Determination of Phosphate by Cathodic Stripping Chronopotentiometry at a Copper Electrode G. L. Lundquist and J. A. Cox1 Department of Chemistry and Biochemistry, Southern lllinois University at Carbondale. Carbondale. lll. 62907
A method for the determination of phosphate based upon stripping of an electrochemically deposited cupric phosphate film has been developed. The film consists of a mixture of Cu(ll) salts of the conjugate bases of H3P04 with the H P 0 4 2 - salt being the major component in the analytical pH range. Linear working curves over single orders-of-magnitude down to 20 ppb phosphate have been obtained. The per cent standard deviation varies from 20% at 20 ppb to within 3% at concentrations above 1 ppm. The detection limit is 10 ppb when a 20-minute electrolysis is used. Chemical dissolution of the film in competition with electrochemical stripping and the appearance of a blank transition due to reduction of cupric ion in the reaction layer establish the detection limit.
Stripping analysis is well established as a method for the determination of trace quantities of certain metal ions in solution; however, application of the method to the determination of nonelectroactive anions has not been extensively reported. Such determinations can be made at an electrode which can be oxidized to an electroactive, sparingly soluble salt of the metal and the anion of interest. A reproducible portion of the anion is concentrated as a salt film a t the electrode by oxidative electrolysis. The salt is subsequently reduced by the application of a constant current or a negative linear potential scan; the coulombs required to reduce the salt are directly proportional to the anion concentration in the original solution if the rate of film formation is constant during the oxidative electrolysis and the salt is completely reduced during the stripping. Previous applications have been limited to determinations of the halides, sulfide, sulfate (after chemical reduction to sulfide), anions of certain organic acids, and oxyanions of molybdenum, vanadium, tungsten, and chromium; see the reviews by Brainina ( I ) and Barendrecht (2) for references. Only mercury and silver electrodes have been employed previously. The detection limits for the ions a t mercury range from 5 X 10-6M for chloride to 5 X 10-8M for sulfide when the oxidative electrolysis is at a potential where dissolution of the metal occurs exclusively by the mechanism of film formation. A t such electrolysis potentials, Roizenblat and Brainina have predicted that the detection limit of the method is primarily determined by the solubility product , n is the of the salt and is proportional to K S p l t nwhere sum of the stoichiometric coefficients in the balanced reaction for salt formation ( 3 ) . Their experimental results substantiate the general prediction. When the electrolysis potential is made more positive and free metal ion formation simultaneously occurs, the detection limit is lowered for ions which tend to form IAuthor to whom correspondence should be addressed. (1) K h . Z. Brainina, Talanta, 18, 513 (1971). (2) E. Barendrecht, "Electroanalytical Chemistry," A . J. Bard. E d . , Marcel Dekker, New York, N . Y . . 1967, Vol. 2, p 53. (3) E. M . Roizenblat and Kh. Z . Brainina, Zh. Anal. Khirn., 19, 681 (1964)
passivating films (3).Under these conditions, 3 X 10-9M chromate was determined at a mercury electrode. The oxidation of mercury to an insoluble phosphate salt has been studied by Lingane ( 4 ) , Christian and Purdy ( 5 ) , and Armstrong e t al. (6). Formation of an insoluble film on copper in the presence of phosphate has also been observed ( 7 ) ; however, a method for the determination of phosphate based upon either the anodic polarographic current or reduction of the salt films has not been reported. In the present study, copper was selected as the electrode because it was anticipated from solubility product data that it would be subject to fewer anionic interferences than mercury in applications to natural water systems. In addition, solid electrodes are somewhat more convenient for field studies.
