the authors gratefully acknowledge his assistance. LITERATURE CITED
(1) De Mars, R. D., ANAL.CHEM.34, 259 (1962). (2) Hamm, R. E., Furse, C. T., Ibid., n. 219. (3) Jones] H. C., “Automatic Coulometric Titrator, ORNL Model Q-2005, Electronic, controlled-Potential,” Method Nos. 1003029 and 9 003029 (R. 6-7-62)] U. S. At. Energy Comm., Rept. TID7015, Supplement 4, in press. (4) Kelley, M. T., Jones, H. C., Fisher, 1-
D. J., ANAL.CHEM.31, 488, 956 (1959). (5) Kelley, M. T., Miller, H. H., Ibid., 24, 1895 (1952). (6) Kolthoff, I. ,,M., Lingane, J . J., “Polarography, 2nd ed., Vol. 11, pp. 519-20, Interscience, New Yo;:, 1952. ( 7 ) Ludwick, M. T., “Indium, pp. q43, 546-79, The Indium Corp. of ilmerica, Xew York, 1959;( (8) .Meit?$, L.,. Polarographic Techniques, I). 260, Interscience. New York. 1955. ‘ (9) Onishi, H., in “Treatise on Analytical Chemistry,” Part 11,Vol. 2, I. M. Kolthoff and P. J. Elving, eds., Interscience, New York, 1962. ~
(10) Phillips, S. L., Morgan, E., ANAL. CHEW33, 1192 (1961). (11) Shults, W. D., “Uranium, Automatic Controlled - Potential Coulometric Method,” Method Nos. 1 219225 and 9 00719225 (1-29-60); U. S. At. Energy Comm., R e p t . TID-7015, Supplement 3, June 1961. (12) Walton, H. F., “Chemical A4nalysis,” p. 409, Prentice-Hall, Sew Pork, 1952. RECEIT-ED for review October 12, 1962. Accepted April 3, 1963. Oak Ridge National Laboratory is operated by Union Carbide Corp. for the U. S. Atomic Energy Commission.
Flame Photometric Determination of Potassium in Unashed Plant Leaves J. L. MASON Research Station, Canada Department of Agriculture, Summerland,
b Direct flame photometry of a suspension of gtound plant material can be used to determine potassium content. The method eliminates the usual steps of ashing and dissolving the ash. Determinations on four samples from a collaborative study gave a mean of 1.52% potassium, compared to 1.50% from 17 laboratories using chemical, flame, or spectrographic methods. Determinations on another set of nine collaborative study samples gave a mean of 1.74% potassium as compared to 1.75% found in the study. Precision was very high, the standard deviation of an analysis in the two sets of samples being *0.0086 and =!=0.0108.
G
ILBERT ( I ) has shown
recently t h a t a powdered material suspended in a moderately viscous medium can be fed directly into a flame photometer and can produce potentially useful spectra. Gilbert used soil ground i n %propanol and made viscous by adding glycerol as a test material. He suggested that the method should have wide application to analytical problems. Thi- paper report< the successful apI)licati\)n of the method to potassium determination in plant material. EXPERIMENTAL
Apparatus. Tlic Akdv;incrti flame photometer I\ a s used (Advanced 111strunients Inc.. Xewton Highlands, IIass.). It i, a double-beam, internal standard iiibtrunieiit designed for ~)otussiuinand sodium deteriiiin:ttion. T h e flame was operated on natural gas and air. Reagent. Glycerol-2-propanol was
874
ANALYTICAL CHEMISTRY
E.
