Similarly the electrochemistry of TPB may be summarized as follows : - 1.55V
TPB
+ e-
-1.lOV
=+=
+e-
TPB‘
=+=
TBP2-
+1.5V
TPC
+ 2e- + 2H+
The first and only two steps of TPB reduction are similar to TPC and TPP. Although the dianion, TPB2-, is isoelectronic with the porphyrinogen, it behaves electrochemically like the other porphin dianions. As Hush (15) has pointed out, the lowest lying vacant molecular orbital of the porphin ring system is nearly doubly degenerate and can accommodate four electrons. If, however, protons become involved, as indicated in the scheme above, the porphin ring system can accept six electrons in forming a porphyrinogen, the product of the fourth electron transfer step. The possibility of acid-base reactions in connection with electron transfer greatly increases the number of possible products. Electrochemical reduction in the presence of proton donors results in protonation of the pyrrole nitrogens 15) N. S. Hush, Theor. Cliim. Acfa,4, 108 (1966).
and the methine bridge. The protons in these positions are quite labile and have a great influence on the course of electrochemical reaction. This is in contrast to peripheral chemical double bond reduction in TPC and TPB. If the corresponding reversible half-wave potentials (one-electron steps) for TPP, TPC, and TPB are compared, it will be noted that they are all approximately the same. If differences in solvation energies can be neglected, this means that the energy of the lowest unoccupied orbitals is not affected greatly by the degree of saturation of the porphin ring system provided that this saturation occurs in peripheralpositions. Since a total of not more than six electrons can be inserted, one expects to see four electrons added to TPC, two to TPB. As the results indicate, this is the case. It would appear from these investigations that peripheral saturation in chlorins and bacteriochlorins cannot, in general, be modified by electrochemical reaction. The only observed exception to this is the conversion of TPB to TPC. RECEIVED for review November 19, 1970. Accepted January 19, 1971. This investigation was supported in part by National Science Foundation Grant GP-9484 and a University Science Development Program Grant from the National Science Foundation to the University of Arizona.
Versatile and Rapid Trace Nitrogen Analysis of Petroleum Materials by Microcoulometry D. R. Rhodes, J. R. Hopkins, and J . C . G u f f y Chewon Research Company, Richmond, Calif. The modifications made to the Dohrmann Nitrogen Analyzer to extend its range to the 0.1 i 0.02 ppm level and to 800 O C (1100 O F ) and higher end point petroleum stocks are described. The most significant changes included a new inlet system, the use of palladium instead of platinum pH-sensing electrodes, and electrical shielding to minimize additional pickup. With these changes, a ten- to twentyfold increase in sensitivity is obtained; and essentially all petroleum materials except crude oils and asphaltenes can be analyzed for nitrogen. The analytical method based on this modified instrument is rapid, about 15 minutes for duplicate results, gives results good to f5% on known samples, and compares as well as could be expected with the Kjeldahl method at these low levels.
