RESULTS
Table 111.
Recovery of Hydrogen Spikes
Hydrogen, ml. (S.T.P.) ReSodium, Total covery, % gram Added founda 0.294 0.465 0.463 100 0.129 0.302 0.316 105 0.121 0.389 0.401 103 0.170 0 550 0.536 97 0.109 0.565 0.603 107 0.340 97 0.056 0.349 0.021 0.177 0.1iO 96 0.020 0.492 0.496 101 0.254 100 0.010 0.254 0.020 0.649 0.653 101 0.027 1.020 1.031 101 0.793 99 0.021 0.799 Corrected for blank of 0.020 ml. 5
period of 10 minutes, and for periods less than this it may have been an overcorrection. However, by neglecting the blank completely, it was possible to show that equilibrium was not attained a t 3 minutes and that not less than 5 minutes heating at 420’ C. was needed to establish equilibrium. Consequently a heating period of 10 minutes at 420’ C. was adapted for all further experiments.
The results of the recovery experiments are given in Table 111. The volume of hydrogen in the sample was calculated from the usual formula: V = Vo(al/ai - 1) - b where V , VO,and b are the volumes of hydrogen in sample, tritiated hydrogen introduced into the ampoule, and apparatus bIank, respectiveIy; al and a2 are the initial and final specific activities of the tritiated hydrogen, The satisfactory agreement between the volumes of hydrogen recovered as determined by this technique with those initially added show that little, if any, isotopic fractionation occurs during the equilibration and gas phase sampling processes. The mean and standard deviation of the recovery experiments was 101 =t 3%. From Table I1 it is seen that the blank was 0.020 ml. and the standard deviation of a single determination was +0.006 ml. Consequently, it is possible to determine 0.040 nil. of hydrogen with a relative error of 1591,. Therefore, if sufficient care is taken when sampling the metal it is possible, using 2-gram samples, to determine the hydrogen in
metal down to the Bp.p.m. level with a relative error of approximately 15%. The experiments on the determination of the equilibration time showed that the temperature could be reduced from the 460’ C., recommended by Holt, to 420’ C. without exceeding the heating period of 4 to 12 minutes, which he found to be satisfactory. The possibility of using an even lower equilibration temperature and possibly reducing the apparatus blank without significantly increasing the time was not investigated. LITERATURE CITED
(1) Evans, C. Herrington, J., Proceedings of Internadional Atomic Energy Conference on the Use of Radioisotopes in the Physical Sciences and Industry, Copenhagen, Paper R.I.C.C. 39, September 1960. (2) “Handbook of Chemistry and Physica,” Chemical Rubber Publishing Co., Cleveland, Ohio, 1958-59. (3) Herold, A., Compt. Rend. 228, 686-8 (1949). (4) Holt, B. D., ANAL. CHEM.31, 51 (19.59).
(51 Wilhams, D. D. et al., U. S. National Research Lab. Memo No. 424 (1955). (6) Wilson, E. J., Evans, C.,U. K. A.E.A. Harwell Report I/M 31, 1954.
RECEIVED for review December 12, 1062. Accepted July 18, 1963.
Fluorometric Determination of Tungsten with FIavanoI RUDOLPH
S. BOTTEI
and
B.
AMBROSE TRUSK’
Department of Chemistry and the Radiation laboratory, University o f Nofre Dame, Notre Dame, Ind.
b Flavonol produces a blue fluorescence with tungstate ion in the pH range 2.5 to 5.5. The curve of fluorescence intensity vs. tungstate concentration is linear between the limits of 6 to 42 pg. of W in 100 mi. of solution. Vanadium, iron, and chrcmium interfere, even in small concentrations, while larger amounts of nickel, cobalt, manganese, and copper can b e tolerated. Other variables have been studied and are reported. A procedure is presented for the analysis of Ni-W alloys. Results b y this method are in excellent agreement with those b y other existing methods.
