solution contains borate, the change toward red is less, and if sufficient boron is present there is a reverse, so that the resulting color is more blue. This color change is the basis for the method. The method as described is remarkably free from interferences. Germanium is the only ion that presents a similar color change; however, approximately 200 times as much germanium as boron is required to produce a given color change. Aside from the possibility of interference from germanium, there are two variables to
be considered: concentration of sulfuric acid and the temperature at which measurements are made. Both of these variables, in view of the extremely small n-orking range over which the absorption measurements are made, appear to have small effect on the sensitivity of the method. The reproducibility of the method, notwithstanding variable room temperatures, appears to be satisfactory. It might be of interest to point out that while the method indicateq the impurity content of the silicon tetrachloride, an intermediate in the silicon production process, it does not neceq-
sarily define the impurity content of the final silicon product in the process. LITERATURE CITED
(1) Johnson, E. A,, Toogood, M. J., Analyst 79, 493-6 (1954). C. S. HAAS R. A. PELLIN~ E. I. du Pont de Nemours & Co.
Newport, ljel.
M. R. EVERINGHAM E. I. du Pont de Nemours & Co. Brevard, N. C. 1 Present address, Motorola, Ino., Phoenix, Ariz.
Spectrophotometric Determination of Traces of Phosphorus in Silicon Tetrachloride Utilizing an Extractive Separation the extraction must be cleaned with ammonium bifluoride after each analysis to remove silica deposits. .411 glrtssware must be thoroughly dried before use. Procedure. Two reagent blanks are carried through the procedure. They are not exposed to silicon tetrachloride but undergo all the manipulations of the procedure. Add exactly 0.50 ml. of concentrated sulfuric acid t o a 125-nil. borosilicate glass flask from an Ultramax buret (Fischer and Porter Co.). Place a TeflonTFE-fluorocarbon resincoated magnetic stirring bar in the flask. Add 25 ml. of silicon tetrachloride from a dry graduated cylinder. Immediately place a Teflon stopper in the neck to prevent loss of fumes. Stir magnetically for 30 minutes. Then allow the mixture to stand for 15 minutes. Decant the SiC14, being careful that EXPERIMENTAL no acid is lost. In this step, the stirring Reagents. AMMONIUM VANADATE. bar is held on the bottom of the flask by a magnet held on the outside surface Mix 11.75 grams of ammonium meta of the flask. Immediately after devanadate and 68 ml. of 60% percantation, add to the flask the assembly chloric acid in 4 liters of water and of distilling head and separatory funnel, filter through #40 Whatman filter containing 50 ml. of demineralized paper. water, as shown in Figure 1. Open AMMONIUM MOLYBDATE. Mix 390.65 the stopcock of the separatory funnel grams of ammonium molybdate in 4 t o allow water to pass down t o the liters of water and filter through #40 flask, wash the bottom of the Teflon Whatman paper. STANDARDPHOSPHORUS SOLUTION. stopper and allow the washings to enter the separatory funnel. When it Use 0.4263 gram of anhydrous diis seen that the water is being definitely ammonium phosphate per liter of soludrawn into the flask, note the time tion. Dilute 10 ml. of this solution t o 1 and allow to digest for 20 minutes liter with water. (1 ml. = 0.001 mg. of with occasional shaking to mix the phosphorus.) contents. Use only demineralized water (10 Wash off the outer surface of the megohms) for mixing reagents and assembly a t the joint of the separatory throughout the analysis for rinsing funnel and the distilling head. Dry by residues, etc. wiping off gently with paper tissue. Apparatus. A critical point in the Remove the separatory funnel and determination is the prevention of wash down the inner surfaces into the loss of phosphorus during the dilution distilling head. Allow the remaining of the sulfuric acid extract. The apassembly to drain while the top is paratus as shown in Figure 1 was used. covered with a clean 50-ml. beaker. Transmittance measurements are Clean the outer portion of the remainmade with a Beckman DU spectroing joint between the flask and distilling photometer using a matched set of 100head as above. Remove the distilling mm. cells. head; with the aid of a magnet a t the The borosilicate glass flasks used for
SIR: The level of phogphorus in silicon tetrachloride is of interest because of the effect of trace amounts on semiconductor properties of elemental silicon. Silicon tetrachloride was first used in the zinc reduction process for commercial production of hyperpure silicon a t the Du Pont Co.’s Newport, Del., plant. Research was therefore undertaken to develop a method to measure trace amounts of phosphorus in silicon tetrachloride. I t was found that phosphorus could be quantitatively extracted from silicon tetrachloride with sulfuric acid, oxidized with perchloric acid to the pentavalent state, and combined with two colorforming agents to give a yellow complex. The intensity of the yellow color was measured with a spectrophotometer.
246
ANALYTICAL CHEMISTRY
Figure 1 .
Dilution apparatus
A. Separatory funnel, 60 ml., 2 4 / 4 0 v 8. Distillation head, 2 4 / 4 0 T C. Erlenmeyer Aark, 125 ml., 24/40-$
side of the flask, rake the stirring bar above the liquid level, wash, and remove, being careful not to introduce contamination at this point. Replace the distillation head on the flask. Add 10 ml. of perchloric acid (60%) to the flask. Place the flask on a hot plate at medium hes,t until fine misty perchloric acid fumes are visible in the bulb of the distilling head. Remove the flask and place 3n a hot plate a t low heat for 10 minutes. Remove the flask and allow to stand for 30 minutes, until it reaches room temperature. Remove the distilling head and add 10 ml. of ammonium vanadate reagent to the flask. Swirl to mix the contents. Transfer the contents of the flask to a 50-ml. volumetric flask. Rinse the distilling flask ,JO that the total amount of solution is not over 35 ml. Add 10 ml. of amnionium molybdate reagent to the volumetric flask. Fill the flask to the 50-1111. mark with demineralized water. Invert the flask 15 times. Allow to stand for 20 minutes to develop the color complex. Pour the contents of the flask into a 50-ml. centrifuge tube. Centrifuge for 30 minutes a t 3030 r.p.m. to separate any gel.
