Results. Relative viscosity measurements in pure water at 2.2 “Cand at pressures up to 1407 kg/cmZhave been made and compared with the pure water data at these temperatures and pressures as reported by other investigators (1,4-7). Correlation was found to be excellent with the repeatability of the K. E. Bett and J. B. Cappi, Nature, 207,620 (1965). R. Cohen, Ann. Phys., 45, 666 (1892). P. W. Bridgman, Proc. Amer. Acad. ArtsSci., 61,57 (1926). N. E. Dorsey, “Properties of Ordinary Water-Substance,” Reinhold, New York, 1940, pp 186, 187.
(4) (5) (6) (7)
roll times ranging from 0.07 to 0.15% average relative standard deviation with a mean of 0.10%. Errors in roll time caused by temperature and pressure on the dimensions of the viscometer were found to be less than 0.10%.
RECEIVED for review April 12,1968. Accepted June 17, 1968. The opinions expressed in this article are those of the authors, and the mention of proprietary products is for identification purposes only. Neither constitutes endorsement by the U. S . Navy or the Naval Establishment at large.
Rapid Determination of Organic Nitrogen with a Coulometric Detector I. J. Oita Research and Development Department, American Oil Co., Whiting, lnd. 46394
FASTAND ACCURATE nitrogen determinations are very important in the petroleum industry. The nitrogen content of petroleum feeds and products can range from a few parts per million to over 5%, and the boiling point can range from 60 “C,as for naphtha, to over 600 “C, as for certain lube additives. Such samples can usually be analyzed by the Kjeldahl method but the long digestion time required limits its usefulness in monitoring process units. Certain modifications of the Dumas (1)and the ter Meulen (2)methods are quite rapid, but neither method can cover a wide range of nitrogen level and sample volatility. R. L. Martin has published a method of determining total organic nitrogen in naphtha and light gas oil by hydrogenation and coulometric titration of ammonia in a unique coulometric cell developed by Martin and Flannery (3). The Dohrmann Instrument Company (4) modified this method to include a nickel metal catalyst and made the apparatus commercially available. We experienced excessive coking with nickel metal at 850-900 “C for certain samples. Martin and subsequently Albert (5) used a nickel-on-magnesia catalyst to convert nitrogen to ammonia. At 260400 “C, acidic gases such as H S and HCl are held on the magnesia; above 400 “C, they are released slowly and interfere in the coulometric titration. Because temperatures over 550 “C are required for the volatilization of heavy cycle oils, lubricating oils, and lube additives, the nickel-on-magnesia catalyst could not be used alone. A combination of 0.6% rhodium on a hydrocracking catalyst and nickel granules at 760 “C “cracks” the heavy samples into lighter components, which are then hydrogenated to ammonia at 360 “C on nickelon-magnesia which also traps the interfering acidic gases. Humidification of the hydrogen carrier stream minimizes coking of the sample on the “cracking” catalyst. Excessive accumulation of coke produces “straggling” effects and reduces the accuracy of ammonia determination. Heavy viscous samples such as lube additives cannot be introduced into the pyrolysis tube with a syringe and septum system. The sample inlet zone must be at least 550 “C to vaporize the sample. Also, because the carrier gas is hydro(1) (2) (3) (4)
I. J. Oita, ANAL.CHEM., 38, 804 (1966). E. C. Schluter, ibid., 31, 1576 (1959). R. L. Martin, ibid., 38, 1209 (1966).
Dohrmann Instruments Co., Advance Product Announcement and Technical Bulletin 508 and 522. (5) D. K. Albert, ANAL. CHEM., 39,1113 (1967).
