542
INDUSTRIAL A N D ENGINEERING CHEMISTRY
from rains. All handling of apparatus should be done in as cool a place as possible and out of direct sunlight. I n direct hot sunlight the end point is vague, and it is difficult and sometimes impossible to obtain reasonable accuracy. Solutions of iodine and starch carried in the field should be protected from light and heat by covering the bottles with thick toweling, which is kept damp with water. Frequency of Determinations An operator familiar with the manipulation of the apparatus can make one complete determination in about 3 minutes.
VOl. 20, No. 5
It is the best practice to make determinations a t 4-or 5-minUte intervals, using the hourly averages for studying the general trend. Such determinations, made a t exactly equal time intervals, will represent a true average of the sulfur dioxide concentrations existing during the period and will also show the amount of variation. If time is not available for practically continuous determinations as just described, they should be made in groups consisting of three or five tests in rapid succession, a t as short intervals as possible, using the average of the group as the true sulfur dioxide concentration.
Quinhydrone Electrode in Amyl Alcohol Solutions’ For Determination of Neutralization Numbers of Petroleum Products Harry Seltz and D. S. McKinney CARNEGIE INSTITUTE
OF
TECHNOLOGY AND DUQUESNE LIGHTCOMPANY, PITTSBURGH, PA.
HE determination of the organic acidity (neutralization number) developed in petroleum oils while in service in turbines, transformers, and oil circuit-breakers is of considerable importance as an indication of the rate of deterioration of the oils. Several methods have been devised. I n the A. S. T. M. method (D188-25T) 20 grams of the oil are placed in a flask with 100 cc. of a neutral 1:l alcoholwater mixture, heated to boiling, and titrated with 0.1 N aqueous potassium hydroxide to a permanent end point, using phenolphthalein as an indicator. The chief objection to this method is that the end points are masked by dark-colored oils or oils which 12 emulsify markedly. Another m e t h o d 2 uses an alcohol-benzene mixture as the s$ 8 solvent and phenolB phthalein as indicator, $4 the titration being m a d e i n t h e cold. 0 b j e c t i o n s to this 0 4 8 /Z /6 method are that the T I M E IN M I N U T E S solvent is unstable, as Figure 1-Change of End Point in Alcohol- sholm in Figure 1and Benzene Solvent Table I. causing high results, and the end point is masked by dark oils.- FFgure 1 was obtained by titrating (colorimetrically), a t intervals of 1 minute, 200 cc. of 1:l alcohol-benzene mixture. The curve shows the total sodium hydroxide necessary to neutralize this amount of solvent a t the times indicated. An electrometric method3 has also been proposed. Ten grams of oil are dissolved in 150 cc. of a mixture of 10 parts benzene, 9 parts 95 per cent alcohol (ethyl), and 1 part of 95 per cent alcohol saturated with potassium iodide. This solution is connected by an agar-agar bridge (containing potassium iodide) to another beaker containing the pure solvent which has been neutrahed with 0.1 N alcoholic potassium hydroxide using phenolphthalein as indicator. Bright platinum electrodes are immersed in the two solutions and 0.1 N alcoholic potassium hydroxide is introduced into the oil solution until the electromotive force of the cell becomes zero. A galvanometer and tapping key are used to determine the end point. The obvious objection to this
T
1 Received November 16, 1927. 2 Holde, “Hydrocarbon Oils and Saponifiable Fats and Waxes,” 2nd ed., p. 69, John Wiley and Sons, Inc. 8 Proc. Am. SOC.Testing Materials, 26, Pt. 1 , p. 282 (1925).
method is that the electrodes as used cannot measure acidity unless some foreign substance in the solution functions in a manner similar to the quinhydrone or azo-hydrazo electrodes, involving the H ion in the electrode reaction. Otherwise, any coincidence of the neutralization end point and a zero electromotive force with this cell must be more or less fortuitous. Also, the solvent is unstable as mentioned above. With these ideas in mind, the authors considered the possibility of utilizing the quinhydrone electrode in a non-aqueous solvent for the determination of the neutralization numbers of petroleum oils. Numerous solvents were tried, but amyl alcohol proved to be the most satisfactory for the following reasons : (1) It has a relatively high dissociating power. ( 2 ) It is an excellent solvent for petroleum oils. (3) Lithium chloride is appreciably soluble in amyl alcohol and the resulting solution is highly conduc ring-a necessary condition for electromotive force measurements of this type. (4) An amyl alcohol solution of potassium or lithium hydroxide can be used conveniently for the titration.
