ACKNOWLEDGMENT
The authors are grateful to E. M. Bickoff and A. L. Livingston for calling attention t o the need for this instrument. LITERATURE CITED
(5) Mavrodineanu, R., Sanford, W. W., Hitchcock, A. E., Coniribs. Boyce Thompson Insl. 18, 167 (1955). (6) Semm, K., Fried, R., Nuturwissenschaften 39,326 (1952).
C. R., DeEds, F., Snence 126, 969 (1957). (2) Brown, J. A., Marsh, M. M., ANAL. CHEM.25,1865 (1953). (3) Livingston, A. L., Bickoff, E. hf., Guggoh, J., Thompson, C. R., Zbid., 32, 1620 (1960). 14) , , Lvman. R. L.. Bickoff. E. hl.. Booth. A. N., Lkngstdn, A. L.,'Arch. Biochem: Biophys. 80, 61 (1957).
MENTIONof specific products does not constitute endorsement by the U. S. Department of Agriculture over similar products not specifically named.
~
(1) Bickoff, E. M., Booth, A. N., Lyman,
R. L., Livingston, A. L., Thompson,
Ammonium Bisulfate Fusion. Other Techniques
Application to Trace Analysis by Spectrochemical and
Cyrus Feldman, Oak Ridge National Laboratory, Oak Ridge, Tenn. P E ~ O C H E Y I S T Soften
dissolve sam-
S ples in order to prepare them for analysis. If the sample is a corrosion scale, refractory oxide, or insoluble fluoride, a potassium pyrosulfate fusion is usually indicated. However, the gram quantities of pyrosulfate involved are often a great inconvenience in performing the subsequent spectrographic, colorimetric, or electrochemical determination. There is no satisfactory way of removing the pyrosulfate alone, so instead the elements of interest are usually removed from an aqueous solution of the fusion cake by precipitation or extraction. This can be done satisfactorily for some groups of elements, but is difficult and uncertain with others--e.g., alkaline earths-and essentially impossible for the alkalies. In any case, the difficulties of collection increase rapidly as the quantity of material collected decreases. In order to make the fusion technique generally useful for trace analysis, a way is needed to remove the fusion agent after use without losing any of the constituents of the sample. A second disadvantage of potassium pyrosulfate fusion is the tendency of the melt to solidify as SO8 is evaporated. If the sample does not dissolve quickly, it may be frozen in place and lost. A third difficulty arises in connection with the determination of impurities in insoluble fluorides. If trace elements are to be determined in an insoluble fluoride and the impurities must be chemically concentrated, a potassium pyrosulfate fusion would render the sample soluble. However, an additional contamination danger now appears in that molten fluoride attacks most nonmetallic crucible materials, and potassium pyrosulfate attacks platinum a t fusion temperatures (>350' C.). A fusion agent was sought which would be effective as a solvent, completely volatile a t hot plate temperatures, noninjurious to platinum or other common crucible materials, and available in sufficient purity.
ATTACK OF PLATINUM. Since N H r HS04 melts a t a comparatively low temperature, most chemical reactions which require the use of platinum can be accomplished without damage to platinum crucibles. If prolonged treatment a t high temperatures is then necessary to dissolve other constituents of the sample, the operation can be transferred to a quartz container. PURITY. NHJlS04 is available as an analytical reagent. Spectrochemical analysis of the residue from the evaporal tion of 46 grams of reagent grade NHJISO4 showed these impurities (Table I). No other metals were detected. Limits of detection for elements not detected ranged from 2 pg. per gram downward. Submilligram amounts of a carbonaceous contaminant were oxidized during the evaporation. Since NHIHSO~is readily vaporized, a blank determination is easily run for all impurities which would be present in the sample preparation.