EXPERIMENTAL Apparatus. The copper electrodes were constructed by heatsealing 22-gauge electrolytic grade copper wire into 6-mm o.d. soft glass using the procedure described by Wheeler (8). Immediately before use the electrodes were cleaned in a 59% (volume) HZS04. 7% "03, and 0.2% HC1 mixed acid bath (8) and activated by performing several deposition-stripping cycles in 0.1FP043- at p H 6.0. The reference was either a Coleman fiber junction saturated calomel electrode (SCE) or a platinum quasi-reference electrode ( 9 ) . The latter was employed for trace level studies. In either case, the reference was isolated from the test solution by a Luggin capillary salt bridge. All potentials are reported us. the S C E . The cell was a 30-ml coarse frit Buchner funnel fitted with a Teflon lid. A three-way stopcock allowed solution deaeration through the frit. maintaining nitrogen flow over the solution during measurements. and removal of solutions by applying suction to the frit. T h e back-pressure was sufficient to prevent leakage during the measurements. Linde Prepurified nitrogen saturated with water was used for deaeration. Magnetic stirring provided convection during the deposition and stripping. For trace level studies, a Metrohm polarographic cell was used because the absence of a glass frit facilitated cleaning. T h e instrumentation was the same as previously described ( I O ) except t h a t a switch was provided to allow a change between the potentiostatic and galvanostatic modes without allowing the indicator electrode to go to open circuit. Reagents. The chemicals used were Matheson, Coleman and Bell A.C.S. Reagent Grade. The Kr\;03 used as the supporting electrolyte was twice recrystallized from water. The water was either doubly distilled from basic permanganate or deionized water which was further purified by passing through an Illinois Water Treatment Company Research Model I1 mixed bed deionizing column. The phosphate solutions were prepared by dissolving KzHP04 adiustina to the aDorouriate D H with dilute " 0 s or K O H . ._. concentrations are reported either in gram formula weights of J J. Lingane, J . Electroana/. Chem.. 12, 173 (1966) G . D. Christian and W . C. Purdy, J . E/ectroana/. Chern.. 3, 363 (1962).
R . D. Armstrong, M . Fieischmann. and J. W . Oldfield, J . Electroan-
a/. Chem.. 14, 235 (1967). M . Sato, K . Matsui. Y . Kuroda. A . Sanbe, and H. Yamamoto, Yarnagata Dargaku Kiuo Kogaku. 10, 23 (1966); Chem Abstr.. 69, 112808a (1968). E. L. Wheeler, "Scientific Glassblowing, ' Interscience. New York, N . Y . , 1954 D. J. Fisher. W L. Belew, and M . T. Keiley. "Polarography 1964," G . H. Hiils, Ed.. Interscience, NewYork, N . Y . , 1966. Vol. 2. p 1043. J. A. Cox and T. E. Cummings. J. E/ectroana/. Chern.. 42, 153 ( 1 973).
~~
~~
~~
Table I. Variation of Stripping T r a n s i t i o n T i m e s w i t h pHfJ TJ
71
PH
4 4 6 7 7 8 9 10 til
5
ET
03 98 10 02 73 83 60 63
sec,
I
-0 12
2 6 5 5 1 0 0 0
v
0 0 22 26 27 16 8 6
2 0 5 3 2 0 0 0
I , , 500 p \ cm-" IC 50
E , I, -0 24 V
\ cm - 2
0 0 8 7 7 5 5 3
E+ 4 , -0 34 v
0 0 0 0 0 2 5 9
0 0 0 0 0 0 8 5
[HIPO~CI
IHP042-1
F (PO4 -1
FIPOF)
[PO43-] F(POa7-j
0.989 0.960 0.650 0.183 0.042 0,003 0.000 0.000
0.004 0.039 0 350 0,817 0.958 0.989 0,951 0.161
0 000 0 000 0.000 0.000 0 000 0.008 0 049 0 839
0 1F POI'
total phosphate per liter (F' P043-)or in p p m ( p p b ) of total phosphate.
