Ions which reacted a t the dropping mercury electrode were those of iron (111), molybdenum(V1) , thallium(1) , and copper(I1). All of these, except copper(II), have half wave potentials sufficiently different from that of uranium(V1) a t p H 5.5 to allow a reasonably accurate analysis to be made, provided the amounts present are of the same order of magnitude as that of uranium(V1). For the polarogram of a solution containing iron(III), Eliz = -0.13; uranium(VI), E,/z = -0.357; and molybdenum, EIiz = -0.82 as shown in Figure 2 (curve 2). Since the El/* of copper was very close to that of uranium(VI), the interference of copper must be eliminated. For this purpose the solvent extraction method of Guest and Zimmerman (2) was tried successfully. One extraction removed uranium(V1) completely from copper and iron. The procedure for the solvent ex%raction recommends starting with 5 ml. of 5% nitric acid solution containing 10 to
30 mg. of uranium oxide. Then 6.5 ml. of aluminum nitrate salting solution (2) are added and the uranium is extracted into 20 ml. of ethyl acetate. The uranium is stripped with water. Sodium perchlorate and EDTA solutions are added such that on final dilution the concentration of sodium perchlorate is 1M and that of EDTA, 0.lM. The p H is then carefully adjusted to pH 5.5, using a p H meter, with perchloric acid and/or sodium hydroxide, and a polarogram taken and read in the usual way. Results accurate to 2 to 3% were obtained. This extraction method does introduce nitrate and aluminum into the solution to be polarogrammed, but these do not interfere as shown. ACKNOWLEDGMENT
The author thanks Daniel Orgeron, who did much of the experimental work, and the National Science Foundation for its financial assistance.
LITERATURE CITED
(1) Cabell, M. J., Analyst 77, 859 (1952). (2) Guest, R. J., Zimmerman, J. B., ANAL.CHEM.27, 931 (1955).
(3) Harris, 15‘. E., Kolthoff, I. M., J . Am. Chem. Sac. 67, 1484 (1945). (4) Imai, Hides, Bull. Chem. SOC.Japan 30, 873 (1957). (5) Kern, D. 31. H., Orlemann, E. F., J . Am. Chem. SOC.71, 2102 (1949). (6) Kolthoff, I. M., Harris, W. E., I b i d . , 68, 1175 (1946). ( 7 ) Kolthoff, I. M., Lingane, J. J., Ibid, 5 5 , 1871 (1933). (8) Kolthoff, I. ,,M., Lingane, J. J. “Polarograph 2nd ed., p. 202, Interscience, k e w York, 1952. (9) Ibid., pp. 462-7. (10) Prfbil, Rudolph, Roubal, A,, Svatek, E., Collection Czechoslov. Chem. Comm u m . 16-17, 561 (1951-2). (11) Rodden, C. J., ed., “Analytical Chemistry of the Manhattan Project,” K.N.E.S., ,Div. VIII, pp. 596-610,
RlcGraw-Hill, New York, 1950.
(12) Souchay, P., Faucherre, J., Anal. Chim Acta 3, 252 (1949).
RECEIVED for review September 23, 1960. Accepted December 7, 1960. Southwest Regional ACS Meeting, Oklahoma City, Okla., December 1960.
High Speed Recording Potentiometric Titrator with Automatic Continuously Variable Rate of Titrant Addition JOHN R. GLASS and EDWARD J. MOORE Research Department, Paulsboro laboratory, Socony Mobil
,A high speed instrument was designed and built to perform potentiometric titration of acid in petroleum products. It titrates the sample rapidly until the end point is approached. It then gradually slows down and proceeds without interruption through the end point. The resulting titration curve is smooth. Instantaneous mixing and rapid response reduce the titration time to 2.5 minutes. The end point overshoot is about 0.2%. Repeatability in aqueous and nonaqueous The solvents i s better than 0.1%. instrument is noise-free and stable. Exceptionally high input impedance and the use of a novel reference electrode make it applicable to titrations in solvents of low conductivity. It is particularly useful for routine titrations of a large number of samples.
