ANALYTICAL CHEMISTRY
1916 (2) Charlet, E. -M., and Harris, R. G., Consolidated Engineering Corp., Pasadena, Calif., Consolidated Group Rept., 74 (Feb.
17, 1950). (3) Friedel, A. R., Sharkey, A. G., and Humhert, C. R., ANAL. CHEM.,21, 1572 (1949). (4) Gifford, A. P., Rock, S. hI., and Comaford, D. J., Ibid., 21, 1026 (1949). (5) Kelley, H. M., Ibid., 23, 1081 (1951). (6) Langer, A., and Fox, R. E., Ibid., 21, 1032 (1949). (7) Lumpkin, H. E., and Thomas, B. W., Ibid., 23, 1738 (1951). ( 8 ) Johnsen, S. E. J., Ibid., 19, 305 (1947). (9) Taylor, R. C., Brown, R. A,, Young, TV. S., and Headington, C. E., Ibid., 20, 396 (1948).
(10) Taylor, R. C., and Young, IT.S., IND. ENG.CHEM.,A N ~ LED.. . 17, 811 (1945). (11) Thomas, B. W., and Seyfried, W.D., Ax.4~.CHEY.,21, 1022 (1949). (12) Washburn, H. W.,Berry, C. E., Robinson, C. F., Gifford, A. P., and Rock, S. XI., Consolidated Engineering Corp., Pasadena, Calif., C.E.C. Poceedings, 5, S o . 4, 6 (1951). (13) Washburn, H. W., Wiley, H. F., Rock, S. If.,and Berry, C. E., ANAL.CHEY.,17, 74 (1945). (14) Young, W. S., and Taylor, R. C., Zhid., 19, 133 (1947). RECEIVED for r e r i e v M a y 8, 1953. Accepted August 18, 1953. Presented a t the Conference on Analytical Chemistry and Applied Spectroscopy. Pittsburgh, Pa., 1953.
Reference Electrode for Potentiometric Titrations in Glacial Acetic Acid R. A. GLENN, Coal Research Laboratory, Carnegie Institute of Technology, Pittsburgh, Pa. HE
q
titration of nitrogen bases in glacial acetic acid with a
Tstandard solution of perchloric acid in the same solvent has proved to be an indispensable technique in the analysis of petro-
leum ( r ) ,shale oil ( r ) , coal hydrogenation oils (IO),pharmaceuticals ( 8 ) , and many other organic nitrogenous compounds (1-4,6). In the titration of colorless compounds the end point may be readily determined by use of color indicators, but with colored compounds i t must be determined potentiometrically. The choice of electrodes for making potentiometric titrations in nonaqueous media, however, presents a problem. The glass electrode, in conjunction with a sleeve-type calomel reference electrode, both with and without a salt bridge, has been used by several workers (6-9), but not without difficulties resulting from the high sensitivity of the system to stray currents and from contamination of the cell by the solution being titrated. To eliminate the use of a salt bridge and the difficulties encountered in the use of the calomel electrode, Fritz ( 9 ) substituted a silver wire coated with silver chloride for the sleeve-type calomel reference electrode. This paper reports further on the use of the silver-silver chloride electrode in conjunction with the glass electrode and how the instability of the electrode pair in the region of the end point may be obviated. When the silver-silver chloride electrode is usedin conjunction with the glass electrode and the two are immersed directly into the solution of the sample being titrated potent io me t r ic a1 1 y , very erratic e.m.f. readings are observed in the region of the end point (see Figure I), thus making its determination doubtful. When the same electrodes are used in a continuous automatic titration using the apparatus of Katz and Glenn ( 5 ) ,a reversal in the change of the observed e.m.f. occurs in the region of the end point (see curve A , Figure 2), which again renders its location difficult. This reversal in the change of the observed e.m.f. results from the fact that the potential of both the glass and the silver-silver chloride electrodes in the region of the end point changes markedly but in the opposite direction.
r, I
VOLUME OF TITRANT ADDED, ml.
Figure 1. Manual Titration Curve Silver-silver chloride electrode used in conjunction with glass electrode Sample. Coal hydrogenation neutral distillate oil
300
/Paraffin
400-
Seal
-
Commercial Silver Silver Chloride Electrode
3 sool Glacial Acetic Acid Saturated with KCI 600-
5 24/40 Silver Button Coated with Silver Chloride KCI Crystals
700-
Glass Wool Plug 0
20
40
€0
80
100
120
CHART DIVISIONS
TITRANT ADDED 1.91 ,ue/div.
Figure 2. A. B.
Continuous Automatic Potentiometric Titration Curves
Silversilver chloride electrode dipping directly into solution being titrated. Sample, quinoline Silversilver chloride electrode placed in isolation cell. Sample, coal hydrogenation distillate oil
Fignre 3. Reference Electrode Isolation Cell
V O L U M E 25, NO. 12, D E C E M B E R 1 9 5 3 By isolation of the silver-silver chloride electrode in a separate compartment, which makes electrical contact with the solution being titrated by means of a liquid-liquid junction through a ground-glass sleeve (see Figure 3), the reversal in the e.m.f. curve is eliminated and a normal titration curve is obtained (see curve B, Figure 2). The silver-silver chloride electrode used is one that is readily available commercially. The liquid in the isolation compartment is the same solvent as used for the sample-i.e., glacial acetic acid--but it is saturated with potassium chloride to increase its conductivity. So far approximately 100 continuous automatic titrations have been made without difficulty using the silver-silver chloride electrode isolated in the separate compartment. The observed e.m.f.’s are essentially reproducible and thus permit the shuttingoff of the automatic titration apparatus by means of a relay operated by the Brown recorder-controller.