RESULTS AND DISCUSSION Mechanistic Study. Cyclic chronopotentiograms obtained a t a copper electrode in stirred 0.1F Po43- as a function of p H are shown in Figure 1. As the p H is varied from 4 to 11, three distinct cathodic transitions can be identified. The variation of the transition times (TI, 72, and 7 3 ) , defined in Figure 1, with p H is shown in Table I, along with the relative equilibrium concentrations of the conjugate bases of H3P04 a t each pH. The correlation be, tween trends in the relative values of 71, 72, and ~ 3 and trends in relative concentration of HzP04-, H P 0 4 2 - , and Po43-, respectively, is evidence that the stripping processes correspond to the reduction of copper salts of these anions. Deviation from direct proportionality of these values probably results from equilibrium shifts in the reaction layer during the anodic half-cycle and errors in the concentration ratios calculated from the thermodynamic equilibrium constants and Debye-Huckel Limiting Law activity coefficients. Below p H 3.5, the absence of a cathodic transition positive of the hydrogen ion reduction indicates that no salt film is formed. Experiments above p H 11 were not attempted because of the appearance of a cathodic transition, presumably the reduction of Cu(OH)2, in the blank (0.1M K x 0 3 adjusted to the test pH with dilute HN03 or
I r
1
Time sec Figure 1. Effect of p H on the cyclic chronopotentiometry of a Cu electrode in phosphate A pH 4 03 6 pH 6 10 C pH 9 60 trl 5 sec l a , 500 P A c m - ' p A c m - * 0 1 F PO.,-
I , 50
KOH) .
The oxidation state of copper in the salt film was determined by constant current coulometry at a Cu-coated Pt electrode. Cu, 1.0 X l o - ' mole, was deposited onto Pt from 0.1M Cu2+ in dilute "Os. After transfer of the plated electrode to a 0.1F P043- solution in 0.1M K N 0 3 a t the test pH, an anodic current of 100 FA cm-2 was applied. Complete oxidation of the copper to the phosphate salt was accompanied by a positive potential transition. By relating the coulombs reguired for the oxidation to the moles of Cu plated onto the Pt, the number of electrons per mole, n, was found to be 2.0 f 0.1; the value was constant over the pH range 4-10 which demonstrates that the conjugate bases all form cupric salts. A control experiment in chloride medium yielded an n value of 1.0 f 0.05 for the known formation of CuC1. The above method for determining the oxidation state of the cation in a salt film would fail if the electrode became passivated prior to complete oxidation of the test metal. That the cupric phosphate salts form a noncontinuous, conductive film ( 1 1 ) is indicated by the effect of anodic electrolysis time. t , ~ ,on 71 and r2. As shown in Figure 2. r l and ~2 initially increase in a regular manner with t,l and then approach limiting values. Electrode passivation can be eliminated as the cause of ( 1 1 ) D. A . Vermilyea. Advar,. Eiectrochem Eiecrrochem € n g . , 3, 211 (1963).
1
2
;
;
Electrolysis Tlme,mir Figure 2. Effect of anodic electrolysis time on sition times of cupric phosphate la, 5 0 0 p A c f r 2 ; I,, 5OpA c m - 2 : 0.01F
the stripping tran-
Pod3--; pH, 6.0
the approach to limiting values since a potential transition at the Cu electrode does not occur during the film formation. Further evidence against passivation is that the calculated number of coulombs stripped in the limiting region of Figure 2, 0.15 coulomb a t 300 sec t,l, is less than the limiting quantity, 0.80 coulomb, in the limiting region of an identical series of experiments in 0.1F Po43-. Evidence that chemical dissolution of the film is the cause of the limiting behavior in Figure 2 was obtained by performing cyclic chronopotentiometry with a controlled delay time a t zero applied current incorporated between
A N A L Y T I C A L CHEMISTRY, VOL. 46, NO. 3, M A R C H 1974
361
Table 11. Effect of Delay Time on the Cupric P h o s p h a t e Stripping Transition Timesa til,
sec
0 .o 3.8 15 .O
45 .O 90 .0 120 .0 180 .o 360 . O
71,
sec
21.6 18.7 12.7 4.2 2.0 0.0 0 .o 0 .o
72,
sec
11.3 10.8 10.8 11.7 11.3 11.3 9.4 6.1
a t e l , 15 sec; I.,,500 PA cm-2; I , , 50 PA cm-2; 0.01F POIS-; pH, 6.0;t D , time a t I = 0 between tcl and application ofI,.