THEIMPORTANCE of trace nitrogen determinations in petroleum stocks was discussed by R. L. Martin ( I ) who described a method in which organic nitrogen compounds are converted to ammonia in a stream of hydrogen. A ter Meulen-type catalyst was used in the pyrolysis-hydrogenation section, and the ammonia was determined coulometrically in a titration cell. The Dohrmann Instruments Company (2, 3) modi(1) R. L. Martin, ANAL.CHEM.,38, 1209 (1966). (2) J. A. McNulty, “New Instrumental Methods of Analysis, Microcoulometric Titrating System,” American Gas Association, Chemical and Engineering Session, Baltimore, Md., May 23, 1966. (3) Dohrmann Instruments Company, Mt. View, Calif., Advance Product Announcement and Technical Bulletins 508 and 522. 556
ANALYTICAL CHEMISTRY, VOL. 43, NO. 4, APRIL 1971
fied the instrumentation, used a nickel metal catalyst, and made the apparatus commercially available. Several groups have reported various applications of this instrument for nitrogen analysis (4-6). This paper describes modifications which extend the useful range to nitrogen levels tenfold lower than previously reported (0.1 f 0.02 ppm) and to high end point stocks. EXPERIMENTAL
Chemicals. Hydrogen was Ultrapure HB(Air Reduction Co.) or hydrogen which has been purified by palladium diffusion (J. Bishop and Co., Model A-1-X, Laboratory Hydrogen Purification Unit). Any tank hydrogen should be passed through a silica gel column. Silica gel used was Baker Analyzed Reagent, Chromatographic Grade; benzene, Spectroquality Reagent, Matheson, Coleman & Bell; nickel metal shot, Matheson, Coleman & Bell; pure nickel wire, 0.23-mm diameter, Wilbur B. Driver Co., Newark, N. J.; sodium sulfate, anhydrous granular, Baker Analyzed Reagent ; potassium carbonate, anhydrous granular, Baker Analyzed Reagent ; platinum plating solution: 3 % chloroplatinic acid (Baker and Adamson); palladium plating solution, 8 % palladium chloride (Matheson, (4) D. K. Albert, ANAL.CHEM., 39, 1113 (1967). ( 5 ) I. J. Oita, ibid., 40, 1753 (1968). (6) H. V. Drushel, Amer. Cliem. SOC.,Dic. Petrol. Cliem., Prepr., 14,No. 3, B 232 (1969).
Coleman & Bell), 7Z hydrochloric acid Reagent Grade, Baker and Adamson). Lead plating solution was made by dissolving the following in 100 ml of water: 10.6 grams of boric acid (Baker Analyzed Reagent); 22 ml of 48.7% hydrofluoric acid (Baker Analyzed Reagent); 15.5 grams of lead carbonate (Baker Analyzed Reagent); 0.02 gram of gelatin (Difco Laboratories). CAUTION: Add PbC03 slowly to the H,BO,HF solution. Carbazole was Eastman White Label and argon, High Purity, Dry, Union Carbide Corp. Instrumentation. The method is based upon a series of physical and chemical changes which convert the nitrogen in organic compounds quantitatively to ammonia and then measure the NHaaccurately. The sample is injected into a hot (700 "C) inlet where vaporization and some cracking occurs in the presence of an excess of flowing hydrogen. The sample then passes into a nickel catalyst zone at 800 "C where pyrolysis is completed and ammonia formed. The products are swept through a K C O a scrubber to remove acidic gases and continue on to a titration cell which absorbs the Ma. The titration cell has a pH-sensing and a H+-generating electrode system first to detect ammonia as a pH change and second to return the pH to tbe original value. The output of a coulometer connected to the cell is presented on an integrating recorder. The critical parts of this apparatus are discussed below. Apparatus. The modified equipment is built around the original nitrogen microcoulometer from Dohrmano Instruments Co. (Model C-2OOA Microcoulometer; Model T-400-H Titration Cell; Model S-200 Pyrolysis Furnace, including Nitrogen Kit 523720). The instrument can be divided into six functional units: inlet, catalyst, scrubber, cell, coulometer, and recorder. Inlet. Figure 1 shows the redesigned inlet, which is made entirely of quartz. The coiled section is made of 6-mm a d . quartz tubing. Hydrogen gas is passed into the inlet and reactor zones through two socket joints. Two clam shell heaters (300 watts, 57.5 volts, Type 84SP, Lindberg HeviDuty Furnace Co., 304 Hart Street, Waterton, Wis. 53094), insulated from the outer tube with asbestos paper, supply heat to the inlet coils to obtain inlet temperatures