Chemical Instrumentation .
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5. Z. LEWIN, New York University, Washington Square, New York 3, N. Y.
T h i s series of articles presents a sulvey of the basic principles, characteristics, a n d limitations of those instruments which find important applications i n chemical work. The emphasis i s on commercially available equipment, and approximate prices are quoted to show the order of magnitude of cost gf the various lypes of design and c a s t r u c t i a .
5. pH Meters (Continued) Coleman Instruments, Inc. One of the oldest manufacturers in the field of pH instrumentation, and a ~ i g nificant contributor to new developments in that field, is Coleman Instruments, Inc., Maywood, Illinois. I t s current manufacture includes both a line-operated direct-reading pH meter, and x hatterypowered, null-halance instrument. The line-operated meter is their Model 18A ($230, complete u i t h electrodes). Its circuit consists of s n electrometer tube (CK571), the grid of which is connected to the glass electrode, while the cathode is connected through a resiqtanre chain (inchding n feedhack resistor) to the reierenee electrode. The tube current determines the potential s t its plate, and this is direct-coupled to the grid of the amplifier tuhe (CK546). The read-out meter responds to the IR-drop in the plate circuit of the amplifier tuhe. Tube voltages are obtained from a, regulated (i.e., stabilized) electronic power supply. Negative feedback stabilizes the output of the amplifier against fluctuations in the power supply voltages or tuhe ehsracteristics. Thus, it can be seen that the circuitry is basically similar to that previously described in greater detail in connection with the Beckman Model H-2. -This instrument is reproducible to 10.02 pH unit, and accurate to +0.05. Range is 0 to 14 in pH and 0 to 1400 millivolts. I t can be equipped to can). out titrat,ions with polarized electrodes, and can he connected to the Coleman Autotrator to control a n automatic t,itration assembly. The null-balance instrument currently being produced by Coleman is their Compax Model 20 pH Electrometer ($200, complete with electrodes). This is a very compact instrument and has severs1 unusual features, which permit it to be simple in operation without sacrificing good orecision and accurrtcv. A sim~lified version of the basic circuitry is shown in Figure 16. Very stahle mercury batteries are used for the heaters of the vacuum tubes and for the pH slideu+re current. The eonstancy of the voltage output of these batteries is sufficiently great that it is not necessary to have 8, zem adjust control for standardizing the slidewire. The only adjustment provided is the bins ~
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adjust, which permits the amplifier to be recalihrated ( c j . Figure 10, and its secompmying discussion of calibration versus standardization) when necessary. I n this, and similar instruments, the stability is such that the bias adjust need not be used often. Calibration of the slidewire is accomplished, in conjunction with 8. standard buffer, by twisting the dial scale physically relative to the slidewire unt,il the d i d marking equal to the buffer pH coincides with the position of balance of the slidewire rontartor.
feature R . C ) . As soon as the condenser voltage reaches the firing potential of the gbw lamp, a gas discharge occurs, and the tube becomes a n excellent electrical conductor. This short-circuits the condenser, causing its voltage to drop rapidly to zero. Since the glow tuhe no longer has s. voltage across its electrodes, the gas discharge ia extinguished, and the condenser begins to charge again. Hence, the glow tube will he observed to flash a t a regular ratc, determined by the rate of charging of the condenser. If the resistance and capacitance are constant, the rate of flashing is determined by the magnitude of the applied voltage. Thus, the rate of flashing of the glow tube in the circuit of Figure 16 ia a measure of the plate voltage of the amplifier tuhe (CK546), and can he used to read out thia voltage in the same fashion as the deflection of a meter needle is used to show visually the magnitude of the voltage applied across the met,er terminals. The Compax is reproducible to zt0.02 pH unit, and nceurat,e to 10.05. I t is graduated for rend-out only in pH units.
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Figure 16. Simplified schematic of the circuit of the Co1em.n Compox Model 20 pH Electrometer.
