by comparison with other nitroso aromatic compounds in similar solvents (3) and comparison of the diffusion current constant with that of the corresponding nitro compound (vide infra). Comparison of reactions 2a and 4 with reactions 2a, 2b, and 3 shows the reason the polarographic diffusion current suffered such a small loss compared to the spectrophotometric measurement is that the indophenol product essentially replaces the nitroso compound as depolarizer. Willis (10) has measured the rate of condensation of 2,6-sylenol with the 4-nitroso-2,6sylenol by photometric means and found it to be 0.49 liters per mole per minute. When this value is used in connection with accurate current-time measurements the decrease in diffu,’ qion current with time can be accounted for solely by differences in diffusion coefficients between the two species. The diphenoquinone is probably polarographically reducible in these solvents although no polarographic wave
is obtained. Spectral measurements indicate that the solubility limit of the compound in these solvents is slightly less than the minimum detectable polarographic limit, 10-6M. I n addition, the quinone is unstable in these acidic solutions and is converted to the reduced form, the diphenol, at a rate approximately equal t o the rate of indophenol formation. At the rotating platinum electrode, the compound yields a wave with a half-wave potential of $0.8 volt vs. the Hg/Hg2S04 reference electrode. This potential is somewhat lower than that obtainable from potentiometric cell measurements. The potential of the diphenoquinone-diphenol syatem in 6:3:1 solvent a t platinum vs. Hg/Hg2S04 reference electrode is approximately +0.8 volt. The system at equilibrium favors the diphenol by a factor of more than 1000: 1 This large ratio coupled with the solubility limits of the two components rendered accurate measurement impossible (9).
LITERATURE CITED
(1) Asai, H . I., Thesis, University of
Illinois, Urbana, Ill., 1959. (2) Bayliss, N. S., Watts, D. W., Australian J. Chem. 9, 319 (1954). (3) Bergman, I., James, J. C., Trans. Faraday SOC.48, 966 (1952). (4)Ibid., 50, 60 (1954). (5) Clark, W. RT., “Oxidation Potentials of Organic Compounds,” Williams and Wilkins Co.. Baltimore (1960). (6) Gowenlock, B. G., Luttke, W., Quarf. Rev. 12, 321 (1958). (7) Hartley, A. M., Asai, R. I., ANAL. CHEM.35, 1207 (1963). (8)Hartlev, A. M., Curran, D. J., Ibid., 35, 686 71963). ‘ (9) Hartley, A. RI., Meerman, G., University of Illinois, Urbana, Ill., unpublished results. (10) .Willis, P. J., Thesis, University of Illinois, Urbana, Ill., 1961. RECEIVEDfor review May 6, 1963. Accepted August 8, 1963. Work supported in part by National Institutes of Health Grant Nos. RG-6495 and AP-97. R. M. Bly also held a National Science Foundation Summer Fellowship for the year 1960-1.
The Tubular Platinum Electrode W. J. BLAEDEL, C. L. OLSON,l and L. R. SHARMA2 Chemistry Department, University of Wisconsin, Madison, Wis.
b The tubular platinum electrode offers considerable promise as an analytical tool, because it permits electrochemical measuremenis to be made on a solution that flows through it. The construction and properiies of the TPE are described. It is easy to fabricate and to use. It is stable, has a well defined geometry, and gives reproducible measurements. Solution holdup is low (2-10 p.). Sensitivity i s high, electroactive substances being detectable at concentrations below 10-8M. The dependence of current upon electrode parameters is derived theorerically and confirmed experimentally.
I
E x p A s D i x G area of continuous analysis, there is an increasing need for -en>ors that will operate continuously in floning streams. .I tubular platinum electrode (TPE) through which the solution flow+ offer,. low solution holdup and high sen-itiFity, and thu. appears suited for continuous elec-
s mi:
1 Present address, College of Pharmacy, Ohio State University, Columbus, Ohio. 2 Visiting the United States as a U.S.A.I.D. (Technical Mission) participant, on leave from the Department of Chemical Engineering and Technology, Punjab Vniversity, India.