T
standard method for the quantitative determination of tungsten is a tedious, gravimetric procedure which has not been changed appreciably since 1895 (3, 6). Dams and Hoste (4) have shown that a t least 0.1% of the tungsten is lost in this method. Various volumetric, colorimetric, solvent extraction, and precipita1 Brother I. Ambrose Trusk, F.S.C. HE ACCEPTED
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ANALYTICAL CHEMISTRY
tion methods, as Fell as various combinations of these, have been proposed. The authors found no record to date of any fluorometric procedure for the quantitative determination of tungsten. In their fluorometric study of the zirconium-flavanol system, Alford, Shapiro, and White ( I ) observed that while zirconium and several other metals gave a fluorescence in acid solution with flavanol, only tungsten, of the 53 ions studied, gave a fluorescence in neutral solution. This paper concerns a study of the fluorescence of the tungstate-Aavanol system and the application of that fluorescence t o the analysis of Xi-147 alloys. EXPERIMENTAL
Apparatus. Fluorescence intensity measurements were made with a Turner manual fluorometer, Model 110. A Corning filter, CS 7-60, was used as a primary filter and a combination of a Wratten CS 2A and a Corning CS 5-61 as secondary filter. The fluorescence spectra were obtained with a Beckman Model DC
spectrophotometer equipped with a Beckman 73500 fluorescence attachment. Reagents. STANDARD TUNGSTATE SOLUTION. A standard solution was prepared by dissolving 2.6779 grams of Na2WOd.2H20 (Baker Chemical, Lot 1532, assayed a t 99.8%) in 1 liter of water. Gravimetric analyses (6) confirmed the concentration to be 0.008100M, or 1.4893 grams of W per liter. This was diluted, 10 ml. to 1 liter, to prepare a solution of intermediate strength which, in turn, was diluted 100 ml. to 1 liter to prepare a working standard solution containing 1.4893 pg, of Rr per ml. FLAVANOL SOLUTION. Eastman Kodak 3-hydroxyflavone (flavanol), lot KO. 6585, was dissolved in Eastman Kodak p-dioxane. The dioxane was purified by distillation after refluxing with sodium for 24 hours. A 0.01% solution was prepared by dissolving 0.1000 gram of flavanol in 1 liter of dioxane. The solution contains about 0.43 pmoles of flavanol per ml. STOCK BUFFERSOLUTIOS.The diluting solution contains 10.0 grams of potassium biphthalate and 8.0 grams of KaC1 per liter.
-
.-2 2 0 m 0)
c
c
-
c (
c 0) c
2 10E
0
a
ii0-
i
0
I
400
I
1
500
450
I
550
X (mp)
Figure 2.
Fluorescence spectra
Solutions prepared as described under "Test Solutions" I
l-0
4.0
1
5.0
PH
Figure 1. intensity
Effect of pH on fluorescent
Test Solutions. Solutions t o be compared are prepar1.d by introducing 10 ml. of either the working standard tungstate solution, the "unknown" solution, or Piater (the blank) and 5 ml. of 0 . 0 1 7 ~ flavanol in dioxane into a 100-ml. volumetric flask and bringing: to volume with stock buffer solution. Analvsis of Ni-W' Allovs. The followihg procedure ?as bekn found t o be effective in the analysis of various Ni-W alloys. The nature and use of these alloys ha3 been described by Luke (6). Weigh out 50- to 100-ma. samdes of the ;eta1 into 200-ml. -Erlenmeyer flasks. Add 20 ml. of "03 and heat, below boiling, until ev2,poration reduces the solution to less than 1 ml. If there are any dark unreacted particles of tungsten in the yellow WOs precipitate, add more " 0 3 and repeat the evaporation. Wash down the flask with about 20 nil. of water. If necessary, heat to dissolve the nickel t,alts. Add one drop of phenolphthalsin and enough 10M NaOH to produce a pink color and then 5 ml. in excess;. With constant swirling, boil for 30 seconds over a burner, wash down the sides, and boil again. Neutralize, while still hot, with HC1 and adjust the end point so that one drop of 1N HC1 just discharges the color. Then add exactly 20 drops (or 1 ml.) of concentrated HCl. After a few minutes everything will be in solution. Allow to stand (or thermostat) until the solution is a t room temperature. Then li-ansfer it to a 1-liter volumetric flask and dilute to volume with water. .4 working standard tungstate solution of the same acirlity as the unknown must also be p-epared. This is done by treating 100 ml. of the intermediate concentrati 3n of the standard tungstate solutior with 20 drops (or 1 ml.) of HCI and diluting to 1 liter with water. T:st solutions of
both the standard and unknown solutions are then prepared according to the procedure described for test solutions. The p H of these test solutions is 3.99. In the above procedure the adaptable range is from 0.6% (100-mg. sample) to 8.5% (50-mg. sample) of tungsten. If the fluorescence of the alloy solutions is beyond the range of linearity of the calibration curve, suitable dilutions can be made and the fluorescence reread. If the fluorescence is too weak, a larger sample must be used. RESULTS