The method of detecting the balance paint in this instrument is unique among current pH metera. A neon glow tube, KE-2, is employed in place of the eonventiond meter movement. I t is characteristic of glow tuhos that they require a certain minimum (threshold) voltage applied to their electrodes before ionisntion of the gas atmosphere inside the tuhe envelope can occur. I t is this characteristic firing potential of the glow tuhe that is utilized as a voltage indicat.or in the Compnx Model 20. The functioning of a glolu-tube osrillator (often also called a velazotion oseilla to^) is illustrated schematically in Figure 17. The glow tuhe is connected in psrsllel with a, condenser that charg~s through a resistance. Initially, the tube is non-canduct,ing. and the condenser charger a t a rate determined by tho time canfitant of the oircuit (time eonatant = time needed to reach I l r of the find voltage =
Figure 17. A. A condenser altoched to a voltage source charger up 01 a rate determined b y the resistance and copocitance of the circuit. B. A glow tube in porollel with the condemer fires whenever the condenser voltage reocher the firing patentiol, thereby discharging the cmdenrer and initiating o new cycle.
Cambridge Instrument Co. A line of pH meters embodying several unique features of design is offered by the Cambridge Instrument Co., Grand Central Terminal, New York 17, New York. These instruments are all ac-powered, and include null-balance, dirert-reading, and continuous-recording typea. Two models of the null-balance type are offered. Both have hsaically the same measuring, circuit and use electron-ray tubes as detectors of the balance point (Continxed on page A664)
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Chemical Instrumentation in lieu of the conventional meter movement. The Laboratory Model ($275, complete with electrodes) has a, reproducibility of *0.02 pH unit and is accurat,e to =t0.05. The Research Model ($485, complete) has a more elaborate electronic power supply (with regulated voltage output), a. pH slidewire that can be set with grertter precisian, and includes R better electrode pair (of the MacInnetRelcher design). I t is reproducible to zt0.01 pH unit and accurate to about the name order of magnitude. The basic circuitry of the Research Model is shorn sehoma,ticdly in Figure 18. r50u
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Figure 18. Simplified schematic of the Combridge Electron-Roy pH Meter, Models L and R.
I n this circuit, the glass electrode is connected to the contml grid of the electrometer tube (657). The reference elcctrode is connected, by way of two slidewires and a variable resistor, to the electrometer cathode. Thus, the difference of potential between ~d and cathode (which determines the magnitude of the current Bowing to the plate) is made up of the following p&uts: (1) the electrochemical potential difference in the cell, E,, plus (8) the voltage drop in the "buffer adjust" slidemire between points 1 and 8, plus (3) the voltage drop along the two "pH" slidewires between points 3 and 4 plus (4) the potential difference along the cathode-biasing resistance hetween points 6 and 8. When a known buffer is in the cell, the "pH" slidewires are set to the proper values, and the reference output (i.e., the "null bitlance" output) is obtained by appropriate adjustment of the "buffer adjust." [In this, and most other instruments, this is also often called the asymmetry potential control, for the adjustment made a t this stage compensates for the internal potential existing in the glass membrane of the sensing electrode.] The role of the other adjustments shown in the circuit should be understandnhle in the light of our previous discussions of similar circuits. The output of the electrometer tube is direct-coupled to an "electron-ray," or "tuning-eye," or "magic-eye" tube, which functions both as amplifier tube and output detector. The construction of such
Chemical Instrumentation a tube is shown in Figure 19. I t has two plates: the first one, called the triode plate, collects some of the current from the cathode stream; the second one, or target, receives the remainder of the tube current. The target is coated with a fluorescent compound which glows brightly when struok by electrons. In making use of this tube, a large resistance is connected between the triode plate and the target; e.g., in the circuit shorn in Figure 18, a 0.5-mogohm resistor is inserted between the two plates. When the tube current is appreciable, electrons flow through this resistance, producing an IR-drop that makes the first electrode (the triode plate) negative with respect to the target. Attached to the triode plate is a thin metal rod that is bent so as to stand close to the target. This is called the ray-control electrode, far if it is negative relative to the target, it deflects same of the electron etream and casts a shadow on the fluorescent screen. The extent of this shadow varies from approximately 100' of the target when the control electrode is much more negative than the target, to 0" when the control electrode is at approximately the same potential as the target. Thus, the extent of opening of the "eye" is proportional to the tube current, which, in turn, is proport~onslto the voltage on the control grid. The "closing of the eye" is a sensitive means of detecting a reference state of the grid potential, and can consequently he used, as in the Cambridge pH meters, as the detector of the balance point of a slidewire potentiometer. CATHODE
FLUORESCENT COATING
,
RAY-CONTROL ELECTRODE
re
TRIODE PLATE
--CATHODE Fiewe 19. Phyricoi layout of the electroder in on electron-ray vacuum tube.
