Table Vlll. Recovery of Nitrate and Nitrite Xitrate K,pg. Sitrite N, Gg. Taken Found Taken Found-’ 10.0 9.88 =I= 0.09 2.00 1.86 zk 0.02 10.0 9.90 f: 0.07 6.00 5.86 i 0.03 10.00 9.88 zk 0.16 10.0 9.92 f 0.07
than 1% of the nitrite at any level. The explanation for this effect is not yet clear but may be due to an increased stal~ilityof nitrite by formation of N204:
terminations by nitration and nitrosation using the usual techniques of linear simultaneous equations to determine each component. The required absorptivity at 324 mp for nitrosoxylenol and 307 mp for nitroxylenol were determined by reaction of nitrite or nitrate alone. The 4: 4 : 1:1 H2S04H3PO~-H20-HOAc mixture has been shown previously (8) to be equivalent to the 5 : 4 : 1 H2S04-H20-HOdc mixture with respect to nitration yields and reproducibility. The sole difference between the two mixtures is a decrease in reagent stability in the latter. Determination by this method is less precise than may be desirable because of accumulated errors in absorbances a t 324 and 3 0 i mp. The reproducibility of nitrosation in the presence of nitrate is clearly better than for nitrite alone (compare Table VI1 and Table V). This effect is evident even when the nitrate concentration is reduced to less
NO+
+ NOa- s
3 2 0 4
(9)
I n any event this phenomenon allows a greater precision in nitrite determination by deliberate addition of nitrate at concentration levels which produce undetectable quantities of 4-nitro-2,6xylenol , A greater precision may be attained by a dual procedure in which both nitrate and nitrite are determined as above and nitrate subsequently determined alone from B second aliquot treated according to the nitrate procedure (8). The results attainable under these conditions are shown in Table VIII. The data of Figure 2 and the corresponding data of Figure 2 of the
previous paper (7) indicate the possibility of determining nitrite in the presence of nitrate by reaction in 4:5: 1 solvent in which nitration proceeds but slowly. Inspection of Figure 2 shows this to be a make-shift procedure at best since the yield of nitrosoxylenol is low. I n this regard it is re-emphasized that higher acidities can be tolerated because the interference of the consecutive reaction to produce indophenol is moderately slow while the oxidation to produce diphenoquinonr is destructive and competitive. LITERATURE CITED
(1) Abeledo, C. A., Kolthoff, I. >I., J. Bm. ChenL. SOC.53, 2893 (1931). ( 2 ) Auwers, K. von, Markowitz, T., Ber. 41, 2332 (1908). (3) Bayliss, N. S.,lV.’atts, D. W., Australian J . Chem. 9, 319 (1956). (4) Boltz, D. F., “Colorimetric Determination of Sonmetals,” Vol. 111, pp. 129-31, Interscience, New York, 1958 (5) Detroit, IT7.J., Hart, H., J d m Cheni. SOC.74, 5215 (1952). (6) Fieser, L. F., J . Am. Chern. SOC.50, 439 (1928). ( 7 ) Hartley, A. RI., Asai, R. I., ANAL. CHEM.35, 1207 (1963). (8) Hartley, A. Rl., Asai, R. I., J . d m . Water W o r k s Assoc. 52, 255 (1960). (9) Sabato, A., Bol. inform. petrol. Buenos Awes 27, 59 (1950). RECEIVEDfor review April 29, 1963. Accepted June 12, 1963. This work was supported in part by a National Institutes of Health Grant RG-6495.
Dicesium Plutonium Hexachloride, a Proposed Primary Standard for Plutonium F. J. MINER, R.
P.
DeGRAZIO, and J.
T. BYRNE
Rocky Flafs Division, The Dow Chemical Co.,
b Since the discovery of plutonium in 1940, no compound of the element has been reported which completely satisfies the requirements for a primary standard, although many have been investigated and, out of necessity, used as working standards. The salt, G2PuCI8, is suggested as a primary standard for plutonium. It can b e easily prepared in a pure state and with a constant composition. It is stable, it has a high equivalent weight, and it is readily soluble in dilute acids.
