Spectrographic Microdetermination of Zinc ALBERT P. VANSELOW AND BERT AI. LAURANCE University of California Citrus Experiment Station, Riverside, Calif.
*
T
are added. This buffer solution contains about 120 grams of sodium citrate (2Na3C6Hs0,.llHz0) and 230 grams of citric acid (C6Hs07.HzO) per liter and is adjusted to a pH of 3.0. The final volume of the solution should be about 40 ml. and the pH about 2.8 to 3.0. The contents of the flask are heated to about 60’ to 70“ C. and the flask is stoppered with a cork containing the hydrogen sulfide inlet tube and an outlet tube. The former extends nearly to the surface of the solution. A slow stream of hydrogen sulfide is passed into the flask as it is being heated. When the contents of the flask have reached the boiling point, the source of heat is removed and the outlet tube is closed. However, the flask is left connected to the hydrogen sulfide generator until it has cooled to room temperature. The flask is shaken occasionally during the cooling period. When cool, the flask is disconnected from the source of hydrogen sulfide, stoppered, and allowed to stand for about half an hour. The contents of the flask are transferred to a 50-ml. Pyrex centrifuge tube; the flask is rinsed once with a few milliliters of 0.05 N formic acid solution saturated with hydrogen sulfide. After the centrifuging, the supernatant liquid is poured off and the precipitate in the tube is washed once with about 10 to 15 ml. of the hydrogen sulfide-formic acid solution. After a second centrifuging, the wash solution is poured off. The precipitate in the tube is dried overnight in an evacuated calcium chloride desiccator. The washing of the precipitate and the drying at room temperature are absolutely essential, since only by so doing can one obtain a loose powdery precipitate which can easily and almost completely be transferred to the crater of the graphite electrode. The dried precipitate in the tube is loosened by means of a small platinum spatula and is transferred as completely as possible to the crater of the electrode. The electrodes used in these experiments were of graphite 0.64 cm. (0.25 inch) in diameter, with the crater 0.32 cm. (0.125 inch) both in diameter and depth.
HE recent development of interest in zinc as a necessary minor element in plant growth creates a demand for a n analytical method capable of determining the minute quantities of this element that are present in vegetable matter. That the methods of spectrography are applicable to, the determination of small quantities of zinc has been demonstrated by Rogers (3). In the method proposed by Rogers, the ash of the plant material is arcked directly, after incorporation of the “reference element.” Since there is a limit both to the amount of ash that can be arcked conveniently and also t o the spectrographic sensitivity for zinc, his method has a limited usefulness. Rogers places the lower limit a t about 0.002 per cent or 20 parts per million in the plant ash. Inasmuch as the concave grating spectrograph used by the authors is not so sensitive as the quartz prism instruments,’ the zinc present in the plant ash must be separated from the main constituents of the ash and concentrated before spectrographic methods can be applied. Experimental I n brief, the method proposed by the authors consists of dissolving the plant ash in dilute hydrochloric acid, adding 2 mg. of cadmium as the sulfate, precipitating the cadmium and zinc with hydrogen sulfide at a p H of about 3, and estimating the zinc in the sulfide precipitate spectrographically. The added cadmium is used as the “internal standard.” SEPARATION OF THE ZINC. A sample of from 2 to 4 grams of the dried plant material contained in a sillimanite combustion boat (100 mm. long and 20 mm. wide) is thoroughly ashed at a temperature of 450” C. in an electrically heated Pyrex-tube combustion furnace. The ash is transferred to a small evaporating dish, 20 ml. of N hydrochloric acid solution being added; the dish and contents are heated on a hot plate until the volume of the solution is reduced one-half. The solution is then filtered, the volume of filtrate and water washings being kept down to about 25 ml. To the filtrate, contained in a 125-ml. Pyrex Erlenmeyer flask, are added 10 ml. of cadmium sulfate solution containing 0.2 mg. of cadmium per ml. In the precipitation of zinc and cadmium sulfides, a modification of the technic of Fales and Ware (1) is used. With bromophenol blue as indicator the solution is adjusted to a pH of about 3.6 with saturated potassium hydroxide solution; 0.5 ml. of 50 per cent formic acid (sp. gr. 1.2) and 3 to 5 ml. of citric acid-sodium citrate buffer solution
SPECTROGRAPHIC EQUIPMENT. The spectrograph (Figure 1) used in this work is a concave grating instrument (made by the Applied Research Laboratories of Los Angeles). The grating, whose radius of curvature is 150 om., is ruled for a space of 5.1 cm. (2 inches) with lines 2.54 cm. (I inch) long; it has approximately 9200 lines per cm. and consequently has a resolving power several times that of the large quartz prism spectrographs. The grating is ruled on speculum metal and is aluminized in vacuum after being ruled; by this means the reflective power is greatly increased, especially in the ultraviolet region. The arc is entirely enclosed, its image being focused on the grating by means of a quartz lens. The camera uses standard 35-mm. negativ? motion picture film. The ultraviolet region from 2370 8. to 4600 A . is photographed on a film 32 cm. long with the slit in one position. A second slit positionjs providedofor photographing the visible spectrum from 4580 A. to 6810 A . on a film 32 cm. long; a disk of Crookes No. 1 glass placed in front of the slit absorbs the second order ultraviolet when the visible region is being photographed.
1 The speed of this grating spectrograph is somewhat less than that of the usual quarts prism instruments because of the fact that i t was designed t o give much higher resolution and dispersion in the visible region than most prism instruments possess.
