, Canada
Recently, domestic and industrial water supplies have become contaminated by organic impurities that cannot be removed by ordinary or oxidative distillation because of steam volatility of the impurities or their derivatives. The results of using a pyrocatalytic distillation system for preparation of ultrapure water for electrochemical and surface chemical work are described. Exacting electrochemical and optical criteria are defined for judging and characterizing the purity of water, with respect to organic impurities, especially with regard to their effects at Pt and H g electrodes.
In recent years, both in North America and in Europe, it is being found impossible to prepare pure water free from organic, surface-active contaminants by means of distillation, even from alkaline KMn04, although previously such a procedure was known to be quite adequate. The organic contaminants now commonly present in many domestic and industrial water supplies are steam volatile and are hence not removed by distillation; also they are not efficiently removed by a “preboil.” That residual distillable impurities might arise after permanganate treatment of water, and affect the electrochemical behavior of Pt, seems to have been envisaged by Formaro ANALYTICAL C H E M I S T R Y , VOL. 45, NO. 8 , JULY 1973
1331
by Russian workers (8). However, this method is unreliable due to the presence of contaminants in most samples of active charcoal, and exposure of charcoal to air generates new sources of contamination from the charcoal itself, Hence, if charcoal is to be used, an elaborate procedure for purification of the charcoal itself is required (8). Also, charcoal is found (9) to lead to contamination of electrode surfaces after this kind of water treatment. In the present paper, we report an essentially simple pyrodistillation procedure for ultrapurification of water for surface and electrochemical work and describe the behavior of contaminated and pure water in relation to electrochemical surface processes a t the Pt electrode. These considerations provide a sound and demanding basis for evaluation of the presence of impurities in water. The principle involved (10) in pyrodistillation ( 1 1 ) is pyrolysis of organic impurities by passage of the steam through a hot column, e . g . , of silica. It is desirable to modify the method (12, 13) by inclusion of oxygen with the steam. In the method to be described, we have adopted a procedure involving catalytic oxidation of organic impurities in steam by provision of a small stream of purified 0 2 which passes with the steam in a recycling system through the silica column containing 90% P t / R h gauze in the heated zone. Unless 0 2 is introduced into the steam and a catalyst baffle is used, oxidation of impurities is inefficient.
EXPERIMENTAL Figure 1. Schematic diagram of pyrocatalytic distillation unit
and Trasatti ( 1 ) but was not previously studied or proved experimentally by them. The criteria by which the presence of such contaminants is indicated are as follows. ( a ) The inability to obtain the correct surface tension of "pure" water, low values being usually obtained. ( b ) The electrochemical reactivity of "water" a t the Pt anode a t potentials where, in really pure solution, no reactions are known to occur (2, 3 ) . (c) The surface blocking of electrosorption of H and OH species at Pt ( 4 , 5 ) due to adsorption of organic contaminants. ( d ) The failure to obtain linear log [currentlpotential relations a t the Hg electrode in the absence of 0 2 and in preelectrolyzed solutions (6) where other ionic contaminants have been previously removed. (e) The indications of macromolecular contaminants by means of light scattering (this is usually due to infection of the water by bacteria). Criteria b and c provide especially