2100
ANALYTICAL CHEMISTRY
trochemical measurements in flowing streams. The TPE also has promise as a tool in hydrodynamic voltammetry. To date, the best defined methods for performing voltammetric measurements in hydrodynamic systems are based on the rotating wire electrode (5, 4), the rotating disk electrode ( 5 ) , and a conical electrode placed in a flowing stream (2). Other hydrodynamic electrodes include a “bypass” electrode (a platinum wire penetrating to the interior wall of a 4-mm. i.d. tube, with an area of 0.3 sq. mm.) ( 6 ) ,and a “string” electrode (a rigidly held segment of platinum wire, 0.2 em. long and 0.02 mm. in diameter, rotated about a center which lies 12 em. from the wire and on its axis) ( I ) . This paper presents the theory and electrochemical properties of the TPE.
Tio is the maximum linear (Le., axial) velocity of the stream flowing through the tube. If C is e1ec)troactiveand if the applied potential is in the diffusion controlled region, the electrolysis current is given by I = n F J , or
I = 2.01 nF
?r
CD3’3”‘8X2’Vd’’
(2)
where n is the number of electrons involved in the electrode reaction and F is the Faraday. For purposes of measurement, it is convenient to use a volume flow rate, V f which may be expressed in terms of R and V 0 . The velocity profile for a laminar stream flowing through a tube is of parabolic shape, where the linear velocity V at any point r from the center axis of the tube is
THEORY
Levich (5) has given a theoretical analy& of convective diffusion to the surface of a tube where the total flux, J , can be expressed as J = 2.01 7r CD2’3R*!3X2‘3V’ol’3 (1) when conditions of laminar flow exist. Here C is the bulk concentration of substance diffusing to the surface, D is its diffusion coefficient, R is the inside radius of the tube, X is its length, and
(3) The volume flow rate may be found by a -imple integration.
PLATINUM CYLINDER
wTUBE, INLET 10/2 B A L L JOINT
Figure 1,
OUTLET TUBE, 6mm. I.D.
Tubular plcrtinum electrode
Equations 2 and 5 yield I as a function
of
v,:
( E = 0.015 inch, S = 1.006 inches). Below a flow rate of 10 ml. per minute, the plots are good straight’ lines, with slopes ranging from 0.32 to 0.35 (Table 11). The median slope is 0.335, giving almost fortuitously good agreement with the theoretical value of 3, according to Equation 6. At about 10 ml. per minute, a break appears in the plots. and the slopes increase to 0.47--0.48 above 10 nil. per minute. I t should be noted that the line segment3 are definitely straight, indicating an abrupt tranaition at about 10 ml. per minute. I t is probable bhat
I = 5.24 X I o j nCD’ 3X2’3V/”3 (6)
Experimental verification of the current dependence upon the parameters expressed in Equation (5 is given in the folloTring paragraphs.
40,
1
I
I
l
l
I
I
CURRENT MEASLIREMENTS
The oxidation of KtFe(CN)6 in 1M ICCI m s used for all current measurements. X stock solution of 1O-zM KiFe(CN)s was prepared by dissolving reagent grade I&Fe(CY)6 in deaerated 1 J I KCl. All other solutions were made by diluting aliquots of the stock solution with deaerated 1X KCl. Gravity feed a t constant :lead provided a constant flow of solution through the TI’E. The outlet of .:he T P E dipped into a small beaker to which a saturated calomel electrode was bridged. The f l o ~rate, Vf,mas obtained by measuring the time for th’: overflow of 25 inl. of solution from the beaker. Flow rate measurements were reproducible t o about 1%. -4 ccnstant’ potential of +0.75 volt 2’s. SCE. well out on the diffusion plateau for the oxidation of IGE‘e(C?;)6, mas applied to the TPE. Currents were measurtd with a Sargent Model XXI polarog-aph. All work was done a t room temperature, with no attempts a t temperz>turecontrol. Dependence of C u r r e n t upon Flow Rate. Figure 2 is a log-log plot of current z‘s. flow rate a t six different
concentrations of %l?e(CK)6 ranging from to lO-cM, for electrode I1
Radius ( E ) , Length (X), inches inches
Electrode
0,020 0.015 0.010 0.010 0 010 0.010
I
I1 I11 Is’ T.
