Gas Chromatographic Determination of Nitrogen in Refractory Materials ROBERT J. HYNEK a r d JOSEPH A. NELEN Allis-Chalmers Research laboratories, P. 0. Box 5 I 2, Milwaukee 7, Wis.
b A new and convenient method has been developed for the determination of nitrogen in refractory materials. The method utilizes a conventional induction furnace for combustion of the matericil in an oxygen atmosphere a t approximately 1500" C. The liberated nitrogen gas is collected in a modified Hempel buret and determined with a gas chromatograph. The new method has been routinely applied to several refractory nitride formulations. These include AIN, BN, CrN, Si3N4, TiN, and ZrN, as well as All\(.SiC and B&i samples.
R
at this laboratory required chemical analysis of the new materials to augment crystallographi: and spectrographic data. Existing methods, with a few modifications, were satibfactory for all of the elemcnts present exwpt nitrogen. The 'on1 cntional sulfuric acid method failchd t o dissolve the new refractories, despi Le extended digestion periods. Fusion with alkali hydroxides to produce ammonia 'iT'as considerably better, but iiicomplete deconiposition frequently ol7curred. Sodium peroxide fusions were always complete, but partial conversion of the nitrides to EFRACTORY SITRIDE RESEARCH
7
MARK A
'MARK B
Figure 1 .
Combustion gas collector
nitrogen gas could be expected, and was observed. EXPERIMENTAL
The conversion of nitridcs to iiitrogeii and the corresponding metal olide by high teinperaturc olidation n as reported by Mellor @), but detailed information was not included. T o confirm this reaction, equipment designed for routine carbon and sulfur determinations was used. A sample of silicon nitride of known composition pas burned in oxygen and the combustion gases were collected in the carbon determinator apparatus. The concentration of nitrogen in these gases was determined (by difference) with a Burrell gas analyzer. The results indicated that nitrogen was quantitatively evolved from the nitride. This preliminary experiment led to the development of a more convenient apparatus for the collection and volumetric determination of the nitrogen. The apparatus, which removed interfering gases by chemical absorption, and the satisfactory results obtained with it, have been described previously ( I ) . Subsequently, a gas chromatograph was found more sensitive and convenient to use for the measurement of the evolved nitrogen. Since the gas chromatograph made possible a direct measurement of nitrogen, it was unnecessary to remove other gases. Consequently, a new and considerably simplified device n as constructed for the collection of combustion gases. This collector consists of a conventional 100-ml. graduated Hempel gas analysis buret joined to a cylindrical glass container which has a three-way stopcock a t the top (Figure 1). During the initial development work, combustions were occasionally incomIjlete. The combustion assembly was eventually modified by shortening the oxygen jct t o provide a more diffuse flon of o\\-gen to the sample. iilso, n m a l l piece of rubber tubing mas used to preient tilting of the jet (Figure 2 ) . Apparatus. CHROIIATOGRAPH. Beckman, GC-2 or equivalent, with a &foot stainless steel column packed with Linde 5-4 Molecular Sieves. h 5-ml. sample loop mas used for this work, but a 10-nil. loop may also be used. COMBUSTION GAS COLLECTOR.-4 100-ml. Hempel gas analysis buret is joined permanently to one end of an S-inch cylindrical glass tube, the ends of which have been tapered to match
the buret taper (Figure 1). A 3-nay stopcock (2-mm. bore) is also joined to the upper end of the tube. (Two of these collectors, one of 600-ml. and one of 900-ml. total capacity, are necded for the ( upper, ungraduated portion. 10-4. PART B. Chromatographic Analysis. Attach the collector stopcock to the sample port of the gas chromatograph with rubber tubing. Flush the port combustion tube, and a quartz jet are with the sample gases, then introduce assembled as shown in Figure 2. The a 5-ml. or 10-ml. sample into a 6-foot jet is shortened and a short length of Linde 5A Molecular Sieve column a t rubber tubing is inserted as indicated. 40” C. under 20-p.s.i. helium pressure. RESZRVOIRFOR GAS COLLECTOR A filament current of 250 ma. and SOLUTION. A 1000-nil. aspirator bottle attenuations of l X , 2X, and 5X will is connected to the lower tip of the gas provide adequate peak heights for most collector with 3 feet of amber rubber nitrogen concentrations. Measure and tubing. The reservoir is filled with record the height and half-height 1000 ml. of a solution containing 200 width of the nitrogen peak in milligrains of hTazS04 and 50 ml. of H2SOd. meters. (If an integrator is used, A few drops of methyl orange are added substitute counts for square millit o impart color to the solution to aid meters.) in reading the buret. Repeat this process for the rest of Procedure. PARTA. Combustion the samples, standards, and blanks. of Sample and Collection of Gases. Duplicate determinations are recomTransfer 100 mg. of 100-mesh sample mended for each category. to a conventional ceramic induction PARTC. Calculation of Results. crucible. Cover with 2 grams of Calculate the area-volume product for iron chip accelerator and 1 gram of each sample, standard, and blank tin accelerator (a scoop may be used determination as follows: here) and place on the combustion pedestal of the induction furnace. Raise the pedestal assembly until the where unit is securely closed. Table I.
