V O L U M E 22, NO. 9, S E P T E M B E R 1 9 5 0 present as hydrofluosilic acid or sodium silicofluoride. Thus, the proctdure may be applied to distillates from the Willard-Winter method ( 2 ) . RANGE, PRECISION, AND REPRODUCIBILITY OF METHOD
To determine the precision and range of this method the procedure given below was employed. Two thorium nitrate solutions were prepared, 0.0157 dl and 0.0032 M , respectively, based on precipitation of thorium A S the hydroxide and ignition to the oxide, The weaker titrant was standardized a t 0.3, 0.9, and 2.0 mg. of fluoride, the stronge- a t 3.0 and 10.5 mg., using sodium fluoride that had been especially purified by Winter ( 2 ) . The experimental results obtained by two analysts using these standardization levels are presented in Table I. The results all fit one of the following equations closely: For the 0.0157 M solution, theoretical factor 1.19 mg. per ml., mg. F = 1.188 mg. per ml. (ml. - 0.15); for the 0.0032 K solution, theoretical factor 0.2415 mg. per ml. in the 0.3- to 0.9-mg. range, mg. F = 0.2406 mg. per ml. (ml. - 0.15), in the 0.9- to 2.0-mg. range, mg. F = 0.266 mg. per ml. (ml. - 0.3). The precision was determined using 0.0123 M titrant. Ten samples of sodium fluoride were titrated a t three levels. The deviation, on the 95% confidence level for a single determination, was * O 013 mg. a t 1.00 mg., 10.09 mg. a t 15.00 mg., and 10.154 a t 30.0 mg. of fluoride. The lower limit of the method using the apparatus and procedure given below is about 0.04 mg. of fluoride using a quartz cell, 50% alcohol, and 0.01 N titrant. The upper limit is above 40 mg. of fluoride using a blown borosilicate glass cell, 40% alcohol, and 0.0125 M titrant. Procedure I(O.1 to 2.0 mg. of fluoride). Place an aliquot of not more than 40 ml. of the sample in the special cell, add 1 to 2 drops of the pH indicator, and adjust the acidity by adding monochloroacetic acid slowly until the yellow indicator color just disappears. Add 5 ml. of buffer solution, 5 ml. of quercetin indicator solution, and sufficient 95% ethyl alcohol to form a Soy0solution, taking into account the alcohol added with the indicator. Make all additions with pipets to keep the alcohol content constant. Place the cell in the modified Klett fluorometer after cooling to 10" to 15" C., and adjust the stirrer to obtain a moderate stirring rate. The fluorometer should have warmed up for 15 minutes. Close the cover of the cell chamber, insert the tip of the buret filled with 0.0025 211 titrant through the hole, and allow it to dip just into the solution. Adjust the balance diaphragm slit of the Klett to about 15 and balance the intensity potentiometer in the usual manner. The galvanometer should be steady on zero; if not, decrease the stirring rate slightly. Add several small increments (usually 0.05 or 0.1 ml.) of thorium nitrate, balancing the galvanometer to zero after each addi-
1197 tion and plotting the observed intensity readings against volume of titrant added. Do this until sufficient points have been obtained definitely to establish a straight line. Allow the titrant to follow slowly into the solution, watching for a sharp increase in galvanometer reading, indicating that the end point is near. .4t this point make several more small additions of titrant (usually 0.05 ml.) with galvanometer adjustment as before. Continue this until suffi1:ient points have been obtained to establish a straight line. Extrapolate the straight lines graphically to the point of intersection which indicates the equivalence point, and read the end-point volume from the graph. Procedure I1 (1.0 to 40 mg. of fluoride). Use Procedure I, but omit cooling of the cell and adjust the alcohol content to 407, instead of 50%. Use 0.0125 M titrant. The first increments may be larger (0.1 to 0.6 ml,), but the increments after the end point should be 0.1 or 0.05 ml. RESULTS AND DISCUSSION
This procedure has given very good results on certain fluoroorganic compounds after treatment by alkaline decomposition methods. Results agreed much more closely than those obtained visually using Alizarin Red S. After one series of distillations for standardization purposes, results by this method applied to distillates from the Willard-Winter separation checked more closely for amounts of fluoride greater than 2 mg. than by the iilizarin Red S method. The method avoids the personal factor of judging subtle visual color changes and takes no longer than the visual procedure. ACKNOWLEDGMENT
The authors wish to thank S. B. Penick & Company for generous supplies of pure quercetin, and H. A. Liebhafsky of the General Electric Company for the gift of a quartz cell. Most of this work was done under contract with the Technical Command, Chemical Corps, U. S. Army. LITERATURE ClTED
(1) i2)
Willard, H. H.. and Horton. C. .1..ANAL.CHEM.,22, 1190 (1950). Willard, H. H., and JVinter,0. B., IND.ENG.CHEM.,AKAL.ED., 5,7-10(1933).
RECEIVED February 11. 1950. Abstracted from a thesis presented by Charles A. Horton for the doctor of philosophy degree in the Horace Rackham Graduate School, University of Michigan.
Spectrographic Microdetermination of Beryllium in Air Dust Samples GEORGE E. PETERSON, GEORGE A. WELFORD, AND JOHN H. HARLEY C . S. Atomic Energy Commission, Health and Safety Division, New Y o r k , N . Y .
