virtually eliminated. Lock-out bars and automatic releases prevent the operator from making a double selection within a given function. The ease of operation has been obtained only at the cost of greater electronic complexity. The authors agree with Dr. Barker in that the operator of an instrument generally has only an imperfect understanding of the operation of the electronic circuits. Therefore, the operation of these circuits should be as automatic as possible. LITERATURE CITED
-0.100
-0.110
L---. 1
D.C.
1 - 1
POTENTIAL
Figure 7. Multicomponent system: 1.238 X 10-3M CdC12, 1.402 X 10-3M Pb(NO&, 1.331 X 1 O-3M TiCI, 9.68 1 X 1 O-jM Cr(N0J3, 0.28M HC104 Square wave polarogram Applied: 1000 c.p.r., 5 mv. peak to peak Initial potential: -0.1 00 volt Scan rate: 100 mv. per minute
is the lowest concentration for which polarograms have been recorded, It does not, however, represent the maximum sensitivity of the instrument. This work was done with a signal of 5 niv. amplitude; a signal Kith 50 mv. is available if desired. I n addition to the elements of control the instrument was designed to operate with a greater flexibility than those previously described. This flexibility
Sensitivity control: 10 G a t e conduction interval: 9 D. C. polarogram (Sargent XXI)
occurs in both the possible variation of the square wave frequency and the choice of position and duration of the open interval of the gate. In spite of the increase in flexibility, the operation of the instrument has been kept simple. With one exception (0-1 volt initial potential control) all controls are quantized and operated by push button switches located on the front panel. Reset error is thereby
(1) ANAL.CHEM.35, 1770 (1963). (2) Barker, G. C., Jenkins, I. L., Analyst 77, 685 (1952). (3) Bauer, H. H., J . Electroanal. Chem. 3, 150 (1962).
(4) Bauer, H. H., Elving, P. J., ANAL. CHEW30. 334 (1958). (5) Booman; G. L., Zbid., 29, 213 (1957). (6) Ferrett, D. Y., Llilner, G. W. C., Analyst 80, 132 (1955). ( i ) Hamm, R. E., ANAL. CHEY.30, 350 (1958). (8) Kelley, 31. T., Fisher, I>. J., Jones, H. C., Ibid., 31, 1 4 i 5 (1959). (9) G. A. Philbrick, Researches, Inc., 127 Clarendon St., Boston 16, Mass. Ampli-
fier descriptions and characteristics available at this address. (10) G. A. Philbrick Researches, Inc., GAP/R Application Brief, X o . D2, April 1, 1960, Boston 16, Mass. (11) Smith, D. E., ASAL. CHEW35, 1811 (1963).
(12) Yasumori, Y.,Japan ilnalyst 8, 361 (1959).
RECEIVEDfor review June 25, 1964. Accepted October 12, 1964.
pH Stat with Digital Readout for Quantitative Chemical Determinations H. V. MALMSTADT and E. H. PlEPMElER Department o f Chemistry and Chemical Engineering, University o f Illinois, Urbana, 111.
b A precise and inexpensive pH stat, which i s stable within 0.002 pH unit, can be rapidly assembled from modular units that have multipurpose laboratory applications. The components, including a new automatic deliveryrefill micropipet, that are not commercially available are described in detail. The instrument provides both direct digital readout for automatic quantitative chemical determinations from data collected early in a reaction and strip-chart recording of complete reaction rate curves. Methods are introduced for the specific and rapid quantitative determinations of urea and glucose. A urea procedure was developed and tested in the 2- to 10-p.p.m. range and a glucose pro34
ANALYTICAL CHEMISTRY
cedure in the 50- to 250-p.p.m. range. The results were reproducible within 1 to 2% for both the urea and glucose procedures. The elapsed time from the start of the automatic pH stat measurement to the direct digital readout was about 3 minutes for each urea or glucose sample.