The direct-reading Cambridge pH meters are based upon a different circuit from those treated thus far. A simplified diagram of the Camhridge pH Recorder ($1531, complete) is shorn in Figure 20. Two electrometer tubes are employed in a balanced arrangement, with the difference in potential between the glass and reference electrodes being impressed on the grids of the two tubes. Due to the IRdrops in the plate resistors, the potentials at the plates (points A and B) are determined by the tube currents, and hence by the grid voltages. The difference in potential between the two plates is fed into t,he recording potentiometer, and a continuous record may be obtained of any voltage variations occurring in the electrochemical cell. A full-wave bridge rectifier (shown in the lower center of the Figure) converts ae to dc to provide the slidewire current. This company also offers 8. non-record(Conlinved on page A886) Volume 36, Number 1 I, November 1959
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Chemical instrumentation ing, direct-reading pH met,er based upon this balanced-amplifier t,ype of circuit. I t is their Indust,rinl Modcl ($350, romplete) and has n reprodueihility of +0.05
resistor (c.g., a platinum wire winding; the kmperature compensator resistance ia not shown in the diagrams presented here) that takes the place of the manual t,emperature adjust, and mtomstically compensates for variations in the temperature of the electrochemical cell. I t is mounted in the solution, alongside the voltage-sensing cloctrodes.
ferent potentials a t the two cathodes (points A and B ) hecause of the different IR-drops in the two cathode hranches. The read-out meter deflects in proportion to this voltage difference.
Photovolt Corporation
Figure 20. Simplified schematic of the Combridge pH Recorder.
and an aocurary of 0.1 pH unit. The direct-reading instruments incorporate in their circuitry a temperature-scn~itiv~
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Simple, direct-reading pH meters arc a specialty of the Photovolt Corp., 95 Madison Avonuc, Xew York 16, Xew 1-ork. All their models have baaienlly t,he same type of circuit, and all are lineoptmted, except for Models 125 and 125B, 1%-hleh are habtrry-powered. A simplified schematic of the circuitry of l'hotovolt pH meters is shown in Figure 21. The circuit eunaists of a halanced pair of electrometer tuhes, v i t h the glass electrnde contrrding one grid potential, whik t,he rcierence electrnde controls thc ot.hrr. [Thc pmecding sentence embodies a form of expression that is common in discussions of electrical circuitry, hot that can he misleading; it should always he clearly understood t,hst the electrodes do not exert their influences independently of each other. Only the d(ffe'el.ence of potential between them has any signifirsnre. This difference is what is sensed by the two contld grids.] Electrons roming from the negative side of the power supply How from point C t o w a d both cathodes. Any imhalanre in the two tube currents results in dif-
Figure 21. Simplified srhemotic of the Photovolt Model 1 10 pH Meter.
There is no standard cell in these instruments; calibration is performed with a standard buffer, and the characteristics (Continued on page Afl88)
Chemical Instrumentation of the ampli6er are controlled by use of a n auxiliary pointer and the "check-point" technique described previously. Model 85 (8135, complete) is the smallest member of the line. It is reproducible to 0.1 pH unit, does not have a millivolt scale, and the temperature and calibration adjusts are the same variable resistor. Model 115 ($175) is next in degree of elaboratenors. I t is reprodueible t o =t0.05 pH units, and has millivolt ranges to 1RW mv. Model 110 ($235) has a larger, more precise meter movement, permitting reproducibility in the measurements to 1 0 . 0 2 pH unit. I t also is marked in millivolts, with ranges to 1600 mv. Model 125 ($195) is battery-powered, although a line-operated power supply is available from the manufacturer that can be used in place of the batteries. The reproducibility is 1 0 . 0 3 pH unit. Model 125B is a modified version of the 125, designed for high sensitivity in the pH 6 to 8 range, which is of special interest in clinical blood chemistry. An indicating galvanometer is employed in place of the conventional meter movement, and the pH scale is expanded. All these pH metera can be adapted to titrations with polarized electrodes (as can, in fact, nearly all commercial directreading pH meters). The amplifier section, consisting of the two clectromctrr tuhes, is hermetically sealed in a plug-in
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U. Figure 22. Illustrating the role of internoi re*irtance in limiting the effective voltage of a bonery. A. An old bonery hor o large internal rerirtonce, m d connot deliver much Current to an external circuit. B. A fresh battery has m a i l internal residmce, and can deliver or much current ar externoi circuit demands.
