sodium ion. Since the sodium cobaltinitrite method for potassium provides no correction for the sodium ion, the results are inaccurate. The influence of sodium on the potassium determination with sodium tetraphenylboron shows little or no effect (Table 11). Other ions that also interfere with the sotliuni cobaltinitrite method are iron, aluminum, calcium, magnesium, and copper ( 4 ) . However, t’hese elements :IS well as manganese, c>obnlt: nickel, mlfate. and phosphate do not interfere wit’li the sodium tetraphenylboron n1ethod ( 6 ) . The determination of potnssium in tlie sodium salts and sodium hydroxide 1))- the sodium tetraplienylboron nietliotl gives results in closer agreement x i t h the flame photometric findiiigs than results obtained with the turbidimetric sodium robaltinitrite method. I n general, t’lie wriation in tlie two methods is greatest when the sodium ion concentration of the reagent is over 40%. Thus, good agreement \vv:is realized by each method for sodium carbonate monohydrate having a sodium content of 37%, whereas the anhydrous salt having R sodium content
of 43.4y0 gave poor agreement. Sodium hydroxide, with a sodium content of 57.5%, showed the greatest variance. The potassium recoverability of the sodium cobaltinitrite method is only half as sensitive as the sodium tetraphenylboron method and is limited in range from 10 t o 20 p.p.ni.I a s shown in Table 111. The salts of the sodium tetraphenylboron complex are well defined, the potassium salt corresponding exactly to the formula KB(C6H5)4.The cobaltinitrite precipitate is usually a mixture of monopotassium disodium cobaltinitrite and dipotnssiuni monos.odium cobaltinitrite ( 7 ) .
for determination of traces of potassium in reagent chemicals. ACKNOWLEDGMENT
The authors wish to thank A. J. Barnard, Jr., E. F. Joy, and E. C. Larsen of the J. T. Baker Chemical Co. for their invaluable assistance in the preparation of this paper. LITERATURE CITED
CHEMICAL SOCIETY, “Reagent Chemicals, ACS Specifications,” 1955. Ibid.. n. 19. -hERICAX
CO NCL USI0 N
The recoverability of potassium and reproducibility of tlie sodium tetraphenylboron method demonstrate the suitability of this reagent as a replacement for odium cobaltinitrite. Good filtering char:wteristics, extremely low solubility of its potassium salt (2.25 x 10-8) ( 5 ) , and the simplicity of the gravimetric method make sodium tetraphenylboron an excellent reagent
Separation and Determi nation of by Liquid-Liquid Extraction
Gloss, G. H.,’Chemist Analyst 42, 50-5 (1955). Snell, F. D., Snell, C. D., “Colorimetric Methods of Analysis,” Vol. 11, 3rd ed., p . 556, Van Nostrand, Yew Tork, 1949. RECEIVED for review October 4,1956. Accepted February 5 , 1957. Division of *4nalytical Chemistry, Fine Chemicals Symposium, 100th Meeting, ACS, Atlantic City, N. J., September 1956.
Neptu nium
FLETCHER L. MOORE Oak Ridge National laboratory, Oak Ridge, Tenn.
b A rapid and quantitative radiochemical method for the determination of neptunium-237 or neptunium239 tracer is based on the liquid-liquid extraction of neptunium(lV) into 0.5M 2 thenoyltrifluoroacetone - xylene. Neptunium is separated free from interferences, both radioactive and nonradioactive. The technique may b e adapted readily to remote control, and is very effective in the purification of neptunium tracer.
-
0
of nuclear reactors of increased neutron flux has stimulated interest in the isolation and determination of the long-lived neptuniuni237 alpha emitter (tliz = 2.2 X 106 years). A method ( I ) had been developed previously for the determination of the neptunium-239 beta, gamma emitter (tliz = 2.3 days); however, because this method allowed approximately 16% of the plutonium originally PERATION
present to follow through the procedure, it could not be used for the determination of neptunium-237. It was necessary to develop a method which would achieve a clean separation of neptunium from fission products, uranium, plutonium. americium, and curium. A carrier-free method was desired to eliminate alpha absorption errors. Experience accumulated in the development of a solvent extraction method for the determination of plutonium (4) using thc chelating agent. 2-thenoyltrifluoroacetone (TTA), suggested that an effective radiocheniical procedure for the determination of neptunium-237 could be devised through the use of this reagent. JIagnusson, Hindman, and La Chapelle ( 3 ) extracted neptunium-237 with 2-thcnoyltrifluoroacetonebenzene away from plutonium and uranium under suitable reducing conditions. It was a l w desirable to use the new method for the isolation and determination of neptunium-239.
