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, (1954). (4)3843 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 O SO8I (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
wire protruding from the glass, and the other a small bead of platinum on the end of a 2-mm. length exposed portion of wire. Electrode areas were measured with the use of a microscope and calibrated micrometer eyepiece. Cell. The polarographic cell consisted of a platinum microelectrode inserted into the melt which was contained in a platinum crucible. S o reference electrode was employed; the platinum crucible served as a massive and nonpolarizable anode (6), connection to the outside of the cell being made with a piece of 18-gauge platinum wire. The crucible was placed in the bottom of a Vycor container measuring 67 X 260 mm. and was held in place with a ring of ceramic wool. The Vycor container extended about 3 inches above the furnace and terminated with a flat-ground flange. The cover was a borosilicate glass head with three standard-taper joints through which a thermocouple, microelectrode, and platinum lead were placed. One of the joints wa3 fitted with a gas inlet tube, through a hich an argon flow was maintained to provide an inert atmosphere in the cell. Since all experiments were conducted a t temperatures in excess of 700" C. it was necessary to cool the upper portion and cover of the container with two jets of compressed air. Kel-F No. 90 lubricant was used in all glass-ground joints. Furnaces and Temperature Control. A multiple-unit electrical furnace, 660 watts, manufactured by the Electrical Heating iipparatus Co., was used for all the work, with the exception of the melt purification. Thp furnace was modified by installing a n auxiliary heating coil in the bottom of the cavity. This coil accounted for 10% of the total heating capacity and was regulated by the temperature controller. Voltage to the main coils was adjusted with a variable transformer such that the furnace operated about 10" C. below the desired temperature. The auxiliary coil was operated through another variable transformer which was connected to the controller. This arrangement allowed the temp;rature to be controlled to within +l C. h stainless steel beaker was placed in the cavity to protect the heating cell in the event the cell broke. The beaker was grounded to a water pipe to remove the possibility of any induced voltages being formed within the cell by the furnace coils. Temperature control was provided by a Minneapolis Honeywell potentiometer pyrometer, Model S o . 156C16PS-21, from which the temperature could be read directly. A Chromel-Alumel thermocouple was used as the sensing element for the controller as well as for measuring the temperature. Miscellaneous Equipment. Polarograms were recorded using a Sargent Model 21 polarograph. Scan rates of 50 to 75 mv. per minute were employed. Applied potentials were monitored occasionally with a Gray hlodel E-3042 potentiometer. Reagents. Potassium and sodium metaphosphates were prepared by 2104
ANALYTICAL CHEMISTRY
I
I
4- 1c. 1
I
I
I
-0.2
I
I
Applied
I
I
- 0.6
-06
-0.4 Polentlol, volts
Figure 1 . Analysis of polarograms for uranium(V1) in fused alkali metaphosphates
fusing the reagent grade alkali dihydrogen phosphates in platinum dishes and heating a t 950" C. for 4 hours. Lithium metaphosphate was prepared by adding the stoichiometric amount of phosphoric acid to lithium carbonate, evaporating to dryness, and fusing in a platinum dish a t a temperature of 950' C. Anhydrous silver metaphosphate was prepared by precipitating the salt from a solution of lithium metaphosphate, filtering, washing, and fusing in a platinum dish a t 950' C. All metal oxides and salts added to the phosphate melts were of reagent grade quality. Argon was passed through copper wire and titanium sponge heated a t 600' C. to remove any oxygen present. The gas was then dried by passing through columns of magnesium perchlorate. Purification of Solvents. Basic impurities were removed by adding a few drops of phosphoric acid and igniting the melts a t temperatures high enough to drive off water and phosphorus pentoxide. The LiP03-KP03 eutectic (64 mole 01,-36 mole yo), having a melting point of 518' C. (g), was used for a solvent in some of the work. The mixture was prepared by melting the required portions in a platinum dish, igniting a t 950' C. for a few hours, and pouring out onto a polished nickel slab to solidify. The product was then crushed and stored in screwcapped bottles. Sodium and lithium metaphosphates form glasses when melted, and if cooled rapidly they do not crystallize; hence the melts are viscous, even a t temperatures several hundred degrees above their so-called melting points. PROCEDURE
Initial work was done using sodium metaphosphate a t 750" C., but the LiP03-KPOs eutectic described previously was used at 730" C. during most of the work.
