Automatic Recording Velocity-Servo Potentiometric Titrcrtor M. T. KELLEY, D. J. FISHER, and E. 6. WAGNER Analytical Chemistry and Health Physics Divisions, Oak Ridge National Laboratory, Oak Ridge, Tenn.
b A versatile automatic, recording potentiometric titrator has been designed with a "thinking" velocity servamechanism to control the rate o f titrant addition automatically and praportionately so that near-equilibrium canditions are maintained throughout the titration. Although designed for the remote titration of highly radioactive materials, it i s equally useful for macroond microtitrations of nonradioactive samples. By appropriate choices of the size o f the syringe and of the concentrotion of the titrant solution, wide ranges of sample size can b e titrated. In macrotitrations of uranium (about 0.2 meq. titrated with 0.1N ferric sulfate) a relative standard deviation o f 0.1% i s obtained. In microtitrations of uranium or iron (about 2 peq. titrated with -0.04N ceric sulfate) the relative standard deviation i s 1.O%.
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desirable in the potentiometric method of analysis to eliminate the need for tedious manual measurements of electrode potential us. titrant volume. Various automatic recording potentiometric titrators have greatly increased the practical value of the method, particularly for replicate analyses. One class of instruments stops delivery of the titrant when a predetermined potential corresponding to the end point is reached. Such potentials are not alT IS
ways known or well defined. Examples of these include the Lingane autotitrator and the Beckman Model K automatic titrator. The Sargent-Malmstadt automatic titrator utilizes a derivative end point. The other class of instruments records the electrode potential vs. the volume of titrant added and the end point is taken to be at the inflection of the automatically plotted curve. This class of titrator tends t o overtitrate because of mixing delay and the time required in some titrations t o reach chemical equilibrium. These overtitration tendencies can he avoided. Robinson's Precision-Dow Recordomatic titrator utilizes the error signal in the recorder t o control the delivery of titrant so that delivery is stopped whenever the error signal exceeds a control value and resumes automatically when the recorder has balanced the input signal (7). The Oak Ridge National Laboratory Model Q-945 automatic recording potentiometric titrator, in addition t o having similar recorder on-off controlled delivery of titrant, utilizes a slow-speed pen balance motor and a unique vacuum tube intermittent balance circuit (4). The new Oak Ridge National Laboratory Model Q-1728 automatic recording velocity-servo potentiometric titrator utilizes an improved, transistorized, intermittent balance circuit, a slow pen balance servomotor in the recorder, and
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Figure 1. Oak Ridge National Laboratory Model Q-1728 automatic titrotor set up for bench top operation
a "thinking" velocity
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to control the rate of addition of titrant automatically, continuously, and proportionately, so that equilibrium conditions are maintained as the end point of the titration is approached even if mixing and reaction times are long. The precision and accuracy of titrations are thus enhanced, but the titrations are performed a t the highest feasible speed. This titrator is designed for general use with potentiometric methods. The titrant delivery unit is connected to the control unit only by electrical CablPs so that it can be used equally well for bench top titrations of nonradioactive samples or for remotely controlled titrations of radioactive samples within a hot cell. Because the delivery unit accepts a variety of sizes of syringes or burets, the instrument may be set up for macro- or microtitrations. This titrator is frequently used for the determination of uranium by the ferric sulfate ( 1 ) and the ceric sulfate (2) methods. The rate of chemical reaction is slow in both cases; hence care must be taken to avoid overshooting the end point. The elimination a t the end point of overtitration and the recording of the true shape of the titration curve have been accomplished; yet the addition of titrant at points removed from the end point is automatically rapid. The titrator is free of drift, because a 100% feedback system is .-
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Figui. _ . . _ . x a l block diagram o f automatic recording potentiometric velocity-servo titrator ORNL Model Q-1728 VOL. 32, NO. 1, JANUARY 1960
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used. It gives the complete titration curve, thus indicating, by changes in the shape of the curve, the deleterious presence of contaminating materials or the need for cleaning of the noble metal indicating electrode. With titrators that stop the titration a t a predetermined end point, such contamination or electrode malfunctioning may be present n-ithout detection, and ruin the accuracy. The next sample may be prepared, while the instrument is titrating one sample automatically. Stirring will be complete and slon- chemical reactions will approach equilibrium before more titrant is added. -4photograph of the titrator is s h o m in Figure 1.
