Direct-reading potentiometer

Because the cell potential is sensitive to the concentrations of the reactants and products involved in the cell reaction, measured potentials can be used, to determine the concentration of an ion. A pH meter is a familiar example of an instrument that measures concentration from an observed potential. The pH meter has three main components a standard electrode of known potential, a special glass electrode that changes potential depending on the concentration of H+ ion in the solution into which it is dipped, and a potentiometer that measures the potential between the two electrodes. The potentiometer reading is automatically converted electronically to a direct reading of the pH of the solution being tested. 

The electromotive force E or EMF) is defined as the maximum difference in potential between the two electrodes (right minus left) obtained when the cell current is zero. The ceE potential is measured usmg a potentiometer, of wliich the common pH meter is a special type. The direct-reading potentiometer is a voltmeter that measures the potential across the cell (between the two electrodes) however, to obtain an accurate potential measurement, it is necessary... 

Borie, W. T. A direct reading potentiometer for measuring and recording both the actual and the total reaction of solutions. Jour. Med. Research, 33, 1916-16, 296. 

McClendon, J. F. A direct reading potentiometer for measuring hydrogen ion concentrations. Amer. Jour, of Physiology, 38, 1916, 186,... 

Fig. 18.1. Standard hydrogen electrode connected to a Zn/Zn " half-cell with a salt bridge and external direct reading potentiometer.

A direct-reading instrument can be made by normalizing Ri to IQ and 1 to 1 V then (volts) = R2 (ohms calibrated as volts). The slide wire is calibrated directly in voltage units. The rheostat AR can be used to zero-adjust the potentiometer for the required normalization. Many commercially available potentiometers are direct reading. Provision exists for adjusting initially to different standard cell emfs. A typical value for is 1.019 V abs, when an unsaturated cell is used. 

The electrical and electronic control system consists of the field devices feedback system, electric motor drive system and the safety and manual logic system. The commands from the micros ar checked in the safety logic system for safe operation before being sent to the fluid power system. The fluid power system operates the device in the correct direction, at the required speed and force, to position it accurately. The feedback device (say potentiometer) is read by the micro and appropriate commands initiated. The device is also connected to the computer independent display on the console. Operation and alarm messages are diplayed on the VDU and logged by the printer. 

To measure the e.m.f. the electrode system must be connected to a potentiometer or to an electronic voltmeter if the indicator electrode is a membrane electrode (e.g. a glass electrode), then a simple potentiometer is unsuitable and either a pH meter or a selective-ion meter must be employed the meter readings may give directly the varying pH (or pM) values as titration proceeds, or the meter may be used in the millivoltmeter mode, so that e.m.f. values are recorded. Used as a millivoltmeter, such meters can be used with almost any electrode assembly to record the results of many different types of potentiometric titrations, and in many cases the instruments have provision for connection to a recorder so that a continuous record of the titration results can be obtained, i.e. a titration curve is produced. 

It is often more convenient to relate the potentiometer reading directly to concentration by adjusting the ionic strength and hence the activity of both the standards and samples to the same value with a large excess of an electrolyte solution which is inert as far as the electrode in use is concerned. Under these conditions the electrode potential is proportional to the concentration of the test ions. The use of such solutions, which are known as TISABs (total ionic strength adjustment buffers), also allows the control of pH and their composition has to be designed for each particular assay and the proportion of buffer to sample must be constant. 

Electrolytic Procedure. The cell is placed inside a vertical-column, Hevi-Duty, multiple furnace and is surrounded by an Inconel heat shield (Fig. 13). The temperature-control thermocouple (Pt vs. 90% Pt — 10% Rh) is located between the crucible and the heat shield and is protected by an outer ceramic tube. A calibration run is initially made without a charge in the crucible. A second thermocouple is inserted through the heat shield and centered on the cover of the electrolytic cell. This thermocouple is read directly by means of a potentiometer the temperature of this thermocouple corresponds closely to that of the melt. The temperature controller is adjusted until the second thermocouple indicates the desired temperature for the electrolysis. ... 

The majority of e.m.f. measurements are made at the present time by means of special potentiometers, operating on the Poggendorff principle, which are purchased from scientific instrument makers. They generally consist of a number of resistance coils with a movable contact, together with a slide wire for fine adjustment. A standard cell is used for calibration purposes, and the e.m.f. of the cell being measured can then be read off directly with an accuracy of 0.1 millivolt, or better. 

As the various parts of the manipulator are moved, changed in potentiometer and encoder readings are directly seen as changes in the ADM image. 

In general, the determination of pH using a pH meter is quite simple. The solution is placed in a small container. The glass and the reference electrode (or the combination electrode) are dipped into this solution so that the glass bulb is dipped completely. The potentiometer circuit is closed and adjusted till the null point is obtained. The pH can then directly be read off the display. Two general precautions must be taken ft) As stated above, the pH meter must be standardized before every measurement, and fii) when not in use, the reference and the glass electrode must be kept immersed in water so that the pH sensitive membrane does not become dehydrated. 

We shall not describe the details of the potentiometer. We may regard it as a galvanic cell, the emf of which can be varied continuously and read from a dial. The potentiometer is connected so that it tries to make a current flow through the cell in the direction opposite to the current generated by the cell reaction. The potentiometer is then adjusted until the current is zero. At this point, the known emf of the potentiometer is equal to the emf of the cell, measured with no current flowing. The emf thus determined is a measure of the ideal maximum amount of work that can be obtained from the chemical reaction taking place in the cell. ... 

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