T’I
Slopes of the Log /-Log V , Plots of Figure 2 Electrode 11, R = 0.015 inch, X = 1.006
inches) Slope ml /min. Below 10 Above 10
&Fe( FV), molarity 10-5
I1 I
Figure 2.
I
I
I
l
l
4 IO Vf, ML/MIN
1
I
40
Dependence of current upon flow rate
Electrode If, R = 0.015 inch, X = 1.006 inches
this transition represents a change from laminar to turbulent f l o ~in the TPE. For flow rates of 10 ml. per minute, in Figure 2, Reynolds numbers are 200-300, far bel017 the critical values of 2000-7000 that are required for turbulent flow in smooth tubes However, the edges of the TPE may be quite irregular, and it iq known that turbulence appears a t protrusions for Reynolds numbers as low a. 20-50. At such low Reynolds numbers, the turbulence is local, being rapidly damped out within short distances past the protrusion, and the flon- in the T P E may still be largely laminar, accompanied by small turbulent end effects. At intermediate Reynolds numbers (100-200), the local eddies may break away from the protrusions. and may be propagated donnbtream, imparting turbulence for large diztances beyond the protrusion ( 5 ) . The Reynolds numbers a t about 10 ml. per minute are just in this intermediate region, ‘ 0 it seems probable that the breaks in the curve3 of Figure 2 repreyent a transition from laminar to turbulent flow Trro observations are in wpport of thi: hypothe&. Khen a kinked platinum wire obstacle was placed close to the inlet end of the TPE, the break in the log I-log V , plot came a t a lom-er flow rate. Electrodes that were not smoothly constructed showed higher slopes, ranging up to 0.40 instead of the theoretical ‘/3.
Much more data similar t o that in Figure 2 was obtained for the other
1.006 1.006 1,006 0,506 0.266 0.104
Table II.
2-X 4 x 5 x 7 X io X
DESIGN AND CONSTRUClION OF THE TPE
Figure 1 gives t’hegeneral form of the TPE. Platinum cylinders were cut from seamless platinum tubing of various diameters (10-mil wall thickness), the ends being finished squarely and bmoothly. In sealing the electrode into the soft glass tubing, care was taken to keep the inside of the assembly smooth, and not to have bumps or twists a t the glass-to-platinum interfaces; but these efforts were not successful in all cases. Several electrodes with the dimensions shown in Table I were constructed. To lend strength, the inlet and outlet tubes were bridged with a 6-mm. glass rod for all electrodes except the shortest. :?or the shortest electrode, the platinum tube was completely embedded in .the glass, electrical contact being rrlade by a platinum leadout wire wrapped around the electrode before sealing,
Table 1. Dimensions of Tubular Platinum Electrodes
10-5 10-5 10-5 10-5
Median
0 32 0.34 0.35 0.34 0.32 0.33 0.335
0.48 0.48 0.47 0.47 0.47 0.47 0.473
electrodes of Table I . Someqof these data are shown in Figure 4. In all cases, the slopes of the log I-log V , plots a t low flow rates were straight lines with slopes ranging from 0.32 to 0.40. -4 definite correlation between the smoothness of the bore and the value of the slope was observed; electrodes whose inlet tubes were smooth in the region of the platinum gave slopes approaching while roughly constructed electrodes gave higher slopes. Dependence of C u r r e n t upon Concentration. The data of Figure 2 are used a t three flow rates (2.5, 5, and 8 ml. per minute) t o give the I os. C plots shown in Figure 3. Examination of Figure 3 reveals two favorable analytical potentialities of the TPE. If other parameters are con-
201,
I
I
1
I
!