Certain substituted, fluorinated betadiketones react with metal ions to form nonionized chelate compounds which are soluble in nonpolar solvents immiscible with water. Many of these ions can be separated from each other because of the strong dependence of the extraction of these chelate compounds in nonpolar solvents on the acid concentration. Thomas and Crandall ( 6 ) report that very few aqueous ions extract appreciably from 0.5X nitric acid. These ions are zirconium(IJ‘), plutonium(1T’) neptunium(IV), cerium(IV), uranium (IV), iron(III), and tin(1V). I n the determination of neptunium23i, the most difficult separation is that of neptunium from the plutonium and uranium which arc usually prescnt in the solutions to be analyzed. Previous workers ( 3 ) have shown that under suitable reducing conditions a solution may contain neptunium(IT’), plutonium(III), and uranium(VI), and the neptunium(1T’) may be extracted I
VOL. 29, NO. 6, JUNE 1957
941
with 2-thenoyltrifluoroacetone-benzene. This paper describes the optimum conditions developed for the quantitative recovery of neptunium-237 or -239 and the decontamination from the usual interferences of other radioelements. REDUCTION OF PLUTONIUM
It was realized that the most difficult separation from alpha emitters would be that of neptunium from plutonium, while radiozirconium mould be the most difficult of the beta-gamma emitters to remove. Plutonium had to be reduced quantitatively to the inevtractable trid e n t state and maintained in this reduced state during the extraction, while neptunium was reduced to the quadrivalent state which was readily extractable into 2-thenoyltrifluoroacetonexylene from lJ1 hydrochloric acid or 1-11 nitric acid. Hydroxylamine hydrochloride, stannous chloride, potassium iodide, and ferrous chloride were tested for reduction efficiency in preliminary experiments. Plutonium(1V) tracer was reduced for 5 minutes at room temperature and then the aqueous phase (1M nitric acid) was extracted for 10 minutes ITith an equal volume of 0.5M 2-thenoyltrifluoroacetone-xylene. Hydroxylamine hydrochloride was the least effective of the reagents tested for holding plutonium in the inextractable trivalent state during the extraction. However, by using very high concentrations of hydroxylamine hydrochloride (4,5X), i t was possible to keep the plutonium approximately 97% in the trivalent state during the 10minute extraction period. Stannous chloride, potassium iodide, and ferrous chloride were effective reductants. Stannous chloride invariably produced emulsions due to hydrolysis. It was decided to develop the method using potassium iodide or ferrous chloride as the reductant for plutonium and neptunium. Potassium Iodide Method. Prex4ous workers (3)produced a plutonium(111)-neptunium(1T') solution by heating a n aqueous solution of 0.1M potassium iodide and 5M hydrochloric acid a t 100" C. for several minutes. Experiments were performed in this laboratory to determine the optimum concentrations of potassium iodide necessary to hold plutonium in the t r i d e n t state a t a n acid concentration suitable for the extraction of the neptunium(1V)2-thenoyltrifluorometone chelate. Tracer concentrations of plutonium(1V) were added to aqueous solutions of varying Concentrations of hydrochloric acid and potassium iodide. The aqueous phases were extracted a t room temperature for 10 minutes with a onehalf volume portion of 0.5M 2-thenoyltrifluoroacetone-xylene. The organic phases were washed for 3 minutes with 942
*
ANALYTICAL CHEMISTRY
an equal volume portion of 1M hydrochloric acid and centrifuged for 3 minutes before counting the plutonium alpha radioactivity extracted.
of neptunium(1V) is exeellent from 1.11 hydrochloric acid but decreases a t higher acid concentrations.
Table I indicates that the use of a n aqueous phase 2 to 2.5M in potassium iodide and 1 M in hydrochloric acid effects a n excellent separation from plutonium.
