Microcoulometric Trace Nitrogen Analysis of Water and Oxygenated Solvents D. R. Rhodes and J. R. Hopkins Chefiron Research Company, Richmond, Cahx 94802
IN THE PAST SEVERAL years, there has been increasing use of microcoulometric titrating systems for the analysis of total nitrogen (1-5). In particular, the Dohrmann (4, 5 ) system is in wide use. In the Dohrmann system, nitrogen species are converted to ammonia over nickel metal catalyst in the presence of excess hydrogen. The ammonia passes into a titration cell and produces a pH imbalance. This hydrogen ion depletion in the electrolyte is detected by the sensor/ reference electrodes, and the hydrogen ion is replaced electrically at the generator electrodes by a current from the microcoulometer. Acid gases from halogen and sulfur species are removed from the gas stream with a packing of K z C 0 3 held at 500 “C. When analyzing oxygen-containing materials, e.g., aldehydes, alcohols, or water, the cell response is first acidic followed by the basic response from ammonia. This acidic interference masks from lO-lOOz of the ammonia response, depending upon the sample type, size, and the amount of carbon on the catalyst. This acidic cell response has been shown to be due to COS(6) produced in the catalyst section
7’
Ball Joints
I/ /I
I
Wool
Standard Samples
Palladium electrode (250 to 500-ohm sensitivity) Nitrogen, ppm Known Found Carbazole in White Oil 1.16 1.16 1.14 1.13
A
I
Table I.
Platinum electrodes (10-ohm sensitivity) Nitrogen, pg N/ml Known Found Carbazole in benzene 24.2 24.3 23.7 23.6 24.6 Carbazole in acetone 29.5 29.9 28.8 29.3 29.1 29.1 Carbazole in isopropyl alcohol 41 .O 41.2 41 .O 41.3 40.6 Monoethanolamine in H20 29.2 29.5 29.0 28.9 29.0 29.1
0.38
I Furnace
Monoethanolamine in H20
0.89
Monoethanolamine in acetone
1.39
Figure 1. Ascarite absorber tube
by the “water-gas” reaction. T o avoid this interference, a method was developed (6) using H 2 0 saturated hydrogen to prevent carbon formation on the nickel catalyst and CaO as a CO, and acid gas scrubber. The water equilibrium conditions in the CaO scrubber and the catalyst section are very easily upset, however; and this makes the method dif(1) D. M. Coulson and L. A. Cavanagh, ANAL.CHEM.,32, 1245 (1960). (2) D. M. Coulson and L. A. Cavanagh, “Microcoulometric Detection in Gas Chromatography,” Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, March 1961. (3) R . L. Martin, ANAL.CHEM.,38, 1209 (1966). (4) J. A. McNulty, “New Instrumental Methods of Analysis Microcoulometric Titrating System,” Chemical and Engineering Session, Baltimore, Md., American Gas Association, May 1966. (5) D . R. Rhodes, J. R. Hopkins, and J. C. Guffy, “Versatile and Rapid Trace Nitrogen Analysis of Petroleum Materials by Microcoulometry,” Division of Petroleum Chemistry, American Chemical Society, San Francisco, Calif., April 1968. (6) R . Moore and J. A. McNulty, “Determination of Total Nitrogen in Water by Microcoulometric Titration,” Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, March 1968.
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ANALYTICAL CHEMISTRY, VOL. 43, NO. 4, APRIL 1971
0.36 0.38 0.31 0.38 0.84 0.88 0.87 0.88 1.43 1.36 1.38 1.41
ficult to use on a routine basis with widely varying sample types. A simpler, but equally effective, method has been in use in our laboratory for some time. A 1-inch packing of 20-80 mesh Ascarite (A. H. Thomas Company, Philadelphia, Pa.) is placed between the exit end of the pyrolysis tube and the capillary inlet to the titration cell (Figure 1). Ascarite, which is sodium hydroxide on asbestos, is a very efficient absorbent for COz and has long been used in microchemical procedures for the determination of carbon (7, 8). It also -
(7) “Comprehensive Analytical Chemistry,” C. L. Wilson and D. W. Wilson, Ed., Vol. l B , Classical Analysis, Chapter VIII, Section 3a,” pp 421-422. (8) A. L. Steyermark, “Quantitative Organic Microanalysis,” Academic Press, New York, N.Y., 1961, pp 221.
