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Electronic Controlled-Potential Coulometric Titrator For Plutonium Analysis
TaJanta, 1960, Vol. 6, pp. 185
to
188. Pergamon Press Ltd. Printed in Northern Ireland
ELECTRONIC CONTROLLED-POTENTIAL COULOMETRIC TITRATOR FOR PLUTONIUM ANALYSIS M. T. KELLEY, H. C. JONES and D. J. FISHER Oak Ridge National Laboratory, * Oak Ridge, Tennessee Summary-An instrument that performs coulometric redox titrations at a controlled-potential is described. A stabilized printed-circuit operational difference amplifier combined with a transistor current amplifier is used to control the potential of the electrode at which the desired reaction occurs. A portion of the electrolysis current is integrated by a stabilized printed-circuit operational amplifier connected as a time integrator, and the integral is read out as a voltage. The instrument is operated completely from an a.c. line and can be operated with either manual or automatic cut-off. The calibration is absolute; results are computed from Faraday's law. Accurate coulometric titration of small amounts of substances with high equivalent weights such as plutonium is possible because of the high stability of the integrator.
THE methods used to control electrode potential and of electronic integration in this instrument are similar to those described by Booman. 1 In this instrument, however, one stabilized operational difference amplifier is used to control the electrode potential. The electrolysis current is controlled by a transistor current amplifier which is driven by the difference amplifier. The transistor current amplifier permits the use of a compact low-voltage cell power supply which can supply the high initial electrolysis current necessary for carrying out a titration in a short period of time. A small portion of the electrolysis current is integrated by a stabilized operational amplifier connected as a time integrator and the integral is read out as a voltage. This instrument contains no batteries and is completely a.c. line operated. Either oxidation or reduction reactions may be performed with good precision and the instrument may be operated manually or automatically. The titration is terminated when the electrolysis current drops to a pre-selected residual value. Stirring and electrolysis rates need not be reproducible. This instrument differs from the ORNL model Q-2005 2 titrator in the integrator input circuitry. Figs. 1 and 2 show the sensitivity switch which has been added along with the 50-ohm power resistor and the 40K input resistor necessary for the 50 X sensitivity increase. The principles of operation of this titrator for reduction titrations are illustrated in Fig. 1. Figure 2 illustrates the operation for oxidation titrations. The input signal to the control amplifier is the algebraic sum of the control potential and the potential of the controlled electrode with respect to the solution as seen through the reference electrode. The source of the control potential is a low-voltage supply which is regulated by a Zener diode and rectified by a silicon diode. A portion of this regulated voltage is selected as the control potential. The electrolysis current is controlled by a transistor current amplifier which in turn is driven by a chopper-stabilized difference amplifier. The difference amplifier, by negative feedback through the cell, maintains its two inputs at equal potential. The difference amplifier is a G.A.P.jR. Model USA-3 printed-circuit universal stabilized amplifier3 modified 2 so that the chopper references against input R which is connected directly to the controlled electrode. The transistor, a germanium PNP • Operated by Union Carbide Corporation for U.S. Atomic Energy Commission. 185
M. T.
186
KELLEY,
H. C.
and D.
JONES
J. FISHER
-:300V
~ i >iceo
r-----------------------{ft~-----------------------------------!+jC~E~L~L~P~O~W~E~R· .. CELL CURRENT"
SU PPLY
,r------------------- ---------, ,,, USA-:3-M:3 1-
--+__.J
CEL L '--__
I
I
TRANSISTOR CURRENT AMPLIFIER
I :
SENSITIVITY
X 50
CHOPPER-STABILIZED DIFFERENCE AMP. L _________ ~0.!4.!'!~L_~~P~~.!..E!l _____
r-- ------"'40k--------- -----.., 400k
'-------------1-,----------"fII\J~
,,
Ion
10,uF USA-:3 - M:3
I I
I I
I : I
J
X10
I
, I
I
,1-:300V ,
"INTEGRATOR BIAS"
+:300V
I
I
CURRENT INTEGRATOR
~------------------------~
I.-Electronic controlled-potential coulometric titrator block diagram: switched for reduction.
