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-
Table I. Recovery of Ammonium Sulfate Digestion Mixture A
M1. H&Oa Present at Start of Boil Period 5.00 7.50
10.00 12.50
State of digest cold Solid Solid Solid Solid Pasty Pasty Liquid Liquid
(?;Hl)zSOd, Grams Sdded Recovered 0.1006 0.0962 0.1004 0,0964 0,1006 0.0976 0.1000 0,0970 0.1002 0.1000 0.1002 0.1001 0,1005 0.1006 0.1000 0,1000
nations nerc given a BO-minute tligebtiori representing the boil period after the clearing of a digestion, and the acid lost during this period was prorated on the basis of a loss of 3.2 grams over a 90minute period (2.1 grams). Therefore, the basic acid requirements are 7.8 grams (4.4 ml.) for digestion mixture A, and 9.7 grams (5.5 ml.) for digestion mixture
n.
The following procedure was used. Digestion mixtures were prepared using 5.00, 7.50, 10.00, and 12.50 ml. of sulfuric acid; 0.1 gram of ammonium sulfate vias added to each. All were boiled for 1 hour, cooled, diluted, and distilled in the usual way. Results of a series of duplicate determinations are given in Table I.
It seems apparent that the critical point lies between 7.50 nil. and 10.00 ml. It can be deduced from these results that the amount of sulfuric acid present in the usual digestion mixture is sufficient unless subjected to a n unduly long heating period or the necessity of using H large sample. Also, as the digestion mixture approaches the composition of the acid sulfate, loss of nitrogen will occur. The physical appearance of the digest also gives a n indication; a loss of nitrogen can be expected if the cooled digest is solid, or nearly so. The amount of acid necessary to deconipose a sample is to some extent dependent upon its structure. The requirements of various types of material (Table 11) were obtained by the follon-ing procoedwe. One-gram samples were digested in a mixture of 10 grams of potassium sulfate and 30 ml. of sulfuric acid (buret), with 0.25 gram of seleniumferrous sulfate (1 t o 1) as a catalyst. After the digestion mixture cleared, boiling was continued for 1 hour. When cool, the digest was diluted with distilled water, transferred to a volumetric flask, and made u p t o 250 ml. An aliquot was taken and titrated with standard alkali. Blanks were also prepared and boiled for 90 minutes. Calculations. The acid used was 95.5$&, with a specific gravity of 1.84. The total 1%eight of acid a t the start is 30 X 0 955 = 52.7 grams of
sulfuric acid
(1)
75
Recovered 95.67 96.02 97.04 97.00
99.80 99.90 100.10 100,00
Table II.
State of digest cold Solid Solid Solid Solid Fluid Fluid Fluid Fluid
Digestion Mixture B (NH4)2SOa,Grams Added Recovered 0.1000 0.0085 0.1005 0.0967 0,1007 0.0982 0.1002 0.0975 0.1008 0.1008 0,1003 0,1004 0.1001 0.1001 0,1003 0.1003
%
Recoverd 95.50 96.17 97.53 97.32 100.00
100.10 100.00 100.00
Sulfuric Acid Necessary for Digestion of Various Substances
Total Grams Net Grams H2S04 Digestion HZSOa/Gram Sample Used Time, Min. of Sample Sulfuric acid 3.2@ 90 13.11 Salicylic acid 75 10 04 15.03 Benzoic acid 75 12 37 95 11.73 Sucrose 8 36 80 13 92 19.57 Anthranilic acid -10 22 Aminosalicylic acid I3 12.87 Acetanilide 15 24 80 18.08 10 87 23.42 Oleic acid 100 22.23 Stearic acid 18 86 95 19 27 22.82 Crepe rubber 100 95 15 87 19.24 Buna rubber 100 11 56 15.11 Light lube oil 105 71 92 15.65 Heavy lube oil 13.75 95 10.38 Leather (chrome tan) 11.49 95 8.12 Wool (flannel) 12.21 100 8.66 Hemoglobin 12.09 95 8.72 Egg albumin 11.73 95 8.36 Blood albumin 19.94 90 16.74 Gelatin Casein 13.04 95 9.67 Corn meal 9.82 100 8.27 Dextrine 10.66 95 7.29 Loss over 90-miniite boil period; all results calculated on prorated loss. Table 111. A l l . HSOn
Present a t Start of Boil Period 5,oo 7.50 10.00
12 50 15.00 17,50 20.00
Grams HzSO, Calcd. 10 0
12 05 6 88 10 73 8 92
13 7$) 17 72 17.94
..