Table 111. Effect of Hydrochloric Acid Concentration on Extraction of Neptunium(1V) HC1, Sf
Table I. Reduction of Plutonium(1V) Tracer with Potassium Iodide in Hydrochloric acid Plutonium Final -Iq. Concn. Tracer HC1, Jf KI, J I Extd., % 0 5
0.15 0.30 0.60 0.15 0 30 0.60 1.00 1.50
1 .o
6 40 4 20
3.20 3.10
2.20
0.70 0.55 0.15
2.00
0.02
2.50
0.01
The extraction of neptunium(1V) as a function of time was studied in a series of experiments. A high-speed motor stirrer (Palo Laboratory Supplies, New York, N. Y.) with a glass paddle gave excellent mixing of the phases. dqueous solutions of 1X hydrochloric acid and 2M potassium iodide containing tracer neptunium-237(IV) were extracted with one-half volume portions of 0.5M 2-thenoyltrifluoroacetone-xylene a t room temperature for varying periods of time. The organic phases aere washed for 3 minutes with a n equal portion of 1M hydrochloric acid and centrifuged for 3 minutes before counting the neptunium-237 alpha radioactivity. Table I1 indicates that a 10-minute extraction period effects essentially quantitative recovery of neptunium.
Table II. Extraction of Neptunium(1V) as a Function of Time Neptunium Time, >fin. Tracer Extd., % 2 5
8 10
86.1 97.7 98.2 99.1
The effect of hydrochloric acid concentration on the extraction of neptunium(1V) was studied next. Aqueous solutions of 2M potassium iodide and varying concentrations of hydrochloric acid containing tracer neptunium-237(IV) were extracted for 10 minutes a t room temperature with one-half volume portions of 0.5M Z-thenoyltrifluoroacetone-xylene. The organic phases were washed as above. Table I11 indicates that the extraction
0.5 1.0
2.0 3.0
Xeptrinium
Tracer Extd., c/o 98 7 99 1 97.5 71.8
Ferrous Chloride Method. K h e n ferrous chloride was substituted for potassium iodide as a reductant, t h e recovery and decontamination of neptunium were equally efficient. Use of ferrous chloride was particularly advantageous, in t h a t a &minute reduction a t room temperature \Tith 0.25-I-f ferrous chloride effects quantitative reduction of the higher oxidation states of neptunium(V,VI). A 20minute reduction period a t 80" C. was found to be necessary when 2A1 potassium iodide was used. Some free iodine may have been produced. The optimum conditions n-ere as follow: An aqueous phase containing the neptunium tracer \vas adjusted to a concentration of 1 M hydrochloric acid1M hydroxylamine hydrochloride0.25M ferrous chloride. The hydroxylamine hydrochloride reduced oxidants such as potassium dichromate which may have been present. After a 5minute reduction period a t room temperature, a 10-minute extraction with an equal volume of 0.5M 2-thenoyltrifluoroacetone-xylene was performed. The recommended procedure given below gave a neptunium recovery of 99 =k 3%. Reagents. Hydrochloric acid, C.P. Hydrochloric acid, 1M. Sitric acid, 10X. Hydroxylamine hydrochloride solution, approximately 5 M . Dissolve 69.5 grams of reagent grade hydroxylamine hydrochloride in 200 ml. of distilled water. Warm to effect solution, if necessary. Ferrous chloride solution, approximately 2iM. Dissolve 40 grams of reagent grade ferrous chloride tetrahydrate in 100 ml. of 0.2M hydrochloric acid. Store the solution in a dark glassstoppered bottle Make fresh solution every 2 weeks. 2 - Thenoyltrifluoroacetone - xylene solution, 0.5M. Dissolve 111 grams of 2-thenoyltrifluoroacetone (Graham, Crowley, and Associates, Inc. , 5465 West Division St., Chicago 50, Ill.) in 1 liter of reagent grade xylene. Standard Procedure. h suitable aliquot of the sample solution is pipetted into a separatory funnel or other extraction vessel. T h e solution is adjusted to a concentration of 1 M hydrochloric acid-1M hydroxylamine hydrochlorideO.25M ferrous chloride by t h e addition of appropriate quanti-
