0.01 p.p.m. compared favorably with the sensitivity of the Aminco-Bowman spectrofluorometer, which was reported to be 0.005 p.p.m. (17). The major problem in recording fluorescence spectra below concentrations of 0.01 p.p.m. is one of refleotcd radiation. The slit settings must be very wide for low concentrations, and consequently many extrancous peaks are recorded along with the fluorescence spectra. Previous authors (17) did not report the slit widths used in the calibration of the Aminco-Bowman instrument, but they were probably larger than,0.5 mm. Noise figures are usually given as R.M.S. values. Since the necessary equipment for such measurement was not available, peak-to-peak values are given. These are much higher than the true R.M.S. measure of the noise level would be. Reproducibility. Reproducibility studies were performed with 1 X 10-4M 2-naphthol solutions in absolute ethyl alcohol over a period of 7 days. The exciting radiation was the 3130-A. mercury line. Before each spectrum was recorded, the instrument was set to read 70 a t the fluorescent peak of a 3-p.p.m. solution of quinine sulfate. The reproducibility data appear in Table 11. The attenuator settings were different for each spectrum. This was to be expected, as the radiation source showed fatigue effects after it was in operation for any length of time. However, as long as the instrument was standardized against some reference solution, the intensities were very reproducible. Fluorescence Spectra. Fluorescence spectra of a variety of organic compbunds have been rezorded using the instrument described. and their
maxima agree well with those recorded by other investigators. As an example, the recorded fluorescence spect r a of 2,3-naphthalenediol in ethyl alcohol a t 298" K. and in E P A ( a mixture of 3 parts of ethyl ether, 5 parts of pentane, and 2 parts of absolute ethyl alcohol) at 77" K. are shown in Figure 7 without correction for spectral response curve of the instrument. These spectra compare favorably with those recorded on a modified Beckman DU (7) and also illustrate the advantages of spectrofluorometry at the temperature of liquid nitrogen.
Table 1.
Determination of Sensitivity
Fluorescent Intensity
Peak to Peak
at -.445 ___
Quinine Sulfate Concn., P.P.M.
Slit Width, Mm.
2 1 0.1 0.01 0.01
0.1 0.1 0.2 0.3 0.2
Mp,
NO&,
Arbi-
Units Same aa Intensity
trary
Units
Off scale
3 2 2 0.5
76
46 44 14
Table II. Reproducibility Studies
ACKNOWLEDGMENT
Attenuator Setting
R.F.I., at 3524 A.
The authors express their gratitude to the National Science Foundation for its support of this project. They are ex-tremely grateful to Samuel Roth for construction of the machined components of the instrument.
0.754 0.759 0.770 0.740 0.750 0.755 0.790
40 41 40 40 41 40 40
Slit width = 0.2 mm.
LITERATURE CITED
(1) American Instrument Co., Silver Springs, Md., Bull. 2778H (1956). (2) Applied Physics Corp., Pasadena, Calif., Bull. CDG7512-3M (1956). (3) Beckman Instrument Co., Fullerton, Calif., Bull. 718 (1957). (4) Farrand Optical Co., Inc., New York, N. Y., Bull. 823 (1957). (5) Freeman, D. C., Jr., White, C. E., J. Am. Chem. Soc. 78. 2678 (1956). (6) Gemmill. C. L.. .$NAL. CHEM. 28.
(11) Lipsett, F. R., Ibid., 49,673 (1059). (12) "M. I. T. Wnvelcngth Tables," Wiley, New York, 1956. (13) Nadeau, G., Joly, L. P., ANAL. CHEM.29, 583 (1957). (14) Ohnesorge, E. W., Rogers, L. B., Spectrochim. Acta 13, 27 (1959). (15) Ibid., p. 41. (16) Slavin, .W., Pittsburgh Conference
on Analvtical Chemistrv and Amlied -Spectroscopy, paper 44, "1958. (17) Sprince, H., Rowley, G. R., Science
~ - - -
\ - - - -
(7) Hercules,'D. M., Rogers, L. B., J . Phvs. Chem. 64. 397 (1960). (8) Hercules, D.. M.,' Rogers, L. B., Svectrochim. Acta 1959.393. (9) 'Kasha, M., J . Opt. koc. Ana. 38, 929
125, 25 (1957). (18) Warren Electronics Co., Bound Brook, N. J.,, "Spectracord Instruction -
Manual." RECEIVEDfor review August 25, 1959. Resubmitted April 18, 1961. Accepted June 14, 1961.