C., Canada
made b y adding 200 ml. of &propanol t o 125 grams of glycerol. Sample Preparation. T h e air-dry plant material was ground in a n intermediate model Wiley mill fitted with a 60-mesh screen a n d thoroughly mixed b y tumbling end-over-end in a jar less t h a n full for 15 minutes. A 1-gram sample was weighed into a n aluminum weighing can and dried in a gravity oven for 48 hours for moisture determination. h 25-mg. sample of air-dry plant material was weighed into a 50-ml. Erlenmeyer flask for flame photometry. Ten milliliters of glycerol-2-propanol reagent was pipetted onto the plant material and the mixture was swirled for a few seconds to obtain mTetting. Fifteen milliliters of lithium internal standard solution (5000 p.p.m.) was pipetted into a 250-ml. volumetric flask. The wetted plant material was transferred into the volumetric flask with deionized water and the volume wa3 made up to 250 ml. The resulting suspension was 1 part of plant material in 10,000. Analysis. T h e flame photometer was calibrated with standard solutions
of potassium ranging from 0.5 t o 4.0 p.p.m. Each standard solution contained 300 p.p.m. of lithium internal standard and 47, by volume of glycerol-2-propanol reagent, the same as t h e plant material suspensions. T h e volumetric flasks containing t h e plant material were swirled a n d allowed t o stand 5 minutes before the slurry was poured into the flame photometer. The galvanometer was balanced, the potentiometer reading recorded, and the potassium content read off the standard curve. The results were corrected for inoiqture content. RESULTS
T ~ v o scts of standard samples of known potassium content were analyzed. Set 1 consisted of four samples (apple, cherry, citrus, and peach leaves) from a collaborative study of analytical methods reported by KenFT-orthy and Miller ( 2 ) . These samples had been analyzed by 16 laboratories using chemical, flame photometric, or emission spectrographic methods. Table I shows
Table 1. Per Cent Potassium in Four Samples from a Collaborative Study as Determined by Conventional Methods and by Flame Photometry of a Suspension Set 1
From collaborative study By flame photometry SpectroMean ________.__ of suspension graphic Source of Chemical Flame (17)” Duplicates Mean leaf material (7) (2) (8) 1.10 1.17 1.18 1.17 1.1s 1,03 Apple 1.10 1.T2 1.71 1.72 1.38 1.67 1.65 Cherry 1.71 1.00 1.01 1.02 1.10 1.16 1.12 1.04 Citrus 2.17 2.17 2.19 2.17 2.07 2.25 2.19 Peach Mean 1.54 1.51 1.49 1.52 1.52 1.52 1.52 One of the 16 laboratories reported two chemical methods. 0
excellent agreement between the trvo sets of data, the mean for the collaborating laboratories being 1.5070 potassium and the mean error for the suspension method +0.02y0potassium. The results of the collaborating laboratories are further divided into groups according to the type of analytical method; seven used chemical methods, eight used flame methoc.8, and two used spectrographic method,j. Statistical tests of the differences between means of the three groups and the new method were made by the anal: The F value obtained w%s0.32, showing t h a t the means n-ere not different according t o t h e test ( F required at P = 0.05 is 3.86, standard error of the mean 0.03s). Set 2 was a group of nine samples from a collaborative study of analytical methods reported by Ward and Heeney (3). This set included strawberry, raspberry, peach, apple, tomato, and corn leayes, grape petioles, brome grass plants, and oat grain. Results for individual samples )?-ere not given by Ward and Heeney. However, the mean value of all samples frclm the nine collaborators was reported as 1.7500 iiotassium. T h e mean error of the results found in this work by direct flamle photometry of a suspension (Tahe 11) was only -0.01% pota,,'w u m . Statistical mea.;ures of the precision of t h e new method were made. The
Table II.
Per Cent Potassium in Nine Samples from a Collaborative Study as Determined b y Flame Photometry o f a Suspension"
Set 2 Sample 2-
3 -1 0.60 2.93 2.11 2.93 2.15 0.61 Duplicate determinations.
1 2.08 2.09
~~~~
5 1.76 1.77
standard deviation of a n analysis was *0.0086 for Set 1 a n d *0.0108 for Set 2. The related relative standard deviations were O.57y0 and 0.69%. DISCUSSION
Direct flame photometry of a suspension is a useful procedure for determining the potassium content of plant material. This is believed to be t h e first application of Gilbert's suggestion of its use for quantitative analysis of powdered materials with little or no preparation. The method is very rapid, since it eliminates t h e usual ashing and dissolving of the ash required when determinations are made on solutions. It produces potassium values similar to those found b y conventional procedures. The precision of t h e analyses is very high. The mrthod should be adalitable t o
6 1.52 1.52
7
8
2.3;1 2.32
0.31) 0.39
9 1.95 1.9-1
a n y flame photometer, provided thc atomizer and burner will pa