ties of these reagents. Potassium iodide may be used as a reductant in 1)lace of ferrous chloride. I n this case, the aqueous solution is adjusted to 1;M hydrochloric acid-1M hydroxylamine hydrochloride-2%' potassium iodide, niised well, heated to approximately 50" C. for 20 minutes, and cooled to room temperature before performing the extraction. Occasionally some free iodine is produced, but this does not interfere with the neptunium recovery. The aqueous phase before the extraction should be approximately 1M in total acid, preferably hydrochloric acid. The nitric acid concentration should be kept as low as is practical. The solution is mixed well and, after a %minute reduction period a t room temperature, it is extracted for 10 minutes with an equal volume of 0.5M 2-thenoyltrifluoroacetone-xylene. ltThenthe two phases have disengaged, the aqueous phase is drawn off and discarded. The organic phase is washed by mixing with an equal volume of 1 M hydrochloric acid for 3 minutes. Wash losses are usually less than 0.1%. After the phases have settled, the aqueous wash solution is discarded, care being taken not to lose any of the organic phase. The neptunium is then stripped from the organic phase by mixing thoroughly for 2 minutes with an equal volume of l O M nitric acid. If the aqueous strip solution is too high in gamma radioactivity for alpha measurement, the last traces of radiozirconium and protactinium may be removed readily by performing a 5-minute re-extraction of the l 0 M nitric acid strip solution with an equal volume of 0.5M 2-thenoyltrifluoroacetone-xylene. The small amount of iron extracted (0.04 mg. per ml. of organic phase in a typical experiment) produces a red color and remains in the organic phase when the neptunium is stripped into 10M nitric acid, effecting an excellent separation of iron from neptunium. Ordinarily, if an aliquot of the organic phase is eJiaporated for a neptunium-237 alpha determination, the small amount of iron present causes negligible selfabsorption of the alpha particles. The aqueous strip solution is drawn off into a centrifuge tube and centrifuged for 1 minute. A suitable aliquot of the strip solution is prepared by conventional methods for either alpha counting ( 2 ) for neptunium-237, or gamma counting (6) for neptunium-239. SEPARATION OF NEPTUNIUM FROM OTHER E1EMENT S
Under the conditions described, an excellent separation of neptunium is effected from aluminum, iron, fission products, thorium, protactinium, uranium, plutonium, americium, and curium. I n the determination of neptunium-237, the final stripping step may be omitted and an aliquot of the organic phase counted for alpha radioactivity. The radiozirconium (beta-gamma radioactivity) in the organic phase ordinarily will not interfere with the alpha measure-
ment. Radiozirconium is the only fission product that extracts into the 2thenoyltrifiuoroacetoncxylene. The stripping solution (10M nitric acid) removes the neptunium quantitatively from the organic phase. Less than lY0 of the radiozirconium is stripped into the 10M nitric acid. If necessary, eoen this small amount of radiozirconium in the final stripping solution may be reduced t o a negligible value by performing a 5minute extraction of the stripping solution with a n equal volume of 0.531 thenoyltrifluoroacetone-xylene. Keptunium remains quantitatively in the aqueous phase. Protactinium behaves like zirconium in the procedure. Typical recoi-ery and decontamination values for neptunium are shown in Tahle IV.
Table IV. Recovery and Decontamination of Neptunium Element yo FOiilld Keptunium 99 3 Plutonium 0 02 dmer,icium 0 02 Ur aniuni 0 04 Zirconium 0 5. a Re-extraction accounts for negligible value.
INTERFERENCES AND GENERAL APPLICATION OF METHOD
K n o m interferences are free sulfate, phosphate, fluoride, and oxalate ions. Tahle V shows the effect of sulfate ion and indicates that free sulfuric acid should be maintained a t a concentration of less than 0.1.Y. The standard procedure \vas employed, n-ith the exception that varying concentrations of sulfuric acid were used in the original aqueous phase. The total acidity was kept 1s.
