Volumetric Determination of Pyridine Bases Using Iron(ll1) Hydroxide Sulfate as Indicator Yoshio Koda Goaernment Industrial Research Institute, Nagoya, Hirate-machi, Kita-ku, Nagoya, Japan
PYRIDINE in aqueous solution is so weak a base that titration using conventional indicators is difficult ( I ) . A method proposed by Schulze (2, 3) has now been improved to permit titration of 0.1N pyridine with a relative error of 0.6%. In Schulze’s method, the sample is treated with iron(II1) chloride and is then titrated with 1N sulfuric acid until the precipitate redissolves. Errors are large when the pyridine concentration is low. If sulfate ion is added either before or along with the addition of iron(III), a fine lemon yellow precipitate of iron(II1) hydroxide sulfate ( 4 , 5 ) is obtained. This precipitate is easily dispersed like turbidity indicators (6) and redissolves completely in a slight excess of acid titrant. EXPERIMENTAL
Reagents. Pyridine was purified by distilling Japanese Industrial Standard guaranteed reagents (JIS G.R.) through a n all glass Claisen flask with a small portion of KOH pellets, collecting the central portions. All other chemicals were of JIS G.R. grade quality. Procedure. WITH SULFURIC ACID TITRANT.Take a portion of the pyridine solution (above 80 mg of base) to be determined and above 1.3 ml of 0.2N sulfuric acid titrant per 10 ml of the solution in a conical beaker; then add about 0.1 ml of freshly prepared 5 % FeC13.6Hp0 solution per 10 ml of solution with stirring. Fine particles of iron(II1) hydroxide sulfate are precipitated, and the solution becomes lemon yellow and turbid. The sulfuric acid titrant is carefully added with constant stirring until the precipitate redissolves; the total amount of sulfuric acid used is equivalent to the amount of the pyridine base. WITH HYDROCHLORIC ACID TITRANT.Take a portion of the pyridine solution (above 80 mg of base) t o be determined in a conical beaker; then add about 0.1 ml of freshly prepared 10% iron alum solution per 10 ml of solution with stirring. Fine particles of iron(II1) hydroxide sulfate are precipitated, and the solution becomes lemon yellow and turbid. Hydrochloric acid, 0.2N, titrant is carefully added with constant stirring until the precipitate has redissolved. RESULTS AND DISCUSSION Effect of Preliminary Addition of Sulfuric Acid. The accuracy of Schulze’s method ( 2 , 3 )was considerably improved by preliminary addition of a small amount of sulfuric acid titrant before treatment with FeC13. Table I shows the dependence of the accuracy on the volume of sulfuric acid (1) 1. M. Kolthoff and V. A. Stenger, “Volumetric Analysis,” Vol. 11, 2nd ed., Interscience Publishers, Inc., New York, N. Y., 1947, pp 53-63. (2) K. E. Schulze, Ber., 20, 3391 (1887). (3) F. P. Treadwell and W. T. Hall, “Analytical Chemistry,” Vol. 11, 9th ed., John Wiley & Sons Inc., New York, N. Y., 1942, p 495. (4) E. Posnjak and H. E. Merwin, J. Amer. Chem. SOC.,44, 1965 (1922). (5) C. Palache, H. Berman, and C. Frondel, “The System of Mineralogy,” J. D. Dana and E. S. Dana. Vol. II,7th ed., John Wiley & Sons, Inc., New York, 1951, pp 608-616. (6) C. Naegeli and A. Tyabji, Hela Chim. Acta, 15,403, 758 (1932).
Table I. Accuracy cs. Preliminary Added Volume of Sulfuric Acid Titrant Error, Preliminary added 20 ml 1Npyridine 20 ml 0.2N pyridine volume of H2S04, rnl with 1N H2SOa with 0.2N H2SO4 +0.9 +3.3 0.0 ... +0.6 0.05 0.0 +1.6 0.1 +1.4 ... 0.2 -0.1 +1.3 0.3 -0.1 ... 0.5 -0.1 +0.8 1.o -0.1 +o. 1 2.0 -0.2 ... 3.0 5.0 10.0 15.0
-0.3
-0.1
-0.1
-0.4
-0.05
-0.1
titrant added in advance. A constant value was obtained when more than 0.5 ml of 1N sulfuric acid titrant or more than 2.5 ml of 0.2N titrant was added in advance. These values are about equivalent to the amount of Fe3+ in the indicator used. The initial addition of sulfuric acid should be above the equivalent amount t o the amount of Fe3+in the indicator. Too small a preliminary volume gives a reddish brown precipitate which does not readily dissolve and may adhere to the walls of the beaker. Titration is then slow. Comparison of Accuracies of Determination Methods. Aqueous 0.1, 0.2, 0.5, and 1.ON solutions of pyridine were titrated with 0.1, 0.2, 0.5, and 1.ON sulfuric acid by three methods: Schulze’s method (A), the method using mixed indicators (0.1 % methylyellow f 0 . 1 methyleneblue in alcohol) ( I ) (B), and the present method (C). All three methods gave satisfactory results when 0.5 or 1.ON solution of pyridine was titrated with 1.ON sulfuric acid, the relative errors being +0.9% (A), +0.3% (B), and + O . l % (C). The relative errors were larger when more diluted solution of pyridine and/or more diluted sulfuric acid titrant was used, particularly in Schulze’s method. F o r example, in the titration of 0.2N pyridine with 0.2N sulfuric acid, the relative errors were +4.1 (A), +0.9% (B), and +0.4% (C). In the range 0.1 t o 1.ON pyridine, results obtained by Schulze’s method using 0.1N sulfuric acid were imprecise, and 4 to 10 high. By other methods using 0.1N titrant, the relative errors were +0.9 Z (B), and +0.6 Z (C). The pH’s of the end points of titration of 0.5N pyridine solution with 1.ON sulfuric acid by the three methods were 2.65 (A), 3.10 (B), and 3.05 (C). Effect of Amount of Indicator. Since the indicator solutions of iron(II1) compounds are acid to some extent, addition of larger amounts lead to a lower result. The p H values of diluted indicator solutions (0.1 m1/10 ml H 2 0 ) were FeC13.6H20 (3.17), mixed indicators (5.62), and 10% iron alum (3.23), respectively. From these data it is understood that the consumption of necessary excess acids will be
