Automatic Titration of Micro Amounts of ChI o ride by Convection Am pe romet ry ANDRE
L. JULIARD
Houdry Process Corp., linwood, Pa. b M i c r o amounts of chloride in concentrated solution can b e determined automatically b y amperometric titration with a pretreated silver cathode and a mercury anode polarized a t 0.1 volt. The end point of the titration in stirred solution is reached when the detector current becomes minimum. Amounts of chloride ranging from 50 to 1000 y in 30 ml. of solution containing from 0.1 to 2 grams of other electrolytes can b e determined accurately in less than 5 minutes with a standard deviation of
10 y.
T
HE AUTOMATIC TITRATION O f micro amounts of chloride in solution containing variable amounts of electrolytes requires a method by which the equivalence point can be detected independently of the concentration of the other electrolytes. In the coulometric method established by Lingane (6) for the automatic titration of micro amounts of chloride, the titration is stopped with a device triggered by the potential of an indicator electrode. Accurate results are obtained with such a method only when the potential of the electrode a t the equivalence point is established for each type of solution prior to the titration, in order to adjust the trigger potential. To perform a titration without preset adjustment, the use of a differential potentiometric method is recommended because it gives a singular point a t the equivalence point, which is more easily detectable than an inflection point. Malmstadt and Fett (7) have already taken advantage of the sharp change of a voltage proportional to the second derivative of the potentiometric titration curve a t the equivalence point for automatic titration of moderately concentrated chloride solutions. Another method to stop the addition of the titrant a t the equivalence point is to feed a mavimum voltage detector with a potential proportional t o the first derivative of the potentiometric titration curve. A titration curve with a sharp V angle a t the equivalence point, like curve 2 of Figure 1, can also be used for automatic determination. The abrupt increase of the indicator property beyond the equivalence point allows the titration to be stopped with the addition of
136
ANALYTICAL CHEMISTRY
a small excess of titrant by means of a device which detects the minimum value of the intensity of the indicator property. This work describes an automatic titrator for micro amounts of chloride in which the addition of the titrant is stopped by a self-balancing niilliroltmeter recorder, used as a minimum current detector. Accurate results are obtained from the recorded titration curve. Amounts of chloride ranging from 50 to 1000 r i n 30 to 35 ml. of solution containing from 0.2 to 2 grams of sodium bisulfate are determined from this curve with a standard deviation of 10 Y.
stirrer with bent blades (Figure 2) allows the solution to be stirred at 1800 r.p.m., R-hich is about three times the conventional rate of stirring, without cavitation or trapping of air bubbles. The minimum current detector is a 2-mv. full scale, self-balancing recording potentiometer with a 1-second full-scale travel time, an adjustable zero setting, and a variable damping circuit. A microswitch attached on an added carriage is pushed forward by the pen carriage as long as the current decreases. The contact of the microswitch is released when the pen carriage moves back approximately 3 mm. after the minimum voltage has been reached. The inertia of the microswitch is slightly increased by the traction of a light n-eight which is suspended vertically
EQUIPMENT
The electrical circuit includes a polarization unit, an electrolytic cell, and a modified self-balancing millivoltmeter recorder used as minimum current detector, placed in series. The polarization unit is a 100-ohm, 10-turn potentiometer which supplies an adjusted voltage of 90 to 180 mv. from a 1.6volt dry cell. The electrolytic cell is a 50-ml. glass beaker, 5 cm. high. The cathode is a 6-cm. silver xire, 1 mm. in diameter, bent in a zigzag within a plane 1 X 1 cm. The anode is a mercury pool which covers the bottom of the beaker. -4
___
~
Figure 2. High speed rotating stirrer
A I B 1B
~
ML
Figure 1.