S
21
02
03
34
Time,sec Figure 3. Effect of the forward electrolysis time and the anodic current density on the cyclic chronopotentiornetry of Cu in phosphate A . l a , 50 p A c r 2 ; I C , 50 p A c m - 2 ; t r l , 0.09 sec; B. l a , 50 p A c w 2 ; I C , 50 p A cm-’; t e i , 0.21 sec; C. l a , 500 p A c r r 2 ; I C , 50 p A c m - * ; trl, 0.025 sec; D. I , , 500 p A cm-‘; IC, 50 p A cm-’; f,,, 0.050 sec. 0.01F po43- ; PH, 6.0
the anodic and cathodic half-cycles. The results are shown in Table 11. The data have two significant features. 71 systematically decreases as a function of delay time until at 120 sec, no electrochemical stripping of the H2P04- salt occurs when the cathodic current is applied. (The potential-time curve recorded a t i = 0 shows a transition at 120 sec corresponding in potential to 71.) Further, as long as some H2P04- salt remains on the electrode, 72 is constant; however, upon its depletion a t 120 sec, 72 begins to decrease. Apparently, chemical dissolution of the salt film occurs; the process is in competition with electrochemical dissolution in the typical experiment. That 72 does not decrease as long as the HzP04- salt is present indicates that the HP042- salt is more insoluble; dissolution of the latter may then be suppressed by the common ion generated as the HzP04- salt dissolves. The relative solubilities of the H P 0 4 2 - and H2P04salts were further investigated by cyclic chronopotentiometry. In Figure 3, the potential-time relationships at pH 6.0 for small values of tel are shown for I,, the anodic current density, equal to 50 and 500 pA cm-2. Whether one or two stripping transitions occur is clearly related to the magnitude of the positive potential excursion during deposition. When 50 PA cm-2 is applied, the potential remains negative of -0.1V for the initial 0.15 second of deposition, and only a single stripping step occurs. The single transition is at the potential of the stripping of the H P 0 4 2- salt (or 72 in Figure 1).A higher 1, or a longer t,, results in a potential excursion positive of -O.lV, and two stripping transitions occur. Thus, the H P 0 4 2- salt is deposited at a less positive potential. The same order of deposition is observed a t pH values where HzP04- is the predominate species; therefore, the dianion apparently forms the more insoluble cupric phosphate salt. Whereas the results in Figure 3 could also be explained by a large crystallization overpotential for the &PO4 - salt, only the relative solubility argument is consistent with both Figure 3 and Table 11. That chemical dissolution occurs upon removal of 1, and thereby causes the plots in Figure 2 to be nonlinear has important analytical consequences. The nonlinearity requires that working curves must be available for the exact electrolysis times used in the analytical determina362
tions rather than simply changing the slope to correct for differences in t,,. In addition, since the plot approaches a limiting value, the analytical readout, the total stripping transition time ( 7 = 71 + 72), cannot be indefinitely increased by lengthening t , ~ .Fortunately, the limiting value is sufficiently high to yield easily measured 7 values upon stripping and, therefore, does not restrict the development of the analytical method. Procedure and Application. Stripping analysis procedures based upon the above mechanism can be categorized according to the deposition mode employed, namely, A) constant current electrolysis, B) controlled potential electrolysis with simultaneous free metal ion generation, and C) controlled potential electrolysis with exclusive oxidation to the salt film. With each of the above, either constant current or linear potential scan can be used for the electrochemical dissolution; however, preliminary experiments with the latter resulted in ill-defined base lines, and the readout must be integrated to produce linear working curves ( 2 ) . In subsequent work, only constant current stripping was used. The primary requirement of a stripping analysis method based upon constant current deposition (mode A) is that the current efficiency must be less than unity. To fulfill this requirement Za must be large enough to cause simultaneous film formation and generation of the free cation. Otherwise. the analytical parameter, T , will be proportional to Za and tel rather than to the test ion concentration. In addition, the sensitivity of the method is found to increase as I, is increased. For example, with a 15-min deposition a t 50 p A