it is unnecessary to pulverize coarse powders. Chunks need only to be crushed to a rough powder; thus, the difficult and contaminating operation of grinding boron can be avoided. KOboron is lost during fusion. Tests made by fusing boric, acid in a quartz flask with water-cooled reflux condenser attached give complete recovery. The reflux condenser is necessary because water is added to the cooled melt and the mixture is boiled for about 10 minutes to dissolve the melt and destroy excess persulfate. From the large quantities of sulfur dioxide liberated during fusion, the
dissolution is assumed to proceed by the following, or similar, reaction 2B
+ KzSzOs
--*.
2 KBOz
+ 2 SOz
but the exact mechanism has not been explored. It is of interest to note that a reaction of this type presents the possibility of determining elemental boron in technical boron by collecting the sulfur dioxide and determining it iodometrically. The solution obtained after destruction of the excess persulfate is suitable for the titrimetric determination of boric acid by the mannitol procedure. If the entire solution from a fusion with 50
grams of persulfate is used in the titration, the reagent blank amounts to less than 0.15 ml. of 0.4N sodium hydroxide. A full description of the titration procedure plus other details will be covered in a later report. LITERATURE CITED
(1) Vasilieva, M. G., Sokolova, A. L., Zh. Analit. Khim. 17, 530 (1962).
A. R. EBERLE M. W. LERNER U. S. Atomic Energy Commission New Brunswick Laboratory New Brunswick, N. J.
Preparation of Carrier-Free Thorium-234 Tracer SIR: An excellent procedure for the preparation of large quantities of carrier-free thorium-234 has been published recently by Berman and coworkers ( I ) . During a recent investigation on the precipitation of submicrogram quantities of thorium by barium sulfate and its application to the fluorometric determination of thorium in biological and ( 2 ) , several mineralogical samp1:s important changes were made in the preparation and use of thorium-234 tracer. Berman used an end-window GM tube through a n aluminum absorber to count the 2.3-m.e.v. beta emission from the 1.18-minute protoactinium-234 (UX,) daughter of thorium-234 rather than the very soft beta radiation of the thorium itself. However, using a 3-inch thalliumactivated sodium iodide well crystal, the activity from the thorium can be determined by gamma counting in 50 ml. of solution with a counting efficiency of about 13%, which is both adequate and much more convenient for tracer studies. Because moist of the gamma activity of thorium-2!34 results from soft gamma rays of onki 63- and 92-k.e.v. energy, each solution should be compared against a stanllard having the same composition and identical height contained in 75-ml. polystyrene vials of uniform diameter and wall thickness. Also, the directions of Berman were scaled up about fivefold to be able to sustain a full-scale con1inuous investigation. Approximately 2 X 105 c.p.m. of tracer was desired per run with a 5minute counting time, so that the statistics of counting would permit the thorium to be accounted for precisely with a standard deviation of about 0.1%. The thorium434 activity present in 1 pound of uranyl nitrate hexahydrate a t equilibrium is approximately 2 X l o 7 c.p.m. and is
sufficient for about 100 determinations under the indicated counting conditions. Half this quantity will be regenerated in 24 days. By alternating three separate 1-pound batches of uranyl nitrate between two columns, a t least 10 runs per day can be maintained indefinitely a t the very liberal level of tracer employed. Berman suggested that little or no interference was to be expected from the impurities present in the uranium. This is probably true even a t the higher concentrations recommended in the present work, if the tracer is used in the hydrochloric acid as it comes from the column. However, sulfate solutions are desired frequently to permit use of sulfuric and nitric acids to oxidize organic material or pyrosulfate fusion to ensure dissolution and conversion of thorium from refractory samples to the ionic state. Small quantities of lead or barium in the presence of sulfates precipitate small quantities of thorium very efficiently ( 2 ) . Consequently, even a t the maximum level of 0.002% lead permitted by ACS specifications, the 9 mg. of lead present in a pound of reagent-grade uranyl nitrate hexahydrate is suficient to precipitate essentially all of the thorium tracer present if added to a solution containing sulfate. The lead is not eluted quantitatively in the first elution, so that the tracer will distribute between the aqueous solution and the precipitate for the first two or three elutions if corrective action is not taken. The present procedure recovers all of the thorium from the insoluble material, results in a perchlorate solution so that the tracer can be employed in solutions containing high concentrations of calcium such as bone ash, and eliminates a small quantity of uranium that comes through the larger columns. Dowex 1is preferred to the resin used by
