as citric acid and iiitrilotriacetic acid caused no interference in the reaction. Relatively large amounts of phosphate and fluoridc interfered. The principal interferences were iron and zirconium. Iron interference can be eliminated either by extraction or by precipitating iron as the sulfide. For quantitative work, the iron sulfide precipitate should be removed by filtration or centrifugation. Zirconium forms a colored complex with the dye in the presence of the masking mixture, if the solution is allowed to stand at room temperature or if the solution is heated. No interference of zirconium was observed if the absorption readings were made within 15 minutes after the color was developed at room temperature. The dye and the thorium-dye complex were stable when left in a boiling water bath for a few hours. The thorium-dye complex develops immediately at room temperature. To utilize the advantage of the difference in the reaction rates, it is important that the absorbance be measured as soon as possible before certain metals such as vanadium, titanium, and zirconium have reacted with the dye. Hydroxylamine hydrochloride or similar reducing agents should be added to prevent oxidation of the dye by oxidizing agents such as cerium(IV), vanadium(V), or uranium(VI). The sequence of the addition of the reagents is important. I n the case of the ceric ion, it makes no difference if the reducing agent is added after the mask-
ing agent mixture. I n the case of the uranyl ion, if the masking agent mixture is added first, followed by the reducing agent, dye, and buffer, a red complex of uranium with Eriochrome Black T is formed. However if the reducing agent is added first and then the masking agent mixture, dye, and buffer, no uranium-dye complex is formed when the buffer is added. Eriochrome Black T can be added before or after the addition of the buffer. The qualitative tests listed in Table I show the effect of reagent addition on color formatmion. Rpectrophotometric titrations of thorium were conducted using Eriochrome Black T solution as the titrant. At 700 mp! a sharp increase in the absorbance was observed after all thorium had been titrated. Neither the dye nor the thorium-dye complex could be extracted by ether, benzene, toluene, carbon tetrachloride, chloroform, and isoamyl alcohol. Eriochrome Blue. Black R gave colored thorium complexes under the reaction conditione, but showed no advantages over Eriochrome Black T. Erio OS, a derivative of Eriochrome Black T which contains no sulfonic acid group, probably could be employed if solvent extraction were desirable. Other 0,0’-dihydroxyazo dyes might be investigated, in order to find a more sensitive or selective reaction for thorium. The addition of 2,2‘,2”-nitrilotriethanol to the EDTA solution is
advantageous for masking purposes, as EDTA is a relatively weak chelating agent for tri- or quadrivalent ions, compared to 2,2’.2”-nitrilotriethanol. The latter reagent is particularly advantageous in masking the interference of the rare earths. The thorium-dye complex has a molar absorptivity of approximately 33,000; the reaction shows R sensitivity of 0.004 pg. of thorium per square cm. per 0.001 absorption unit a t 700 mp, according to the notation of Sandt.11
(4). ACKNOWLEDGMENT
The authors thank the Research Corp. for an F. Gardner Cottrell grant to support this work. LITERATURE CITED
(1) Cheng, K. L., “Proceedings of the
International Symposium on Microchemistry 1958,” pp. 465-73, Pergamon Press, London, 1960. (2) Korbl, J., Pribil, R., Chemist-Analyst 45, 102 (1956). (3) Rodden, C. J., “Analytical Chemistry of the Manhattan Project,” Chap. 2, McGrsw-Hill, New York, 1950. (4) Sandell, E. B., “Colorimetric Determmation of Traces of Metals, 3rd ed., Interscience, New York, 1959. ( 5 ) Snell, F. D., Snell, C. T., Snell, C. A., “Colorimetric Methods of Analysis,” Vol. IIA, Van Nostrand, Princeton, N. J., 1959. RECEIVEDfor review July 23, 1959. Accepted July 18, 1960. Division of Analytical Chemistry, 135th Meeting, ACS, Boston, Mass., April 1959.
Microdetermination of Nitrogen in Rocks and Silicate Minerals by Sealed Tube Digestion F. J. STEVENSON Departmenf o f Agronomy, University o f Illinois, Urbana, 111.
b Kjeldahl digestions for the determination of nitrogen in rocks and silicate minerals were carried out in sealed tubes with concentrated sulfuric acid. The time required for maximum release of nitrogen varied with temperature of digestion. For analyses of silicate minerals and igneous rocks, a minimum digestion time of 90 minutes a t a temperature of 420” C. is recommended; for sedimentary rocks, the digestion time can b e reduced to 60 minutes. The accuracy and precision of the proposed method were considerably better than when digestion was performed in Kjeldahl flasks.
