Anion Exchange Study of a Number of Elements in Nit ric- Hy d rofl uo ric A c id Mix t ures Analytical Applications of the System EDMUND A. HUFF Argonne National Laboratory, Argonne, 111.
b The behavior of 19 elements on a strong base anion exchange resin was studied at variable nitric ( 1 N to 12N) and hydrofluoric (0.2N to 5 N ) acid mixture concentrations. Volume distribution coefficients were obtained b y spectrographic analysis of column effluent fractions. The application of this system to the analysis of tantalum for trace impurities or' for the determination of tantalum as a trace impurity in other matrices has been demonstrated.
S
of the behavior of elements on a strong base anion exchange resin in h:;drochloric (9), nitric ( 3 ) , arid hydrofluoric (4) acids have been published. The reported results have been invaluable in the selection of optimum anion exchange separation conditions. Adsorption charact.,eristics for a number of elements, along with suggested analytical separation techniques, have been reported by several authors (6, f 0 ) using hydrochloric-hydrofluoric acid mixtures on a strong base anion exchanger. To extend the scope of the ion exchange separation methods and to investigate the behavior of elements in mineral acid mixtures, anion complex formation of metals was investigated using nit ric-hydrofluoric acid solutions. This system was a suitable solvent for a number of elemcnts of the fourth and fifth rows of the periodic table-i.e., NbI T a , IT-that tend to hydrolyze, precipit'ate, or polymerize even a t fairly high mineral acid concentrations. The nitric acid nietlium has been used in the tlet'ermination of impurities in plutonium ( 2 , R ) , neptunium (IZ), and thorium ( 5 ); the hydrochlorichydrofluoric acid em has been applied wcvwfully to the analysis of tantalum Jor trace impurities (6). Exatnination of the absorption data in nitric-hydrofluoric acid mixtures indicated an alternate procedure for the deterinination of impurities in tantalum as well as the evaluation of tantalum as TSTEJIATIC SURVEYS
a trace impurity in ot,her matrices. This system offered the advantage over the hydrochloric-hydrofluoric acid separation in that most impurities could be eluted without a change in acid mixture (.omposition. Furthermore, high hydrofluoric acid concentrations were not required, resulting in lower blank corrections.
EXPERIMENTAL
Materials. Dowes 1 X 4 , 100 to 200 mesh, strong base anion eschange resin was supplied by I3io-Rad Laboratories, Berkeley, Calif., in the nitrate form. I t was oven dried a t 100" C. for several hours and stored in airtight plexiglass containers. Ion exchange columns were Iirepared from commercial ' h c h polyethylene tubing. One-gram Imrtions of the dry resins were transferred quantitatively as a water slurry to each column! containing polypropylene wool as the solid phase support. The resin bed was conditioned by thorough washing with the appropriate acid mixture. Polypropylene and polyethylene laboratory equipment was w e d throughout, the investigation. .kcid mixtures were prepared by dilutions of standardized reagent grade chemicals. Procedure for Adsorption Studies. Metal stock solutions were prepared in a dilute nitric-hydrofluoric acid mixture. Each standard contained several elements in concentrations ranging from 200 to 1000 f i g . : 1n1.~ depending on their sensitivities in series copper spark excitation ( 7 ) . of five ireconditioned columns was used simultaneoudy at 1.Y, 3 S , 5.Y, 8.V; and 1 2 5 nitric acid containing a constant concentration of hydrofluoric acid. Three separate runs a t 0 . 2 S , IS,and 5 s hydrofluoric acid were made. .In aliquot of a 0.1-ml. stock standard was added t o each column Effluent fractions were collected in calibrated Imlyethylene vials in 2-nil. increments. continuous elution technique was subsequently used, adding a new stnnr1:Lrtl after 5 fractions of the prec-eding metal aliquot had been collected. Volume increments taken for analysis were increased progressively for elements