Berman because of its lighter color, making the uranium band head more easily visible. EXPERIMENTAL
Apparatus. The ion exchange column is of conventional design, with a column 52 inches long and 2 inches in inside diameter. h 2-liter reservoir is attached t o t h e top, with a coarse fritted disk and a S o . 2 stopcock a t the bottom. The column is packed with about 4 pounds of reagent-grade Dowex 1-X4, 50-100 mesh, in the chloride form, backwashed, and allowed to settle as uniformly as possible. A light plug of glass wool is placed a t the top of the column. Hydrochloric acid, 9.6JI (4 to l), is passed through the column a t a rate of about 10 to 15 ml. per minute until the water has been displaced completely. Procedure. Dissolve 1 pound of reagent-grade uranyl nitrate hexahydrate in 100 ml. of concentrated hydrochloric acid and evaporate the solution to dryness. Dissolve the uranyl chloride cake in 500 ml. of 9.6M (4 t o 1) hydrochloric acid and add the solution to the column of Dowex 1-X4 in the same concentration of acid. Pass the uranium solution slowly into the resin and collect the effluent in a 500-ml. graduated cylinder. When the bright yellow uranium solution has dropped to the top of the resin column, continue the elution with 9.6M acid a t a flow rate of about 10 to 15 ml. per minute until the thorium234 activity begins t o come off. Approximately 800 ml. of effluent will precede the activity and can be recycled through the column. The next 1600 ml. will contain about 95% of the activity. The elution curve of the UX, is very similar to that shown by Berman ( I ) . With subsequent elutions, the activity begins to come off almost immediately and the entire eluate should be collected. At least four or VOL. 36, NO. 3, MARCH 1964
675
five consecutive elutions can be obtained before significant breakthrough occurs. Small quantities of uranium are eliminated effectively during the subsequent separation. When a significant breakthrough appears certain during a n additional elution, elute the uranyl chloride from the column with 2 or 3 liters of water. Evaporate the solution t o dryness, dissolve the uranyl chloride cake in 500 ml. of 9.6M hydrochloric acid, and put the uranium back on the column after xashing the latter with 9.6M acid. Place the combined hydrochloric acid effluents containing the thorium234 tracer in a 4-liter beaker, add 2 or 3 boiling chips, and evaporate the solution to about 25 ml. Transfer the solution t o a 250-ml. Erlenmeyer flask and rinse the beaker with a little concentrated hydrochloric acid to reccver most of the activity. Add the rinses t o the flask but do not transfer the boiling chips. Add 3 ml. of concentrated sulfuric acid carefully around the sides of the flask and evaporate the solution to fumes. When nearly t o fumes, add a few drops of nitric and perchloric acids t o oxidize a small quantity of organic matter from the resin. Add 2 grams of anhydrous sodium sulfate and heat the flask over a blast burner with continuous swirling
until a clear pyrosulfate fusion is obtained. Cool the flask, and add 25 ml. of water, 0.2 ml. of a ferric perchlorate solution containing 1 mg. per ml. of iron, and 2 drops of 30% hydrogen peroxide. Heat the solution to boiling and add 8M sodium hydroxide until the solution becomes alkaline, as indicated by the precipitation of ferric hydroxide and a few drops in excess. Boil the solution for 2 or 3 minutes to dissolve any lead sulfate that might be present and to decompose excess hydrogen peroxide, but ignore the typical flocs of silica that will generally be present from the resin. Transfer the solution to a 40-ml. conical centrifuge tube and centrifuge at 2000 r.p.m. for 2 minutes. Discard the supernate. Add 1 ml. of 72% perchloric acid and 15 ml. of water to the centrifuge tube. Warm the solution if necessary to dissolve the precipitate and filter the solution through a 2.5-cm. hardened paper in a small Hirsch funnel to remove the silica. Wash the tube and filter paper with a little water. The resulting solution will contain approximately 2 X lo7 c.p.m. (gamma) from 1 pound of uranyl nitrate hexahydrate containing thorium234 in equilibrium. Half this quantity can be obtained every 24 days. Prepare a working solution containing about 2 X lo5 c.p.m. per milliliter or less by appropriate dilution of the stock solution in 1% perchloric acid.
