Modified Joliot Apparatus for Study of the Electrodeposition of Radioactive Materials W. H. POWER1 and J. W. HEYD M o u n d Laboratory, Monranto Chemical Co., Miamirburg, Ohio
An apparatus for continuously recording the deposition or dissolution of radioactive material upon a radiationtransparent electrode during potential change is described. Deposition of the radioactive element is detected by an increase i n the ionization current record of an ionization chamber adjacent to the electrode; dissolution of the deposit is accompanied by a decrease in the ionization current. Potential control of the cathode or anode is effected by a variable electronic potential controller. The reversibility of electrode potentials is determined by alternately making cathodic and anodic potential sweeps. For reversible systems, the cathodic inflections occnr at the same potentials as the anodic inflections. The use of the apparatus for recording deposition and dissolution potentials of polonium in 4.7N hydrochloric acid is described.
transparent gold or platinum foil, approximately 0,0001 inch thick, is mounted between the ball joints to serve both as the bottom of the solution vessel and as the cathode (or anode). With this arrangement, gas bubbles formed by irradiation of the electrolyte tend to rise from the bottom electrode rather than to remain on its surface and disrupt electrical contact between the solution and electrode. Contact with the potential-controlling circuit is made by means of a gold washer 0.010 inch thick in contact with the foil. Neoprene gaskets prevent leakage of liquid between the glass and metal surfaces, and the assembly is held together by a modified ball joint clamp. -4platinum
SECT ION
REFERENCE SECT13N
c
ONVENTIONAL methods of investigating the electrochemical properties of materials with high specific radioactivity are limited by the interference of decomposition products produced by irradiation of the solvent. For instance, poloniumplated electrodes in contact with perchloric or sulfuric acids can be polarized by an esternal current source, either positively or negatively, with many times the coulombic equivalent of the polonium itself without permanently changing the electrode potential. Such behavior indicates that the potential is controlled by solvent decomposition products rather than by the poloniumpolonium ion couple. The method of Joliot (2) is designed to detect deposition of a radioactive material without interference caused by inactive decomposition products. By this method, concentration of radioactive material on the cathode (or anode) is indicated by means of an ionization chamber placed adjacent to the radiationtransparent electrode. An increase in ionization current as the electrode potential is made more negative indicates that radioactive material is concentrating a t the cathode. The ionization current is independent of electrochemical reductions other than those of the radioactive substances themselves, Tl-hile the cell currents, as measured by polarography, may include the electrolytic reduction of solvent decomposition products. Also, the measurement of ionization current rather than cell current may esteiid the range of detection of deposition in very dilute solutions. Joliot's method has been modified so that the potential of a radiation-transparent electrode may be uniformly varied and the ionization current in the vicinity of the electrode continuously recorded. Inflections in the graphic record of the ionization current as a function of electrode potential indicate deposition or dissolution of radioactive material.
CURRENT CONTROL
AMPLIFIER
- - - - - - - -1
I
DIFFERENCE AMPLIFIER
A N !
TRVNCPTED
REFERENCE F - ELECTROOE
JOIN SPEEDOMAX.
8-
11 I
'
- 1
. AMPLIFIER ELECTRO-
Figure 1. Potential control and deposition detecting apparatus
300K
300K
300K
301Y
APPARATUS
The solution vessel shown in Figure 1is fabricated from two truncated baII joints. ,4n alphaI Present address, Inorganic Chemicals Division. Monsanto Chemical Co., Everett Station, Boston 49, Mass.
Figure 2.
523
Electronic potential controller
ANALYTICAL CHEMISTRY
524
wire anode (or cathode) and reference electrodes are inserted through a rubber stopper a t the top of the cell. To prevent radioactive contamination in working areas, the cell is enclosed in a hood unit equipped with glove ports. The section belom the ball joint extends through the floor of the hood, so that the ionization chamber can be inserted from belom. I n studying the deposition behavior of noble radioactive elements, such as polonium, stable anodes, such as mercury, cann,3t be used, since spontaneous reduction of polonium occurs a t the mercury surface. An inert mire anode (or cathode) and an e1e:tronic potential controller (Figures 1 and 2 ) are therefore used to control the alpha-transparent electrode potential.
parent electrode iz compared manually with the potential of a referen,re electrode. The potential is made more negative by actuating the motor-driven potentiometer. The ionization current is continuously recorded on a strip chart and the potentials are manually recorded on the same chart when measured. As the potential sweep is practically linear with time, intermediate potentials between readings can be determined by interpolation. After an increase jn ionization current is recorded, indicating a polonium deposition, the direction of potential sweep may be reversed. A decrease in the ionization current indicates dissolution of the electrode deposit. IONIZATION CURRENT us. CELL CURREKT
I 01 ----CELL
CURRENT
4m
1.0 09 0.8 Q7 OS 0.5 0 4 0 3 0.2 0.1 0 -0.1 CATHODE POTENTIAL va. Ag / Ap C I I H GI
Figure 3.
