Performance of Interference Filters in Simple Flame Photometer W. G. SCHRENK and B. L. GLENDENING' Kansas Agricultural €xperiment Station, Manhattan, Kan.
The performance of interference filters in a simple flame photometer has been studied with respect to sodium and potassium determination in plant tissue. Magnesium depresses the intensity of emission of both sodium and potassium when interference filters are used for spectral isolation. Sodium and calciuni have only a slight effect on potassium. Calcium enhances the sodium emission, whereas the effect of potassium on sodium is negligible. Sodium and potassium may be accurately determined in plant tissue, if standards containing calciiim and magnesium in quantities approximating that present in plant tissue are prepared. The standard deviation from the mean i n a series of 16 alfalfa samples was 1.90% for sodiuni arid 2 . 2 4 7 ~ for potassiiim.
T
HE availabilit\- and properties of interference-type blters
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, K I25
Figure 2.
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Typical calibration curves for sodium and potassium
-1cylindrically concave mirror of stainless steel was mounted behind the light source and a convex lens was used to focus t l i ~ energy on the photocell. A variable slit was placed b e h e e n the convex lens and the photocell. The slit was 1 em. wide (masimum) and 1 em. high. It was used to adjust the quantit\- of radiant energy permitted to pass through the appropriate filter and strike the photocell. il schematic diagram of the apparatu.: is shown in Figure 1. EXPER131 ENTAL
Calibration Curves. The response of the instrument to varving concentrations of sodium and potassium was determined, i n the range encount'ered in work mith plant and animal tissue. Sodium standards were prepared from reagent grade sodiuni c!iloride. For potassium, reagent grade potassium chloride wa' used. A11 chlorides used for standards Ivere stored in a desiccator and weighed in weighing bottles fitted with ground-glass coverp. Typical calibration curves with no interfering substances present are shown in Figure 2. Calibration points could be duplicated to = k l . O % galvanometer deflection; therefore, determination of the influence of extraneous elements appeared desirable. Effect of Interfering Ions. The principal mineral constiturrit~ of plant tissue n-hich were studied for mutual effects include calcium, magnesium, potassium, and sodium, although many other mineral constituents are present in smaller quantitie.G. Figure 3 presents graphically the effects of added sodium, calcium, and magnesium on potassium when present in the test solution as chlorides. These solutions all contained potassium a t a concentration of 0.40 mg. per ml. Separate solutions containing potassium plus calcium, pot>assium plus sodium, and potassium plus magnesium were used to obtain these data. Sodium and calcium exhibit little effect on the intensity of emission of potassium. Magnesium, however, has a depressing effect on the intensity of emission from potassium. These data indicate the necessity for including magnesium in the standarch to compensate for this effect or removal of magnesium during sample preparation. Figure 4 illustrates the effects of calcium. potassium, and magnesium on sodium rea.dings. Separate solutions were prepared containing sodium plus calcium, sodium plus potassium, and
Diagram of optical si-steni of flame photometer
Cylindrical stainless steel mirror Condensing lens Variable slit Interference filter
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B. C. D.
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immediately buggest the possibility of using them in flame photometry. These filters are characterized by narrox- band I\ Idth. steep slopes on the band pass curve, high transmittances .it the wave-length maximum for n hich they are constructed, and availability of filters with peak transmittances a t the wave lengths riqeful for sodium and potassium analyses. Each of the filters transmits a band of frequencies considerably n'wrower than usual color filters; however, the nature of the c instruction indicates t h a t frequencies harmonically related to the fundamental frequency of the filter also are transmitted. Thus, a second-order filter for sodium (589 mp) also transmits energy of a wave length corresponding to its third and fourth order, or at approximately 393 and 295 mp, respectively. Similar effects are noted for filters Jvith peak transmittance. a t other wave length>.
Figure 1.
minationa. These filters, 2 inches square in size, were sec-ondorder filters, available from Bausch and Lomb Optical Co. They were mounted in blocks which facilitated their use with the other equipment. .I General Electric barrier layer photocell, Type 88 X 565. \\'as used to convert the radiant' energy into electrical energy. The energy from the barrier layer photocell was used to activate a sensitive galvanomet,er which had a sensitivity of 0.007 p a . per mm., a resistance of 362 ohms, and a period of 3 second* (Rubicon Co., Type 3402").
E . Barrier layer photocell 6. Galvanometer S. Burner of flame attachment, .Model 10,300
The results of a n investigation of the use of these filters in a *imple flame photometer for the determination of sodium and potassium in samples of plant tissue are reported in this paper. The availabilitv of the older type Beckman flame excitation unit in many laboratories, where i t has been replaced by the newer Model 9200 flame attachment, may make the use of the apparatus described in this paper of interest. EQUIP-WENT A Y D 4PP4RATUS
Flame Photometer. The excitation source used was the Beckman Model 10,300. This unit uses the glass aspirator type of atomizer and a heated spray chamber from which the sample is fed into the flame of the burnrr. Gas pressure [vas maintained a t 5 em. of water pressure, oxygen pressure a t 40 inches of water, and air pressure a t 25 pounds per square inch Under these conditions sample feed rate was 0.203 gram per minute Two interference filters, with peak transmittance at 589 and 768 mGj respectively, were used for sodium and potassium d e t e r 1 Present address, Kansas State Board of Health, D i ~ i s i o nof Laboratories, Topeka, Kan.