w
_1--
TITRANT
Titration curves suitable for automatic titration
A . Equivalence point
E - D. Minjmum change in indicator property required to trigger device rrhich stops titration B - A , B’ - A . Titration error due to delay in response of trigger device
through a pulley. The sensitivity of the recorder, working as a microammeter, is adjusted with a variable 100-ohm resistor placed in parallel. A 500-ohm resistor that can be substituted for the cell is used to calibrate the recorder. The titrant delivery device is a 10nil., high quality glass syringe, lubricated with micro-fine graphite. The piston of the syringe is moved \\-ithout
backlash by a screw mechanism linked through a clutch to the chart driving shaft of the recorder. The gear ratio is adjusted so that the displacement of 1 division on the chart, equal to inch, corresponds exactly to the delivery of 0.010 ml. of titrant. A turn counter with a sensitivity of 0.1 turn, geared to the driving mechanism of the syringe, measures the volume of titrant delivered to the nearest 0.001 ml. The titrant is
I
, I
I
I
0 10
0 05
0 20
0 I5
0 25
M L 0100 M AgNO,
Figure 3. Effect of chloride addition to 30 ml. of solution containing12 grams of sodium bisulfate Chloride Added,
Curve
0 35 142 354 io9
1 2 3 4 5
~ - -
1
~~
d
I
005
-
OD
0 15 ML TITRANT
0 20
0 25
Figure 4. Accommodation of cathode with successive titration 1. New bright silver electrode 2. Second run 3. Fifth run
delivered a t the rate of 0.060 ml. per minute into the solution through a tapered capillary tube with a tip dianieter of less than 0.1 mm. PROCEDURE
Cathode Pretreatment. The silver electrode is activated by electrolysis. The nire is dipped for a few seconds in a 1 to 1 nitric acid solution (until its surface becomes dull white) and ivashed immediately with water. The cell is filled with 20 nil. of 5% sodium bisulfate and 10 ml. of 2mM sodium chloride solution. The electrodes are polarized a t 130 mv. and the solution is stirred. Chloride is then slowly precipitated by adding a 0.1X silver nitrate solution with the syringe until an excess of the titrant equal to approximately 20% of the equivalent amount of chloride has been added. The amperometric current, practically nil a t the beginning of the titration, increases more and more rapidly during the titration, drops before reaching the equivalence point, and increases abruptly after it. After the cathode has been submitted to this treatment and washed with water only, amperometric curves similar in shape to those of Figure 3 are obtained. The initial current and the acuteness of the V angle a t the equivalence point increase after successive titrations. The cathode remains fully active for many xeeks, or recovers its full activity after one titration when kept nonpolarized in the titrated solution with a slight excess of silver ion. Calibration. The syringe is filled with a 0.1M silver nitrate solution. Just enough mercury to cover the bottom is introduced into the cell. Then 10 ml. of a standard 2.00 m M sodium chloride solution containing 150 grams of sodium bisulfate per liter is pipetted into the cell and diluted t o 30 t o 35 ml. with water. The solution is stirred. The voltage impressed to the cell is adjusted between 90 and 140 mv., so that the initial current reaches approximately 200 pa. The titration is started 1 minute later when the current, which first drops rapidly, becomes stable. Ripples in the detector current are reduced below 1 pa. by firmly fixing the cathode close to the stirrer and by keeping the beaker free from vibration. Determination. The sample, containing between 0 and 2 mg. of chloride and from 0.1 to 2 grams of a supporting electrolyte such as sodium sulfate, is diluted with water t o approximately 30 ml. The acidity is adjusted with sulfuric acid (1 to 10) to a pH of approximately 1. The titration is then performed in the same wag as the calibration. More than 20 determinations with samples containing about 1 mg. of chloride may be performed successively without refilling the syringe. In the fully automatic titration the motor that drives the chart and activates the syringe is stopped by the rupture of the contact between the pen carriage VOL. 30, NO. 1, JANUARY 1958
137
Table I. Accuracy of Chloride Determinations
0.1 1.5
Correction for Delay in Detection Minimum and for C1 Corrected Current, in NaHS04, Vol., MI. MI. M1.
Vol. Taken,a MI.
Molarity of NaHSOd 1.5
Vol. of 0. lOOM AgN03 at
+ 2.OOmM NaCl
10.0 10.0 10.0 3.0
2.0
1.0
a
0.003 0.006 0.205 0.069
0.003 0.006 0.006
0.044
0.003 0.003
0.023
Theoretical Vol.. MI.