INTERMITTENT BALANCE PRINCIPLE
The intermittent balance feature of this titrator is a n improved version of the one previously developed for use in the ORKL Model Q-945 titrator which does not h a w the velocity servo or the remotely operated titrant delivery unit features. The original intermittent balance circuit has been described by Kelley and con-orkers ( 4 ) . Intermittent balance and the proportional velocity servocontrol and remote operation features particularly distinguish this titrator from other instruments ( 5 ) . The original Dow titrator also had a separate titration assembly ( 7 ) , but this feature was not retained in the Precision-Dow instrument. The functional block diagram of the titrator (Figure 2) shows the relationship of the intermittent balance circuit to the nhole instrument. The block diagram is divided up into the following sections: titrant delivery unit, control unit, and potentiometric recorder. The circuits are shon n in Figures 3 and 4. After two stages of voltage amplification, the error signal is taken out of the recorder amplifier and passed through a transistorized gate circuit. The gate is opened and closed by an unsymmetrical niultivibrator to give the intermittent balance feature. The error signal is intermittently fed hack a t the gain control point into the remainder of the Brown pen drive amplifier. The pen can drive towards balance only while the gate is open. The titration takes place through the end point with the titrant being added in very small, discrete increments. The size of these increments is approximately inversely proportional to the rate of change of electrode potential. More and siiialler increments are automatically added than would be feasible in a manual titration. The recorded curve, which is a plot of electrode potential us. the volume of titrant added, has the usual inflection at the end point and can be treated as if it were a strictly continuous record, because the steplike increments are very small. 62
ANALYTICAL CHEMISTRY
GATE CIRCUIT
The gate circuit (Figure 3) consists of the following elements: KPN transistor switch Q 2, PNP transistor switch Q 1, cathode follower (first half of V 5 ) , and grounded grid amplifier (second half of Ti 5 ) . The 60-cycle square wave error signal from the recorder is brought into the gate through connector Cn 3, fed through a cathode follower, through a grounded grid amplifier, and back into the recorder via connector Cn 4. The signal is amplified to about 125% of its original value. When the “intermittent-continuous” switch, S 4, is not actuated, the gate is always open, because the transistor switches are both open and the signal is seen continuously by the recorder pen drive servosystem. When switch S 4 is closed, the N P N transistor is driven open and closed by the cathode current from the multivibrator, V 6. When the NPK switch is driven closed, it simultaneously drives the PNP switch closed. When the PNP transistor “bottoms”-i.e., when i t is closed-its emitter to collector resistance is very low so that the input t o the grounded grid amplifier is nearly at ground potential. During the interval the transistor switches are closed, the recorder does not receive enough signal to actuate it, so that its pen remains stationary whether it is balanced or not. The value of the plate resistor for the cathode follower (connected to pin 1 of 1’ 5 ) is shown as 100 K on the circuit diagram (Figure 3). This value may have to be reduced to as lorn as 25 K for some Type 2x270 transistors. If the recorder pen creeps slowiy in the presence of a n error signal rather than remaining stationary. while the transistors are switched on, the value of this resistor should be reduced to less than 100 kilo-ohms. VELOCITY SERVOSYSTEM PRINCIPLES
-4s shown in the block diagram (Figure 2), the recorder error signal is not only used intermittently or continuously to balance the recorder, but also when amplified and rectified constitutes one comniand signal to the velocity servosystem. The other command signal, the s p e d control, determines the mauimum rate of titrant delivery, which is attained when the recorder is at balance. I n the absence of appreciable recorder error signal (recorder balanced), the modified servomotor drives the tachometer at a constant equilibrium rate such that the tachometer output balances the speed control command signal. The diode (V1 b of Figure 3) and antiback-up control prevent a large error signal from reversing the direction of rotation of the velocity servomotor. The setting of the servo sensitivity control determines thi- minimum value of recorder error signal sufficient to halt the velocity servomotor completely. I n the presence of error signals greater than this value, from an unbalance of about 1% of full scale, the velocity-