Oil Company, Inc., Paulsboro, N, 1.
slow to reach equilibrium. It is also adaptable to remote operation with radioactive samples and is, therefore, somewhat complicated. A simpler instrument which is particularly useful for high speed titrations in nonaqueous solvents is described in this paper. To attain a high speed, it is necessary that the rate of chemical reaction and electrode response be very rapid and that instantaneous mixing be provided. The instrument will perform titrations involving moderately slow reactions about as well as most instruments, but it may give low results for the very slow reactions that can be
T
HE MOST GENERALLY useful potentiometric titration devices are those which automatically plot the curve. These are revienTed by Phillips ( 3 ) . A comparison of the various types with a recently developed versatile model is discussed by Kelley, Fisher, and Wagner ( 3 ) . Their instrument was particularly designed t o perform accurate titrations when cell reactions were very
494
*
ANALYTICAL CHEMISTRY
Figure
1.
Mechanical arrangement
handled by the instrument described by Kelley et al. Our instrument uses modern electrometer tubes to measure the potential of a glass electrode and continuously varies the rate of titrant addition so that the rate of change of electrode potential is fairly constant. Titrant addition is not intermittent. The rate varies continuously during the titration. A derivative of the signal from the electrodes controls the magnetic braking of the titrant motor. APPARATUS
A diagram of the mechanical arrangement of the apparatus is shown in Figure 1. A Leeds & Northrup Type A, Model S, 10-mv. recorder was used. This recorder is no longer manufactured; however, any recorder of 50 mv. or better sensitivity, and a response time of 2 seconds or less can be used. The recorder is mounted in a cabinet relay rack about 22 X 22 X 67 inches. Mounting the cabinet on casters makes the instrument an accessible unit which can be easily moved to wherever it is needed. Also, there is ample space in the cabinet for the circuitry and the titrant reservoir. Titrant Feed System. The synchronous recorder chart motor is re-
-
l-l/2'+
t 10-
n n
1 d..h Figure 2.
1/16'THICK
Stainless steel stirrer
placed by a two-phase variable speed reversible motor ( S o . F P E 25-11, 115/115 volt, 60 cycle, 2 pole) obtained from Diehl Manufacturing Co., Somerville, N. J. This motor drives the chart a t a variable speed. It also drives a shaft passing through the side 3f the cabinet. A worm gear on the ahaft drives the screw that raises the plunger of the syringe. Thus, the travel of the chart is proportional to the amount of titrant fed by the syringe. The gear ratios are chosen so that a 50ml. syringe feeds about 1 ml. of titrant per inch of chart travel, and the maximum speed of chart travel (for a motor speed of 3500 r.p.m.) is about 20 inches per minute. The syringe is refilled by reversing the motor. A solenoid clutch disengages the chart drive when the motor is reversed. Limit snitches prevent overtravel of the syringe. Glass solenoid valves (obtained from Houston Glass Fabricating Company, Houston, Tex.) admit titrant to the syringe and pass it to the titration vessel. Capillary tubing of 2-mm. bore, fitted with 12/2 ball joints, conveys the titrant. Gas bubbles are automatically purged from this small bore tubing. If the titrant reservoir is placed below the syringe, gas pressure should be supplied to it to raise the titrant and thus prevent bubbles from forming in the titrant lines. The solenoid valves are convenient for rapid macrotitrations. However, they leak about 0.001 ml. per minute and can be opened by pressure. The auxiliary Teflon stopcock should be closed when performing microtitrations. The number of milliequivalents of titrant dispensed per division of chart travel is determined by titrating a standard sample.