to 700 "C. These heaters can be connected in series to the original inlet electrical heating leads. To make the inlet septum, one end of a Teflon (Du Pont) Swagelok union is cut off and reamed out to lipinch diameter. The septum is inserted between the nut and ferrules. A '/&ch copper tubing water coil wrapped around the inlet downstream from the septum cools the injection port. Coulometer. The Dohrmann microcoulometer is basically an ac amplifier with a chopped input and output. Because the output to the cell is 180" out of phase with respect to the input to the cell, a common ground can be used for the two sets of electrodes. The coulometer bas a high and low gain mode. For the nitrogen system, a high gain of 2500 is used. Recorder-Integrator. A high input impedance Dohrmann (R-100) recorder is used with this coulometer. An input filter, a ?r circuit containing two 100-pf condensers and a 500-ohm resistor, is necessary between the coulometer and the recorder. Normally, the disk integrator supplied with the Dohrmann recorder has been used to obtain the area of the nitrogen peaks. However, a Varian Digital Integrator, Model 475, was also evaluated. Generally, this unit gave results within a range *3% of the results obtained from the disk integrator. Below 10 ppm, the spread becomes larger, usually about +6%. Use of the Varian Digital Integrator gives a sipnificant increase in the number of analyses which can be made by a technician. Also, the errors introduced from counting the strokes of a disk integrator are eliminated. Cell. The pH of the electrolyte in the cell is measured with a palladium black-sensing electrode us. the Pb/PbSOn/O.O03M Na2SOI reference electrode. The palladium electrode is
0 , .
. .-
Figure 1. Inlet section installed in modified Dohrmann Furnace prepared from palladium foil (0.7 cm by 0.7 cm and 0.005 cm thick). Care must be taken to make sure the platinum wirepalladium foil spot weld is covered. Otherwise, a mixed potential will be obtained; and the platinum will drive the palladium electrode to its potential. Because of the expansion of palladium upon absorption of hydrogen, a glass cover is usually unsatisfactory. A satisfactory seal can be made with molten low molecular weight polyethylene nr wax. Best sealing is obtained if the cap is preheated. The sensing electrode should be plated with a dark coating of palladium. With very heavy coatings, entrainment of the plating solution sometimes occurs, and with too light a coating, the electrode is easily poisoned. Even with heavier coatings, the electrode slowly loses its activity over a period of time because of adsorption of poisons on the surface. It can be regenerated by heating to red heat or dipping in concentrated nitric acid and then replating. Normally, the electrode is prepared for plating by electrolyzing the palladium (negative) and platinum (positive) electrodes in 1M H2SOafor 45 minutes at 200 mA. Then the palladium electrode is palladized for about 4 seconds at 10 mA in 8 % PdCL and 7 % HCI plating solution with a palladium anode. After the palladium electrode has been plated, it must be immersed in a hydrogen-saturated solution for several (2 to 6 ) hours before being used. This amount of time is required for the electrode to become saturated with hydrogen. The lead reference electrode at this concentration of Na2S04 should have a potential of about -0.28 volt us. the SHE. The experimental value obtained in this cell is about -0.26 volt. The difference in the two voltages is a reflection on the difficulty of making accurate lead reference electrodes. Mixed potentials due to oxygen and the various solid states of lead are obtained except under the most rigorous conditions. If there is any question about the reliability of the lead reference electrode, it can be quickly checked by immersing a saturated calomel electrode (SCE) into the electrode compartment and measuring the voltage. The lead electrode should be about 0.50 volt more negative than SCE. When the lead reference electrode fails, its voltage usually shifts in the positive direction. ANALYTICAL CHEMISTRY, VOL. 43, NO. 4, APRIL 1971
557
otherwise, high and noisy base lines will be obtained. Heating the catalyst to about 1050 “C in flowing hydrogen for 20 minutes will remedy this condition.
