Acidimetric Titration of Metal Acetates in Glacial Acetic Acid A. T. CASEY' and K. STARKE2 Department of Chemistry, University of British Columbia, Vancouver 8, B. C.
Acetates of 18 metals can be titrated potentiometrically in glacial acetic acid with an accuracy comparable to that obtained b y conventional analytical methods, although iron, aluminum, and chromium give unusual results. Most of the metals can easily b e dissolved as acetates or, alternatively, as nitrates, and the nitrate radical decomposed with acetic anhydride.
b
R
EACTIONS in
glacial acetic acid have been studied so extensively in the past 30 years that recently the principles of chemistry in anhydrous acetic acid could be summarized (4). Interest in this solvent has also yielded practical dividends in analytical chemistry. I n the inorganic field acidimetric titrations of various salts have been performed according to the scheme MOCOCHa
+ HC10,
=
PIlClOl CHdCOOH
+
The purpose of this investigation was to gather additional information concerning the estimation of metals by titration in glacial acetic acid. EXPERIMENTAL
Apparatus. A Becknian Model G p H meter was equipped with glass and saturated aqueous calomel electrodes. For maximum sensitivity, the latter was of t h e sleeve type. T h e solution inside t h e electrode was changed frequently t o prevent contamination by acetic acid. The calomel electrode could not be used to titrate solutions of lead or silver, because precipitates of their chlorides froze the ground-glass surface of the electrode. I n such cases, a lithium acetate salt bridge was used or, alternatively, a silver-silver chloride electrode was used as the reference electrode. A Fisher Senior Titrimeter with the same electrode systems was used for a series of determinations on the same metal. Solvents. Glacial acetic acid C.P. was purified by adding t h e calculated
Present address, Department of Chemistry, Universitl of Melbourne, Carlton N. 3, Victoria, ustralia. * Present address, Department of Chemistry, University of Kentucky, Lexington, KY. 1060
ANALYTICAL CHEMISTRY
amount of acetic anhydride t o react with the mater present (2), and then distilling twice using a 30-inch helixtype column. The fraction boiling at 117-118" C. was collected; samples melted a t 16.5" C. (lit. 16.6" C,). Reagent grade 1,4-dioxane was purified according to Vogel ( 8 ) and distilled twice. The fraction boiling at 100.5101.5" C.was collected; samples melted a t 11.6-11.9" C. (lit. 12' C.), Titrants. -4 0.1N solution of perchloric acid in glacial acetic acid was obtained by mixing 31 ml. of a 60% aqueous solution of perchloric acid with 500 ml. of purified acetic acid. Then 33 ml. of chilled acetic anhydride were added and the solution was made u p t o 2 liters with acetic acid. The solution stood overnight, or better 2 to 3 days, to allow the anhydride to react with the water present. This solution was used with both indicator and potentiometer. A 0.1N solution of perchloric acid in 1,4-dioxane was obtained by diluting 31 nil. of a 60% aqueous solution of perchloric acid to 2 liters with dioxane. An occasional brown color did not influence the acid strength. Because i t contained a small amount of water, this solution could be used only in potentiometric titrations. Indicator. A 0.5% solution of crystal violet in glacial acetic acid i m s satisfactory. One drop was sufficient for 55 ml. of acetic acid. T h e color change was not sharp, passing from blue-violet through various shades of blue and green t o yellow. T h e blue-green end point was especially difficult to detect when the titration produced a precipitate, as nith potassium and ammonium acetates. The best procedure here was to titrate a sample to the end point potentiometrically, then add indicator and use this solution as a standard. Standards. Sodium carbonate, or preferably potassium acid phthalate (6), may be used as a primary standard. Two milliequivalents were dissolved in 55 ml. of glacial acetic acid and titrated with the perchloric acid solution either potentiometrically or with indicator to a blue-green end point. Water acts as a weak base in acetic acid. I t s presence in the solution when titrating potentiometrically can be tolerated up to 1.5% but i t causes erroneous results when using an indicator. To minimize absorption from the atmosphere, a lipless beaker was used, closed by a thin rubber stopper through which R small hole was punched to admit the
