Precision Coulometric Titrator G. E. GERHARDT, H. C. LAWRENCE, and J.
S. PARSONS
Research Division, American Cyanamid Co., Bound Brook,
.in instrument for carrying out coulometric titrations in an industrial laboratory within 0.1% precision and accuracy gives constant currents up to 450 ma. The electric time clock is operated by a frequency standard, so that the instrument can be used on industrial power systems where the frequency is not rigidly controlled.
A
S ISSTRUMENT was needed for performing coulometric
titrations a t constant currents of several hundred milliamperes so that as much as 1 meq. of material could be titrated in a reasonably short time. The constant current supply of Reilley, Adams, and Furman (6) was modified to obtain currents up to 450 ma. I t was necessary to employ a tuning fork source as a frequency standard for operation of the electric time clock, as available plant power frequency variations caused errors in time measurements of 5 0 . 5 to 1% relative. Fry and Baldeschwieler (3)indicate that utilitv system power may vary by zt0.2 cycle, resulting in 3=0.370 error in time Craig, Satterthwaite, and Wallace ( 2 ) show that frequencr errors in time are less than 0.27& for the Pittsburgh area. INSTRUMENT4TION
The circuit for the constant current supply is shown in Figure 1. Both the power transformer and choke in the rectifier section are rated at 500 ma. A pair of 866/866A rectifier tubes was chosen, because of their high current capacity and their inherently better regulation characteristics as compared with vacuum rectifiers. The single L section choke input filter yields a satisfactory ripple value. The thermal delay relay (30 seconds) is used in the interest of rectifier life. The current control section is an exact duplication of Reilley's circuit with the addition of two series regulator tubes and additional lamp bulbs (series load resistors) to handle the higher currents encountered. Physically, the rectifier scction was constructed on a separate chassis and connected to the control section by a four-conductor cable. iill output connections and controls with the exception of the main power switch are located on the controller chassis. A 500-ma. meter was provided for rough indication of the current level. A Riverbank standard tuning fork (Cenco, 60 cycles, 5 p.p.m
Table I.
Accuracy of Time
Time Interval (Betveen Station W W V Signals), Minutes 5 5 5 5 4 10 15 20 25 60 60
Table 11.
Electric Time Clock (RIodel 8-1) (Operated by Standard Tuning Fork) Error in Seconds -0.21 -0.05 -0.27 -0.02 -0.04 -0.03 -0.14 -0.07 -0.08 -0.10 +O. 08 -0.46 -0.79
N. 1. precision, rated a t 40-volt, 2-watt output) was employed for precise frequency control in operating the time clock. Thp power amplifier of the Cenco Riverbank source was redesigned for 115-volt output. A separate power supply for the above power amplifier was constructed as a separate unit together with the direct current source for operation of the clock clutch. The schematics of the revisions are not given, since a commercial frequency unit (Type 2005, American Time Products, Inc.) is available in a convenient package for direct operation of the clock. A 65-ma, selenium rectifier, a 10-henry choke and a 10 X 10 pfd condenser were used in the construction of the direct current source for the time clock clutch. The electric time clock operated by the above frequency standard (Model S-1, Standard Electric Time Co.) has a capacity of 60 seconds with 0.01-second divisions and is provided with a direct current clutch having a start-and-stop operation error of +0.002 second. Another clock (Standard Electric Time Co., SM-60), which was operated by plant power, was used to indicate minutes. The accuracy of the timer was checked by comparing with signals sent out by the Bureau of Standards Station WWV. The circuit for the various component parts of the titrator i. shown by the block diagram in Figure 2. These components were assembled in a standard 19-inch rack cabinet. Figure 3 i. a photograph of the front panel showing the various controls. The equipment was provided with a connector for using the Beckman autotitrator as an end-point detector for automatic coulometric titrations as suggested by Lingane ( 4 ) . The current was determined accurately by measuring the I R drop across 3 standard resistor in series with the electrolvsis cell. PERFORMANCE OF IVSTRUWEYT
The data in Table I show errors in time to be less than 0.1% (relative) in all cases where the timer was operated with a standard tuning fork. Most of this error for the short time periods is probably due to human error in starting and stopping the clock a t the beginning and end of the Bureau of Standards time signal. The constant current supply maintains a current constant to considerably less than 0.1% for all current levels. At the 263-ma. setting, current measurements made during actual coulometric titrations over a 4- to 6-hour time period were all much less than 0.1% (maximum spread). Table I1 indicates the accuracy of milliequivalents determined from the current and time measurements as compared with the milliequivalents of silver deposited by passing the current through a silver coulometer. The performance of the titrator for the coulometric titratidh of Bureau of Standards potassium dichromate with electrolytically generated ferrous ion ( 1 ) iq shown by the results of Table 111. The end point for the first five titrations shown in Table 111 was
Table 111. Titration with Electrolytically Generated Ferrous Ion Potassium Dichromate Current, Taken, Meq. >fa, 149.7 0.6261 0.6261 149.7 0.6261 149.7 149.7 0,6261 0.6261 148.6 149.7 0.6261n 149.7 0.6261a a 100 s t a r t s and stops.
Comparison of Current X Time Relationship with Silver Coulometer Data
No. of Maximum Current, Measure- Spread, 3'% Ma. ments Relative 30 0.09 157.1 265.8 25 0.04 14 0.04 296.3 12 0.05 434.0
lfeq.