sensitive indications of surface-active organic impurities in water and will be described in more detail below. Since organic impurities cannot be removed by a simple electrolytic procedure using a mercury cathode, which is effective (6) only for removal of heavy metal ions from water or for reduction of 0 2 , it is necessary to consider other methods. Sorption onto charcoal has been considered by Parsons (7) and a procedure has been described ( 1 ) L. Formaro and S. Trasatti, Electrochim. Acta, 12, 1457 (1967). ( 2 ) F. G . Wili and C. A. Knorr, 2. Elektrochem., 64, 258 (1960). (3) H . Angerstein-Kozlowska, B. E. Conway, and W. B. A. Sharp, J. Electroana/. Chem., in press April (1973) ( 4 ) S. Gilman, J. Phys. Chem., 67, 78 (1963). (5) S. Gilman, Trans. Faraday Soc., 61, 2456: 2561 (1965). (6) A. M . Azzam, J. O'M. Bockris, 8. E. Conway, and H . Rosenberg, Trans. Faraday Soc., 46, 918 (1950). ( 7 ) R. Parsons and F. G . R. Zobel. Trans. Faraday SOC.,62, 3511
(1966). 1332
ANALYTICAL CHEMISTRY, VOL. 45, NO. 8, JULY 1973
Apparatus. Figure 1 shows a schematic diagram of t h e apparat u s with dimensions of the essential parts. The only critical factors are the size of the boiling flask and the rate of boiling in relation t o the diameter and length of t h e pyrocatalytic column. The latter is made of silica ca. 18-mm internal diameter a n d 0.9 m in length. It is heated to 750-800 "C by means of a non-uniformly wound 1-kW furnace made from Kanthal wire. Forty squares of 100 mesh 90% Pt-10% R h gauze are mounted in a baffle arrangement within a 20-cm zone of the silica tube maintained a t 750800 "C (see G in inset of Figure 1). All glass was cleaned in 98% HzS04 and washed with water before glass-blowing operations were commenced in the fabrication of the apparatus. The boiling flask is removed a n d filled with 1.2 1. of water doubly distilled from alkaline K M n 0 4 . The still is then operated so as to recycle the water through the catalytic furnace for 48 hr with purified 0 2 passing through a t about 1 mi sec-I. The rate of distillation may be observed in the trap T. This trap is provided with a Soxhlet type overflow syphon 0 which returns ca. 100-ml samples to the still every hour. This recycling aspect of the still is believed t o be important and ensures t h a t in a 48-hr distillation there is a mean four-fold extent of recycling and exposure of the water t o the catalytic pyro treatment for a volume of 1.2 1. in the boiler. T h e above rate of distillation corresponds t o ea. 0.1 mol min-1 or ca. 35 ml steam per sec through the column. The cross section of the column is ca. 3 cm2 so t h a t the linear velocity of the and drain ( 0 ) steam + 0 2 is ea. 10 cm sec-1. A return trap (T) was adopted so t h a t the water could experience pyrocatalytic distillation for several cycles before samples were taken. When samples S are withdrawn, a somewhat higher rate of distillation may be allowed, as required. T h e water remaining in the boiling flask is used for washing cells before experiments are set up. (8) N . P. Berezina and N . V. Nikolaeva-Fedorovich, Elektrokhimiya, 3,
3 (1967). (9) D . A. Jenkins and C. J . Weedon, J. Electroanal. Chem., 31, 13 (1971 ) . (10) Canadian Patent No. 675,829, December 10, 1963, Ciass 202-29, J. Konikoff, to General Electric Co., Schenectady, N. Y . ("Recovery of Potable Water from Metabolic Wastes"). (11) J. O'M. Bockris and A. K. M . S. H u q , Proc. Roy. SOC.,Ser. A , 237, 227 (1956). (12) E. E . Criddle, Canadian and U.S. Patents applied for. (13) E. E. Criddle. "The High Oxygen Potential on Platinum Using incinerated Air and Pure Water." presented at the Electrochemical Society Meeting, Miami (1972): Symposium on Oxide-Electrolyte lnterfaces, in course of publication, The Electrochemical Society, N. Y . , 1973.