I
RECENT yeari, many materials have been proposed and evaluated for use as primary analytical standards for plutonium. The material most widely used as a working standard has been metallic plutonium. The problem of obtaining metal of known purity has been overcome recently by a pluto-
N
1218
9
ANALYTICAL CHEMISTRY
GoIden, Colo.
nium metal standard, available in limited quantities from the National Bureau of Standardb, vhich is 99.97% pure. Other difficulties associated with the use of the metal as a standard haT-e been dibcussed (16). Anhydrous plutonium sulfate has been suggested, but has been considered too hygroscopic to prepare and store in a stoichiometric form ( 2 , 6). Recent n-ork has indicated that it can be prepared. under special conditions, in a form that is not so hygroscopic (17, 19). Plutonium wlfate tetrahydrate has been evaluated and i i propoied a? a primary analytical standard for plutonium (16). A number of other compounds have been found deficient: plutonium tetraiodate because i t contains coprecipitated iodic acid ( 2 , 4 ) ; hydrated plutonium fluoride because it hydrolyzes after several weeks (10); plutonium tetranitrate pentahydrate because it appears to be hygroscopic (5, 22) ; plutonium dioxide
because i t is difficult to obtain in a stoichiometric form (27) and even more difficult to dissolve ( 1 6 ) ; and organic compounds because some have uncertain compositions and are susceptible to long-term changes in composition as a result of the plutonium alpha flux (22). Anderson has described (1) the preparation of the anhydrous double salt, dicesium plutonium hexachloride. Kooi, Weisskopf, and Gruen have noted that this double salt is nonhygroscopic and have recommended i t as a starting material for production of plutonium metal ( 1 3 ) . Gruen pointed out these properties to the present authors in 1958 ( 7 ) . EXPERIMENTAL
Reagents and Apparatus. Plutonium metal, 99.8 yo purity. Average atomic weight, 239.12. Cesium chloride, American Potash a n d Chemical Corp., technical grade,
+
96% minimum assay, 0.1 t o 0.3% potassium, 0.1% sodium, 2 t o 4% rubidium. Ceric sulfate, 0.05N solution in 1N H2S04 prepared from Ce(HS04)4 and standardized against individually weighed portions of SBS standard h 2 0 3 using ferroin as indicatx (3, 26). Humidistats. Humidistats of various relative humidities were prepared by charging desiccators with saturated solutions of salts or, in one case, a sulfuric acid solution. The salts and the acid solution used znd their relative humidities a t 25' C. ale: BaC12.2H20, 90%; ("I)zSO~, 81% ; M g ( C d L d " . 4H20, 65%; ?tIg(iY03)2.6HzO, 53%; ' H2S04, 15%; LiCl-HzO, 11% 60 wt. % (14, 24). Preparation of (k2Puc16. Ten grams of plutonium is dissolved in 20 ml. of 8 N HCl contained in a 250ml. beaker. After ccmplete dissolution, and while the solution is still warm from the dissolution, 10 ml. of concentrated "03 is slowly added t o oxidize the plutonium to Pu(1V). Fifteen grams of CsCl is dissolved in 350 ml. of concentrated HC1. The plutonium solution is poured slowly and with constant stirring into the CsCl solution. The cs2Pu(&, which precipitates immediately, is separated by centrifugation and mashed three times with concentrated HCl. Washing more than three times causw the Cs2PuCls particles to disperse, producing a very fine material that will riot settle during centrifugation. After washing, the cslPUcl6 is placed in an oven a t 110' C. until dry. This procedure has bzen scaled down for use on as little as 1 gram of plutonium. For final batches containing more than 10 grams of plutonium, precipitates from several individual batches containing no more than 10 grams of plutonium are mixed together by slurrying in concentrated HCl, then centrifuged and dried.
Deviations from this srocedure which would alter the CljPu ratio can result in a nonstoichiometric compound. This is discussed below. Initial n-ork used ethanol for the final washing instead of concentrated HC1. Recently availakle ethanol, however, has contained en organic impurity that has caused high result, in the plutonium assay. Analysis of Cs&(&,. The plutonium was determined using the Rocky Flats volumetri: assay method (S), modified to include the reduction of plutonium before the titration. Either a Jones (20) or a lead (11) reductor can be used. For a lead reductor, 1.5 grams of G2PuC16is dissolved in a solution containing 28 ml. of concentrated HCl, ' 0 nil. of concentrated H2804, and 50 ml. of water and this solution is paszed through the reductor a t the rate of 25 ml. per minute, The reductor is washec with 100 ml. of 0.lN HCl and the combined solution is titrated with standa-d O.05N ceric sulfate ( 3 ) . For a Jones reductor, the C\~PiiC16is dissolved in 25 ml. of 2.V
Table 1.