Solvent. I n the previous work with flavanol (1, 2) a 0.017, alcoholic solution was used. With tungsten the fluorescence was stronger when flavanol was dissolved in dioxane. When stored in well sealed dark bottles, this solution is satisfactorily stable for a t least three months. Effect of pH. The curve of fluorescence intensity us, p H is shown in Figure 1. The steepness of this curve emphasizes the importance of keeping all solutions, whose fluorescence is to be compared, a t the same pH. However, it is not convenient or necessary that the p H be exactly 4.2. When the various aliquots are prepared as listed under "Test Solutions," the p H is satisfactorily stabilized a t 4.08. Ionic Strength. The fluorescence intensity decreases as the salt concentration increases from zero to 0.05Y but is constant in the concentration range 0.05 to 0.5N in SaC1. The 8.0 grams of NaCl per liter in the stock buffer solution guarantees a salt concentration in excess of 0.1N. Concentration of Flavanol. K i t h a given amount of tungstate thc fluorescence intensity increases with increasing flavanol concentration until a plateau is reached. Using 5 nil. of the working standard tungstate in a
50-ml. volumetric flask, for example, the maximum fluorescence (us. a blank of equal flavanol concentration) is reached with 4 ml. of flavanol solution. Fluorescence Spectra. When solutions were prepared in stock buffer solution, the fluorescence spectrum of flavanol alone had a peak a t 490 mp and that of flavanol with tungstate had a peak a t 460 mp (see Figure 2). The Corning filter, CS 5-61, used in the secondary, transmits 66% a t 460 mp and 30% a t 490 mp so a satisfactory difference between these colors can be measured. The Wratten CS 2.1 filter was used to eliminate stray exciting light (shorter than 415 mp). Effect of Time. After 20 minutes fluorescence of the test solutions is stable for a t least 3 hours. Effect of Temperature. The fluorescence intensity of the test solutions decreases 2.57, per degree rise in temperature. Because the proposed method makes use of a standard tungstate solution for every determination, any possible temperature effect is compensated when all test solutions are a t room temperature. Calibration Curve. The curve of fluorescence intensity us. tungstate concentration is linear between the limits 6 to 42 pg. of tungsten with 5 ml. of flavanol in 100 ml. of test solution. It deviates negatively above and below these limits. Interferences. Some metals which occur in Xi-W alloys or some other tungsten alloys were studied for possible interference. S o n e was found to increase the fluorescence intensity. Nickel(I1) can be tolerated t o at least 300 X the tungsten concentration; Co(I1) to 300 X W; Mn(I1) to 100 X W; Cu(I1) to 30 X W; Mo(V1) to at least 10 X W; Fe(II1) to 0.5 X W; Cr(II1) to 0.3 X W; and vanadium suppresses seriously even as low as 0.025 X W. The only tolerable maskVOL. 35, NO. 12, NOVEMBER 1963
0
1911
I. Analyses of Ni-W Alloys psnr7: Tungsten found, % Table
No. of
FluoLuke’s X-Ray rescence method met’hod Fluorescence analmethod yses (6) (6) 0.49 0.98 1.49 2.00 2.44 2.94
0.47 0.98 1.49 2.00 2.45 2.94
0.48 f 0.04” 0.98 f 0.04 1.49 i 0.04 2.01 f0.05 2.44 f 0.04 2.93 f 0.21
6 9 6 6 6 9
Precision expressed as standard deviation. Q
ing agent was the cyanide ion and this is effective only for vanadium. No other metals were studied. Formula of the Complex. The complex formed between tungstate and flavanol, as determined by Job’s method, is in a 1 to 1 ratio. Analytical Applications. It follows from the data on interferences t h a t two types of alloys must be recognized in the analytical applications of the fluorescence of the tungstate-flavanol system. One type would include those alloys containing metals, in favorable
concentrations, which do not suppress the tungstate fluorescence. These require no separation of the metals. The analysis for one such alloy, composed of nickel and tungsten, is described in the experimental section of this paper and the results are presented in Table I. The second class of alloys does contain interfering metals, particularly iron, chromium, and vanadium. These require an isolation of the tungsten or masking of the interference prior to the fluorescence measurement. This second class of alloys is presently under intensive study and results will be published later. DISCUSSION
The advantage of the fluorometric method as appfied to Ni-W alloys is its simplicity. No tungsten can be lost in a separation process. The longest step in the procedure is the dissolution of the alloy and this is common to most wet methods. Some tungsten could be lost by incomplete dissolution of tungstic acid in the boiling NaOH solution. The alkali treatment, as outlined here, works well and is similar to that used by Luke (6).