Weighed amounts of the metal oxides or salts were added to weighed amounts of solid solvent and the mixtures fused in a platinum crucible until all the oxide or salt had dissolved. The crucible, with its contents, was then placed in the Vycor container and polarograms were taken a t a microelectrode. A weight dilution method was used in some of the work. A prepared solution was poured out onto a polished stainless steel slab in the form of beads which were then weighed and added to the pure solvent as needed. The microelectrodes were inserted until the glass tip just touched the surface of the melt. At the high temperatures employed some conduction of the glass itself was observed, but such errors were minimized by not inserting the glass into the bulk of the melt. If too rapid scanning rates were used peaks were observed for the reduction of metal ions in the melt; consequently, slow scan rates were employed. Scan rates of 50 mv. per minute were used during most of the work. Some of the work was done in an atmosphere of air, but an inert atmosphere of argon was employed whenever easily oxidized species were being determined. RESULTS AND DISCUSSION
Limiting Electrode Processes of Melt. The two melts had a potential span of 0.95 st 0.01 volt between the solvent decomposition processes. When the melts were electrolyzed with two platinum electrodes, the electrode reaction a t the anode appeared t o be 2 POa-
O2
+ PZOS+ 2e-
(6)
Continuous evolution of gas occurred a t current densities above 5 ma. per sq. cm. Identical results were obtained with graphite anodes. The cathodic process was complicated and appeared t o involve the reaction 4 POI5 eP 3 PO4-3 (7)
+
+
Reactions yielding products such as phosphite and phosphide ions could
Table I. Relationship between Limiting Current and Concentration of Uranium(V1)
Concn., C, mg. U/gram
id
(corrected for residual)
NaPOa 2.38 5.02 7.41 9.91 12.97 15 50
/ Iu -02
2.56 2.63 2.56 2.55 2.66 2.65 Av. 2.61 Rel. std. dev. f 2 37,
-1.0 I
I
-0 4
I
I
-0 6
1
-0 8
1
-IO
Applied Potentlo I volts
Figure 2.
id)C
6.1 13.2 19.0 25.3 34.6 41 6
Analysis of polarograms for copper in fused alkali metaphosphates
occur, but the only one observed was the evolution of phosphorus, which ignited spontaneously in the air. The white fumes obtained during the burning of the gas bubbhhs evohed at the cathode were collected, dissolved in water, and gave a positive t e 4 for phosphoric acid DelinarskiI and Kaptbova stated that the cathodic reaction was the deposition of alkali metal, but this was not observec, even a t current densities as high as 260 ma per sq. em Zinc metal reacted with the melts to produce phosphorus ,tnd a solution of the zinc salt. Oth1:r active metals cyhibited the same behavior, including aluminum, iron, and nickel Platinum cathodes were severe y attacked when phosphorus was evolved, but no discoloration or corrosion was noted during the polarographic work. This could be due to the formation of other products a t lower current densities I n the following discussions the reported half-wave potentials of redox \ystems were taken from the cathodic current-voltage curves, obtained in solutions containing the metal in its highest oxidation state only. When securing anodic or anodic-cat ?odic curves, the 1 oltage scan was started at a n initial liotential mfficiently positive to obtain thc anodic decomposition of the solvent a t the microelectrode Thib was neces.ai y since the current-voltage curve, .tarting at zero voltc, applied between tn o platinum electrodes, will originate on the wave itself when both species of a reversible redox couple are present. Thus, for the anod c and composite curves shown in Figures 2, 3, and 4, the point at which the solvent anodic curve originated mas taken as an arbitrary “zero applied volts” in order to make the curves and their half-wave potentials directly comparable to those obtained in other solutions. This procedure proT ided reproducible results in the ab>ence of a true reference electrode.