absorbs the acid gases resulting from the pyrolysis of halogens and sulfur while it passes ammonia. The position of the absorbent keeps it at approximately room temperature and allows the operator to watch for depletion of capacity for CO? or water. Ascarite is used without any pretreatment, and there is no water or gas equilibrium to establish or maintain. Also, it is not necessary to humidify the hydrogen to the cell and furnace. Samples can be run in any sequence, whether they are hydrocarbons, water, aldehydes, or alcohols. There
is no base carry-over to the titration cell, so Ascarite is effective whether analyzing high nitrogen samples using platinum electrodes or low level nitrogen samples using palladium electrodes (5) and very high sensitivities. The accompanying Table I illustrates some typical standard samples. RECEIVED for review November 2, 1970. Accepted December 3, 1970.
Simple Technique to Determine Vapor Adsorption Capacities of Molecular Sieves D. P. Roelofsen Laboratory of Organic Chemistry, Unicersity of Technology, Julianalaan 136, Delft, The Netherlands THE ADSORPTION ISOTHERM of a molecular sieve characteristically consists of a steep section at low relative pressure pips followed by a nearly horizontal section up to a relative pressure of about 0.8, proving complete filling of the adsorption cavities; finally the adsorption isotherm increases again because of capillary condensation on the outside surface of the crystals. Therefore the saturation capacity of a molecular sieve can be obtained satisfactorily using a onepoint method in the horizontal part of the isotherm. This paper describes a vapor adsorption apparatus of simple construction for the above purpose in which several samples can be investigated at the same time. Thus this technique offers distinct advantages over more conventional volumetric or gravimetric adsorption methods ( I ) when saturation caDacities of molecular sieves are to be determined. An adsorption apparatus with a similar aim but of a different construction has been mentioned (2) but no technical details have been published. APPARATUS
The apparatus (Figure 1) consists of two glass-bells, a coupling with Viton O-ring seals, and a holder with six adsorption tubes. In the upper glass-bell the holder with the glass adsorption tubes, each with a sealed bulb filled with activated molecular sieve, is held at a temperature TI. This is accomplished by forcing thermostated water through the jacket of the glass-bell and in addition through a circular groove in the brass coupling (Figure 2). In the lower glassbell liquid adsorbate is thermostatically controlled at a temperature T2 < TI. Using published vapor pressure data one can adjust pips above the samples by choosing TI and T,. By connecting an oil-type vacuum pump to the Serto valve, the apparatus is evacuated to remove interfering gases and to increase the adsorption velocity. On molecular sieve samples only water, of the gases possibly present, will seriously interfere with the adsorption of the vapor a t the temperatures and pressures used. To remove water completely, vacuum was applied during a few minutes, thus purging the apparatus with the vapor of the liquid adsorbate. Any last traces of water may be removed by activated 3A molecular sieve pellets in a porcelain crucible. 3A sieve has a very high (1) D. M . Young and A . D. Crowell, “Physical Adsorption of Gases,” Butterworths, London, 1962. (2) G. R . Landolt. Abstracts of Papers, TECH 007, 158th ACS National Meeting, New York, September 1969.
UPPER GLASS-BELL
IO
Crn
1
i
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WATER 11
c G R O U H O SURFACE
--A
BAR MAGNET ADSORPTION TUBES
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SAMPLE IN S E A L E D BULB
PERFORATED SHEET-BR4SS
VACUUM / D R Y AIR
GROUHO SURFACE
CILIHORICAL
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CLAMP
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LOWER G L A S S - B E L L
Figure 1. Exploded view of adsorption apparatus ANALYTICAL CHEMISTRY, VOL. 43, NO. 4, APRIL 1971
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