FIG.
r--JV\/VIo~
.. CELL
+ 300V
~
'--'_>_'-"c"'e"--_ _ _ _ _ _ _ _~
CURRENT"
CONTROLLED ANODE (Pt GAUZE)
- -------,
"" "CONTROL,. POTENTIAL
-+__J
CE LL '--__
,, I
ZENER DIODE CONTROL POTENTIAL SOURCE
TRANSISTOR : CURRENT I AMPLIFIER I I
,
:
L__________C~~:!:.R_O_L_~~~L~~I!:'! ______ J
SENSITIVITY
X50
DIFFERENCE AMP.
r---------4b"k-------- - -- - ------: 400k
X40
IOn.
"INTEGRATOR BIAS" I
CURRENT INTEGRATOR
~----------------------------~
FIG. 2.-Electronic controlled-potential coulometric titrator block diagram: switched for oxidation.
Electronic controlled-potential coulometric titrator for plutonium analysis
187
high-voltage power transistor, Motorola Type 2N375, is provided with a heat sink to prevent overheating and loss of control or damage to the transistor itself. The intrinsic current, Iceo, of the transistor which flows in the collector circuit with a zero base current is supplied by the G.A.P./R. Model R-IOOB power supply3 through an alternative path and does not flow through the cell. In this way it is possible to attain low background currents with this instrument. Cell currents as high as 300 rnA can be delivered by this instrument, but with the small cells that are used the current is limited to 25 rnA by a series resistor. TABLE I.-PROCEDURE FOR CONTROLLED-POTENTIAL COULOMETRIC TITRATION OF PLUTONIUM
(1) Pipet the Pu sample into 8-10 ml1M HCIO. in cell.
(2) Position cell; start helium flow; start stirrer. (3) Reduce automatically at +0.135 V vs. Hg-Hg 2S0.-Li 2 SO. to background current of 5 flA. C4) Zero the integrator. (5) Oxidize automatically at +0.435 V vs. Hg-Hg 2 S0.-Li.SO. to background current of 5 ItA. (6) Measure readout voltage. (7) Calculate weight of Pu oxidized.
The cell current meter has four calibrated linear current ranges and a logarithmic range. At the end of a titration, automatic cutoff is provided by the use of a meter relay with lower-limit contacts that switch power to a normally closed d.c. relay in the cell circuit. The four calibrated current ranges have overload protection furnished by a shunt diode. On the logarithmic current range the meter can be caused, by adjustment of a potentiometer in series with the meter, to read linearly on any portion or all of the lower half of the meter scale and logarithmically on the remaining upper portion of the scale. By this means it is possible to follow an entire titration on one current range while retaining the sensitivity of the lower portion of the scale for accurate manual or automatic cutoff. The current integrator is a conventional Philbrick analog computer circuit. 3 With the sensitivity switch in the X 1 position, 1/40,000 of the cell current is integrated. The lower limit of the instrument at this sensitivity, for a relative standard deviation of 0.1 per cent, is approximately 4 C of electricity (about 5 mg U) used in the electrolysis. With the sensitivity switch in the X 50 position, 1/800 of the cell current is integrated. The lower limit, for a relative standard deviation of 0.1 per cent, at this sensitivity is approximately 0.3 C (0.6 mg Pu). The chief obstacle to extending the sensitivity limit of the instrument seems to be drift of the integrating amplifier. While the full voltage output of the integrator is being monitored, the drift rate may be adjusted