(1948). ~ --_,. -
(10) Klerk, A. D., Spruit, F. J., Ibid., 46, 556 (1956).
VoIumetric Determination of Tungsten Determination of Tungsten in Tungsten-Nickel Alloys C. L. LUKE Bell Telephone laboratories, Inc., Murray Hill, N . J .
b A new volumetric method for the determination of tungsten is proposed. Sexivalent tungsten is reduced to the trivalent state in strong hydrochloric acid solution containing ammonium chloride by boiling with granular lead and then passing through a lead reduc"or. Ttie reduced tungsten is caught in ferric iron solution, and the ferrous iron produced is titrated with standard dichromate solution. The method has been applied to the determination of 0.5 to 5y0of tungsten in tungsten-nickel alloys.
S
WORKERS have attempted to reduce sexivalent tungsten to the t,ivalent state, preparatory to an oxidimetric determination, but Someya (8) appears to be the first to achieve quantitative reduction. Someya, and subsequently Holt and Gray (Y), reduced the tungsten in HCI solution with a lead-mercury amalgam. However, reduction with an amalgam is inconvenient and not at all suitable for routine use. It seemed desirable, therefore, t o reinvestigate the possibility of reducing by boiling with metal as for EVERAL
the determination of tin (P), o r l b y passing the tungsten solution through a column of granular metal. Accordingly an investigation was undertaken using aluminum, zinc, cadmium, or lead as the reducing metal. Unfortunately all attempts to obtain quantitative reduction of tungsten failed. However, as much as 50 mg. of tungsten could be reduced 99% by boiling with granular lead and then passing through a lead reductor. Since the per cent reduction was uniform over the entire range up to 50 mg., the negative error could be VOL 33, NO. 10, SEPTEMBER 1961
1365
eliminated by standardizing the oxidant solution against tungsten. As a result, a very satisfactory volumetric determination of tungsten has been developed in which the reduced tungsten is caught in ferric iron solution, and the ferrous iron produced is titrated with standard dichromate solution, During this investigation the paper by Geyer and Henze (6) came to the author's attention. These workers could not obtain complete reduction of tungsten with granular zinc, but were able to do SO by the use of powdered zinc and a cadmium reductor. The proposed lead reduction-dichromate method has been adapted t o the determination of 0.5 to 5% of tungsten in tungsten-nickel alloys. I n this analysis the sample is dissolved in HNO,, all nitrates are converted to chlorides, and most of the cations are removed on an ion exchange resin. The re making traces of interfering metals are removed as hydroxides, and the tungsten in the filtrate is determined by the proposed volumetric method. APPARATUS
Ion Exchange Column. Prepare a column from 16- to 50-mesh Amberlite IR-120 (H) cationic resin supported on a small pad of glass wool (6). Wash .the resin with 75 ml. of HC1 (1 2) and then with water until the effluent is neutral to litmus. After using the column for 10 analyses, free the resin of nickel by repeating the acid plus water wash. Because of the possibility of contamination of the resin with silicic acid. DreDare a fresh column after about 30 analyses. Lead Reductor. Obtain a Jones reductor whose reservoir has a capacity of 120 ml. and whose column is 2 cm. O.D. by 20 em. in length ( 1 ) . Insert a small loose pad of glass wool into the bottom of the dry reductor. Fill the column to a height of about 18 cm. with pure granular lead (test lead) that passes a 10-mesh sieve but is retained on a 20-mesh sieve. Dissolve 15 grams of NH4C1 in 150 ml. of hot HCl (2 1) and pass the solution slowly through the reductor. Then drain out the solution and replace with HC1 (1 1). If, in the course of use, the rate of dissolution of the lead becomes excessive as a result of the couple action caused by the deposition of traces of more noble metals on the lead, drain out the HCl (1 1) and pass a mixture of 50 ml. of HCl (1 9) plus 2 ml. of H202 (30%) through the reductor. When this solution has drained out completely, wash out the soluble lead salts with hot water and replace the water with HC1 (1 1). Receiving Flask. Fit a 1-liter conical flask with a three-holed rubber stopper. Two of the holes should have diameters equal to that of the stem of a 25-ml. buret and of the lead reductor, respectively. The third
+
+
+
+
+
+
1366
ANALYTICAL CHEMISTRY
hole should hold a glass inlet tube for introducing nitrogen gas. REAGENTS
