Gasometric Determination of Nitrogen in Refractory Metal Nitrides of Groups IVB, VB, and VIB L. P. MORGENTHALER' and R. P. MENlCHELLl Western Electric Company, Inc., Engineering Research Cenfer, P.
b The
nitrides of the metals of groups
VB, VB, and VIB can b e quantitatively reacted with an excess of dry chlorine gas below 600' C. in a sealed reaction tube to form the corresponding metal chloride and nitrogen gas. Subsequent absorption of the excess chlorine by strong caustic solution and trapping of the residual nitrogen in a gas micro buret allows a quantitative estimation of the nitrogen to be made. The technique is theoretically applicable to the determination of nitrogen in other metals and alloys.
D
of nitrogen in the refractory nitrides of the metals of groups IVB, VB, and VIB has presented difficulties to the analyst. Incomplete solubility of the compounds makes Kjeldahl analysis difficult and high dissociation temperatures and low reactivities make vacuum or inert gas fusion methods questionable. I n the method described here, use is made of the reactivity of the nitrides of these metals with chlorine gas a t elevated temperatures to form the corresponding halides and to release nitrogen. Kitrogen trichloride is not formed as it is unstable a t the reaction temperature ( 3 ) . The unreacted chlorine is absorbed by potassium hydroxide solution and the nitrogen is measured directly in a calibrated gas micro buret. All of the chlorides of these metals are volatile at temperatures below 300' C. except for chromium trichloride which reportedly sublimes a t 1300' (1). A11 of the metals and the metal nitrides react with dry chlorine at temperatures below 800' C. ( 2 ) . The reactions of the metals and the nitrides with chlorine are exothermic. The exact temperature of the reaction is variable and depends on the rate of heating, the state of subdivision of the sample, and the intensity of light incident on the reaction tube. ETERMINATION
EXPERIMENTAL
Apparatus. Figure 1 is a diagram of the reaction tube. Dimensions of the tube are dictated by the necessity 1 Present address, Department of Chemistry, Arizona State University, Tempe,
Aria.
570
ANALYTICAL CHEMISTRY
9'nm OD
? -
0.Box 900, Princeton, N. J.
Pyrex to Vycor 22 h m gyded seal /Vycor Tu
08540
2 4 / 4 0 Vycor
1
15mm ID Pyrex capillary tubing -
+Y > lm- P 24/40' Pyrexi v /I 7= C api I r y 'Hih l=__F?-
-
_ n>
Outer Joint
Hi h Pressure !topcock
Figure 1 .
Ia Pressure Stopcock
Reaction tube for nitrogen determinations
of thermal isolation of the stopcocks. High pressure stopcocks were obtained from h c e Glass, Inc., and were lubricated with Kel-F grease. Other lubricants hardened on exposure to chlorine causing streaking and air leaks. The capillary section of the reaction tube was calibrated with mercury in increments of 0.01 ml. This enabled the volume of gas to be estimated to the nearest 0.002 ml. Tygon tubing used in this analysis is of two types, both readily available. Type S22-1 tubing is the more resistant to attack by the chlorine gas while type R-3606 tubing is the more resistant to attack by the alkaline hypochlorite solution. Reagents. Chlorine, obtained from Matheson Co., was used after prolonged expulsion of air entrapped in the gas cylinder. Although reportedly a minimum of 99.5y0 purity, after approximately 7 pounds of chlorine were allowed to escape from a 40-pound cylinder at least 99.99y0 of the gas was absorbed by potassium hydroxide solution. The chlorine was dried before use by passing it through a sulfuric acid bubbler. Each new cylinder was purged until a zero blank was obtained. Metal nitrides were prepared according to Verkhoglyadova, Dubovik, and Samsonov ( 4 ) from finely divided metals obtained from the Kern Chemical Corp. (Los Angeles). Tantalum nitride was obtained from W. Guldner of Bell Telephone Laboratories (Murray Hill, N. J.). The composition of each of the nitrides was confirmed by x-ray diffraction and all were estimated to be greater than 90% single phase. Tungsten nitride was not used in these experiments because of the difficulties encountered in preparing single phase materials. Freshly prepared 20y0 potassium hydroxide solution was used for the chlorine absorption because of the accumulation of hypochlorite ion in the caustic. I n the presence of some of the metal oxides formed by the hydrolysis of the halides, the hypochlorite
is catalytically decomposed and oxygen is given off. Some difficulty was experienced with oxygen produced in this manner, and it was found that 200 ml. of caustic solution could be used for five analyses without any appreciable oxygen formation. PROCEDURE
The sample of powdered metal nitride was weighed into a small porcelain boat. The sample weights, 1 to 5 mg., were chosen so that the final volume of nitrogen collected would be 0.15 to 0.30 ml. The sample boat was inserted well back in the Vycor section of the combustion tube which was sealed into the buret section with Apeizon W wax. The ends of the combustion tube were fastened to the chlorine cylinder and a water trap with type S22-1 Tygon tubing. Chlorine was passed through the combustion tube rapidly for 15 minutes. Both stopcocks were closed. Heat was provided by a small laboratory burner placed immediately below the sample boat. Heating was stopped as soon as the boat was completely empty by visual inspection. The total heating time required was 1 to 2 minutes for all of the compounds investigated except for CrN which required approximately 5 minutes for total sublimation of the sample. The area of the combustion tube being heated was kept to a minimum to keep the pressure in the tube as low as possible and to minimize heat transfer to the stopcocks and the ground joint. Heating of the tube was begun only after adequate safety precautions had been taken, The possibility of the combustion tube exploding is always present, although in approximately 200 experiments in this laboratory, no combustion tube has fractured. Because of the high toxicity of chlorine and its great oxidizing power, great care must be taken in handling of the gas. All experiments should be performed in an adequate fume hood and a gas mask should be available in case of accident. Any material sus-