RESULTS AND DISCUSSION
B
f
6 mm Pressure
6 mm Pressure Decrease
I
I
I
I
W,
2
4
6
8
10
I
12
Time, M i n u t e s
Figure 2. Effect of hydrogen pressure change on base line. Bias change on 30-ohm sensitivity. Pressure change on 100ohm sensitivity ( a ) Platinum black-sensing electrode, 90 mV bias (b) Palladium black-sensing electrode, 70 mV bias
Because the pH is held at about 6 (-0.36 volt us. SHE), there should be approximately 0.1 volt difference between a platinum black and the lead reference electrode. The palladium black electrode should have a bias of from 0.01 to 0.1 volt depending upon the amount of hydrogen absorption. Dohrmann recommends the use of a 0.04% Na2S04 electrolyte. Higher concentrations result in sluggish response. The response of the system also depends on the pH in the cell. If the pH is made more basic than 6.0, there is greater sensitivity but less stability. If the pH is more acid than 5.5, the system becomes more sluggish. At a pH of 6.0 and with 0.04 % Na2S04,the ac resistance between the platinum-sensing electrode and the lead reference electrode should be about 600,000 ohms and 200,000 to 300,000 ohms between the two generating electrodes. These resistances can be used to show proper connections and the absence of bubbles in the side arms. Catalyst. The catalyst is nickel shot held in place by matted plugs made from nickel wire. Regeneration is periodically needed and is accomplished by passing oxygen (approximately 200 cc/minute) over the catalyst at 800 “Cfor 10-20 minutes. Argon is used (400 cc/minute for 5 minutes) to sweep hydrogen from the tube before oxygen introduction and to remove oxygen before restarting the hydrogen flow. Oxidation of the nickel occurs during the regeneration, and it takes approximately 15 minutes to reduce the oxide back to the metal with hydrogen. New catalyst must be heated to about 1050 “C with hydrogen (400 cc/minute) to remove sulfur from the metal. Air should not be allowed to contact hot catalyst; 558
ANALYTICAL CHEMISTRY, VOL. 43, NO. 4, APRIL 1971
This work was directed toward increasing the sensitivity and the useful sample boiling range of the method described by Martin ( I ) . To accomplish these objectives, investigations were made to determine how to decrease noise, increase sample size, lower blanks, and to establish optimum catalyst conditions. These various factors will be discussed separately below. Noise. Base-line noise and drift are the factors which most severely limit this instrument for low level nitrogen analysis. The instrument as received from the manufacturer could only be used on a day-to-day basis down to 2-5 ppm nitrogen with precision and accuracy of i10 %. Recorder base-line noise can be divided into three kinds : fast, moderate, and slow. Fast noise is usually associated with electromagnetic pickup between the high impedancesensing electrodes and nearby electrical devices. It can be remedied by using the T filter described earlier in the input to the recorder and by placing a grounded metal box around the titration cell. Moderate noise is associated with stirring effects, pressure variations, and voltage regulation. Adjustment of the electrolyte level to about 1 cm above the electrodes and adjustment of the magnetic stirrer to near maximum rate gives the best performance. The most significant contribution to moderate noise comes from hydrogen partial pressure fluctuations when platinumsensing electrodes are used. The platinum electrodes respond to small changes immediately. Palladium, with its unusual properties toward hydrogen gas, damps out these abrupt changes to a high degree. A properly prepared palladium electrode decreases this type of noise about tenfold. Palladium can absorb many times its volume of hydrogen gas either directly from a hydrogen-saturated solution or by electrolysis to form hydrogen gas at the palladium electrode surface. During absorption, three distinct regions are obtained ( a , a /3, /3). Among other ways, these can be characterized by a change of potential of the electrode as a function of hydrogen content of the metal phase. The potential of the electrode during these phase changes follows the Nernst relationship for hydrogen pressure. For the purpose of this work, two factors are important: First, there is a significant plateau region (a /3) where large changes in hydrogen content of the metal take place for small changes of hydrogen pressure; and, second, the rate of attainment of equilibrium during hydrogen pressure changes is slow, even in the /3 phase, because of the large amount of hydrogen which is absorbed in the metal. These effects are shown schematically and discussed by Ives and Janz (7) and Lewis (8). Under the conditions used for analysis, the metallic states of the palladium black-sensing electrode which are desired are the a /3 and the /3 phase. When the palladium electrode is first connected to the coulometer, very noisy base lines are obtained; and a very low bias is required (0-20 mV). This first stage of hydrogen absorption ( a phase) lasts 1 to 4 hours. The base line then becomes significantly quieter, and a higher bias is required for optimum performance (a
+
+
+
+
~
(7) D. J. G. Ives and G. J. Janz, “Reference Electrodes, Theory and Practice,” Academic Press, New York, N. Y., 1961, p 113. (8) F. A. Lewis, “The Palladium Hydrogen System,” Academic Press, New York, N. Y., 1967.