Table V. Effect of Sulfuric Acid on Extraction of Neptunium(lV) with 0.5M
2-Thenoyltrifluoroacetone-Xylene Xeptunium( I V ) H2SOr, 1%Tracer Extd., % 0 0.13 0.25 0.38
99.3 96.6 93.1 87.2
Xeptunium(1V) is unstable in nitric acid (3) and previously it has been customary t o destroy the nitric acid when preparing neptunium(1V) solutions. However, data in Table VI show that neptunium(1V) tracer can be extracted essentially quantitatively by the procedure given above from nitric acidhydrochloric acid misturee or even from nitric acid alone. The neptunium
Table VI. Effect of Nitric Acid on Extraction of Neptunium Using Standard Method XeDtuniiirii ?racer Aq. Phase -1cidity Extd., c L 1 0.5-11 H?*03-0.5.21 HC1 99 5 2. 1X HSOI 99 5 3. 0.234 HN03-0.8JI HC1 99 4 4. 1-11"03 09 2
tracer stock solution for experinienta 1 and 2 (Table VI) was originally contained in 2 X nitric acid and probably IT as chiefly neptuniuin(Y). The stantlard procedure n as used, except that thc original aqueous acidity was altered as indicated. I n experiments 3 and 4 (Table VI) the neptunium tracrr stock solution \vas hexavalent neptunium originally contained in 0.1.V potassium dichromate1.5M nitric acid. The results of these tIvo experiments indicate that neptunium tracer may be recovered essentially quantitatively when introduced into the method as neptunium(V1). Khile the extraction of neptuniuin(IV) tracer from a nitric acid medium appears t o be very effective when the standard procedure is used, it is recommended that hydrochloric acid be used whenever possible, as in the preparation of dilutions, because there is some tendency for the formation of extractable ferric ion in the nitric acid system. K h e n it is not known what interferences are present in solution with the neptunium tracer, the neptunium(IT-) tracer can be carried on 0.5 to 1.0 nig. of lanthanum precipitated as the hydroxide. Then the precipitate iq dissolved in 1 M nitric acid, lanthanum fluoride precipitated, and this precipitate dissolved in several drops of 2-11 aluminum nitrate and dilute hydrochloric acid. The aqueous solution may be treated by the standard extraction procedure. Usually only the lanthanum fluoride precipitation is necessary to remove neptunium tracer from many interferences. If it is desirable to use a yield correction for neptunium, which obviously i- necessary if much involved chemistry is rniployed, neptunium-237 is useful f-r determining the neptunium-239 recovery 11hilc neptunium-239 m:iy iic iiietl t o 1iiilir:itc. the neptunium-237 yield. The liquid-liquid extraction technique described haa beeii applied succtssfully for several years t o the purifiLntion and isolation of neptun~uni-237and neptunium-239. ACKNOWLEDGMENT
The author gratc,fully acknon-ledges VOL. 29, NO. 6 . JUNE 1957
e
943
the able assistance of John Slessinger, Ray Druschel, and the personnel of the Radioisotope Analytical Laboratory under the direction of Edward K y a t t for assistance in testing the method described in this paper. LITERATURE CITED
(1) Hudgens, J. E., Jr.,
F.S. Atomic
Energj- Commission Confidential Rept. MonN-13 (September 1945). (2) Jaffey, H., “The Actinide Elements, G. T. Seaborg, J. J. Katz, eds., National Nuclear Energy Series, Division IV, Vol. 14A, 596636, McGraw-Hill, New York, 1954 (3) Magnusson, L. B., Hindman, J. C., La Chapelle, T. J., U. S. Atomic Energy Commission Declassified Rept., ANL-4066 (October 1944). (41 lloorc, F. L., Hudgens, J. E., Jr.,
5.
U. S. htomic Energy Commission Secret Rept. ORNL-153 (September 1948). ( 5 ) Reynolds, S. A,, Record Chem. PTogr., 16, No. 2, 102 (1955). ( 6 ) Thomas, J. R., Crandall, .H..W., U. S. Atomic Energy Commission Secret Rept., CN-3733 (December 1946). RECEIVEDfor review October 11, l!I56. Accepted December 31, 1956.
Acid Requirements of the Kjeldahl Digestion R.
B.
BRADSTREET
The Bradsfreef Laborafories, Inc., P.O. Box
b In studying the acid requirements of a Kjeldahl digestion, acid indices have been calculated from which optimum conditions of digestion may b e determined. Data are presented showing that loss of nitrogen occurs as the digest approaches a solid state. Sodium sulfate requires a higher ratio of acid to salt than an equivalent amount of potassium sulfate or potassium sulfate-sodium thiosulfate mixture, thus making this salt unsuitable for use with refractory compounds.