servosystem stands a t rest and waits for the recorder t o rebalance before adding reagent. I n the presence of smaller error signals, when going into a titration break, the rate of addition of titrant will be proportionately reduced, because the servomotor will drive the tachometer a t a rate such as t o minimize the difference between its output and the algebraic summation of the two conimand signals. The velocity-servo amplifier isaBrom-n amplifier modified as shown in Figure 3 to use 12BH7 output tubes instead of the usual 12AU7 tubes so that the servomotor may deliver higher torque. The servomotor is a Brown balance motor modified mechanically so that the rotor shaft extends out of it as well as the usual gear reduction output shaft (6). The tachometer is coupled to the rotor shaft and the synchro generator to the gear reduction output shaft of the modified servo motor. During titrations, the synchro generator drives in unison and a t rapid shaft speed synchro receivers geared, respectively, to the syringe and the recorder chart. KO significant angular lags of the synchros have been observed. AUXILIARY EQUIPMENT
Auxiliary equipment intended for use with the titrator includes a screwoperated, remotely controlled elevator to position the titrant delivery unit vertically with respect to a stationary titration vessel, a remotely controlled motorized lab-jack, which can be used alternatively to adjust the vertical position of a rotating microtitration vessel beneath the stationary mounted titrant delivery unit, a p H indicator (Leeds & Sorthrup Cataiog No. 7664) used as a preamplifier, and an electrical heating unit for the titration vessel assembly used for the titration of uranium by the ferric sulfate method. If the rotating titration vessel assembly is not used, the titration vessel is supported on a standard. commercially available magnetic stirring unit. Detailed dran ings of these auxiliary devices (other than the preamplifier) are available (6). A photograph of the titrator set up for the titration of about 30-mg. quantities of uranium by the ferric sulfate method is shown in Figure 1. The screw-operated elevator, heating unit, and magnetic stirrer are also shown in Figure 1. T-arious titration vessels and electrode s y s t e m can be used with the titrator. For pH titrations with glass electrodes, the input impedance of the modified Brown recorder is not sufficiently high for best results. To obtain rapid response n ith negligible dead space, a p H indicator is used as a preamplifier. The p H electrodes are connected to the preamplifier and the titrator input leads are connected to the recorder jack of the przamplifier. The internal connection to the recorder jack is shifted from the standard 20-mv. tap to the full -1.4-volt output of the
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VOL 32, NO. 1, JANUARY 1960
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direct current amplifier. The URC of this commercial instrument a8 n prcamplifier is Icss expensive than building special input stages of very high impedance for the recorder, 8.9 was done with the QRNL llorlel Q-945 titrator (4). For titrations with electrode systems of lower impedance, it is not neccsswy to usc the preamplifier, but it does amplify the signal somewhat, andso its use results in increased scnsitivity. Some advantage is thereby gained, if the titration does not have a large potential break at the end point. Titrant Delivery Unit. The titrant delivery unit is s1lon.n i n Figure 1. T h c function of this unit is to drliver titrant to n titration vessel nt a vnriable rate t h a t is controlled by the titrator. -4thrce-way stopcock connects thc syringe cithcr to B dclivery tip which has nn 0.005-inch orifice or to a reagent supply rcservoir for filling. The tlcliwry tip is iminersed in thc solution