Limit of Detection
0
Table Ill.
Between and Within Set Variance of Nitrogen Sensitivity Factors
F ratio
Series 75 mg., Between set 100 mg., Between set 75 to 100 mg., Within set Expressed in -4VP units (1
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Mean diff .a
Variance
0.000584 (10 D.F.) 0 031 0.000842 (10 D.F.) 0 ,035 0.016 0.000109 (11 11.F.) per m g . N , X
ANALYTICAL CHEMISTRY
(5%)
Calcd.
Theor.
5.36 7.73
2.80 2.86
= area under curve, sq. mm. A V = volume of collected gases, ml. Attn. = instrument attenuation S = gases injected, ml.
Subtract the blank AVP value from each sample and standard AVP value. Divide the net standard A V P value by the milligrams of nitrogen present in the standard to obtain the nitrogen sensitivity factor, then calculate the nitrogen content of the sample as follows:
TON = -Avp4 x 100 WP where AVP, = net sample ares-volume product W = sample weight, mg. F = nitrogen sensitivity factor. DISCUSSION
A review of the data accumulated over a 7-month period has been made to establish limits of application and precision. Wilson (3) recommends that the standard deviation of the blank be reported when a new method is proposed to facilitate its evaluation without detailed experimentation. The standard deviations of the blanks in the two ranges of application (5y0 N) were calculated by the method for duplicate values (4). (This method was used because the range of individual blank values exceeded the mean difference of duplicate values by a factor of nearly 10.) The standard deviations of the blanks in the low and high ranges are, respectively, 1 0 . 1 3 ( 5 D.F.) and ~k0.085(11 D.F.) AVP units. To assure a 98y0 probability that the nitrogen detected is actually present, a &fold multiple of the blank standard deviation for the low nitrogen range must be used ( 3 ) . Multiplying, (h0.13 x 3 = 0.39), and converting to % N (dividing by sensitivity factors obtained in the low nitrogen range), produces three limits of detection as shown in Table I. These limits clearly reflect their inverse proportionality to the sensitivity factors. Since the sensitivity factors vary as a fiinction of chromatograph response and of oxygen purity, a single limit of detection cannot be given. The limits in Table I indicate the capabilities of the proposed method, however. The possibility of differences in nitrogen evolution in the high range as a function of sample size was also investigated. Two samples sizes, 76 m g . and 100 mg., were tested. The data obtained from 24 replicate analyses of a silicon nitride (36.15% h’) are shown in Table 11. The calculations summarized in Table I1 clearly show that either sample size may be used, since the calculated variance ratio and Student “t” values do not exceed theoretical values a t the 5% probability level. The data in Table I1 also clearly reflected other operative effects. A com-
parison of sensitivity factor variations as a function of sets, summarized in Table 111, shows a Eignificantly larger difference occurring between sets of analyses than between sample sizes within a set (mean differences between sets are greater by a factor of two and calculated variance ratios exceed the theoretical at the 5% probability level). This is a reflection of the effects arising from daily fluctuatioru of ambient temperature and pressure and of instrument controls. Thei%eeffects are controlled and minimiiied by including standards and blanks with each analysis of a group of unknowiis. The evolution of EL nitrogen-bearing gas other than nitrogen was observed only in the case of boron nitride or refractories which contained boron nitride. This was anticipated from a comment in (2) pointing out that some nitric oxide is produced upon oxidation of boron nitride. The formation of this secondary by-produe‘, was readily observed by the appearance of the reddishbrown vapor of NnOa in the collector apparatus. A colorimetric estimate after absorption of the N20a vapor in potassium hydroxide indicated that only 0.17y0 of the nitride nitrogen was converted to nitric oxide during combustion. This bias is automatically elim-
Table IV. Analysis of Various Nitrides and Nitride-Containing Refractories
Sample Amorphous boron BitPn 87932 BsSi 974-25-1 BsSi 974-18-1 AlN-Sic (impure) A1N (impure) CrN5 (impure) ZrN (impure) AlN -Sic A1N Sic A1N. s i c A1N.SiC A1N-Sic SisNc (36.2% N) 0
b
Nitrogen, Yo 1.65, 1.65 3.81, 3 . 9 2 0.84, 0.81 0.43, 0.44 7.11, 7 . 0 2 10.8, 1 1 . 3 9.6, 9 . 7 11.4, 1 1 . 4 29.6, 29.7 27.4. 2 7 . 3 29.0; 28.8 30.9, 30.9 29.7, 30.6 36.4, 36.6
With Kjeldahl method: 9.4% N. With Kjeldahl method: 11.5% N.