The assumption that the platinum crucible served as a nonpolarizable electrode was justified by the fact that its surface area was a t least 1000 times that of the microelectrode. Uranium. Keighed portions of U308were added to both metaphosphate melts and produced a well defined wave which was partially anodic in character. The solidified solution was emerald green, indicating the presence of uranium(IV), hut the characteristic yellow-green color of the uranyl species Tvas produced when the melts were heated in the open air and then solidified. The solid exhibited the typical fluorescence of uranyl salts when exposed to ultraviolet light. I n another experiment addition of uranyl nitrate to the metaphosphate melts produced reduction waves which were completely cathodic. The nitrate appeared to decompose immediately upon addition. Typical curves are shown in Figure 1. The half-wave potential is -0.3 volt. i Plots of applied potential us. log -. ad - 2
I
- 0.2
U(V1)
+ 2 e-
e U(1V)
(8)
The limiting current was proportional to concentration. Table I s h o w the relationship of diffusion current as a function of concentration, using a microelectrode of 3.0 sq. mm. area. The reproducibility of the data is better than that usually obtained in fused salts; however, the high viscosity of the melt undoubtedly minimizes the usual large errors due to convection. Copper. Cupric oxide produced reduction waves in both melts. iiddition of cuprous oxide or cuprous chloride caused a n anodic wave t o appear as shown in Figure 2. T h e i plots of applied potential us. log had least squares dopes of 0.198 and 0.182 in the sodium metaphosphate and eutectic melts, respectively, compared to theoretical values of 0.203 and 0.199 for a 1-electron reduction process. KO deposition of copper metal was observed at the highest concentration
I
-0 4
Applied
Figure 3.
also shown in the figure, gave least squares slopes of 0.110 and 0.08i in sodium metaphosphate and the LiP03KP03 eutectic, respectively, compared to theoretical slopes of 0.102 and 0.0995 for a 2-electron electrode reaction. The reduction appeared to be
-0.6
-08
-ID
Polenliol, v o l t s
Analysis of polarograms for iron in fused alkali metaphosphates VOL 35, NO. 13, DECEMBER 1963
2105
I.
l O m g V205/9 LiPO3-KPO3
2
Same, after reduction
Electrode crea
I
LiP03-KP03 solvent t o give the deposition curves shown in Figure 5 . KO analysis of the waves could be made, since the upper portion of the curves became obscured by the cathodic decomposition of the solvent. The stability of the silver metal in the melt indicates t h a t a silver-silver metaphosphate reference electrode could be used for future studies in this melt. Sulfur(V1). Two waves were produced when sodium sulfate was added to the melts. The first wave appeared to be the reduction of sulfate ion to sulfite ion, and the second the reduction of sulfite to sulfur or sulfide ion. Liu (11) showed that the electrolysis of sulfate melts produced sulfide ion instead of sulfur, but the poor reproducibility of the second wave in the metaphosphate solvents frustrated any attempts to deduce the nature of the product formed. A few milligrams of silver sulfate were added to another portion of the melt, and polarograms of the kind shown in Figure 6 were recorded. The second wave was well defined and was accompanied by the deposition of a black compound which was soluble only in hot, concentrated nitric acid. Analysis of the waves by means of a plot of E us. log -.d for the first wave Zd’l and a plot of E us. log (id- i) for the second wave gave least squares slopes of 0.094 and -0.0328, respectively. The theoretical slopes should be 0.102 and -0.0338 for a 2- and &electron process, respectively. The deposit, which wag apparently silver suliide, could be stripped off anodically by reversing the voltage scan a t approximately -0.8 volt. The stripping curve, shown by the dotted line in Figure 5, indicates that the electrode process taking place here is reversible. Two possible reactions yielding silver sulfide are
0 7 mrn2
I
I
-04
Applied
Figure 4.
