ANALYTICAL CHEMISTRY
1754
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 (1) Cooske, W. D., and firman. N. 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)
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
1755
V O L U M E 27, N O . 1 1 , N O V E M B E R 1 9 5 5 a more accurate, but also more elaborate, instrument for integrating current in controlled potential electrolysis. Chemical integrator8 or coulometers such as used by Saehelledy and Somogyi (8)in their original work on coulometric titrations, although accurate, are inconvenient. Wheatley (0) described low-inertia motors for use a8 electramechanicd integrators. These are permanent magnet direct current motors with negligible friction, low brush contact resistanoe, and low iron losses, so that the speed of the motor is a linear function of the driving voltage. Furthermore, the time constant is small (about 10 milliseconds), so that little error is caused by starting and stopping.
INTEGRATOR
MOTOR COMER
manufacturers indicate that the speed of the motor with no load varies approximately 0.35y0per 10' C. Another possible source of error is the load on the motor which is due to the mechanical counter. A load an the motor reduces the speed range within which the speed is linear with respect to voltage input. A load also increases the current necessary to operate the motor. According to the manufacturers, a current of 0.225 ma. is reauired to omrate the motor a t its nominal 24-volt meed with no load. For current levels lorer than 75 ma. the series resistor ( R ) in Figure 1 should be greater than TO ohms, so that the motor will be operating on the linear portion of its speed-voltage curve. Data obtained st current levels of 12.5 t o 13.7 ma. over a period of several months with EL 1000-ohm series resistor giLve a calibration factor with a standard deviation of iO.lS% (calculated from 26 determinations). Lou-er currents were not studied. The motor-counter unit should not be mounted near iron, or other magnetic material. By supporting the unit on the base of an iron ring stand, the calibration factor was decreased 1.5%. The unstabiliaed direct current sources, employed for the coulometric titrations described in this paper, were used because of their availability a t the time this work was done. A selenium rectifier bridge with choke and condenser for filtering has since
ws SOURCE
di
Figure 1. Block
Integration of current a i d the I R drop or voltage across a fixed resistor in series with the electrolysis cell to supply power to the motor-counter unit. By keeping the resistance of the series resistor constant, the speed of the motor, or counts per second, is a linear function of the current flowing through the series resistor. The motor may be calibrated to read quantity bf electricity (coulombs per count). CALIBRATION O F INTEGRATOR
A blook diagram of the manual coulometric titration cirouit is shown in Figure 1. The integrating motor (Electro Methods, Ltd., Stevensge, Herts, England, type 913, 24 volts with mechanical counter) was connected across an approximately 70-ohm fixed setting of a 100-ohm, 100-watt potentiometer resistor. A photograph of the motor-counter unit is shown in Figure 2. The resistor was kept immersed in a reservoir of transformer oil, t o prevent i t from being heated excessively. A constant-ourrent supply and a timer opersted by a standard frequency tuning fork (S,7) were used t o calibrate the integrator. An accurately measured constant current was passed through the 70-ohm resistance for an acourately measured time, and the corresponding count on the integrating motor was read. The Calibration factor was expressed by the following equation:
Figure 2. Integrating motor-counter unit
Table I. Calibration Faotor Date
515/54
8/11/54 9/21/54
10/11/54
Calibration data obtained are presented in Table I. Data in Table I indicate that variations in the calibration factor of the motor lead to an uncertainty of +0.13% (standard deviation) for integration of current and time. The difference of 0.15% in the average value for the factor a t the 260- to 270and 138- to 154ma. level is due t o a slight deviation from linearity of the motor speed-voltage curve. Deviations from linearity become greater than 0.5% when the motor is operated a t an input voltage below 5 volts. Data in Table I also indicate that the reproducibility of the factor is better than 0.1% for a series of determinations on a given day. The variation on different days m w be due to an effect of temperature. The
Current, Ma. 261.2 261.3 261.4 261.4 261.3 272.6 272.6 273.1 262.6 262.7 262.5 262.8 268.6 265.6
Time.
