A Thermometric Titrator with Direct Temperature Readout W. L. Everson Shell Development Co., Emerycille, Calif. IN THERMOMETRIC TITRATION, it is convenient and often desirable to know the actual temperature of the solution. A Wheatstone bridge circuit is described which, in conjunction with the strip-chart recorder used for recording the titration curve, permits direct reading of temperature over a 12 -degree range. It can also be used simply as a sensitive, accurate thermometer, Linde, Rogers, and Hume ( I ) were the first to employ continuous titrant addition and continuous temperature indication. To indicate temperature, they used a thermistor in an equal-arm Wheatstone bridge circuit; bridge output voltage was displayed on a strip-chart recorder. For the most part, later workers in the field have used variations of the same basic circuit. Tyson, McCurdy, and Bricker ( 2 ) described a differential titrator in which the voltage output of the bridge represented the temperature difference between the indicating thermistor and a reference thermistor. Circuits using the derivative principle, to permit automatic end point detection, have been described by Zenchelsky and Segatto (3) and utilized by Priestley (4). The problem of nonlinearity of voltage output with temperature is not severe when the temperature change is small, as is generally the case in aqueous titrations. Nonlinearity of the bridge itself compensates somewhat for thermistor nonlinearity. Jordan and Alleman (5) claimed their circuit to be linear within 1 or better over a range of *l " C. In nonaqueous work, where changes of 5"-10° C are not uncommon, need was felt for more accurate temperature indication and for greater freedom from the need for calibration in terms of a reference thermometer. Several ways have been described for improving linearity of Wheatstone bridge output with temperature (6-8). The circuit described here is based on the ideas of Nordon and Bainbridge (8). Thermistor current is held essentially constant by making two adjacent arms of the bridge very high in resistance compared to the thermistor and the remaining arm. Change of thermistor resistance with temperature is linearized by shunting with a resistor whose optimum value is given by rs
=
rT
(B (B
- 2T)
+ 2")
where rs is the shunt resistance, rT the thermistor resistance at absolute temperature T (the midpoint of the operating range), (1) H. W. Linde, L. B. Rogers, and D. N.Wume, ANAL.CHEM., 25,404 (1953). (2) B. C. Tyson, Jr., W. H.McCurdy, Jr., and C. E. Bricker, Ibid., 33, 12 (1961). (3) S . T. Zenchelsky and P. R. Segatto, Zbid., 29, 1856 (1957). (4) P. T. Priestley, Analyst, 88, 194 (1963). 29,9 (1957). (5) J. Jordan and T. G. Alleman, ANAL.CHEM., (6) A. B. Littlewood,J . Sci. Instr., 37, 185 (1960). (7) E. Pitts and P. T. Priestley, Ibid., 39,75 (1962). (8) P. Nordon and N.W. Bainbridge, Ibid.,p. 399. 1894
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ANALYTICAL CHEMISTRY
Table I. Circuit Constants Component 10K thermistors 3K thermistorse 480K 120K r1, r2 raa, rdS 7500 2250 3925 1170 r4 r d a , 18-19 72.6 23.1 19-20 72.9 23.2 20-2 1 73.2 23.3 21-22 73.4 23.4 22-23 73.6 23.5 23-24 73.8 23.5 24-25 74.0 23.6 25-26 74.1 23.6 26-27 74.0 23.6 27-28 73.8 23.5 28-29 73.7 23.4 1'4b, each unit 7.35 2.33 rk 10,410 3,130 1.34 V mercury 5 4 batteries used 5, 2.5, 1 C 4, 2, 1" c Sensitivity for fullscale deflectionb Other values: rap = 25K, 1-turn pot; r, = 500-ohm, 10-turn pof. S-1 and S-2 are Centralab PA-300 miniature rotary tap switches; S-1A is a Grayhill 10-tap switch. 4 Calculated values. b For 1-mv recorder.