The dissimilarity in behavior of cell current and ionization current during cathodic polarization of an electrode in a polonium solution in hydrochloric acid is shown in Figure 3. Although the ionization current base line remained flat bet\\-een potentials of 1.0 to 0.49 volt positive to the silver-silver chloride reference electrode, the cell current undement a decided increase, starting a t 0.8 volt. This current was probably associated with oxygen reduction. The final cell current inflection a t -0.1 volt was associated with hydrogen reduction. An ionization current inflection, caused by the deposition of polonium, occurred a t 0.49 volt. Additional ionization current inflections a t potentials more negative than 0.49 volt were recorded in subsequent experiments.
Ionization current and cell current relationship
The electronic potential controller is a modification of an early version of the controllers used to maintain a constant solution to cathode potential during electrodeposition ( I ). The fixed reference potential is replaced by a variable potential from a motordriven potentiometer, R l ; a phase inverter stage is added to permit anodic-potential sweep; and a variable bias, R3, is added to ensure maximum lineaiity of operation for VS. The nine single-pole double-throw switches are shown in anodic-potential sweep position; they may be ganged for ease of control. R l is a high-resistance Kohlrausch slide-wire. The synchronous motor drive is provided with several speeds and a reversing gear. 222 ermits adjustment of the voltage span covered by Rl. The ias from R3 is normally approximately 1.5 volts. The potentials B1, B2, and C may be supplied from batteries or mell-regulated power supplies; B 2 must be isolated from ground, so that the alpha-t,ransparent electrode can always be maintained a t ground potential. The tube heaters are supplied with 6.3 volts alternating current regulated by means of a Sola constant voltage transformer. The tubes may have to be selected for low grid current: Type 6hK5 or GQ7 can be substituted for Type GAG5. The initial solution to alpha-transparent electrode potential is set by the adjustment of R 3 and R4, and by the initial position of Rl. The linear change in the voltage of the grid of V2, as R1 is traversed, produces a linear change in the current through Va. This changes the solution t o alpha-transparent electrode potential, which change is impressed on the grid of VI. Hence the difference amplifier, VIand VZ, remains balanced as long as the change in the solution to alpha-transparent electrode potential is linear. Any nonlinearity in the potential change of this electrode, due to polarization or amplifier nonlinearity, results in unbalance of the differential amplifier. This causes the current through Va t o change in direction and magnitude t o restore linearity. The corrective action is rapid, and the potential of the alpha-transparent electrode changes linearly Tyith time. An ionization chamber is mounted directly below the platinuni foil (Figure 1). The ionization chamber consists of a rylinder, fabricated of platinum 0.005 inch thick, 0.32 inch in diameter, and 2.5 inches long. A platinum mire, 0.030 inch in diameter, rounded at one end, and located a t the axis of the cylinder, is used as the positive electrode. The voltage developed across a lo1'ohm resistor placed in series with the ionization current circuit is fed t o the input of a vibrating reed electrometer (Applied Physics Corp.) and is continuously recorded by a Speedomax potentionieter connected t o the electrometer output. If desired, both cell current and ionization current can be recorded simultaneously on a multipoint recording potentiometer. Cell currents can be measured by placing a resistor in the external current circuit and measuring the potential drop across the resistor. PROCEDURE
After the cell containing polonium solution has been arranged as shown in Figure 1, the open-cell potential of the alpha-trans-
SIGNIFICANCE OF IONIZATION CURRENT INFLECTIOXS
A hypothetical ionization current record of the deposition of a radioelement having two oxidation states in solution is shown in Figure 4. At point A , the electrode potential is made negative by an external controlling circuit until a deposition occurs a t B. The direction of potential sweep is reversed a t C, so that the potential
@rP---
L
FQTENTIPL CWNGING TOWARD NEGP;TIVE POTENT!AL CHANGING TOWARD POSITIVE VALUES
I POTENTIAL
Figure 4. Idealized electrodeposition behavior of radioelement having two oxidation s tates in s o h tion
*z a 0 3
POTENTIAL CHANGING TOWARO NEGATIVE U4LUE5 FROM LEFT TO RIGHT
2
0
POTENTIAL CHANGING TOWRO POSITIVE VALUES FROM LEFT TO RIGHT ,
Q
N
s
I Figure 5.
POTENTI&
v6.