1031
ANALYTICAL CHEMISTRY
1032 sodium plus magnesium. Potassium has no measurable effect in the range studied. Calcium enhances the reading for sodium, whereas magnesium causes a depressing effect. Thus, it seems important to include calcium and magnesium in the sodium standards, a t the approximate level that they occur in the unknown samples, or use some other technique to compensate for the effects produced by these elements. Comparison of Methods. A4sa result of the preliminary data obtained standard solutions for sodium and potassium were prepared to contain calcium and magnesium a t levels approximating those found in plant tissue. Using these standards the sodium and potassium contents of a number of samples Tvere determined by different procedures. The data in Table I were obtained by comparing the present method nith the flame excitation technique previously reported b r Schrenh and Smith (9).
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Figure 3 . Effects of added sodium, calcium, and magnesium on potassium readings Potassium concentration is 4.0 mg. of potassium per 10 ml.
Figure 4. Effects of added calcium, potassium, and magnesium on sodium readings Sodium concentration is 0.20 mg. of sodium per 10 ml.
Plant tissues were prepared for analysis by drying and grinding to pass through a 40-mesh screen. One gram of the sample was weighed and ashed a t 600' C. The plant tissue averaged 9.5% ash on a dry weight basis (determined to =t0,02% by the usual weighing procedure). T h e ash was dissolved in a minimum amount of dilute hydrochloric acid, evaporated t o dryness, and diluted to a known volume with distilled water. Samples were filtered to remove insoluble residue. Aliquots of each solution thus prepared were analyzed in duplicate using the filter arrangement presented in this paper and also the method reported by Schrenk and Smith ( 9 ) . The average value for 29 different samples of plant tissue analyzed for sodium by the filter method was 0.53%. The same samples analyzed for sodium by the method of Schrenk and Smith was 0.56%. n-hile the standard deviation between the two methods was 0.08. Standard deviations were calculated by the formrila
S.D. =
dF
Thirty-two sampler of plant tissue were analyzed for potassium by both techniques. The filter instrument produced an average while the spectrographic procedure (9) gave an value of 1.04%'0, average value of 1.16YG, The standard deviation between the two procedures was 0.16.
The data in Table I1 compare the filter procedure described in this paper, the official ;10.1C ( 1 ) method for potassium involving precipitation as potassium platinochloride and a flame method using the Beckman Model DU spectrophotometer equipped M ith the neLver flame photometer attachment, Model 9200. The seven samples containing relatively low levels of potassium show close agreement among the three methods. The average potassium content of these samples was 0.34% as determined by the AOAC ( 1 ) method, while the two flame methods both averaged 0.33%. Agreement between methods for samples containing higher levels of potassium-Le., alfalfa and soybeans-was not so good. Data obtained by precipitation averaged higher than those obtained by the t n o flame methods. The average on these samples was 1.52Oj, when determined by the AkOAiC procedure, while the Beckman flame average Isas 1.32% and the apparatu? described in this paper gave values averaging 1.23%. These differences, hon ever, are similar to those obtained b r other investigators ( 7 ) . Agreement between the two flame methods is good a t all potassium levels. The same standards were used in calibrating the two different flame instruments. Precision of Method. T o determine the reproducibility of the filter instrument, a single sample of alfalfa was prepared for analysis. Aliquots were run each time a set of samples was analyzed. Sixteen determinations were made, each on a separate day. The results of this method of replication are given in Table 111. DISCUSSION
The use of interference filters a i t h simple equipment to indicate relative intensity of emission from the flame appears to offer a promising method for the determination of sodium and potassium in samples of plant tissue. Some interferences occur in the flame; therefore, standards should be prepared containing the interfering ions a t appropriate concentrations or the interfering ions must be removed. The usual precautions concerning flame photometry must be taken. Solutions should be free from suspended material and
Table I. Comparison of Two Flame RIethods for Determining Sodium and Potassium
Table 11.
Concn., %-Littrow Filter 0 56 0 53 1 16 1 04
No. of Samples 29 32
Element Sodium Potassium
Std. Der. 0 08 0 16
Comparison of Three Rlethods for Determining Potassium" % Potassium
Sample Alfalfa 1 Alfalfa 2 Alfalfa 3 Corn 1 , grain Corn 2, grain Corn 3, grain Wheat 1. grain Wheat 2, grain Wheat 3, grain Wheat 4, grain Soybeans 1 Soybeans 2 Soybeans 3
AOAC 1.60 1.14 1.31 0.30 0.29 0.26 0.35 0.38 0 41 0.39 1.64
1 69 1.76
Beckman flame 1.32 0.97 1.19 0.32 0.31 0.30 0.32 0.39 0 37 0.34 1.35 1.40 1.48
Flame 1.25 0.93 1.15 0.33 0.31 0.30 0.30 0.38 0.35 0.34 1.28 1.35 1.38
These standards included calcium and magnesium.