A
s
A MEANS of determining nitrogen, the Kjeldahl macromethod is ii convenient and relatively simple procedure requiring no complicated equipment or technique. However, eyen after 73 years, comparatively little has been published on the mechanism of the reaction. Hot, concentrated sulfuric acid acts as a weak oxidizing agent. Severtheless, the condition existing in tlie boiling acid is favorable to reducl d be tion; otherwise, nitrogen ~ ~ o u not recovered as ammonia, but probably would be lost as nitrogen. It is quite likely that the oxidation-reduction range, within which nitrogen is reduced to ammonia, is a narrow one. This may be an explanation of loss of nitrogen w1iic.h sometimes occurs when oxidizing groups or halogens are present in a compound. Charring generally takes place in the hot acid, and the resulting carbon acts as a reducing agent. Oxidized forms of nitrogen are only partially reduced under these conditions, and are usually subjected to a pretreatment with a suitable reducing agent. Another factor to be considered is the temperature, which a t all times should be high enough to induce and ensure pyrolytic decomposition of the sample. Various factors are involved in the Kjeldahl digestion--e.g., acid required,
944
ANALYTICAL CHEMISTRY
I , Cranford, N. J.
digestion temperature. boiling rate, salt addition, catalysts, reducing agents, salicylic acid or related compounds, and length of boiling period. The acid requirements are. naturally, based on whatever modification is being used. As an example. in a digestion mixture involving the use of salicylic acid and thiosulfate, the following factors must be considered: conversion of potassium sulfate to the acid sulfate, of sodium thiosulfate to sodium acid sulfate, of salicylic acid to carbon dioxide and water; loss of acid through volatilization during the entire digestion and boiling period; decomposition of the sample; and minimum excess of acid a t the end of tlie determination.
If 10 grains of potassiuni sulfate are used, the conversion to potassium acid sulfate li,YO4 H2SO4 2KHSOa
+
-+
requires 5.6 grams of sulfuric acid. When 5 granis of sodium thiosulfate pentahydrate are wed as a reducing agent, 3.87 grams of acid are required
There is also a secondary reaction which takes place in the hot concentrated acid, converting the sulfur liberated from the thiosulfate into sulfur dioxide: Y
+ 2H2S04
-+
3SOs
+ 2H2O
This reaction requires 12.1 grams of sulfuric acid. Salicylic acid, as such, is converted into carbon dioxide and water
and 1 gram of salicylic acid consumes 10 grams of sulfuric acid. The loss of acid during digestion and the boiling period is dependent upon the boiling rate and the total digestion time. With a n over-all boiling time of 90 minutes-representing a 30-minute
clearing time and 1 hour’s additional boil-an average loss for several runs was found to be 3.2 grams, calculated as sulfuric acid, for a mixture of 10 gram. of potassium sulfate and 30 ml. of sulfuric acid. At this point, 31.6 grams of sulfuric acid, or 18 nil. (based on acid strength of 95.5% and a specific gravity of 1.84) have been used for conlersion to acid salts and decomposition of salicylic acid. I n addition, assuming a total elapsed time of 90 minutes, 3.2 grams of sulfuric acid, or 1.8 ml., are lost through volatilization, bringing the total volume of acid used t o 19.8 nil. This, then, leaves 9.2 ml. of acid for excess and for decomposition of the sample. Khere the material involved is high in nitrogen and the amount of sample is smalle.g., 0.1 to 0.2 gram-the amount of sulfuric acid used does not present any difficulty, but n i t h materials lon in nitrogen (11hich necessitate large samples), this amount of acid map not be sufficient for either the sample or for maintaining a proper excess. Both Self ( 8 ) and Carpiaux (3) have reported loss of nitrogen when the final digest nas solid. Self further recommended that at least 15 grams of acid be present a t the end of the digestion. I n a series of determinations, the minimum amount of acid necessary to avoid 10s‘. of nitrogen was found t o be actually less than 15 grams. Two typical digestion mixtures were used: d. 10 grams of potassium sulfate and sulfuric acid; B. 10 grams of potassium sulfate, 5 grains of sodium thiosulfate pentahydrate, and sulfuric acid. An equivalent amount of sodium sulfate (2.86 grams) was substituted for the thiosulfate. This is representative of the condition existing after the thiosulfate has been oxidized to sodium sulfate. The amounts of acid necessary for conversion of the sulfates to the acid sulfates are 5.6 grams for mixture A and 7.6 grams for mixture B. I n each case, the volume of sulfuric acid was varied from 5.00 nil. to 12.5 ml. A11 determi-