Stoichiometry of CszPuCl~
Plutonium analyses Chloride analysesBatch Cs2PuClG KO.of 3 f sn; No. of 2 f SI>, No. in batch, g. aliquants 70 pu aliquants 70 c1 1 33 12 33.321 f 0.012 S o t determined 2 250 11 33.316 f 0.015 4 29.63 f 0.04 3 115 11 33.320 f 0.013 8 29.64 f 0.04 4 95 9 33.310 f 0.013 9 29.64 zk 0 03 5 330 10 33.309 f 0.008 Kot determined 6 100 11 33.324 f 0.009 Kot determined 100 12 33.321 f 0.010 S o t determined Theoretical 33.320 29.64 In all batches except 5, there is no statistical difference between mean and theoretical yo Pu at 95:; C1. In batch 5 , tlierr is B difference st both 96 and 9 9 % CI.
-
H2S04 and this solution forced through the reductor a t 25 ml. per minute. The reductor is washed with 100 ml. of 2N H2S04 and the combined solution is titrated as described ( 3 ) . The spectrophotometric end point of the titration was somewhat sharper when the Jones, rather than the lead, reductor was used. Chloride mas determined using a volumetric semimicroprocedure. -\ 200-mg. sample of CszPuCl6 is dissolved in 40 ml. of 0.2N " O B containing 5% sodium citrate dihydrate to prevent hydrolysis of the plutonium. The chloride is titrated with O,lLII Ag?;O3, standardized against specially dried KC1 (26). The end point is determined potentiometrically using either a mercury sulfate or double junction calomelsilver billet electrode pair. Prior to each titration the silver electrode mas immersed in concentrated aqueous S H s and then treated with a HXOs bolution containing N a S 0 2 ( 1 2 ) . Buoyancy corrections were applied to the weights of samples of c.2Pucl6 taken for plutonium analyses. Comparable corrections were too small to be applied to samples of Cs2PuC16taken for chloride analyses or for samples of 2%s203 or plutonium metal taken for standardizing ceric solutions. RESULTS AND DISCUSSION
Yield. When the preparation method described was used, filtrate losses were less than 1 gram per liter, which is in agreement with the reported solubility data of Cs2PuCls (13). Stoichiometry. The analytical results on seven batches of Cs2PuC16are summarized in Table I. For making the statistical calculations reported in the table, a t test n-as used, in which the experimental mean value was compared with t8hetheoretical value. The same test was used in making coniparable comparisons throughout this report. The Cl/Pu and Cs/Pu ratios of the solutions from which CspPuCls is Irecipitated were studied to see if they
Table 11. Effect of CI/Pu and Cs/Pu Mole Ratios in Solution on Stoichiometry of Precipitated CS~PUCIG
Expt . To. 1
Mole ratio in solution Cl/Pu Cs/Pu 76 c-
2
3
80
(i
-4 6
9-1.
?
6
i i
100 120
3 3
2 3
Theoret. C1 in cP21'Uc1P, ppt., "To
81.8 91.9
92.3 96.4
100.1 100.0
influenced the stoichiometry of the precipitated cs2Puc16 (Table 11). -1 CI/Pu ratio of a t least 100 to 1 is necessary in order t o obtain a stoichiometric product. This high ratio is probably required to convert the plutonium to the hexachloride complex. Electrical migration measurements indicate that in 7 5 HC1 only 93YG of the plutonium is present in an anionic form (8). This is increased to 99.7% in 1 0 X HC1. Two anionic chloride compleles are known, PuC15- and Puc16-2. It is logical that a high Cl/Pu ratio is required for the precipitation of stoichiometric Cs2PuC16, since a high concentration of chloride would be required for complete conver * I
Cs*PuCls
1
.4~203 Pu
I
(’S2PUCl@
c
n
AS&
Pu
7 5 7 5
__
J
HC1
0.2 43.4 55.2 1.2
of Ceric Sulfate
aliquanls
12
-
Normality f 81) 0 05021 i 0 00ooI
0 05019 f 0 00003 0 05039 f 0 00001 0 05040 f 0 00003
0.05046 =t0.00001 0.05049 it 0.00002 0.05049 f 0.00001 0 05989 f 0 00006 0 05981 zk 0 00010 0 05989 zk 0 00005
a In all solutions except C, there is no significant difference between normality values obtained using various standards a t Q5yoC1. In Solution C. a signifimnt difference exists at 9.59‘ C1 betheen nurniality values obtained using CssPuCls and As20s and 9gCh C1 between normality values obtained using CsnPuCls and Pu.