I
I
I
\
t 15
c, MOLARITY x 105 Figure 3.
Dependence of current upon concentration Data from Figure 2
VOL. 35, NO. 13, DECEMBER 1963
2101
40---y;Q I
R 0010''
I I Figure 4.
4 IO Vf, ML/MIN
40
Dependence of current upon electrode length Electrodes 111, IV, V, VI
trolled, the relationship beheen I and C is linear and reproducible, which means that I may be used as a very convenient measure of C. Sccording to Figure 3. a 10-511 &FelCN)& soIutiGn gives a current of 1 t o 2 pa'.; a t moderate flow rates. This current does not show the fluctuations that a dropping mercury electrode or a rotating platinum electrode does. Fluctuations in the T P E current appeared to be much less than ITG, which means that concentrations below 10-'Ji should be detectable (vide infra). It is of interest that I depends linearly upon C also in the region of high flow rates, above 10 ml. per minute. Dependence cf Current upon Lergth. Figure 4 is a series of log I log V , plots similar to thoje of Figure 2, but for a set of electrodes of differing lengths. Data mere taken from Figure 4 at two flow rates (2.5 and 5 ml. per minute) and plotted on a log-log basis in Figure 5 to show the effect on current of length of the electrode. The log I-log X plots of Figure 5 are straight lines with slopes of 0.61, in reasonably good agreement with the theoretical ialue of 2;3, according t o Equation 6. Since the exponent of X must be obtained from data from different electrodes, the precision can-
40r
I
i
I .I Figure 5.
.4 I.o X, INCHES
Dependence of current upon electrode lenglh Data from Figure 4
2102
4 .O
ANALYTICAL CHEMISTRY
not be expected t o be as good as the precision with which the exponent of V , is determined (Figure 2 ) , where all data were taken for a single electrode. Dependence of Current upon Radius. Figure 6 is an I t's. C plot for electrodes I, 11, and 111,of radii 0 010, 0.015, and 0.020 inch, respectively. I t may be seen that data for all three electrodes fall upon the same straight line, showing that I is independent of the electrode radius, in accord with Equation 6. Calculation of D. According to Cquation 6 the slope of an I us. C plot permits a calculation of D. The three plots in Figure 3 give 0.75, 0.73, and 0.73 X sq. em. per second for flow rates of 2.5, 5, and 8 ml. per minute. The plot of Figure sq. em. per second, 6 gives 0.70 X which may be regarded as an average of the data from three 1-inch electrodes of different radii. These values are very consistent, indicating that D is independent of flow rate. The median value of 0.73 X sq. cm. per second is in good agreement with von Stackelberg's value of 0.63 X sq. cm. per second ( 7 ) , indicating that the conditions under which Equation 6 holds may be approached fairly closely with good electrodes. It would be expected that values of D calculated from TPE data would be high; any turbulence would bring electroactive material to the electrode faster than accounted for by Equation 6, resulting in higher currents and higher apparent values of D. Actually, D-values can also be calculated from data other than that in Figures 3 and 6. Values of D calculated from Figure 5 fall a t about 0.9 x 10-5 sq. em. per second, which is in line with the greater turbulence that is suspected t o occur in the shorter electrodes used to obtain the data of Figure 5 . Proof of High Sensitivity. The high sensitivity of the T P E was shown bf preparing a "blank solution" containing 0.10Jf KC1 and 0.1OM Na2HP04, neutralized to pH 7.5 with NaH2P04. A portion of the blank solution was then made 10-3LV in K3Fe(C;c')6. Two 100-fold dilutions and a twofold dilution of the 10-311f KaFe(ChT)6n-ith the blank solution gave a