z
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avoided if an appropriate volume of iron(II1) solution is used as an indicator. A practically constant titer was obtained with less than 0.2 ml of 5 % iron(II1) chloride (or 10% iron alum) solution per 10 ml of solution. In the titration of 20-ml portions of 0.2N pyridine with 0.2N HCl in the presence of 0.2 ml of 5 % FeCl3.6Hz0 and from 0.1 to 1.0 gram of KzS04,all results were accurate to within 0.5%. Similar results were obtained when 0.2 ml of 10% iron alum replaced both FeC13 and KzS04 was added to 20-ml portions of 0.2Npyridine and titrated with 0.2NHC1. Interferences. In both the method using sulfuric acid titrant and the method using hydrochloric acid titrant, neutral salts which react with Fe3f to form precipitates like Pod3-
disturb the determination. Small amounts of NaCl, NaN03, MgS04, K2Cr207,Hz02,and Lipon-F (a commercial neutral detergent) do not interfere. The method has been successfully applied to a- and ypicoline and also to pyridine (Py) isolated from RuOl 2Py (7) and ZnClz.2Py. e
RECEIVED for review October 30, 1969. Accepted March 19, 1970. (7) Y . Koda, Znorg. Chern., 2, 1306 (1963).
Use of Ammonium Fluoride in Determination of Zirconium and Other Elements by Atomic Absorption Spectrometry in the Nitrous Oxide-Acetylene Flame A. M. Bond Department of Inorganic Chemistry, University of Melbourne, Parkville, Victoria, 3052, Australia DURINGTHEIR DEVELOPMENT of the high temperature nitrous oxide-acetylene flame as a means for improving the sensitivity of the atomic absorption analytical method for certain metals forming refractory oxides, Amos and Willis ( I ) observed that fluoride enhanced the absorbance of zirconium in this flame. This observation was later confirmed by Bond and O’Donnell (2) and by Sastri and coworkers ( 3 , 4 ) . Fluoride has also been used to improve the sensitivity of the spectrographic determination of zirconium in the carbon arc (5). Enhancement effects in atomic absorption spectrometry can be either physical or chemical in origin. However, for fluoride interaction with the zirconium system, chemical intrepretations have been favored (2-4). Bond and O’Donnell ( 2 ) observed that the enhancement of zirconium absorbance by the ammonium ion had very similar characteristics to that caused by the fluoride ion. More recently, Bond and Willis (6) reported that a considerable number of other nitrogen-containing compounds also caused an enhancement of zirconium absorbance in the nitrous oxideacetylene flame and that the effect appeared to involve the formation of a zirconium-nitrogen bond. The effect of the ammonium ion was, furthermore, found to be additive to the enhancement provided by fluoride (2,6). Even in the nitrous oxide-acetylene flame, the sensitivity and limit of detection of zirconium are not particularly good. It was therefore felt that use of a suitable concentration of ammonium fluoride would improve the atomic absorption (1) M. D. Amos and J. B, Willis, Spectrochim. Acta, 22, 1325, 2128 (1966). (2) A. M. Bond and T. A. O’Donnell, ANAL. CHEM.,40, 560 (1968). (3) V. S. Sastri, C. L. Chakrabarti, and D. E. Willis, Can. J. Chem., 47, 587 (1969). (4) V. S . Sastri, C. L. Chakrabarti, and D. E. Willis, Talanta, 16, 1093 (1969). ( 5 ) A. A. Frishberg, Russ. J. Appl. Spectrosc., 5 , 8 (1966). (6) A. M. Bond and J. B. Willis, ANAL.CHEM., 40,2087 (1968).
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method for zirconium determination by utilizing the enhancement effects of both the ammonium and fluoride ions. A fairly high concentration of ammonium fluoride should be tolerable, without any matrix interference due to formation of solid material in the flame, because ammonium fluoride should readily decompose to give the highly volatile products, NH3 and HF. Interelemental and chemical interferences in the nitrous oxide-acetylene flame are quite widespread (1, 7-9). In an unpredictable fashion, certain elements or compounds can cause either suppression or enhancement of the absorbance of the element being measured. As described later in this paper, many of the interferences in the determination of zirconium are completely eliminated or greatly suppressed by use of ammonium fluoride. Thus the possible use of ammonium fluoride to improve the atomic absorption method for determination of zirconium in the nitrous oxide-acetylene flame has been investigated in detail. Its use in improving the determination of some other elements forming refractory oxides has also been examined. Results, optimum concentrations, and further considerations of the mechanism of the ammonium fluoride enhancement are also reported. EXPERIMENTAL
Reagents. All chemicals used were of reagent grade purity. Zirconium solutions were prepared from zirconium oxychloride octahydrate (ZrOClz.8Hz0). All solutions contained 0.006M potassium chloride to suppress ionization, which can otherwise be appreciable for many elements in the nitrous oxide-acetylene flame (8). Sufficient hydrochloric acid was added to each solution to maintain all zirconium species in solution. (7) J. B. Willis, Appl. Opt., 7 , 1295 (1968). (8) D. C. Manning, A t . Absorption Newslett., 5 , 127 (1966). (9) W. Slavin, A. Venghiattis, and D. C . Manning, ibid., p 84.