p phase). If the electrode has been properly plated and if it has not become poisoned, it then changes to the p phase. This is reflected in a bias of 60-70 mV and stable base lines which are sluggish to hydrogen pressure changes. An experimental comparison was made on the effecb of Hzgas pressure change in a cell with a platinum black- or a palladium black-sensing electrode (Figure 2). A length of 1/4-inchpolyethylene tubing with two T joints was connected to the cell cap. One T joint was connected to an open-ended manometer. A finely adjustable valve and a short piece of soft gum tubing was connected to the T joint at the end of the polyethylene tubing. The pressure in the cell was adjusted to 6 mm greater than atmospheric pressure by closing the soft tubing with a clamp and adjusting the valve. After a stable base line was obtained on the recorder, the pressure in the cell was quickly changed by releasing the clamp. The pressure could be quickly increased by placing the clamp back on the tubing. The investigation was carried out with conditions essentially identical for both electrodes. The same equipment, including cell bottom, was used in both cases. The one variable which was different was the bias voltage which had to be used for each electrode. The correct bias for an electrode depends upon electrode activity, upon the concentration of electrolyte, and, in the case of palladium, upon the hydrogen-palladium phase. A bias change procedure can be used to determine the optimum bias for an electrode. A small bias change will result in a change of pH in the cell electrolyte. The resulting generation of protons or hydroxyl ions will be displayed on the recorder as a peak. The peak should be relatively sharp with a minimum of tailing or overshoot. The bias change procedure was used to adjust the bias until both electrodes had about the same sensitivity (Figure 2). A bias of 70 mV was required for the palladium electrode compared to a bias of 90 mV for the platinum electrodes. The palladium electrode was in the p state during this test. The effects of pressure change on the recorder base line of the two electrodes are shown in Figure 2. The base line for the platinum electrode is rapidly and significantly changed for a 6-mm pressure change; whereas, the base line for palladium is disturbed much less. Because both types of electrodes follow the Nernst relationship
an increase in pressure must result in a generation of protons. The smaller base line change or proton generation reflects the sluggish response of palladium to pressure changes. Depending on the rate of hydrogen outgassing from the palladium foil, enough protons will be generated eventually to bring the system back to equilibrium. Slow noise or drift is usually associated with temperature or pressure changes in the titration cell. The metal shielding box helps maintain temperature, and close control of hydrogen pressure and flow rates minimizes this drift. Exposure of the hot nickel catalyst to air results in high and noisy base lines when the unit is put back in operation. This basic gas is presumably ammonia and comes from the nitrides formed when air has contacted the catalyst. Sample Size and Inlet Conditions. The sample size recommended for use with the original inlet was 10 111. Larger sample sizes result in significant amounts of oil at the outlet of the catalyst tube. Larger samples can be injected but at very slow rates. In order to use larger samples, the inlet shown in Figure 1 was developed which, with appropriate
Injection sequence 1 2 3 4 5 6 7 8 9 10 11
Table I. Effect of Coking (1.5 ppm N Standard) Found, Sample size, PPm N 1.51 1.50 1.43 1.42 1.46 1.33 1.38 1.29 1.22 1.20 1.14
J .!
