Determination of Potassium Iodate Sampled from Sodium Nitrate-Potassium Nitrate Eutectic Me1ts at 250 “C by Coulometric Back-Titration in Aqueous Medium R. B. Fulton and H. S. Swofford, Jr. Department of Chemistry, University of Minnesota, Minneapolis, Minn.
IN CONJUNCTION with studies of the oxidation of iodide at platinum electrodes in the KN03-NaN03 eutectic melt at 250 “C (I) it was necessary to develop a method for the quantitative determination of positive oxidation states of iodine. The method described in this paper was developed for the determination of iodate, but could be adapted to the determination of other stable positive oxidation states of iodine. In principle, the iodate (or another positive oxidation state) is reduced to iodine with iodide in acidic aqueous solution, a known excess of arsenic(II1) is added, and the excess arsenic(111) is determined by a back-titration with coulometrically generated iodine to an amperometric end point. This method has been applied to the determination of samples in the range 24 pequiv to 68 pequiv of potassium iodate with a relative error ranging from 0.4 to 1.4%.
55455
Table I. Determination of KIO, in the Melt by Coulometric Back-Titration
z
pequiv added 00.0 26.0 24.1 32.8 32.6 55.4 49.2 60.4 54.7 68.1 a
pequiv found4 -0.04 26.2 24.3 33.1 32.6 55.2 49.2 61.2 55.3 68.6
Relative error ... 0.77 0.83 0.92 0.00 -0.36 0.00 1.32 1.09 0.73
Each value listed in the average of at least two determinations.
EXPERIMENTAL Apparatus. The constant current for the coulometric back-titrations was supplied by a Sargent coulometric current source, Model IV. The generating anode was a platinum wire (area = 0.5 cm2) while the counter cathode was a platinum wire isolated in a sintered glass sealing tube. End-point detection was achieved with a rotating platinum indicator electrode (area = 0.1 cmz) in conjunction with an SCE reference (also isolated in a sintered glass sealing tube). A Sargent Model XV Polarograph served to impress zero volts between the rotating platinum indicator electrode and SCE reference and was also used to record the amperometric endpoint. Reagents. Reagent grade chemicals were dried and used without further purification. The standard arsenic(II1) solution was prepared from primary standard arsenous oxide and its normality was checked coulometrically. The aqueous generating solutions were prepared by dissolving 0.1 mole of potassium iodide and 0.1 mole of sodium bicarbonate in 1 liter of water followed by adjustment of the pH to 8.0 with acid, and a trace amount of arsenic(II1) was added to reduce any iodine formed by the air oxidation of iodide. The excess arsenic(II1) in the generating solution was titrated just prior to the titration of the unknown samples. Procedure. Solutions of potassium iodide were prepared by adding weighed quantities of dry potassium iodate directly 100 ml). A small quantity to the eutectic melt (volume (2 to 3 ml) of this solution was then drawn into a coarse sintered-glass sealing tube and forced out into a previously tared beaker. After cooling, the beaker and sample was reweighed and the exact volume of melt withdrawn was calculated from the known density (1.96 gram/ml) of the melt at 250 “C (2). The melt sample was dissolved in oxygen free water and transfered to a volumetric flask. Two milliliters of 0.1N potassium iodide was added and the solution was acidified (1) H. S. Swofford, Jr., and J. H. Propp, ANAL.CHEM., 37,974 (1965). (2) H. S. Swofford, Jr., Ph.D. Thesis, University of Illinois, 1962.
with two drops of 1 N sulfuric acid. Exactly 10 ml of standard arsenic(II1) was then added and the solution was diluted to the mark. Aliquots of this unknown solution were taken for the analyses. In practice, 50 ml of the generating solution containing a trace of arsenic(II1) was added to the cell and titrated to an amperometric endpoint. Then an aliquot of the unknown solution was added and the solution was again titrated to an amperometric endpoint. The number of microequivalents of iodate present in the original melt sample was calculated from the data. RESULTS AND DISCUSSION Because arsenic(II1) cannot be readily oxidized by iodate (3),it was necessary to reduce the iodate with iodide in acidic aqueous solution according to the reaction
The iodine formed is then reduced with an excess of arsenic(111) as shown in Equation 2
+ As(Il1) = 31- + As(V)
(2) Finally, the excess arsenic(II1) is back-titrated with coulometrically generated iodine. Before preparing a solution of potassium iodate in the melt, the residual nitrite was removed by controlled potential electrolysis at +0.85 V us. the usual Ag/Ag(I) (0.07M) reference electrode (4). This was deemed necessary because in aqueous acidic medium nitrite oxidizes iodide according to the reaction shown below (5) 13-
~~
(3) H. A. Laitinen, “Chemical Analysis,” McGraw-Hill, New York, 1960, p 402. (4) H. S. Swofford, Jr., and P. G. McCormick, ANAL.CHEM., 37, 970 (1965). (5) I. M. Kolthoff and E. B. Sandell, “Textbook of Quantitative Inorganic Analysis,” 3rd ed., Macmillan, New York, 1952, p 599. VOL. 40, NO. 8, JULY 1968
1375
2N02-
+ 21- + 4Hf
= 2N0
+I2
+ 2H2O
(3)
and would, therefore, introduce positive errors in the analyses. Table I presents the data obtained for the determination of potassium iodate in the eutectic melt by this method. With reference to column 3, the relative error in the analyses is about 1%. However, because duplicate aliquots generally agreed to-within a few tenths of one per cent, the error observed most probably reflects the difficulty in measuring the total volume of the melt (-100 ml) and is not a result of errors inherent in the method.