Titration Cell. For high speed titrations i t is particularly important that no concentration gradient exist in the solution; this requires instantaneous mixing which is approached by driving the stirrer (Figure 2) at 3450 r.p.m. by a 0.05-horsepower induction motor (No. TM7354; Herbach & Rademan, Philadelphia, Pa.). The stirrer is connected to the motor by a Tygon tubing coupling (Figure 1). This insulates the metal stirrer from the rest of the system and prevents it from interfering as a third electrode. A hole through the Micarta cover of the beaker serves as a bearing. This cover also supports the titrant inlet and the electrodes. A hinged section clamped by a thumbscrew permits the glass electrode to be removed easily. The 250-ml. titration beaker is used in determining acid and base numbers by ASTM D664 (1). It is held by a spring between the conical upper and lower supports. The lower support can be depressed and swung away to remove the beaker. It is locked rigidly in place by rotating the handle of the clamp. The supports for the motor and titration cell should be strong and rigid to hold these parts securely and prevent vibration. The interference of atmospheric gases is about the same with violently agitated solutions as with gently agitated ones. In both cases the solution comes rapidly to equilibrium with the gas above it. A sealed cell purged with an inert gas should be used if a titration is subject to atmospheric interference. Microtitrations can be performed with the instrument by using smaller syringes and titration cells. These are often preferable because of the small sample, titrant, and solvent requirements. Electrodes. One of the most difficult potentiometric titrations t o perform, either manually or automatically, is ASTM D664, because the toluene-isopropanol solvent has high resistance. A suitable indicating electrode for this titration is the all-purpose glass electrode, Beckman 41262. Because the usual types of reference electrodes have a high and erratic junction potential and require considerable maintenance, the reference electrode shown in Figure 3 was developed. The silver mire is coated with Tygon paint to eliminate corrosion above the liquid level 17hich would lead to erratic potentials. The exposed silver is coated with silver chloride by electrolysis in 1W HC1 for 24 hours a t 10 Ma. A cylinder, cut from a piece of Coors porous porcelain plate (Scientific Glass Apparatus Co., Bloomfield, N. J., KO. P-7210), is fitted snugly into the end of a borosilicate glass tube. It is sealed with a flame, care being taken not to overheat the joint. Use of a large diaphragm and an electrolyte
NEOPRENE STOPPER TYGON T U B I N G TYGON PAINT
ELECTROLYTE
1 ik
COORS
POROUS PORCELAIN
Figure 3.
Reference electrode
compatible with the titration solvent minimizes the junction potential. Thus, a steady potential is maintained even in a violently agitated toluene-isopropanol solution. With this solvent the electrolyte used in the reference electrode is 0.2M tetramethylammonium chloride in isopropanol. The electrolyte leaks a t the rate of a few milliliters per day through the porous porcelain diaphragm. The only maintenance required is replacement of electrolyte each day that the electrode is used. Complete draining of the electrolyte and drying of the electrode are not harmful. ELECTRONIC CIRCUIT
A wiring diagram of the apparatus is shown in Figure 4. The input tubes are CK 5886, the miniature type usually used with an ionization chamber for radiation measurements. This tube is available from radio supply houses. The amplifier can be powered by any regulated power supply having a variation less than 0.1% for a 10% line voltage change. The one used by us was based on a radio handbook design. We believe the use of a commercially available power supply would be more practical. We have used a Hamner H-106 power supply for this type of circuit in a nonrecording titrator. Ordinary radio components are used for the rest of the circuit. The 115 volt a.c. for the instrument is obtained from the terminal board of the recorder. The power switch and fuse of the recorder serve the entire instrument. The solenoid valves are controlled by a separate deck on switch S1. The pen motor circuit is c.onnected to another deck of S1 so that the pen circuit is active when the instrument is in the titrate position. The glass electrode is plugged into a Beckman-type socket. The grid lead of V1 is suspended in air, contacting nothing except the polystyrene-insulated spring to which it is soldered. This VOL. 33, NO. 4, APRIL 1961
495
AC
TEST POINTS
1
REGULATED POWER SUPPLY
I
‘GI