11 11 11 26 26 26 51 51 51 11 11
hydrogen flow rates, can handle at least 5 0 - ~ 1samples. The 4-foot heated quartz coil acts as a vaporization and samplehydrogen mixing chamber. If temperature and flow rates are not carefully controlled in the inlet section, severe coking can occur on the glass tubing. Hydrogen flow rates into the inlet should be at least 200 cc/ minute with larger sample sizes. The temperature of the entire inlet can vary depending on the types of samples to be run, although about 700 "C is satisfactory for essentially all petroleum materials. However, the temperature along the quartz tubing cannot vary significantly. In an early design of the inlet, the front portion was heated to 700 "C while the back portion just preceding the catalyst section operated at about 400 "C. Table I shows the results for a 1.5-ppm standard of carbazole dissolved in silica gel-treated isooctane (Hz flow rate of 200 cc/minute). Each successive injection resulted in the formation of a noticeably thicker coke deposit in the cooler section. Catalyst. The function of the catalyst section is to complete the pyrolysis and to convert hydrocarbon compounds to gaseous products and to give ''100%" conversion of nitrogen to ammonia. There are several possible catalysts that can be used. The ter Meulen catalyst described by Martin ( I ) can be used with materials which boil below about 500 "C. There is some question about the activity of the catalyst over a period of time and for various types of samples. If the catalyst activity decreases significantly, the catalyst section must be replaced. It has the advantage of not needing a separate scrubber section for acid gases. Nickel catalyst has several advantages over the ter Meulentype catalysts. The higher temperature of the nickel catalyst means that the catalyst section does not limit the kind of samples that can be analyzed. Another advantage is the ease of regenerating the catalyst by oxidation. One disadvantage is its lower surface area. Care must be taken in adding large samples to avoid coking of the catalyst. Until the new inlet system was introduced, sample sizes were limited to 10 p1 because of insufficient mixing of the samples with hydrogen gas. McNulty (2, 3) has found that the nickel catalyst operates best around 850 "C outside temperature for the original Dohrmann equipment. At lower temperatures, coking occurs on the catalyst. At higher temperatures, nitrogen reacts in a manner to suggest strong bonding with the catalyst. This is shown by significant broadening of peaks as a series of samples is injected. The new inlet system requires that the outside temperature of the catalyst section be reduced to 800 "C because of the higher temperature of the hydrogen gas entering it. ANALYTICAL CHEMISTRY, VOL. 43,
NO. 4, APRIL 1971
559
3 9 2 PPM
n
&
38.4 PPM
A
A
-
0 05 PPM
2 MINUTES
+----+
B
TO.l
I
Figure 3. High level nitrogen sample 5 ~1 of a furnace oil, 20-ohm sensitivity
MILI.IVOLT
Figure 4. Low level nitrogen samples of petroleum sample, 450-ohm sensitivity (b) 30 pl of petroleum sample, 400-ohm sensitivity (a) 30 ~1
Because of the high temperature of the catalyst section, one might first suspect that there would be significant decomposition of ammonia gas to nitrogen gas. However, the fact that synthetic standards (carbazole in isooctane and benzene) come within =t5 % of their true value indicates that conversion of ammonia to nitrogen is insignificant. Two possible reasons for this are the large excess of hydrogen in the reaction tube and the short residence time of ammonia in the catalyst section. Injection Techniques. The lowest level of nitrogen that can be determined is also dependent upon the needle blank. This blank is a function of need!e pretreatment. An appreciable peak is obtained from a needle which has been exposed to laboratory air for a period of time. Reinjecting the needle several times through the septum into the hot region at frequent intervals causes the peak to decrease and to finally reach a relatively constant size. This final value is due to carrying some of the septum on the needle into the hot portion of the inlet. The needle blank due to the septum usually corresponds to 0.5-1.5 nanograms of nitrogen for a Wilkens silicone rubber injection septum. For a IO-mg sample size, this amounts to a correction of from 0.05-0.15 ppm nitrogen; for 30 mg of sample, a correction of 0.020.05 ppm