1222
0
ANALYTICAL CHEMISTRY
(1) Anderson, H. H., “The Transuraniuni Elements,” Natl. Nuclear Energv Series, Div. IV, Vol. 14B, p. 793, McGrawHill, S e w York, 1949 (2) Bvrne, J. T., Pflug, J. L., Rocky
Flats Division, Dow Chemical Co., unpuhlished data, 195i. (3) Caldwell! C. E., Grill, L. F., Kurta, R. G., Miner, F. J., Moody, N. E., AIiAL. CHEhl. 34, 346, 859 (1962). (4) Dawson, J. R., Elliott, R. M., Brit. At. Energy Research Establishment, Rept. AERE C/R 1207 (1953). ( 5 ) Drummond, J. L., Welch, G. A . , J . Chem. Soc. 1956, 2565. (6) Ibid., 1958, 3218. (7) Gruen, D. RI., Argonne Xational Laboratory, Argonne, Ill., private communication, 1958 18) Hind,man, ,J. C., “The Artinidc E ~ P inents, Katl. Nuclear Energy Series, IXv. IV, 1-01, 14‘4, p 337, MrGrawHill, Xem Tork, 1954. ( 0 ) Johnson, A J , Vejvoda, E., .4h 41’. CHEM31, 1643 (1959). (10) Jones, JI. kl., U. S. At. Energy Comm , Rept. HW 30384 (1953). (11) Kolthoff’, I. >I., Belcher, R , “Volunietxic Anal) SS,” 5’01. 111, p. 16, Interscience, N r ~ r1 ork, 1957. (12) Kolthofi, I. AI , Fumlan, X. H., “Potentiometric Titrations,” 2nd ed., p. 145, Wiley, Sew York, 1947. (13) Kooi, J., Weisskopf, E.,Gruen, D. M., J . Inorq. Sml. Chem. 13, 310 (1960). (14) Lange, S . A , , ed., “Handbook of Chemistrj ,” 9th rd., p. 1420, McGra\vHill, S e w York, 1956. (15) I’cnneinnn, R. A , , Kwnan, T. K.,
S. At. Energy Comm., Rept. NAS-NS 3006 (1960). (16) Pietri, C. E., ANAL.CHEY.34, 1604 (1962). (17) Pietri, C. E., TJ. S. At. Energy Comm., Rept. NBL 170, 5 (1961). (18) Ibid., NBL 177, 22 (1962). (19) Ibid., NBL 188, 3’3, (1962). (20) Rodden, C. J., ed., “Analytical Chemistry of the Manhattan Project,” Natl. Nuclear Energy Series, Div. VIII, Vol. 1, p. 57, McGrsw-Hill, Xew York, 1950. (21) Soklina, L. P., Gel’man, A. D., U.
Russian J . Inorg. Chem. 5 , 487 (1960) (Engl. trans. by The Chemical Society). (22) Staritzky, E., ANAL.CHEX.28, 2021 (1956). (23) Staritzky, E., Truitt, -4. L., “The Actinide Elements,” Xatl. Tuclear Energy Series, Div. IV, Vol. 14A, p. 813, McGraw-Hill, New York, 1954. (24) Stokes, R. H., Robinson, R. A , , I n d . Eng. Chem. 41, 2013 (1949). (25) Vogel, A. L:, “Textbook of Quantitative Inorganic Analysis,” 2nd ed., p. 250, Longmans, Green, London, 1951.
(26) Ibad., p. 304. (27) Waterbury, G. R., Douglass, R. XI., Metz, C. F., ANAL.CHERI.33, 1018 (1961). (28) Zachariasen, IT. H., Acta Crust. 1, 268 (1948). RECEIVEDfor review October 22, 1963. Accepted Mav 22, 1963. Work done under AEC Contract AT(29-1)-1106. Presented in part at the Sixth Conference on Analytical Chemistry in Nuclear Reartor Technology, Gatlinburg, Tenn., Oct. 9-11, 1062.
