July 15, 1931
INDUSTRIAL AND ENGINEERING CHEMISTRY
321
Potentiometric Titration in Non-Aqueous Solutions I-Diff esential Method for Determining Oil Acidity' Beverly L. Clarke, Leland A. Wooten, and K. G . Compton BELLTELEPHONE LABORATORIES, INC.,463
HE increasing importance of the accurate determination of the acid number on lubricating and insulating oils, as well as on a variety of other materials, such as asphalts and nitrocellulose products, which are insoluble in water, has occasioned a number of attempts to devise satisfactory methods. None of these has been completely successful. The several proposed variations (2, 4) of the colorimetric method have had a limited applicability to oils giving a nearly colorless solution. Kremann and Schopfer (6) made an early attempt to apply electrometric principles to the problem, dissolving their fats or fatty acids in ethyl alcohol and using a cell consisting of a calomel reference electrode joined through a sodium chloride bridge to an electrode comprising a piece of bright platinum dipping into the unknown solution and flooded with a stream of nitrogen or air. Another method (1) uses as solvent a benzene-alcohol mixture containing potassium iodide, and instead of the usual calomel half-cell, employs a beaker of the neutralized solvent electrically bridged to the unknown solution. The end point is taken as corresponding to a zero e. m. f. between two bright platinum wires of which one is immersed in each solution. Seltz and McKinney (7) suggested for the potentiometric titration of oil acidity a method using amyl alcohol saturated with lithium chloride as ti solvent and also as the reference electrode, quinhydrone being added to both half-cells. Bright platinum electrodes were used. I n a subsequent paper (8) amyl alcohol was abandoned in favor of butyl alcohol, also saturated, however, with lithium chloride, and in place of the cumbersonie reference half-cell first used, a silver-silver chloride electrode was specified. Although this method seemed to be the most satisfactory that had been recommended, the troublesome technic suggested for preparing the reference electrode and its short life, together with the failure of the authors to specify necessary details concerning the preparation and care of the alcoholic caustic solution used as reagent, seemed to reduce the usefulness of the procedure for routine purposes. It was also discovered that the preliminary saturation of the butyl alcohol-with lithium chloride greatly reduced its solvent powers for oils and produced such a viscous mixture that thorough mixing during the course of a titration was difficult and slow, resulting in drifting, uncertain potential differences. It will be clear from the foregoing that the several potentiometric methods proposed for oil-acidity measurement differ essentially in the solvent, the nature of the reference electrode, and the presence or absence of an accessory substance like quinhydrone. I n a systematic effort to effect improvements in these directions, we have perfected a method which, while comprising no radical departures from standard potentiometric practice, nevertheless can be presented in a refinement of manipulative detail which was absent in the otfher published methods examined. Although the butyl alcohol recommended by Seltz and Silverman (8) has been adopted as the oil solvent, it has been found necessary to make elaborate specification of detail concerning the storage and use of this solvent, and the drastic reduction in the lithium chloride concentration, for the reasons given in the preceding para-
T
1 Received April 15,1931. Presented before the Division of Physical and Inorganic Chemistry at the 81st Meeting of the American Chemical Society, Indianapolis, Ind , March 30 to April 3, 1931.
WEST
ST.. NEW YORK. N. Y.
graph, increased the resistance of the electrode system so that ordinary instruments became rather insensitive. This has made necessary the adaptation of an electrical measuring instrument, the thermionic titrometer, so that i t will permit a direct reading of the differential of the titration curve in systems having a high resistance. This instrument, which will have a wide usefulness in a laboratory doing potentiometric work, may be economically constructed and requires no special skill for its operation. The method here proposed shares with other potentiometric methods the advantage that opaque as well as colorless oils may be titrated. Description and Use of Apparatus The apparatus employed consists of a thermionic titrometer (6), an electrode system, a titration cell, and a reservoir for storage of the alkali. Since the success of the method depends to a large extent upon the quality and characteristics of the apparatus, a rather detailed description will be given. THERMIONIC TITROMETER-The thermionic titrometer is an instrument designed to give the differential titration curve directly. It combines ease of operation with such high sensitivity that it will operate satisfactorily with solutions having electrical resistances as high as 100 megohms (about 10 times the specific resistance of ethyl alcohol or benzene).