ferricyanide solution that had the same composition as the blank, but in addition contained nominally 5 X 10-8111K3Fe(CN)6. The blank was pumped with a peristaltic pump (Model 500-1200, Harvard Apparatus Co., Dover, Mass.) through the TPE (0.040-inch i.d., and '/z inch long) from a two-way stopcock that permitted alternation between the blank and the 5 X 10-8M ferricyanide. A potential of 80 mv. was applied across the T P E (cathodic) and a saturated calomel electrode. The current through the T P E passed through ~t 10,000-ohm resistor, and the voltage drop was measured with a microvoltammeter (Model 425A, Hewlett-Packard Co., Palo Alto, Calif.) and recorded (hIodel G-10, Varian Associates, Palo Alto, Calif.). Figure 7 is a record of the response
2oF' '
I
l
l
3
t
A i
c, Figure
i
/
t-
MOLARITY x 105
6. Nondependence of current upon electrode radius Electrodes I, II, 111
when the solution is switched from blank to ferricyanide and back to blank again. The residual current n-as 0.012 l a , too large to balance out with the recorder zero adjustmeut, so a bucking voltage was applied to the recorder to brine: the blank solution resuonse onto the chart. The stabilitv of the steadv-state currents is such "that concentrations well below IO-*-lf K3Fe(CS)6 should be easily detectable. It m-as not possible to dilute 16Fe(CN)8 solutions. Attempts to do so led to loss of the ferrocyanide a t dilutions a t about 10-6V, probably owing to air oxidation. The KCI-Sa2HP0, buffer solution was chosen as a supporting electrolyte because of prior experience with the stability of KSFe(CN)e in this medium. CONCLUSIONS
A principal advantage that accrues to the use of the TPE in a flowing solution is high sensitivity, since transport of
FE D
m
Figure 7.
Response of TPE to 5 ZX 10-*M KaFe(CN)B
Steady-state current with blank solution 5 X 1 O-8M ferricyanide introduced C. 5 X 1 0 - 8 M ferricyande reaches electrode D. Steady-state current with 5 X 1 O-SM ferricyanide E. Blank solution introduced F. Blank solution reaches electrode G. Steady-state current with blank solution TPE, 0.040-inch id., and '/e inch long; residual current, 0.01 2 pa.; flow rate, 8.4 ml./min. A.
B.
electroactive material t 3 the electrode is aided greatly by convection. h typical TPE can detect concentrations of electroactive substances below 10‘8M in streams of moderate I elocity. Other advantages are simplicity of construction and reproducibility of measurements, under turbulent as well as laminar flom conditions. For continuous analvsis in flowing streams. the low holdup volume (2-1OupI.) may ’prove to be a great advantage. Xt applied potentials .A7ellout into the diffusion limiting regia?, experimental curreiitb obtained with the TPE are in
agreement with theory. N o measurements have been made in the potential limiting region, where the dependence of current upon electrode parameters is not yet clear. ACKNOWLEDGMENT
Thanks are extended to W. J. Wheeler, for special instructions in preparation of the electrodes. LITERATURE CITED
(1) Jordan, J., ANAL. CHEM.27, 1708 (1955). ( 2 ) Jordan, J., Javick, R. A,, Ranz, W. E., J . Am. Chem. SOC.80,3846 (1958).
(3) Kolthoff, 1. M., Jordan, J., Ibd.9 76, 3843 (1954). (4) Laitinen, H. A., Kolthoff, I. M., J. Phys. Chem. 45, 1079 (1941). (5) Levich, V. G., “Physicochemical
Hydrodynamics,” Prentice-Hall, Englewood Cliffs, iY.J., 1962. (6) Muller, 0. H., J . Am. Chem. SOC.69, 2992 (1947). (7) Von Stackelberg, M., Pilgram, M. Toome, V., 2. Elektrochem. 57, 342 (1953).
RECEIVEDfor review May 22, 1963. Accepted August 27, 1963. The Partial support of this work by grant No. AT (11-1)-1082, from the Atomic Energy Commission, is gratefully acknowledged.
