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.
Percentage Composition of Tetraphenylarsonium Hexafluorophosphate Percentage Carbon Hydrogen Phosphorus Arsenic Fluorine Determined 54.37 3.89 6.01 13.95 21.80 Calculatedo 54.56 3.82 5.86 14.18 21 -58 From the formula (C~HE.)&PF~.
Table 1.
Table 11.
Gravimetric Determination of Hexafluorophosphate as (CeHs)AsPFa (a) Determination with no interferencee present,. Wt. of KPFa, No. of
AV. % w; Std. dev. gram detns. 6 0.05263 78.93 0.18 5 0.05441 78.82 0.07 0.03682 79.66 8 0.32 ( b ) Determinations all on solution 3 with various interfering ions present. Each sample contained 0.03682 gram of KPF6. Wt. KF, Wt. NazPOSF, Wt. Pu’asPO,, No. of granis gram gram detns. Av. % Std. dev. 0.1507 ... ... 4 79.29 0.06 1.3218 ... 4 79.44 0.18 0.2644 0.3732 0.66 6 79.57 0.21 Av. of all 14 samples 79.45 3fr 0.20 % PF;, theoretical % ’ = 78.76.
Solution 1 2 3
-
WAVELENCTH
Mp
Figure 1.
Absorption spectrum of a 2 X 1 0 3 4 solutiorm of tetraphenylarsonium hexafluorophosphate
for abiorbance meaiurements throughout the Concentration range of IO+ to 10-GM as this was Found to be approximately the cor centration of a saturated solution of the hexafluorophosphate salt. A stock solution of tetraphenylarsonium chloride, which had been standardized by the method of Willard and Smith (4),was used to prepare a series of standards which co Y-ered the concentration rangc 2 X to 5 X 10-6M to test the obedienc: to Beer’s law. The absorbance of each of these solutions was measured a t 220 mp on a Beckman DU quartz spectrophotometer, equipped with a photomultiplier tube detector, and when plotted against concentration gave a straight line. From a least, squares analysis of the data the absorbtiy-ity was found to be 35,700 liter/mole-crn. with a standard deviation of 220 liter/mole-cm. An estimate of the solubility of tetraphenylarsonium hexafluorop‘iosphate was obtained by measuring the absorbances of several saturated solu1,ions. The tetraphenylarsonium hexafluorophosphate used for the solubility determinations was dried a t 105’ C , 2nd the solubility was found to be 1.22 X 10-6 moles per liter with a standard deviation of 0.10 x IO-’ moles per liter at 20’ f 0.1’ C. Procedure. -4n aliquot of potassium hexafluorophoqphate solution containing 36 to 55 rng. of the salt is taken. The solution is made basic with ammonium hydroxide. Final concentrations of tmmonium hydroyide in the range 5 to 11M were used succeLsfully in quantitative determinations; howe~w.,qiinlitative tests indicate that solutions a$ dilute as 1.5M will flocculate the precipitate satisfactorily. The volume of solution should be approxirna1,ely 50 ml The solution is then warmed to near 50’ C. and about twice the equivalent amount of 0.01 5-11‘ trtraphenylzrsonium chloride is added slowly with stirring After standing for approximately 30 minutes the precipitate is fi1tcri:d with a sintered glass crucible of medium porosity and washed with 50 ml. of dilute ammonium
Table lil. Comparison of Results for the Nitron, Amperometric, and Gravimetric with Tetraphenylarsonium Chloride Procedures for Hexafluorophosphate Ion
Av. 97,
Method Grav. Nitron Te trapheqylarsonium chloride Amperometric Gravimetric Theoretical
PF, No. of found detns. 79.34 4
Std. dev. 0.15
z9.69 19.53 78 * 76
0.55 0.26
11 22
hydroxide using 5- to 10-ml. portions for each wash. The precipitate is dried to constant weight a t 105’ to 115’ C. and weighed as (C6H5)48sPFg. RESULTS AND DISCUSSION
Three stock solutions of potassium hexafluorophosphate were prepared and aliquots of these solutions were analyzed. Solutions numbered 1 and 2 were made from the same batch of recrystallized potassium hexafluorophosphate and solutions 3 from a different batch. Table IIa contains the result8 obtained by the analysis of these solutions when no interfering ions were present. Solution 3 was analyzed gravimetrically with Nitron and amperometrically with tetraphenylarsonium chloride. The Nitron and amperomctric procedures used were described earlier (1). A comparison of the results obtained with the three methods is made in Table 111. Any anion which forms a precipitate with tetraphenylarsoniuin chloride will interfere with the determination: typical examples are permanganate, perrhenate, perchlorate, bromide, and iodide. The most likely occurring ions
in hexafluorophosphate salts are: phosphate, monofluorophosphate, difluorophosphate, and fluoride. Of these ions only the difluorophosphate ill give a precipitate with the tetraphenylarsonium ion, and as shown in Table IIb none of the others will interfere with the determination. However, the interference of the difluorophosphate ion is not serious as White has shown that its interference can be eliminated by making the solut,ion basic and boiling for a few minutes whereupon the difluoro- is hydrolyzed to the monofluorophosphate ion which does not interfere (3). Interference studies wcre made by analyzing aliquots of solution 3 to which varying amounts of potassium fluoride, sodium monofluorophosphate, and sodium phosphate had been added. The results are given in Table IIb. A large molar excess of these salts did not result in any interference with the determination. A very easy and straightforward gravimetric procedure for hexafluorophosphate has been described. Eight samples can be run from start to finish This method is in 6 to 8 hours. faster for large numbers of samples than the amperometric method and the tetraphenylarsonium chloride is a much simpler reagent to use than Nitron, since it is stable and its solutions can be kept for indefinite periods of time. LITERATURE CITED
(1) Affsprung, II. E., Archer, V. S., ANAL. CHEM;35, 976 (1963). ^. W., Muller, E., Bcr. 63, 1058 ( 2) Lange,
(l9W).
(3) White, Wayne E., “Encyclopedia of Chemical Technology,,’ Vol. 6, p. 716, Interscience EncvcloDedia. Inc.. New York, 1951. “
1
I
(4) Willard, 13. A,, Smith, G. M., IND. ENG.CHEM.,ANAL.ED. 11, 186 (1939). RECEIVEDfor review June 28, 1963. Accepted August 9, 19ti3. Work supported by the National Science Foundation. VOL. 35, N O . 12, NOVEMBER 1963
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