Figure 1-Diagram of Apparatus TI-Western Electric 102-G vacuum tube Tp-Western Electric 101-D vacuum tube Er-135-volt battery En-135-volt battery Ea-18-volt battery E4-6-volt battery, lead accumulator Rt-15,OOO-ohm fixed resistor R1-0 to 10,000-ohm variable resistor IL-1000-ohm fixed resistor S -Shunt for meters, adjustable so that full-scale deflections correspond to currents of 0.1, 1, 10, and 100 milliamperes, respectively M-Direct current milliammeter
Essentially, the instrument, which is an improvement on that originally developed by one of the authors (6), consists of a vacuum tube circuit so arranged that any change in the potential of an indicator electrode upon the addition of an increment of reagent causes a large deflection of an indicating meter. The schematic diagram in Figure 1 shows how this is accomplished. The electromotive force of the cell to be measured is impressed upon the grid of the 102-G tube, T I . An increase or decrease in the voltage of this cell will cause a change in the current flowing through the resistor RI,and therefore a change in the potentid of the grid of the 101-D tube. Owing to the amplifying power of the 102-G tube, this second grid potential change is much greater than that of the first tube. Consequently, a much greater change of plate current will occur in the second tube, and will be indicated by the meter M . Experience in this laboratory has indicated that the Western
ANALYTICAL EDITION
322
TO THERMIONIC
RESERVOIR
PURE MERCUR NARY MERCU
AGAR-AGAR
BRIDGE
OIL SAMPLE DISSOLVED IN BUTYL ALCOHOL WITH 005 GR. QUINHYDRONE
Vol. 3, No. 3
lays while waiting for the eircuit to reach a steady state. The curvature of the voltage-meter deflection curve is so slight, and the portion actually used for the graphical estimation of the end point so small, that this curve may be considered a straight line. ELECTRODE SYSTEM-The reference electxode was a saturated calomel half-cell, which made contact with the solution through an agar-agar salt bridge. The agar-agar mixture (7) was added while hot to a tube 15 cm. by 6 mm. in diameter, one end of which was drawn out to a capillary, until the tube was about one-half full. The remainder of the tube was filled with saturated potassium chloride solution. The assembled half-cell is shown in Figure 2, While it was necessary 00casionally to replace the agar-agar bridge, the same halfcell was used for a period of more than twelve months. The indicator electrode consisted of a bright platinum wire dipping into the non-aqueous solution to be examined, to which quinhydrone had been added. The platinum wire was cleaned after use by heating in the oxidizing tip of a Bunsen flame. All titrations were carried out in an atmosphere of nitrogen, which served both to stir the solution and to exclude carbon dioxide. R E A O E N T ~solution A of potassium hydroxide in butyl alcohol (0.05 N ) was used as a standard reagent in all titrations. The alcohol used for this purpose was freed from aidehyde by treatment with silver nitrate in alkaline solution. Water was removed by refluxing with quicklime followed by distillation in an atmosphere of nitrogen. The potassium hydroxide was of c. P. quality prepared by crystallization from alcohol. After preparation the solution was stored in a Pyrex bottle made light-proof by painting with black lacquer and protected from atmospheric contamination by towers of
PLATINUM
--_-_-_&=--=) (
Figure 2-Assembled Cell
All resistors, tap switches, and connections in the instrument are well made so that no erratic changes take place in the circuit constants. As far as possible, all connections are soldered. The number of variable controls has been reduced to the shunt on the meter and the adjustment for setting the meter needle to zero, thus simplifying the operation of the circuit. To prepare the instrument for a titration the meter, M , is short-circuited by means of one of the shunts a t S, and the various batteries connected. The electrodes are then placed in the circuit and RP adjusted until the meter reads zero on the most sensitive scale. I n case erratic fluctuations of the meter needle are encountered, i t is necessary to shortcircuit the meter, disconnect the electrodes, and connect the terminals of the instrument together. If the trouble disappears when the meter is again set at zero on the most sensitive scale, the trouble is in the electrodes. If the trouble remains, it is due either to faulty connections in some part of the circuit, or to faulty batteries. The batteries should be replaced with new ones, and the circuit traced for possible bad contacts. There are certain important precautions to be exercised in the use of the titrometer. Whenever the electrodes are removed from the solution or disconnected from the instrument, the meter should first be short-circuited to prevent its being damaged Furthermore, the meter should not be set on the most sensitive scale while an increment of reagent is being added to the solution in the titration cell. Old and badly used batteries and those having a low capacity should not be used, as they will tend to cause drifts or erratic fluctuations of the meter needle. Tf the instrument is in daily routine use, the batteries should be left permanently connected to avoid de-
cc. MH
Figure 3-Titrations
of Benzoic Acid Dissolved in Butyl Alcohol ,
soda lime and alkaline pyrogallol. The buret was connected to the alcoholic solution by means of a siphon so that the solution was never exposed to oxygen or carbon dioxide. This solution was standardized by titration against 100-mg. portions of Bureau of Standards’ benzoic acid dissolved in the prescribed solvent. The standard alkali solution prepared and stored in this way was found not to vary appreciably in strength over a period of ten weeks. The solvent used for oils and fats was n-butyl alcohol. A saturated solution of lithium chloride in butyl alcohol was prepared by adding an excess of the salt to a portion%f the alcohol and refluxing for several hours. The saturated solution was stored in a Pyrex bottle made light-proof as described. The quinhydrone used in all titrations was obtained from the Eastman Kodak Company.
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INDUSTRIAL AND ENGINEERING CHEMISTRY
July 15, 1931
Recommended Procedure Dilute 10 cc. of the saturated lithium chloride-butyl alcohol mixture to 110 cc. with the pure solvent and dissolve in i t approximately 50 mg. of quinhydrone. Add this solution to a 250-cc. wide-mouthed Erlenmeyer flask containing approximately 10 grams of the oil sample. Shake until solution is complete. Carry out the titration as follows: Place titration cell in position (Figure 2) and allow a stream of nitrogen to bubble through the solution for 10 minutes before starting titration. By means of the two-way stopcock fill the buret with standard alkali. Connect electrodes to titrometer, as shown in Figure 1.