cm-2 on a Cu electrode in 1 ppm Po43- a t pH 6.0, the resulting film is stripped in 1.2 sec when Z, the cathodic current density, is 50 pA cm-2. When Za is 500 pA cm-2, 7 is 24 sec. In more concentrated solution, the effect of Za is less marked. Apparently a large flux of Cu2+ is needed to obtain a supersaturated solution in the reaction layer when the phosphate concentration is low. Currents higher than 500 FA cm-2 were not employed because the increased rate of electrode dissolution resulted in irreproducible results. The accompanying area change would alter the current densities. In addition, the Cu2+ which is generated will consume a portion of I, and may also precipitate phosphate in the bulk solution. The optimum value of I , is determined by two factors. The total stripping transition time, which theoretically is inversely proportional to Z, must be an easily measured value. Also, Z, must be high enough to make the previously discussed chemical dissolution negligible. The appropriate value of I, is determined for each order-of-magnitude of phosphate concentration by selecting ICin a range such that the inverse proportionality with T occurs. Table I11 lists the typical values used in this study. The values in ‘Fable I11 were determined at p H 6.0, but the results are applicable over the p H range of 5-10. In
ANALYTICAL CHEMISTRY, VOL. 46, NO. 3, M A R C H 1974
-2 B
Table 111. Electrolysis Conditions for t h e D e t e r m i n a t i o n of Phosphate by Cathodic Stripping Chronopotentiometry at pH 6.0 POa3-, ppm
0.02-0.10 0.10-1 .o 1.o-10 .o 10-100 100-1000
IC,P A
I , , MA cm-2
500 500 500 500 500
cm-2
t e l , min
5 10 20 50 50
10.0 5 .O 2.0 1.0 0.5
c
i
Mid-range Blank r , 7 , sec See
3.0 3 .O 2.0 0.1 0.1
10 15 35 25 40
33
~
Table IV. Variation of Transition Time w i t h
4i
53
60
Time, sec
Phosphate Concentration POa3-F ppm
0.02 0.04 0.06 0.08 0.10 0.20
I,
4.1 6.5 9.7 12.2 15.8 29.8
secC
d= 0 . 8 f 0.9
+ 0.8
* 1.0 f1.2 0.9
&
POa3-? ppm
T, see
0.1 1.o 5 .O 10 .o 100 .o 500.0
3 .O 10.5 15 .O 19.8 35.5 49.8
a I , , 500 p A em-2; I,, 5 p A em-2; t,l, 10 min. I,, 500 p A em-2; ern-2; tpi,6 min. Standard deviations based on S trials.
IC,50 pA
-
fact, for concentrations of Po43- below 10-2F, accurate control of pH within this range is not important. For example, with 5 ppm Po4:’-, the T values at pH 6.15, 7.24, 8.39, and 9.78 are 30, 28, 27, and 28 sec, respectively, using the values in Table 111. Even at pH 5.00, the result is only 15% low. For the present analytical studies, the pH was adjusted to 6.0. Single order-of-magnitude working curves prepared using the conditions in Table I11 are linear and pass through the origin. Typical results are shown in Table IV. When a single set of conditions is used over a wider concentration range, the curve shows a positive deviation from linearity. The latter curve is useful only for determining the order-of-magnitude of phosphate. When tel is increased to 20 min, the detection limit of the stripping method is 10 ppb which compares favorably with the commonly used spectrophotometric procedure described by Murphy and Riley (12). Repeated trials on a single sample demonstrate the reproducibility of the method. For example, with 8.0 ppm Pod3- a typical set of T values is 49, 49, 50, 51, 51, 51, 52, 52, and 51 sec. The increase of 7 with trial number is probably a result of an increase in roughness factor of the electrode. Most determinations were performed by the Method of Standard Addition as it requires only linearity of readout us. concentration and consecutive trial precision. When deposition mode B, defined above, is used, the results are identical to those obtained with mode A as long as the potential is controlled at the same value as that established during the constant current electrolysis. When the quasi-reference electrode is used, exact potential control is not possible, and the results are less reproducible. In a set of experiments with 100 ppm Po43- the mean and standard deviation of T in 6 trials with mode A was 24 f 0.5 sec whereas with mode B, using the quasireference electrode, they were 22 f 2 sec. With deposition mode C, the electrolysis potential depends on phosphate concentration. To determine the electrolysis potential a positive linear potential scan of 0.1 V s e c - l is applied to a Cu electrode in the stirred test solution to the onset of electrode oxidation (arbitrarily taken where the current is 10 pA cm-2). When the phosphate (12) J Murphyand J P Riley,Ana/ Chim Acta. 27, 31 (1962).