buret tip. Allowance was made for any temperature change by assuming that the O.1N solution in acetic acid or dioxane had the same coefficient of expansion as the pure solvent. Reagents. The metal acetates used nere of the best obtainable purity and xyeie used as received with the following exceptions: Anhydrous sodium acetate TYBSdried for an hour a t 110' C. Ammonium acetate Tyas recrystallized from methanol or acetic acid, washed v i t h ether, and diied in a desiccator a t IO-mm. pressure. The acetates of iron, chromium, aluminum, manganese, copper, tin, antimony, and bismuth were prepared by heating the nitrates nith glacial acetic acid and sufficient acetic anhydride to decompose the nitrate and react with any mater of crystallization. The chloroacetates were prepared by the method of Kastle and Reiser ( I ) . Procedure. A conr.enient solution for titration contained about 3 meq. of t h e salt in 40 nil. of acetic acid. It was occasionally necessary t o warm t o effect rapid solution. T h e solution T T ~ then S chilled and titrated immediately. The acetates of sodium, potassium, lithium, ammonium, strontium, and barium gave equally good results with indicator or potentiometer, The acetates of magnesium, calcium, manganese, and cadmium may be titrated potentiometrically but not with indicator. S o satisfactory end point could be obtained when titrating acetates of cobalt and nickel with perchloric acid in acetic acid. Reproducible but poorly defined end points \yere obtained using a solution of prrchloric acid in dioxane. Zinc acetate is insoluble in acetic acid and must be determined indirectly by adding a measured excess of perchloric acid solution and back-titrating with standard sodium acetate solution. Silver acetate is only slightly soluble and must be determined in the same way. I n addition, for both these metals, a silver-silver chloride electrode was used as the reference electrode. The acetates of aluminum, Chromium, and iron gave anonialous results. The acetates of copper, tin, antimony, bismuth, and uranium behaved as if they were undissociated in acetic acid and could not be titrated.
800
1
I
40
I
I 1
Ooo (CH,COO),G
CH, COO Na
-
32
'
W
f
END POINT 24
9
I-
3
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v)
16
b
3 r
v
8W,
a 1a
400 50
100 125 PER CENT NEUTRALIZED
0
75
100 125 50 75 00 125 PER CENT NEUTRALIZED Figure 2. Titration of calcium acetate and cobalt
50
AV (ARBITRARY UNITS)
Figure 1. Titration of sodium acetate in glacial acetic acid (0.075N solution) with 0.1 N perchloric acid
75
acetate in glacial acetic acid
(0.075Nsolution) with
0.1 N perchloric acid
Titration and differential curves
The chloroacetates were titrated in the same manner as the corresponding acetates, except for ammonium chloroacetate. I t s solution changed strength continuously, possibly as a result of conversion into glycine, and n-as not titratable. Analyses. T o check accuracy of t h e titrations, each salt was determined by conventional quaiititative methods (9). Gravimetric analyses: sodium as magnesium uranpl acetate; potassium as perchlorate; lithium as sulfate; strontium and barium as sulfate; calcium as oxide; magnesium and manganese as ammonium phosphate; zinc, cadmium, cobalt, and nickel as pyridine thiocyanate complex; lead as molybdate and silver as chloride. Ammonium was determined volumetrically by treating nith a stronq base, and distilling into excess standard acid, RESULTS
Representative titration curves are shown in Figures 1 and 2. The potential break was not sharp enough to determine the end point accurately from the graph, so the more precise method of plotting 4E/AV, the change in potential per unit volume, against AV, the volume added near the end point, was used. One such curve accompanies the titration curves. If the differential curve is not symmetrical, the end point may be found by extrapolation ( 3 ) assuming that in a small region about the end point dE/dV varies linearly with V . For all but the most basic of the acetates, the sensitivity of the titration did not justify extrapolating the volume to fractions of a drop.