... . . . . ... ... , . .
... ...
Recovery of Ammonium Sulfate Using Sodium Sulfate
State of Digest Cold Solid Solid Solid Solid Solid Solid Solid Solid Solid Solid Pasty Pasty Fluid Fluid
(NH&30a, Grams Added Recovered 0.1083 0 0.1001 0 0.1029 0 0 1014 0 0.1001 0 0975 0 1005 0 0977 0.1006 0 0987 0.1000 0 0979 0 1011 0 0979 0 1007 0 0972 0.1036 0 1018 0 1028 0 1003 0.1004 0 1003 0 1020 0 1009
and a t the end 0.049 X S X ml. of alkali = grams of sulfuric acid The difference betn-een Equations 1 and 2 represents the amount of acid used by the sample and lost on boiling. The blank determinations showed an average loss of 3.2 grams of acid during a 90-minute boiling time. Prorating gives a value of 0.036 gram of sulfuric
ci:
/c
Recovered 0 0
0
0
97.45 (17.21 !58 12 97.90 96.78 97.01 98.28 98.37 99.92 99.87
acid lost per minute. Therefore. the amount of acid used by the sample is Eq. 1 - Eq. 2 -
(0.036 X time in minutes) (3)
Self has stated that carbohydrates require approximately 7.5 grams of acid per gram; proteins, 9.0 grams per gram; and fats, 17.8 grams per gram. Comparison with similar compounds in Table VOL. 29, NO. 6, JUNE 1957
945
I1 shows that these amounts are more or less in agreement. From time to time the use of other snlts has been advocated, particularly sodium sulfate ( I , 2, 5, 6, 9). Houever, where a relatively high temperature is necessary, it iq unsatisfactory because solid digests are obtained unless a relatively large excess of sulfuric acid is present. The increase in the acid to salt ratio results in a lower temperature and thus eliminates the uSe of this sulfate for the more refractory compounds. A series of determinations similar to those made with potassium sulfate was run using 12.8 grams of sodium sulfate (equivalent to 10 grams of potassium sulfate and 5 grams of sodium thiosulfate pentahydrate) . The results (Table 111) indicate that the same condition exists as with potassium sulfate: When the cooled digest is solid, there is a loss of nitrogen. The effect of salt addition, naturally, is to elevate the temperature. The importance of the consequent higher temperatures has been pointed out by Lake (4) and Perrin ('7). Figure 1 shows results obtained by progressive addition of potassium sulfate to 30 nil. sulfuric acid and to 30 ml. of sulfuric acid containing sodium sulfate equivalent to 5 grams of sodium thiosulfate pentahydrate. The mixtures were brought to a steady boil, and the temperature mas taken with a thermometer enclosed in a thermometer well in the Xjeldahl flask. If these temperatures are plotted against the ratio of acid (volume) to salt (weight), the curves shown in Figure 2 are obtained. By selection of the proper acid to salt ratio, or acid index, the conditions of digestion can be regulated to obtain optimum temperature. The data in Table I show that, under the conditions stated, certain acid indices euist, beyond
946
ANALYTICAL CHEMISTRY
which it is unsafe to go. I n calculating the ratios after digestion, an arbitrary loss figure of 0.0355 gram per minute calculated as sulfuric acid has been assumed. The value for sulfuric acid lost during the stated boil periods is not exact, and an actual value determined on individual samples would undoubtedly vary from the value used here. The digestions represent the boiling period after clearing of a digestion, when all organic matter has presumably been destroyed and nitrogen is present as ammonium sulfate. K i t h a 60minute boil period, the calculated loss will be 60 X 0.0355 or 2.13 grams of sulfuric acid (equivalent to 1.22 ml. of 95.5% acid). At this point in the total digestion period, this nil1 be the only loss. Consequently, the volume of acid a t the end of 60 minutes of boiling will be the original volume minus this loss. The calculated values are shown in Table 117. Examination of these data shows that the critical point beyond which nitrogen will be lost is represented by an acid index of 0.88 for digestion mixture A, and 0.68 for digestion mixture B. For optimum conditions the acid index a t the end of the digestion should not be lower than these limiting values.