U
PH STAT has greatly increased in recent years, primarily by biochemists for enzymatic reactionrate studies, and several instruments have been described (1-3, 9-13). The primary objective, as indicated by most reported results, has been to obtain complete reaction rate curves by maintaining the p H stationary a t a preSE OF THE
selected value with continuous neutralization of the acid or base formed during the reaction; a recorded plot of milliequivalents (or volume) of acid or base us. time provides a reaction rate curve. Xeasuring the initial slopes of the recorded curves often provides data related to the concentration of a specific species that reacts under controlled conditions. However, there has not been any emphasis in the literature on utilizing initial rate information from a p H stat for direct and rapid quantitative determinations. The instrument described here provides direct digital readout of initial rate data that are proportional to concentration. New procedures for the determination of
urea and glucose are introduced. The excellent stability of' this pH stat (better than 0.002 p H unit over several hours) is important for obtaining high sensitivity in quantitative determinations by the rate methods. The high precision and stabilitl of the expanded ranges of the p H :,tat are possible because of the true null-point servo operation of the direct-reading p H recording electrometer, which is patterned after a recently described instrument (4). The complete instrument is readily assembled from the unmodified inexpensive Heath pH recording electrometer, a Heath sei-vo recorder, and newly designed auxiliary components. The modular constiruction has the obvious advantages that basic units which have multipurpose laboratory applications are not tied up permanently in one instrument, and modifications and updating of basic units are easily accomplished. A simple optical control sensor is clipped onto the carriage rod, and as the pen carriage moves into and out of a narrow slit of light, a relay is closed and opened, The closing of the relay triggers the series of events which provides automatic readout of the total number of microliter aliquots deliL ered during a preset time interval (including an estimate to the nearest one tenth of an aliquot), and, if desired, provides the recording of the complete reaction rate curve on a standard 100-mv. potentiometric recorder. The designs of the control sensor and control devices for providing the digital readout and/or recording from an unmodified potentiometric recorder are described in detail. It was necessary to develop an automatic delivery-refill micropipet Fvhich would have high accuracy, simplicity, rapid response, and would not generate electrical transients that would foul the sharp response characteristic of the pH stat. The quantitative data presented indicate that a sensitive and stable p H stat has considerable promise as a direct readout measuring system for the important urea determinatiun. Only synthetic urea samples were used. The results in the 2- to 10-p.p.m. range were reproducible to about 1% and required less than 3 minutes of elapsed time to obtain the final result. The pH stat procedure for urea is based on the rate of hydrolysis of urea in the presence of urease; the pH is maintained constant a t about p H 6.2 by adding small increments of dilute HC1 to neutralize the ammonia formed; the number of increments of 0.0055 HC1 added during a preset time interval (about 2l/* minutes) is counted by the digital readout system, and the count is directly proportional to urea concentration.
The glucose procedure is based on the rate of its selective oxidation in the presence of glucose oxidase to form gluconic acid and hydrogen peroxide. The pH is maintained constant a t pH 6.5, by adding small increments of 0.002.V S a O H to neutralize the gluconic acid formed during the first part of the reaction. As in the urea method, the number of aliquots of reagent delivered during a preset time interval is counted and the count is directly proportional to the glucose concentration. This procedure has some advantages over the previous quantitative glucose reaction rate procedures because the only reagent besides the glucose oxidase is the stable sodium hydroxide solution for neutralization and no complex catalyzed secondary reactions are involved. However, the pH stat method for glucose is not so sensitive in its present form as a few of the ultrasensitive procedures recently described (5-8), and it has not been tested for interferences in specific samples. Samples with inherently high buffer capacity would cause poor sensitivity. Both the urea and glucose procedures are presented primarily t o illustrate the possibilities of using a digital readout pH stat t o obtain rapid quantitative results from reaction rate information during the initial stages of a reaction. Further work is in progress for determining urea and other constituents in specific materials by the p H stat methods illustrated herein. I n general terms, it is demonstrated that quantitative determinations through systems' control can be provided by a very precise and stable p H stat. INSTRUMENTATION
The basic modular components of the p H stat are shown in Figure 1. The Heath (Benton Harbor, Nich.) Model EUW-301 p H recording electrometer is easily adapted for providing the p H control signal by clipping an optical control sensor onto the pen carriage shaft. S o other modifications are necessary. As the pH of the solution which is contained in a temperature-regulated cell changes from a preset pH value, the edge of the pen carriage on the EUW-301 moves out of the light path and a relay closes, causing a small aliquot of neutralizing reagent to be delivered from the micropipet. Simultaneously with the delivery of an aliquot of reagent, the digital readout unit steps one full count, and the meter, which will display the fraction of an aliquot at the end of the analysis time interval resets to zero. Also an additional voltage increment equivalent to one chart division (1% of full scale) in sent from the stepper t o an unmodified potentiometric recorder which plots the
RECORDING pH ELECTROMET HEATH EUW-301.