ceptible to oxidation by chlorine should be absent from the hood and t,he surrounding area. Xft'er cooling, the large st'opcock of the combustion tube was fastened to a type R-3603 Tygon tube leading to a reservoir containing 20y6 potassium hydroxide solution. & i l l of the air in the system below the large stopcock was removed by gentle flexing of the tubing. The combustion tube was raised to a vertical position and the large stopcock opened to allow the potassium hydroxide solution to absorb t,he chlorine. Gentle swirling of the apparatus appreciably speeded the absorption. When the syst,em had reached equilibrium the calibrated capillary t'ube was read wit'h the reservoir height adjust,ed to bring the trapped gas to atmospheric pressure. The weight of nitrogen found was calculated from the ideal gas law. Experimentally, it was unnecessary to correct the gas volume for the vapor pressure of water above the alkaline hypochlorite solution. The t80t.alelectrolyte concentration in the liquid immediately below the nitrogen is high because of the chlorine absorption and cannot readily be determined. If the vapor pressure of the water were as high as 10 mni. of Hg, an error of less than 0.3q;b would be introduced into the final percentage. It was also unnecessary to correct the pressure of nitrogen found for the capillary rise in the gas buret as t'his is of the order of 2 mm. of Hg. RESULTS A N D DISCUSSION
The results of the nitrogen analyses of the metal nitrides are given in Table I. Agreement of the analytical results with the theoretical composition of the nitrides is excellent, showing less than 5% error for all except molybdenum nitride. Because of the large discrepancy between the theoretical nitrogen content for MosS and that found, a n indellendent analysis of the sample was performed by LeDous and Co. (Teaneck, K. J,) using a micro Kjeldahl technique. Results: 5.7y0r\; by micro Kjeldahl; 5.5y0 r\; by chlorination in this laboratory. This amount of nitrogen corresponds to a compound of the formula MojS2 a hich has a theoretical composition of 5.51y0 S. If the single phase nature of the material as determined by x-ray diffraction and the agreement between the Kjeldahl analyses and the data obtained in this laboratory is considered, it is possible that the classical formula of the lower molybdenum nitride, XloJ, may be in error. Examination of the individual result\ for a given analjsis showed a symmetrical distribution around the mean value indicating that deviations from the mean were caused by random
Table 1.
Compound TiN
vs
Cr?;
ZrN
SbN
Mo*N HfN
Tax
Results of Nitrogen Analyses in Refractory Metal Nitrides
N, 7cj
s,%,
theoretical 22.6 21.6 21.2 13.3 13.1 6.80 8.6 7.2
found 19.8 6.6 20.0 11.5 10.6 5.5
rather than procedural errors. Estimates of error in each of the four esperimentally determined parameters (pressure, temperature, gas volume, and sample weight) indicate that a relative standard deviation of 7y0 could be the maximum present. The value of the relative standard deviation was exceeded only by the results from the analysis of molybdenum nitride. Metal oxides did not cause interference in the determination of nitrogen. No reactions have been detected between the oxides of the metals examined and dry chlorine a t elevated temperatures in this laboratory with the exception of vanadium, molybdenum, and chromium. Weighed samples of the oxides were heated in a stream of dry chlorine a t approximately 800" C. for 15 minutes. Only the oxides of vanadium, molybdenum, and chromium showed any appreciable weight loss as indicated in Table 11. The vanadium, molybdenum, and chromium compounds were probably volatilized as oxychlorides as no deposit of metal chloride was observed in the cool section of the reaction tube which was always the case when the metal nitride was heated similarly. An error may be introduced into the analysis if a n appreciable amount of these oxides is present, oxides would form the oxychloride and a corresponding amount of oxygen. This method of analysis for nitrogen is not theoretically restricted to the metals discussed here. Iron, cobalt, nickel, aluminum, and magnesium all react completely with chlorine a t temperatures below 800" C. and so it should be possible to analyze these materials for nitrogen. This technique has been particularly useful in the analysis of the nitrogen content of thin films of tantalum metal and tantalum nitride on vitreous substrates. The total time per determination varies somewhat depending on the time required for total absorption of the chlorine. Generally, absorption is
Re1 ztd deviation, c*
Variance 0.6
/c
5 3 2 7 5
0.5 0.2
0.9 0.5 0.5
7.7
13 2 5
0.0
6.5
0.1
Table II. Weight Loss of Heating Oxides in Chlorine for 15 Minutes at
800" C. Weight Oxide TiOg
108%
7c
(1
100 15
