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INDUSTRIAL AND ENGINEERING CHEMISTRY
Acknowledgment The authors are indebted to the International Nickel Company of Canada, Ltd., for a generous gift of platinum metals.
Literature Cited (1) Claus, C., Pharm. Centr., 1846, 833. J . C h e m M e t . Mining SOC. fl. Africa, 25, 296 (2) Cooper, R. (1925). (3) Feigl, F., “Qualitative Analysis by Means of Spot Tests,” P. 185, Leipzig, AkademiyFhe Verlagsgesellschaft, 1931. (4) Fulton and Sharwood, Manual of Fire Assaying,” p. 225, New York, McGraw-Hill Book Co., 1929.
VOL. 8, NO.2
Ibid., p. 228. Gilchrist, R., Bur. Standards J. Research, 9, 547 (1932). Ibid., 12, 291 (1934). Graham, K. L., S.African Mining Eng. J . (March 19, 1927). Krauss, F., and Deneke, H., 2. anal. Chem., 67, 86 (1926). Noyes and Bray, “Qualitative Analysis of the Rarer Elements,” p. 320, New York, Macmillan Co., 1927. (11) Paal and Amberger, Ber., 1378 (1907). (12) Wichers, E., Gilchrist, R., and Swanger, W. H., Trans. Am. Inst. Mining Met. Eng., 76, 602 (1928). (13) Zhemchushnii, S. F., Ann. inst. platine, 5, 361 (1927). (5) (6) (7) (8) (9) (10)
RECEIVED October 29, 1936.
A Thermionic Titrimeter without Batteries HOBART H. WILLARD
s
AND
0. B. HAGER, JR., University of LMichigan, A n n Arbor, Mich.
bridge a t and near balance, the use of self-biasing, and the TCE the vacuum tubes in a Wheatstone bridge circuit use of a twin-type tube to obtain the advantage of identical used by Hiltner (3) and others (1, 4-7) are capable of vacuum and thermal fluctuations for both triode systems, simultaneously acting as rectifiers of alternating current and together with a heater-type cathode, the usual fluctuations in as statically controlled variable resistors for the bridge, it the voltage of commercial lines have so little influence on the appeared possible to power a thermionic voltmeter with ordevice as to be entirely negligible for potentiometric titrations dinary alternating current. Trial not only showed it to be but great enough to prevent better than about 0.1 p H precipossible but also showed that the mere substitution of altersion in pH measurements. nating for the usual direct current power enabled the device As the name implies, the titrimeter is more useful and ret o measure alternating as well as direct current applied voltliable for making titrations than for measuring equilibrium ages. voltages, as in pH measurements with hydrogen and quinhyMany vacuum-tube voltmeters have been described for drone electrodes. The titrimeter passes sufficient current chemical purposes since their introduction by Goode ( 2 ) . (0.1 to 0.5 microampere) so that it will not read the zero curThis multiplicity of offerings is not indicative of a generally rent voltage of such cells, but it can be employed to determine unsatisfactory performance of vacuum-tube voltmeters but pH by using standard buffers to determine the small correcrather is the result of advances in electrical engineering which tion due to this small current. It cannot be used with the enable succeeding authors to offer operational improvements glass electrode. of real value. More recently Hiltner (3) introduced a novel This titrimeter is distinguished from all others based on the method of reducing the grid current to an exceedingly low principle of the Wheatstone bridge by the fact that the tubes value, negligible so far as titrations are concerned. His in the arms of the bridge not only serve as variable resistors method required no manual adjustments, as contrasted to but also simultaneously rectify the alternating current used previous methods of regulating the grid current. He atto power the device and thus do away with all batteries and tained alternating current operation by the conventional the necessity of a rectifying tube and filter system, The remethod of rectifying the alternating current before applying sult of this dual function of the tubes is a remarkably simple it to the tubes. I n the interim between presentation of the circuit that requires fewer parts and is therefore simpler and titrimeter described in this paper a t the 88th meeting of the more economical to construct, more