ACKNOWLEDGMENT
The samples of Ni-W were generously furnished by C. L. Luke of the Bell Telephone Laboratories, Inc. LITERATURE CITED
(1) Alford, W. D., Shapiro, L., White, C. E., ANAL.CHEM.23, 1149 (1951). (2) Coyle, C. F., White, C. E., Ibid., 29, 1486 (1957). (3) Cremer, F., Eng. Mining J. 59, 345 (1895). (4) Dams, R., Hoste, J., Tulunfu 8, 664 (1961). ( 5 ) Hillebrand, W. F., Lundell, E. F.,
Bright, H. A., Hoffman, J. I., “Applied Inorganic Analysis,” pp. 690-3, Wiley, New York, 1953. (6) Luke, C. L., ANAL. CHEM.33, 1365 (1961).
RECEIVEDfor review July 1, 1963. Accepted August 8, 1963. Division of Analytical Chemistry, 145th Meeting, ACS, New York, N. Y., September 1963. Work sponsored in part by the National Science Foundation under a Science Faculty Fellowship to Brother Ambrose Trusk, F.S.C., and in part by the United States Atomic Energy Commission under Contract AT( 11-1)-38. The Radiation Laboratory of the University of Notre Dame is operated under contract with U. S. Atomic Energy Cornmission.
Gravimetric Determination of Hexafluorophosphate as Tetraphenylarsonium Hexafluorophosphate HAROLD E. AFFSPRUNG and VERNON S. ARCHER Deparfment o f Chemistry, The University of Oklahoma, Norman, Okla.
b A method for the gravimetric determination of hexafluorophosphate is described. The precision of the determination and the solubility of the precipitate are given. A comparison of the results with those obtained with the gravimetric Nitron (4,5-dihydro1,4-diphenyl-3,5-phenyIimino1,2,4triazole) and the amperometric tetraphenylarsonium titration procedures is made. The method is simple and direct and has no serious interferences from ions commonly occurring with the hexafluorophosphate.
T
wo METHODS for the determination of hexafluorophosphate ion have been developed : a gravimetric procedure with Nitron (4,5-dihydro-1,4-diphenyl-3,5-phenylimino- 1,2,4-triazole) as the precipitant (2), and an amperometric titration with tetraphenylarsonium chloride as the titrant (1). The amperometric titration of hexafluorophosphate ion depends upon the formation of a precipitate and the sub1912
0
ANALYTICAL CHEMISTRY
percentages. On the basis of these data and the further agreement with other determinations of the percentage of hexafluorophosphate ion, the assumed empirical formula for the salt appears to be correct. The tetraphenylarsonium ion absorbs light in the ultraviolet region because of the presence of the benzene rings in the ion. Because the absorbtivity of the benzene ring is very large in this spectral region, it appeared that a spectrophotometric method could be used to analyze for the tetraphenylarsonium ion in solubility determinations of the hexaEXPERIMENTAL fluorophosphate salt. The salt is quite insoluble and a sensitive method is Tetraphenylarsonium chloride was needed if the solubility is to be deobtained from K and K Laboratories. termined. Preliminary scans of transPotassium hexafluorophosphate, sodium mittance us. wavelength were made on monofluorophosphate. and difluorophosa Beckman DK-1 spectrophotometer phic acid were donated by the Ozarkusing a 10-6M tetraphenylarsonium Mahoning Co., Tulsa, Okla. The pochloride solution and a saturated solutassium hexafluorophosphate was retion of the hexafluorophosphate salt. crystallized by a procedure described The spectra of the two solutions were previously (1). identical in the 200- to 300-mp region. An elemental analysis of the precipiAn example is given in Figure 1 which tate gave values in good agreement with presents the spectrum of a 2 X 10-5M the theoretical values computed from of tetraphenylarsonium hexathe assumed formula ( C ~ J ~ A S P F ~ solution . fluorophosphate. The flat shoulder at The analytical data are given in Table 220 mp w w chosen as the wavelength I and are compared with the theoretical sequent reduction of excess tetraphenylarsonium ion a t the mercury drop. From the appearance of the titration curve i t appeared that the method could be adapted to a gravimetric procedure if the precipitate could be obtained suitably pure. The work described below has shown that a gravimetric determination of hexafluorophosphate with tetraphenylarsonium chloride gives very good results and that the precipitate is pure and quite insoluble.
