Increasing the Sensitivity of the Beckman Zeromatic pH Meter Jack J. Keegan and George Matsuyama, Scientific and Process Instruments Division, Beckman Instruments, Inc., Fullerton, Calif. RECISE
potential measurements using
(200 X 10-0 ampere), Equation 2 can be written desired full scale sensitivity (volts)
P a variety of high resistance elec-
trodes are of constantly increasing interest. The common instrument available for potential measurements of high resistance systems is the pH meter. Many of these meters are not so sensitive as is required, for example, in the measurement of the pH of blood or of sodium ion concentration using glass electrodes, such as the Beckman 78137V or 78178V electrodes. However, the operating principle of the Beckman Zeromatic p H meter allows its millivolt sensitivity to be increased by simple external additions without extensive internal changes aa suggested by T. A. Taulli [ANAL.CHEM.32, 186-9 (1960)l. OPERATING PRINCIPLE
OF
METER
The Zeromatic pH meter operates as a feedback amplifier, the purpose of which is to give a voltage reading on a meter without drawing an appreciable current, making possible the measurement of the potential of a high impedance electrode system without polariz-
Table
1.
Feedback Resistances for Scale Expansion
Scale Expansion
Desired, Mv. (Full Scale)
Ohms
1x
1400
7000
2x
700 280 140
3500 1400 700
(standard scale) 5x
10x
Ein,
Rfb,
Rfb
Figure 1 .
Feedback amplifier
Input voltage to be measured. Eamp. Amplifier input voltage I,. Amplifier output current Rib. Feedback resistor Eout. Amplifier output voltage €in.
ing the electrodes. The operating principle of a feedback amplifier is shown schematically in Figure 1 . Suppose a voltage Ei. is injected with the polarity as shown. Since R f b is a very low resistance compared to the amplifier input resistance, most of the voltage would a t first tend to apnear as E,,,. However, the amplifier upon receiving this positive signal immediately produces current, I,, which gives rise to a potential in the opnosite sense (negative feedback) through the feedback resistor. The amplifier very quickly adjusts itself, so that Eout q u a l s E,, minus Eamp.Tf the gain of the amplifier (ratio of E,,,, to E,,,) is high, E,,, is negligible compared with Eout. Thus El, and E,,,* are essentially equal and variations in the gain do not show up in the meter. Since the output current through the meter is the same as the current through the feedback resistor, the Ohm's law equation may be written as
Equation 1, solved for the value of the feedback resistor required for any desired sensitivity, becomes
Table II. pH Scale Expansion
T$"dl., 0 5 10 15 20 25 30 35 40 45 50 55
60 65 70 75
80 85
90 95 100
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Nernst Coefficient, Mv./pH
Rfi, Ohms
54.2 55.2 56.2 57.2 58.2 59.2 60.1 61.1 62.1 63.1 64.1 65.1 66.1 67.1 68.1 69.1 70.1 71.1 72.1 73.0 74.0
ANALYTICAL CHEMISTRY
379 386 393 400 407 414 42 1 428 435
442 449 456 463 470 477 484 49 1 498 505 511 618
EXPANSION O F MILLIVOLT SCALE
It is very simple t o change the feedback resistor in the Zeromatic pH meter, because this instrument was designed to operate with an externally connected thermocompensator acting as the feedback resistor. Therefore, no internal modifications are necessary. By plugging the proper resistor in the thermocompensator jacks on the rear terminal board and operating the instrument for automatic thermocompensation (depress AUTO button), the sensitivity can be increased. The proper feedback resistance for any desired sensitivity can be selected by using Equation 2. Since I , (full scale) for the Zeromatic p H meter is 200 pa.
=
200
x
10-
(3)
Table I illustrates the application of this equation to a few examples of sensitivity increase on the millivolt range. The resistors used for Rfi should be within 0.5% of the calculated resistance, since the accuracy of the scale expansion depends on the accuracy of the resistance. Asymmetry control allows 0 mv. to be set a t any point on the scale, whether the meter is operated on the sensitive ranges or on the standard scale. By shorting the meter input one can set 0 mv. a t any desired point. For example, with R f b = 700 ohms, setting zero a t the right end of the scale allows readings from 0 to 140 mv. to be made; zero a t the left extreme gives a scale of 0 to - 140 mv. ; zero a t midscale gives a =t'iO-mv. scale, etc.
