D. E. Sellers, S. S. N. Tang, and R. E. Van Afta
Southern Illinois University Carbondale
I I
Analysis of Tartar Emetic A photometric titration
In connection with the development of new experiments for an introductory course in instrumental analytical methods, the authors sought a photometric titration experiment which would fulfill two principal objectives. An experiment was desired which would be suitable for use with any of several common types of photoelectric colorimeters, while utilizing a chemical reaction in which some difficulties are encountered in the application of conventional titrimetric techniques. Potassium permanganate was selected as the titrant for use in the proposed experiment because of its relative ease of preparation and standardization, as well as the known optical properties of both the reagent and its reduction product in acid solutions. The latter properties permitted the use of a standard filter and guaranteed a rapid increase in measured absorbance of the sample solution after the end-point has been reached. The antimonyl-antimonic system was selected as the system to be measured, since the difficulties in visual detection of the permanganate endpoint in the system are well known (1). Although various titrimetric methods for the determination of antimony have been described (t, 3, 4, etc.), most of these methods have some disadvantage; they may be either tedious or inaccurate, or both. The permanganate titration of antimony also possesses the added problem that the reaction vessel should be chilled to 5-10°C in order to minimize the error in the end-point introduced by the presence of hydrochloric acid ( I ) . The determination of antimony in tartar emetic by photometric titration with potassium permanganate proved to meet the original objectives, while producing student results consistent with good analytical practice.
540 mp filter, and manual stirring. A 10-ml Schellbach semimicro buret, with the tip extending just below the sample solution surface, was used for titrant delivery. Reagents and Chemicals. The unknown samples analyzed were Soluble Antimony (Tartar Emetic) obtained from Thorne Smith, 1847 North Main Street, Royal Oak, Michigan. The potassium permanganate solutions were prepared by conventional methods from Merck reagent and standardized against Kational Bureau of Standards primary standard sodium oxalate. All other reagents were C.P. grade. Procedure. Accurately weigh and transfer a 0.1-0.2 g sample of the previously dried tartar emetic unknown into the colorimetric cell. Add 20 ml of 1: 1HC1, stirring to effect dissolution, if necessary, and dilute to a total volume of 50 ml. Insert the stirrer into the cell and position the cell in the instrument cell holder in the light path. Using an appropriate filter (approximately 525 mfi), measure the absorbance of the sample solution, as directed by the instrument operating instructions. Add 0.5 ml of t,he titrant and stir the solution for about 15 see. Measure and record the absorbance and titrant volume delivered. Repeat this operation with 0.5-ml increments of titrant until the first significant change in absorbance is observed. Then continue the titration in a similar manner, using 0.1-ml increments of titrant until four or five additional readings have been obtained. Plot the resultant data, with absorbance as ordinate and volume of titrant as abscissa, and determine the endpoint of the titration from the intersection of the two straight lines obtained (Fig. 1). Calculate the per cent antimony in the sample in the conventional manner.
The Experiment
Results
Apparatus. Photometric measurements were made with a Fisher Electrophotometer equipped with colorimetric titration assembly, 60-ml rectangular cell, and 525 mp filter. Alternatively, a Klett-Summerson photoelectric colorimeter was used, with a similar cell, The authors wish to express their appreciation to the Graduate Council of Southern Illinois University far financial assistance during the course of this investigation.
408 / Journal of Chemiccrl Education
I t has been reported (5) that the permanganate titration of antimony1 ion yields quantitative results "he" the sample solution contains no more than 20-30 Per cent by volume of a 1: 1 mixture of HC1 and HzSOt or an equal amount of HC1. Consequently, a series of 23 experiments was conducted in which tartar emetic samples containing from 8 to 30 mg of antimony were each dissolved in approximately 40 ml of distilled
Toble I. Analysis of Tartar Emetic Unknowns b y Photometric Titrotion with 0.1 00 N KMnOl Solution
water, followed by the additiou of 10 ml of a 1 : 1 mixture of HC1 and H1S04. The sample solutions were then titrated with 0.100 N KMnOI following the photometric titration procedure previously described. Although the average deviation of this series of measurements was 1 3 . 3 relative per cent, the reproducibility of individual results was poor.
Sam~le size Sb (mg)
Analyzeda
(% Sb)
Foundb (% Sb)
9.43 18.42 13.61 19.36 22.44 19.75 16.82 9.98 24.79 49.58 25.55
11.26 15.53 16.97 20.25 24.11 25.63 27.38 29.39 29.39 29.39 31.00
11.15 15.48 16.92 20.28 24.47 25.78 27.54 29.33 29.47 29.76 31 44
Average
Relative error
(%I
11.0 +08 +l.2 f 2.8 11.5 +0.6 +0.7 +O.S
1.3 f 1.3 11.9 f1.3 f
Analysis 6gures by Thorne Smith, 1847 North Main Street, Royal Oak, Michigan. * Each value represents three or more individual analyses. a
0
1.O
2.0
3.0
4.0
Volume 0.100 N KMnO., ml. Typical student photometric titration curve for SbOf
Figure 1. versus MnOc.
5.0
- H+
A second series of 43 experiments was then performed in which similar samples ranging from 9 to 50 mg in antimony content were dissolved in 20 ml of 1: 1 HCI, diluted to about 50 ml and again titrated in the same manner. The average deviation of this series of measuremeuts (Table 1) was 1 1 . 3 average relative per cent, a figure consistent with that normally expected from instrumental measurements, while the reproducibility of individual results was satisfactory. The experiment described has been performed by approximately 50 students over the past two years, using both the Fisher Electrophotometer and the
Klett-summer so^^ calorimeter, yielding results consistently in agreement with those shown in Table 1. The permanganate solutions used have been standardized against h ~ o w utartar emetic samples, as well as primary standard sodium oxalate, by both conventional and photometric methods; the results obt,ained are independent of the method of standardization. The titration usually requires about 30 min for completion after the sample has been dried. Duplicate analyses are rarely necessary, owing to the accuracy and precision of the end-point detection. An advantage to the instructor is the fact that a wide range of suitable solid samples is commercially available at low cost, precluding the necessity for unknown preparation. Literature Cited (1) HILLEBRAND, W. F., ET. AL., "Applied Inorganic Analysis," 2nd ed., John Wiley & Sons, Inc., New York, 1953, p. 280. (2) SHAW,L. I., WHITTEMORE, C. F., AND WESTBY,T. H., I d . Eng. Chem., Anal. Ed., 2, 402 (1930). (3) PIERCE,W. C., HAENISCH,E. L., AND SAWYER,D. T., "Quantitative Analvsis," 4th ed.. John Wilev & Sons. I&., New York, 1958, p. 302. (4) WILLARD,H. H., AND YOUNG,P., J. Am. Chem. Soe., 50, 1322 (1928). (5) PIETER~, H. A,, Anal. Chim. Acta, 2, 270 (1948).
Volume 39, Number 8, August 1962
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