Cation Exchange Resin. Wash 16to 50-mesh IR-120 (H) cationic resin in a large glass column with HCI (1 2) until the effluent comes through colorless. Then wash thoroughly with water until the effluent is neutral to litmus. Remove most of the water from the resin by suction, Keep the damp resin in 8 brown bottle. Ferrous Iron Solution (0.0.5N). Dissolve 10 grams of ferrous ammonium sulfate (Fe(NH4)z(SO&.6 HzO) 95). Keep in 500 ml. of H2S04 (5 in a stoppered bottle. Ferric Iron Solution. Dissolve 10 grams of ferric ammonium sulfate (FeNH4(S04)z~12Hz0) in 20 ml. of HC1 and then add 80 ml. of water. Diphenylaminesulfonic Acid Solution (0.005M).Supplied by G. Frederick Smith Chemical Co., Columbus, Ohio. Standard Tungsten Solution (2 Mg. of W per MI.). Dissolve 3.588 grams of pure NazW04.2Hz0 in water in a 1-liter volumetric flask and dilute t o the mark. If the purity of the sodium tungstate is in doubt standardize the solution by the usual gravimetric WO, method. Standard Potassium Dichromate Solution (0.0lN and 0.05N). Dissolve 1 or 5 grams of K2Cr20, in 2 liters of water. To standardize these solutions dilute 2.0- or 10.0-ml. portions, respectively, of standard tungsten solution (2 mg. of W per ml.) to 25 ml. in a 125ml. conical flask. Add 5 or 8 grams of NH4C1, respectively, and then reduce and titrate with the appropriate dichromate solution as directed in the Procedure. Correct for the indicator blank.
+
+
PROCEDURE
Preparation of Receiving Solution. Transfer 500 ml. of water, 25 ml. of H3PO4, 5 ml. of ferric iron solution, plus 5 drops of diphenylamine sulfonic acid solution to a 1-liter conical flask. Cap with a 3-holed rubber stopper and arrange to pass pure nitrogen gas a t a rate of about 500 ml. per minute into 1) the flask. Drain the HCl (1 in the reservoir of the lead reductor to the level of the granular lead. Then insert the stem of the reductor into the flask through the appropriate hole in the rubber stopper. Keep the nitrogen flowing through the flask throughout the reduction and titration of the tungsten. Reduction and Titration of Tungsten. Obtain up to 50 mg. of sexivalent tungsten in solution in the presence of a small excess of alkali (ca. 0.4 gram of KOH). Dilute to 25 ml. in a 125-ml. conical flask. Add 8 grams of NHICl (or 5 grams of NH4Cl if less than 25 mg. of tungsten is present) and a few crystals of S i c (as antibump). Cover and heat nearly to boiling to dissolve the salt. Add 25
+
ml. of HC1 and heat to fairly vigorous boiling. Add about 10 grams of pure, unscreened, granular lead (test lead). Cover and boil vigorously for 2 minutes. Decant the hot solution to the reservoir of the reductor. Without delay, rinse the flask and lead and decant to the reservoir with 3 successive &ml. portions of concentrated HC1. Drain the solution through the reductor a t a rate of about 20 ml. per minute. When the solution has drained so that its level reaches the lead, wash down the inside wall of the reservoir with about 10 ml. of HCl (1 l), added from a wash bottle, and again allow to drain to the lead. Repeat this wash with two successive 15-mL portions of water and 1). finally with 15 ml. of HC1 (1 When the level of the latter solution has reached the lead withdraw the stem of the reductor and add a small drop (ca. 0.05 ml.) of ferrous iron solution (0.05N) through one of the holes in the rubber stopper. Then insert a 25-ml. buret and titrate the warm solution immediately with 0.05N K2Crz0, solution (or 0.01N solution if less than 5 mg. of tungsten is present) until a definite lavender-purple color that persists for a t least 15 seconds is obtained. Correct for the indicator blank. To determine this blank dissolve 8 or 5 grams of NHhC1 in 50 ml. of HC1 (1 1) and carry through the reduction and titration as directed above using KzCr20, solution of the appropriate concentration. Determination of Tungsten in Tungsten-Nickel Alloys. Transfer 0.1000 gram of the sample t o a 125ml. conical flask. Add 2 ml. of "Os, cover, and heat gently until dissolution of the sample is complete. Evaporate to dryness on a low temperature hot plate. Cool and add 5 ml. of HC1. Cover and heat. to dissolve salts from the flask walls and to expel most of the brown fumes. Add 1 ml. of formic acid and continue to heat until the HNOa has been destroyed and spraying ceases. Remove the cover and evaporate just to dryness on a low temperature plate. Toward the end of this evaporation reduce tlie applied heat so as to avoid loss of the sample by splattering. Finally blow out the last traces of HCl from within the flask. Wash down the