323
Table I-Titration of Benzoic Acid Dissolved in Butyl Alcohol POTASSIUM HYDROXIDE DEFLECTION (-AE/ AV) Titration 1 Titration 2 CC. 17.05 22 20 17.10 30 35 17.15 44 43 17.20 60 67 17.25 80 76 17.30 95 88 17.35 100 94 17.40 75 85 17.45 78 70 17.50 68 58 17.55 60 52 17.60 51 51 17.65 48 43 17.70 46 37 17.75 43 40 17.80 40 36 17.85 34 40 17.90 38 33
.
Table 11-Reproducibility POTASsI u M HYDROXIDE
TITRATION
of Method
cc.
DEVIATION MEAN
FROM
1 2 3 4 5
17.34 17.34 17.39 17.37 17.35
-0.02 -0.02 4-0.03
+O.Ol -0.01
Mean
17.36
A0.02
Table 111-Titrations of Two Used Lubricating Oils POTASSIUM HYDROXIDE DEFLECTION (- AE/ AV) Titration 1 Titration 2 CC. 0.15 50 0.26 46 56 0.35 100 74 0.45 152 160 0.55 260 250 0.65 280 330 0.75 150 184 0.85 120 125 0.95 93 150 1.05 70 78
...
CC. K W I
Figure 4-Titrations
of Two Lubricating Oils
With Rz adjusted until the meter needle rests at zero on the most sensitive scale, set the meter on the next higher scale, Add exactly 0.5 cc. of the reagent. Record the equilibrium deflection of the meter needle as microamperes and then set the needle back to zero. Read the buret and add a second 0.5-cc. portion of the reagent, recording the deflection as before. * Proceed in this manner until a maximum deflection has been passed. This will constitute a rough titration to locate the region in which the maximum occurs. Repeat the titration using a fresh sample of proportionate weight, but running the reagent into the solution until within about 0.5 cc. of the buret reading a t which the maximum should occur, as indicated in the rough titration. When this point is reached add the reagent dropwise in equal increments of approximately 0.1 cc., carrying the titration well beyond the maximum. The volume of reagent plotted against the deflection of the meter in microamperes will give curves similar to that shown in Figure 3. The peak marks the equivalence point. After a little experience has been gained with the apparatus, the end point may be estimated without plotting the curve or recording the deflections. For the highest precision it is recommended that the data be plotted. Within certain limits the precision is determined by the size of the increment (3). A titration should be performed each day to determine the acidity of the solvent. This consists in following exactly the above procedure, except that the oil is not added. All titrations are corrected by subtracting the value of the blank. Experimental Data I n Table I are given titration data obtained by titrating two 100-mg. portions of benzoic acid dissolved in 110 cc. of butyl alcohol. Plots of these data are shown in Figure 3. Table I1 shows that the reproducibility of the method on standard benzoic acid is of the same order of magnitude as the error of reading the buret. Plots for the titration data on two used lubricating oils, given in Table 111,are shown in Figure 4.
The data in Table I V show the close concordance obtainable between our method and the A. S. T. M. colorimetric method (2) in cases where the latter procedure is applicable. These data are especially convincing since the colorimetric results were obtained independently of the authors at the laboratory of A. E. Flowers of the De Lava1 Separator Company, whose courtesy in permitting the use of them is greatly appreciated. Data on Used Oils by Electrometric and Colorimetric Methods A. S. T. M. BELLLAB. COLORI- ELECTROOIL SAMPLE HISTORY METRIC METRIC DIPF 1 Used transformer oil 1.52 1.56 -0.04 2 Artificially sludged spindle oil 0 . 2 9 0.27 4-0.02 3 Artificially sludged turbine oil 0 . 0 3 0.03 +O.OO 4 Used turbine oil 0.83 0.81 +0.02 5 Used crankcase oil 0.19 0.16 10.03 6 New turbine oil 0.21 0.18 +0.03
Table IV-Comparative
Mean
ja0.02
There has been occasion to adapt this method to problems where organic solvents other than butyl alcohol are required, and the details of these adaptations will be published in a future paper. Acknowledgment The authors are indebted to A. E. Ruehle for performing part of the laboratory work on this problem, and to D. E. Avery for similar services in the early stages of the investigation. Literature Cited (1) Am. SOC. Testing Materials, Proc. A m . SOL.Tesling Malerials. 25, Part I, 282 (1925). (2) Am. SOC.Testing Materials, Tentative Standards, p. 378, 1930 (3) Clarke and Wooten, J . Phys. Chem., 33, 1468 (1929) (4) Evans and Davenport, IND. ENG.CHEM.,Anal. Ed., 3, 82 (1931). (5) Gelbach and Compton, Ibid., 2, 397 (1930). (6) Kremann and Schopfer, S e i j e , 6, No. 35-38 (1922). (7) Seltz and McKinney, IND. ENG. CHEM.,20, 542 (19; >) ( 8 ) Seltz and Silverman, Ibid., Anal. Ed., 2, 1 (1930).