Table
IV. Acid Index
M1.
bll. H2SOd
after Acid 60-Min. before Digestion Index Digestion Digestion Mixture A 5.00 0.50 3.78 0.75 6 28 7.50 1.00 8.78 10.00 11.28 1.26 12 50 Digestion Mixture B 0.39 3.78 5.00 0.58 6.28 7.50 0.78 8.78 10.00 0.97 11.28 12.50
30 mi. of sulfuric acid
0.38 0.63 0.88 1.13 0.37 0.49 0.68
0.88
The amount of acid necessary for the decomposition of the sample will vary. Table I1 shows some actual results, and Figures 3 and 4,some calculated values. The total amount of acid will vary depending upon the size of the sample, ease of decomposition, rate of heating, over-all time of digestion, use of salicylic acid or related compounds, and the like. A reasonable estimate of the amount necessary can be made from actual data or calculated values, and an acid index
ACI!
Figure 1 . Effect of salt addition on temperature
Acid Index
/ 54-T
9>-10
Figure 2. Temperature vs. ratio of acid to salt
selected to give optimum conditions throughout the digestion period. LITERATURE CITED
(1) Alcock, R. S., Analyst 71, 233-4
(1946). (2) Ashton, F. L., J . SOC.Chenz. I n d . 56, 1014-T (1937).
(3) Carpiaux, E., Bull. SOC. chiin. belg. 27, 13-14 (1913). (4)Lake, G. R., ANAL.CHEJI. 24, 1806 (1952). (5) Latshatv, W. D., J. Ind. Eng. Chenc. 6, 586 (1915). (6) Messman, H. C., Cereal Chem. 9, 357 (1932). ( 7 ) Perrin, C. H., AUL. CHEM.25, 968 (1953).
(8) Self, P. B. W., Pharm. J . 8 8 , 384 (1912).
(9) Umbreit, W. W.,Bond, V. S., I s u . ENQ.CHEM.,AXAL. ED. 8, 276-8 (1936).
RECEIVEDfor review N a y 29, 19.56. Accepted February 13, 1957.
TantaIum Determination in Presence of Niobium by Precipitutio n with N-Benzoyl-N-pheny Ihyd roxyla mine ROSS W. MOSHIER and JAMES E. SCHWARBERG Wright Air Development Center, Wright-Patterson Air Force Base, Ohio b A quantitative gravimetric determination o f tantalum in the presence of niobium, titanium, and zirconium can b e made by precipitation with N-benzoylN-phenylhydroxylamine. The sample, containing no more than 50 mg. of tantalum oxide, is dissolved in 200 ml. of solution containing 0.01 6 to 0.020 mole of hydrofluoric acid acidified with sulfuric acid to pH 1 .O =t 0.1, and the tantalum i s precipitated b y adding a hot aqueous solution o f Nbenzoyl N - phenylhydroxylamine. The resultant mixture, allowed to stand in a t a p water bath, i s filtered after 2.5 hours, washed with a saturated solution of the reagent, and ignited a t 900" C. One dissolution and reprecipitation are necessary for samples containing less than 10 mg., and two for samples containing more than 10 mg. of niobium pentoxide. The error in the determination i s 0.2 mg. of tantalum oxide. Titanium and zirconium show negligible interference compared to niobium.