Quinhydrone Electrode4 in Amyl Alcohol Solution
Quinhydrone is an equimolecular combination of hydroquinone and quinone. In water solution, in the presenceiof 4 Kolthoff and Furman, “Potentiometric Titrations,” p. 220, John Wiley and Sons, Inc.
Fi$ure 2-Diagram
of Electrochemical Cell
INDUSTRIAL A N D ENGINEERING CHEMISTRY
N a y , 1928
(1) Bright platinum electrodes 1 to 2 sq. cm. in area. ( 2 ) Galvanometer (Leeds & Northrup) ; resistance, 1095 ohms; sensitivity, 41 megohms; period, 3 seconds. (3) Potentiometer (Leeds & Northrup); range, 0 to 1200 millivolts; accuracy, 0.5 millivolt with resistance less than 10,000 ohms. The galvanometer contained in this instrument was replaced by that specified in ( 2 ) to give greater sensitivity. (4). Titration vessel. Wide-mouthed glass bottle, 400 cc. capacity fitted with a stopper perforated for reference half cell (S/s-inch test tube), one electrode, buret tip, nitrogen inlet and outlet. A rapid flow of gas through the apparatus eliminates the necessity of a mechanical stirrer.
t 0.4
to.:
? 3
L
$0.1
0
-0.1
543
4
/2
6
I6
20
c c.o f KOH Figure 3-Titration Curves A-blank. B-benzoic acid; C-stearic
acid
H ion, the quinhydrone electrode can be considered to function according t o the following electrochemical equation :
+ 4- ~ ~ S C ~ H ~ O Z H Z
CeH4024- 2H Quinone
Hydroquinone
The electromotive force of the single electrode will be given byzthe following expression: I
O.O~Q
[quinone] [ H I ? E = E o -t 2log [hydroquinone]
where Eois a constant which depends on the reference electrode and the equilibrium constant of the cell reaction. It is evident, since [quinone] = [hydroquinone], that the electromotive force of the quinhydrone electrode changes in the same way with change of the H-ion concentration as does the hydrogen electrode. Table I-Sodium Hydroxide Necessary to Neutralize 200 cc. of 1 : l Alcohol-Benzene. Indicator, Phenolphthalein TIME TOTAL NaOH ADDED TIME TOTAL NaOH ADDED Mmutes Mg. Minutes Mg. 9 1.23 0 10 1.65 11 1.87 12 2.18 13 2.48 3.10 14 3.52 15 16 4.33 4.95
REAGEKTS--XOhent. Isoamyl alcohol, boiling point 128132' C., J. T. Baker c. P., saturated with lithium chloride. Alkali for titration. Potassium or lithium hydroxide dissolved in amyl alcohol, 0.1 or 0.50 N . Quinhydrone. Eastman Kodak Company, melting point 189-170" C., 0.05 gram to 125 cc. of amyl alcohol. This volume is used for each titration of oil. If desired, the quinhydrone may be prepared by the method given by Kolthoff and F ~ r m a n . ~ dgar-agar bridge. 1.2 grams agar-agar plus 2.0 grams lithium chloride dissolved in 100 cc. distilled water. REFERENCEELECTRODE OR HALF-CELL-The electrode is contained in a S/s-inch test tube (1.6-em.) having a hole approximately inch (6 mm.) in diameter blown near the closed end. When the electrode is prepared the hole is closed with a short piece of rubber tubing slipped over the test tube and the agar solution poured in to a depth of about 1 inch (3 cm.). After the agar has set, approximately 25 cc. of the solvent, containing quinhydrone, are added. The platinum electrode, held in a stopper, is then inserted in the tube. When the electrode is used in this form, it is necessary to reverse the leads to the potentiometer during the titration. since the electromotive force passes through zero. To avoid this, 1 gram of benzoic acid can be added to each 100 cc. of solvent used in the reference electrode so that the H-ion concentration is always greater than in the titrated solution. This is not recommended when titrating oils of very low neutralization number, as diffusion of the benzoic acid may give high results. If titrations are run in sequence, the reference half cell may be used without change during four or five runs. The electrode immersed in the oil-solvent mixture should be washed and ignited before each run.