nitrogen. The Hamilton silicone rubber septum (Catalog No. 18228) gives about the same needle blank. An alternative technique which eliminates the septum blank can be used. This is done by inserting a small diameter stainless steel or Kovar tube through the septum and into the inlet until the top is just beyond the carrier gas inlet. The diameter of the tube is of such a size that the syringe needle will just fit inside. Sample addition is carried out by putting the needle just inside the tube, allowing the base line to come to equilibrium, injecting the sample, and withdrawing the needle until the tip is just inside the tube. The needle is left in the tube until the peak is finished. It is necessary to block the tube with the needle because basic impurities in hydrogen gas cause a small amount of continuous proton generation. If the flow rate through the cell is significantly changed, the base line shifts. The tube technique takes somewhat more time than the normal method; however, if the needle is kept clean, all needle blank is eliminated. This improves the precision considerably at low levels. Even with this new inlet, large samples cannot be injected rapidly. Usually, no smoke or oil is noticed with large, rapid injections; but the resulting nitrogen peaks may overshoot significantly below the base line after the peak. This is probably due to the formation of species such as ethylene which react with the palladium electrode surface or hydrogen adsorbed on the electrode surface. Usually, for samples greater than 20 p1, an injection rate of about 1 pl/sec consistently gives best results. 560
0
ANALYTICAL CHEMISTRY, VOL. 43, NO. 4, APRIL 1971
High level samples with relatively high end points, such as residua or lubricating oil blends, are also sensitive to injection rates except in a different manner. Decreasing the rate of injection of 10 pl of sample from a “tapped” injection to an injection time of 1 second causes as much as a 10% decrease in determined nitrogen for these types of samples. Visual observation of injections show that a “tapped” injection gives a very fine spray; whereas, a 1-second injection results in the formation of large droplets on the needle tip. The rapid injection agrees well with Kjeldahl values. As a general rule, as the nitrogen content and the end point of the sample become lower, the injection rate of samp’es less than 10 p1 in size becomes less important. Injection of 10 pg nitrogen/ ml carbazole in benzene standard and other low boiling point materials shows little sensitivity to injection rate. As mentioned earlier, about a 30-second injection rate is used for 30 p1 of low level samples. Analyses. Several types of known standards were used to check the efficiency and reproducibility of the overall system. These included carbazole in spectroquality benzene (20 pg nitrogen/ml), carbazole in white oil (20 pg nitrogenlml and 0.9 pg nitrogenjml), and imidazole in water (14.0 pg/ml). All of these standards fell well within +5% of the expected values. These results, along with the fact that essentially 100% recovery is obtained from direct injection of ammonia into the cell, clearly indicate that the cell-coulometer combination is electrochemically 100% efficient. Not all nitrogen compounds give 100% recovery as pointed out earlier by Martin (1). Azo compounds such as azobenzene and 4phenylazodiphenylamine gave 80 and 89 % recovery, respectively. One class of compounds represented by 5-chloro1,2,3-thiodiazole gave less than 1 recovery. The presence of the sulfur attached to nitrogens must promote the formation of nitrogen gas. Figure 3 shows the current-time curve of a typical petroleum stock. The peaks are finished in about 5 minutes and have good reproducibility. The amount of nitrogen is determined by integrating the area under each peak (current-time) and converting coulombs to micrograms of nitrogen. All of the results reported here are absolute coulometric measurements. No empirical calibration factor has been introduced. Peaks at much lower levels of nitrogen are shows in Figure 4. The size of the peak compared to the noise level of the background is excellent. The peaks are sharp with relatively small amounts of tailing. Table I1 shows the results on a wide variety of samples analyzed by the modified Dohrmann microcoulometer and by a modified Kjeldahl-Indophenol ( 9 ) method. These in(9) E. D. Noble, ANAL.CHEM., 27, 1413 (1955).
