ANALYTICAL CHEMISTRY
1688 Evans, D. E. M , , Tatlow, J. C., J . Chem. SOC.1955, 1184-8. (5) Fishel, J. B., Haskins, J. F., I n d . Eng. Chem. 41, 1374-6 (1949). (5) Harvey, D., Chalkley, D. E., Fuel34, 191-200 (1955). (7) James, A. T., M ~ QChemist . 26, 5-10 (1955). (8) James, A. T., Research (London) 8,8-16 (1955). (9) James, 1.T., Martin, A. J . P., Brit. M e d . Bull. 10, 170-6 (4)
(1955).
S.A , , Burow, F. H., A N A L . C H E Y . 28, 1510-13 (1956). 111) Littlewood. A. B.. Phillim. C. S. G.. Price, D. T., J . Chem. SOC. 1955, 1480-9. (12) Martin, A. E.. Sinart, J., S a t w e 175, 422-3 (1955). (IO) Lichtenfels, D. H., Fleck, ~I
(13) Osborne, J. A., Adamek, S., Hobbs, IT. E . , ANAL.CHEY.28, 211-15 (1956). (14) Patton, H. W.,Lewis, J. S., Proceedings of Third National Air Pollution Symposium, Pasadena, Calif., 1955, pp. 74-9; ANAL.CHEY.27, 1034 (1955) (abstract). (15) Patton, H. W.,Lewis, J. S., Kaye, R. I , I b i d . , 27, 170-4 (1955). (16) Purnell, J. H., Spencer, 11.S., Sature 175, 988-9 (1955). (17) Touey, G. P., ANAL.CHEV.27, 1788-90 (1955). RECEIVED for review January 9, 19%. Accepted July 0, 1956. Presented a t the Tobacco Chemists' Research Conference, Korth Carolina State College, Raleigh, N C., October, 6 1955.
Titration of Copper Oxinate in Glacial Acetic Acid CHARLES H. HILL, HAN TAI, A. L. UNDERWOOD, and R. A. DAY, JR. D e p a r t m e n t o f Chemistry, €mory University, Emory University, Ga.
Organic precipitants are very useful for separating metal ions, but gravimetric determinations involving direct weighing of the precipitated metal chelates are often beset by difficulties, including not only the usual gravimetric tedium, but also errors arising from dubious weighing forms which exhibit uncertain hydration, decomposition, and volatility. It is thus of interest to investigate methods other than gravimetric for the measurement of these analytical precipitates. In the present study, it is shown t h a t copper can be determined by applying nonaqueous titrimetry to the copper oxinate precipitate. Cupric ion is precipitated from aqueous solution with oxine, the copper oxinate is dissolved in glacial acetic acid, hydrogen sulfide is bubbled through the solution to precipitate copper sulfide, and final13 the oxine solution is titrated with perchloric acid to a potentiometric end point. The method yields very satisfactory results.
600-
*5500
1500-
35 ML OF HCIO, Figure 1.
B
ECAUSE organic precipitants are, in general, weak acids and/or bases, it was reasonable to investigate the applica-
tion of nonaqueous titrimetry t o these reagents and their metal complexes. One might hope thereby t o retain the attractive features of organic precipitants in effecting separations while circumventing some of the difficulties attending their gravimetric use. Some preliminary studies along these lines with 8-quinolinol (8-hydroxyquinoline, oxine) and dimethylglyoxime have been reported by Fritz (3). M7hen oxine is dissolved in glacial acetic acid and titrated potentiometrically with a solution of perchloric acid in glacial acetic acid, an excellent end point is obtained (Figure 1). If, on the other hand, a metal oxinate rather than oxine itself is dissolved in acetic acid and titrated, the end point is very poor (Figure 1). This has proved true in the case of iron, copper, aluminum, magnesium, and a number of other metals (4). Studies of the extents t o which the various metal oxinates dissociate in acetic acid, and the dissociation of the resultant metal acetates, would be needed t o explain the difficulty completely. Presumably the following equilibria are involved:
+ nHO.\c
+ M(OAc), HOx f HO;lc % (HpOx)+ + OAC-
M(Ox),,
%
nHOx
40
Titration curves of oxine and metal oxinates 1. Oxine Aluminum oxinate Copper oxinate
2. 3.