DIRECT DIGITAL READOUT
STEPPER
.--i ____._____. THE RMOSTATED CELL (SHIELDED)
,
;/TIME
u c
MTIONS
Figure 1 . Basic building blocks of the digital readout and recording pH stat
complete reaction rate curve of volume us. time. After the pen on this recorder
reaches full scale (100 aliquots), the stepper resets to zero causing the pen to fly back to zero and continue to record increments, traversing the scale as many times as necessary to plot the entire curve. The dotted line to the recorder block (Figure 1) indicates that it is not necessary to connect a recorder when using the p H stat for only quantitative determinations utilizing initial first-order reaction rate data and direct digital readout. Specific characteristics of each component and device follow. Model EUW-301 pH Recording Electrometer. The data obtained in this work indicate a combined electrode-instrument short-term stability which is significantly better than 0.002 p H unit. The EUW-301 was switched to a span of 1 p H unit full-scale, each 0.1-inch movement of the pen representing 0.01 pH unit. A change of 0.001 pH unit can be detected. The apparent changes of pH about a control point can be recorded at any time by inserting chart paper, but this would not be done for routine work. The pH of a solution can be read or recorded at any time throughout the analysis because the normal functions of the pH recording electrometer (4) have not been modified in any may. This feature is particularly valuable during the preliminary investigation of an analytical procedure. Glass Calomel Electrodes. A combination glass electrode-calomel reference electrode (Thomas Co., Philadelphia, Pa.) was used for most work. It mas necessary to change the glass electrode connector from the old-style Beckman connector to their modern connector (used by Beckman, Corning, and some Coleman electrodes) so that it would fit the Heath Xodel E U f - 3 0 1 pH electrometer. The convenient size of the combination electrode (about '/&xh diameter) makes it VOL. 37, NO. 1, JANUARY 1965
35
ideal to fit into small beakers. The glass membrane has a low resistance (less than 100 megohms) and a fast response time, and it proved very stable. An Ingold combination pencil-type electrode was tested but it had about 1000megohm resistance and a slow response time of about 4 to 5 seconds. A miniature Leeds & Northrop glass electrode and reference electrode were used and the response to pH changes was also slow and somewhat erratic. Response characteristics can, of course, vary from electrode to electrode. Optical Control Sensor. An optical sensor, illustrated in Figure 2, detects a change in pH from its present control value. When the edge of the pen carriage on the ELW-301 moves out of a light beam (prefocused G.E. No. 222), the light, passing through a narrow slit, hits a small photoconductive cell (600 Series, Clairex Corp., ?;em York, K.Y.) which is wrapped in black electrical tape except for a narrow strip of the photoconductive surface. The change in light intensity changes the resistance from several megohms to a few kilohms,
To
Battery
Figure 2. Optical control sensor TO CENTRAL CONTROL UNIT
Ph
_.'
PH
and this resistance change is utilized to trigger both the addition of an aliquot of reagent and subsequent events. Room light does not affect the operation because the light beam is sufficiently more intense than room light and the cell is shielded by both the pen carriage and the metal strip containing an intermediate slit. The control sensor has another pen carriage shaft groove (not shown in Figure 2), symmetrical to the one shown and placed on the other side
k
of the pen carriage to allow pH control of solutions containing either acid- or base-producing reactions. The EUW301 reversing switch could be used instead of reversing the optical sensor. The circuitry which provides the necessary control signals following from the resistance change in the optical sensor is discussed under the Central Control L-nit. Central Control Unit. Once a departure from the initial pH of the
I.
7
,TI ,T2
I
115 v.ac. 60 CW
Figure 3. Relays K1, K2, K 3 , K4, K 5 are 110 volt dx., 5000
36
ANALYTICAL CHEMISTRY
Central control circuit
a.