Significant hydrothermal distillation of Pt occurs if the furnace is maintained above ca. 800 "C, as indicated by the appearance of a dark deposit in the cooler tube above the silica column after several months use. Rates of distillation of Pt in 0 2 are significant down to 800 "C (14). The product which distills appears to be PtOz (15, 16). In order to avoid absolutely the possibility of the presence of traces of Pt in the pyrodistilled water used for electrochemical experiments, a further regular distillation is advisable for certain purposes, e.g., for work at Hg electrodes. However, there have been no indications of the presence of soluble or colloidal Pt in the pyrodistillate as far as the experiments at Pt are concerned; e.g , no change of H accommodation occurs. The effectiveness of the pyrocatalytic distillation system was apparent as soon as it was set up and used. For a lengthy period prior to adoption of this procedure, various methods of water treatment were employed without success, judged on the basis of the criteria discussed below in terms of the behavior of the Pt electrode. Previously, for 15 years, double distillation of water from alkaline permanganate had been adequate, cf. (6). Similar difficulties were reported from other labs in the Ottawa area commencing in 1971 and, around this time, related problems were discussed at meetings in the U. S. A . and Europe. In surface chemical work, difficulties with surface tension and surface presbure measurements have arisen ( 17, 18). Procedure. The potentiodynamic method ( 1 ) was employed to examine the behavior of the Pt electrode in impure and in the ultrapurified water solutions. This method is especially suitable for indicating and quantitatively characterizing diffusion-controlled reactions of redox couples and for determining both the characteristick of surface reactions at Pt electrodes and the coverage by adsorbed species involved (2, 4, 19, 20). Potentials were scanned with a linear voltage sweep between 0.05 and 1.4 V E H . A Wenking potentiostat and a Servomex function generator were employed in the usual way (2, 3). The apparatus was an all-glass cell of the type described previously ( 3 ) for high-purity electrochemical work. It was cleaned in pure HzS04 and' then washed with redistilled water several times before commencement of runs. A hydrogen reference electrode in the same acid solution was used as a basis for the scale of potentials referred to as E H in this paper
I00
c
A n
Figure 2. Potentiodynamic current-potential profile for a typical Pt electrode in pure aqueous 1N H2S04 solution at 25 "C; The sweep rate = 50 m V sec-l
Water from the second stage of K M n 0 4 distillation prior to 1971. Also indicated are the principal structural features (2. 3) of the i-V profile characteristic of Pt-surface processes in clean aqueous solutions
3
RESULTS AND DISCUSSION Behavior of Pt. The necessity for ultrapure water in electrode-kinetic and electrosorption studies has been recognized (6) for many years. Schuldiner (21) has directed attention to the desirability of conducting such work in controlled-atmosphere rooms. Until recently, however, the most common impurities giving rise to difficulties in electrochemistry have been depolarizing heavy metal ions and/or 0 2 which can be removed by preelectrolysis. Difficulties with organic impurities that cannot be removed by ordinary oxidative distillation have only recently been encountered. New contaminants in local water supplies are either not oxidized at all in the boiling alkaline K M n 0 4 or are only partially oxidized to derivatives which are not retained by alkali on distillation. In the distillate, these impurities are often found t o be reduced and reoxidized in a redox couple at the electrode surface or become chemisorbed a t the electrode. At the Pt electrode, between the potential limits 0.0 and 1.23 V E H corresponding t o the standard electrochem(14) G. C. Fryburg and H . M . Petrus, J . Electrochem. SOC.,108, 496 (1961). (15) C . B. Alcock and G . W. Hooper, Proc. Roy. SOC.,Ser. A , 254, 555 (1960). (16) E . K . Rideal and 0. H. Wansbrough-Jones,Proc. Roy. SOC.Ser. A , 123, 202 (1929). (1 7) B. Pethica (Unilever Research Laboratories), private communica(18) (19) (20) (21)
tion at Summer School of Surface Chemistry, R u d e r Boskovic Institute, Rovinj, Yugoslavia, June 1972. A. D. Bangham and M . W. Hill, Nature (London). 237,408 (1972). B. E. Conway, 6 . MacDougall, and H . Angerstein-Kozlowska,J . Nectroanal. Chem.,39, 287 (1972). B. E. Conway, E. MacDougall. and H. Angerstein-Kozlowska, J. Chem. Soc., Faraday Trans. I , 68, 1566 (1972: see also J . Weber, Y . B. Valil'ev,a n d V. S. Bagotskii, Elektrokhimiya, 5, 323 (1969). S. Schuldiner and T. Warner. J. Electrochem. SOC.. 112, 212 (1965).
( b ) Other impure distilled water
( a ) Redistilled tap water
Figure 3. ( a ) Typical i-V profile for a solution containing an impurity oxidizable in a diffusion-controlled process ( s w e e p rate 50 mV sec-'). ( b ) Typical i-V profile for a solution containing
chemisorbable oxidizable impurities. Also shown are the progressive H blocking effects that arise in repetitive cycling over a restricted potential range from 0.05 to 0.5 V EH (sweep rate 50 mVsec-l) ical thermodynamic limits of stability of water, no continuous Faradaic oxidation or reduction processes can occur, except when oxidizable or reducible impurities are present. Under non-steady-state conditions, e . g . , in cyclic voltammetry (2, 3, 22), time-dependent, nonsteady currents flow at the Pt electrode and correspond to the following processes which arise in response to the linearly varying ( c f . polarography) potential using 1N aqueous HzS04 solutions at 25 "C and sweep rates in the range 0.005 to 0.5 V sec-1 (see Figure 2): Anodic sweep direction: ionization of adsorbed H (0.0 to 0.40 V) in three distinguishable peaks, electrodeposition of surface oxide (>0.75 V) (broad, three-peak region). Cathodic sweep direction: reduction of surface oxide (from >0.75 V to 0.6 V) (single peak), redeposition of atomic H (0.40 to 0.0 V) ( 2 peaks). (22) P.