0.000
0.004
0.000
0.000
0.000
0.199 0.065 0.041 0.020
0.200 0.060 0.040 0,020
DISCUSSION
Subsequently diluted with water to 30 ml.
and the microswitch carriage when the current increases approximately 2 pa. over the minimum value. RESULTS
Figure 3 represents a set of recorded amperometric titration curves carried out under experimental conditions. For each of these titrations the voltage impressed t o the cell and the sensitivity of the recorder were adjusted to obtain the maximum deflection of the recorder a t the beginning of the titration without raising the polarization voltage above 170 mv. Figure 4 shows successive recorded amperometric titration curves for 30 ml. of 0.7mM chloride solution obtained with silver and mercury electrodes. The first titration was performed with a new bright silver wire not treated with nitric acid. The following titrations were performed with the same silver cathode after washing with water only. Figure 5 shows titration curves, with and without acid addition, for 50 nil. of a solution 1.00niM in sodium chlo-
potential between the two electrodes in the automatic, noninterrupted potentiometric titration of the same solution. Figure 7 compares the shape of the titration curves for 30 ml. of 0.7mM chloride containing 2 grams of sodium sulfate with and without the addition of gelatin. Table I gives the results of a series of titrations of solutions containing known amounts of chloride and variable amounts of sodium bisulfate.
ride and 0.1M in sodium sulfate. In these experiments a silver wire, 6 cm. long and 1 mm. in diameter, was used as an anode instead of the mercury pool; the titrant was added by hand from a 1-ml. microsyringe instead of automatically. Figure 6 shows the potentiometric and amperometric titration curves for 30 ml. of 0.7 m M chloride solution containing 2 grams of sodium sulfate obtained with an activated silver electrode and a mercury electrode. The titration curves were performed simultaneously on the same solution. Curve 2 is the interrupted recorded amperometric titration curve. Curve 1 shows the potential between the electrodes measured each time 30 seconds after the interruption of the amperometric titration and the disconnection of the polarization voltage. A curve similar to curve 1 is obtained by recording the
The chemical literature reveals some abnormal electrolytic behavior of halogens in amperometric titrations. Kowalkowski and coworkers (4) have observed an abnormally high indicator current, greater than 50 pa,, prior t o the end point in the coulometric titration of iodide when the equivalence point is detected by the current impressed through two silver electrodes. Kolthoff and Kuroda ( 3 ) have shown that the indicator current increases more rapidly with the addition of silver nitrate than can be expected from the solubility of silver chloride in the amperometric titration of chloride with the rotated platinum electrode. Samson (8,9) has observed a high indicator current before any addition of silver ion in the amperometric titration of chloride with silver-silver chloride electrodes. Too rapid an increase of the indicator current before the equivalence point during the amperometric titration of chloride with the rotated platinum electrode has been attributed by Laitinen and Kolthoff (6) to the depolarizing
100
-200
20 -
~
1
100
500
3C0 MICROLITER
Figure 5.
01 N A q +
Effect of acid
1. Without acid addition 2. With 0.1N nitric acid 3. With 0.1N phosphoric acid
138
ANALYTICAL CHEMISTRY
.
~
0 20
0 IC ML
Figure 6. trode and 1. 2.
~
030
TITRANT
Open circuit voltage between pretreated silver elecmercury electrode during titration Potentiometric manual titration Interrupted recorded amperometric titration of same
solution
effect of colloidal silver chloride appearing during the titration. Samson (8) also explains the abnormally high amperometric current as the result of a reduction of silver chloride. The following observations suggest that the abnormally high initial current reported in the experiments described above results from the catalytic reduction of a product generated a t the anode, and that the decrease of the amperometric current near the equivalence point results from an increase of the open circuit voltage of the cell a t that point. The residual current of the chloride solution becomes abnormally high only after the cathode has already been used in the silver amperometric titration of a chloride. The grayish smear which appears on the electrode during this operation acts as a catalyst on the cathodic reaction responsible for the residual current. Traces of organic substances markedly reduce the residual current, as shown by Figure 5. The poisoning effect of gelatin on the activated cathode is so strong that the initial current remains about one half of its original value in the subsequent titration even though the cathode has been washed with water. The residual current becomes abnormally high only when the solution contains hydrogen ions in addition to the chloride ions, as shown by Figure 5 . The current increases with hydrogen ions up to 0.01N, and with chloride ions up t o 0.001N concentration. Dissolved oxygen does not participate t o the high residual current, as can be expected from the low polarization voltage applied to the electrodes.
I
-
1
Figure 7.