Decant the clear yellow solution into
a dry 50-ml. volumetric flask. Allow the flask to stand 30 minutes to reach room temperature. Fill a 100-mm. spectrophotometer cell with the solution and read at 430 mp using a 0.05-mm. slit width. Determine the amount of phosphorus present in the sample by reference to the standardization curve prepared as described below. Standardization Curve. Transfer suitable aliquots of the standard phosphorus solution (0.001 mg. per ml.) t o borosilicate glass flasks. Add 50 ml. of demineralized water and 10 ml. of 60% perchloric acid. Follow the outlined procedure for the regular analysis. Plot absorbance X lo4 against the level of phosphorus. DISCUSSION
The phospho - vanado - molybdo yellow complex has long been used to determine phosphorus content of many compounds. The development and stability of this color complex have been established by Racicot (1) and Snell and Snell (2).
Sulfuric acid treatment of silicon tetrachloride has been used for purification purposes at various times. This extraction can be adapted to quantitative analytical procedures a t very low concentration levels. In the 5-p.p.m. range, recoveries are essentially quantitative with detection limits of better than 15 p.p.b. on the silicon basis. A key to this method has also been the prevention of loss of phosphorus halides on dilution of the extractant with water. The particular dilution apparatus described satisfactorily retains the phosphorus and makes quantitative spectrophotometric measurements possible. LITERATURE CITED
(1) Racicot, E. L., ANAL. CHEM. 23,
1873-75 (1951). (2) Snell, F. D., Snell, C. T., “Colorimetric Methods of Analysis,” 11, p. 672, Van Nostrand, New York, 1954. W. A. LANCASTER E. I. du Pont de Nemours & Co.
Newport, Del.
M.R. EVERINOHAM E. I. du Pont de Nemours I%Co. Brevard, N. C .
Detection of Submicrogram Amounts of FJuoride on the Ring Oven with lrhorium Chloranilate SIR: The advantag?s of the ring oven for detecting trace quantities of cations and anions are well established (10). However, the only method recommended for detecting fluoride ion by this technique (9, 11) is not useful because chrome azurol S is no longer available from the manufacturer (Y), although it may still 138 purchased from Roboz Surgical Instniment Go., Washington 7, D. C. Feigl (4) has recommended two reagents, zirconium-alizarin (2) and zirconium azoarsonate (9), for detecting fluorids via spot tests. The former did not prove satisfactory when applied to the ring oven because the yellow color produced by destruction of the zirconiurr-alizarin complex cannot easily be seen and the complex itself is not thermdly stable. The zirconium azoarsonate method requires a specially prepared paper which did not prove practical for m e with the ring oven. Consequently, other reactions useful for detecting fluoride a t the sub partsper-million level were sought. Other dye methods (1) for fluoride were not tested because of the dependence of these methods upon closely controlled experimental conditions, which experience has shown cannot alivays be achieved when working with the ring oven. Two recently developed ree,gents based upon lanthanum (6) and thorium chloranilate
(8),both of which react with fluoride by precipitate exchange to produce a highly colored complex, were evaluated for use on the ring oven. Both reagents produced satisfactory methods for fluoride. The method for thorium chloranilate is described here because it is more sensitive. A recent method (12) based on the reaction of fluoride with iron(II1) rhodizonate and reaction of the liberated rhodizonate with silver ion t o form a blue-black precipitate was not tested. EXPERIMENTAL
Whatman No. 40 filter paper (5.5 em. or larger in diameter) is extracted with water in a Soxhlet extractor for 4 hours and carefully dried. The washing procedure described by West, PAPER HOLDER
FILTER PAPER
es3
Llacer, and Cimerman (1.9) is also satisfactory. About 50 mg. of thorium chloranilate (Fisher Scientific Co.) is ground into the center of the paper to cover a spot about 1 cm. in diameter, the paper is quickly inverted, and the excess reagent is tapped off. A glass ring is a convenient device for confining the reagent to the area. A micro spatula (A. H. Thomas, Catalog No. 9007-C) may be used to grind the reagent into the paper. The paper is mounted between two paper weights (National Appliance Co., Part S o . 5120), as shown in Figure 1, and the entire assembly is placed on the ring oven (National Appliance Co.), the surface temperature of which is 140’ C. -4 solvent pipet (National Appliance Co., Part No. 5117), modified by forming a concave depression in the upper end, is placed on the ring oven and the paper is adjusted so that the pipet touches the center of the spot. The prepared paper is washed rapidly with 0.25 ml. of ion-exchanged water added through the solvent pipet with a Kline antigen pipet. After the paper has dried, it is removed from the oven and placed on a glass annulus. At this stage the paper should contain a narrow violet or brown ring about 3 cm. in diameter. The fluoride solution to be analyzed, adjusted to pH 7 and free of interfering cations, is spotted on the paper in the center of the reagent. Care must be taken to ensure that the wet spot does not cover an area greater than that
a
Figure 1. Assembly for extraction of thorium chloranilate spot
VOL. 36, NO. 1, JANUARY 1964
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