unit which can be pulled out of the in(Continued on page A670)
Chemical Instrumentation strument and replaced with a new one with a minimum of inconvenience if tmuble should arise. I n Model 125 and 125B a very convenient means is pmvided for the checking of the characteristios of the instrument batteries. By means of a position on the main switch, the read*ut meter can be disconnected from the amplifier and connected across one or the other of the batteries, in series with a known resirtance, so that the test is made while the batteries are under load. If the battery is old, its internal resistance will he large, and it will not he capable of delivering much current to an external circuit. This is illustrated diagrammatically in Figure 22. Stated in other terms, it can be asserted that although the open-circuit voltage of an old battery may be close to its face value, the criterion of usefulness is not this, hut its voltage under load. Hence, in testing a battery, it must be allowed to deliver n current of the same order as, or greater than, that which will be drawn under actual use conditions. A vacuum tube voltmeter draws negligible current when measuring voltages. Therefore, when checking a battery with a VTVM, the battery should first be hunted with a n appropriate resistance, i.e., the resistor should he placed in parallel with the VTVM in order to draw current from the battery.
Leeds and Northrup Company A well-designed pH meter based upon the conversion of the dc voltage of the eleetrochemioal cell into an se signal, followed by amplification and rectification, is the Model 7664 pH Indicator of
mRecorder
Figure 23. Simplified schematic of the design principle d the Leedr and Northrup Model 7664 pH lndicotor. Actual circuit hor more rtoger of amplification, a regulated power supply, ond additional adjustable controls; in actual circuit the amplifier is connected a s a cathode follower lie., grid of second itoge coupled to dropping resistor in cathode circuit of Rr3t stage).
Lee& and Northrup Co., Philadelphia 44, Pennsylvania. A simplified diagram
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Chemical Instrumentation -~
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of the main features of the circuit of this instrument is given in Figure 23. The converter, shown in the upper left of the diagram, cont.ains a vibrating reed that is kept in motion by an ac-energized coil (not shown in the diagram). Each time the reed makes contact t o the left, the input condenser, C, charges up; each time i t contacts on the right, the condenser discharge8 (through R ) . Thus, an alternating signal is transmitted to the grid of the first stage of amplifiestion. The variations in tube current generated by this ac signal produce fluctuations in t,he potential of the plate, and these are transmitted through the interstage coupling condenser to the grid of the next tube. The output tube's current. flows up through resistors AB and LIP; the latter resistor introduces the feedback ~tahilization. Its role can be understood a8 follows.
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Figure 24. Basic principle of the inverse feedbock circuitry employed in the Leeds and Northrup Model 7664 p H Indimtor. R, is the feedbock resistor.
The signal to be amplified is E,; the tots1 input signal on the grid of the first. stage of the amplifier is the net potential difference: [ D to El [Gto HI [Ref. Eleebode to Glass Elecbode] Points G a r d H are variable contacts for use in the standaidiaabion and cdibration of t,he device, and may be disregarded for our present purposes. Thc IR-drop from D to E creates a voltage that is in opposition t,o E,. Hence the net input to the amplifier is the signal, Ex, minus a jraction qf the output. This is the necemary condition for negative feedback, as described in detail previously ( e j . Figure 8). A simplified diagram of the basic idea involved in this circuitry ia given in Figure 24. The feedbaok current flowing through resistor AR in Figure 23 croates the voltage drop that can he fed into a recording potentiometer to draw a record of the pH variations in the system; in Figure 24, the feedback current itself is shown as the reed-out signal. [ I t may be noted, a t this point, that the feedback signal from the amplifier is se, and must he converted hack to dc of the correct polarity in order to be applied in opposition to the input electrode voltage. This is accomplished hy an electronic converter circuit that has not been shown in the previous Figures. Figure 25 shows the role of the electronic converter in the over-all circuitry of the instrument.] In the Model 7664, the input amplifier (Continued on page A678)
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Chemical Instrumentation tube is a 12AX7, specidly selected for low noise level and high stability. All tubes are operated wit,h filtered rtc on the heaters, to minimise noise in the output. The instrument is available without the recording potentiometer for $309, eomplete with electrodes. The reproducibility is &0.02 pH unit; maximum limit of error is dz0.07 pH on the meter, 1 0 . 0 2 pH a t the recorder output. It may he used as a millivoltmeter with ranges up to 1400 mv. The input current is less amperes. than 1 X 1OWL2 This manufacturer also has avsilable a null-balance, battery-powered electrometer-type pH meter. It is Model 7663 ($535, complete) and is based on a single vacuum tube, with a sensitive galvanometer for detection of the "null" balance. Reproducibility is 1 0 . 0 1 pH
unit; input current is of tho order of 10-L8amperes.