All capacitors above 1 I f . are electrolyticr rated at 150 W V D C
7
IN1084
R8 IK
R 9 IOK
UNITS T E N S
-Figure 4. Relay
One-hundred increment stepper circuit
K6 is 35 volt d.c., 500Oil. Capacitors C6, C7, C8 are 150 WVDC; C13, C 1 4 are 20 W V D C
solution is detected, an air valve is turned on allowing air a t 25 1i.s.i. to flow to the piston of the micropipet which then injects an aliquot of neutralizing reagent,. Simultaneously, the digital readout unit is advanced one full count. and its meter, which will indicate the fraction of an aliquot. added a t t,he end of the ana'lysis t'iine, is reset to zero. Also the st,epper is advanced one unit. The air valve is returned to its original position aftert he aliquot. has been added. These operations are programmed by the central control unit shown in Figure 3. The resistance of the photoconductive cell in the optical sensor is a part' of a voltage divider (R1 and Rx), and the volt'age across Rx is applied between t,he grid and cathode of a 2D21 thyratron tube T1. When the pen carriage is in t'he light beam, the light on the photoconductive cell is low and the resistance Rx is a few megohms. Resistance R 1 is small lanough so that most of the voltage of battery B1 appears across Rx and t'he grid of the thyratron is sufficiently negative with respect to the cathode so that it does not conduct during the positive half-cycle of the l l b v o l t 60-cycle plate voltage. When the light beam enters the cell, the resistance Rx drops to a few kilohms. The negat'ive voltage at the grid is now sufficiently low with respect to the cat'hode so the thyrat'ron c0nduct.s and relay K1 is energized closing cont,acts a-b and d-e. The closing of contact,s a-b causes capacitor C2 to discharge
through t'he coil K12 which turns the air valve on, initiating the reagent delivery. Capacitor C2 is later charged up through R2 and D1. The closing of contacts d-e energizes relay K 2 , which holds itself closed via contact's f-g by making contacts h-i. Relay K2 opens cont,acts h-j t80 interrupt the charging path of C2. This prevents the charging of C2 which could reactivate the pipet, prematurely if a spurious noise pulse caused t'he pen carriage to jump in and out, of the light beam while the pipet was completing its cycle. Capacitor C4, which was initially shorted to ground by contact's ?i-m of K2, charges through R4 via contacts k-2,unt'il t.he plate voltage across T 2 reaches the point where conduction begins. Until that time there is no current in relay K 3 and it is not energized. This programmed delay allows t'he pipet to complete half its cycle before K 3 is energized closing contacts v-w which energize coil K11 returning the air valve to its original position, causing the air in the pipet piston to be exhausted to the atmosphere. This delay is controlled by adjusting R4. Resistance divider R5 and battery B1 also control the delay, but are adjusted so that the thyratron will not conduct until the voltage across C4 is high enough t.o provide sufficient current through K 3 to hold it, closed briefly. When thyratron T 2 conducts, C4 is discharged through K3 until the voltage drop across t'he tube drops below its ionization potent8ial. During the dis-
charge of C4, contacts f-g of K 3 are opened bo as to break contacts h-i and break the circuit which holds K 2 closed. .\lthough K 3 is held on long enough after contacts f-g open t o allow C5 to discharge t o the point where K 2 opens, it must not be held on so long as to allow K4 to open if at this time K 1 has not returned to its normal position (by the pen carriage moving back into the light path). Relay K 4 opens a brief time after K 2 opens because the voltage required to hold it shut is lo~rerthan the voltage required to hold K 2 shut. Relay K 4 has been turned on by the closing of contacts q-r of relay K 5 and holds itself closed via contacts s-u. K5 and K7 of the direct digital readout unit (Figure 5 ) are in parallel with K 2 via contacts s-t and close at the same time K 2 does. Upon closing, K5 closes contacts n-p which advance the stepper (Figure 4) one unit. Relay K 5 also closes contacts q-r to energize relay K4. Relay K 4 in turn opens K 5 and KT, which as a result are only energized briefly. Relay K 4 holds itself closed by contacts s-u until contactsf-g are opened and C5 discharges sufficiently. The quick release of K 5 by K 4 allows capacitor C6 of the stepper to begin charging again via contacts n-o while the rest of the control unit completes its cycle. This also provides a momentary shorting by contacts f'-g' of relay K7 (Figure 5) across the integrating capacitor Cl1 in the direct digital readout unit. The control unit cycle has been cornVOL. 37, NO. 1,