and B the thermistor constant. Further, if suitable values are chosen for the input voltage and high-resistance arms, bridge output voltage per degree can be made exactly equal to the sensitivity of the recorder, providing direct readout. Figure 1 shows the schematic diagram for a thermometric titrator incorporating these ideas. Table I gives circuit data for 10,000- and 3000-ohm precise YSI thermistors (Yellow Springs Instrument Co., Yellow Springs, Ohio), a 1-mV stripchart recorder, and a range of 18"-30" C. Only the 10,000ohm thermistors have been used to date; data for 3000-ohm thermistors are calculated and have not been checked in actual construction. However, use of the latter offers some potential advantage; there is less chance of error from use of a recorder with too low input impedance and the consequent Kirchhoff distribution effects resulting from excessive bridge output current. Recorder input impedance should be at least 0.5 megohm/millivolt, preferably higher. Correct bridge voltage setting is provided by the calibration resistor, rk, whose resistance is that of a thermistor at 24" C. The bridge is first balanced at this temperature at 1O sensitivity with the Zero Adjust control, r,,; the range switch is then set to 23' and the recorder adjusted to full-scale deflection with the Span Adjust control, rsp. These adjustments are repeated once or twice, as the controls interact slightly. A thermistor is now plugged into J-l and the range switch
Ext
J-1
r3 s
To Taps on
S-1,S-lA = Range Switches S-2 = Sensitivity Selector S-3 = DPDT S-4 = SPST J-1 = Indicating T h e r m i s t o r 5-2 = Reference T h e r m i s t o r 5-3 = Output to R e c o r d e r
S-1A
B = rl - r4 = rsp = ro = rk =
1,3417 Mercury Batteries See Table 1 Span Adjust Z e r o Adjust Calibration R e s i s t o r
Figure I. Schematic circuit diagram of constant-sensitivity thermometric titrator
rotated to bring the recorder pen on scale. The temperature of the solution is the range switch setting plus the reading of the recorder chart (assumed to have a 0-100 scale). Temperature will be indicated correctly over the entire working range. For lower sensitivity, selected with S-2, full-scale deflection corresponds (for 10,000-ohm thermistors) to 2.5 or 5.0 degrees above the base temperature shown on the range switch, and the chart is read accordingly. For higher sensitivity, S-2 is set to "EXT" and an external voltage is supplied-e.g., 67.5 volts for 0.1" sensitivity. The decimal range switch, S-IA, is now used to supplement S-1; the temperature is the sum of S-1, S-lA, and the chart reading. For differential thermometric titration (Z), the reference thermistor is plugged into J-2 and S-4 is closed. The range switches are now inoperative. The bridge is balanced by immersing both thermistors in the same solution and adjusting r, and S-3.
The principal factors which influence accuracy are thermistor variation, and the accuracy with which the circuit resistances, especially those in r4a, duplicate the actual resistance-temperature curve of the thermistors used. For YSI precise thermistors, the manufacturer claims that variation in resistance at a given temperature is within 10.5 At 25" C and 1 O sensitivity, a deviation of 0.5 % from the nominal value corresponds to an error of 0.12" 6. Actual tests, in which two YSI thermistors were compared with a sensitive mercury thermometer, showed agreement within +0.06" to -0.02" over the entire range; for the 20"-27" range, maximum deviation was i0.03", only slightly greater than the uncertainty in reading the comparison thermometer. The following points may be of assistance in construction: the value of the dropping resistors (rl, rz) is not critical, but they should be closely matched before attempting other circuit adjustments. This is best done by connecting them temporarily in a bridge circuit with a reversing switch; resistance VOL. 39, NO. 14, DECEMBER 1967
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1895
of the other two arms is unimportant provided the recorder trace is brought on scale. Resistance is then added to the arm (rl or r ~with ) the lower value until, on reversing their position, the recorder trace remains at the same setting. Wire-wound resistors, preferably of the 1 precision type, are recommended for all fixed resistors except those used to adjust the values of the individual units in rda and r4b. These values can be adjusted by shunting precision wire-wound resistors with ordinary carbon resistors of considerably higher value. For the 10,000-ohm data of Table I, the r4a resistors were adjusted by shunting 75-ohm wire-wound resistors with carbon resistors having values of 2000-6200 ohms; it was possible in this way to match the given values within 0.1 ohm. The r4bresistors were made by shunting 7.5-ohm wire-wound resistors with 360-ohm carbon resistors; matching was
within 0.05 ohm for each unit, and within 0.1 ohm for the total combined resistance. Several titrators of this general design have been built and have proved convenient and satisfactorily accurate for general temperature measurement as well as for thermometric titration. The entire circuit, including the mercury batteries, can be housed in a standard 4- X 7- X 12-inch aluminum box. Construction time is perhaps eight to 16 hours, depending on facilities available, experience, and degree of effort in fitting the resistors to the correct values. Circuit data can easily be calculated for other thermistor, recorder, and range combinations. There is no inherent reason why range should be limited to 12 O C, but extension of range would probably involve more complicated circuitry. RECEIVED for review July 3, 1967. Accepted August 31, 1967.
