Complexometric Determination of Fluoride w it h Ce rium(III) STANLEY S. YAMAMURA, MAXINE ELLIOTT KUSSY, and JAMES E. REIN Atomic Energy Division, Phillips Petroleum Co., ldaho Falls, ldaho
b A volumetric method i s presented for the determination of fluoride in inorganic samples containing high levels of metal ions as well os the anions nitrate, chloride, and perchlorate. After preliminary separation of the fluoride b y pyrolysis, cerous fluoride is precipitoted stoichiometrically a t p H 1.75 with o measured excess of cerium(lll), and the excess cerium i s back-titrated with EDTA to an arseneazo-cresol red end point. Sulfate and large amounts of borate interfere.
P
ANALYSES for fluoride in complex inorganic samples invariably involve preliminary separation of the fluoride, usually by the familiar Killard-Winter distillation procedure or the more recent pyrohydrolysis procedure introduced by Warf, Cline, and Tevebaugh ( 5 ) . llilligram levels of fluoride are measured titrimetrically based on a complesing or precipitating reaction with a cation except for the specific case where fluoride is the only anion volatilized, permitting a n alkalimetric titration. Titrations based on cation reaction are reliable only for low salt' solutions under carefully controlled empirical conditions ( 4 ) . High concentrations of anions, such as nitrate and chloride vihich volatilize in both the Killard-Winter distillation and high temperature hydrolysis, interfere. Earlier investigations in this laboratory and elsewhere ( I ) had shown that the tibrations of the rare earths with (ethylenedinitri1o)tetrancetic acid (EDTA) to an arseneazo [3-(2-arsonophenylazo) - 4,s - dihydrosy - 2,7naphthalenedisulfonic acid, trisodium salt] end point \vert> not affected adversely by the presence of large amounts of common anions. -2method based on precipitation of fluoride with a n excess of a rare earth followed by a titrimetric determination of the unprecipitated excess was considered not to be subject to the aforementioned adverse factors especially if the precipitation were quantitative. The use of cerium (III), which forms a highly insoluble fluoride ( 6 ) )was investigated. The proposed method, involving it pyrolysis separation and a cerium pre~~ijiitation-EDTA titration measurenirnt, has been applied to a variety of
RESENT
samples containing fluoride coniplesing metals such as zirconium, uranium, aluminum, and beryllium, and high concentrations of nitrate and chloride. EXPERIMENTAL
Apparatus and Reagents. The pyrolysis apparatus is similar to t h a t described by Powell and Menis (3). Dilute sodium hydroxide was used in the water tower to remove carbon dioxide. .4 platinum liner supported in a quartz tube was employed; however, the all-quartz apparatus of the above authors appears equally satisfactory. The volatilized fluoride was absorbed in sodium hydroxide contained in a 50-ml. centrifuge tube support,ed on a lab jack. Reagent. grade chemicals were used throughout, The cerium solution (0.075M) was prepared by dissolving 65.14 grams of cerous nitrate hexahydrate in 500 ml. of 0.01M nitric acid, filt,ering through a 0.45-micron Millipore filter paper, and diluting to 2 liters with 0.01.M nitric acid. This solution mas standardized against EDTA. The EDTA\ solution (0.05M) was prepared in water and standardized against a zinc+ solution prepared from the pure metal. The arseneazo indicator was prepared as a 1% (w./n.)solid mixture in sodium chloride. Procedures. SEPARATIOS O F FLUORIDE B Y PYROLYSIS. Pipet a sample containing between 5 a n d 40 mg. of fluoride into a platinum boat half filled with tungstic oxide. If the sample is a solid, mix t h e sampli. intimately with t h e accelerator in A small mort'ar, then transfer to the boat ivith t h e aid of additional small portions of W03. Pipet 1 ml. of saturated aluminum nitrate evenly along the entire length of the boat a i d cover with additional tungstic oxide. Pack firmly with a spatula. Immerse the pyrolysis exit tube into a 50-ml. centrifuge tube containing 20 ml. of 2111 carbonabe-free sodium hydroxide. Insert the boat into the reaction tube (200" C. or less) and adjust the air flow to 2.5 liters per minute. Heat rapidly to 950" C. and maintain at this temperature for 10 minutes. Lower the receiver vessel and rinse the exit tube by directing a stream of water from a squeeze bottle up into it. DETERMINATION OF FLUORIDE. Add 2 drops of lilf chloroacetic acid to the 0.5 pyrolyzate and acidify to p H 3 with nitric acid using a p H meter. Cool to room temperature by immersing
