Table 1.
Spectrophotometric Determination of Acetic Acid in Acetic Anhydride Samples
Composition Acetic anhydride Acetic anhydride 0 . 5 % water Acetic anhydride 1.OY0 water Acetic anhydride 2 .O% water Acetic anhydride 3 . 0 % water Acetic anhydride
+ +
+ + + 4 . 0 % water
Moles of acetic acid/mole of acetic anhydride, total
Moles of acetic acid/mole of anhydride (due t o added water)
Water, % Added Found
0.0685 0.1285
0.0600
0.502
0.52
0.1780
0.1095
1.046
0.97
0.2900
0.2215
2.023
1.85
0.4320
0.3635
3.054
3.20
0.5100
0.4415
4.028
3.89
acetic acid present due to added water will be the total acetic acid minus the acid originally present in the anhydride solution. The measured acetic acid content and the amount actually present due to the added water are shown in the table as per cent water to give an idea of the accuracy claimed by this method. Acetic acid content in acetic anhydride has also been checked up by a second method (1) and is found to be 3.36y0 by weight. The latter method works smoothly but the accuracy regarding the acid content of the anhydride may not be as good as the present method a t this range because it is dependent on the difference between the tw7o fairly large titer values. LITERATURE CITED
(1) Critchfield. F. E.. Johnson.' J. B.. ANAL.C H E ~28, . 436 (1956). (2) Palit, S. R., ANAL. CHEM.33, 1441 (1961). (3) Palit, S. R., Chem. Ind. London 1960, p. 1531. >
acetic acid content of the acetic anhydride solution is easily calculated. As an independent check of the result, known weights of water are added to acetic anhydride to obtain a series of solutions of known extra acetic acid content (reagent C). Measurements are made on them by a similar procedure and separate Curves (not shown) are drawn for each solution and the total acetic acid (for solutions c) are calculated by the same procedure as above.
RESULTS AND DISCUSSION
As obtained by this method, acetic acid present in the anhydride sample is found to be 0,0685 mole per mole of acetic anhydride or 4.03% by weight (Table 1). To have an independent check of the accuracy of this Procedure, different percentages Of water are added to the anhydride and allowed to generate equivalent quantities of the acid. The
,
BHAIRAB CHANDRA MITRA PREMAMOY GHOSH SAKTIR. PALIT Indian Association for the Cultivation of Science, Jadavpur, Calcutta-32, India ONEof the authors (B. C. M.) received a scholarship from the Council of Scientifir & Industrial Research, Government of India.
Dissolution of Elemental Boron SIR: Elemental boron is generally dissolved by either a sodium carbonate fusion or by treatment with nitric acid with or without hydrogen peroxide and fusion of the insolubles remaining with sodium carbonate. Other less frequently used methods involving the use of solutions of such oxidizing agents as ceric sulfate, potassium periodate, iodate, permanganate, or dichromate are summarized by Vasilieva and Sokolova ( I ) , who also describe a hydrogen peroxide-potassium persulfate procedure. In work being carried out by the authors on the analysis of boron, none of these methods has been found to be completely satisfactory. The carbonate fusion is slow and is ordinarily limited to less than 300-mg. samples. Unsuccessful fusions are often obtained as a result of flaring, creeping, etc. Nitric acid dissolution results in the presence of nitrate in the final solution, and nitrate interferes seriously in one of the methods being developed for the determination of boron. Early in the present work, a composite method was 674
ANALYTICAL CHEMISTRY
devised in which the sample is treated with slightly less than the calculated amount of nitric acid, then sodium carbonate is added and, after evaporation, the residue is fused in platinum. A solution essentially free of nitrate is obtained but significant quantities of platinum, which also interfere, are present. The method, however, is convenient and potentially useful. Of the remaining dissolution methods, only the hydrogen peroxide-potassium persulfate mixture (1) was considered suitable for testing. Samples of finely divided pure boron weighing less than 200 mg. dissolved satisfactorily within an hour leaving only traces of residue that had to be fused with carbonate. However, with samples consisting of coarser particles or containing relatively large amounts of carbide, dissolution proceeded with difficulty and roughly half of 200-mg. samples remained undissolved after repeated reagent addition and prolonged boiling up to several hours. I n these cases, fusing the insolubles is nearly as troublesome as fusing the entire sample.
Powdered elemental boron can be dissolved smoothly and rapidly by fusion with potassium persulfate for 15 to 20 minutes in a quartz flask. KOattack on the quartz is apparent. Large quantities of persulfate must be used: 50 grams for 400-mg. samples; 60 grams for 800-mg. samples. Boron carbide in the sample dissolves readily but a slightly longer heating period is necessary. I n fusions made with boron carbide, 300-mg. samples dissolved satisfactorily in 50 grams of persulfate. High-temperature carbide, fired at greater than 2400" C., merely required longer heating periods. In an occasional sample, traces of material remaining undissolved when the cooled melt was taken up in water have been identified by x-ray diffraction as boron nitride. When nitride is present, the solution is filtered, the paper is moistened with sodium carbonate solution and ashed, and the residue is fused with 2 grams of sodium carbonate. This fusion presents none of the problems associated Tvith the original sample. Powdered samples must be used but
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 an 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 the 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
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