to essentially zero, but it will not remain constant over a long period of time. One week after being adjusted to zero, the drift rate increases (with the readout switch in the X I position) to approximately 0.3 m V/5 min. This makes it necessary to check daily, and, if necessary, re-adjust the drift rate. A new instrument is now being designed specifically for work at the 0.3 C level and lower. It is anticipated that the new instrument will have much better drift characteristics at the high sensitivities necessary for work with small amounts of substances with high equivalent weights. A method for the determination of plutonium with this titrator has been developed
188
M. T. KELLEY, H. C. JONES and D. J. FISHER
by W. D. Shults. A brief outline of his procedure is shown in Table I. The precision that he obtains with various amounts of plutonium is shown in Table II. It can be seen by a comparison of the drift rate with the readout values in Table II that the precision can be seriously affected by the drift rate at low concentrations. This makes the new instrument now being designed, with its anticipated lower drift rate, attractive for use at lower concentrations. TABLE H.-PERFORMANCE ON X 50 SENSITIVITY (Integrator drift <;0.3 mY per 5 min.) Pu titrated, ~f1g
1000 250 50
Readout, ~mV
ReI. std. dev., (%)
500 120 24
0.1 .3 1.5
Detailed procedures for the controlled-potential coulometric determination of plutonium have beenpublished4 ,5. Acknowledgement-Tables I and II and the information on sensitivity limits and drift rates were obtained from W. D. Shults, Analytical Chemistry Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee. REFERENCES G. L. Booman, Anal. Chem. 1957,29, 213. • M. T. Kelley, H. C. Jones and D. J. Fisher, Anal. Chem. 1959,31,488 and 956. 3 G.A.P./ R. Electronic Analog Computers (Catalog data sheets USA -3 and R-100B.) G. A. Philbrick Researches Inc., Boston 16, Mass. 4 W. D. Shults, B. B. Hobbs, E. L. Blevins and P. F. Thomason, Progress Report on the Analysis of Dissolver Solutions by Controlled-Potential Coulometric Titration. O.R.N.L., ORNL-2776, 1959. 5 W. D. Shults, Controlled-Potential Coulometric Titration of Plutonium-Application to P RFR Samples. O.R.N.L., ORNL-2921, 1960. 1
to
188. Pergamon Press Ltd. Printed in Northern Ireland
ELECTRONIC CONTROLLED-POTENTIAL COULOMETRIC TITRATOR FOR PLUTONIUM ANALYSIS M. T. KELLEY, H. C. JONES and D. J. FISHER Oak Ridge National Laboratory, * Oak Ridge, Tennessee Summary-An instrument that performs coulometric redox titrations at a controlled-potential is described. A stabilized printed-circuit operational difference amplifier combined with a transistor current amplifier is used to control the potential of the electrode at which the desired reaction occurs. A portion of the electrolysis current is integrated by a stabilized printed-circuit operational amplifier connected as a time integrator, and the integral is read out as a voltage. The instrument is operated completely from an a.c. line and can be operated with either manual or automatic cut-off. The calibration is absolute; results are computed from Faraday's law. Accurate coulometric titration of small amounts of substances with high equivalent weights such as plutonium is possible because of the high stability of the integrator.