I
I
1
1
1
I
I-m 5 5 O L
1
"450
,
400//
5
IO 15 ML. OF HCIO,
, 20
1
Figure 2. Titration of copper oxinate solution after precipitation of copper sulfide
V O L U M E 28, N O . 11, N O V E M B E R 1 9 5 6 Table I.
T i t r a t i o n of Copper Oxinate w i t h Perchloric Acid
Copper Oxinate, Mg. Taken Found 225.5 255.1 260.7 312,s 324.4 369.5 374.8 392.9 401.2 471.6 $96.2 005.0 519.6 536.9 539.0 550.9 591.3 743.5 867.5 881.3 886.4
226.0 264.3 260.4 314 6 324.7 370.5 373.9 389.3 401.5 473.7 497.4 504.9 523.6 ?36.9 044.1 553.4 594,s 743.4 872.3 881.4 891.5
Deviation
%
hIg. 0.5 0.8 0.3 1.8 0.3 1.0 0.9 3.6 0.3 2.1 1.2 0.1 4.0 0.0 5.1 2.5 3.5 0.1 4.8 0.1 5.1
0.22 0.31 0.12 0.67 0.09 0.27 0.24 0.92 0.07 0.44 0.32 0.02 0.76 0.00 0.94 0.45 0.59 0.01 0.55 0.01 0.57
Av. deviation 1 . 8 Standard deviation 2 . 6
0.20
Table 11. Precipitation of Copper from Aqueous Solution Followed by Nonaqueous T i t r a t i o n Taken
Copper, M g . Found
72.2 72.2 72 2 72.2 96.2 96.2 96.2 96.2 96.2 120.3 120.3 120.3 120.3 120.3 120.3 120.3 144.4 144.4 144.4 144.4
71.8 72.0 72.1 72.4 95.6 95.9 95.9 9B.O 96.6 119.5 119.F 119.r 119.7 119.9 120.1 120.5 143.5 143.5 143.9 143.9
Deviation hlg. 0.4 0.2 0.1 0 2 0 6 0.3 0.3 0.2 0.4 0.8 0.7 0 6 0.6 0 4 0 2 0.2 0.9 0.9 0.5 0 5
A v . deviation 0 . 4 Standard deviation 0 . 5
% 0.55 0.28 0.14 0.28 0.62 0.31 0.31 0.21 0.42 0.67 0.58 0.50 0.50 0.33 0.17 0.17 0.62 0.62 0.35 0.35 0.32
The equilibrium constants that would be needed for a complete interpretation of the problem are not a t hand. A workable egress from this difficulty is applicable in cases where the metal in question forms a sulfide that is insoluble in glacial acetic acid. When hydrogen sulfide is bubbled through the solution, the difficulty clears up, as shown in the following reaction for the case of copper: Cu(0Ac)n
+ HaS e CUS+ + 2HOAc
The precipitated metal sulfide does not interfere with the potentiometric titration, and the other product is, of course, the eolvent itself. Unfortunately, copper and cadmium are the only examples from a large number of metals tested where the sulfide precipitation, the solubility of the oxinate in acetic acid, and an unambiguous formula for the oxinate are all favorable for an accurate determination. In this paper, a workable method for the determination of copper based upon the above principles is presented. Briefly summarized, the method is as follows: Cupric ion is precipitated from aqueous solution in the usual manner with oxine, and the copper oxinate is collected by filtration, washed, and then dissolved from the filter with glacial acetic acid. Hydrogen sulfide is bubbled through the acetic acid solution, and the solution is finally titrated with a standard solution of perchloric acid in glacial acetic acid. The precipitation of copper oxinate, and the separations which can be accomplished thereby, have been thoroughly discussed
1689 ( I ) . Thus it was not necessary to investigate this phase of the method, and attention was centered upon the actual determination of the copper oxinate. The titration proposed here is very much faster than drying and weighing the precipitate, and apparently a t least equally reliable. It is somewhat faster and probably more accurate than the bromometric titration of oxine. EXPERIMENTAL PROCEDURES