-0.6
- 0.8
-10
Potential, volts
Analysis of polarograms for vanadium in fused alkali metaphosphates
level, 2% copper by weight. The half-wave potential is about -0.4 volt. The heights of the waves appeared to be proportional to concentration. Iron. Both ferric and ferrous oxides produced waves in the two melts. The waves for ferrous oxide were anodic when care was taken to exclude oxidation in the air. When air was admitted into the cell the waves slowly shifted upward until they were identical to ones produced by ferric oxide. Typical polarograms and the analyses of the waves are shown in Figure 3. The least squares slopes of the plots were 0.179 and 0.216 for waves in sodium metaphosphate and the LiPOsKP03 solvent, respectively, implying a 1-electron electrode reaction. The half-wave potential is about -0.37 volt. Vanadium. Vanadium(V) oxide was dissolved in a metaphosphate melt by heating in a n electric furnace under an atmosphere of air. Curve 1 of Figure 4 shows a polarogram obtained with this solution. The initial anodic wave was largely obscured by the anodic dissolution of the solvent. However, a well defined cathodic wave was observed. On cooling, a yellow glass was formed. When this experiment was repeated, using a Meker burner to heat the melt, a green glass was formed on cooling. The polarogram is shown by curve 2 of Figure 4. Apparently the gases from the burner were able to reduce the vanadium in the melt to a lower valence state. The addition of a small piece of zinc to the original oxidized melt caused a shift of the polarogram from type 1 to 2. The anodic dissolution of the solvent obscured the first wave and made an analysis impossible. The second wave showed a least squares slope of 0.193, corresponding to a 1-electron electrode 2106
I
I
1
-0.2
ANALYTICAL CHEMISTRY
reaction. The half wave potential was -0.46 volt. mhen vanadium(V) oxide was added to the melt it went into solution, accompanied by the evolution of gas. A portion of the oxide decomposed to give vanadium(1V). This was confirmed by allowing the melt to solidify, dissolving a portion in water, and testing for the presence of reduced species. The yellow-orange glass dissolved to form green solutions which contained both vanadyl and vanadate species. Solutions of the reduced melt were cooled, dissolved in water, and found to contain only vanadium( IV). I t must be assumed that a small amount of vanadium(II1) must have oxidized during solution of the glass in water. Thus the initial wave should be due to the vanadium(V)-(IV) system while the second wave, a t half-wave potential -0.46 volt, is due to the vanadium(1V)(111) system. Silver. Weighed portions of silver metaphosphate were added to the
+ SOS-l + 6 e- +
2 Ag+
Ag,S
+ 3 0-’
(9)
200-
150-
I. 2. 3. 4.
Solvent background 4.6 mg Aq] 8.6 mq Ag
9.5 mg Aq 5. 13.0 mg Aq
Figure 5. Currentvoltage curves for silver in fused LiP03-KP03 eutectic
6. 19.2 mg Ag
i
i
De‘ orom
Of
Lipo3-
5 E 5
V
50.
I
-0.4
I
I
,
I
-0.6
~
0.8
Applied Potential, volts
8
I
-1.0
I
--1 ---L
-32
Pole-tiol
Appl e d
Figure 6.
Table II.
- 0.8
-06
-04
I
-IO
volts
Analysis of polarograms of silver sulfate in fused Nap03
Least Squares Slopes Measured from Plots of Log
i r-7 vs.
Applied
Potential for Polcirograms of Metallic Ions and Sulfate in Fused Alkali Metaphosphates Observed Theoretical -
K'aPOa LiP03-KP03 solvent, 750' C. solvent, 73U" C. U(V1) 0.110 0 os7 Cu(I1) 0.198 0 .182 Fe(II1) 0. l i Y 0,216 Y(IV) 0.193 so4-2 0 094 0 090 so,-2 - 0,033" . . a From plot of log ( i d - i) LS. E . Ion
Table 111.
electrons in of electrpde wave(s) reaction uaos Anodic-cathodic 2 UOZ(NOs)a 6H20 Cathodic 2 (d.1 CUO Cathodic 1 cupo, CuCl Anodic 1 Fe103 Cathodic 1 FeO Anodrc 1 V205 (d.) (1) Cathodic 1 (2) Cathodic 1 Yature
9
MnO2 (d.) NazSO, Cr03 (d.) KgCrO, (d.) CoCla
NiO
PbO
SbzOs ZrOn
LiPO,-I