Preliminary experiments showed that the quinhydrone :&rode functions satisfactorily in amyl alcohol solution saturated with lithium chloride. N o attempt was made to evaluate Eo against any standard reference electrode, since the cell was to be used only in titrations and the inflection point in the electromotive force curve taken as the end point. One objection to the use of the quinhydrone electrode is that in alkaline solutions the quinhydrone is oxidized rapidly with the formation of weak acids. To inhibit this oxidation a cell was constructed so that the entire titration could be' carried out in an atmosphere of nitrogen. This is not absolutely necessary for ordinary purposes, however, since the end point is always passed before the oxidation becomes appreciable. f
Apparatus and Reagents ELECTROCHERlICdL CELL-Figure 2 shows diagrammatically the design which was found to be satisfactory. The details of the set-up are as follows:
cc. o f A-highly
KOH
Figure &Titration Curves oxidized oil; B-turbine oil No. 1 ; C-turbine
oil No. 2
INDUSTRIAL A N D ENGINEERING CHEMISTRY
544
Vol. 20, No. 5
of these curves furnish a satisfactory end point. The end points are found more accurately by plotting the change in electromotive force per cubic centimeter of base added ( AE/Acc.) against cubic centimeters of base, as shown in Figures 5 and 6. The maxima in these curves determine the end points. Table 11-Blank
AE
.-
N/20 KOH
ACC
cc.
0 0.05 0.10 0.18 End point 0.11
Titration of 125 cc. Amyl Alcohol (Curves A , Figures 3 and 5) E.M.F. N/20 KOH E.M.F. Volts cc. Volts -0.011 0.25 $0.213 +0.018 0.32 +O. 234 +O. 089 0.40 4-0.245 4-0.154 0.50 +0.259
CC.
of 0.1221 Gram Benzoic Acid (Curves B ,
Table 111-Titration N/20KOH
cc. 0
C.C. of
KOH
Figure 5-Determination of End Points by Plot of AE/Acc. against cc. of Base A-blank; B-benzoic acid; C-stearic acid
Figures 3 and 5) N/20 KOH E.M.F. cc. Volts 19.0 4-0 080 19.2 +0.090 19.4 +0.104 19.6 $0.117 19.85 +0.144 20.0 $0.163
E.M.F. Volts -0,079 +0.012 +0.031 +0.047
10.05 15.0 17.0 18.0 +0.056 18.5 +0.066 End point 20.30 CC.
Table IV-Titration E.M.F. Volts 0 -0,099 10.0 -0.o50 14.0 -0.012 15.0 -0,007 End point 16.8 cc.
N / 2 0 KOH
Procedure
cc.