The abnormal residual current appears only when the anode is close to the cathode. This current increases markedly when the solution is vigorously stirred. With a polarizing voltage adjusted to observe approximately the same increase of current with the addition of the titrant after the equivalence point, the initial current drops from 200 to a few microamperes when the anode is separated from the cathode by an electrolytic bridge instead of being dipped in the same beaker. With the two electrodes dipped in the same beaker, the current increases up to five times when the stirrer turns a t 1800 r.p.m. instead of standing still. -4 similar amperometric titration curve, but with a less abrupt change in the current a t the equivalence point, is observed when a large silver surface is substituted for the mercury pool as an anode. The species which carries the current a t the beginning of the titration is probably a complex of the anode, 31. The reaction generating the residual current could be the following one: At the anode: M
molecular diameters on a mercury electrode during the convection polarography of chloride solution has been observed by Kolthoff and Jordan ( 2 ) . That the decrease of current near the equivalence point results from an increase of the open circuit voltage of the cell is shown by Figure 6. The potentiometric titration curve established by measuring the potential between the activated silver and the mercury electrodes shows an increase of the potential from 50 to 92 mv. near the equivalence point. A similar phenomenon, but with a decrease instead of an increase of voltage near the equivalence point, has been observed by Clark ( 1 ) in the potentiometric titration of chloride with silver and amalgamated silver as indicator electrodes. During the titration the potential os. S.C.E. of both electrodes follows the stretched S shape of a regular potentiometric titration curve. The difference of potentjal between the two electrodes follows a shape similar to a first derivative potentiometric titration curve because the potential of the anode near
1-
+ 1 2 C1- + H + -1
HMCl?
+e
?
1
-4t the cathode: HMC12
+ e +RI +
These formal reactions explain a t the same time the role played by the chlorine and hydrogen ions and the importance of convection current in the generation of the high residual current, The formation of a halide film of a few
i
I
-_
0 10 ML 0 IOOM AgNOa
0
20
Effect of gelatin 1. Without gelatin 2. With 100 p.p.m. of gelatin
the equivalence point is less affected than the potential of the cathode by the increase of the silver concentration a t that point. TThen mercury is used as anode, this behavior is the result of the greater solubility of mercurous chloride os. silver chloride. When silver-silver chloride is used as anode, this behavior is the result of the permeability effect of the silver chloride layer on the diffusion of silver ions toward the silver surface. The increase of the open circuit voltage near the equivalence point is great enough to reverse the direction of the current near that point when the electrodes are polarized below 90 mv. a t the beginning of the titration, Accuracy. The accuracy of the method depends on the accuracy of the measurement of the volume of titmnt delivered by the syringe, and on the delay in the response of the indicator current. Thrce conclusions can be deduced from Table I. The titration end point for 30 ml. of a chloride-free solution is reached with 0.003 ml. of 0.liM silver nitrate. This blank, corresponding to 10 y of chloride, is due mostly to the delay in the response of the indicator device. A volume of 0.003 ml. (0.006 - 0.003) of 0.1M silver nitrate is necessary to titrate the amount of chloride present as impurities in 10 ml. of 1.5M sodium bisulfate. VOL. 30, NO. 1 , JANUARY 1958
139
Amounts of chloride ranging from 70 to 700 7 with variable amounts of sodium bisulfate dissolved in 30 ml. of solution can be determined by convection amperometry with a standard deviation of 10 y when the correction factors corresponding to the delay in the response of the indicator device and for the amount of chloride present in the supporting electrolyte are subtracted from the volume of titrant. ACKNOWLEDGMENT
The author gratefully acknowledges the assistance given by Yalter J.
Microdiffusion of
Savournin and Theodore Snydernian in the experimental part of this n-ork, and by Jack R. Grider for reviewing the present paper. Thanks are also due t o Houdry Process Corp. for permission to publish this paper.
(1954). (7) Mdmstadt, H. V., Fett, E. R., Ibid., 27, 1757 (1955). ( 8 ) Samson, S., Anal. Chim. Acta 13, 473
(1955). (9) Samson, S., Nature 172, 1042 (1953).