-
I
of the less familiar metals in special alloys has intensifietl investigations of the analytical chemistry of the Group IVa, Va, and T-Ia ni&ls of the periodic system. Tlie main separative methods cmployed in the determination of niobium and tantalum minerals were critically reviewed by Atkinson, Steigman, and Hiskey (2). The methods of Schoeller (26) and LIarignac ( I S ) and modifications of the Schoeller method are inadequate (20). Since the publication of Schoeller's book (16) on the determination of these minerals, modifications have shortened the time of analysis and improved the accuracy. Slavin and Piiito (do), for example, \+ere able to cut to one third the 15-day Schoeller time. Tlowever, the tantalum values muqt be NCREASED USE
corrected for residual titanium, and tin must be removed first. According to Fucke and Daublander (79,tantalum can be separated from niobium by precipitation with phenylarsonic acid from a sulfuric acid-hydrogen peroxide solution. The separation is not strictly quantitative, and tin, zirconium, and hafnium interfere. Dupraw (5) used n-propylarsonic acid successfully to separate tantalum from niobium, titanium, and tungsten in an oxalic acid-sulfuric acid solution. Titanium interferes above a titaniumtantalum ratio of 1 to 1. Khen less than 10 mg. of tantalum is present or when titanium is present, tannin must be added t o obtain quantitative tantalum precipitation. Zirconium and hafnium interfere (6, 8). Jaboulay (11) first separated mixed tantalum-niobium oxides from steel by repeated pyrosulfate fusion and ammonium polysulfide treatment, then applied potassium hydroxide fusion with extraction by aqueous sulfuric acid-hydrogen peroxide solution, and recovered niobium from the filtrate. Stockhauseii and Zall (22) applied a modified hypophosphite method ( I ) for separating tantalum from the mixed niobium-tantalum oxides recovered in steel analysis. Gillis and coworkers (9) separated tantalum from niobium by repeated precipitation with ferroin (o-phenanthroline-ferrous sulfate complex) from a hydrofluoric acid solution. This method requires subsequent removal of iron from the tantalum precipitate. The method proposed here separates tantalum quantitatively and reliably from niobium, titanium, and zirconium by two to three precipitations with Nbenzoyl-AT-phenylhydrovylamine (tantalon) from a hydrofluoric acid-sulfuric acid solution a t p H 1.0. The operating time is considerably less than that of other methods and thP precipitate is
uncontaminated by the other metals, thus eliminating the necessity for corN - Benzoyl - N - phenylhyrections. droxylamine was recommended as an analytical reagent for the precipitation of metals by Bamberger ( 3 ) . It has been used for determination of iron, alumhum, titanium, and copper ( l 7 ) , for tin ( I 5 ) , and colorimetrically for vanadium (18), and recommended as superior to cupferron, when applicable, because it is much more heat- and lightstable. Shome (19) points out its p H sensitiveness in gravimetric use. Bamberger (3) and Shome (17) give detailed methods for its preparation. By these methods, white needles were obtained in a 40 to 50% yield, nith a melting point of 120-21' C. crystallized from water. The main by-product is the a,p-dibenzoyl derivative. S-BenzoylA'-phenylhydroxylamine on potentiometric titration with perchloric acid shows a basic dissociation constant, pK, of 9.1. APPARATUS AND REAGENTS
Instrumentation. A Beckman Model G p H meter was used for p H measurements, and a Beckman Model DU spectrophotometer was used in the quantitative determination of contaminants in precipitates. A Suclear hkasurements Corp. PC-1 proportional counter was used for all beta counting. Tungsten stock solution. Dissolve 5.69 grams of sodium tungstate dihydrate (J. T. Baker Chemical Co.) in water and dilute to 500 ml.; contains 4 mg. of tungstic oxide per ml. Molybdenum stock solution. Dissolve 4.915 grams of ammonium heptamolybdate (Merck & Co., Inc.) in water and dilute to 500 ml.; contains 4 mg. of molybdic oxide per ml. Cupferron reagent, 0.2M. Dissolve 31 grams of cupferron (G. Frederick Smith Chemical Co.) in water and dilute to 1 liter. Store in the refrigerator. VOL. 29, NO. 6, JUNE 1957
947