Polarography in Fused Alkali Metaphosphates ROY D. CATONl and HARRY FREUND Department of Chemistry, Oregon State University, Corvallis, Ore.
b Polarograms were obtained with a cell consisting of a platinum rnicroelectrode inserted in a melt contained in a platinum crucible. No reference electrode was employed; the platinum crucible served a:, a massive and nonpolarizable anode. Electrolyses were carried out in fused Nap03 at 750” C., or LiP03-Na1’03 at 730’ C. in which a potential span of 0.95 volt was available between the solvent decomposition processes. Twenty oxides and compounds were studied. UaOs, CuO, FeO, Fez03, and V Z O ~ gave redox waves. Silver was the only species that could be reduced to the metal. These data are interpreted with respect to probable electrode reactions.
F
alkali metaphosphates are well known for their ability to dissolve metal oxides; moreover, they have been used from lime to time to “open up” complex minerals to make them water-soluble. The sodium metaphosphate bead test hE,s been used for some time as a means of qualitative analysis in determinative mineralogy, since the fused salt enters into chemical combination with many metal oxides to give characteristic colcrs ( 1 4 ) . Little is known concerning the nature of the species present when metal oxides are dissolved in such sol vents, although some work is now being done by Soviet electrochemists. Andrseva (1) determined the decomposition potentials of a series of metal oxides dssolved in fused sodium metaphosphate and fused Present address, Depmtment of Chemistry, University of New Mexico, Albuquerque, N. >I. USED
sodium pyrophosphate a t 1000” C., and Delimarskii and Andreeva (3, 4) used sodium metaphosphate as a solvent in studies of galvanic concentration cells. DelimarskiI and Kaptsova (6) conducted a polarographic study of solutions of titanium dioxide in molten sodium metaphosphate and found that a two-step reduction wave was obtained. The two steps were ascribed to reduction of titanium(1V) to titanium(II1) and thence to the metal. Most work on decomposition potentials involved relatively concentrated solutions of the oxides, however, and no discussion or evidence of the formation of intermediate oxidation states was given by the authors. Preliminary experiments in this laboratory indicated that intermediate oxidation states did exist, and that the electrode reactions of metallic ions in more dilute solutions did not necessarily involve simple deposition of the metal. The chemistry of alkali metaphosphates is complicated by the fact that the compounds are polymerized in varying degrees (8). The molecular formulas of most of the metal metaphosphates have not been determined; therefore, only empirical formulas are used throughout the discussion in this section. The metal oxide, when dissolved in a metaphosphate melt, probably undergoes one of the following types of reactions:
+ XaPO, SaM2POr (1) ?VI0 + Sapol SahlPOI (2) MrOs + 3 KaPO, 2 MPO4 + Na3P04 (3) MOI + 6 NaPO, hi20
-+
--t
4
-+
2 NasPO4
+ M(PO&
(4)
Other equations could be written for oxides having different formulas. The reactions are oversimplified, since various complex ions are probably formed. Van Wazer provides ample evidence for the complexity of phosphate systems (16). Many reactions are acid-base in nature, as exemplified by the reaction of sulfates in the melt: MS04
+ NaP03
-+
+
X ~ ~ ~ P SO8 O I (5) The techniques of polarography in molten salts have been amply described and reviewed by several workers (7, 10, 1.2) and are not discussed here unless deviations from established practices were made. The objective of the present study is an evaluation of the use of fused alkali metaphosphates as a solvent in which to conduct electrochemical studies. The polarographic work was undertaken to lay the groundwork for future e.m.f. studies and possible coulometric determination of metals dissolved in the melt. EXPERIMENTAL
Microelectrodes. Platinum microelectrodes used to obtain the polarograms were constructed by sealing 30-gauge wire in Supremax glass tubing. This tubing is manufactured by the Jenaer Glaswerk Schott of Mainz, West Germany, and was obtained from the Fish-Schurman Corp., New Rochelle, N. Y. It appears t o be nearly identical to Corning No. 1720 glass, an aluminosilicate glass used for ignition tubing. Good seals between the glass and the platinum wire were obtained, probably because of the small diameter of the wire used. Two types of electrodes were used, one consisting of a small tip of straight VOL 35, NO. 13,
DECEMBER 1963
2103