inside of the flask with 20 ml. of water and heat to dissolve nickel salts. Add 1 small drop of a 1% ethanolic solution of phenolphthalein and then individual pellets of KOH, with swirling, until the pink color of the indicator persists. Then add 1 pellet in excess. [In all, 4 pellets of KOH (ca. 0.35 gram) will be required and once the required amount has been established the addition of phenolphthalein can be omitted.] Heat carefully to boiling on a flame for 15 to 30 seconds while swirling to ensure complete dissolution of the WOa by the alkali. Add 2 ml. of H F (1 9) from a plastic graduate, swirl, and then wash down the inside of the flask with 3 ml. of HC1 (1 9). Swirl and heat to dissolve all of the Ni(OH)2 both in the solution and on the flask wall. Ignore traces of insoluble salts of manganese, cobalt, or
+
+
+
-
+
+
aluminum. Cool immediately to room temperature in a cold water bath. Without delay, add 5 grams of cation exchange resin and allow to stand with occasional swirling for 5 minutes. Then decant the solution as completely as possible to the reservoir of the ion exchange column, retaining as much of the resin in the flask as possible. Allow the solution to pass through the column at a rate of about 10 ml. per minute into a 200-ml. conical flask. Wash down the 125-ml. flask with about 5 ml. of water, swirl, and decant to the reservoir just as the level of the solution therein reaches the resin bed. Drain to the resin, wash down the reservoir with water, and again drain to the resin. Then continue to wash-decant the resin in the 125-ml. flask with 5-ml. portions of water until the total volume of solution collected in the 200-ml. flask amounts to about 60 ml. Stop the flow, discard the resin in the 125-ml. flask and add 1 drop of phenolphthalein solution to the effluent. Add individual pellets of KOH, with swirling, until the pink color of the indicator persists and then add 5 pellets (ca. 0.4 gram) in excess. Heat to gentle boiling for 30 seconds to coagulate the precipitate and then filter the hot solution through a 9-cm. medium porosity paper (e.g. No. 40 Whatman). Wash the flask twice and the paper once with water, allowing to drain completely between washings.. Discard the precipitate. Add S i c and boil to a volume of 25 ml. Add 5 grams of NH4C1and then reduce and titrate the tungsten as directed above using 0.01N K2Cr207. DISCUSSION
Volumetric Determination of Tungsten. Metallic lead is a weak reducing agent ordinarily, but in the presence of HC1 its reducing power is increased, presumably as a result of a shift in the electrode potential because of the complexing action of the chloride ions on the dissolved lead. Experiments in the reduction of tungsten have shown that about 97% reduction can be obtained in HC1 (2 1) by boiling with granular lead and then passing the hot solution through a lead reductor. However, if a sufficient amount of a salt such as NH4C1, KC1, or KBr is added to increase the halide concentration in the solution the reduction can be made 99% complete. The reduction is more readily accomplished the smaller the amount of tungsten present and the greater the amount of lead surface available. In the proposed procedure the addition of 5 grams of NH4C1 makes it possible to obtain 99% reduction of as much as 25 mg. of tungsten, and, by using 8 grams of the salt, as much as 50 mg. of tungsten can be handled. However, increasing the acid or salt concentration, time of reduction, or lead surface over that recommended in the proposed
+
procedure did not lead to higher than 99% reduction. When less than 5 mg. of tungsten is should to be determined 0.01N KZCr207 be used in the titration. Because of the fleeting nature of the end point obtained with this dilute solution it is best to make a separate calibration against standard tungsten solution rather than to depend upon volumetric dilution of a standardized 0.05N K&2rzO, solution. When 8 grams of NH4Cl is used in the redaction, difficulty may occasionally be encountered due to precipitation of salts in the lead reductor. This can be 1) prevented by passing hot HCl (1 through the reductor just before use. An easier way to eliminate the trouble, when less than 50 mg. of tungsten is present, is to reduce the amount of NH4C1used. When a newly prepared lead reductor is first used, the reduction of a given amount of tungsten is not quite as complete as i t is in subsequent analyses. Apparently the etching of the lead by the hot HC1 results in a greater area of surface available for reduction. However, after this etching