Figure 4. Wiring diagram spring contacts the center wire of the glass electrode lead. The threaded part of the socket that contacts the shield of the glass electrode lead is grounded. The stirring motor, titration stand, recorder case, and cabinet are also grounded. Principles of Operation. The potential of the glass or other sensing electrode is applied t o the grid of V1. Drifting is minimized by V2. The output of VI is amplified by V3A and V3B to give an output to the recorder. The output of V3A and V3B is differentiated by the 1-mfd. condensers and the I-megohm resistors. The resulting rate of change of electrode potential is amplified by V U and V4B and further by V5A and V5B. It is then applied to the grids of the two 6L6 tubes, increasing their plate current. This passes through the control winding of the motor, magnetically braking it. The control winding is also supplied with 80 volts a x . from the Stancor transformer. The other motor winding is supplied with 115 volts a.c. These alternating current voltages are not varied, Therefore, high torque is obtained even a t low speed. The speed of the motor varies inversely with the direct current passing through the control winding. The screen potential of the 6L6 tubes is adjusted so that the motor will never come to a complete stop but will add the titrant continuously and slowly a t the end point. Thus, the motor runs rapidly before and after each end point but slowly while passing through each end point. This keeps the rate of change of electrode potential with time relatively constant, as is ideally done by an analyst in performing a manual titration. Controlling the motor speed by the usual method of varying the alternating current voltage to the control phase was also tried. With this type of opera496
ANALYTICAL CHEMISTRY
tion the motor would come to a complete stop. With nonaqueous solvents the resulting increments were large and the titration curves were difficult to interpret. Similar difficulties had previously been experienced by us when attempting to use incremental addition with the commercial model of Robinson’s titrator (4). To obtain a smooth curve, it was necessary to add the titrant continuously. The circuit described above was a simple and effective means of accomplishing this. Calibration. R2 is adjusted so 580 mv. (measured by an external potentiometer connected to the test points) exists across the 10-turn potentiometer, R1. The electrodes are then immersed in a buffer solution of 0.05M sodium tetraborate (pH 9.18), and R4 and R5 are adjusted so that the recorder reads the desired pH and the reading is shifted 9 pH units by nine turns of the potentiometer, R1. A recorder range of 0 to 15 pH was found most useful for ASTM D664. Any desired pH or millivolt range is readily obtained by adjustment of either R1 or R4 and R5. These enable the instrument to be used with various electrode and solvent combinations. R3 is used as a fine adjustment to correct for differences between various electrodes. Speed Control. R6, acting through T75A and V5B, varies the grid bias on the two 6L6 tubes. This varies the rate of change of electrode potential needed to slow down the motor a t the end point. Thus, the speed control is used to set the rate of the electrode potential change suitable for the reaction involved. The rate of titrant feed is varied automatically during the titration so that the set rate of electrode potential change with time is maintained. The potential of the indicating electrode becomes more positive or more
negative depending upon the type of titration. The instrument may be used for either type of titration by reversing the input to the 6L6 grids with the “Up Scale-Down Scale” switch. This corrects for the reversal in the direction of electrode potential change. PERFORMANCE
The instrument performs titrations very rapidly. In determining acid and base numbers in oil (ASTM D664), the titrant is fed a t a rate of 20 ml. per minute while the pH is changing slowly and a t abcut 3 ml. per minute through the end point. The results obtained are in agreement with those obtained at a rate of 1 ml. per minute on a commercial model of Robinson’s titrator (4) and with those obtained by an even slower manual titration. Samples that take as long as 20 minutes to titrate with the commercial instrument have been titrated in less than 5 minutes with the new titrator. The important factors that make this high speed possible are: violent agitation, fast response of the input circuit, and continuous variation of the rate of titrant addition. No irregularities are produced by the turbulent agitation. Use of a reference electrode having a large contact area and filtering condensers on the plates of V1, V2, V3A, and V3B remove the stirrer noise. The noise level of the instrument is negligible, being comparable to that of the recorder. Thus, the titration curves obtained are remarkably smooth with sharp inflection a t the end point. They resemble the theoretical curves given in textbooks. The speed of response to pH changes is essentially the 2-second response time of the recorder. This delay and slow electrode response could cause the end point of the titration curve to occur a t a point beyond the true equivalence
point. Hon-ever, poor mixing or slowness of reaction could raise the concentration of unreacted titrant abnormally a t the electrodes and thereby shift the end point of the titration curve ahead of the true equivalence point. These opposing errors tend to cancel each other. In any case, the remaining error is cancelled in the standardization. Acids were titrated in both aqueous and nonaqueous (ASTM D664) solvents at the normal speed and a t the slowest speed setting to estimate the amount of overshooting The normal speed titration took 2.5 minutes; the slow speed titration took 8 minutes. With the aqueous solution the fast titration consumed 0.16% more titrant than did the slow titration. With the nonaqueous solvent the fast titration used 0.20y0 more titrant than did the slow titration. The precision of the instrument was checked by replicate titrations of 50-ml. portions of aqueous 0.05M potassium acid phthalate with 0.1N KOH titrant. Each of the titrations required 2.5 minutes. The standard deviation of 26 determinations was 0.064%. A similar series of 8 determinations of cyclohexanepropionic acid in nonaqueous ASTM D664 titration solvent