+
EXPANSION
OF
p H SCALE
The measurement of pH involves measurement of the electromotive force of a suitable galvanic cell, commonly consisting of a glass electrode and a reference electrode. The potential of this cell varies with the pH and temperature of the solution in which the electrodes are immersed. In Table 11, the Nernst coefficient is the change in the cell potential when the p H changes by one unit a t temperatures from 0" to 100" c. To expand the pH scale using the Zeromatic pH meter, two simple schemes are available. One alternative is to expand the millivolt scale as described above. A convenient expansion is to 140 mv. full scale. By comparing the potential of the cell when the electrodes are immersed in the sample solution with the potential when the electrodes are immersed in a standard buffer a t the same temperature, and dividing the change in potential by the Nernst coefficient a t the temperature of the solutions, one can calculate the difference in pH of the two solutions. A second alternative is to expand the scale so that the full scale potential corresponds to 1.4 pH units (1.4 times the Nernst coefficient). In Table 11 are presented the values of the feedback resistor necessary to give the desired scale expansion a t various temperatures. If either of these schemes for expanding the pH scale is used, it is
can he used to make fine adjustments. Some typical circuits for this purpose are shown in Figure 2. Since no attempt is made to supply temperature compensation, the pH valves in Figure 2 are only approximate (see Nernst coefficientin Table 11). LIMITATION
Figure 2.
Scale shifting circuits
important to remember that the pH meter expands potential measurements around zero, which corresponds to pH7onthemeter. Theasymmetrycontrol allows the zero potential reading to heshiftedtoeitherendofthemeterscde. Thus, if the scale is expanded to 1.4 pH units full scale and the pH meter asymmetry control is adjusted so that the meter needle is a t 7 with the electrodes immersed in pH 7 buffer, the pH meter scale actually corresponds to pH values of 6.3 to 7.7. If the meter needle is adjusted to 0 with pH 7 buffer, the scale corresponds to pH 7.0 to 8.4. If the meter needle is adjusted to 14 with pH 7 buffer, the scale corresponds
to pH 5.6 to 7.0. Because of the limited range of the asymmetry control, the Zeromatic pH meter with scale expanded to 1.4 pH units full scale is useful in the range of about 5.6 to 8.4. To use this expanded scale at pH values outside this range, it is necessary to introduce a scale shifting potential into the reference lead. For example, if it is desired to have pH 4 at the center of the meter scale at 30" C., one must apply a potential of 3 X 60.1 or 180 mv. to the reference electrode. Simple voltage dividers, using a flashlight cell or mercury cell, can he made to apply approximately the necessary voltage and the pH meter asymmetry control
There are limitations to which the sensitivity can he increased. Since the Zeromatic pH meter is a line-operated instrument and there is sure to be a certain amount of line "noise," 140 mv. full scale is as far as the sensitivity should he increased with standard operation. It is possible to increase the expansion to 35 mv. full scale by using a constant voltage transformer to stabilize the line voltage. By using the transformer and increasing the pulse speed of the relay in the Zeromatic, an expansion to 7 my. full scale may be used. To increase the pulse speed, changes must he made in the circuit: Replace R28 with a %megohm resistor and R29 with a 68,M)O-ohm resistor. Common 5% 0.5-watt carbon resistors can he used. This modification increases the relay pulse rate to approximately 5 pulses per second.
Adapter for Alpha Counting with Conventional l o w level Beta Counter M. 1. Gonshor, J. E. Green, and R. E. Wood, Kennecott Research Center, Kennecott Copper Corp., Salt Lake City, Utah
WING the publication of
U. S. Geological Survey Bulletins No. 1097 A and 1097 B, eonsiderahle interest has developed in mining exploration circles in the age dating of rocks by the lead-alpha method [Larsen, E. S., Jr., Keevil, N. B., Harrison, H. C., Bull. Geol. SOC.Am. 63, 1045-52 (1952)l. An improvement in technique for alpha radioactivity measurements is described in the present paper. Since the appropriate accessory minerds for this determination, zircon in our case, and monazite or xenotime, are difficult and expensive to separate, only a minimum is collected. For t,his
r
Disassembled components
for alpho counting
reason also, the same sample must be used for the alpha count and for the lead determination. This means that a sample presentation system for alpha counting is required such that the entire sample can he recovered and arced in the spectrographic measurement for lead. This being the case, the use of mixed sample and scintillator or 4 pi alpha counters is ruled out. A standard Model 314 Packard Instruments Tri-Carb low level liquid sriutillation spectrometer was adapted for the detection and counting of alpha radiation. A prerequisite was that any alterations in the equipment be such that a conversion from beta to alpha detection or the reverse he accomplished with a minimum of effort, time, and equipment. Figure 1 shows the components of the final unit, disassembled to show construction details. Figure 2 is a photograph of the assembled unit. The assembled unit is shown mounted on a tripod which has one vertical leg and two slanted legs. This construction permits placement of the entire device in the corner of the liquid scintillation counter freezer chest. To change the Packard unit from low level beta radiation measurement to alpha detection, it is
only necessary t o remove a preamplifier from its standard position in the freezer and place it On the
~~~~i hop^^^^^^ adjustment of the multiplier
high voltnge to t,hat required for the particular tube in use are all that is required on the control panel. The cables connected to the unused preamplifier of the l'acksrd 314 unit are
.
Figure 2. Assembled adapter unit proportion Fig. 1 VOL 33, NO. 9, AUGUST 1961
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