in cold water. Add exactly 10.0 nil. of 0.07534 cerous nitrate and adjust the p H to 1.75 with dilute ammonium hydroxide or nitric acid. Transfer quantitatively to a 50-ml. volumetric flask using water adjusted to p H 1.75 with nitric acid for the rinses. Do not dilute to volume. Digest in a boiling water bath to precipitate the cerous fluoride, cool, and dilute to volume with the water acidified t o p H 1.75. Centrifuge about 40 ml. of the slurry to settle the precipitate. Alternatively, centrifuge the 50-ml. volumetric flask supported in a large diameter centrifuge cup. Pipet 25.0 ml. of the clear supernatant solution into a 150-ml. beaker. .%dd 50 ml. of water, 10 drops of pyridine, 2 drops of 0.2y0cresol red, and a sufficient amount of the arseneazo-salt mixture to produce a purplish red color. Titrate immediately with 0.05X EDTA to a change from red to yelloworange. If the end point is overrun, quickly pipet a n additional 5.0- or 10.0-ml. aliquot of sample into the beaker and continue the titration. Calculate the fluoride concentration from the amount of cerium precipitated. RESULTS AND DISCUSSION
Separation by Pyrolysis. Po\\ ell and Xenis (3) adopted the name pyrolysis for their modified version of the ( 5 ) pyrohydrolysis original Warf method. I n pyrohydrolysis, the hydrolyiis of fluoride salts is accomplished with superheated steam which also serves to purge the system. I n the revised method, moist oxygen rather than steam is employed iesulting in minimum sample dilution. The important variables of this technique have been discussed adequately ( 3 ) . Some of the more important factors and revisions made in the present study bear emphasis. Quantitative recovery of 5- to 40mg. portions of fluoride was obtained by pyrolyzing a t 950" C. for 10 minutes using a moist air flow of 2.5 liters per minute. KO significant difference between oxygen and air was observed Small quantities of fluoride adsorbed on the inner walls of the reactor vessel exit tube, nhere temperature is lower, are removed by a jet of water directed through a long hypodermic needle inserted into the delivery tube. The addltion of aluminum nitrate to the sample controls the release of fluoride from volatile samples and also serves as an accelerator for others. VOL. 33, NO. 12, NOVEMBER 1961
1655
Precipitation of Cerous Fluoride. Effect of Acetate, Chloroacetate, and p H . Stoichiometric precipitation of rare earth fluorides requires p H control. I n an acetate-buffered medium, serious coprecipitation of lanthanum acetate has been reported (2). Attempts to precipitate cerous fluoride by p H control only (with no acetate present) revealed a n estremely slow precipitation rate, too slow for analytical usefulness. With acetate buffering, precipitation was fast and the precipitate settled rapidly.
Table
II.
Anion None Acetate Chloride Dihydrogen phosphate Perchlorate Silicate Sulfate
Table
I.
Precipitation of Cerous Fluoride
Effect of pH and Various Ion Concentrations DigesIon ti% 70 Time, Added, Mmoles Recovery Rfin. PH Acetate" 2.0 0 1 Incompl. PPtn. 3.0 3.25 3.5 4.5
10 0 1
10 0.1 1.0 10 0.1 1.0 0.1 1.0
101 2
Incompl PPt?. 104 3 101 7 105 5 120 0 102.0 108.7 104.1 115.5
CNoroncetateh 1.50
0.1 1.0
1.75
0
2.00
0.1 1.0 0
2.75
0.1 1.0 0.1 1.0
Incompl. PPtnd
100,o 101.9 102.4 100.2 99.8
Interferes
10 20 0.13 0.26 0.65 2 5 0.13 0.32
100.0 100.2 100.2 99.5 98.8 113 134 100.0 100,o
24-mg. portions of fluoride precipitated a t pH 1.75 using 0.1 mole of chloroacetic acid and 30 mmoles of sodium nitrate.