gave a standard deviation of 0.08%. Thus, the instrument performs about as well with the nonaqueous as with the aqueous solution. The current drawn by the grid of the input tube of the instrument was measured by connectins a 100,000megohm resistor between the two electrode sockets. The grid current was calculated from the shift in the pH reading that occurred when this resistor was shorted: picoamperes
=
pH shift X 5.8 X 1O’O
ohms
The current drawn by this instrument was 0.05 picoampere. Six popular commercial pH measuring instruments drew from 5 to 200 times as much current. The low grid current (high input impedance) makes the instrument particularly suitable for titrations in solvents of low conductivity. The balanced arrangement of the input tubes and the use of regulated direct current for the filaments stabilize the circuit. After a warm-up time of 30 minutes the drift rate is about 0.1 pH unit per hour. Checking a t the test points and adjustment of R2 is normally done monthly. The output of
this type of amplifier is not strictly linear; the maximum error is about 0.1 pH unit at the extremes of the scale. This is negligible for titration work. The instrument has been used to titrate 5800 samples over a period of 14 months. In this period the contacts of switch S-1 required cleaning once and one 12AX7 tube in the power supply failed. It is planned to install O.lFf. condensers across the switch contacts to minimize arcing. ACKNOWLEDGMENT
The authors thank S. L. Duncan for his design of the regulated power supply. LITERATURE CITED
(1) ASTM Standards on Petroleum Prod-
ucts and Lubricants Method D66458, “Neutralization Number by Potentiometric Titration,” Am. SOC.Testing Materials, Philadelphia, Pa , 1960. ( 2 ) Kelley, M. T., Fisher, D. J., Wagner, E. B., ANAL.CHEM.32, 61 (1960). (3) Ph;flips, J. P., “Automatic Titrators, pp. 55-65, Academic Press, Sew York, 1959. (4) Robinson, H. A,, Trans. Electrochem. SOC.92, 445 (1947). RECEIVEDfor review October 6, 1960. Accepted January 4, 1961.
Determination of Acridine by Potentiometric Titration in Acetic Acid P. RAMACHANDRA NAIDU and V. R. KRISHNAN Chemical laboratories, Sri Venkateswara University, Tirupati, South India
b Acridine dissolved in glacial acetic acid has been determined potentiometrically b y titration with perchloric acid. The method is rapid and more convenient than other methods described. Only the presence of bases of comparable strength interferes with the titration.
A
is of interest as a coal-tar component, spot-test reagent, fluorescent indicator, and the parent substance of a number of pharmacologically active compounds (1). Very few methods are available for its estimation. In the gravimetric method, it is weighed as the bisulfite compound (5). In volumetric estimations, it is precipitated as the picrate and the product titrated with methylene blue (a), or an alcoholic solution of the compound is titrated directly with sulfuric acid (4). Titrations carried out in our laboratory by the second method gave results which were not reproducible. CRIDINE
Weak bases show an increase in strength when dissolved in acid solvents; therefore acridine (pK 5.6) should exhibit a greater relative base strength when dissolved in acetic acid. The volumetric estimation of acridine has been investigated by potentiometric titration with perchloric acid in glacial acetic acid medium. EXPERIMENTAL
Reagents. Glacial Acetic Acid. Reagent grade glacial acetic acid was refluxed with the amount of acetic anhydride required to react with the water present for about 8 hours. Refluxing was continued for another 6 hours after the addition of 2% by weight of chromic anhydride. The acid was distilled rapidly using a fractionating column, and the fraction boiling a t 115-16” C. was collected. Perchloric Acid. A solution in acetic was prepared by a modification of the method of Hall and Conant (3). A 60% aqueous solution was added slowly to the requisite amount of
chilled acetic anhydride to react with the water present, and diluted further to the desired concentration with the glacial acetic acid described above. This solution of perchloric acid was standardized by potentiometric titration with a standard solution of sodium acetate in acetic acid (3). Chloranil (Eastman Kodak). Hydrochloranil prepared from chlorani1 by reduction with phosphorus and iodine. Acridine. A British Drug Houses laboratory reagent sample, melting point 110’ C. Procedure. Samples of acridine were weighed out and dissolved in glacial acetic acid t o yield ca. 0.1N solutions. A mixture of chlorani1 (0.6 gram) and hydrochloranil (0.8 gram) was added and a platinum rod, inserted in the solution, served as the indicator electrode; a saturated calomel electrode (S.C.E.) was used for reference and a saturated solution of lithium nitrate in glacial acetic acid functioned as the salt bridge. The solution was titrated rapidly with standard perchloric acid and the change VOL 33, NO. 4, APRIL 1961
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