A Small Volume Flow Cell for Simultaneous Polarographic, Coulometric, and Spectrophotometric Measurements Jiri Janata and Harry €3. Mark, Jr. Department of Chemistry, The Unicersity of Michigan, Ann Arbor, Mich. 48104
FLOW CELLS used for electrometric measurements have often been used in industrial control processes and long duration automatically controlled laboratory experiments ( I ) . Also, cells in which the electrolysis solution is set in motion by a stream of an inert gas, such as nitrogen, have been employed in certain polarographic analyses (2); and flow systems for the determination of the electron spin resonance (ESR) spectra of electrogenerated free radicals have proved to be very valuable in electrode reaction mechanism studies and free radical investigations (3). We have employed these flow system principles for the construction of a small volume flow cell in which polarographic, coulometric, and spectrophotometric measurements can be made simultaneously. This flow cell proved to be very helpful in studying the electrode reactions of the aromatic hydrocarbons, corannulene (4) and 4,5-methylenephenanthrene (5). DESCRIPTION OF FLOW CELL CONSTRUCTION
This device consists of two main parts: An all glass flow cell (Figure 1, projection in direction of light path) and a light tight metal mounting case (Figure 2) which fits into the sample cell compartment of a Cary Model 14 UV-VIS Spectrophotometer. (If no optical measurements are necessary for a particular coulometric study, the glass flow cell can be used without the metal frame.) The flow system was designed to have as small as possible solution volume (approximately 10 ml) so that it is possible to conserve sample and solution. The electrolysis solution is set in motion in the flow cell by a stream of an inert gas (usually purified nitrogen) which enters the cell through the inlet, L. (1) Z . P. Zagorski, in “Progress in Polarography,” Vol. 2, P. Zuman,
Ed., Wiley, New York, 1962. (2) P.Zurnan and 0. Manousek, J. Heyrovsky Polarographic In-
stitute, Prague, private communication, 1967. (3) R. Dehl and G. K. Fraenkel, J. Chem. Phys., 39, 1793 (1963). (4) J. Janata, J. Gendell, C.-Y. Ling, W. Barth, L. Backes, H. B. Mark, Jr., and R. G. Lawton, J. Am. Chem. Soc., 89, 3056
(1967). ( 5 ) J. Janata, J. Gendell, R. G. Lawton, and H. B. Mark, Jr., University of Michigan, Ann Arbor, Mich., unpublished data,
1967. 1896
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ANALYTICAL CHEMISTRY
The bubbles of gas rising in the spiral tube carry along the solution, which results in a uniform counterclockwise flow (provided a uniform gas flow is maintained). The gas bubbles then separate from the solution in the cell head (the gas escapes from the cell through outlet, M) and the solution then flows into the lower part of the system toward the working electrode. The dropping mercury electrode (DME), indicated by symbol A , an auxiliary gas inlet, B, and the salt bridge from the external reference electrode, C , are introduced into the cell head through the Teflon cell cap, D . A flexible polyurethane strip, E, is wrapped around the neck of the cell head to hold it in the center of the split Teflon ring, F, which is then slid sideways into the top of the metal case. A slotted metal piece, N , is then slid into the metal case top. This arrangement gave a flexible yet firm support for the glass body of the flow cell. Although this flow cell has been used only for polarographic (DME) studies, a platinum wire micro electrode, G, is sealed permanently into the glass wall of the top compartment (or section) for possible electro-oxidation studies. The lower part of the flow cell is compartmented. The Pt auxiliary electrode, I , is contained in one of the compartments which isolates the electrolysis solution under study from mixing with the solution of the auxiliary compartment. The auxiliary electrode compartment has two stopcocks which allow for separate draining and filling of this compartment. A sintered glass disk, W,separates the auxiliary electrode and flow section of the cell, but, of course, it allows electrical contact between the two compartments. The exhaustive electrolyses take place at the mercury pool electrode, J . The electrolysis solution flows across the mercury pool electrode surface, into the quartz optical flow cell, K (Precision Cells Inc.), and, then, up to the upper cell head for recirculation. It was found convenient (to prevent accidental dropping of the optical cell) to cement the ground glass inlet and outlet joints of the optical cell with a small amount of epoxy resin. Experimentally it was found that the time interval between initiation of production of colored product at the mercury pool working electrode and its subsequent arrival at the optical cell was about 0.5 to 1.0 sec using a nitrogen glass flow of 150 to 30 ml/min, respectively. The rate of flow of the nitrogen gas pump can be regulated by the stopcock, 0.