fool-proof, and more conAMERICANCHEMICAL SOCIETYin September, 1934, and the venient to use. It does not easily get out of order. One of present time, during which it was thoroughly tested and dethese instruments has been in frequent use for more than a veloped, a circuit very similar to Hiltner’s has been offered year without giving any trouble. These advantages make it by Garman and Droz (1). They used alternating current particularly suitable for chemists, who are usually not fapower through the usual means of first rectifying and filtering miliar with thermionic circuits. it. Their chief contribution is introducing into chemical Much experimental work has influenced the selection of the literature and chemical vacuum-tube apparatus the principles following circuit constants and materials as the best cornproofmu-balance in combination with a Wheatstone bridge circuit. mise with regard to sensitivity, stability, ease of operation, This rather elegant method of eliminating the effects of line low grid current, convenient assembly, and general availavoltage variations is probably effective, but considerable use of bility of parts. the more simple circuit to be described has shown that it is stable enough for purCircuit Materials p o s e s of titration. This T U B E .RCA-53. RCAE; paper p r e s e n t s a much 6A6 is identical in charactersimpler modification of the es istics except that it has a 6.3Hiltner voltmeter, adapting volt heater; otherwise it can be substituted for RCA-53 the circuit to alternating without change. c u r r e n t power without a METER. Weston microspecial rectifying tube and a m m e t e r , type 440, scale using American v a c u u m 0-150, 75 divisions, 3.5-ohm tubes. coil. I I T R A N s F o R M E R . Since Because of the inherent the power requirements are FIGURE1. WIRINGDIAGRAM stability of the Wheatstone
MARCH 15, 1936
ANALYTICAL EDITION
low, any good radio power transformer designed for midget sets and capable of delivering 10 m a . at 220 volts for the heater will be satisfactory. S h o u 1d the high-voltage secondary of t h e transformer to be used be higher than 220 volts, the excess is dissipated by Rg (Figure 1). GRID S W I T C H . Any type of doublepole double-throw switch can be used. A F e d e r a l anticapacity switch can be recommended f o r i t s n e a t appearance, e a s e of assembly, and ease of operation. F I X E DRESISTORS. R, d e t e r mines the grid current; its value depends on the characteristics of t h e t u b e , a n d since Applied -Potential these vary it is imp o s s i b l e to give FIGURE 2. CALIBRATION CURVE any definite value. Four tubes required values between 50,000 and 200,000 ohms. The determination of this value is the only adjustment the builder of the titrimeter is required to make. This resistor is very conveniently handled when designed to be inserted in grid leak mountings, thus facilitating adjustment whenever a new tube is put into operation. R, is a wire-wound resistor having 1000 ohms and capable of dissipating about 3 watts. Only a small fraction of this amount of power will actually be dissipated, but a wide margin is recommended as an aid in preventing a noticeable temperature rise which would change the resistance and cause drifts in the zero setting of the titrimeter. Rs is a wire-wound resistor whose value will depend on the value of the high-voltage winding of the transformer being used. It should be capable of dissipating three watts and have a resistance as given by the equation Rg = 480 (ET- 220), where ET = transformer voltage. R8 is a shunt resistor for the microammeter and has the value of 20 ohms. VARIABLE RESISTOR.R3 is a midget-type radio potentiometer whose resistance is 2000 ohms. It must be wire-wound. R4 is a midget radio potentiometer whose resistance is 50 ohms. It must also be wire-wound. If a radio rheostat is chosen for R4 it should be the type with no “off” position. The settings of R3 and R4 have no bearing on the accuracy or sensitivity of the titrimeter, but the value of R4 for measuring sensitivity depends on the effective resistance of the tubes and of the meter, and the value given is the proper one for the parts recommended.
