Arthur Bradley and Harold T. Peterson, Jr.'
Associated Nucleonics, Inc. Garden City, N. Y.
Solubility of Neodymium Oxalate by Coprecipitation with Pm-147
Radioactive tracers are becoming increasingly useful in t,he determination of the solubility of various substances. For the determination of the solubility of very sparingly soluble compounds, the radioactive tracer technique often represents the only practical method. Unfort,unately it is not always possible to obtain readily a radioisotope with a convenient, half life. If, however, a radioisotope of another element of closely related chemical properties can be substituted, coprecipitation may tend to distribute the tracer element between the solution and solid phases in the same proport,ion as the carrier. In such a favorable case, only the residual radioactivity of the solution due to the tracer need he measured to give information on the solubility of the carrier. The relationship may be expressed as follows: Let S
=
solubilitv of carrier salt, mdml
If t.he coprecipitation distribution factor
=
Table I.
1, then
carrier (soln.) - tracer (soln.) carrier (ppt.) traoer (ppt.) - S 2-8
temperature overnight. The precipitate was then separat,ed by filtration and aliquots of the filtrate were evaporated on planchets for counting. Because of the very slight solubility of the oxalate, it was necessary to use a relatively high concentration of the Pm-147 tracer: usually one-half millicurie per determination, or 5 X lo7 dpm/ml before pre~ipit~at~ion. This resulted in lo4 to 2 X lo5 dpm/ml in the filtrate, depending on the initial amount of neodymium added. The oxalic acid concentration was high enough (0.1M) to insure a fairly constant excess a t all neodymium levels. This was necessary so that each filtrate aliquot would have about the same self-ahsorption count,ing correction. Under the conditions employed, the thin (1 mg/cm2) layer of oxalic acid on each planchet reduced the apparent counting efficiency to about one-fourt,hof that of a weightless sample. Blank det,erminations to establish the counting correction were an essential part of the experimental program. Results of Solubilitv Determinc~tians
Pm-147 tracer, I)pm/ml X 10'
(T)
NdiJ -Net activity in filtrate (1 ml+ Carrier, Dpm X lo-' mdml (1) Cpm X lo-J'' (vF
--2II' - 1 1
--
165 (Omitted) 87 24 8.0 (Omitted) 45 22 16.4 214
Since S and y are small compared to x and T,respectively, In the ideal system, therefore, a summation of z, y point,s a t a fixed T would constitute a rectangular hyperbola on linear coordinate paper or a straight line of unit slope on a log log plot. If the experimental result,~ are found to take this form it would indicate that the tracer does coprecipitate with the carrier with a di~t~ribution coefficient of 1. Moreover, the carrier solubility . (S) . . could then be determined from the x?/ product, according to equation (1). This paper describes a measurement of the solubility of neodymium oxalate by co~reci~itation with promethium-147. This experiment illustrates the method suggested above, and by comparison with the known value, permits an evaluation of Pm-147 as a general tracer for neodymium and possibly other rare earths. Neodymium ('-loo mg) was precipitated from an excess of aqueous oxalic acid containing Pm-147 adding neodymium in so'ution, the volume being 25 ml. The react,ion mixture was warmed t,o 80" for an hour and allowed to stand a t room
' Present
address: Frick Chemical Laboratories, Princeton University, Princeton, N. J.
398 / Journal of Chemical Education
8 mg of o x a k acid,
The result. of a series of precipitations carried out starting with 0.56 mc of Pm-147 each are given in ~ ~ arid b plotted l ~ with log log coordinates in Figure straight line fitting the experimental The x, y points (x = mg/ml of neodymium before precipitation, = dpm/ml of Pm.147 in filtrate) had a slope of 1.15 in fair agreement. wit,h t,he value of 1.0 implied by equation (1). The zy product common to all points on the unit slope (dashed line) was 2 , 10Pmg-dpm /rnll. The solubilit,y(8)mas derived from equation (1). I?,=
Since T S
=
=
87' = 2.5
x
10"
5 . 0 X 10' dpm/ml,
5.0 X lo-' mg/ml = 3.5 X 10-%ole/liter
of !idta
-
Assays
X
T=5.0x10
DPMIML
A ~ = 2 . 10'5 ~DPMI ML
i
DASHED LINES HAVE UNIT SLOPE \
pMI4' Figure 1.
IN FILTRATE, DPM / M L x
(Y')
Log log plot of ~olvbilitydata.