has been accomplished no further increase in the per cent reduction occurs. In view of this it is recommended that the lead in a newly prepared reductor be treated with hot HCl-NH4Cl solution before it is used. When the acidity is high and large amounts of NH4Cl. KCl, or KBr are present, the dichromate-diphenylamine sulfonate titration of trivalent tungsten (7) cannot be used because the indicator is irreversibly oxidized. Phenanthroline is not destroyed under such conditions and hence the ferrous iron produced in the titration of the trivalent tungsten can be successfully titrated by the ceric sulfate-phenanthroline method (6). Unfortunately, this method cannot be used when phosphoric acid is employed to prevent precipitation of sexivalent tungsten and to bleach the yellow color of the ferric chloride. During the titration a white precipitate appears a t the point of contact of the ceric sulfate and the phosphate-containing solution. This ceric phosphate compound tends to redissolve fairly well if the oxidant is added rapidly, and hence little or no error is made in the titration of ferrous iron. However, if the titration is deliberate the resolution of the precipitate is incomplete and very high results in the titration can be obtained. By using as little acid and salt as possible and by titrating in a large volume i t is possible to prevent the destruction of the diphenylamine sulfonate indicator provided that bromide is absent. To obtain a satisfactory end point in the titration in strong HCl solution it is essential that some ferrous
+
iron be present and that sufficient H J P O ~be added to complex the ferric ion produced. Analysis of Tungsten-Nickel Alloys. Perhaps the most convenient way to determine tungsten in the tungstennickel alloys used in the manufacture of vacuum tubes is to analyze by a photometric method (4) or by an x-ray fluorescence method. To use the latter method, however, i t is necessary to calibrate against standards that have been analyzed by some chemical method. Because of this and because x-ray techniques for the analysis of powdered and granular samples are not entirely satisfactory, a good chemical method must be available. The gravimetric method can be used but is time consuming, especially when a double precipitation of the tungsten is required ( 3 ) . For this reason it has seemed worthwhile to investigate the possibility of adapting the lead reduction-dichromate method to the analysis of tungsten-nickel alloys. Attempts were made to reduce and titrate the tungsten in the presence of the nickel after first removing iron, aluminum, chromium, and titanium with ammonium hydroxide. However, satisfactory results could not be obtained because of difficulties resulting from the tendency for the nickel to be deposited on the lead during the reduction. This deposition occurs in strong HC1 solution because the position of the electrode potentials of lead and nickel are reversed because of complex formation. When deposition of nickel occurs, the effective reducing power of the lead is lowered as a result of the lower hydrogen overvoltage of the deposited nickel and because of the depletion of the available reducing surface. Moreover, large amounts of lead dissolve and subsequently precipitate as phosphate, thus obscuring the end point. Because of these difficulties it is necessary to remove all the nickel before attempting the reduction of the tungsten. In contrast to nickel, cobalt does not appear to be deposited on the lead, in any appreciable amounts, during the reduction. Several methods of removing the nickel were tested but abandoned, for various reasons, in favor of an ion exchange separation similar to the one proposed by Black and Bonfiglio (6). If sufficient H F is added to convert the tungsten into a soluble complex, no loss of this metal occurs during the removal of the nickeI on the ion exchange resin. Neither citric acid nor tartaric acid can be used to complex the tungsten because high results in the tungsten titration are invariably obtained when these organic acids are used. Apparently some of the organic VOL 33, NO. 10, SEPTEMBER 1961
1367
material present can be oxidized by dichromate. Metals such as vanadium, molybdenum, chromium, iron, titanium, and
Table 1.
Volumetric Determination of Tungsten
Milligrams of Tungsten Added Found 1.oo 2.00 5.00 10.0 15.0 20.0 30.0 40.0 50.0
Table
11.
1.02, 1.01 1.99, 2.00 5 . 0 0 , 5.00 1 0 . 0 , 10.0 15.0 , 15.0 2 0 . 0 20.0 30.0 , 3 0 . 1 40.0 , 4 0 . 1 50.0 , 50.0
.
Investigation of Interference
Metals Added, Mg.
