AIDS FOR THE ANALYST Microdetermination of Nitrogen in Volatile Compounds. William J. Schenck’, Eli Lilly & Co., Indianapolis, Ind. determination of nitrogen in volatile compounds is a difThe standard Dumas procedure will repeatedly give low results for volatile compounds because the compound will usually be swept through the combustion tube in the initial weeping-out process. One method that was used to overcome this difficulty was simply to seal the volatile liquid in a capillary and explode the capillary in the combustion tube with heat. This will give good rewlts if care is taken. HE
Tficult and sometimes impossible task.
1
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3
6 _- -_-_--I-----
o\ Figure 1.
7
Modified Apparatus for Microdetermination of Nitrogen
.1. Capillary inserted here
8. Pressure applied here to break capillary
sufficient sample has been drawn into the capillary, it is removed from the liquid, wiped clean, and cooled by being placed on a piece of dry ice, and then the open end is sealed by touching it in the flame of a microburner. After the sample has reached room temperature, it can be weighed. After weighing, the capillary is scored very lightly with a glass cutter and inserted into the copper tube a t point A in Figure 1. Great care must be taken a t this point in inserting the capillary and copper tube into the combustion tube. After the initial sweeping-out process is completed, the capillary is broken by pressing up a t point B. The combustion tube is then warmed, starting as close as possible to the stopper. The analysis is completed as it would be for any ordinary sample. The length of the capillaries used for the samples was approximately 4.5 cm. The bore of the capillaries was about 0.5 mm. As the samples occupied a proximately 1 cm. of the capillaries’ length, this left a volume o?air (trapped in the capillary when the sample capillary is sealed) of about 0.0075 cc. Considering that 79% of the air in the capillaries was nitrogen and inert gases, this should give 0.006 cc. of gas in a blank determination. Using distilled water as the sample in three blank determinations, no measurable blanks were found. By first burning samples of acetanilide, then breaking a capillary with distilled water as the sample in the combustion tube, an increase in volume was found. For three determinations, blanks of 0.004, 0.002, and 0.004 cc. were found. These blank volumes were disregarded because of the margin of error in the determinations. Using very small samples, blank determinations would have to be taken into account. The results from this method were favorable (Table I). The greatest drawback t o this method is that the sample must be handled with great care, SO that the capillary will not be broken before the analysis is begun.
1. Sample capillary 2. Combustion tube 3. Copper tubing i s beat 3’ t o 5 O a t this point 4. Copper tubing is perforated a t this point to allow complete sweep4. ing out of combustion tube 5. Rubber stopper 6. 15- to 20-cm. length of copper tubing, 2.5-mm. diameter, 1-mm. bore 7. Rubber tubing 8. Carbon dioxide inlet
Use of Toggle Valves and Teflon in Hydrocarbon Analysis. R. A. Brown and D. J. Skahan, The Atlantic Refining Co., Pa*
The more volatile compounds, however, will explode so violently that they may still go through the combustion tube without being burned. Splintered glass is also left mixed in with the ~“able copper Oxide rendering it for future determinations. A modification in the manipulation of the sample has been developed to overcome this difficulty.
T HAS
been the practice in this laboratory to determine C, to C5
1 components of liquid hydrocarbons by mass spectrometer analysis of distilled fractions. Four ]ow temperature columns
operated by one analyst are used for this In practice, two fractions are collected, a CI-Ca gas and a C5 PROCEDURE liquid. Accurate analysis of these cuts depends upon taking representative samples for the mass spectrometer. Gases, in One end of a capillary is sealed and the opposite end drawn out particular, must be thoroughly mixed prior to sampling. This is to a fine hair; the capillary is cleaned with a piece of normally done by three strokes of a Toepler pump, compressing cooled, and weighed. After weighing, the capillary is warmed along the sealed end, then the open end is dipped into the liquid. and expanding it with mercury. Errors due to poor mixing are On cooling, the liquid will be drawn UP into the capilk’. After further reduced by using capillary tubing for manifolds and col1 Present address, University of Southern California, Los Angeles, Cahf. lecting the distillate gas in a single gas bottle. Mixing by the Teopler ensures that the small volume of gas originally in manifold lines will be mixed with that from the bottle. Table I. Comparison of Results Using Four Different .Methods Konrepresentative samples Nitrogen Found Capillary Nitrometer can also occur as a result of sealed Reading cross contamination, princiCapillary explode’d Corrected, Open sealed in combus- Present Present pally through sorption of hydrocapillary, one end, tion tube, method, Method, Theory, carbons in stopcock grease. Compound % % % % cc. % Allylamine 52.5- 53.5 6.55 18.86 24.32 24.26 0.705 24.54 Compounds absorbed from a 12.88 21.49 24.38 24.46 0.824 given sample subsequently de11.45 26.55 24.52 24.38 0.792 sorb, to appear as impurities in aec-Butylamine 45.2 2.53 7.62 19.1ti 18.99 0.538 19.15 7.60 8.27 18.93 18.89 0,402 following samples. To mini6.56 7.09 18.87 19.04 0,697 mize such contamination glass n-Butylamine 77 - 79 10.39 11.99 13.44 13.57 0.332 13.58 10.99 13.38 13.19 13.42 0,538 manifolds equipped with spheri12.01 13.27 13.50 13.49 0.366 cal joints have been periodically Pyridine 115 -118 9.39 19.20 16.74 17.48 0.365 17.71 disassembled and cleaned, but 12.48 15.31 13.49 17.80 0.536 15.39 14.22 17.53 17.74 0,532 this procedure has never been Isopropylamine 33 - 34 8.58 17.40 23.35 23.29 0.859 23.70 satisfactory, owing to shuttrace 21.41 23.43 23.52 0,613 17.51 14.90 23.42 down time and the fact that 23.56 0.883 ptopcockgreasewasstillpresent.
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