ADVANTAGES O F A M M O N I U M BISULFATE
Each requirement is satisfied to a large extent by ammonium bisulfate, NHBSO4. (The normal sulfate spatters, and should not be used.) EFFECTIVENESS. NH+HSO4 can perform most of the fusions for which K&O7 is usually used, although the ammonium salt does not attack some refractories if they have been fired a t very high temperatures (TanOs, ALOa, BeO). Some of these can be dissolved by prolonged (1 to 2 hours) fusion with ammonium fluoride in a covered platinum crucible. The normal fluoride, "9, is more effective than the bifluoride, NHJIF2. The bisulfate has both acid and oxidizing effects; it can dissolve many metals directly, including some, such as Zr and Nb and their alloys, which ordinarily require the acid HF for dissolution. ELIMINATION FROM SYSTEM.This compound melts to a clear, colorless liquid a t 146.9' C.; it can be vaporized smoothly and completely a t hot plate temperatures ( 2 O O O t o 300' C.). LIQUIDITY. Under a tight-fitting watch glass, NHJISO, can be kept molten indefinitely a t temperatures up to -475' C. I t boils quietly a t higher temperatures, but does not solidify. A stubborn sample can thus be treated as long as necessary. Once the sample has been dissolved, the beaker can be fitted with a ribbed watch glass and the fusion agent eliminated with no loss of metallic constituents. Volatile acidic oxides which are normally lost from pyrosulfate fusions (B203, AszO8) would be lost in this case also. The less volatile oxides, such as Moo3 and W03, are retained.
Table I.
APPLICATIONS
The following are examples of analyses in which the ammonium bisulfate's felicitous combination of properties has been of decisive importance in preparing samples for spectrographic analysis. PLUTONIUMANALYSIS. Two milligrams of an impure plutonium oxide residue were submitted to our high radiation level analytical facility for quantitative determination of all detectable impurities. The sample was fused with NHJlSO4 in a small quartz beaker, the bisulfate evaporated, and the sulfate residue dissolved. Then the
Impurities in NHlHSOd
(pg. metal per gram of NH4HS04)
AI
Be
Ca
Cr
Cu
Fe
0.36
0.02
1.7
0.16
0.08
0.96
Mg 0.30
Mo
Ka
0.23
8.7
Ni 0.36
VOL. 32, NO. 12, NOVEMBER 1960
Sr 0.5
1727
plutonium was rerr.oved by selective solvent extraction and the remaining solution analyzed spectrographically. The entire chemical operation was performed in the original beaker, so that no trace elements were lost other than those mentioned’above. ANALYSISOF UF,. When UFI was fused with NH4HB04at a low temperature in platinum, the fluoride was quickly eliminated and the uranium oxidized. Evaporation of the bisulfate left an easily soluble uranyl sulfate. This was dissolved, the uranium extracted, and the aqueous layer analyzed spectrographically. DISSOLUTIONOF METALSWITHOUT HF. Hydrofluoric acid is necessary to dissolve some metals. The con-
tinued presence of the fluoride ion is sometimes undesirable after the dissolution, but this ion is often difficult to eliminate without some serious disadvantage to subsequent operations. This situation arose, for example, in the determination of trace impurities in niobium-rare earth alloys. The use of H F would have given insoluble rare earth fluorides; the fluoride would then have been diffcult to eliminate without rendering the niobium insoluble, losing trace elements, or both. These alloys dissolved with little difficulty in molten NHJISOI. In this case, however, immediate evaporation of the bisulfate would have left an insoluble residue. The melt was therefore dissolved and the major con-
stituents were precipitated with gaseous NHa, centrifuged, and redissolved in concentrated HCl or oxalic-nitric acid mixture. The supernate, containing the bisulfate and the impurities soluble in ammonium hydroxide solution, was treated with excess sulfuric acid and evaporated to remove the ammonium salts, and the two portions then were recombined. This reagent should be especjally valuable where subsequent determination, spectrographic, colorimetric, or electrochemical, would be hampered by the presence of gram quantities of fusion agent. PITTSBURGHConference on Analyticai Chemistr and Applied Spectroscopy, PittsburgK, Pa., March 3, 1960.