Seconds 300.0
300.0 300.0
300.0 300.0 300.0 330.1 2400.0 300.0 600.0 180.0 240.0 300.0 360.0
Count 40.39 40.39 40.40 40.38 40.39 42.14 46.37 337.01 40.51 81.05 24.32 32.42 41.45 49.75
Factor. Meq./Count 0.02010 0.02011 0.02012 0.02013 0.02011 0.02011 0.02011 0.02010 0.D2016 0.02015 0.02010 0.02016 0.02013 0.02013 0.02013
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Std. der. = &0.11%
138.8 135.9
9/23/54
139.5
139.7 153.9 .-3.9 3.9 3.9, 3.6 3.7 3.7
10/11/54
lO/l!
~
300.1 230.1 360.1 270.0 360.1 600.0 300.1 420.1 360.1 300.1 420.0
21.36 16.40 19.36 28.52 47.53 23.77 33.27 28.43 28.44 33.20
0.02021 0.02020 0,02019
0.02014 0.02013 0.02013 0.02014 0.02016 0.02017 0.02015 0.02016 Std. dea. = *0.14%
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1756
ANALYTICAL CHEMISTRY
recommended that the output voltage of the source be 100 t o 200 volts, so that a fairly high resistance may be used in series with the electrolysis cell in order to prevent excessive fluctuations of the current. PERFORMANCE OF INTEGRATION MOTOR IN COULOMETKIC TITRATIONS
Titration of Dichromate. The coulometric titration of 9stional Bureau of Standards potassium dichromate with electrolytically generated ferrous ion ( 2 ) was used to test the motor. An unstabilized source of direct current (about 40 volts) obtained from the rectifier section of a potentiostat was used. A series resistor was adjusted to give 140 to 150 ma. through the electrolysis cell. The titration procedure and cell design of Cooke and Furman ( 2 ) were employed with the exception that the total volume of the titration solution was increased to about 140 ml., and a perforated cylindrical platinum electrode cathode (2 inches high by 1 inch in diameter) was used. The titration was carried out in an atmosphere of nitrogen and the end point was determined with a platinum-tungsten electrode pair. The results of titrations with 25 ml. of 0.0252V potassium dichromate (based on weight of Bureau of Standards K2Cr207diluted to volume in calibrated glassware) are shown in Table 11. The data were corrected for ferrous ion in the ferric ion reagent. Data given b j titrations 6 and 7 where the current was switched “off” and “on” 400 to 500 times indicate that this factor causes little, if any error. Automatic Macrotitration of Chloride. The integrating motor was used in conjunction with the Beckman automatic titrator for the automatic coulometric titration of chloride with electrolytically generated silver ion. The basic circuit shown in Figure 1 was used except for a double pole-double throw relay and the Beckman titrator in place of toggle switch ( 8 ) to terminate the end point. Lingane ( 4 ) has described the method of connecting the relay to the Beckman titrator. Uncontrolled direct current power (about 350 volts) was obtained from the rectifier section of a constant current sup ly ( 3 ) . A large series resistor was adjusted to give a current orabout 260 ma. through the electrolvsis cell. The electrolysis cell design was similar to that recommended by Lingane ( 4 ) except that the large stirrer which is provided with the Beckman titrator and a 600-ml. beaker were employed. Titration was carried out in the solution obtained from a 15gram sodium peroxide Parr bomb combustion after neutralization with a slight excess of nitric acid, dilution to 150 ml., and addition of 300 ml. of alcohol (formula 3a, denatured). The iso-
Table 11. Manual Titration of Dichromate with Electrolytically Generated Ferrous Ion Yo Recovery Titration 1
100.04
2
100.06
3
99.99
4
100.03
5 6 (500starts and stops) 7 (400 starts and stops)
100.09 100,og
100.12 Av. 100.06 S t d . dev. = Z’Z0.05