The data from the titrations were used to calculate the percentage purity of the metal acetates. These results are compared with those of conventional methods in Table I. DISCUSSION
The titration curves show the same changes when proceeding from strong to
Table
I.
u-eak bases as is found in nater. Acetates of the first and second group metals show a pronounced potential break a t the end point, n hile those of metals usually considered as weakly basic in n a t e r shon- a less pronounced break. An exception is ammonium acetate, which, in acetic acid, is as strong R base as potassium acetate.
Titration in Glacial Acetic Acid with 0.1 N Perchloric Acid
Salt Titrated Purity Found by Titration, -Acetates Sodium 99.59 99.55 99.50 Potassium 99,56 99.41 99.49 Ammonium 97.58 98.08 97.58 Lithium 99.73 99.70 99.77 Magnesium 99.82 99.69 99.71 Ca 1ci u m 98,95 99.06 99.00 Barium 99.40 99.47 99.49 Strontium 99,65 99. GO 99.49 Zinc 97,73 97.50 97.26 Cadmium 98.72 98.25 98 .. 65 .~ Manganese 99.69 99.31 99.38 Cobalt 91,35 91.05 91.17 Kickel 93.61 93.30 93,21 Silver 99,72 100.13 99.62 Lead 98.36 98.49 98.57 Iron 11.365 11.3gb ll.5gC Chromium 11.71~ 11.50b 11.4-P Aluminum 11.06' 11.23b 11.15c Chloroacetates Sodium 97 96 98 13 98 07 Potassium 97 83 99 00 99 01 Barium 97 36 97 30 97 46 Magnesium 97 35 97 46 97 33 Each figure is mean of a t least two concordant determinations. titration 3 meq./40 ml. e 3 meq./45 ml. ' 6 meq./45 ml. 9 meq./45 ml.
Purity Found by Conventional Analytical Methods, 70 99.58 99.48 97.73 99.75 99.85 98.96 99.41 99.41 97.40
98 . - An _.
99.62 91.06 93.29 99,83 98.30 98.96 99,65 99.03
99.52 99.47 97.95 99.60 99.74 99.05 99.46 99.46 97.30 98.72 99.73 91.00 93.42 99.76 98,33 98. 90 99.43 99.15
98 10 98 00 98 96 98 89 97 50 97 35 97 49 97 46 Usual concentration for
VOL. 31, NO. 6 , JUNE 1959
0
1061
It might be inferred from the work of Pifer and Wollish (5) that certain salts of antimony and bismuth could be titrated by this method, if they reacted basic in glacial acetic acid. The curves obtained for various salts of each of these metals, and also of copper, tin, and uranium, have the same form as the solvent alone, and no titration is possible. It would also be inferred that aluminum and iron can be titrated normally, because their acetates react basic in glacial acetic acid. These experiments show that the curves for aluminum, chromium, and iron have end points indicating only 11 to 12% purity, while conventional gravimetric analyses gave results close to 1 0 0 ~ o . These end points are reproducible and the potential breaks are more clearly defined than those for certain other metals whose curves yield normal results. d possible explanation of this is the formation of complexes of the type [lI,(OH)z(CH~COO),] CHsCOO Such complexes have been isolated when M = Cr or Fe ( I O ) and have been reported t o exist in solution when hf = A1 (7’). Titrating such a complex with perchloric acid would yield the per-
chlorate of the complex cation-Le., only one third of an equivalent of perchloric acid would be consumed per gram-atom of metal. This would yield a result of 11.11% of the theoretical, not far from the value obtained, The values remained essentially constant over a threefold increase in concentration, indicating a definite compound and not an equilibrium mixture. Throughout a titration the potential between the electrodes remained constant for periods u p to 20 minutes as long as no moisture entered the system. If aliquots of the same solution were titrated, the curves and position of the end point wpre essentially identical. Each figure in Table I represents the mean of a t least two concordant determinations. The table shows that the accuracy and reproducibility of the results are as good as in conventional methods. Thus. it may be concluded that the accuracy and precision of these estirnations are sufficient to warrant further study of the glacial acetic acid system. The first problems are the introduction into the organic phase of metals and their compounds not directly soluble in acetic acid, and their subsequent separation and estimation. The results of such a study will appear in a later paper.