gen, no appreciable amount of it should be released during the sample introduction step. A novel sample inlet system was devised to permit introduction of viscous or solid samples without opening the system to the atmosphere. The sampling system devised by Nerheim (6) is not designed to operate at 500-600 “C and the metal-to-glass connection it requires can lead to leaks. EXPERIMENTAL
A schematic diagram of the apparatus is shown in Figure 1. Hydrogen is the carrier gas, but as nitrogen is used periodically during maintenance work on the system, a nitrogen cylinder is permanently connected to the main hydrogen line. The humidifier is a steel sample bomb about 14 X 2 inches, wrapped with a heating tape so that it can be heated to 95 “C. Details of the inlet system and reaction tube are in Figure 2. The sample container is a tin capsule, 5 X 13 mm, Laboratory Equipment Co., part No. 501-59. The sample inlet is an 8-mm bore borosilicate glass stopcock with a silicone rubber septum on one end. The vaporizing zone is heated to 600 “C with a resistance wire which is connected to a Powerstat. The furnace for the “cracking” zone is normally kept at 760 “C, but it should be capable of reaching 900 “C. The hydrogenation zone furnace is kept at 360 “C but it should have a maximum temperature of 600 “C. The titration cell is described in detail by Martin (3). Both the coulometer and the titration cell are available from Dohrmann Instrument Co., San Carlos, Calif. The hydrocracking catalyst is 0.6z rhodium on a support such as silica-alumina cracking catalyst. The calculated amount of rhodium chloride is dissolved in the maximum amount of water (predetermined by titration) that the support can absorb without appearing “wet.” The rhodium solution and the support are mixed, air dried, and pelleted to oneeighth inch pellets. The pellets are calcined at 550 “C for two hours and ground to 16-35 mesh. The nickel chips were reagent grade, also 16-35 mesh. The nickel on magnesia catalyst is the modified ter Meulen catalyst described by Holowchak, Wear, and Baldeschwieler (7). Details of the catalyst preparation are given by Martin (4) also, Because a vertical reaction tube is used in this method, final reduction of the catalyst can be carried out in the reaction tube. The shrinkage “channeling” that can occur in a horizontal tube is thereby avoided. (6) A. G. Nerheim, ANAL.CHEM., 36,1686(1964). (7) J. Holowchak, G. E. C. Wear, and E. L. Baldeschwieler, ANAL.CHEM., 24, 1754 (1952). VOL 40, NO. 1 1 , SEPTEMBER 1968
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Figure 1. Schematic diagram of apparatus The electrolyte is a solution of 5 grams of reagent grade sodium sulfate in 1 liter of ion exchange water adjusted to pH 5.9-6.0. Procedure. The sample size is chosen to obtain about 0.005 mg of nitrogen for a full scale peak. The sample is weighed into a small tin capsule on a microbalance with a small vial as a base to hold the capsule upright. The open end of the capsule is crimped and folded over in such a way that the total length is about 10 mm. Volatile samples such as naphtha or refinery waste water can be either weighed into a tin capsule or injected from a long needle syringe Table I. Analyses of Typical Motor Oils and Additives Nitrogen Dumas Kjeldahl Coulometric Sample Motor oil base stock 46 PPm 44 ppm New motor oil A 368 ppm 367 ppm New motor oil B 1019 ppm 1008 ppm 980 ppm 970 ppm Diesel lube oil Motor oil additive A 1.53% 1.5575 Motor oil additive B 0.5875 0.56% Motor oil additive C 0.108% 0.10675 Motor oil additive D 6.2275 6.0075 Table 11. Nitrogen Determinations of Gasolines Nitrogen, ppm Kjeldahl Coulometric 47.4 30.6 26.5 15.6 8.5 3.4 1.0 0.4
44.2 33.4 24.2 14.0 8.0 4.0 2.0 0.7
Table 111. Nitrogen Determinations of Water Samples Nitrogen, pprn Sample Kjeldahl Coulometric Refinery Waste Water A 33 30, 33 B
C D AgNO, (82 ppm taken) KNOJ (10 pprn taken)
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ANALYTICAL CHEMISTRY
13 11 15
13, 13 12,ll 15, 16 80, 80, 83 10, 11, 10
Figure 2. Hydrocracking-hydrogenation tube through the silicone rubber septum in the stopcock into the vaporization zone. The titration cell is filled to a depth of 2 inches with the electrolyte. The usual flow rate is 300 ml per minute, but if the sample is especially high in nitrogen, 100 ml per minute may be used. The slower flow rate broadens the peak and thus keeps it within the span of the recorder. The cell is normally operated with a 100-mV bias. However, the bias can be moved a few millivolts either way to obtain an optimum generation of titrant. The sensor electrode is positioned at the reference electrode outlet. The range control is generally set at 4 ohms but a higher setting can be used for trace determinations. The capsule is placed in the inlet stopcock and by turning the stopcock 180°, the capsule is dropped into the vaporization zone. The amount of nitrogen in the sample is calculated by comparing its peak area with the peak area of known standard sample. Whenever the lower loop of the pyrolysis tube is filled, the pool of molten tin must be removed by sucking it up through a borosilicate glass tube. The stopcock must be removed and the hydrogen gas must be substituted with nitrogen gas. Also, the titration cell must be disconnected from the pyrolysis tube in order to avoid sucking the electrolyte into the catalyst zone. RESULTS
Nitrogen values of some typical motor oils and oil additives are shown in Table I. These samples range from about 40 ppm to over 6z. Motor oil additives C and D were analyzed by the Dumas method because they contained a thiodiazole compound, the nitrogen-to-nitrogen linkage of which causes low results with the Kjeldahl method. All of the coulometric values were within 5 % relative of the Kjeldahl or Dumas values. The precision of these methods for these samples are about 4z relative. Nitrogen determinations of volatile gasoline samples are shown in Table 11. These results confirm the finding of Martin ( 3 ) that the accuracy is within l ppm for the 0-10 ppm range and about 2-4 ppm in the 20-50 ppm range. Table I11 shows the results obtained on aqueous solutions and refinery waste waters. Inorganic nitrates are determined quantitatively as shown by the results obtained on silver and potassium nitrate. Although all of the samples in Table I11 were clear solutions, samples with large solid suspensions can be handled easily in a tin capsule.