Figure 25. Schematic of the Leedr and North. rup Model 7664 pH Indicator, showing relo. tion of electronic converter to feedbock and input m g e s of circuitry.
Macbeth Corporation
The first line-perated, direct-reading pH meters were made by the Macbeth Corp., Newburgh, New York, and their instruments are still commercially available. The current models include the Model A Laboratory pH Meter, Modol T titratmn pH Meter, and Model I Industrial pH Meter. These instruments are based upon a circuit utiliaing two vacuum tubes in s. balanced arrangement, with the glass electrode connected t o one control grid, and the reference elect,rade t o the other. The read-out meter measures the difference in plate potentials of the two tubes (compare this with Figure 21, in which the meter measures the difference in cathode potmtisls, in s n otherwise similar arrangement!. The power supply is regulated, i.e., ita output voltage is electmni d l y stabilized to remain relatively constant in spite of moderate fluctuations in the line voltage. The reproducibility of the Model .4 is rt0.05 pH unit; accuracy is +0.1. Tho Model T is similar, hut has additional voltage stabilization of the power supply, and is designed for convenient applioation t o potentiometric titrations, including "dead-stop" titrations. Model I is also similar. but with ~rovision for attaching recorders or controlling mechanisms to be operated by the pH meter. Analytical Measurements, Inc.
A very convenient, light-weight (3pound), miniaturized pH meter is available from Analytical Measurements, Inc.. Chatbam, New Jersey. Their Analytical Pocket pH Meter ($125) utilizes a aingle vacuum tube in a simple electrometer
G.E.;
Figure 26. Schemotic of the type of circuit employed in the Analytical Pocket pH Meter.
circuit, with a 25-0-25 microampere meter measuring the tube current. The circuit IS shown schematirrslly in Figure 26. The tube current flows up through the meter ss shown by the arrow, It. Tho small 3-volt battery sends an electron current through the meter in the opposite sense, as shown by arrow la. The slidewirc contactor is adjusted to make these two currents equal s t pH 7, whereupon the needle shows no net deflection and ~ t a n d s In its center position. (Continued on page A6741
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Chemical Instrumentation Philips, Eindhoven Thc pH meters of the Dutch manufacturer Philips, Eindhoven, are available in the 6.S. through Philips Electronics, Inc., Mount Vernon, New York. The line includes three direct-reading instruments. Model P R 9401 ($377) is a conventional battery-ponered electrometer circuit with three stages of s,mplificnt,ion and negative feedhaek st&hilieation. The reprodi~eibilityand accurary sro hoth &0.02 pH unit. Input current is not more than 2 X 10-I' amperes. Model P R 9400 ($925) is a high precision instrument based upon a vibrating condenser which converts the deetrochemical voltage into an nc signal that, is amplified, then rectified and read on a meter movement. Feedback stahilization is employed. Reproducibility and accuracy are +0.01 pH unit. The input current is less than 10F2 amperes. The instrument is line-operated, with electronic regulation of the power supply output voltage. Modd P R 9402 ($820) is based on the same circuitry as the 9400, hut is specifically designed for compactness, remote indication, and control applications in industrial installations.
Other Manufacturers Additional pH instrumentation from abroad is available in the U. S. through the following soorcee: C. A. Brinkmann and Co., Great Neck, New York ( p H metem of Metrohm Ltd., S ~ i t z e r l a n d ) ; I