Figure 4. Cathodic stripping chronopotentiometry of chloride and phosphate at a Cu electrode A . 1.5 pprn CI-; 6. 10 ppm P 0 4 3 - ; C. 1.5 ppm C I - , 10 ppm P 0 4 3 - . t e ~ , 30sec; la, 5 0 0 1 A c m - 2 ; I C . 2 0 p A c m - * : pH, 6.0
concentration is below 5 ppm, no evidence of film formation is observed, i.e. with Z, equal to 5 pA cm-2, both the blank and test solutions show T equal to 1 sec. Likewise, an investigation of C1- at Cu showed a detection limit of 70 ppm C1- by mode C US. 0.5 ppm by modes A and B. The former figure agrees well with the theoretical detection limit ( 3 ) . As previously mentioned, these authors demonstrated that simultaneous generation of the free cation of the electrode metal lowers the detection limit for cases where passivating films are formed, but the results for C1- and P043- a t a copper electrode demonstrate that the same effect is observed for certain conducting film systems. Mode C was not used in the applications because of the high detection limit. Four categories of interferences are anticipated in the application of the method: agents which form strong copper complexes, substances which are reduced in the same potential range as the copper phosphate salts, other anions which form insoluble copper salts, and surface active agents which can interfere by preventing salt formation. To determine the effect of complex formation, stripping analyses were run in the presence of varying concentrations of EDTA. In test solutions containing 5 ppm Pod3a t pH 6.0, the concentration of EDTA necessary to decrease T by 5% is l X 10-3M which indicates that interference by complex formation should not be significant in general. Interference by complex formation was important in the method development, however. When 0.1F citrate or acetate solutions were used as buffers, film formation was not observed. The use of more dilute buffer solutions was not attempted, since they would not eliminate the need to check the pH of the test solutions prior to analysis. Dissolved oxygen and Cu2+ generated during the anodic half-cycle interfere with the determination of Po43- as they are reduced in the same potential region as cupric phosphate. The reduction of 0 2 occurs at -0.35V in 0.1M K N 0 3 a t pH 6.0. This process, which limits the negative potential excursion in stirred solution when I , is below 25 pA cm-2, precludes an accurate measurement of T. Therefore, for determinations below 10 ppm Po43-, the solutions must be deaerated. When 1, is 5 PA cm-2, stripping experiments performed in stirred blank solution show a 3-sec transition at ca. -0.10 V. The transition is due to the reduction of Cu2+ present in the reaction layer since the transition time does not increase with tel. The blank transition is significant because it, along with the previously discussed chemical dissolution, establishes the detection limit of the method. ANALYTICAL C H E M I S T R Y , VOL. 46, NO. 3, M A R C H 1974
363
Over a limited concentration range, C1- interferes with the determination of phosphate by forming CuC1. When I, is 500 pA cm-2, a CuCl film is formed a t C1- concentrations greater than 0.5 ppm. A typical example is shown in Figure 4. The reduction of CuCl occurs a t a potential coincident with 71 for the phosphate stripping so the determination must be based upon 72; however, 72 is decreased from 15 to 13 sec in this mixture which contains 10 ppm P043-and 1.5 ppm C1-. The decrease in 72 is significant when the C1- concentration is greater than 0.5 ppm and exceeds 10% of the phosphate concentration. Apparently the presence of CuCl interferes with the nucleation and/or growth of the phosphate film (11). The stripping method was compared to the spectrophotometric procedure for the tietermination of phosphate in a set of five identical grab samples of lake water. The
samples were stabilized by addition of 1% (volume) chloroform and were filtered through 0.45-p Millipore membrane prior to use. The mean and standard deviation were 52 f 20 ppb by the electrochemical method and 58 f 15 ppb by spectrophotometry. The relatively high standard deviation of the former is probably the result of surface active agents in the samples. In the present state, the method is primarily useful for determinations in turbid or colored samples where the spectrophotometric procedure is subject to interference. Received for review August 22, 1973. Accepted November 5, 1973. This research was supported by the United States Department of the Interior Office of Water Resources Research Allotment Grant A-041-ILL.