ACKNOWLEDGMENT
The authors gratefully acknowledge the financial support of the International Kickel Co. of Canada given to one of them (A.T.C.) in the form of a threeyear research fellowship. LITERATURE CITED
(1) Kastle, J. C., Keiser, B. C., Am. Chem. J. 1 5 , 471 (1893). (2) Kendall, J. C., Gross, G. H., J. Bm. Chem. Soc. 43, 1426 (1921). (3) Kolthoff, I. M., Laitinen, H. A,, “pH and elect rot it ration^,'^ Longmane, Green, New York, 1948. (4) Maass, Gunther, Jander, Gerhart, Fortschr. chem. Forsch. 2, 619 (1953). ( 5 ) Pifer, C. W., Wollish, E. G., ANAL. CHEM.24,519 (1952). (6) . . Seaman. William, Allen. Eueene. Zbid., 23,592 (1951): (7) Spacu, G., Popper, E., Kolloid-Z. 103, 19 1194.11. (8) Vbgei,.’ A. I., “Practical Organic Chemistrv.” Lonrtmans. Green. New York, 19iS. (9) Vogel, A. I., “Quantitative Inorganic Analysis,” Longmans, Green, New York, 1948. 110) Weinland. R. F.. Reihlen., H.., 2. anorg. Chem’82, 426’(1913). Y
I
-
*
I
RECEIVEDfor review June 9, 1958. Accepted December 12, 1958.
Derivutive Chronopote nt io metry REYNOLD T. IWAMOTO’ University o f California Radiafion laborafory, livermore, Calif.
b Derivative techniques are described for the determination of transition times in chronopotentiometry. One based on following the potential between two working electrodes operating a t different current densities with a Sargent Malmstadt automatic titrator appears to be the best. Transition times for poorly defined chronopotentiograms have been successfully evaluated b y the differential technique. Automatic determination of transition time with the titrator is possible under favorable conditions. Limitations of chronopotentiometry for investigating very dilute solutions of electroactive substances are discussed.
D
techniques have been successfully utilized for the determination of end points in numerous analytical methods. The evaluation of transition times in chronopotentiometry (3-4, 6) is often complicated because ERIVATIYE
Present address, Department of Chemistry, University of Kansas, Lawrence, Kan.
1062
ANALYTICAL CHEMISTRY
the E1:4 value of an electroactive substance lies very close to the decomposition potential of the supporting electrolyte solution, or the individual E1f4’s of mixtures lie very close to one another. This difficulty is evident when dealing with dilute solutions of electroactive substances whose chronopotentiograms can only be obtained with low current densities. At these densities, the foot of the decomposition wave of the supporting electrolyte solution is magnified, and appears earlier, merging with the wave for the electroactive substance. This apparent shift of the foot of the wave is also observed in polarography when higher current sensitivities are used. Furthermore, the decomposition of the supporting electrolyte solution may become the favored electrode process over that of the oxidation or reduction of the electroactive substance. The problem with respect to mixtures of two or more components is similar, except that succeeding potential holdups are due to processes involving electroactive substances. A problem unique to single-drop
chronopotentiometry is size and arrangement of electrodes. A differential derivative technique is proposed as a means of resolving the problem of evaluating transition times and of size and airangement of electrodes in single-drop chronopotentiometry. The characteristics and performance of the two-electrode and the conventional three-electrode derivative techniques are also presented. The potential of the working electrodes during the electrolyses was followed by a Sargent Malmstadt automatic titrator coupled to a potentiometric recorder. The derivative chronopotentiograms were characterized b y a n initial second derivative signal and one for each transition in potential arising from the superimposition of an additional electrode reaction. EXPERIMENTAL
Reagents. Reagent grade chemicals were used. Stock solutions of potassium ferrocyanide, ferricyanide, and iodide, and cadmium, zinc, lead,
