968
ANALYTICAL CHEMISTRY
from this value is 1:0.854. The error in the angle is 0.3%, and the error in the ratio is 0.6%. Figure 6, when completed, would appear like Figure 4,if the fundamental spherical projection were rotated around the right-left axis until point c was at the north pole-that is, the center of the projection. All great and small circles in a stereographic projection represent real circles of the spherical projection. However, owing to the tangent function of the stereographic projection, the geometrical center of a circle on the stereographic projection is not the projection of the center of the real circle. The projection of the center of a real circle i s the point equidistant from all points on the projected circle in terms of angular, not linear measure. A compilation of determinative angles for all the crystals reported in the literature is now being prepared in England and its release is expected a t an early date. It is based upon the system devised by Barker (1) and is well discussed by Donnay (3). This should greatly enhance the value of these methods of crystallography in the identification of crystals found in electron microscopical samples. SUMMARY
The silhouette angles (practically the only analytical data available to the electron microscopist) of crystals observed on electron micrographs can be correlated with interfacial angles. Agreement of calculated with measured angles establishes the identity of the crystals. Furthermore, the silhouette angles may . methods represent be used to calculate axial e l e m r ~ ~ t sThese the determination of physical rrnstants with the electron microwope. ACKNOWLEDGMEYTS
The authors wish to express their apprwiation to T. G. Rochom for his suggestion of the problem and interest in the application of crystallographic concepts to elertron microscopy. They wish to thank J. D. H. Donnay for his adviw and encouragement in the preparation of thia paper. They also are indebted to their co-workers for suggestions m d criticisms.
Figure 6. Stereographic Projection of Calcite Using Electron RIicroscope Goniometric Data LITERATURE CITED
(1) Barker, T. V., “Systematic Crystallography, an Essay on Crystal
Description, Classification, and Identification,” London, Thomas Murby & Co., 1930. (2) Dana, E. S., “A Textfoook of Mineralogy,” 4th ed., rev. by W. E. Ford, New York, John Wiley & Sons, 1932. (3) Donnay, J. D. H., “Crystallochemical Analysis,” Vol. I, Chap. XIII, “Physical Methods of Organic Chemistry,” ed. by Arnold Weissberger,New York, Interscienre Publishers, 1945. (4) Donnay, J. D. H., and O’Brien, W. A , , IND. ENQ.CHEM.,ANAL ED., 17, 593-7 (1946). RECEIVED February 3, 1948. Presented in part before the Electron Microscope Society of America, Pittsburgh, Pa., December
7, 1946.
Volumetric Determination of Microgram Quantities of Acid-Soluble Sulfur LEONARD P. PEPKOWITZ’ University of Calgornia, Los Alamos ScientiJic Laboratory, Los Alamos, IV. Mex.
I
N CONJUNCTIOK with one phase of the research activities at Los Alamos, it was necessary to develop a rapid method,
more accurate than available colorimetric methods, for determination of acid-soluble sulfur on the microgram scale. The method is presumably applicable to all materials that are soluble in hydrochloric acid and release hheir sulfur as hydrogen eulfide. As given, it is applicable to the microgram range, but may be used for determining milligram amounts of sulfur if 0.1 N eelutions are employed. The successful application of the modified standard distillation method to the microgram scale, despite the fact that the small amount of sulfur is determined by difference, depends primarily on two factors: oxidation of the sulfide to sulfate rather than to sulfur as in the usual procedure S-40C1- + SO,-4C1- and the inherent accuracy of the iodometric end point. Unfortunately, stoichiometric relationships are not borne out experimentally. The reason for this is not known at present. 1 Present address, General Electric Researoh Laboratory, Sohenectady, N.Y.
+
+
Quantitative results are obtained by determining the titer valuas of the reagents against known amounts of sulfide. PRINCIPLE OF METHOD
The basic reactions involved are:
REAGENTS AND APPARATUS
Calcium Hypochlorite (1, 2). Dissolve6 to 10 grams of U.S.P. calcium hypochlorite, depending on the chlorine content, in 250 ml. of distilled water, shake well, and filter. Dilute the filtrate to 1liter and store in an amber bottle in the dark. Under these conditions, the solution is stable. This solution is approximatel 0.1 N . For use on the microgram scale, dilute t o 0.01 N eacz day before use and redetermine the titer.
' V O L U M E 20, NO. 10, O C T O B E R 1 9 4 8
969
A procedure for determining acid-soluble sulfide on the microgram scale is presented. The sulfur is distilled as hydrogen sulfide from acid solution and absorbed in a measured excess of calcium hypochlorite. The sulfide is oxidized to sulfate and the excess hypochlorite determined iodometrically. Stoichiometric relationships are not borne out experimentally. Quantitative results are obtained by determining titer values of reagents against known quantities of sulfide.