A duo study of the effect of p H and acetate on the precipitation of cerous fluoride (Table I) showed that coprecipitation of cerous acetate increased with increasing p H and with in creasing acetate concentration. Below p H 3, the precipitation rate is very slow. It is apparent that the controlling species is the acetate ion rather than the undissociated acid. Chloroacetic acid with :t pK, value of 2.86 compared to 4.75 for acetic acid was selected for evaluation. As shown in Table I, the degree of coprecipitation
Table 111. Reliability of Cerium Precipitation, EDTA Titration Procedure, and Over-all Procedure, Including Pyrolysis
100.1 103.0
Incompl. PPtn.
Standard Deviation for Single No. of DeterFluoride Deter- Average minaTaken, mina- Recovery, tion, Rlg. tions Mg. Mg. Ce pptn, and EDTA titn. proc."
100.9 104.5 103 1 106.2
Chloroacetateb
1.75
Sodium nitrate" 0 Incompl. PPtn.
103.4 107.9
104.3 100.0 100.0 100.7 101.0
10 10 10 60 10
a Single determination using 24-mg. portions of fluoride. b Sverage of two or more deterniinations using 24mg. portions of fluoride. c Complete precipitation after digesting in boiling water for approximately 45 minutes. d Complete precipitation after 10 to 15 minutes digestion. Chloroacetic acid concentration increased from 0.1 to 1 nmiole 0
1656
2 5 12 24 2
c%; Kecoverya
'3'3.9C
0.1 1.0
2.00
Millimoles Added ...
100.6
3.50
00 30 60 0 30
Tetraborate
Effects of Diverse Ions
ANALYTICAL CHEMISTRY
6.00 12,oo 24.00 36.00 42.00
3 8 12 9 4
5,95 11.92 24.00 36.15 42.11
0.018 0,049 0,037 0.098 0,053
0
6 6 6 6 6 7
24 09 36.27 5.96 12.07 31,85 39 65
Table IV.
0.39 0.44 0.12 0.20 0.40 0.53
Cerium fluoride precipitated at pH
1 . 7 5 using 0.1 mmole chloroacetic acid and 30 mmoles sodium nitrate. b Represent results of randomly selected
control standards simultaneously analyzed with routine samples 1,s a trained chemist.
Analysis of Various Types of Samples _Fluoride, _ _ _ _ ~NgL-
Zr, Al(S03)-HNOaF*
NaF-HaB03c SH4HFz-.41(NO,)a NaF-Be(N03h-Al(rYIO3h;
Over-all procedure" 24.00 36,OO 6 . OOb 12.OOb 32. OOb 40 O O h
is much less compared to that obtained with acetate and fast precipitation (10 to 15 minutes) is still obtained at pH 1.75. The condition selected was p H 1.75and 0.1 mmole of chloroacetic acid. Salt Effect. T h e rate of precipitation of cerous fluoride is also dependent on ionic strength. T h e effect of sodium nitrate concentration a t p H 1.75 and 2.0 is shown in Table I . Because of t h e high levels of nitrate encountered in this laboratory, the sodium nitrate concentration was maintained at' 0.5M or greater. Solubility. Weaver and Purdy (6) reported a value of approximately 1x for the solubility product' of cerous fluoride. Gnder the precipitution conditions of the procedure, quantitative recovery was obtained to a 6-mg. total fluoride level (see Table 111). Effects of Diverse Anions. T h e effects of v:irious common anions, determined by niialyxing kiiown amounts of fluoride containing the ions as the ammonium or sodium salt, are reported in Table 11. Large amounts of chloride and perclilorate and low levels of borate and silicate are without effcct. Sulfate and phosphate interfere seriously in tlie l m cipitation j however, phosphate docs not distill in the pyrolysis ( 3 ) . Reliability. The reliability of the precipitation and complexonietric titration portion of the procedure is reported in Table 111. The d a t a reprcsent the results of eupeiinients performed over a period of several nio n t 11R . The reliability of the over-all procedure including pyrolysis is shown in Table 111. The computed standard deviation for a single deterniinatioii based on 6 replicate determinations :it the 24- and 36-mg. levels of fluoridc n x s
Taken
Recovered.