To minimize grid current the following procedure is used: A fairly sensitive galvanometer (a pointer type is satisfactory) is connected to the e. m. f. binding posts and the grid switch is thrown to include the galvanometer in the grid circuit. R, is then varied between wide limits when attached to first one grid and then the other. When R, is attached to one of the grids the galvanometer will show minimum current for some certain value of R,. This minimum current must under no condition exceed 0.5 microampere, and if the proper value of R, is used the minimum can be made 0.1 microampere and even less. In a later paper it will be shown that a grid current no greater than this has no effect on the titration cell.
It is important that the positive voltage being applied in measurements be connected to the same grid to which R, is connected. A reversal of applied polarity immediately causes a grid current of several tenths of a microampere to
145
flow through the grid circuit. It is of course necessary to adjust the connection of the microammeter so that the negative binding post is connected to the plate corresponding to the grid to which R, is connected. The voltage to be measured is connected to the e. m. E. terminals, but the bridge circuit must first be balanced by throwing the grid switch to connect the two grids directly, and adjusting RBand R4 until the meter shows zero current. Upon throwing the grid switch to include the e. m. f. between the grids, the bridge will be unbalanced by this potential to the extent shown by the reading of the meter. If a meter other than that specified is used, its resistance must not be greater than 50 ohms unlese one is willing to sacrifice sensitivity. The sensitivity to be expected with the above materials is 0.22 microampere per millivolt impressed on the e. m. f. terminals. Various factors combine to make repeated readings of the same impressed voltage show a maximum variation of about 0.75 microampere, so that one cannot read with certainty closer than about 4 millivolts of applied potential. Higher sensitivities than that given can be obtained by decreasing the value of R,, the sensitivity being about double a t zero ohms. However, there is a loss of stability as sensitivity is gained by this method and the difficulty of controlling the grid current is increased. Higher sensitivities may also be obtained with tubes of types 47,2A5, and possibly 2A3, by using them in a two-tube circuit, but this is more than counterbalanced by the difficulty of maintaining calibration, by the necessity for using large resistors, by the greatly increased cost, and by other disadvantages. As a direct-reading voltmeter it must be calibrated if the meter specified is used. Figure 2 shows a calibration curve. The calibration will not change until the tube emission becomes weak after very long use. The zero point should be checked before beginning a titration or making a voltage reading. The apparatus must be calibrated separately for measuring direct and alternating current voltages. The applied alternating current voltage to be measured must be phased properly. If the alternating current volta,ge being measured is of the same frequency as that applied to power the voltmeter and if it is also in phase or 180” out of phase with the voltage used for power, connecting the applied potential one way to the voltmeter causes no response, whereas the reverse connection results in meter readings proportional to the applied alternating current voltage.
Summary A simple thermionic voltmeter without batteries has been described which operates from the alternating current lighting circuit, and is particularly convenient for potentiometric titrations. It is sensitive to 3 or 4 millivolts and the grid current, which is usuallynot over 0.1 microampere, has no effect on the titration cell.
Literature Cited Garman and Droz, IND.ENO.CHEM.,Anal. Ed., 7, 341 (1935). Goode, K. H., J. Am. Chem. SOC.,47, 2483 (1925). Hiltner, Werner, Chem. Fabrik, 6, 111 (1933). Nottingham, J. Franklin Inst., 209, 287 (1930). (5) Stadie, W. C., J . Bid. Chem., 83, 477 (1929). (6) Stadie, O’Brien, and Lang, Ibid., 91, 243 (1931). (7) TBdt, F., 2. Elektrochem., 34, 594 (1928).
(1) (2) (3) (4)
RECEIVED October 1, 1935. Presented before the Division of Physical and Inorganio Chemistry at the 88th Meeting of the American Chemical Society, Cleveland, Ohio, September 10 to 14, 1934. From a portion of a dissertation submitted by 0. B. Hager, Jr., to the Graduate School of the University of Michigan in partial fulfillment of the requirements for the degree of doctor of philoeophy.