A Simple, Wet Oxidation Procedure for Biological Materials 1. 1. Reitz, W: H. Smith, and M. P. Plumlee, Animal Science Department, Purdue University, West Lafayette, ind.
of plant or animal W matter for subsequent mineral analyses is superior to dry ashing in ET OXIDATION
many respects. In an excellent review article Middleton and Stuckey (3) point out that in dry ashing there is the possibility that certain elements might be absorbed or combined with other ash constituents or even with the material of the crucible, and that contamination by dust might occur. A large number of wet oxidation procedures using some combination of nitric, sulfuric, or perchloric acids have been reported in the literature. However, many af these methods are time consuming and require special equipment and careful attention by the operator to be effective and safe. Although perchloric acid is an effective aid to digestion, its introduction in large quantities is potentially dangerous as it can react with explosive violence under certain conditions. The method described for wet oxidation using nitric, sulfuric, and a small amount of perchloric acid requires very little attention by the operator and no specialized equipment. Perchloric acid is added dropwise near the end of the oxidative process. In this respect the method is similar to that used by Gettler and Bastian ( 2 ) for the preparation of tissue or blood for zinc analysis, and to that of Feldstein and Klendshoj (1) in preparing tissue samples for mineral analysis in toxicology studies. However, all these workers used Kjeldahl flasks in which to carry out the oxidations. PROCEDURE
All the quantities mentioned in the
1728
0
ANALYTICAL CHEMISTRY
subsequent directions are for samples of approximately 5 grams, dry weight. For other quantities the proportions should be altered accordingly. Weigh the sample accurately into a 250-ml. Erlenmeyer flask and add 35 ml. of codcentrated nitric acid. All the acid a n be added a t once unless the sample is such that it foams badly on the addition of the acid. I n this case, add only about 5 ml. of acid; allow this to react before adding the remainder. Set the flask on a hot p h t e using the lowest temperature setting in a hood to digest very slowly for about 4 to 6 hours. At this stage of the digestion, the solution is a clear yellow and the initial volume is not markedly reduced. Cool the sample overnight, whereupon any fat that is present solidifies. If there is a significant amount, carefully filter it off by pouring the sample through a thin layer of glass wool in a stemless funnel. Return the clear filtrate to the Erlenmeyer and wash the glass wool several times with a small amount of concentrated nitric acid. Add 2 ml. of concentrated sulfuric acid and return the flask to the hot phte to heat rapidly. If the sample has a tendency to lLbump”or splatter during this step, lower the heat and cover the flask with a stemless funnel until the sample boils smoothly. The sample needs no careful watching until the volume is well reduced and a frothing action sets in. Charring a p pears rather suddenly. If a number of samples are being run simultaneously, the charred sample can be removed from the heat a t this point and the oxidation completed a t a later time if desired. To the hot charred sample carefully add dropwise a digestion mixture consisting of two parts of 72% perchloric acid and one part of concentrated nitric acid. By adding the. perchloric acid
dropwise, the danger of violent explosion is eliminated and the material is oxidized quickly and completely About 5 minutes are required for this step. The color of the mixture gradually turns from black to brown to colorless as the digestion mixture is added and heating is continued. Only 0.5 to 3.0 ml. of the digestion mixture are necessary to clear the material. When the sample is clear, heat it strongly at the fuming stage for an additional 10 to 15 minutes. Only the white or slightly yellow mineral residue and a small amount of sulfuric acid are left in the flask. Remove the sample from the heat, cool, and add about 10 ml. of distilled water and 3.0 ml. of concentrated hydrochloric acid. Boil the contents gently to accomplish solution and then pour into a volumetric flask of appropriate size. A blank on the reagents can be prepared by evaporating the same quantity of the acids as used in the unknown. For very accurate mineral work it is advisable to use redistilled acids. Directions for this redistillation have been described ( 4 ) . Samples of various feedstuffs, animal tissue, and milk have been oxidized successfully and subsequently analyzed for calcium, phosphorus, iron, and zinc using the described method of oxidation. The procedure appears suitable for a wide spectrum of trace mineral analyses, and hence would be a helpful tool for many laboratories carrying on this type of investigation. LITERATURE CITED
(1) Feldstein, M., Klendshoj, N. C., Analyst 78,43-7 11953). ( 2 ) Gettler, A. O., Bastian, R., Am. J . Clin. Pathol. 17,244-9 f1947). (3) Middleton, G., Stuckey, R. E., Analyst 78,53242 (1953). (4) Piper, C. S., “Soil and Plant Analysis,” p. 306, Interscience, New York, 1944.