to avoid the deliver). of discrete drops and oriented so that reacting titrant is stirred against the indicator clr~ctrodc. The syringe can be any stantlard 1Iicrometric unit that ha5 crtpacitice raiigiiig from 0.2 to 10 nil, per inch of travel. Tlic plunycr of tile syringe is coupled by means of n leadscrew and a right-anglc worm gear to thc output shaft of a synchro receivcr (6). This synchro receiver is controllcrl by the same eynchro generator that drives the chart-clrive synchro receiver. The syringe phingcr can be retracted by the titrator to refill the syringe with titrant. To protect the glass syringe, electric limit witches are provided at both its extremes of movement. An alternative titrant delivery unit has been built for use with this titrator. It consists of a Gilmont micropipet-buret of I-ml. capacity (Emil Greiner Co.) coupled by a Tight-angle worm gear to a synchro receiver. The Gilmont device has a digital counter built into i t . This unit is interchangeable with the regular unit shown in Figure 1. It is filled aftcr immersing the delivery tip in a beaker containing standard titrant. The syringe type delivery unit shown in Figure 1 is preferred. Recorder. The Brown Electronik potentiometric strip chart recorder is modified t o measure potentials at high input impedance, to balance intermittently, and to deliver a n error signal t h a t commands the velocity servo-controlled rate of delivery of titrant. Robinson ( 7 ) and Lingane (6) also used a recorder both to plot and control the titration. This titrator, however, by means of the velocity servo, diminishes the size of the individual increment of titrant addition in inverse proportion to the size of the error signal that becomes appreciable as an end point is approached. The recorder 1 1 ~ sa 1-volt span. By means of a Helipot, which is located on the recordcr, the reference point of the recorder slide-wire can be shifted 80 that the zero of the 1-volt span falls anywhere withia the range of - 2 to +I volts, The recorder is standardized by manually eniaging the standardizing mechanism, because the usual syn-
Figure 5. Typical titration of eo. 32 mg. of uranium by 0.1 N Fef3 by means of the ORNL Model Q-1728 automatic titrator
chronous chart-drive motor is not used. The scale of thc recorder is not graduated in volts or pH units, but by use of a voltagc standard or p H buffers one may prepare calibration curves to read potential or pH directly from the recorder scale. For titrations it is not necessary to know absolute values of electrode potential or pH, and i t is therefore considered practical to eliminate the expense of absolute calibrated recorder scales. The usual 1.&volt working battery is not used. A lineoperated, silicon diode rectified, Zener diode regulated, power supply furnishes a 10-ma. working current to the slidewire of the recorder. Details of all mechanical and electrical changes made to the recorder arc fully described (6). Operation of Titrator. The following controls are located on the front panel. The toggle switch t h a t is labeled "power" applies line and slidewire voltages t o t h e Brown recorder and line voltage to the amplifier in the control unit, t o t h e synchro motors, and to the titration assembly. A neon lamp is illuminated when the power switch is thrown t o the on position. The "titrate-stand-by" switch 1s in one of the power leads to the modified servomotor of the velocity servo unit. When this switch is in the stan?-by position, the velocity servomotor is disabled so that the synchro generator that i? connectod to the titrant delivery and the.chart drive synchro receivers remalns stationary. When it is in the titrate position, the velocity servomotor is energized and will obey the commands of i$ amplifier provided ale0 that the fill-titriite') switch is in the titrate position. When the syringe plunger actuaka the forward limit switch, this power