Potassium Iodide, 0.1 iV. Sodium Thiosulfate, 0.1 AT. Dilute to 0.01 S for use. Starch Indicator Solution. Standard Sodium Sulfide Solution. Prepare a stock solution containing 1 mg. per ml. If the large clear crystals of sodium aulfide are hand-picked for weighing, the calculated value will closely approximate the standardization value. Standardize the 6tock solution against standard iodine-potassium iodide and sodium thiosulfate. Dilute to desired range before use, checking the standardization of the stock solution immediately before dilution. Pyrex Still (Figure 1). I-Ml. Buret, calibrated in hundredths.
chlorite solution. ildd sufficient hydrochloric acid through the side arm, A , to effect solution of the sample. Wash the acid down with a small amount of water and immediately connect the air stream. Aidjustto a moderate rate, so that the flow is rapid enough to prevent sucking back when the still cools. Heat the solution in the flask to 80 O to 90 O C. Best results are obtained if the solution is not boiled. Sweep for 5 minutes, longer sweeping time increases the blank value. I t was found experimentally that all the hydrogen sulfide is carried over in 5 minutes. I t is important to be sure that the sample is in solution before starting the sweep interval.
Table I. PROCEDURE
Recoveries of Known Amounts of Sulfide without Distillation
S - - Taken,
S - - Found,
Determine the titer by adding 100 micrograms of sulfide sulfur % Micrograms Micrograms to 2 ml. of 0.01 N hypochlorite in a 25-ml. Erlenmeyer flask. 10005 1010 101 .o This is done conveniently by employing a calibrated 100-micro100.0 100.0 100.0 50.0 50.0 100.0 titer syringe pipet. After a minute or so, add 2 ml. of 0.1 N po20.1 100.5 20.0 tassium iodide followed by 2 drops of concentrated sulfuric acid. 5.0 6.3 126.0 Allow the reaction to proceed for 5 minutes, so that the iodine re5 0.1 N reagents employed. action will go to completion. Titrate the liberated iodine with 0.01 N thiosulfate, adding 2 drops of starch indicator just before the end point. Denote this volume of thiosulfate as a. Similarly, determine the volume of thiosulfate required to titrate 2 ml. of 0.01 N hypoAt the conclusion of the determination, lower the receiving 100 y flask, disconnect the tip, and wash inside and out into the rechlorite without added sulfide, A . Then, the titer K = -j-q* ceiving flask with a small quantity of water. Add 2 ml. of 0.1 N potassium iodide and 2 drops of concentrated Introduce the weighed sample into the distilling flask and assulfuric acid and mix thoroughly. Allow 5 minutes for the iodine semble the apparatus. Pipet 1 ml. of 0.01 iV hypochlorite into reaction to go to completion and titrate with 0.01 N thiosulfate, the 25-ml. receiving flask and add a little distilled water, so that adding 2 drops of starch solution just before the end point. Dethe tip of the condenser tube is below the surface of the hypotermine the blank by running through the distillation procedure without added sulfide. If m ml. of thiosulfate me used to titrate the residual TO hypochlorite, then
Q P
y
s--
= (B
- m)K
where B is the volume of thiosulfate required to titrate the 1 ml. of 0.01 N hypochlorite carried through the blank determination. DISCUSSION
* SOLN.
25
Figure 1. Pyrex Distillation Apparatus
Table 1 gives some typical recoveries of known amounts of sulfide added directly to 0.01 iV calcium hypochlorite. The values reported are individual values and not averages. Table I1 presents the recovery of known amounts of sulfide carried through the complete procedure including the distillation. In order to efTect a complete distillation a t the microgram level, it is necessary to sweep out the hydrogen sulfide from the
ANALYTICAL CHEMISTRY
970 Table 11. Recoveries of Known Amounts of Sulfide Including Distillation S - - Taken, Micrograms
5 - - Found, Micrograms
%
50.0
49.0 19.2 5.1 0.9
98.0 96.0 102.0 90.0
20.0 5.0 1.0
l), proved extremely effective. No trace of sulfide or silver ion could be detected in the air stream. The small blank correction is necessary if the distillation is performed. The blank is positive and constant and is apparently caused by the destruction of a small quantity of hypochlorite during the distillation. No blank correction is necessarv if the sulfide is added directly to the hypochlorite. LITERATURE CITED
warm solution with a stream of air. Air from the usual compressed air line contains relatively large amounts of sulfur, which is remoyed by bubbling the air through the silver nitrate solution. A simple and very effective way to prevent the air stream from carrying over any entrained silver nitrate is to back up the silver nitrate scrubbing flask with a standard U-tube filled with Ascarite. This arrangement, plus a simple safety valve (Figure
(1) Kolthoff, I. M., and Sandell, E. B., “Textbook of Quantitative Inorganic Analysis,” New York, Macmillan Co., 1943. (2) Kolthoff, I. M., and Stenger, V. A., IND.ENG.CHEM.,ANAL.ED.. 1, 79 (1935). RECEIVED January 29, 1948. Based on work performed a t Los Alarnos Scientific Laboratory of the University of California, under Government Contract W-7405-eng-36. The information will appear in Division V o! the Nuclear Energy Series (Manhattan Project Technical Section) as part of the contribution of the Los Alarnos Laboratory.