JANUARY 1965
* 37
c;: 1
@
K 7
d-c relay
From CENTRAL CONTROL UNIT
From STEPPER
@ L
II
''
R12 I Me
'\
'a-
lyf
CII
e"
- 5
f'
II f
\
'\ \
_---------
L
-7
I
it
relay R13 u'
IK
T
P'
r'
COUNTER
R14
K9 d-C relay
CIO
s3
I'
s I (rrmotd - -X
y1 Sb 0'
Figure 5. Relays
K7,K8, K 9 are 1 IO
pleted when K3 energizes briefly. However, a t this time the pen carriage may not have moved back into the light beam and relay K1 may therefore still be closed. The K2-K3 cycle will be repeated until K1 is de-energized. Relay K4 remains energized during this time and K 5 and K7 do not recycle. The recycling does not affect the air valve which remains in the off position. One-Hundred Increment Stepper. The unit in Figure 4 provides 100 equal voltage steps which may be fed to a recorder to provide a stepwise digital reaction rate curve. I t s output i s also used by the direct digital readout unit to help provide a readout to a fraction of an aliquot a t the end of the analysis time, when the total number of aliquots is less than 100. The unit resets itself after 100 steps allowing subsequent groups of 100 steps to be recorded also. I n thia way a reaction rate curve may be followed regardless of the number of aliquots needed. Two stepping relays (Type MER, Guardian Electric) are used to 38
ANALYTICAL CHEMISTRY
a'
s4
S6
3
Direct digital readout circuit
volt d.c.,
5000 12; K10
is 1 15 volt, ax.
provide the voltage steps by stepping along a voltage divider of 1% precision resistors. The voltage across the voltage divider is supplied by a Zener regulated lo-volt, power supply. The voltage doubler configuration of D5, D6, C13, and C14 provides a filtered d.c. voltage of about, 18 volt,s from t'he 6.3-volt r m s . output of the filament transformer. The voltage divider having a total resistance of 1 niegohm is placed across the out'put. terminals D and E to provide the 100-mv. full-scale signal for a recorder. The 10-volt output of the stepper eliminates any problems caused by slight drifting of the unstabilized operational amplifier (.-I in Figure 5) to which this signal is fed in the direct, digital readout unit. The large volt'age also keeps the current through resistor R12 (Figure 5 ) significantly higher than the grid current of the input tube of the operational amplifier. Relay K 5 operated by t,he cent'ral control unit discharges capacitor C6 via contacts n-p energizing coil K13 which advances the Units st,epping relay each
C 9 is 150 WVDC
time a pipet addition is made to the reaction cell. Coils K13, K14, K15, and K16, about 100 ohms each, ordinarily operated by 115 volts a.c., are instead operated by discharging capacitors through them. This minimizes electrical noise as well as mechanical noise. Xcroswitches S8 and S9 have been mounted on each relay t o reset them when they have reached their tenth step. The microswitches are activated by a piano wire which has been connected to the ratchet wheel of the relay by a small bolt placed through a hole conveniently drilled by the manufacturer. The other end of the piano wire is threaded through a hole drilled in ths microswitch lever, and is bent so that tension on the wire will actuate the switch. The length of wire is adjusted so that the microswitch is thrown when the ratchet wheel is moving from the tenth position to the deventh position (9 t o 10, and 90 t o 100). Switch s8 causes the advance of the Tens relay at the same time it resets the Units relay by discharging
capacitor C7 through coils K14 and K15. The closing of microswitch S9 charges capacitor C8, through resistor RIO, which eventually reaches a voltage sufficient to cause relay K6 to close. Relay K 6 closes contacts c’-e’ discharging capacitor C8 through coil K16 which resets the Tens relay. This adjustable delay allows the stepper to provide a brief readout at the onehundredth step before the Tens relay is reset. I n this way part of each hundreth step appears a t the hundredth division (full scale) and part at the zero position on the chart paper. In setting the delay time the full-scale response time of the recorder, 1 second in the Heath ETW20X recorder, should be taken into account. The step1)er may be reset to zero a t any time by manually closing S8 and S9 in that order. Direct Digital :Readout Unit. X direct digital readout t h a t is proportional to concentration is provided by using a n electroinechanical counter to count the number of reagent aliquots added during a preset analysis time interval. A simple integrator circuit estimates and indicates the nearest one tenth of a n aliquot hypothetically adided between the final addition and the end of the