lead is opened so that tlrc syringe docs not break. Jt'hen the piungcr has reached its forward limit, the opcrutor may throw thc titrate-stnntl-l)y switch to the stand-by position, turn tlic stopcock so as to connect tlie syringe to tlic titrant reservoir (or immerse the tip of the Gilmoiit buret in a heaker of standard titrant), and then throw the filltitrate switch to the fill position. This actuates relays wliicli switch tlic syringe drivc synchro rcceivcr from the vclocity servo synchro generator to the syringefill synchro gcncrator, which is d r i w i by a I3odine motor in a tiircction such that the syringe plunger is causcd to retract nrid refill the syringe or hurct with titrant. \Vhrn tlic plungcr rcachcs tlie rear limit switch, the relays will be deenergized so that the synchro receiver driving the plungcr of thc syringe is again conncctcd to the synchro generator driven by the velocity servosystem. The operator then turns the stopcock so as to reconnect the syringe to the titrant delivery tip, and throws the fill-titrate switch and the titrate-standby switch to the titrate positions, whereupon tlic titration is resumed and proceeds nutomatically. The setting of the titration speed control determines the maximum rate of titrant delivery in the absence of a deterring error signal from the I3rou.n recorder amplifier. The intcrniittentcontinuous switch actuates a multivibrator-controlled, transistorized g a t ing circuit. If it is thrown to the intermittent position, the signal that goes to the position servomotor driving the en of the recorder is turned on and off gy the gate. This is particularly desirable (luring some titrations in order to allow for relatively longer reaction times. When the switch is in the continuous position, thc multivibrator is disabled and balancing of the recorder is continuous. The servosensitivity control is adjusted so that the velocity servo will be halted by error signals corresponding to small uw balances of the recorder, which. occur in the presence of a rapidly changing input signal (3). The following controls are located a t the rear of the control unit. The trigger control adjusts the frequency of the multivibrator. It is set so that the recorder pen, in intermittent balance, proceeds across t h e chart by steps each equal to 0.5% of full sede (8). The antibaek-up control IS adjusted so that the velocity servo remains stationary in the prescnce of large error (unbalance) signals (3). The green and red pinjacks are used for oscillographic observation of t h e wave form of the error signal at, respectivelv, the input and output of the ate. The procedure for adjustment of these controls has been described in greater detail (8). The following controls are located on the Brown recorder. The "zero-adjust" control allows t h e 1-volt span of the recorder to be shifted to the range that is appropriate for the particular titration performed. The zeru of the 1-volt span can be set a t any point within the range of -2 to 4-1 volts. The Brown recorder is standardized manually by VOL 32, NO. I , JANUARY 1960
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depressing the standardize button for about 30 seconds. This operation is repeated until the recorder pen will return to the same point on the chart each time after the button is released. The recorder should be standardized once per day of use. The “chart drive” switch should be in the on position at all times. Stepwise Procedure. A detailed stepwise procedure for t h e use of this titrator including its installation and its routine operation has been written (8) * Maintenance Manual. Instrument service and adjustment instructions and a trouble shooting guide for the maintenance of this titrator have been written ( 3 ) . Fabrication Details. Complete mechanical and electronic drawings have been prepared for this titrator for use in maintenance operations and for shop fabrication (6). Applications a n d Performance. This titrator is being used routinely for titrations of acid r\-ith base, uranium with ceric sulfate solution ( 2 ) , uranium with ferric sulfate solution ( I ) , and iron with ceric sulfate solution. For titrations of uranium the indicator electrode is a gold wire. A platinum wire “shielded” reference electrode inserted in the syringe tip is used when ceric sulfate is the titrant. -4 high temperature calomel electrode is the reference electrode for the titrations with ferric sulfate. Titration of iron(I1’i was carried out with -0.04N ceric sulfate. About 0.1 meq. (8 mg.) of iron mas titrated using a 5-ml. per inch syringe and the chart travel was about 11 inches with a relative standard deviation of 0.15’%,. About 2 weq. (100 y) of iron were titrated using a 0.2-ml. per inch syringe with a relative standard deviation of 0.6%. I n the microtitration of 2 peq. of uranium with -0.04,V ceric sulfate using a 0.2-ml. per inch syringe, a relative standard deviation of 1.2% was obtained. When about 9 ueq. (I mg.) of uranium 11-as titrated using a 0.5ml. per inch syringe, a precision to 0.2% was obtained. Many titrations of nonradioactive uranium samples with 0 . 