After a blank titration is made on the solvent, 125 cc. of amyl alcohol, saturated with lithium chloride and containing 0.05 gram of quinhydrone, are placed in the apparatus. The reference electrode is prepared as indicated above. The necessary electrical connections are made, nitrogen is bubbled through the container to remove air, and a voltage reading taken. Small increments of potassium (or lithium) hydroxide solution are added from the buret. Readings are taken after each addition of alkali, the solution being agitated by the nitrogen stream until the voltage becomes constant. The potassium hydroxide solution is then standardized by dissolving 0.1221 gram of benzoic acid in 125 cc. of solvent and titrating as above., Increments of 1 or 2 cc. of alkali can be added a t the start and reduced to 0.05 cc. near the end point. (The end point is indicated by a relatively large change of voltage for a given addition of alkali, amounting to nearly 0.3 volt per cubic centimeter of alkali as the end point is approached.) I n the determination of neutralization numbers of petroleum oils, 10 grams of the oil are dissolved in 125 cc. of solvent and the titration is carried out as described above. A larger sample of oil is not recommended, as the conductivity-and therefore the sensitivity-of the cell is materially reduced. Data and titration curves for the blank, benzoic and stearic acids, and three petroleum oils are given in Tables I1 to VI1 and Figures 3 and 4. It is apparent that the inflection points
N/20 ROH
cc.
E.M.F.
vozts
0.0 -0,124 5.0 -0.026 6.5 +0.008 7.0 +0.035 7.5 +O.OiS End point 7.8 cc.
VOllS
+0.207 +0.243 i-O.275 +0.330 +0.366 'rO.385
CC.
VOllS
CC.
VOZlS
4-0.019 +0.029 +0.041
16.8 17.2 17.4 17.9
+0.130 +0.309 +n.33o $0.360
+0.066
of 2 Grams of Highly Oxidized Oil Figures 4 and 6 ) E.MF. S / 2 0 KOH E.M.F. N/20 KOH Volts cc. volt5 cc. -0.018 5.0 4-0.119 7.0 +0.018 5.5 +0.158 8.0 f0.037 6.0 $0.215 9.0 +0.062 point 6.25 cc,
Table VI-Titration
E.M.F.
16.0 16.2 16.4 16.6
N/20 KOH 0.0 2.0 3.0 4.0 End
KOH CC. 20.2 20.4 20.6 21.1 22.0 23.0
of 0.2347 Gram Stearic Acid (Curves C, Figures 3 and 5) X/20 KOH E.M.F. 5 / 2 0 KOH E.M.F.
Table V-Titration c-. c.
"20
(Curves A , E.M.F. Volts +0.312 +0.390 +0.427
of 10 Grams Turbine Oil No. 1 (Curves E , Figures 4 and 6) E.M.F. N/20 KOH E.M.F. N/20 KOH Volts Volts CC. CC. $0.180 7.7 $0.103 8.5 7.9 +0.125 9.4 8.1 f0.146 10.4 +0.166 11.4 0.340 8.3
::"33
of 10 Grams Turbine Oil No. 2 (Curves C, Figures 4 and 6) N/20KOH E.M.F. Ai/20KOH E.M.F. cc. Volts cc. Volts +0.210 +0.058 6.3 5 0 + O . 235 5.5 +0.109 6.5 +0.291 7.0 +0.131 5.7 + O . 329 7.5 +0.156 5.9 +0.351 $0.183 8.0 6.1
Table VII-Titration X/20 KOH
cc.
E.M.F. VoZts -0.094 -0,022 -0.OOi
0 2.0 3.0 +0.008 3.5 4.0 +0.021 1-0.037 4.5 End point 6.1 cc.
Summary of Results
1
I
2
1
4
I 6
I
8
I
lU
I
/e
C.C. o f KOH Figure 6-Determination of End Points by Plot of AE/Acc. against CC. of Base A-highly oxidized oil; B-turbine oil No. 1; C-turbine oil No. 2
The method is applicable to the determination of the neu tralization numbers of petroleum oils and to the titration of free fatty acids. Duplicate determinations check consistently within 1.0 per cent when the titration is 1 cc. or more. For example, calculation of the factor for the alkali used in the determinations given above, yields for benzoic acid 0.0495 N , and for stearic acid 0.0493 N , a difference of 0.5 per cent. On another solution of alkali factors of 0.0454 N and 0.0452 N were obtained using benzoic acid; whereas colorimetrically, using phenolphthalein in ethyl alcohol, a factor of 0.0435 N was obtained. Acknowledgment The authors wish to thank Max Hecht, chief chemist of Duquesne Light Company, for valuabIe suggestions.