LITERATURE CITED
(1) Clark, Walter, J . Chem. SOC. 1926, 749-75. (2) Kolthoff, I. M., Jordan, Joseph, J . Am. Chem. SOC.77, 3215 (1955). (3) Kolthoff, I. M., Kuroda, P. K., ANAL.CHEM.23, 1306 (1961). (4) Kowalkowski, R. L., Kennedy, G. H.,
C, through Cg
Farrington, P. S., Ibid., 26, 626 (1954). (5) Laitinen, H. A., Kolthoff, I. M., J. Phys. Chem. 45, 1079 (1941). (6) Lingane, J. J., ANAL.CHEM.26, 622
RECEIVEDfor review July 17, 1956. Accepted August 10, 1957. Division of Analytical Chemistry, 129th Meeting, ACS, Dallas, Tex., April 1956. Delaware Valley Regional Meeting, Philadelphia, Pa., February 16,1956.
Organic Acids
LAWRENCE M. MARSHALL and FRANCIS T. FOX Robert A. Taft Sanitary Engineering Center, Public Health Service, U. S. Department of Healfh, Educafion and Welfare, Cincinnati 26, Ohio
b Rate of escape of normal aliphatic acids from C1 through Cg varies directly as the carbon chain length during microdiffusion from an aqueous medium. The differences in rate of transfer of the isomers of butyric and valeric acids are presented. Microdiffusion of formic, acetic, and propionic acids from nonaqueous media reflects the relative vapor pressures of the three aliphatic acids in the pure state.
nates between the normal and branched isomers of butyric and valeric acids and possibly even characterizes the branched isomeric pentanoic acids. Although the observed values for microdiffusion from aqueous media for formic, acetic, and propionic acids were insufficiently different to permit characterization of these acids, such data in nonaqueous media were more discerning. MATERIALS
D
the separation of compounds found in the particulate matter filtered from urban air, it was recognized that many classical methods for identification of organic compounds were difficultly applicable to the quantity of organic acids encountered. The chromatographic mobilities of the normal and branched isomers of butyric and valeric acid are similar ( I ) , even when the procedure of Corcoran ( 2 ) is used, which separates the norma1 aliphatic acids elegantly from formic to decanoic. The possibility was considered that the physical laws that govern the microdiffusion of acids in a Conway cell are similar to those underlying the volatilization of organic acids with water vapor described by the Duclaux (4,6) constants. The expected advantage of the diffusion procedure would be its applicability to amounts of organic acids too small for the classical distillation approach. Data reported here indicate that the rate of escape of volatile organic acids having five carbons or less reflects the behavior on distillation defined by the Duclaux numbers. Of particular interest was the observation tha0, in aqueous media, the procedure discrimi140
URING
ANALYTICAL CHEMISTRY
The organic acids were purchased from Matheson, Coleman and Bell. The purity, as determined through the neutral equivalent, was greater than 98Yc for all acids except acetic and formic. When acetic and formic acids were separately chromatographed on silica gel ( 2 ) , titratable acidity was confined exclusively to the corresponding effluent zones for these acids.
disk to reduce the possibility of its contaminating the contents. The cells were shaken on a platform-type shaker a t about 90 cycles per minute. A sheet of asbestos supported the cells sufficiently high above the floor of the shaker to prevent transfer of heat from the shaker motor to the cells. At the end of 1, 2, or 3 hours, the solutions were transferred quantitatively to Erlenmeyer flasks and titrated with 0.0015N sodium hydroxide which contained 10 mg. of thymol blue per liter as an internal indicator. After a weighed quantity of the original acid solution had been titrated as the standard, the percentage of acid remaining in the Conway cell equaled: 9Or
I
I
I
PROCEDURE
Aqueous Media. I n the outer well of a tared Conway cell (Microchemical Specialties) having a total capacity of 6 to 7 ml. was placed 100 f 20 or 200 =t40 mg. of an approximately 2% aqueous organic acid solution. Mass was used as the index of sample size to avoid difference in the volume of one acid-e.g., acetic-and another-e.g., valeric-delivered by pipet. Although the acid solutions were unlike in density, this difference could not influence the data, when each acid solution was standardized on the basis of mass rather than volume. After sodium hydroxide pellets were placed in the center well of the Conway cell, it was covered with a plastic top. The petroleum used as the seal was applied to the outer edge of the cover
80
c W
0
cc a W
\ ISOVALERIC+\
\
I I
1.5
\ 1 2.0
TIME, H R .
Figure 1. Isomeric acid pairs remaining in outer vesseE of Conway cell after shaking a t 20' C.
Sample mass, 100 i 20 mg