Some additional points were obtained with half the initial Pm-147 content, or 0.23 mc/25 mi. The zy product common to all points on t,he best, unit slnpe was 1.1 X lo-*, leading to a value of 3.1 X 10-Bmole/ liter for 8. These results are good approximations of the solubility of Nd+3 in unbuffered 0.1M oxalic acid solutions. Using Nd-147 as the tracer, Crouthamel and Martin2 found a minimum solubility of 1 . 5 X 10-6 mole/liter a t an oxalate activity of lo-'. Under the condit,ions reported here the oxalate ion activity would be approximately which corresponds to a Nd+3solubility of 3 X m~le/liter.~ Apparently the coprecipitation of promethium and neodymium oxalates was very nearly ideal (i.e., the distrihution coefficient between the solid and solution phases equals 1.0). This is not surprising considering that the two salts must have very similar crystal st,ruct,ures and solubilities. I t would certainly be of interest to measure the solubilities of some other rare earth compounds using this tracer. Where great. accuracy is not required, it may often be convenient to employ Pm-147 instead of a tracer radioisotope of the same element for the preliminary study of a new compound in the lanthanide series. Due to the weak (0.22 Mev) beta emission, t,here is essentially no external radiation from glass vessels containing millicurie amounts of Pm-147, yet a G-M counter detects millimicrocurie amounts on a planchet with ease. Stock solutions need rarely be assayed, due to the chemical stability in weak acid and the long half life (2.6 years). This isotope is attractive from a safety standpoint, and very liberal tolerances have been established for ingestion.' The metabolism of rare earths is such that an accidental ingestion of as much as 10 mc of Pm-147 would scarcely lead to the permissible body burden being exceeded. CROUTHAMEL, C. E., A N D MARTIN, 1). S., J . 4 n t . Chew. Soe., 73, 569 (1951). Ibid., p. 572, Figure 2. U. 8. National Bureau of Standards Handbook 69, p. 6 1 . V ~ r s u sA. ~ ,M.,ROWLET, K
30, 1605 (1958).
, A N D GORDON, L.,Anal. Chem.,
The aluminum planchets had a circular depression 1.25 in. in diameter and 0 . 1 in. deep. They were counted in close proximity to an Anton 210T G-M tube, on the top shelf in a lead-shielded sample chamber, with about 0.1 in. clearance from the end-window. Five-minute counts were recorded on an Atomics 1040A scalar for all planchets. The background, usually 25-30 cpm, was subtracted. Table 1 gives the net counts and corresponding assays for fourteen determinations. The stock I'm-147 solution was assayed by comparison with a known standard, and found to have 28 + 2 microcuries/ml. When an appropriate dilut,ion was counted under the same conditions used for t,he filtrate aliquots in the solubility det,erminations, it gave 10yo of the calculated decomposition rate, or a counting efficiency for a weightless sample of 0.10. A number of additional planchets were prepared with t,his same dilutiou plus 8 mg of oxalic acid (equivalent to 1.0 ml of t,he 0.1 M filtrate). The average count rat,e then observed was reduced by a factor of four to the equivalent, of 2.5% of the theoretical, with a precision of 0.5%. Assays of filtrate aliquots were therefore made by dividing the measured cpm by a counting efficiency correction factor of 0.025, and the reported dpm values may be considered precise to +20%, a source of error due primarily to the variations in self-absorpt,ionof the oxalic arid residue in the planchets. Referring to Figure 1, the filtrate activity point at 1 mg/ml of Nd+3 lies above the best slope fitting the remaining points. At this concentration level about half of t,he oxalic acid was carried out with the precipitate, and the remainder on the planchet was therefore only about 4 mg. The counting efficiency for such a case was not measured, but it would undoubt,edly he higher than 0.025. I t is evident that a more accurat,e assay correction factor mould tend to make this experimental point a bet,ter fit t,o the theoretical unit slope.
Coprecipitation Theory
llecent work in the field of coprecipitation of rare earth oxalates has heen ~nmmarizedby Feibush, Rowley, and G o r d ~ n . These ~ authors discussed the experimental results in terms of a model in which the solution was in equilibrium with the surface of the growing rryst,al, not t,he ent,ire solid phase. The corresponding distribution coefficient,^ were constant over a wide range of t,racer/carrier ratios at low concentrations. However, these coefficients were not in agreement wit,h the calculated values derived from the kuown solubility product,^, nor were they always mutually consistent. Thus, t,he product of the Kd/Yt and Yt/Ce fact,ors should have equalled that of Nd/Ce, which was not observed. In addition, the distribution coefficients mere significantly affected by the rate of the react,ion, and most of t,he data were gathered a t concentration levels where the precipitation was incomplete eveu after several days. The present investigation indicates t,hat in the limiting case of immediate and essentially quantitative precipitation, the Pm/Nd distribution factor is a constant approaching 1 .O. Volume 37, Number 8, August 1960
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