lated cathode compartment was kept filled with a similar chloridt3free solution. For the tit’rations described, in this paper, the chloride was added to the solution after the combustion so as to eliminate any possible effect of incomplete combustion on these studies. The Beckman was set a t f0.240 volt (standard calomel electrode). The anticipation was set to provide for the addition of several hundred increments in the vicinity of the end point, With integration of current and time, the large number of “starts” and “stops” did not cause significant error. The large number of increments had further advantages in preventing overstepping of the end point. allowing time for equilibrium to be established and requiring less care in the positioning of thc electrodes in the cell. Results of automatic titrations are presented in Table 111. Two mmual titr:ttioiis with leas than 10 starts and stop.; and wit,h 1 meq. of chloride present gave 99.4 and 99.6y0 recovery. The slope of the titration curve in the vicinity of the end point, when 1 meq. of chloride is titrated, leads to an error of about 0.4% chloride for 10 mv. At a current level of 260 ma. 1 meq. of chloride requires about 370 seconds of generation time or 49.50 counts as read on the integrator. The end point, used t o obtain the values for chloride in Table I11 may be slightly in advance of the true end point ar the titrator was set to st,op a t 4-0.240 volt (S.C.E.), whereas the end point as determined from a manual curve n-as +0.253 volt, (S.C.E.). Automatic Semimicro Titration of Acid. The automatic. t i t r a tion of acid with electrolytically generated hydroxide was also studied. .it a current itvrl of 13 ma., it tl-as necessary to replace R in Figure 1 xith a 1000-ohm resistor so that the I R drop \\ ould he sufficient to operate the motor at, a favorable speed. The constant-current supply and accurate timing equipment, ( 3 ) and t,he Beckman titrator were used. The cell, a 100-nil. weighing bottle, contained a magnetic stirrer and a rubber stopper. The latter held a cylindrical cathode of platinum gauze (52-mesh, 1.8 cm. in diameter by 1.5 cm. high), glass and calomel electrodes, tubes for keeping the titration solution under an atmosphere of nitrogen and one leg of a U-tube (25 cm. by 8 mni. in diameter), which was filled with 3% agar in 1M sodium chloride. The other leg of the U-tube dipped into a separate vessel containing 1M sodium chloride and a platinum anode. Titrations were carried out on 10-ml. portions of st,andar(i 0.0LV hydrochloric acid in about 50 ml. of 0.LV sodium chloride, Reagents were prepared with boiled dist,illed water and the tit,rxtion vessel TTas kept under an atmosphere of nitrogen. The Beckman titrator was set at pH T.O. Results in Table IV indicate excellent agreement betweeii values determined from current. and time measurements and h y means of the integrating motor. For the t.itration in Table I V the anticipation was set so that. 132 to 240 increments were ohtained. The pH of the solution after the completion of the titration was 7.0 to 7.1. Current was passed for about i30 seconds at about 13.2 ma. Approxi-. mately 75 counts were obtained. The calibration factor of t,he motor was 0.001325 meq. per count.
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Table 111. Automatic Titration of Chloride Chloride Added, Gram
Number of Determinations
0,0356
15
0.0709
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~
Recovery 99.6 99.9 99.7
1 1
0,0532
Std. Dev. 0.3
%
..
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% ’ Recovery Titration 1
2 3 4 5 6 7
Time and current 100.2 99.9 99.7 99.8 99.8 99 7 99.8 99. 84
AI. Std. dev. = Z’Z0.18
Integration
100.2
Av. 9td. d e r .
=
99.8 99.8 99.7 99.8 99.8 99.8 99.85 10.14
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The authors are indeht,ed to G. E. Gerhardt and H. C. IAW. rence for assistance in t,he construction of theelectricalequipmciit, LITER.ATURE CITED (1) R e t t ,
..
____ Table IV. Titration of Acid
ACKNOWLEDGMENT
K.,Morris, G., a n d X o c l i , W., ,IXAL. CHEY.,26,
784 (1954)
Analyst, 79, 607 (1954). TAL. CHEM., 22, 896 (1950), (2) Cooke, TV. D., and Furman, N. €I (3) G e r h a r d t , G. E., Lawrence, H. C., a n d Parsons, J . S., I b i d . , 27, 1752 (1955). (4) Lingane, J. J., Ibid., 26, 622 (1954); “Electro-analytical Cheinistry,” Interscience, N e w York, 1953. ( 5 ) Meites, L.. ASAL. CHEM., 24, 1057 (1952). ( 6 ) Ibid., 27, 1116 (1955). (7) Parsons, J. S., and S e a m a n , W., Ibid., 27, 210 (1955). (8) Szebelleds. L.. a n d Somogyi, Z., 2. anal. Chern., 112, 313, ~ L ’ . J , 332, 385, 391, 395, 400 (1938). (9) Wheatley, B. M., Brit. J . Radiol., 26, 382 (1953). RECEIVED for review February 16, 1955. Accepted August 8, 1955. Pre. *ented before Analytical Group, S o r t h Jersey Section, ACS, Meeting-in. Sliniature, Newark, N. J . , January 24, 1955.