1 S ferric sulfate solution have been made with this instrument. Using a 3-ml. per inch syringe, 0.05 meq. (6.5 ma.) of uranium was titrated with a relative standard deviation of 0.257,; the chart travel was 3.82 inches. If the amount of uranium is increased to about 0.2 meq., the precision is 0.09%. The titrations of about 30 mg. of uranium with 0.1.Y ferric sulfate are routinely run with a 5-ml. per inch syringe. The Special Analyses Laboratory a t ORKL has been performing these titrations with this instrument, and has used statistical control charts 66
ANALYTICAL CHEMISTRY
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since M a y 1955 to keep the method in control. During the entire period beginning in May 1955 the control limits at the 95y0 probability level have been rt 0.20% and these control limits are based upon a relative standard deviation of 0.10%. .4n example of the titration curves obtained by them is shown in Figure 5 . The concentration of titrant reagent, the syringe capacity, and the amount of sample titrated are so chosen that about 12 inches of chart travel appear between the end points. The titration takes place in 9 minutes. The control chart for 1958 is shom-n in Figure 6. ACKNOWLEDGMENT
The use of performance data obtained by the work of W. R. Laing, B. J. Ginocchio, Bryce Philpot, G. I. Gault, J. T. Gass, and W. L. lladdox is acknowledged with thanks. The control chart (Figure 6) is based on data obtained by K. C. Lannom and B. J. Ginocchio. The mechanical modification t o the Brown servomotor in the velocity servosystem was made by H. L. Hemphill. Work was done by G. A. Holt, C. C. Courtney, and G. E. Tipton in the layout and fabrication design of the titrator. The titrant delivery mechanical dram ings (syringe type) were revised by H. C. Jones and W.L. Maddox. The titration vessel heater was built by H. C. Jones. The Gilmont delivery unit was built by W. L. Maddox. The &-1728 mechanical and electronic drawings were prepared by Mark Bowelle and coworkers. The transistorized gate and Zener diode power supply were developed with the assistance of H. C. Jones and R. W.Stelzner.
LITERATURE CITED
(1) Cooper, J. H., “Uranium, Potentio-
metric Ferric Sulfate Method,” Method Kos. 1 219222 and 9 00719222 (8-3-53), ORNL Master .Inalytical Ifanual, TID-7015 (Section l ) , Office Tech. Services, U. S. Dept. of Commerce, Kashington 25, D. C. (2) .Corbin, L. T., Cooper, J. ,H., ‘[Uranium Potentiometric Ceric Sulfate Method,” Method Nos. 1 219221 and 9 00719221 (3-25-54), ORSL Master Bnalytical Manual, TID-7015 (Section I), Office Tech. Services, U. P. Dept. of Commerce, Washington 25, D. C. (3) Fisher, D. J., “Instrument Service Instructions ORSL 1Iodel Q-1728 Velocity-Servo Automatic Potentiometric Titrator,” RS-1728, -4nalytical Instrumentation Group, -4nalytical Chemistry Division, Oak Ridge Satl. Lab., Oak Ridge, Tenn., Ang. 12, 1958. (4)Kelley, M. T., Horton, J. L., Tallackson, J. R., Miller, F. J., Proc. In&. Soc. Am. 7, 63 (1952). (5) Lingane, J. J., “Electroanalytical Chemistry,” Chap. 8, Interscience, Kew York, 1953. (6) Oak Ridge Natl. Lab., Oak Ridge, Tenn., ORXL drarrings Q-1728-1 through 15. (7) Robinson, H. 4.,Trans. Electrochem. Soc. 92,445 (1947). (8) Wagner, E. B., “;iutomatic Potentiometric Titrator, Velocity Servo, Model Q-1728,” Method Nos. 1 003028 and 9 003028 (6-21-56), ORSL Master Analytical Manual, TID-7015 (Section l), Office Tech. Services, U. 6. Dept. of Commerce, Washington 25, D. C. RECEIVEDfor review October 23,. 1958. Accepted October 5 , 1959. Division of Analytical Chemistry, 135th Meeting, ACS, Boston, Mass., Bpril 1959. Instrument Society of America Symposium on Laboratory Analytical Instrumentation, Southwide Chemical Conference, Memphis, Tenn., December 7, 1956. Work performed under contract Pio. TT7-74O5eng-26 at Oak Ridge liational Laboratory, operated by Union Carbide Nuclear Co. for the U. S. Atomic Energy Commission.