Time, Seconds 1805.0 1500.0 918.3 720.1
Meq.,
2Silver
96 5 Coulometer 2.938 2.937 4.132 4.132 2.822 2.819 3.238 3.234
Difference, % Relative f0.04 0.00 -0 09 +0.12
1752
Time, Seconds 408.0 403.2 404.2 404.3 406.2 404.2 405.3
Potassium Dichromate Found, hleq. 0.6231 O.fi254 0.6264
0.6271 0.6253 0.6269 0.6278
Error,
70
-0.16 -0.11 +0.13 4-0.16 -0.13 +0.13 f0.27
V O L U M E 27, NO. 1 1 , N O V E M B E R 1 9 5 5
1753
v3 6AS7G
f2 I
L 1c '
866/ 8 6 6 A
1
1 +
I
9V,,V&,H
1
trs.
TSF F i g u r e 1. C i r c u i t f o r Constant C u r r e n t Supply
R20.
Rzi.
R??. Ria. R24. R2a.
R26. R21 t o R32. Raa t o Raa.
100-ohm, 1-watt carbon resistors 470,00O-ohm, 2-watt carbon resistors 12,000-ohm, 25-matt wire-wound resistors l5,000-ohm, 25-watt mire-wound resistor 10,000-ohm, 25-watt wire-wound variable resistor 100,000-ohm, 20-watt wire-wound resistor 50,000-ohm. 20-watt wire-wound resistor 2700-ohm, 1-ivatt carbon resistors 7500-ohm; two l5.000-ohm, 20-watt mire-Tronnd resistors in parallel 4000-ohm: four 4000-ohm, 20-watt wire-wound resistors in series-parallel 2000-ohm: four 2000-ohm, 20-watt wire-wound resistors in series-parallel 1000-ohm; four 1000-ohm, 20-watt wire-wound resistors in series-parallel 70,000-ohm, 7-watt wire-wound potentiometer 10 000-ohm 7-watt wire-wound potentiometer 1750-ohm: ' eight ,3500-ohm: 20-watt wire-wound resistors, four paralleled in series with four paralleled & w a t t , 115-volt tungsten lamps 25-watt, 115-volt tungsten lamps
Cl.
C?. L1.
T.. T,. Td.
10-mfd.. 1000-volt d . r . oil ranaritor 2-mfd., '600-volt d . c . capacitcr-----10-henry, 500-ma. choke, UTC-CG 108 or equivalent Filament transformer, 2.5-volt a t 10A, UTC-CG 34 or equivalent Power transformer, 500-0-600-volt a t 500 ma., 6.3-volt a t 5.4, 6.3-volt a t 3A, UTC-CG 431 or equivalent Filament transformer, 6.3-volt a t 4.4, UTC-CG 33 or equivalent S.P.S.T. toeele switch
6AC7, s h u n t control tube Vo, 6J6, series regulator tubes
ANALYTICAL CHEMISTRY
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FREQUENCY STANDARD
++
CURRENT SUWLY
AMPLIFIER
4AAV 1 k , ~ ''oAc
TITRATOR
SWITCH
Figure 2.
Block circuit diagram for coulometric titrator
determined potentiometrically with stopping and starting the current shout ten times in the vicinity of the end point. For the last two titrations in Table I11 t.he current was interrupted 100 times. These two values indicate that this large number of starts and stops do not cause appreciable error. The instrument was found to perform very satisfactorily for a large number of coulometric titrations with electrolytically generated titanous ion ( 5 ) . Figure 3.
ACKNOWLEDGMENT
The authors wish to express their appreciation to N. Howell Furman, professor of chemistry, Princeton University, and G . L. Royer, formerly director of analytical chemistry, for their interest an d many helpful suggestions. ~
~
~~
LITERATURE CITED
H., ANAL.CHEM..22.89 6 (1950). (2) Craig, R. S.,Sstterthwaite, C. B., and Wallace, W. E., Ibid.. 20, 5,55 (1948).
Coulometric titrator
(3) F ~E.. n,i., and Baideschwieier. E. L., IND. END.C n m , ANAL. ED.. 12, 472 (1940). (4) Lingane. J. J.. ANAL. CHEM., 26, 622 (1954). (5) Parsons, J. S.. and Seaman, W., Rid., 27, 210 (1955). (6) Reilley, C. R.,Adams. R. N.. and Furman. N. H., Ibid.. 24. 1044 (1952)
(1) Cooske, W. D., and firman. N.
RECE~VE iorrerier D February 16. 1955.
Accepted August 8, 1955.
Coulometric Titrations with low-Inertia J. S. PARSONS, WILLIAM SEAMAN,
and R. M. AMICK'
Research Division, American Cyanamid Co,, Bound Brook,
A small electromeehanieal motor for integrating current and time in a coulometric titration has given satisfactory performance in the titration of dichromate, the automatic titration of maom quantities of chloride, and the automatic titration of 0.1-meq. quantities of acid, with eleotmlytieally generated ferrous ion, silver ion, and hydroxide ion.
C
0ULUM"IKIti titration equipment reportea in rhe litera-
ture ( 4 ) has consisted of various regulator circuits for maintaking current constant and a good electric timer as the main 1
Present address. Princeton University. Princeton, N. J.
N. J. components in the circuitry. A small motor, which integrates current and time, makes it possible to perform conlometric titrations with an unstabilized current. Coulometric titration equipment could be simplified and made less expensive by eliminating the need far a precisely controlled constant current supply, an accurate timer, and a frequency standard to drive the timer where frequency fluctuates as it may with industrial plant power (3). Recently, Aett, Morris, and Nock ( 1 ) reported the use of a low-inertia integrating motor for coulometric titrations a t high current levels. Meites ( 5 ) has used a relay for integrating current a t law levels. This requires calibration for each current level and has a blank count. More recently, Meites ( 6 )reported