Possible
Tungsten, Mg. Added Found 1.00
MA: i
2.00 5.00 5.00 5.00
0.99 2.01 5.00 4.99 5.02
AI, 1
5.00
5.00
2.00
5.00 5.00 2.00 2.00 4.65 1.86 1.98
Mgj 1
co, 5 cu, 5
Ti, 1 Ti. 0.1 Fe, 1 Cr. 1
Cr; i Cr, 0 . 1
5.00 5.00 5.00 2.00 5.00
2.00 2.00
5.00
copper must be absent a t the time of the reduction and titration of tungsten since they would cause high results. The first two metals mentioned are not found in nickel, but traces of chromium and small amounts of iron, titanium, and copper are usully present. Nickcl and most of the other cationic metals present in tungsten-nickel alloys are virtually quantitatively removed by the resin under the conditions specified in the recommended method. However, titanium is strongly complexed by fluoride and accompanies the tungsten. Moreover, part of any aluminum and chromium that is present in the sample
1368
ANALYTICAL CHEMISTRY
may also be found in the effluent from the resin separation. For this reason i t is necessary to resort to a subsequent hydroxide separation to remove titanium and chromium before proceeding. Occlusion of tungsten in the hydroxide precipitate is negligible; nor is tungsten occluded to any extent in the traces of insoluble aluminum fluoride or basic compounds of cobalt or manganese that may remain after dissolving the Ni(0H)Z in HF plus HC1 just before the ion exchange separation. Of the metals normally encountered in nickel alloys, the only one that causes serious trouble in the proposed mcthod is chromium. If more than traces of this metal are present, low recoveries of tungsten are encountered (Tables I1 and 111). This may be due to the formation of a complex chromium-tungsten cation which goes onto the resin. Complexing the tungsten with fluoride tends to decrease the deleterious effect of the chromium, but no method has been found for completely eliminating this interference. Fortunately nickel usually contains only traces of this metal.
Table 111.
Tungsten Added, Mg.
Cr addcd
added
1.00 2.00 4.00 5.00
0.96 I .90 3.91 4.90
1.01 1.98 3.98 4.98
Various aliquots of standard tungsten solution (2 mg. of W per ml.) were diluted to 25 ml., 5 or 8 grams of NH&1 was added, and the tungsten was then reduced and titrated. In the titration of the 1-, 2-, and 5-mg. samples, 0.01N K2Cr20T was used. The results are shown in Table I. Various aliquots of standard tungsten solution, 0.1 gram of nickel as chloride, plus, in some instances, small amounts of the chloride of a metal normally present as an impurity in nickel were evaporated to dryness and the mixtures were then analyzed for tungsten. The results obtained are shown in Table 11. Various aliquots of standard tungsten solution plus 1 ml. of ”03, 0.1 gram of Ni, 2 mg. of Co, and 0.1 mg. of Cu, Al, Ti, Fe, Mn, Mg, and Cr (all nine metals as chlorides) were evaporated to dryness. Nitrates were destroyed with HC1 plus formic acid, and the amount of tungsten in each sample was then determined, The analyses were repeated except that no chromium was added to the samples. The results obtained are shown in Table 111.
Tungsten Found, hlg. No Cr
Tungsten-nickel alloys of varying tungsten content were prepared in these laboratories from pure tungsten and pure nickel. The samples were analyzed by the proposed method and also by an x-ray mcthod that had been calibrated using standards whose tungsten content had been detcrmined gravimetrically. The results obtained are shown in Table IV.
Table IV.
Sample EXPERIMENTAL
Analysis of Synthetic Mixtures
No. 1 2 3 4 5 6
Analysis of Tungsten-Nickel Alloys
Tungsten Found, yo Proposed XXjmethod method 0.49 0.98 1.49 2.00 2.44 2.94
0.47 0.98 1.49 2.00 2.45 2.94
LITERATURE CITED
(1) Am. SOC. Testing Materials, Phil-
adelphia, Pa., “AYTM Methods of Chemical Analysis of Metals,” p. 16
(1956). ( 2 j Zbid., p. 323. (3) Ibid., p. 304 (1960). (4) Andrew, T. R., Gentry, C. H. R., Metallurgia 60, 175 ( 1959). (5) Black, H., Bonfiglio, J. D., ANAL. CHEM. 33,431 (1961). (6) Geyer, R., Heme, G., 2. anal. Chem. 172,409 (1960).
(7) Holt, M. L., Gray, A. G., IND.ENG. CHEM.,ANAL.ED.12,144 (1940). (8) Someya, K., 2. anorg. u. allgem. Chem. 145,168 (1925).
RECEIVEDfor review January 16, 1961.
Accepted July 10, 1961