Standardization. Although generally not necessary for tracer work, the solutions can be standardized by gamma counting after determining the counting efficiency for a given set of conditions as follows. Dissolve 1 gram of pure uranium metal or U3O8 at least several months old in the appropriate acid, add 1.0 ml. of 6.6% barium chloride dihydrate solution, and evaporate to dryness. Fuse with 3 grams each of anhydrous sodium and potassium sulfates and 3 ml. of concentrated sulfuric acid. Reprecipitate, wash, and dissolve the barium sulfate in alkaline diethylenetriamine pentaacetic acid as described in another publication ( 2 ) , and count under the desired conditions. Calculate the counting efficiency by dividing the observed counting rate per gram of natural uranium by 7.36 X lo5,the activity of uranium-238 and therefore of thorium234 per gram of natural uranium in secular equilibrium. LITERATURE CITED
S. S., McKinney, L. E., Bednas, M. E., Talanta 4, 153 (1960). (2) Sill, C. W., Willis, C. P., ANAL. CHBM.36, 622 (1964). CLAUDE w.SILL U. S. Atomic Energy Commission Idaho Fulls, Idaho (1) Berman,
Improved Determination of Strontium-90 in Milk by an Ion Exchange Method SIR: During analysis of several thousand Public Health Service milknetwork samples with the procedure previously reported (C), two useful modifications were developed which are described here. The procedure consists of storing the milk samples with formaldehyde preservative for the ingrowth of the yttrium-90 daughter of strontium-90, adding yttrium carrier, and then passing the milk consecutively through cation and anion exchange resin columns. The alkaline earth ions in milk are replaced by sodium ions in the cation exchange column, after which the yttrium is retained as an anionic complexprobably of citrate-in the anion exchange column. The effluent milk is discarded and the yttrium complex on the anion exchange resin is destroyed with hydrochloric acid. The yttrium, eluted with the acid, is precipitated as the oxalate, and the radio-yttrium is measured with a low-background beta counter. One modification in the original procedure is the inclusion of solvent extraction after the yttrium elution to remove 676
ANALYTICAL CHEMISTRY
the lanthanum-I40 daughter of barium140. These fission products, being so short-lived that they are usually detected in the environment only within one-half year after their formation, were not in milk when the procedure was first developed and applied. They were identified in milk soon after the resumption of atmospheric nuclear testing in September 1961 (6). At yttrium-90 and lanthanum-140 levels much higher than those occurring in milk, the two radionuclides may be distinguished from each other by their different halflives (64 us. 40 hours, respectively) and types of radiation (lanthanum emits gamma rays whereas yttrium does not). At the low levels encountered in milk, however, differentiation by decay properties is inaccurate. Solvent extraction from 14N nitric acid into 100% tributylphcsphate was utilized because effective lanthanum removal nith little yttrium loss had been demonstrated (1). The procedure was further modified to obtain higher yttrium yields. Initially, the maximum yttrium yield was 65% and average yields were 5570. Yttrium retention OII the resin was
improved from 80 to 99% by adding sodium citrate to the milk. The per cent eluted in 35 ml. of hydrochloric acid was increased from 81 to 967& by decreasing the amount of anion exchange resin and by thoroughly stirring the resin during elution. Other losses were minimized by small procedural changes, so that the overall average yield was increased to 86%. EXPERIMENTAL
Reagents and Apparatus. Use ion exchange resins and apparatus described in the original procedure (4), except for the following changes in dimensions: upper rolumn is 5 cm. in diameter and contains 170 ml. of Dowex 50W-X8 resin; lower column is 1.9 cm. in diameter and contains 15 ml. of Dowex 1-X8 resin. Prepare yttrium carrier by dissolving 25.5 grams of yttrium oxide in 100 ml. of concentrated nitric acid. Add water, adjust t o pH 2 with XHdOH, and dilute to 2 liters. Measure concentration of yttrium in carrier solution as yttrium oxalate. Prepare strontium carrier by dissolving 50 grams of strontium nitrate in water, adding 1 ml. of concentrated