exhibiting higher distribution coefficients. The concentration of an element in each fraction was determined spectrogralihically by the copper spark method ( 7 ) . Samples were evaporated to dryness to remove free nitric acid and then reclissolxd in 1 ml. of hydroclilorichydrofluoric acid mixture A 0.1-nil. aliquot was evaporated on a freshly machined copper electrode lmir. The spectra were recorded i n apl)ropriate wavelength regions and evaluated by visual comparison to standard plates. The estimated accur:iry of concentrations was judged to be within a factor of two of the amount iiresent. Procedures for the Analysis of Tantalum. A 0.20- to 0.60-gram tantalum sample was dissolved in a nitric-hydrofluoric acid mixture in a Teflon heaker. T h e solution was evaporated to 1 ml. or les. and its acid concentration adjusted to allpro si mat e 1y 1.Y nitric- 1A' 1 1 r o fl u or ic acid. The sample was then transferred to a 1.5-cm. polj.etliylene column containing 6 to 10 grams of Donex 1 x 8 anion exchange resin which had been lireconditioned with a 1 s nitric-1.y hydrofluoric acid solution. The imllurities were eluted with 150 ml. of the acid mixture and evaliorated to dryness. The residue ab rediswlved in 1 rnl. of hydrocEiloric-hy~l~ofl~i~~ric acid and analyzed by the coIq)er siiark method. Vibual corn1)arison evsluation of concentrations \\-it11 an estimated accuracy of a factor of two was found to be satisfactory foi, most elements. h selected number of inllJurities were determined den,~itonietrically,using cobalt as an internal standard. Xccelited spectrochemical calibration procedures were used in the conbtruction and evaluation of analytical \vorkinp curves (1'). The adsorbed tnntaluni was then eluted x i t h a 12.V riitric-52V hytlrofluoric acid mixture. Although some re,Gn d2gradation W R F n o t i c d in the strong nitric acid ~rietliuni.tlic columns lirovtti c,fficient for -evt,rnl -ep:iwtioris. .I siniilar procedure to that dcscribcd above !vas applied to the deterillination of tantalum as a trace irnliurity in a number, of niatrices. A column cuiitairiing 2 to 3 granis of 1)ones I x 8 rcsin was used to adsorb the tantaluni impurity while the matris wai bcing eluted with a laV nitric:-1 .\- hydrtrfluoric acid 111ix t 11re. VOL. 36,
NO. 10,
SEPTEMBER 1964
1921
ttd No A d s
No Ads - No Adsorption SI Ads-Slight Adsorption Curve a - 0.2 N H F Curve b - 1.0 N HF Curve c - 5.0 N H F
No A d s b,c
No Ads
m
t-i-l SI Ads
H No a d s o,b
SI Ads
H
Re
SI Ads
No A d s c
Figure 1. Volume distribution coefficients for elements from nitric-hydrofluoric exchange resin
RESULTS AND DISCUSSION
Absorption Studies. T h e experimental results for 19 elements are summarized in Figure 1. Elements t h a t eluted quantitatively in the first 2-ml. fraction are indicated by ‘ X o . Ads.” T h e appearance of a trace concentration in the second fraction led to the classification of “Sl. Ads.” Distribution coefficients were computed from the equation Ztigated that indicated an increase ACIDS. The tendency of Di(II1) to form in adsorption with an increase in hydronitrate anion complexes is shown fluoric acid concentration. This result was consistent with the observations graphically in Figure 1 . An increase in hydrofluoric acid concentration result.ed made by Faris on the behavior of this in a corresponding decrease in volume particular metal in hydrofluoric acid (4)and hydrochloric-hydrofluoric acid distribution coefficients. METALSADSORBED FROM EITHER mixtures (6). ACID. [:(VI), Pd(II), and Re(V1I) Tantalum Analysis. Table I shows a compariqon of impurity concentraretained the general form of their nitric acid adsorption functions. There tioni in tantalum obtained by the use was, however, a general decrease in of the hydrochloric-hydrofluoric and adsorption wit,h the addition of hydronitric-hydrofluoric acid elution sys-
Table
__.
Element
2 Ba
Be Ca co Cr cu Fe Ga Hf In Li Mg Mn Mo Na Sb Ni Pb Sn Sr Ti U
v
W Zr
I&