THE methods used to control electrode potential and of electronic integration in this instrument are similar to those described by Booman. 1 In this instrument, however, one stabilized operational difference amplifier is used to control the electrode potential. The electrolysis current is controlled by a transistor current amplifier which is driven by the difference amplifier. The transistor current amplifier permits the use of a compact low-voltage cell power supply which can supply the high initial electrolysis current necessary for carrying out a titration in a short period of time. A small portion of the electrolysis current is integrated by a stabilized operational amplifier connected as a time integrator and the integral is read out as a voltage. This instrument contains no batteries and is completely a.c. line operated. Either oxidation or reduction reactions may be performed with good precision and the instrument may be operated manually or automatically. The titration is terminated when the electrolysis current drops to a pre-selected residual value. Stirring and electrolysis rates need not be reproducible. This instrument differs from the ORNL model Q-2005 2 titrator in the integrator input circuitry. Figs. 1 and 2 show the sensitivity switch which has been added along with the 50-ohm power resistor and the 40K input resistor necessary for the 50 X sensitivity increase. The principles of operation of this titrator for reduction titrations are illustrated in Fig. 1. Figure 2 illustrates the operation for oxidation titrations. The input signal to the control amplifier is the algebraic sum of the control potential and the potential of the controlled electrode with respect to the solution as seen through the reference electrode. The source of the control potential is a low-voltage supply which is regulated by a Zener diode and rectified by a silicon diode. A portion of this regulated voltage is selected as the control potential. The electrolysis current is controlled by a transistor current amplifier which in turn is driven by a chopper-stabilized difference amplifier. The difference amplifier, by negative feedback through the cell, maintains its two inputs at equal potential. The difference amplifier is a G.A.P.jR. Model USA-3 printed-circuit universal stabilized amplifier3 modified 2 so that the chopper references against input R which is connected directly to the controlled electrode. The transistor, a germanium PNP • Operated by Union Carbide Corporation for U.S. Atomic Energy Commission. 185
M. T.
186
KELLEY,
H. C.
and D.
JONES
J. FISHER
-:300V
~ i >iceo
r-----------------------{ft~-----------------------------------!+jC~E~L~L~P~O~W~E~R· .. CELL CURRENT"
SU PPLY
,r------------------- ---------, ,,, USA-:3-M:3 1-
--+__.J
CEL L '--__
I
I
TRANSISTOR CURRENT AMPLIFIER
I :
SENSITIVITY
X 50
CHOPPER-STABILIZED DIFFERENCE AMP. L _________ ~0.!4.!'!~L_~~P~~.!..E!l _____
r-- ------"'40k--------- -----.., 400k
'-------------1-,----------"fII\J~
,,
Ion
10,uF USA-:3 - M:3
I I
I I
I : I
J
X10
I
, I
I
,1-:300V ,
"INTEGRATOR BIAS"
+:300V
I
I
CURRENT INTEGRATOR
~------------------------~
I.-Electronic controlled-potential coulometric titrator block diagram: switched for reduction.
FIG.
r--JV\/VIo~
.. CELL
+ 300V
~
'--'_>_'-"c"'e"--_ _ _ _ _ _ _ _~
CURRENT"
CONTROLLED ANODE (Pt GAUZE)
- -------,
"" "CONTROL,. POTENTIAL
-+__J
CE LL '--__
,, I
ZENER DIODE CONTROL POTENTIAL SOURCE
TRANSISTOR : CURRENT I AMPLIFIER I I
,
:
L__________C~~:!:.R_O_L_~~~L~~I!:'! ______ J
SENSITIVITY
X50
DIFFERENCE AMP.
r---------4b"k-------- - -- - ------: 400k
X40
IOn.
"INTEGRATOR BIAS" I
CURRENT INTEGRATOR
~----------------------------~
FIG. 2.-Electronic controlled-potential coulometric titrator block diagram: switched for oxidation.
Electronic controlled-potential coulometric titrator for plutonium analysis
187
high-voltage power transistor, Motorola Type 2N375, is provided with a heat sink to prevent overheating and loss of control or damage to the transistor itself. The intrinsic current, Iceo, of the transistor which flows in the collector circuit with a zero base current is supplied by the G.A.P./R. Model R-IOOB power supply3 through an alternative path and does not flow through the cell. In this way it is possible to attain low background currents with this instrument. Cell currents as high as 300 rnA can be delivered by this instrument, but with the small cells that are used the current is limited to 25 rnA by a series resistor. TABLE I.-PROCEDURE FOR CONTROLLED-POTENTIAL COULOMETRIC TITRATION OF PLUTONIUM
(1) Pipet the Pu sample into 8-10 ml1M HCIO. in cell.
