PERMANGANATE STANDARDIZATION WITH A BECKMAN SPECTROPHOTOMETER I. E. RANSFORD, S. I. University of Ssnta Clara, Santa Clara, California
S T U D E N T S of quantitative analysis often lose valuable time in standardizing several portions of 0.1 N KMnOa solution. The following spectrophotometric method is suggested as a time-saver that might be employed by the instructor when a Beckman model B Spectrophotometer is available. Besides saving time, this method affords the student an opportunity, under the direction of his instructor, of gaining a nodding acquaintance, a t least, with a useful spectrophotometric method. Also, the instructor may employ it as an introduction to a discussion of this method as applied to other' quantitative determinations.
PROCEDURE
The transmittancy of a KMn04 solution of approximately 0.1 N is compared by means of a calibration curve with the transmittancy of a KMn04 solution of known strength. The solution may then be used, employing the factor obtained from the calibration curve, or may be adjusted to the exact normality by suitable dilution. Preparation and Standardization2 of Solution. Weigh on a watch glass approximately 3.2 g. of reagent-grade KMnOr crystals. Dissolve in 1.0 1. of distilled water, and heat in a covered beaker for 2 hours on a steam bath. Allow the covered beaker to stand overnight, filter through a sintered-glass filter, and store in a dark, glass-stoppered bottle. Formation of a brown crust on the inner surfaces of the bottle indicates a change in the composition of the solution. Dissolve 20 g. of reagent-grade NaOH pellets in 80 ml. of water and cool to room temperature. Weigh, to the closest 0.1 mg., three 0.2-g. portions of dried arsenious oxide of known purity into 250-ml. wide-mouth Erlenmeyer hsks. Add 10 ml. of the cooled NaOH solution, and allow to stand 8-10 minutes with occasional swirlina, or until the sample is dissolved. Dilute to 100 ml. and add 10 ml. concentrated HC1 and 1 drop 0.002 M potassium iodide or iodate solution. Titrate with KMnOl solution until a faint pink color persists for 30 seconds or more. Run a blank determination. From the net volume of permanganate, corrected for buret calibrations and the blank, and from the weight and purity of the standard compute the normality of the permanganate. The average deviation should not exceed one art ster thousand. 'LINGANE,J. J., AND J. W. COLLAT,Anal. C h m . , 22, 166 (1950).
PIERCE, W. C., and E. L. HAENISCH, "Quantitative Analysis," John Wiley & Sons, New York, 1948, pp. 214, 216. , 2
Calibration of standard^.^ In a series of three 500-ml. volumetric flasks place, respectively, 2.0, 2.5, and 3.0 ml. of the standardized 0.1 N KMn04 solution; dilute to volume with distilled water and mix thorough'y. The normalities of these standards are approximately N/2000 times the factors 0.8, 1.0, and 1.2. Prepare just before use and proceed immediately as follows. Transfer a sufficient port,ion of each calibration standard to one of a pair of matched (preferably quartz) cells. Set the distilled water in the other cell a t 100 per cent transmission on 530 mp and record the transmittancy of the calibration standards. The transmittancy of the solution with the factor 1.2 should be approximately 49 per cent. Plot the observed values on semilog graph paper using the factors 0.8, 1.0, and 1.2 as ordinates so that each division unit represents a factor change of 0.01. The points fall on a straight line. Routine Tests. Potassium permanganate stock solution, approximately 0.1 N, should be prepared in the same manner as in the calibratiou procedure. Dilute 2.5 ml. of this stock permanganate solution to 500 ml. with distilled water. Place a sufficient quantity of this diluted solution in one of the matched cells. Measure the transmittancy of this sample against distilled water set a t 100 per cent transmission on a wave length of 530 mp, and obtain the factor from the prepared calibration curve. Multiply the normality of the standard solution by this factor to obtain the normality of the stock solution. N of stock solution = factor X N of standard solution REMARKS
The temperature of the distilled water and solutions must be the same during all transmission measurements. The stock solution (if factor is greater than 1.0) may be diluted to the proper normality (0.1 N ) by application of the following formula factor
=
ml. of stock solution to he diluted to 1000 ml.
It is preferable, however, to use the permanganate solution without further dilution, and to correct results by means of the factor. Better results seem to be obtained if the KMnO, solutions are filtered with sintered-glass filters. Such filters may be cleansed of Mn02 residue with the usual dichromate solution. 3 FISTER,H. J., "Msnual of Standardized Procedures for Spectrophotometric Chemistry," Standard Supply Carp., New York, 1950, method P45.1.
