Improved Constant-Current Supply for Coulometric Analysis CHARLES N. REILLEY, RALPH N. ADAMS, AND N. HOWELL FURMAN Princeton University, Princeton, N . J .
1RECENT
publication ( 2 ) described a three-tube power supply designed as a constant-current or constant-voltage source for coulometric titrations. This circuit has given excellent service in this laboratory for over two years, but it suffers from the disadvantage that the current can be varied only in fixed steps through light bulb resistors. An extensively improved circuit is described herewith which gives continuously variable constant curA
rent from about 0.6 to I50 ma. with equal or better regulation than before. This circuit appears to have several distinct advantages over the recent one of DeFord, Johns, and Pitts ( 1 ) . It is operated from a 110-volt line source and requires no external direct current voltage supply; floating-type grounds eliminate precautions against common grounding between cell and external voltage
3
Figure 1. CI.
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Schematic Diagram of Constant-Current and Constant-Voltage Supply
2-mfd., 1000-volt capacitor 10-mfd., 1000-volt capacitor %mid., 600-volt capacitor 10-henry, 200-ma. filter reactor 2-ampere fuse 6-watt, 110-volt tungsten lanips 0-200-ma. meter 100-ohm, IO-watt resistors 12,000-ohm, 25-watt resistors 470,000-ohm, 5-watt resistors 100,000-ohm, 20-watt resistor l5,000-ohm, 25-watt resistor 10,000-ohm, 25-watt resistor, variable 50,000-ohm, 20-watt resistor 470,000-ohm, 2-watt resistor 3000-ohm, I-watt resistors 70,000-ohm, IO-watt resistor, variable 10,000-ohm, IO-watt resistor, variable Composite resistance 1750 ohms; eight 3500-ohm, 20-watt resistors, two sets of 4 parallel resistors, s e t s in series Composite resistance: 7300 ohms; two l5,000-ohm, 20-watt resistors in parallel Composite resistance, 4000 ohms; four 4000-ohm, 20-watt resistors in series-parallel Composite resistance, 2000 ohms; four 2000-ohm, 20-xvatt resistors in series-parallel Composite resistance, 1000 o h m s ; four 1000-ohm, 20-watt resistors in series-parallel See text S. P. S. T. toggle switches 3-circuit, 5-position switch %position ii$-itch D. P. D. T. toggle switch 425-0-426, 165-ma. power transformer
1044
V O L U M E 2 4 , NO. 6, J U N E 1 9 5 2
1045
supply. A constant-voltage output is incorporated from which very small currents (for titrations in the microgram region) can be obtained through use of appropriate resistances. Complete and continuous coverage of the region 0.6 t o 150 ma. is easily obtained by direct panel controls, and fine adjustment makes possible exact settings to any predetermined current value within t h e above limits. Higher currents can be obtained by slight modification of the power supply.
'I'ahle I.
This improved circuit was designed for current outputs only up to 150 ma., since higher generation rates are seldom really necessary in coulometric titrations. However, currents as high as 500 ma. may be easily attained b y slight modification of the rectifier circuit. Heavier duty components-Le., transformers, chokes, and mercury-vapor rectifier tubes-could be used in the rectifier circuit for this purpose. As the 6XS7 tube is limited to a maximumof 250 ma., the extra current can be bypassed through a resistor, R,, or through additional 6887 tubes in parallel. The latter method effects best regulation of high currents. RESULTS
Performance of Constant-Current Circuit Stability over I-Hour Period
Mean Current, h l a 0.637 20.278 28.797 33.765 65 191 100 455
.\laxlmul'l 1)eviation 7
0 02 0 02 0 01 0 04 0 0-1 0 04
Mran Deviation,
c7c
0.01 0.01 0.00 0.02 0.01 0.02
The circuit, shown in Figure 1, contains the basic elements of the original design together with the 6J6 and 6.4C7 tubes and the through R r z )used for continuous variation of the resistors ( R I G constant current. There are three outputs, constant-voltage, medium constant-current, and high constant-current. The upper limit of the medium constant-current output is limited to about 40 ma. b y RIB. The lower limit of the high output is about 28 ma. as set by the minimum current passed by way of the 6SL7 tube and the associated resistors when no light bulbs are in the circuit. I n using the high output, the light bulbs and the (6J6, 64C7, RI6-Rz2)circuit can operate in conjunction to give any intrrmediate current setting. Resistors RIB through RTJ are of high wattage and are coupled in series-parallel to give minimum heating during operation. The coupling of these resistors is indicated in the legend accompanying Figure 1. Temperaturecompensated resistors could be substituted. Resistors RIB and Rlr are the coarse and h e adjustments for the current variation. All grounds indicated in Figure 1 are of the "floating" t-ype-i.e., are insulated from the cabinet chassis.