Apparatus and Reagents. A Beckman Model H p H meter with a glass electrode and a sleeve-type calomel electrode was used for the potentiometric titrations. The titrant was O.IN perchloric acid in glacial acetic acid, prepared according to Fritz ( 2 ) . This solution can be standardized against potassium acid phthalate; the same normality was obtained in this way as with pure oxine as the standard. All materials were reagent grade. Copper oxinate was prepared according to directions by Flagg (1). Standard copper solutions were prepared from electrolytic cop er foil in the approved manner. A small lecture cylinder ofhydrogen sulfide is a convenient source of this gas, although a Kipp generator can be used. Titrations of Pure Copper Oxinate. To test the reliability of the titration itself, apart from possible errors in the precipitation of copper as oxinate, accurately weighed portions of pure copper oxinate were dissolved in glacial acetic acid. Very roughly, 50 ml. of acetic acid is required to dissolve each 100 mg. of copper oxinate. The range studied was from about 200 to 900 mg. of copper oxinate. Hydrogen sulfide was bubbled through the solution for about 3 minutes. The precipitated cupric sulfide was coarsely granular and settled to the bottom of the beaker. The supernatant solution was yellow when the precipitation of copper was complete; otherwise green. It was not necessary to separate the precipitate from the solution, nor did excess hydrogen sulfide interfere in any way. A few drops of acetic anhydride were added to the solution just prior to the titration, to eliminate water. Traces of water are not objectionable, but larger amounts make the end point less sharp. The solution was titrated potentiometrically with the standard perchloric acid solution. The end points in these titrations were rather good, as may be seen from the typical titration curve shown in Figure 2. Volumes of titrant were calculated by the differential method (6). The results of these titrations are summarized in Table I, where it may be seen that the titrations are apparently free of any important error. Combined Precipitation and Titration. To establish the over-all reliability of the method combining separation of copper from aqueous solution as the oxinate with the nonaqueous titration, a second series of experiments was performed. The copper was precipitated from aliquots of a standard cupric nitrate solution according to directions given by Flagg ( I ) . The precipitates were collected, washed, and dissolved in glacial acetic acid. Copper was precipitated with hydrogen sulfide, and the titrations were performed as in the first series. The quantities of copper ranged from about 70 to 150 mg. The results of this study are presented in Table 11, where again it is indicated that the method is sound. ACKNOWLEDGMENT
Charles H. Hill is indebted to the X‘ew York Community Trust for a fellowship during the period when the work was performed. LITERATURE CITED
(1) Flagg, J. F., “Organic Reagents,” pp. 157-89, Interscience, New York, 1948. (2) Fritz, J. S., “Bcid-Base Titrations in Nonaqueous Solvents,” p. 13, G. Frederick Smith Chemical Co., Columbus, Ohio, 1952. (3) Fritz, J. s.,ANAL.CHEM. 26, 1701 (1954). (4) Tai, Han, h1.S.thesis, Emory University, 1955. (5) Willard, H. H., Merritt, L. L., Jr., Dean, J. A., “Instrumental Methods of Analysis, 2nd ed., pp. 209-10, Van Nostrand, New York, 1951. RECEIVED for review April 26, 1956. Accepted July 13,1956. Most of the data presented in this paper are taken from a thesis submitted by Charles H. Hill in partial fulfillment of the requirements for the degree of master of arts, Department of Chemiatry, Emory University, 1956.