T
RE CONVENTIONAL digestion pro-
cedure for the determination of
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ANALYTICAL CHEMISTRY
nitrogen in rocks and silicate minerals by the Kjeldahl method often leads to unacceptable values. Losses of nitrogen occur through bumping and thermal destruction of ammonia. The difficulty of obtaining satisfactory values for nitrogen in igneous rocks and silicate minerals is augmented by the fact that the small amount of nitrogen ordinarily present is so intimately combined with mineral matter that prolonged periods of digestion are required for its liberation. This enhances the possibility of contamination with ammonia from the atmosphere. The nitrogen associated with rocks and silicate minerals is now known to exist, in part, as ammonium ions held within the lattices of silicate structures (6, 7). Rayleigh (6)pointed out that the nitro-
gen in rocks was “. . . capable of conversion into ammonia by heating with alkali, and may be described as ammoniacal nitrogen.” Scalen (6) used four different extraction procedures in an attempt to remove quantitatively the ammonium from igneous rocks and found that none of the procedures gave reproducible values. To avoid uncertainties incident to digestion of rocks and silicate minerals with sulfuric acid by the regular Kjeldahl method, the author digests such samples with sulfuric acid in sealed tubes. The use of sealed tubes for digestion eliminates losses of nitrogen through bumping and prevents contamination with ammonia from laboratory air. I n addition, digestion in sealed tubes allows complete oxida-
470'C. '-40
-
W
W
ORTHOCLASE 20
z O dI0-
37OOC.
-
GRANITE ROCK
0-O TIME IN MINUTES
Figure 1 . Effect of time and temperature on nitrogen release from orthoclase feldspar and granite rock by sealed tube digestion
tion of heterocyclic nitrogen compounds without addition of a catalyst or other extraneous substances (2, 8). APPARATUS
Digestion was carried out in Vycor and borosilicate glass tubes. The Vycor tubes were 45 to 50 mm. long, having an inner diameter of 13 mm., a wall thickness of 1.2 mm., and a volume that approximated 7 ml. The borosilicate tubes were 55 to 60 mm. long, having an inner diameter of 17 mm., a wall thickness of 1.2 mm., and a volume that approximated 12 ml. Before use, the tubes were cleaned thoroughly with chromic acid solution, after which the open end was not touched, to avoid contamination. Vycor glass tubes were used a t temperatures above 420" C., borosilicate glass tubes a t temperatures of 420' C. and below. Heating was performed in a muffle furnace equipped with a thermocouple. To protect the furnace in case of explosion, the tubes were placed in a protective casing consisting of a 4-inch length of 3/4-inch iron pipe provided with two screw caps and a small vent hole near the top. REAGENTS
Boric acid solution, 2%, to which the mixed indicator of M a and Zuazaga (4) was added. The indicator consisted of a mixture of bromocresol green and methyl rrd. Kessler's reagent. PROCEDURE
Place a 50- t o 500-mg. sample of finely ground (100-mesh) rock or silicate mineral in the digestion tube. Add 1.5 ml. of concentrated sulfuric acid and seal the tube with a gas-oxygen torch. Place the tube in the protective casing and insert in the muffle furnace. After heating for the desired time and at the proper temperature (see section entitled Results and Discussion), remove the casing from the furnace and cool. Apply a sharp small flame to the tip of the tube until it opens as a result of slight internal pressure. For highly carbonaceous materials, the tube should be opened behind a protective shield; for igneous rocks, silicate minerals, and most sedimentary rocks, the pressure within the tubes a t room temperature