Ag/AgFI/ HCI ( 4 . 7 N l
Deposition and dissolution of polonium clean platinum 7.0 X 10 -6 M Po in 4.i" IICl
011
525
V O L U M E 28, NO. 4, A P R I L 1 9 5 6 now becomes increasingly more positive. The radioelement starts dissolving from the electrode a t point D. The potential sweep is reversed again a t E , and a cathodic inflection is recorded a t F . If the electrochemical system is reversible, potential D should agree with potential F. It would not necessarily be expected, however, that the initial deposition potential, B , should agree vith D and F , since at B no equilibrium between the reduced form of the radioelement and its ions would have been established. If, after F , the potential is made increasingly more negative, a second inflection a t G indicates that the radioelement has been reduced either from a second oxidation state or to a solid phase different from that of the first couple. The potential is reversed a t H , and the inflection a t I indicates that deposition no longer occurs from the second oxidation state, but the increase i n the ionization current a t potentials more positive than H indicates that deposition is still occurring. A t J,the potential of the electrode has become more positive than the most positive couple, and dissolution occurs. If the potential couples are reversible and if negligible material has been removed from the solution, potentials G and I should be identical and potentials D,F , and J should be the same. PERFORiMANCE
Tracings of typical ionization current values, recorded during Lhe polarization of a platinum foil electrode in 7 X 10-6~11 polonium-210 solution in 4.7N hydrochloric acid 3re shown in Figure 5. Starting at A , the potential was made more negative until an ionization current inflection at B indicated the polonium was being deposited. This first deposition on fresh platinum was not +eproducible. Anodic polarization was started at C, the curve reached a masimum, and the dissolution of polonium started a t D. The direction of potential sweep was again reversed a t E, and a cathodic inflection was recorded at the same potential, F , as that of the previous anodic inflection, 0.50 volt. Polonium deposition or dissolution inflections could be reproduced any number of times by fising the electrode potential more negative or more positive than 0.50 volt. When the potential w3s allowed to become more negative than 0.27 volt, a second cathodic inflection was recorded, suggesting a second oxidation state of polonium. The anodic inflection a t 0.27 volt could not
be resolved, but the anodic infleetion at 0.60 volt still remained. When the potential was made 0.70 volt positive to the reference electrode and then decreased, a third cathodic inflection a t 0.56 volt was recorded. DISCUSSION
The nonreproducibility of the potential of the initial polonium deposit on fresh platinum may be explained by alloy formation between the metals. The shift of subsequent deposition potentials to more positive values than that of the initial deposit is simiIar to the behavior of silver deposition from very dilute solutions reported by Rogers, Krause, Griess, and Ehrlinger, (5). These authors found that for concentrations of silver insufficient to cover a platinum electrode ( l O - 7 M ) the resulting deposition curves often shifted to a more “noble” potential than that predicted by the Kernst equation. Inflections a t 0.56 volt were noted only after anodizing the platinum electrode, so this potential was probably associated with platinum reduction and the carrying of the chemically similar element, polonium. Anodic dissolution never completely removed polonium from platinum electrodes. ACKNOWLEDGMENT
The authors wish to thank P. E. Ohmart and W, €1. Baker for assistance with the instrumentation. LITERATURE CITED
(1) Heyd, J. W., Ohmart, P. E.. Hites, C. E., “Four Unit Plating Controls, PC-4 and PC-5,” U. S.Atomic Energy Commission MLM-467 rev. (Aug. 1 , 1949). (2) Joliot, F., J. c h b i . phus. 27, 119 (1930). (3) Rogers, L. B., Xrause, D. P., Griess, J . C., Jr., Ehrlinger, D. B., Trans. Electrochem. SOC.95, 33 (1949). RECEIVED for review J u l y 21, 1955. Accepted December 8 , 1955. Abstracted from U. S. Atomic Energy Commission Report RILM-SOS, October 18, 1953. ;\found Laboratory is operated b y Monsanto Chemical Co. for t h e U. S. Atomic Energy Commission under Contract AT-33-1-GEN-53.
Analysis of Phosphorus Compounds Rapid Hydrolysis of Condensed Phosphates in Volumetric Analyses EDWARD J. GRlFFlTH Research Department, lnorganic Chemicals Division, Monsanto Chemical Co., Dayton, O h i o
The step that requires the most time in the analysis of phosphates by p€I titration is hydrolysis of the condensed phosphates to orthophosphates. The time required for hydrolysis has been reduced from a mininluxn of 8 hours to lees than 1 hour by boiling the sample to dryness from strongly acidic solutions. Alkali nielal chlorides are added to the acidic solutions to prevent recondensation of the orthophosphate when heated to dryness. No loss in the quality of the results is caused 1,y the more rapid method.
T””
analysis involved in a quantitative determination of the various species of condensed phosphates in complex mixtures invariably requires a hydrolysis step. I n colorimetric analysis ( 2 ) the hydrolysis step is relatively rapid, but the volumetric analysis of condensed phosphates presents several problems not erkountered in the other methods. If a large excess of a volatile acid is used to hasten the hydrolysis of the phosphates, this acid must be removed or neutralized before the phosphate may be titrated. The neutralization of the escess acid with concentrated {
caustic is not recommended because of thc danger of carbonate contamination and the ultimate interference in the analysis. Carbonate not only titrates as a weak acid but also canses tinstable end points. A difficulty involved in removing the volatile acid by boiling it away occurs during the final stages of the evaporation and appears to be caused by a small aniount of recondensation of the orthophosphates as the solution boils to dryness. This difficulty may be avoided by adding sufficient sodium chloride or potafisium chloride t o the sample to be analyzed to furnish an excess of alkali metal ions a t the completion of the reaction. When potassium chloride is present, the orthophosphates do not recombine to form condensed phosphates, but instead the hydrogen of the orthophosphate interacts with the chloride ions to form volatile hydrochloric acid which is boiled away. The residue of this reaction is a mixture of orthophosphate and any excess of sodium or potassium chloride which was originally added to the solution. The orthophosphate is slightly more basic than would have resulted from the hydrolysis if the strong electrolyte had not been added.