Delahay, "New Instrumental Methods in Electrochemistry." In-
terscience, New Y o r k , N . Y . , 1954.
ANALYTICAL CHEMISTRY, VOL. 45, NO. 8, JULY 1973
1333
Figure 4. Dependence of the i-V profile at Pt on sweep rate (5 and 50 mV sec-') showing the role of diffusion effects
in the impurity
behavior
-
Impurity oildotion
Blocking c4 On
Blocking of U Impurity
daposltlon
Figure 5. Effect of stirring on the i-V profile at Pt in the H adsorption and surface-oxidation regions (1) Unstirred
solution; (2) Npstirred solution. Sweep rate 50 rnV sec-'
Between the limits of desorption of H and the beginning of electrodeposition of OH in the anodic sweep, only a region of constant double-layer charging current arises. The i-V profile for a typical Pt electrode in pure aqueous 1N HzS04 a t 25 "C made up from water doubly distilled from alkaline KMn04, before the present difficulties were encountered, is shown in Figure 2. When organic impurities are present, several new and important features of the i-V profile arise, depending on the nature and reactivity of the impurity; they are as follows: (i) the blocking of the H adsorption/desorption peaks due to adsorption on Pt metal sites; (ii) the blocking of the initial stages of Pt surface oxidation in the potential region 0.75 to 1.05 V; (iii) reactivity w i t h or o n the surface oxide a t higher potentials; (iv) the modification of the normally constant double-layer capacity in the potential range 0.35 to 0.75 V E H in a n anodic-going sweep. In addition to i-iv, (v) new electrochemical reaction currents may arise in the double-laytr charging potential region (0.35-0.75 V E H ) , and/or (vi) a t the commencement of surface oxidation of Pt a t 0.75-0.9 V E H . 1334
ANALYTICAL CHEMISTRY, VOL. 45, NO. 8, JULY 1973
In these cases, as also in i-iii, the currents which arise are diffusion controlled. They give rise to new peaks in the i-V profile which increase with the square root of sweep rate (22) and with stirring. The latter two properties of such peaks enable them to be specifically associated with impurities, since the peaks for surface oxide formation and reduction, and for H, are linear in sweep rate because only surface processes are involved. (vii) A different type of impurity arises in some cases and exhibits reactivity in the chemisorbed state (19, 20). Known examples are acetonitrile (19, ZO), other nitriles (19, 20), unsaturated hydrocarbons ( 4 ) , thiourea (19, 20), etc. In such cases, strong blocking of the H and OH electrochemical adsorption processes occurs and the impurity may, in some cases (19, 20), undergo reversible oxidation and reduction without desorption as the potential is cycled in anodic and cathodic repetitive sweeps. Typical electrochemical behavior of impure water is shown in Figures 3a and 3b; in Figure 3b, the result of cycling the electrode only over the H adsorption region is also shown, and appreciable blocking of H electrochemi-
B l o c k i n g of OH
Figure 6. Progressive removal of c h e m i s o r b e d impurities tial r a n g e (1.05 t o 1.4 V E H
at a
Impurily
Pt e l e c t r o d e in 1N aqueous H2S04 at 25 “C with c y c l i n g over t h e poten-
The number of cycles, from 1 to 100, is indicated on the series of i-V profiles. SweeD rate 50 mV sec-’
sorption arises within 5 min because the normal cleaning effect due to oxidation of the electrode surface beyond 0.8 V is not occurring. When impurity currents arise from a diffusion-controlled faradaic oxidation of trace substances in solution, the impurity effects on the i-V profile are: ( a ) dependent on sweep rate as shown in Figure 4 for 5 and 50 mV sec-1 (the higher sweep rate leads to relatively lower impurity diffusion current, since currents for the surface-oxidation process increase linearly with sweep rate s while diffusion currents increase only as s1 2 ) ; and ( b ) dependent on solution agitation as shown in Figure 5 , where curve 1 for unstirred solution shows relatively smaller impurity effects than curve 2 for a N2-stirred solution. A different type of impurity effect arises when the impurities are chemisorbed on the electrode surface. In such a case, progressive improvement in the z-V profile arises on cycling between +0.05 and 1.3 V as shown in Figure 6. This improvement