Potentiometric Measurement of Copper in Seawater with Ion Selective Electrodes Raymond Jasinski, Isaac Trachtenberg, and Dmetro Andrychuk Texas Instruments Incorporated, P. 0. Box 5936, Dallas, Texas 75222
The data reported confirm the thesis that, with proper precautions in sample handling and measurement, the ionic cupric copper concentration of seawater can be monitored with ion-selective electrodes. The measured cupric ion content is identical to the total soluble cupric copper content: when the electrode response is calibrated directly in seawater, to account for the inorganic complex ion distribution; when organic chelating agents with high stability constants relative to cupric ion are absent; and when the proper Nernstian slope is used in calculation. At and below 1 ppb copper, account must be taken of the injection of soluble copper (or silver) into solution by the sensor electrode. Seawater samples from three different sources were studied. The analysis of an open ocean water was straightforward. However, analysis of the near shore waters showed abnormalities which can be explained by the presence of small quantities of chelating agents.
Recent advances in analytical electrochemistry have resulted in commercially available ion selective electrodes which are reasonably specific to some of the trace metal cations of interest in the characterization of seawaters ( I ) , and which have sensitivities comparable to the concentrations of these cations in seawater (nominally 10-aM) (2). Heretofore, it has not been considered possible to measure copper directly in seawater by this potentiometric technique (3, 4 ) . Considerable instability in the measured potentials as well as a general lack of sensitivity a t the parts-per-billion concentration level have been reported ( 4 ) . Recently, the use of such electrodes for the measure( 1 ) J. Ross, in "Ion Selective Electrodes," R. Durst, Ed., Chapter 2, Nat. Bur. Stand. ( U . S . ) Spec. Pubi. 314 (1969). (2) J. Riley and G. Skirrow, "Chemical Oceanography," Vol. 1 , Academic Press, New York. N . Y . , 1965, p 164. (3) T. Warner, Mar. Tech. SOC.,-6thAnn. Preprints, 2, 1495 (1970). (4) R. Durst, Paper No. 330, 142nd National Meeting, Electrochemical
Society, Miami Beach, Fla., October 1972.
364
ANALYTICAL CHEMISTRY, VOL. 46, NO. 3, MARCH 1974
ment of copper in natural waters has been described ( 5 ) . However the buffer system employed is not compatible with seawater, resulting in the loss of soluble cupric ion from the sample, presumably by coprecipitation with the alkaline earth fluorides. The data presented in the present paper imply that many of the problems reported for measurements of copper in seawater have been due to alteration of the cupric ion content of the sample by the containers (6) and, in some cases, by the electrodes themselves. This paper will show that, with the proper precautions, direct measurements of copper in seawater can be made with ion selective electrodes. In fact, it is possible thus to study the solution contaminating processes with these electrodes. An evaluation was then made of the response of the commercial cupric ion selective electrode in three different seawaters, in terms of intrinsic electrode sensitivity (limit-ofdetection); electrode dynamics ( e . g . , response time, stirring dependencies. electrode stability); naturally occurring chelating agents and possible interfering metal cations.
EXPERIMENTAL Orion Model 94-29 cupric ion selective electrodes were used in this study. Most as-received electrodes had to be equilibrated in seawater before potentials representative of the soluble cupric ion concentration could be obtained. The equilibration time period did vary with electrode. and in some cases was of the order of 24 hours. The Orion double junction Ag/AgCl electrode was used as reference with 0.1N K N 0 3 as the outer filling solution. Glass barreled. single junction electrodes gave rise to a slow, continuous increase in t h e potential measured by the copper electrode, presumably due t o t h e slow contamination of the test solution by soluble silver from the reference electrode. Potentials were readout to within 0.1 mV on a Corning Model 101 digital voltmeter. All measurements were made in polyethylene and Teflon beakers and bottles to minimize contamination of t h e seawater sample. As a precaution, all containers were first acid washed; on oc(5) M . Smith and S. Manahan. Ana/. Chem., 45,836 (1973). (6) J. Alexander and E, Corcoran, Limnoi. Oceanogr.. 12, 236 (1967).