is not sufficient to cause any hazard. After opening, place the tube in a drying oven a t 100" C. for 15 minutes t o facilitate expulsion of carbon dioxide, sulfur dioxide, and other gases. Transfer the contents of the tube to a onepiece micro-Kjeldahl distillation apparatus ( I ) , using 8 to 10 ml. of ammonia-free water. Add 8 ml. of 45% sodium hydroxide solution and distill the ammoiiia into a suitable receptacle, as described below. A blank, consisting of 1.5 ml. of concentrated sulfuric acid, is treated in exactly the same manner as the unknown. For samples containing less than 50 pg. of nitrogen, collect 15 ml. of distillate in a colorimetric tube containing 0.25 ml. of 0.05N sulfuric acid solution. Add 1 ml. of Nessler's reagent and, after 30 minutes, read absorbance or transmittance a t 435 mp, using a suitable spectrophotometer. The amount of nitrogen is then read from a standard curve prepared for that purpose. For analysis of samples containing more than 100 pg. of nitrogen, collect the distillate in a 125-ml. Erlenmeyer flask containing 5 ml. of boric acid solution. I n this case, nitrogen is estimated by titration with standardized 0.01N sulfuric acid solution. RESULTS AND DISCUSSION
The rate and completeness of digestion in sealed tubes were tested by determining the effect of temperature on the amount of nitrogen recovered from a sample of orthoclase feldspar and a granite rock. Preliminary work showed that about two thirds of the nitrogen in each of these samples existed as lattice-bound ammonium ions. The results (Figure 1) show that the time required for maximum recovery of nitrogen decreased with increasing temperature. .4t the temperature recommended for digestion by the regular Kjeldahl method (370" C.), maximum recovery of nitrogen was obtained ~ i t ha 120-minute digestion period. The temperature chosen for subsequent work was 420" C., since borosilicate glass tubes could be used. T o ensure complete removal of nitrogen, a minimum digestion time of 90 minutes is recommended. To determine if the nitrogen in sedimentary rocks were released at rates comparable to those for the orthoclase feldspar and granite rock, a study was made of the effect of time on nitrogen release from two shales. Shale A, taken a t a depth of from 2264 to 2269 feet, was classified as Chattanooga >hale overlaying Devonian sand. This shale had an organic carbon content of 12.1% and a carbon-nitrogen ratio of 29. Shale B, taken from a depth of 1976 to 2018 feet, was from a Mississippian horizon. This shale had an organic carbon content of 0.66% and a carbon-nitrogen ratio of 13. The results of the study (Figure 2) show that
z.:b7;i;
.\o 0 W
SHALE A
0 L1:
SHALE 8
5 02 z
0
60
120 I80 0 60 TIME IN MINUTES
120
180
Figure 2. Effect of time on nitrogen release from two shales by sealed tube digestion at 420" C.
the time required to obtain maximum release of nitrogen from the shales was shorter than that required for t h e orthoclase feldspar and granite rock (Figure 1). Approximately 90% of the nitrogen in the shales was recovered within a digestion time of 15 minutes; however, a n additional 30 minutes was required t o release the remaining nitrogen. For the determination of nitrogen in shales. digestion at 420" C. for a minimum of 60 minutes is recommended. Evidence that digestion in sealed tubes effected complete oxidation of the organic matter in the shales was indicated by the fact that
Table I. Comparison of Nitrogen in Igneous Rocks and Silicate Minerals by Two Digestion Procedures
Sample Orthoclase feldspar
Nitrogen Content, pg. /Gram Sealed tube Kjeldahl digestiona digestion*
-4v. Granite
Av. Porphyry
Av. Pegmatite
Av. Mica
Av. b
Std. dev. Std. dev.
40 41 41 42 43 41.4 30 31 32 32 35 32.0 44 46 46 46 48 46.0 19 19 21 21 22 20.4 102 102 103 104 108 103.8
29 34 37 41 48 Av. 3 8 . 0 19 20 26 28 42 Av. 27.0 34 35 36 53 54 Av. 42.6 14 17 18 25 28 Av. 20.4 70 74 87 92 107 Av. 8 6 . 0
= 1.73. = 9.94.
VOL. 32, NO. 12, NOVEMBER 1960
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Table 11. Comparison of Nitrogen in Sedimentary Rocks (Shales) by Two Digestion Procedures
Sample Shale A
Nitrogen Content, % Sealed tube Kjeldahl digestion0 digestiona
0.417 0.421 0.431 Av. 0.423 Shale B 0.049 0.051 0.053 Av. 0.051 SM. dev. = 0.007.