usually corresponds to slow oxidation of the chemisorbed impurities in the Pt-oxide region (0.75 to 1.2 V ) . A good example of this type of behavior arises in ref (23) where S chemisorption was investigated a t Pt electrodes. When this occurs, the total charge Qa passed in the anodic-going sweep is greater than t h a t (Qc) in the cathodic-going one in the oxide-reduction region (Figure 6). This is because ( a ) adsorbed impurities usually block, to some extent, electrosorption of OH and 0 species a t Pt and ( b ) give extra charge arising from their own electrochemical oxidation above ca 0.95 V. Hence, Qa is initially greater than Qc but with progressive cleaning of the surQc and the difference, a t a clean electrode, is face Qa less than the experimental uncertainty of ca. 2% in mea-
-
(23) T Loucka, J Electroanal C h e m , 31, 319 (1971)
surement of Q values. (Only a t relatively high potentials a t clean electrodes does Qa become somewhat greater than Qc, due to incipient dissolution of Pt ( 2 4 ) ) .In various earlier works, Qa/Qc values appreciably greater than 1 have been quoted but this effect must be attributed to the presence of impurities. In some cases, after cycling t h a t removed chemisorbed impurities (Fig. 6), other impurities remained in solution t h a t gave rise to diffusion-controlled peaks or diffusioncontrolled modification of the i-V profile in the surfaceoxide region (as in Figures 3a, 4, and 5 ) . Figure 7 shows the exactly reproducible i-V profile which can be obtained after only a few cleaning cycles when the pyrodistilled water is examined. The i-V profile undergoes no further change with progressive cycling and the structure of it remains independent of stirring or sweep rate down to 5 mV sec-I, which is an exacting requirement since a t low sweep rates, diffusion-controlled trace impurity currents are always relatively more important in relation to the surface-oxidation/reduction currents. In all these experiments, the supporting electrolyte was 0.1 or 1N H2S04 made up from the best B.D.H. “Aristar” grade acid available. T h a t the source of impurities referred to in this paper was not the acid itself is indicated by ( a ) previous satisfactory experience with this source of H2S04 and ( b ) t h a t with the catalytic pyrodistilled water this acid shows no impurity effects dependent on acid concentration. The shape of this profile of Figure 7 can be used as a n absolute criterion of purity since it is always reached after various ultimate stages of purification and is identical (24) D. A . J. Rand and R. Woods, J. €/ectroana/. Chem., 35, 209 (1972). A N A L Y T I C A L CHEMISTRY, VOL. 45, NO. 8, J U L Y 1973
1335
1 N H,SO,
~n PYRO-CATALYTIC
75- 0
5'
50-
4
25-
I-
:
a
0-
(L
25-
50-
3
75-
100125
-
150 -
Figure 7. Potentiodynamic i-V
v
profile for Pt electrode in 1N aqueous H2S04, 25 "C,in the pyrodistilled water The sweeD rate = 50 mV sec-'
with profiles obtained in many experiments prior to 1971 when impurity problems started to become significant. Another exacting test of solution purity is whether the H coverage, measured by the integral of the i-V profile under the H peaks, remains constant with time (20) when the potential sweep range does not include the surface-oxide region, L e . , when the normal surface cleaning processes that arise by electrooxidation in this potential range (0.8-1.2 V) are not occurring. In water from alkaline KMn04 distillation after 1971, the H coverage suffered 30% diminution due to impurity adsorption already in a period of 2 min (cf. Figure 3b), indicating substantial concentrations of adsorbable impurities. In the pyrodistilled water, the H coverage measured by the integral under the i-V profile in the H region a t Pt remained constant to within 5% over sewral hours when the potential sweep range did not include the surface-oxide potential region. Also, this small extent of H blocking was little influenced by stirring. The conditions for satisfactory water purity, as indicated from studies of the oxidation behavior of Pt electrode surfaces, may now be summarized as follows: (i) maintenance for a t least 1 hour of surface coverage (or equivalent charge) of H and resolution of the H peaks when potentiodynamic sweeping is conducted over