Acid-Base Equilibria in Tertiary Butyl Alcohol LELAND MARPLE and JAMES S. FRITZ Institute for Atomic Research and Department of Chemistry, Iowa State University, Ames, Iowa
b The dissociation of acids in tertiary butyl alcohol has been studied b y potentiometric, spectrophotometric, and conductimetric methods. Values for the over-all dissociation of perchloric and picric acids arid several tetrabutylammonium salts were estimated by the Fuoss-Kraus -reatment of conductance data. Potentiometric studies were carried out (at constant ionic strength to minimize activity coefficient variations. An acidity scale was established from potentiometric measurements a t a glas,s electrode and conductance values of dissociation constants. A method was developed for the evaluation of the over-all dissociation constant of weak acids using potentiometric data for hydrogen ion activities and conductance data for the corresponding anion activities. Over-all dissociation constants are reported for perchloric acid, picric acid, 2,4-dinitrophenol, and benzoic acid. Apparent dissociation constants from potentiometric measurements a t a constant ionic strength were determined for hydrotiromic, nitric, hydrochloric, picric, and p-toluenesulfonic acids.
T
ERTIARY BUTYL ALCOHOL has been shown to be a c r y useful solvent for the titration of weak acids with tetrabutylanimoniuni hydroxide (14). This solvent is espwially useful for differentiating titrations involving carboxylic acids and phenols because of the flat slope in the buffer region of the titration curve. I n solvents such as acetone, acetonitrile, and pyridine, the titration curves have 2, steep slope in the buffered region, owilig to tJhe interniolecular association of the acid anion with the free acid (4, 10, 16, 17). Evidently, this association is not appreciable in tertiary butyl alcohol.
Because of its excellent solvent characteristics, fundamental information on acid-base equilibria would be very useful. Taking into account previous investigations of acid-base equilibria in alcoholic solvents ( I , 2, 11-13, 15) and solvents of low dielectric constant (6, a),we felt that the main reactions in\ olved in the dissociation of an acid HX are: HX 8 H+X- (ion-pair formation); H+X- e H + X- (ionpair dissociation). The corresponding equilibrium constants are: K,= (H+ X X-)/(HX); K,j = (H+)(X-)/ (H+X-). The formation of ion pairs in tertiary butyl alcohol is undoubtedly significant (e), but probably not extensive. The over-all dissociation of strong electrolytes in dilute solutions (10-3M to 10-5M) was expected to be large on the basis of titration data. Since i t is convenient to interpret potentiometric data in terms of the over-all dissociation of a species, we adopted the use of the over-all constant for our work. This constant is defined (9) for a n acid as K H X = (H+)(X-)j (HX) (H+X-). I n the case of very low dissociation of the solute, (HX) (H+X-) will be very nearly equal to CEX (CHXis the analytical concentration of solute). T h r plan of research was to determine KH\ for 1-arious acids in trrtiary butyl alcohol by potrntiomrtric, bpectroor conductometric l)hotometric, methods. K e nerc able to apply the method of Fuoss and Kraus (5) in the determination of dissociation conitants from conductance data. I’nfortunately, conductance measurements for solutions of weak acids could not be obtained because the solutions were not sufficiently conductive. Potentiometric measurement was at first believed to be the best method for evaluation of dissociation constantb of weak acids. However, K H X values
+
+
+
varied with CHX unless a fairly high (lO-*M) concentration of tetrabutylammonium perchlorate (Bu4SC104) was added to keep the ionic strength constant. This was a n indication that the activity coefficients of the ionic species are markedly dependent upon the ionic strength. Subsequent analysis of conductance data did show that activity coefficients varied even at very low (10-5AU)concentrations of solute. While the addition of a large amount of Bu4NC104 does keep the activity coefficients constant during the potentiometric measurement of acidity of a weak acid, it complicates the simple equilibrium of HX as shown below
+
HX*H+ XTJ ClOi- TJ BuiK HClOi Bu4SX
(11
+
(For h p l i c i t y , ion-pair formi of solutes mill be omitted from all subsequent discussion.) 4 modified constant, K’Rx, can be applied to this equilibrium K‘HX= { [H+] (HClOa)) X { fX-1 (BurNX)J/(HX) (2)
+
+
Here, [H I] is the actual concentration of hydrogen ions (solvated with water) in solution (and not the activity). For a solution of HX in excess 13uciYC104,thr. expression for K”X bcromcb KlHX = [ [H+] (HC10,) ] ?, (HX). Values of the quantity ([H’j (HCIOJ) arc determined from c.m.f. mcawrcmeiitb by reference to the concentration-e.m.f. curve for perchloric acid. K”x may alao be evaluated from spectral measurements of the quantity { [X-] (BUISX) } . Another expression for the over-all constant KHy can be obtained by substitution of the equations IH+l = ( W + I (HClOa)l KHCQI/IKacio, ( S t H + ) (ClOi-)l ( 3 )
+
+
+
+
VOL. 35,
NO. 9, AUGUST
+
1963
1223
