A Servo-Controlled pH Stat Makio Murayama, Joseph M. Conlon, and Grant C. Riggle, The National Institutes of Health, Public Health Service, U. S. Department of Health, Education, and Welfare, Bethesda, Md.
of pH stats in bioT chemical investigations has been reviewed by Jacobsen et al. (2), in HE SUBJECT
which they described a variety of meter relays (photoelectric or capacity-operated types) useful for sensitive control. These devices maintain constant pH during a chemical reaction by the addition of an acid or an alkali. They generally use solenoid-operated burets or motor-driven syringes. They prcvide “go-no go” delivery, injecting a fixed quantity of reagent per signal. This principle of operation does not take into account the problem of overshooting the predetermined pH value upon delivery of the reagent into the reaction vessel. The instrument described is an improved model in which the output signal from the pH meter, used to indicate the solution normality, is converted through a servo amplifying system into a proportional rate of chemical reagent delivery. Thus, a continuous method of injection is achieved and improved control of p H accomplished. Control of 0.01 pH unit is maintained. The instrument is used in the study of protein structufes and amino acid sequences. INSTRUMENTATION
A line-operated pH (electrometer) meter (Leeds & Northrup, Model 7664-A1) using conventional glass elec-
Figure 1.
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trodes (Beckman Instrument Co.) provides a 20-mv. output signal used to control the preselected pH of the solution contained in a temperaturecontrolled reaction vessel (Figure 1). This signal was originally intended to drive a recording potentiometer and is taken out of the meter through the outlet plug for the recorder. The signal is algebraically added to the output of the “set pH” 10 turn potentiometer. When this sum is zero, the motor does not drive and no reagent is added. A change in pH of solution or a manual change of the ‘‘set pH” dial will cause a signal which drives the motor and adds reagent. The solution is mixed bv conventional magnetic stirrers. The *difference between a battery-supplied reference signal and the pH signal is fed into the servo amplifier (Minneapolis-Honeywell, Brown, No. 357504-3 with converter 354210). The speed of the servomotor (Minneapolis-Honeywell, Brown, No. 364949-1 144 RPM) driving the micrometer-syringe buret (‘AGLA’ brand, Burroughs Welcome and Co., Tuckahoe, N. Y.) which contains the reagent to be added is directly proportional to the difference signal (at the slow speed used). A damping circuit is added to prevent the servomotor from hunting as the solution pH approaches the predetermined value. A small portion of the motor control voltage is added to a 3-volt 60-cycle a.c. signal and rectified. Th’s signal is velocity proportional. The d.c. level is removed and a portion of this signal is
applied as negative feedback to the servo imput. Since only one reagent is pumped, motor rotation is unidirectional and the feedback signal is unidirectional. Operation in this manner constitutes a highly sensitive regulator rather than a bidirectional servo. The servomotor is connected to the syringe-micrometer buret through a mechanical drive system (a pawl and a yoke) similar to that described by Jacobsen and LBonis ( I ) . One innovation is added: a one-way clutch (Formsprag, Model F-02) placed between the servomotor and gear train driving the micrometer-syringe; this permits the potentiometer attached to the gear train to be driven in one direction only. Recording System. A stable d. c. potential is applied to a potentiometer (Helipot Corp., Model A 10,000 ohms, 10 turns) geared to the buret drive. The applied potential is adjusted to the full span required by the recorder. Any standard millivolt recorder may be used. The recorded signal is a measure of the rate and the total volume of reagent injected into the reaction vessel. LITERATURE CITED
(1) Jacobsen, C. F., LBonis, J., Compt. rend. trav. lab. Caarlsberg, Slr. chim. 27, 333 (1951). (2) Jacobsen, C. F., LBonis, J., Linderstrom-Lang, K., Ottmen, M., “Methods of Biochemical Analysis, Vol. 4, pp. 171-210, Interscience, New York, 1957.
A new servo-controlled pH stat which provides a continuous method of injection and improved control of pH
ANALYTICAL CHEMISTRY