12 00 '24 00 :36 00 2 i 00 24 00 12 00 24.00
12 02 24 14 35 86 23 71 23 83 11 88 '23 95
24 00 24 10 NaF-Hh 0 3 0 23 95 NaF-HCIOra 24 00 24 00 24 1'3 NaF-HCla Average of two or more determin:itions. * Contained 100 mg. each of Zr and .\l and 20 mmoles of nitrate. c Each sample contained 0.5 mniolrb boric acid. d Each sample contained 1 mmole of beryllium nitrate. ~ E u c hsample contained 2 ml. of thc concentrated acid. 0
0.42 nig. Results of randomly submitted control standards simultaneously analyzed with routine samples b y a t r a i n d chemist are also shown in Table
111. Types of Samples Analyzed. A variety of samples containing such divt,ise ions as beryllium(II), alumin u m ( I I I ) , borate, nitrate, chloride, and jwrchlorate were analyzed by the over-all procedure. As shown in Table IT,n o interference is noted.
ACKNOWLEDGMENT
The authors appreciate evaluation aid given by John H. Sikes. LITERATURE
CITED
(1) Fritz, J . S., Oliver, R. T., Pieirzyk, Piei.rzyk, D.J., A X A L . CHEM.30,1111 (19%). ( 2 ) hleyer, R. J., Schulz, W., Z. anyew. Chem. 38, 203 (1925). (1925) (3) Powell, R. H., Menis, O., ANAL. CHEU.30, 154 1549 (1958). ( 4 ) Simons, J. H., € "Fluorine Chemigtry,"
Vol. 2, pp. 83-9, Acade~nic Press, New York, 1954. (5) Warf, J. C., Cline, W.D., Tevebaugh, R. D.. ANAL.CHEM.26.342 11954). (6) Weaver, J. L., Purdy, \V: C., 'Anal. Chim. Acta 20,376 (1959). RECEIVEDfor review April 28, 1961. Accepted July 10, 1961. Fourth Conference on Analytical Chemistry in Nuclear Reactor Technology held October 12-14, 1960 at Gatlinburg, Tenn. The Idaho Chemical Processing Plant is operated by Phillips Petroleum Co. for the U. 9. Atomic Energy Commission under Contract X o . AT( 10-1)-205.
Analysis of Thallium Amalgams WILLIAM
T.
FOLEY and JUDITH M. OSYANY
Chemistry Departmenf, Sf. Francis Xavier University, Antigonish, Nova Scotia
b A sample of thallium amalgam is dissolved in nitric acid and, after removal of the mercury by precipitation with formic acid, the thallium is estimated either b y titration with iodate or by a coulometric titration with electrogenerated bromine. An electrolytic method of preparing pure thallium sulfate is described. Formic acid is proposed as a reducing agent for thallium(lll).