Owing to the astigmatic properties of the concave grating, it is difficult to use the revolving logarithmic sector of Scheibe and Seuhausser (4). This difficulty has been overcome by Hasler and Lindhurst (8) by the introduction of a halfcylindrical shutter immediately in front of the photographic a m . This shutter has its axis of rotation a t the center of the cylinder and is so mounted that this axis is parallel to the plane of the film and normal to the plane of the light beams forming the spectral lines. The theory of such a shutter is given in the paper of Hasler and Lindhurst ( 2 ) . The type of spectrograms obtained with such a revolving sector is shown in Figure 2, in which picture there are two such spectra with an iron comparison spectrum between them. The distance between the points a t which a spectrum line fades out, L , is measured with a scale mounted in the eyepiece of the comparator microscope. The theory of such a shutter leads to the following simplified expression :
FIGURE 1. COSCAVEGRATIIVG SPECTROGRAPH
240
JULY 15, 1936
ANALYTICAL EDITION
241
ESTIMATION OF ZINC ~ i PURE i SOLUTIONS.Solutions containing 2 mg. of cadmium as the sulfate and varying amounts of zinc as the sulfate were precipitated, washed, dried, and arcked in the manner described in this paper. The spectrograms were measured, and from the values of the ratio LCd/ LZn the recovery of zinc was obtained from the curve of Figure 3. The results are presented in Table 11. TABLE 11.
SPECTROGRAPHIC ESTIMATION OF ZINC FROM PCRESOLUTIONS
Zinc Taken
FIGURE2. SPECTROGRAMS OBTAINED WITH SHUTTER
RECOVERED
Zinc Found
1Mg.
Xg.
0.01 0.025 0.05 0.075 0.10 0.20
0 . 0 1 0 and 0 . 0 1 2 0 , 0 2 3 and 0.022 0 057 and 0 . 0 4 8 0 . 0 7 8 and 0 . 0 7 8 0 090 and 0 . 1 0 8 0 1S5and . . .
RECOVERY OF ZINC ADDEDTO PLANT TISSUE.Varying amounts of zinc as the sulfate were added to the ash resulting from the ignition of 2 grams of dried orange leaves (No. HALF-CYLINDRICAL 358). Similar amounts of zinc were added to 2 grams of the leaf tissue (No. 358) before ignition. The results of these experiments are given in Table 111. T.4BLE 111. RECOVERY OF ZINC ADDEDTO PLANT TISSUE BEFORE AND A ~ T E R IGNITION
in which L1 and Lz are the lengths of the blank spaces between the points a t which the two lines fade out, and I I and Zz are the intensities of the two lines being compared. CALIBRATION OF THE SECTOR.For the spectrographic estimation of zinc, the “internal standard” method was used.. Cadmium was chosen as the internal :tandard for several reasons: (1)The cadmium line a t 3252.5 A. is of the same type is conveniently situated, and is as the zinc line a t 3345.0 i., of approximately the right intensity when 2 mg. of cadmium are co-precipitated with the zinc. (2) The volatilities of cadmium sulfide and zinc sulfide are sufficiently similar so that the two elements are vaporized a t the same time; this is a rather important point since the “wandering” of the arc is a possible source of serious error if the two elements are not vaporized simultaneously. (3) Cadmium does not occur in plant materials in amounts sufficient to augment appreciably the 2 mg. added as internal standard. Mixtures with known ratios of cadmium sulfide and zinc sulfide were made up. Placed in the cavity of a graphite electrode, 2.5 mg. of each of these dry powders are arcked as the lower and positive electrode with a direct current of 7 amperes and 150 volts. Twenty seconds are sufficient to vaporize the sulfides completely, but all exposures were made for one minute. A slit width of 0.05 mm. was used, and the sector revolved a t about 500 r. p. m. The results of the spectrograms of these ratio powdersoare shown graphically in Figure 3. The zinc )ne a t 3345.0 A. is compared with the cadmium line a t 3252.5 A,, and in the figure the ratio LCdlLZn is plotted against the square root of the number of milligrams of zinc present for each 2 mg. of cadmium TABLE, I. EFFECTOF CURRENT ON RATIOOF INTENSITIES OF CADMIUM 3252.5 AND ZINC 3345.0 LINES Current, llmperes 4 to 5 4 to 5 7 to 8 7 to 8 11 t o 12 11 t o 12
LCd/LZn 1.25 1.27 1.30 1.28 1.25 1.27
The data in Table I strongly indicate that the pair of lines used in this investigation are invariant. I n each case, 2 mg. of cadmium and 0.1 mg. of zinc as the sulfides were used.
Zinc Added
Recovery of Zinc Added before Ignition
.Mg.
Mg.
Sone 0.05 0.10 0.15 0.20
0.006 0.058 0.096 0.152 0.193
Recovery of Zinc Added after Ignition
.
Mo
.
Mg. 0.010
M g
0.062 0.10s
0 057
0:04%
0.108 0.144 0.212
0.152 0.203
:
0 :i 7 6
O.OQ6
As shown by these data, the recovery of zinc added to the plant tissue before ignition is quite satisfactory. Only in the case of the highest amount of zinc used (0.2mg.) is there any indication of a loss due to ignition, and these few determinations can scarcely be considered conclusive. TABLEIV. ZINC CONTENT OF CITRUS LEAVES (Parts per million on air-dried basis) Sample No. Zinc P p . m. 346 2 0 358 3 0 356 8 0 370 12 5 374 20 379 27 368 30 294 85
I n Table IV are shown a few typical results of the analyses of citrus leaves. The zinc content is expressed as parts per million on an air-dried basis. 20 I
I
I
1
I
15
10
a34 e 0
EASTMANORTHO EASTMANS S PAH AGFA PLEWCHROME.
05