The constant-current circuit was t,ested as before (b) by measuring the voltage drop across a standard resistance. After an init,ial wwm-up period of 30 minutes, the maximum current deviations observed over a 5-minute period \\-ere all less than O . O l ~ o . For all settings, the change in current was only 0.01% or less, when an electrolytic cell of 100 ohms w a ~shorted. The degree of regulation over a 1-hour period is shoivn in Table I. The current ranges studied are representative of all possible control settings. The constant-voltage source was tested as previously described ( 2 ) by using a 1-megohm resistor in the external circuit t o give a current of about 0.5 ma. The mean current deviation over a 1hour period n-as 0.017, and shorting the cell resistance of 100 ohms resulted in a current variation of 0.02%. A 5% change in line voltage produced a 0.1% variation in t,he output current. This effect can be eliminated by inserting a constant-voltage transformer. LITERATURE CITED
(1) DeFord, D. D., Johns, C. J., and Pitts, J. S . , 4 ~ . 4 CHEM., ~ . 23, 941 (1951). 12) Reilley, C.N., Cooke, IT. D., and Furman, N. H., Ibid., 23, 1030
(1951). RECPIVED for review October 3, 1951. Accepted Januitry 11, 1952.
Temperature-Composition Relation of Mixtures of 2,4- and 2,6-Dichlorophenols G . J . .JANZ AND T. H. DONNELLY Walker Laboratory, Rensselaer Polytechnic Institute, Troy, N . Y .
OTH 2,4- and 2,B-dichlorophenols are obtained when phenol and o-chlorophenol are chlorinated t o the dichlorophenol stage (3, 4). T h e relative amounts of these two isomers in the dichlorophenol product' thus obtained may vary greatly, depending on the conditions of the chlorinatioii. A study of the temperature-composition diagram for the system 2 , 4 and 2,6-dichlorc+ phenols was of i n k & t,o determine the applicability of thermal analysis for measuring the relative amounts of the two isomers in thc, chlorination product. Two simple laboratory methods, the cooling-curve and the thu\v-melt methods of thermal analysis (b), Tvere used t o investigate t,his system. The cooling-curve method is the simplest and moRt useful method for studying systems ITithout formation of' solid solutions but with possible formation of a binary eutectic. In such cases the tpmperature-time relation is follo~vedas thi. molten sample cools under cuntrolled conditions, and the cooling cut've shows definite arrests a t the melting point (the point of incipient, frewing) of the mixture, and a t the eutectic temperature. I n the thaw-melt method, the behavior of the sample is olistsrved as the temperature is gradually raised from a point below the eutectic temperature. T h e thaw point or temperature of firsstwetting of the sample is taken as the melting point of t'he eutectic mixture. T h e melting point for the mixture (the temperature a t which the last traces of solid melt) is also readily olisrrvcd by this method. I n this st,udy both methods were used to tfrtc,rmine the solid-liquid equilihrium diagram, since in the
cooling-curve method the equilibrium is approached from the liquid state, and in the than--melt mrthod, from the solid state. COOLIXG-CUHVE RIEASI!HI.:RlERTS
The 2,4- and 2,6dichloro~~h~nc1ls \rere prepared by chlorinat,ion of o-chlorophenol and separated by fractional distillation in a 100-plate Stedman column. The tTvo i.sonitw were recrystallized repeatedly from low boiling petroleum ether. The melting points and analysis of the purified isonitBrs \veri': Z,ltiichlorophenol, melting point 43-45" C. (literatuw 43-45" C.), c-hlorine analysis, found 43.80, 43.7795, calculatcltl! 43.56%: 2,6-tiichlorophenol, nielt,ing point 65-66" C. (lit,er:tture 6 5 6 7 (1. j , chlorine analysis, found 43.69, 43.47'%, calculated, 43.56%. The mixtures used for the cooling curve measurements were made hy direct weighing from these specimens. T h e apparatus for the measurements consisted of a \vide test tube (15 m m . X 15 cm.) provided with a therniomet,er reading in fifths of degrees and a glass