The main advantages of the coulometric method are speed and high sensitivity. Nitrogen values can be reported within 15 minutes after the sample is accepted. The catalyst has a long life. Over 400 analyses, mostly of motor oils and additives, were carried out on a single catalyst filling over a period of three weeks. The catalyst would have had even longer life if the bulk of the samples were volatile samples such as gasoline or municipal sewage. The hydrocracking catalyst can be regenerated at 900 “C with air but the hydrogenation catalyst cannot be regenerated. The high sensitivity of the coulometric titration makes the method ideal for trace nitrogen determination. It is possible to redesign the pyrolysis tube and to alter the relative amounts of catalysts to suit a specific type of sample. For example, waste water analysis would require a very small amount of
hydrocracking catalyst and a small amount of hydrogenation catalyst at a lower temperature. The entire volume of the pyrolysis tube can be kept very small so that a sharp narrow peak is obtained. In this case, the peak height alone can be used to calculate nitrogen content. On the other hand, fertilizer samples in the 10-20x nitrogen range might require a larger amount of hydrogenation catalyst to ensure complete hydrogenation and to provide the necessary adsorption time to broaden the peak so that it would be within the recorder span. The design and dimensions here are a compromise to obtain the maximum versatility. RECEIVED for review April 15, 1968. Accepted June 20, 1968. Presented at Division of Analytical Chemistry 155th Meeting, ACS, San Francisco, Calif., April 1968.
Use of Powder Covered to Enhance Sensitivity in Detec,tion of Aerosols via Internal Reflection Spectrometry N. J. Harrick Philips Laboratories, Briarcliff Manor, N . Y. 10510
SAMPLE HANDLING often precludes the measurement of the optical spectra oia transmission and internal reflection spectroscopy of very small droplets-e.g., 0.01 microliters-not because of the limited amount of sample material but because of the difficulty of sample handling for such minute quantities. It is well known that in many cases one advantage of internal reflection over transmission is the ease of sample handling. This paper describes a method of easily and efficiently utilizing the sample in recording the spectra of aerosol droplets, as well, via internal reflection spectroscopy. For internal reflection spectrometry, the sample is utilized most efficiently when it is spread out on the surface of the internal reflection element in a thickness less than the effective thickness ( I ) of the sample material. Capillary action between closely spaced plates or between a wire mesh and the surface of the internal reflection plate has been employed for spreading liquid droplets. It is, however, difficult to collect and inject small droplets between plates. The wire mesh approach, where the droplet penetrates through the mesh, has the advantage that the entire surface of the internal reflection plate is readily accessible; however, there are strict requirements on spacing and flatness of the plate and the wire mesh making this a difficult structure to assemble. In addition, severe attenuation of the infrared power may occur for reflection from areas where the mesh is bonded to the internal reflection plate. It is furthermore found that small droplets will sometimes not penetrate the wire mesh to reach the plate surface. The disadvantages of the wire mesh structure are eliminated by the use of powders on the surface of the plate, in place of the wire mesh, for spreading the sample. It should be recalled that nonabsorbing powders placed on the surface of an internal reflection element do not scatter light for internal reflection (2), as they do in transmission. (1) N. J. Harrick, “Internal Reflection Spectroscopy,” Interscience, New York, 1967. (2) N. J. Harrick, and N. H. Riederman, Spectrochim. Acta., 21, 2135 (1965).
Experiments were carried out to determine the optimum particle size and number of powder layers required to give maximum spreading of 0.01-pl droplets of water on Ge and Si surfaces. Droplets of controlled and uniform size were generated by flicking a thin needle through a reservoir of the solution in question, in a manner similar to that described by Wolfe (3). Aluminum oxide was chosen as the powder because it has a relatively low refractive index, is inert, and is readily available already sized as a polishing abrasive. The powders were deposited on the surface of the Ge plate by precipitating from a water slurry in the following way. A known amount of powder is placed in a known amount of liquid and the mixture is agitated. The internal reflection plate is placed a t the bottom of a container. The agitated mixture is then poured into the container and the powder is allowed to precipitate. The clear water is then siphoned and the plate is thereafter permitted to dry. The same mixture is always used and the layer thickness is determined by the height of the liquid-powder mixture in the container above the plate. For example, it was found that for powder of three microns in diameter, a mixture consisting of 0.1 gram of powder and 90 cc of water will yield one layer of powder per cm height of liquid in the container. Except for the very fine powders-e.g., 0.5-p diameter-the powder layers deposited in this way adhered rather strongly to the surface and remained on the surface even under running water. It was necessary to swab the surface to remove the powder. The powder covered plate can thus be used repeatedly without reprocessing for detecting droplets providing the droplet evaporates from the surface. (Droplets of 0.01 p1 of water evaporate from the powder covered surface in about five to ten seconds, depending on the humidity.) It is evident from the curves in Figure 1, that even though the practical long wavelength cut off of sapphire (A1203)as a window material is about 7 p, powdered A1203 can be used as an agent for spreading aerosols out to beyond 11 p , Apparently even after 80 reflections, the effective thickness of the (3) W. R. Wolfe, Rev. Sci. Instrum., 32, 1124 (1961). VOL. 40, NO. 1 1 , SEPTEMBER 1968
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