0.367 0.369 0.380 Av. 0.372 0,047 0.049 0.051 Av. 0.049
the residues were white and finely divided. Having determined the conditions for maximum recovery of nitrogen from silicate minerals, and from igneous and sedimentary rocks, the perfected method was compared with conventional digestion in micro-Kjeldahl flasks. I n addition to the samples described previously, porphyry, pegmatite, and mica samples were used. The procedure adopted for the micro-Kjeldahl analyses wm the one recommended by the Association of Official Agricultural Chemists (1). The data obtained (Tables I and 11) show definitely that digestion in the sealed tubes was superior to digestion in the Kjeldahl flasks. The somewhat erratic values obtained by digestion in micro-Kjeldahl flasks resulted from bumping, contamination with ammonia from laboratory air, and deposition of mineral material along the sides of the flasks. Contamination with ammonia from laboratory air was particularly serious for the samples with low nitrogen contents (orthoclase feldspar, granite, porphyry, and pegmatite). For the mica, the low value obtained by digestion in the Kjeldahl flasks was attributed to the tendency of the mineral to adhere to the sides of the flask above the sulfuric acid layer, resulting in incomplete digestion, Considerable swelling of the
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ANALYTlCAL CHEMISTRY
mica was observed during digestion in sealed tubes, a condition which undoubtedly facilitated removal of interlayer ammonium ions. For shale A, the residues from digestion in Kjeldahl flasks mere dark colored, indicating incomplete oxidation of organic matter. As shown in Table 11, larger amounts of nitrogen were recovered from shale A by digestion in sealed tubes, as compared to conventional digestion in micro-Kjeldahl flasks. That the borosilicate tubes were sufficiently strong to withstand the pressures developed at 420” C. was demonstrated by the fact that, except for shale A, a sample was seldom lost in the muffle furnace during the course of this investigation. Difficulty was encountered with shale A because of its high organic matter content. To determine the suitability of borosilicate glass tubes for digestion of carbonaceous shales, varying amounts of shale A were subjected to digestion a t 420” C. The tubes withstood breakage so long as the sample taken for analysis contained less than 12 mg. of carbon (equivalent to 0.4 mg. of nitrogen). The carbon-nitrogen ratio of most sedimentary rocks is about 12; hence, the upper limit of nitrogen generally will be higher than 0.4 mg. I n addition to being suitable for determining nitrogen in rocks and silicate minerals, sealed tube digestion was suitable for determining nitrogen in soils, sediments, and stony meteorites. With respect to soil, the technique was particularly useful for the analysis of subsurface soil. Nitrates are not included in the values obtained by the proposed method; however, this is not a serious problem, as most geologic specimens contain insignificant amounts of nitrates (5, 6). Nitrogen is lost during digestion by the regular Kjeldahl method when the temperature exceeds 400” C. The use of sealed tubes permits digestion a t temperatures considerably higher than 400” C. without loss of nitrogen (2, 3, 8). Under the conditions em-
ployed in this study, there was ha loss of nitrogen when ammonium sulfate was heated with 1.5 ml. of sulfuric acid at 470’ C. for 5 hours. Grunbaum et d.(3) found that as little as 10 pl. of water for each milliliter of gas space retarded thermal destruction of ammonia a t elevated temperatures. The sulfuric acid used in this study probably contained sufficient water to retard decomposition of ammonia a t the temperatures employed. The perfected method is both rapid and accurate. A technician can perform many determinations in a day without having to repeat any of them because of a lack of precision, or loss of sample. For the analysis of large numbers of samples, multiple digestions can be performed simultaneously. If desired, the time required to complete an analysis can be reduced by increasing the temperature of digestion; ’ however, when doing so, Vycor glass tubes must be used. ACKNOWLEDGMENT
The author expresses his appreciation to Clyde Smith, who performed many of the determinations, and to the National Science Foundation, which provided financial aid in the form of a research grant. LITERATURE CITED
(1) Assoc. Offic. Agr. Chemists, “Official and Tentative Methods of Analysis,” 8th Ed., Section 37.9, p. 805 (1955). (2) Grunbaum, B. W., Schaffer, F. L., Kirk, P. L., ANAL. CHEM. 24, 1487
(1952). (3) Grunbaum, B. W., Kirk, P. L., Green, L. G., Kock, C. W., lbid., 27, 384 (1955). (4) Ma, T. S., Zuasaga, G , IND.ENG. CHEM.,ANAL. ED. 14,280 (1942). (5) Rayleigh, L., Proc. Roy. SOC.(London) 170A,451 (1939). (6) Scalen, R. S:, Ph.D. thesis, Univ. of
Arkansas, “University Microfilms,” Ann Arbor, Mich., Microfilm No. 59-1379
(1959). (7) Stevenson, F. J., Science 130, 221 (1959). (8) White, L. M., Long, M. C., ANAL. CHEM.23, 363 (1951).
RECEIVEDfor review April 18, 1960. Accepted July 11, 1960.