A
~ I M P L E precise
method of deterniining thallium in thallium amalgams nas needed in a current study of the diffusion coefficients of thallium in thnlliiun amalgams. The method of Richard. and Daniels ( 2 ) was not suitable. 111 preliminary experimcnts the prescwce of formic acid caused no interfvrence with the determination of macro amounts of thallium(1) by the iodate method ( 3 ) . I n another set of experiments microsamples of thallium(1) \I ere oxidized coulometrically with the use of electrogenerated bromine ( I ) and, after conversion to the sulfate, these samples were reduced with hot dilutc formic acid and again estimated with clectrogenerated bromine. The results showed t h a t formic arid was a good redwing agent for thallium(II1). REAGENTS
Thallium Sulfate. The thallium sulfate was purified by the folloiving procwliiie, which is less demanding on the time and attention of the 01)erator than the procedure of Richards and Daniels ( 2 ) . Thallium metal was dissolved in nitric acid, and the nitrate was conv c r t d to the sulfate, which was filtered off and dried. A sample of this salt was dissolved in water and the pOH of this solution was brought to 1 or
slightly less with sodium hydroxide. The thallium metal was electrolyzed into a mercury cathode b y means of a potentiostat a t a cathode potential of -0.51 volt us. thc hydrogen electrode. The reference electrode was mercurynierruric oxide. At this potential, lead remains in solution because Eo for the reduction of lead plumbite is -0.54 volt. From time to time more thallium sulfate was added to the electrolyte; hydrazine sulfate was used as a n anodic depolarizer. Enough salt was used to ensure a n amalgam concentration of about 20%. The amalgam was transferred t o filter paper in a funnel and, after i t was carefully washed with water, the amalgam mas transferred to a boiling flask. Slightly less than the stoichiometric amount of nitric acid, based on the reduction of the acid t o nitric oxide and on the oxidation of thallium, was added, and the mixture was heated under reflux. Enough water was added to dissolve the thallium nitrate and the mercury was filtered off. The solution now contained only thallium(1) and possibly trace amounts of salts of more noble metals. To free the thallium from any remaining impurities, the solution was adjusted t o pH 9.5 with ammonium hydroxide, and 4 grams of silver nitrate was added. The solution was electrolyzed at constant current. The current density was partially governed by the stirring and +he necessity of obtaining an adherent : h e r deposit at the cathode. With the conditions prevailing, the current density was 3 X l o + amperc per sq. cm. Thallium oxide appearrd as a very adherent deposit at the cathode. From time t o time more silver nitrate was added as needed, and the electrodes were removed occasionally to recover the products of electrolysis. The electrode containing the thallium oxide was immersed in a hot dilute solution of formic acid, which soon converted it to thallium(1) formate. The thallium formate was evaporated to dryness on the water bath and thcn, by
means of sulfuric acid, it was converted to thallium sulfate. The thallium sulfate was crystallized and dried. Mercury. RIercury was purified by drawing air through mercury which was covertd by a dilute solution of nitric acid. I t was then distilled to remove noblr metals. Thallium Metal and Thallium Amalgam. T h e method of Richards and Daniels (2) was used to prepare thallium metal from thallium sulfate, and the preparation of thallium amalgam was also according t o the procedure described by these authors. The amalgam was stored under hydrogen. The other chemicals used were analytical grade reagents. PROCEDURE
A weighed sample of amalgam was dissolved in 8111 nitric acid. Enough sulfuric acid mas added to ensure conversion of the nitrate to the sulfate, and the solution was evaporated on the water bath until the cover glass was dry. The salt rvas transferred to a 250-ml. round-bottomed flask. To remove a n y hydrolyzed mercury salt from the walls of the beaker, the latter was rinsed with 10 ml. of a 20% solution of formic acid, and the rinsings were added to the flask. A water-cooled condenser was attached t o the flask; the contents were heated to a gentle reflux, and this heating was continued until the mercury gathered in one or two globules. The reflux condenser was necessary t o prevent mercury from steam distilling from the reaction flask. The mercury was freed from the solution by filtration, and the beaker containing the filtrate was placed on the water bath until the volume was reduced to 15 to 20 ml. The procedure followed next was dependent on the sample size. If the sample was a macrosample, the solution was transferred to an iodine flask and sufficient concentrated hydrochloric acid was added to ensure a concentration of 3h' VOL. 33, NO. 12, NOVEMBER 1961
1657