analysis time interval. The circuit is shown in Figure 5 . The time interval for counting is controlled by a 3-inch diameter cani driven by a synchronous motor which opens and closes microsnitches 82, 83, 84, and 87. The cani rclquires 5 minutes for one complete revolution. Time intervals up to 220 seconds may be obtained with the present arrangement by positioning microsmitch S4. The time intervals reproduce within 0.1 second. I n some cases it is desirable to allow a reaction to proceed for a short period of time before the counting begins. The pre-analysis time can be adjusted from 0 to 1 minuie with the present 90’ indentation cut into the cani. After the pre-analysis time has occurred, the timer holds briefly and begins the analysis time intervsl in synchronization with the next reagent addition. The first addition is purposely not counted by the counter. The switches, relays, and cain in Figure 5 are shown ready t o begin the pre-analysis time. The sample is added to the reaction vessel and switch S1 is simultaneously closed manually. This closes the circuit to the synchronous motor via terminal a, and contacts k‘-l’, na’-n’, and b“-e”, and the cam moves until the round edge of the groove closes contacts h’-i’ and j’-k‘ of niicroswitches S2 and 83, respectively. Microswitch S3 stops the motor by opening contacts k’-l’. With the next addition of neutralizing reagent the central control unit energizes relay K7. Relay $3is in parallel with
K7 via contacts h’-i‘ and p’-y‘ and R13 stops the integration and the meter which ensures that K9 closes after K7 reading remains fixed a t the fraction of an aliquot hypothetically added from closes. When relay K9 is energized, the the last addition to the end of the motor is activated through contacts analysis time interval. The opening of 8 ’ 4 and j’-k’ and the analysis time contacts e”-f” by relay K10 opens the interval begins. Relay K9 also shorting circuit of the integrating completes its own circuit via contacts capacitor C11 to prevent the resetting of q ’ 4 t o hold itself closed throughout the analysis time interval. Capacitor C10 the meter t o zero as further aliquots are added. supplies enough current to K9 to pull it Because the integrator provides a completely closed during the transition linearly increasing voltage when its inwhen contact y’ is not in contact with put is a constant voltage, a source of either p’ or T ’ . constant voltage must be used which The closing of contacts a”-z’ by relay depends on the time interval between K7 would ordinarily cause the counter the final aliquots. This voltage is to advance for the first addition, except adjusted so the integrator will cause the contacts u’-u’ are still open. The closing meter reading to move from 0 to 1.0 of contacts d-t‘ by K9 completes the hot between final aliquots regardless of the side of the circuit to relay K8. When A time between these additions. K7 releases again, it completes the constant voltage source which is directly ground circuit to K8 via contacts y’-z’ proportional to the initial concentration and K8 closes. Relay K8 then comof the rate controlling reactant has the pletes its own circuit via contacts desired characteristics for the input to ~ ’ - 2 and ’ remains closed. The counter the integrator. For instance, at some circuit is now complete and will operate given time after the initiation of a first each time K7 closes contacts a”-z’, order reaction, the time between until microswitch S4 opens contacts aliquots is inversely proportional t o the b“-e”, breaking the common connection concentration of the rate-controlling to the counter. Diode 0 7 and capacitor reactant. .Isthe initial concentration of C9 provide a filtered d.c. voltage to the controlling reactant is doubled, the operate the counter and hold K9 shut. aliquots are added twice as fast to After the nieasurement time interval, maintain pH control. If the additions iiiicroswitch 84, which was held closed are being made twice as fast as by the cam, is released as it drops off the previously, the constant voltage input sharp corner of the groove cut in the to the integrator must be twice its cam. This stops the motor by opening previous value so that the meter contacts b”-c” and opens the circuit to moves from 0 t o 1.0 twice as fast as it the counter via b”-c” so that it stops previously did. counting. Microswitch S4 also closes The output of the stepper is an easily c”-d” energizing relay KlO which stops available source of the input voltage t o the integrator as described in Readout of the integrator. For a first-order lliquot Fraction below. The stepper reaction the number of aliquots added has continued to