Spectrochemical Determination of Beryllium Increased Sensitivity of Detection in the Cathode Layer JACOB CHOLAK AND DONALD M. HUBBARD Kettering Laboratory of Applied Physiology, University of Cincinnati, Cincinnati, Ohio A spectrochemical method for the determination of beryllium in biological material (1) has been revised to take advantage of the increased sensitivity of detection made possible by volatilization from the cathode of the direct current arc. In quantitative procedures the method is sensitive to 0.001 microgram of beryllium in the arc, while as little as 0.00025 microgram can be detected in unweakened spectra. The most useful analytical range, employing the beryllium line at 2348.6 A. extends from 0.005 to 0.30 microgram per ml. of prepared solution. The method described earlier is used to supplement the present method within the range of 0.2 to 2 micrograms beryllium per ml., and a separate working graph for the line pair beryllium 2650.8 A., thallium 2580.2 A. is used for the range 2.0 to 30 micrograms of beryllium per ml. of solution.
T
HE determination of beryllium in biological material by a spectrochemical method ( 1 ) has proved feasible in the case of samples in which the absolute quantity of beryllium present in the arc is not less than 0.02 microgram. Smaller quantities have been detected frequently, but, in general, detection was not certain when the quantity of beryllium was below the value cited above. Failure to detect beryllium with certainty by this method, in the large proportion of the available samples of the urine of persons known to have been exposed to dusts and vapors of beryllium and its compounds, has indicated that the concentration of beryllium in the urine of such persons is commonly very small. If analysis of the urine is to be a useful tool for estimating the extent of individual or group exposure to beryllium compounds, it must be possible to detect a quantity on the electrode equivalent to 0.01 microgram in the original aliquot of 50 ml. of urine taken for analysis. Actually, a spectrographic method poqsessing a sensitivity of 0.001 microgram of beryllium or less in the arc would be desirable, for then, on isolating and concentrating beryllium by the procedure outlined previously ( I ) , it would be possible to detect as little as 0.1 microgram of beryllium per liter of urine. A remarkable increase in the sensitivity of the spectrographic detection of beryllium is obtained when the sample is volatilized from the negative electrode of the direct current arc (2). This type of excitation has made it possible to detect with certainty somewhat less than 0.001 microgram of beryllium in the arc, and has led to the development of a spectrochemical method particularly adapted to the determination of very small quantities of beryllium. This paper is confined to a description of the
changes introduced into the spectrographic procedure, and to a discussion of the factors that influence the ultimate sensitivity of detection and the accuracy of analysis. PROCEDURE
Spectroscopic Buffer Solution. Fifteen milliliters of a stock solution simulating the salt content of urine ( I ) are mixed with 50 ml. of hydrochloric acid (specific gravity 1.19) and 5 ml. of thallic nitrate solution (1 ml. = 5 mg. of thallium), and the whole is diluted to 500 ml. Preparation of Samples. Samples are prepared in exact accordance with methods described previously ( I ) , except that the weaker spectroscopic buffer solution, described in the preceding paragraph, is substituted for that described in the earlier paper. Spectroscopic Procedure. Poytions of 0.2 ml. of the prepared solutions are placed in the craters of each of two graphite rode (0.63 cm. in diameter and 3.76 cm. in length, with craters 3 mm. in diameter and 10 mm. in depth). The impregnated rods are dried in an oven at 110” C. and each is used as the lower (negative) electrode of the arc. The upper, positive electrode consists of an unimpregnated rod of similar shape and size. The arc is operated from a 110 volt direct current power main, with sufficient ballast in the line to draw 10 amperes. A constant arc gap of 5 mm. is obtained by maintaining the projected image of the arc between two properly spaced marks on a screen. The light of the arc is so focused on the slit of the spectrograph that the.glowing “catho.de layer” just enters the top of the sllt, while llght from the incandescent anode is Prevented from entering the instrument by the housing of the slit. [The cathode layer may also be focused on the collimator lens of the spectrograph by means of the system of convex lenses, screens, and diaphragms described by Strock (4.1 The width and length of the slit are the same as those indicated previously, and the rotating step sector is also used as before ( 1 ) . Eastman S o . 33 plates are used to photograph the spectra during