Two important conclusions can be drawn from this study: that potassium iodate is stable in the melt at this temperature for periods of at least hours, and that a method has been developed which can be used to determine iodate (and other stable positive oxidation states of iodine) sampled from tMs fused salt medium. RECEIVED for review November 8, 1967. Accepted May 9, 1968. The authors thank the Procter and Gamble Co. for their support of this work.
Atomic Absorption Analysis of Ferrites Solomon L. Levine Systems Development Division, International Business Machines Corp., Poughkeepsie, N . Y. 12603 MANYVARYING TECHNIQLJES have been used for the quantitative analysis of ferrites, but most of the existing techniques are difficult or quite time consuming. MnMgZn and NiCo ferrites can be analyzed in 3-8 hours by a combination of extraction, complexation, and chromatography (1). NiZn ferrites have been analyzed by X-ray fluorescence spectroscopy after, among other things, corrections for matrix effects have been made (2). MnZn ferrites can be analyzed by wet chemistry in one hour after some tedious sample preparation (3). Many of the wet chemical techniques used for various ferrites (including ion exchange separations and titrations) have been summarized (4). A tedious 8- to 12-hour wet analysis of this type has been performed in IBM’s SDD Poughkeepsie Materials Technology Analytical Laboratory on MnZn ferrites, by photometric end point detection for the zinc (5). This laboratory is particularly concerned with MnZn, MnCu, LiNi, and Cu ferrite compounds. The existing methods for Li are good but require great care and time; procedures for analyzing these ferrites are lengthy and therefore impractical for use on a quality control basis. Atomic absorption spectrophotometry (AA) has rapidly become an important analytical tool because of its sensitivity, speed, good precision (in some cases as good as 0 . 2 x at ppm levels), and accuracy when proper precautions in standard preparation and dilution are taken. The AA analysis of ferrite compounds described below is rapid and accurate enough to be used on a control basis. EXPERIMENTAL Reagents and Solutions. AA standards, except for Li and Na, were prepared by dissolving the appropriate spectrographically analyzed or electrolytically pure metal in concentrated HCl or 1 :1 HC1-HN08, evaporating to 3-4 ml and diluting to 1 liter with distilled water. LiN03 (Mallinckrodt,
( 1 ) V. Barcanescu, E. Potamin, and S. Calugareneau, Rev. Chim.’ 15, 561 (1961); Chem. Abstr., 64, 1351a (1965). (2) K. Date and S . Mori, Nut/. Tech. Rept. (Japan), 10, 449 (1964); 11,441 (1965); Chem. Abstr., 63, 14033h (1965). (3) H. N. Nikol’shaya and T. N. Siminova, Zaoodsk. Lab., 31, 545 (1965); Chem. Abstr., 63, 2369f(1965). (4) M. Fritsch, “A Survey of Various Chemical Methods Used in the Analysis of Ferrite Compounds,” presented at the American Ceramic Society Meeting, Washington, D. C., May 1966. ( 5 ) H. J. Golden and S.L. Levine, Analytical Letters, 1,39 (1967).
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ANALYTICAL CHEMISTRY
AR) and NaCl (Mallinckrodt, AR) were recrystallized and dissolved in water. The stock 1 gram/liter (1000 ppm) solutions were diluted as needed in the preparation of standards and known solutions. Apparatus. The Perkin-Elmer Model 303 atomic absorption spectrophotometer was used for this work. Except for aluminum, where an NzO-acetylene flame and nitrous oxide burner head was used, an air-acetylene flame was used with the Boling 3-slot premix burnerjatomizer assembly. Resonant wavelengths, slit widths, lamp currents, and fuel flows were those recommended for the element in question except for Fe, where a lamp current of 35 mA was used because of “noise” at lower currents. A multi-element CrCu-Mn-Co-Ni lamp was used for these elements; Ca-Mg and Zn-Ca lamps were used for Ca, Mg, and Zn. In all other cases, single-element hollow cathode lamps were used. pH measurements were taken on the Beckman Research pH meter using a glass electrode us. a saturated calomel reference electrode.
(a,
RESULTS AND DISCUSSION
Calibration plots for the major constituents of the ferrite compounds in question were determined each time data were taken, except for a few cases where two standards were used and unknowns were determined by direct proportion. The replicate determinations of these plots indicate that the practical upper limits of the six major elements under the conditions were used as follows: Fe, 15-18 ppm; Li, 12-15 ppm; Ni, 25 ppm; Mn, 15-18 ppm; Cu, 20 ppm; and Zn, 4-5 ppm. Deviations from Beer’s law were noted at higher concentrations. Solutions containing the major constituents of the ferrites (LiNi, MnZn, and MnCu) in question were prepared from the AA standards and analyzed using the calibration plots. The results, shown in Table I, compare favorably with the known amounts. The largest errors, in ppm, are in the most concentrated solutions-e.g., 121 ppm of Fe in solution IIIthese solutions were rediluted in order to work on the straightline portions of the calibration plots. The bulk of the error in these cases may be dilution errors. In any event, the results indicate an average accuracy of about f3 (relative) in this ppm range; this is quite good for AA. The precision of the
x
(6) Perkin-Elmer Corp., “Analytical Methods for Atomic Absorption Spectrophotometry,” Nonvalk, Conn., 1966.