pairs (4), the relative standard deviation of all of the pairs in the 5y0 N range (mean = 27,2% N), the relative standard deviation is 1.43%. The proposed method offers a relatively simple solution to the problem of rapidly evaluating refractory nitride materials. After the blanks and standards have been run to establish the nitrogen sensitivity factor (30 to 40 minutes), the elapsed time of an analysis in duplicate is 15 minutes. The equipment needed is generally found in analytical laboratories, requiring only minor modiha tions. LITERATURE CITED
(1) Hynek, R. J.,, Nelen, J. A,, “Volumetric Determation of Nitrogen in
inated, however, by calibration with a boron nitride standard. The proposed method haa been applied to a variety of refractory materials. These include carbides, nitrides, oxides, phosphides, and silicides of aluminum, boron, chromium, silicon, titanium, zinc, and zirconium. Individual results typical of some of these materials are shown in Table IV. Using the method recommended for duplicate
Refractory Nitrides,” Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, March 1961. (2) Mellor, J. W., “A Comprehensive Treatise on Inorganic and Theoretical Chemistry,” Vol. WI, p. 97, Longmans, Green and Co., New York, 1947. (3) Wilson, A. L., Analyst 86, 72 (1961). (4) Youden, W. J., “Statistical Methods for Chemists,’’ p. 17, Wiley, New York 1951.
RBCEIVED for review February 8, 1963. Accepted July 5, 1963.
Gas Chrcmatographic Technique for Determination . of Hydridic and Active Hydrogen in Organic Materials Microanalysis of Borane Compounds and Ako hols IHOR LYSYJ
and
R. C. GREENOUGH
Rockefdyne Division 0:‘ North American Aviafion, Inc., Canoga Park, Calif. The technique described here is based on the chemical liberation of active or hydridic hydrogen in a microreaction cell incorpcrated in a gas chromatographic flow system and measurement of the hydrogen gas band in a nitrogen gas carrier b y thermoconductivity detection. The differential in thermal conductivity between hydrogen and nitrogen was detected with a Teflon-coated hot wire detector. This technique was used for the determination of hydridic hydrogen in borane compounds and for the determination of hydroxyl hydrogen in alcohols. The hydrogen gas was formed from the borane compounds b y acid hydrolysis and from the alcohols b y reaction with lithium aluminum hydride. The sample sizes were in the milligram range, and analysis was completed in less than 5 minutes.
of active hydrogen plays an important role in the analysis of organic compounds. By this determination, it is possible to obtain information on the presence and amounts of hydroxyl, amino, or carboxyl groups. A number of methods for the determination of active hydrogen involve the use of methyl magnesium iodide or lithium aluminum hydride and manometric measurement of the liberated hydrogen (1, 8, 10, 11). I n other methods, an excess of a standard solution of lithium aluminum hydride is back-titrated with a standard solution of alcohol (4). The microcombustion method requires only a few milligrams of the sample, but the technique is laborious and time consuming (9). A more recent method involves exchange of the active hydrogen with deuterium followed by an assay of the isotopic hydrogen content of the sample (3). DiETERMINATION
borane has also been used as the hydridic agent for the liberation of hydrogen (7). A specially designed gas chromatograph with an elaborate valve system and a system of membrane switches has been designed for the analysis of active hydrogen after liberation by means of lithium aluminum hydride reagent (2). The main advantage of the method described is that it can be applied for the analysis of a variety of organic niaterials and borane compounds using conventional commercially available equipment-Le., gas chromatograph of any make with a gas sampling valve of any design. The method described dealr uith determination of two kinds of chemical hydrogen. the hydridic type, such as that found in the compounds containing ti boron-hydrogen bond, and the active hydrogen type, such as that in hydroxyl, amino, and carbox11 groups. Both VOL. 35, NO. 1 I , OCTOBER 1963
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