advance throughout during the analysis time is directly the reaction regardless of the condition proportional to the initial concentration of the digital readout unit, and if its outof the rate-controlling reactant. The put is being recorded no part of the stepper output at the end of the analysis reaction rate curve is lost. READOUTOF ALIQUOT FRACTIOK.time is therefore proportional t o the initial concentration, and can be fed The fraction of an aliquot is read from directly to the input of the integrator. the meter face, Figure 5 . The reading Resistor R12 has been made large of this adjustable voltmeter (R16 in enough so that the load current from the series with the 1-ma. current meter) is voltage divider of the stepper circuit is controlled by an integrator so that at negligible. Also the voltage output from the tiiiie of addition of the final aliquot the stepper has been made sufficiently the meter is set to zero by the molarge so that the current passing through mentary shorting of capacitor C11 via R12 is a few orders of magnitude larger contacts e”-f” by contacts f’-g’ when than any grid current supplied to the relay K7 closes momentarily. The integrating capacitor C11 by the grid of meter reading then increases linearly the input tube of the operational with time a t a rate such that just prior amplifier, -4 (Heath inodel EUW-19.4). to the addition of the next aliquot the Since the stepper voltage output is meter reads 1.0. At any time between not continuous and the time between the these two additions the meter reading additions themselves is not infinitely will be approximately equal to the short the results obtained in practice are fraction of an aliquot hypothetically only approximate. However only 1070 added up to that time. If the analysis accuracy is required to provide a readtime interval ends between these two out to the nearest one tenth of an additions of aliquots, switch S4 closes aliquot . contacts c”-d” which energizes relay The resistor R16 is adjusted during K10 via contacts j’-k‘ and 8’-t’. Rethe initial setup of equipment so that sistance R12 is thus shorted to ground the meter reads 1.0 for the time interval by the closing of contacts h”-i”. This VOL. 37, NO. 1, JANUARY 1965
39
between the final steps. This is easily done by running a sample and visually following the maximum meter reading near the end of the analysis time. Resistor R16 is adjusted so that as the last addition is made the meter reads 1.0. Because the time interval may vary about 5% in either direction an eye average of the maximum readings of the last few steps may produce a better setting than just the last step itself. Once R16 is set, however, it need not be reset for succeeding runs unless the number of milliequivalent5 per aliquot added is changed or until a different species in another cheniical system is measured. As the last addition is made, relay K7 resets the integrator output to zero by shorting C11 via contacts f’-g’. The integrator output voltage moves the nieter pointer linearly toward 1.0. At the end of the analysis time interval, microswitch S4 closes contacts c”-d” energizing K10, via contacts j‘-k‘ and s’-t’, which stops the integration by shorting R12 to ground via contacts i/‘-h’I . If this time occurs one fourth or half way between additions the meter will read 0.25 or 0.5, respectively, indicating the fraction of an aliquot which is hypothetically added since the last addition of an aliquot. Relay K10 also opens contacts e”-f” which opens the shorting circuit of C11 SO that the integrator will not be reset by K7. The dicisions on the nieter face are such that 1.0 may be set as only a fraction of full scale, and only a fraction of the available amplifier output voltage is used. The available output voltage is limited by loading the amplifier with resistor R l 5 so that if the operational amplifier goes into limit b e h e e n runs the meter is not overloaded. RESETTIKG.After the analysis time the cam may be automatically reset to its pre-analysis time position by niaking contacts m’-o’ by manually throwing switch 85. ilIircoswitch S7 is already closed. Contacts s‘-t’ and j’-k’ are still closed. The motor will therefore turn the cam until S7 opens, at which time S 5 is returned to its original position making contacts m’-n’ and the tinier is ready to start another run. During the resetting, switch S3 will have broken j‘-k’ and made k‘4‘ but the motor will continue to turn since contact 1’ is hot via terminal (a). If no pre-analysis reaction time is desired the timer is reset by closing switch 86. Then when S3 is in the grove of the cam, S1 is momentarily opened t o release relays Z