0
K E of the important tools in the prevention of beryllium
poisoning is the monitoring of the concentration of berylHum in the air. The critical concentration for outside air may be as low as 0.01 microgram per cubic meter ( 4 ) ,and it is necessary to measure a t the level of 0,001 microgram per cubic meter. Even with a sensitive spectrographic method, the necessity for taking large volume air samples leads to many difficulties in the analytical process. The most widely adopted method for spectrographic analysis for microquantities of beryllium is that of Cholak and Hubbard ( 2 , 3 ) . Barnes, Piros, Bryson, and Wiener ( 1 ) have described a method for urine, tissue, and electrostatic separator dust samples. The general methods of Steadman (9) and Feldman ( 5 ) have also been applied to the determination of traces of beryllium. All these methods had certain disadvantages for the particular type of
samples run in this laboratory, and it was necessary to develop a suitable procedure. The method is not sufficiently sensitive to be useful in the analysis of urine and tissue samples. Further work is being done on the development of a more sensitive, yet reproducible method. .4ir dust samples run in this laboratory are collected by drawing known volumes of air through paper filters. Depending on the atmosphere sampled, varying types and amounts of impurities are collected with the beryllium. This makes it necessary to use chemical separations to obtaln the beryllium in relatively pure solution, not because of spectral interference, but in order t o obtain a more uniform matrix for excitation. Any of the usual dilution methods reduces the sensitivity below the required minimum. Although the techniques of air sampling are outside the scope of
A N A L Y T I C A L CHEMISTRY
1198 this ]xiper, certain features of the sampling influence the analytical method and must be descrihrd. Three types of filters are in common use, the Whatman KO. 41 ashless filter paper, the Rline Safety Appliances Company's all dust paper S o . 2133 and Type S unimpregnated filter. The first filter, in a 10-cm. (4inch) diameter, allows a sampling rate about 0.7 cu. meter per minute, but clogs rapidly in dusty atmospheres. The second paper allows a sampling rate of 1.0 to 1.3 cu. meters per minute and does not clog so rapidly. The Type S filter, because of the large area of the pleated structure, allows a sampling rate of about 2 cu. meters per minute and is very resistant to clogging. However, it has an ash content of over 1% (125 mg. per filter), which adds to the impurities in the eample. Blanks run on these papers show up to 0.3 microgram of beryllium per filter.
Table I. y
Precision of Spectrographic Procedure y Be Found Yo Relative Error
Be per M I . 0.10 0.15 0.20 0.50
1 .o 1.5 2.0 5.0 7.5 10 20 50
100 150 200
0.13 0.18 0.22 0.72 0.4 1.0 1.6 1.4 1.8 2.6 4.8 5.8 6.9 9.0 8.6 22 19 64 51 44 47 116 107 100 190 220
30 20 10 44 20 0 7 7 10 30 4 16
8 10 14 10 5 28 2 12 6 16
7 0
Av.
27 10 13
-__-
The addition of nitric acid is repeated until all organic matter is destroyed. The solution is transferred to a 50-ml. platinum dish, 2 ml. of hydrofluoric acid are added and taken to dryness on a sand bath. To the dry samples, 4 ml. of hydrochloric acid are added, the sample is transferred to a 50-ml. centrifuge tube, and the volume is adjusted to about 20 ml. I t is neutralized to the first precipitate of iron or aluminum hydroxide with ammonium hydroxide, and 2 r.11. of glacial acetic acid and then hydrochloric acid arc added dropwise, until the precipitate dissolves. Then 5 ml. of 12% oxine in glacial acetic acid and some paper pulp are added. The p H is adjusted to 6 with ammonium hydroxide, using test paper as indicator, and the sample is centrifuged at about 3000 r.p.m. for 5 minutes. The liquid is decanted through a loose-textured paper into a 125-ml. sepawtory funnel, the precipitate is washed with water, and the washings are added to the original filtrate. The filtrate and washings are extracted with 10 ml. of chloroform to remwe excess oxine, repeating if the organic layer is not colorless. The aqueous phase is transferred to a clean 50-ml. centrifuge tube and the p H adjusted to 7. If a visible precipitate forms at this point, the removal of iron and aluminum was incomplete and the oxine separation must be repeated. If the solution is clear, 1.0 ml. of an aluminum solution (2.5 mg. of aluminum per m].) is added, centrifuged, and the supernatant liquid is discarded. The precipitate is dissolved in 0.5 ml. of 1 to 3 sulfuric acid and rinsed into a graduated 1 5 ml. centrifuge tube with water. I t is evaporated to a volume of 1.0 ml. under an infrared lamp. SPECTROGRAPHIC PROCEDURE
Spectrograph. Baird 3-meter grating, 50-micfon slit. Source Unit. Baird with Sola constant voltage transformer. Lower Elect,rode. National Carbon Company 0.25-inch (0.6-cm.) nominal diameter graphite, regular grade, 6.3 by 50 mm., with a 4.5mm. cup, 5 mm. deep. Upper Electrode. Same type, sharpened to a 45" point. .Excitation. Direct current arc, 10 amperes, 2 minutes, Densitometer. Baird nonrecording. The cupped electrodes are waterproofed with a solution of
Table 11. Type of Filter None
Recovery of Beryllium from Synthetic Samples y
Be Added 0 1.0 5.0
No. 2133
0 0.5 1.5 5.0
The total volume of air sampled may range from 100 to 20,000 cu. meters. This means that at the 0.001 microgram per cubic meter level from 0.1 to 20 micrograms of beryllium would be present.
25 0
REAGENTS
Be Found
% Recoverya