1917 ACKNOWLEDGMENT
The writer wishes to thank Joy S. Wolfarth for making the titration with the Macbeth titration-pH meter. LITERATURE CITED
(1) Fritz, J. S., “Acid-Base Titrations in Non-Aqueous Solvents,”
Columbus, Ohio, G. F. Smith Chemical Co., 1952. (2) Fritz, J. S.,ANAL.CHEM.,22, 1028-9 (1950). (3) Ibid., 24, 306-8 (1952). (4) Fritz, J. S.,and Keen, R. T., Ibid., 24, 308-10 (1952). (5) Katz, M., and Glenn, R. A,, Ibid., 24, 1157-63 (1952). (6) Narkunaa, P. C., and Riddick, J. A , , Ibid., 23, 337-9 (1961). (7) Moore, R. T., et al., Ibid., 23, 1639 (1951). (8) Pifer, C. W., and Wollish, E. G., Ibid., 24, 300-6 (1952). (9) Seaman, I+‘., and illlen, E., Ibid., 23, 5 9 2 4 (1951). (10) TS’ittman, G., Angew. Chem., A60, 330-3 (1948). RECEIVED for review hlay 26, 1953.
Accepted July 29, 1983.
Photometric Determination of Copper in Gasoline J. K. LIVINGSTONE AND N. D. LAWSON Petroleum Laboratory, E. I . d u Pont de Nemours & Co., Inc., Wilmington, Del. appears in gasoline in very minute quantities, usC ually less than 0.5 mg. per liter and, because of its pro-oxidant effect, is detrimental to gasoline stability. In order to handle the
light path from the 50 mm. used here will result in changes in precision and accuracy.)
problem of copper contamination of gasolines intelligently, it is necessary to be able to measure accurately the amount of copper present. A revien of the literature reveals that only a few methods deal with the determination of copper in gasoline. A qualitative test was described several years ago ( 4 ) . Another method ( 5 ) , designed for the estimation of copper in gasoline by visual matching of yellow copper diethyldithiocarbamate solutions, was adapted later ( 2 ) for use with a photoelectric colorimeter but is not sufficiently sensitive or accurate for many purposes. This appears to be due principally to incomplete extraction of the copper from the gasoline and the high transmittancy range a t R hich measurements are made. A rapid photometric method for the quantitative determination of copper in gasoline has been developed that is applicable to concentrations of 0.025 to 5 mg. of copper per liter of gasoline with an accuracy within 10% when using a 200-ml. sample of gasoline. I t was found during the development of this method that the normality of the hydrochloric acid solution had a considerable effect on the completeness of the extraction of the copper from the gasoline. I t was found that 0.1A‘ acid removed the copper much more readily than the 4.V acid recommended in earlier methods The use of multiple extractions also improved the recovery of the copper. Furthermore, if the transmittance measurements were made on solutions of the copper complex in an organic solvent such as carbon tetrachloride, as suggested by Sandell ( S ) , more consistent re.sults were obtained. The use of chloroform to remove residual color from the alkaline solution of copper is an improvement over the use of the previously recommended tert-amyl alcohol, as chloroform does a more effective job. Throughout the method the transferring of the solutions containing the copper from one container to another is minimized in order to reduce the possibility of the loss of copper.
REAGENTS
OPPER
APPARATUS
The tranmnittarice measurements were made with a Lumetron Model 402-E photoelectric colorimeter equipped as follows: Picture projection lamp, G.E. T-8, 100-watt. Filter, monochromatic, narrow band, No. 440 (transmittance peak = 440 mp). Absorption cell, 50 mm., 70-ml. capacity. (Any change in
Hydrochloric acid, 0.1N. Dilute 8.3 ml. of concentrated hydrochloric acid (37%) to 1000 ml. with copper-free water, double-distilled from glass distillation apparatus. Sodium diethyldithiocarbamate solution. Dissolve 1 gram of sodium diethyldithiocarbamate in copper-free water, doubledistilled from glass distillation apparatus, and dilute to 1000 ml . Standard copper solution for preparing the standard curve (1 ml. = 0.005 mg. of copper). Dissolve 1.9645 grams of C.P. copper sulfate pentahydrate ( C U S O ~ ~ ~ Hwhich ~ O ) ,contains 0.50 gram of copper, in copper-free water, double-distilled from glass distillation apparatus, and dilute to 1000 ml. Dilute a 10-ml. aliquot of this solution to 1000 ml. Ammonium hydroxide, concentrated, C.P. (28% minimum). Chloroform, C.P. Carbon tetrachloride, C.P. C.P.
PROCEDURE
Extraction with Acid. Pipet 200 ml. of the gasoline to be tested into a 500-ml. separatory funnel and add 30 ml. of 0.1N hydrochloric acid. Shake the solution vigorously for 3 minutes,
Table I.
Recovery of Copper Added to Iso-octane Copppr”, Ilg.
Added t o 200 ml. of iso-octane b
Found in final 100 rnl. of of CCh solution
0,005 0,005
0.005 0.005
0.025 0.028
0.025 0.025
0.050 0,050
0.051
0.050
0.050
0.050
0.051
0.075 0.075
0.074 0.074
0.050
Copper-%ethyl hexoate (C~sHrno~Cu), assay 17.46% copper, dkeolved in acetone t o obtain gasoline-soluble copper standard. b Iso-octane (2,2,4-trirnethyl pentane) treated with three 30-rnl. portions 0.1N HC1 before copper addition.