(2) Position cell; start helium flow; start stirrer. (3) Reduce automatically at +0.135 V vs. Hg-Hg 2S0.-Li 2 SO. to background current of 5 flA. C4) Zero the integrator. (5) Oxidize automatically at +0.435 V vs. Hg-Hg 2 S0.-Li.SO. to background current of 5 ItA. (6) Measure readout voltage. (7) Calculate weight of Pu oxidized.
The cell current meter has four calibrated linear current ranges and a logarithmic range. At the end of a titration, automatic cutoff is provided by the use of a meter relay with lower-limit contacts that switch power to a normally closed d.c. relay in the cell circuit. The four calibrated current ranges have overload protection furnished by a shunt diode. On the logarithmic current range the meter can be caused, by adjustment of a potentiometer in series with the meter, to read linearly on any portion or all of the lower half of the meter scale and logarithmically on the remaining upper portion of the scale. By this means it is possible to follow an entire titration on one current range while retaining the sensitivity of the lower portion of the scale for accurate manual or automatic cutoff. The current integrator is a conventional Philbrick analog computer circuit. 3 With the sensitivity switch in the X 1 position, 1/40,000 of the cell current is integrated. The lower limit of the instrument at this sensitivity, for a relative standard deviation of 0.1 per cent, is approximately 4 C of electricity (about 5 mg U) used in the electrolysis. With the sensitivity switch in the X 50 position, 1/800 of the cell current is integrated. The lower limit, for a relative standard deviation of 0.1 per cent, at this sensitivity is approximately 0.3 C (0.6 mg Pu). The chief obstacle to extending the sensitivity limit of the instrument seems to be drift of the integrating amplifier. While the full voltage output of the integrator is being monitored, the drift rate may be adjusted to essentially zero, but it will not remain constant over a long period of time. One week after being adjusted to zero, the drift rate increases (with the readout switch in the X I position) to approximately 0.3 m V/5 min. This makes it necessary to check daily, and, if necessary, re-adjust the drift rate. A new instrument is now being designed specifically for work at the 0.3 C level and lower. It is anticipated that the new instrument will have much better drift characteristics at the high sensitivities necessary for work with small amounts of substances with high equivalent weights. A method for the determination of plutonium with this titrator has been developed
188
M. T. KELLEY, H. C. JONES and D. J. FISHER
by W. D. Shults. A brief outline of his procedure is shown in Table I. The precision that he obtains with various amounts of plutonium is shown in Table II. It can be seen by a comparison of the drift rate with the readout values in Table II that the precision can be seriously affected by the drift rate at low concentrations. This makes the new instrument now being designed, with its anticipated lower drift rate, attractive for use at lower concentrations. TABLE H.-PERFORMANCE ON X 50 SENSITIVITY (Integrator drift <;0.3 mY per 5 min.) Pu titrated, ~f1g
1000 250 50
Readout, ~mV
ReI. std. dev., (%)
500 120 24
0.1 .3 1.5
Detailed procedures for the controlled-potential coulometric determination of plutonium have beenpublished4 ,5. Acknowledgement-Tables I and II and the information on sensitivity limits and drift rates were obtained from W. D. Shults, Analytical Chemistry Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee. REFERENCES G. L. Booman, Anal. Chem. 1957,29, 213. • M. T. Kelley, H. C. Jones and D. J. Fisher, Anal. Chem. 1959,31,488 and 956. 3 G.A.P./ R. Electronic Analog Computers (Catalog data sheets USA -3 and R-100B.) G. A. Philbrick Researches Inc., Boston 16, Mass. 4 W. D. Shults, B. B. Hobbs, E. L. Blevins and P. F. Thomason, Progress Report on the Analysis of Dissolver Solutions by Controlled-Potential Coulometric Titration. O.R.N.L., ORNL-2776, 1959. 5 W. D. Shults, Controlled-Potential Coulometric Titration of Plutonium-Application to P RFR Samples. O.R.N.L., ORNL-2921, 1960. 1