a restricted potential range, e.g., +0.05 to t-0.75 V, i , e . , not including the region where surface oxidation arises; (ii) cathodic and anodic charge balance to within 2% indicating ( 3 ) absence of blocking of the oxide layer and absence of oxidation reactions in the oxide formation potential region; (iii) absence of any diffusion-controlled, and hence sweep-rate dependent (22), peaks in the double-layer region (0.40 to 0.75 V) or over the surface-oxide formation region (>0.75 V), similarly, absence of any stirring effects on the shape of the anodic i-V profile; (iv) absence of any slant of the whole i-V profile about the zero-current base line due to faradaic oxidation processes in the anodic sweep and faradaic reduction processes in the cathodic one; (iv) maintenance of the shape of the i-V profile down to 5 mV sec-l without appearance of spurious peaks or blocking effects in the
1336
ANALYTICAL CHEMISTRY, VOL. 45, NO. 8, JULY 1973
H or surface oxidation potential regions; (v) resolution of the three distinguishable peaks ( 3 ) in the anodic surface oxidation i-Vprofile for Pt between 0.75 and 1.1V EH. Behavior at Hg. The behavior a t Hg is also a useful guide to the presence of organic impurities, but criteria as specific and exacting as those which can be formulated for the Pt electrode are more difficult to define. However, the presence of adsorbable organic impurities usually results in a sigmoidal shape of the log [current]-potential relation for cathodic Hz evolution a t Hg and there is usually hysteresis between the results for,cathodic- and anodic-going changes of potential due to irreversibility in the adsorption of the organic impurity. For neutral organic molecules, maximum adsorption tends to occur near the potential of zero charge (ca. -0.23 V EH in aqueous &So4 solution) and this influences kinetic results, particularly a t low-current densities in the case of the hydrogen-evolution reaction. These conclusions only apply in the absence of depolarizing impurities such as heavy-metal ions or 0 2 which can normally be removed previously by preelectrolysis as described in earlier work (6). An Optical Criterion for Purity. It has recently been found that bacterial contaminants with appreciable populations per milliliter can grow in high-purity deionized water (25) and in hospital-distilled water (26, 27). Particulate contaminants in these waters are readily detectable by observation of the Tyndall effect produced by directing the beam of a special illuminator, previously described ( 1 3 ) , through a sample of water. Normally, the concentration of light-scattering particulates in deionized and in distilled water increases after one or two days of storage. However, pyrodistilled water, prepared as described in this paper, was found to be stable in a borosilicate glass vessel, with respect to development of light-scattering impurities, when particulates in the distillate were initially low (13). In the present tests, water doubly distilled from alkaline permanganate gave a strong Tyndall beam; however, the product of catalytic pyrodistillation in trap T showed only a very faint Tyndall beam indicative of a pure, stable product. Presumably, when all steam-volatile impurities are removed by oxidation, development of bacterial contamination (25-27) is greatly retarded.
ACKNOWLEDGMENT The first three authors gratefully acknowledge support for work on electrochemical processes at P t - and Hg-electrode surfaces on Defence Research Board (Canada) Grant No. 5412-01 and on an Environment Canada contract. In the course of these two research projects, the preparation of pyrodistilled water became necessary, and its electrochemical characterization was investigated. We also acknowledge the assistance of J. C. Currie in setting up in our laboratory two further stills of the type described. Received for review October 20, 1972. Accepted January 29, 1973. (25) A. J . Bryce, "Identification of a Recurring Bacterial Contaminant in a Spacecraft Water System," presented at the 9th ATM. American Association for Contamination Control, 1970: also released as NASA-CR-73431. (26) M. S. Favero, L. A. Carson, W. W. Bond, and N. J. Petersen, Science, 173, 836 (1971). (27) M. S. Favero. Science, 175, 8 (1972).