Table 111. Precision of Flame Determination of Sodium and Potassium in Alfalfa Element Na K
s o . of
Range,
Mean,
Detn. 16 16
0 078-0 091 1 16 -1 35
0 085 1 25
%
%
Std. Dev., % of Mean 1 90 2 24
V O L U M E 2 7 , NO. 6, J U N E 1 9 5 5 the atomizer fuel rate maintained constant. Since the galvanameter reads emissions continuously, any change in fuel rate while the equipment is in use is easily observed. I n order to minimize the effects of organic materials (b-4, 6, 8) and of anions (b, 3, 5, 6, 8),the procedure has been standardized to include ashing of samples followed by treatment with hydrochloric acid. This treatment tends to reduce all samples to a common matrix and makes the preparation of standards a simpler problem. Because calcium has considerable effect on the emission of sodium, different standards are used for determining sodium in grasses 8 s compared to legumes. Legumes may contain more than ten times as much odclcium as grasses. The sensitivity of the arrangement described is not so high for potassium as for sodium. I n plant materials this is not a serious factor. Inspection of the response curve of the barrier layer photocell indicates that the potassium lines near 768 mp used for analysis are in a region where photocell response is falling rapidly. Response, however, remains sufficiently high for most work, since lcvels of under 1 p,p,m. of sodium and 10 p.p m. of
1033 potassium are readily detectable. Use of a variable slit in front of the barrier layer photocell makes adjustment simple far different concentration ranges. LITERATURE CITED
( I ) Assoe. Offio. Agr. Chemists, "Official and Tentative Methods of Analysis," 7th ed., p. 99, 1950. (2) Baker, G. L.. and Johnson. L. H., ANAL.CHEM.,26,465 (1954). (3) Berry. J. W., Chsppell, D. G., and Barnes, R. B., IND.END. CHEM., ANAL. ED., 18, 19 (1946). (4) Caton, R. D., Jr., and Bremner, R. W., ANAL.CHEM.,26, 805 (1954). (5) Fox, C. L.,Jr.,lbid., 23,137 (1951). (6) Inman, W. R., Rogers, R. A,, and Fournier, J. A,, Jbid., 23, dR? Sl) _ I _ \I _l"Q-^,.
(7) Johnston. B. R., Duncan, C. W., Lawton. Kirk, and Benne,E. J., 3. Assoe. Ofic.Agr. Chemists. 35,813 (1952). (8) Parks. T. D., Johnson, H. 0.. and Lykken, L., ANAL.CHDM..20, 822 (1948). (9) Sehrenk, W. G., and Smith. F. M.. IMd.. 22,1023 (1950). RECEIVED for review July 10, 1954. leoepted January 24, 1955. Conwibution No. 505, Department of Chemistry, K a n s a ~ . 4 g r j d t u r dEwerinrent Station. Manhattarn.
CRYSTALLOGRAPHIC D A T A
95. Triphenylacetic Acid Contributed by 0. W. A D A M S and W A L T E R C. MCCRONE, Armour Rosearch Foundation of Illinois Inslitute of Technology, Chicagol6, 111.
sium bromide solutions) 1.2293 (x-ray). Because of experimental difficulties the accuracy of the flatation density is very limited. OPTICALPROPERTIES Refractive Indices (5893 A,; 25' C,). B = 1.6156 i 0.002. w = 1.6794 0.002. Sign of Double Refraction. Negittive.
+
i Structural Formula for Triphenylacetic Acid
RIPHENYLACETIC acid is solrr uble in benzene, glacial acetic acid, acetone, and most common organic solvents. It
is insoluble in water. Best crystalsfor study were obtained from nitrobenzene or benzyl alcohol (see Figure 1). CRYSTAL MORPHOLOGY CrystalSystem. Hexagonal. Form and Habit. Most common forms are hexagonal prisms of second order with basal pinecaids { 0001 1. Axial Ratio, a : c = 1:0.9285. Cleavage. Fusion indicates eleaTage perpendicular to the c ax,& X-RAYDIFFRACTION DATA CellDimensions. a = 14.27; c = 13.25 A. Formula Weights Per Cell. 6 (6.091 calculated from x-ray data). Formula Weight. 288.33. Density, 1.248 j= 0.020 (flotation in aqueou8 potas-
I Figure 1. Characteristiccrystals of triphenylacetic acid from nitrobenzene on microscope slide
Figure 2. Orthograph of typical crystal of ti.r acid
. L I c