Table 11. Nitrogen by Modified Dohrmann Dohrmann No. of End point, Analysis ppm Kjeldahl “C Sample type Processing 3 0 . 2 8 i 0.02 0 . 2 5 445 product Processing 15 11.5 i 0 . 4 12 520 product Processing 3 0.19 i. 00 0.21 product 545 Processing 4 0.08 i. 0 . 0 2 540
Vacuum bottom >540 Lubricating .oil blend .Butene gas Olefin-amine copolymer MW >lO,oOO
1
1715
1740
1
1730
1740
1 1
352 19
340 16
4
1160 =t18
1080
cluded vacuum bottom cuts, atmospheric residua, solventdeasphalted oils, lubricating oil blends, hydrofined and partially hydrofined stocks and products, etc. The results gen-
erally fall between +lox of Kjeldahl. With no sampling problems, the reproducibility and accuracy of the KjeldahlIndophenol method for most petroleum samples above 10 ppm nitrogen are about =tlOz. Two types of petroleum material did not give good recovery. Crude oils gave nitrogen results up to 35z lower than that obtained with the Kjeldahl method. Asphaltenes, which were analyzed by dispersing in benzene, gave very poor recovery. When the sample was injected, a fine spray of black particles was noticed in the outlet. Evidently the solvent flashed from the dispersed asphaltene particles, and they passed essentially unreacted through the catalyst section. High level samples (>150 ppm) were determined after dilution in spectroquality benzene. Isooctane was originally tried, but a number of samples precipitated after a short period of time. When some heavier type samples, including carbazole standards, were dissolved in benzene or isooctane and left standing out in light, the nitrogen level as determined by the Dohrmann microcoulometer decreased with time. Evidently, light catalyzes a photochemical reaction is which nitrogen is converted to a species which might absorb on the glass walls of the container as they are formed. RECEIVED for review September 30, 1970. Accepted January 8, 1971. Presented at the Division of Petroleum Chemistry, 155th National Meeting, American Chemical Society, San Francisco, Calif., April 1968.
Solvent Effects on Protonation Constants Ammonia, Acetate, Polyamine, and Polyaminocarboxylate Ligands in Methanol-Water Mixtures D. B. Rorabacher, W. J. MacKellar, F. R. Shu, and Sister M. Bonavita Depcrrfment r$ Chemistry, W a y n e Sttrte Unicersity, Detroit, Miclz. 48202 The protonation constants for ammonia, acetate, and eight multidentate polyamine and polyaminocarboxylate ligands have been determined as a function of solvent composition in methanol-water mixtures containing 0-99% methanol (by weight). The data have been obtained by means of potentiometric titrations using conventional glass and calomel electrodes to measure pH*, the nonaqueous equivalent to pH as referenced to the standard state in the identical solvent composition. I n agreement with previous observations, the protonation constant for ammonia passes through a minimum in the region of 6 5 7 0 % methanol (wt/wt) while acetate exhibits a continually increasing value with increasing methanol content. The amine nitrogen and carboxylic oxygen donor atoms i n the multidentate ligands exhibit corresponding behavior with slight deviations apparent for the nitrogen atoms as a result of increasing substitution and charge effects. The general behavioral patterns are interpreted in terms of electrostatic effects, base solvation, and proton solvation as a function of the solvent composition.
THEPOTENTIAL ADVANTAGES of utilizing nonaqueous solvents for improving analytical methods involving complexation have been noted previously ( I ) . However, the lack of reliable data for protonation and complex stability constants in other (1) A. Ringboni, “Complexation in Analytical Chemistry,” Interscience, New York, N. Y . , 1963, p 14.
than aqueous media has largely hindered the exploitation of nonaqueous approaches except on an occasional empirical basis. In connection with our current interest in solvent effects (2, 3), we have undertaken the measurement of protonation equilibria for a number of analytically important ligands, principally of the polyamine and polyaminocarboxylate families, in methanol-water solvents ranging from pure water to 99% methanol (by wt). In so doing we have taken advantage of recent advances in the understanding of these solvent mixtures, including the establishment of thermodynamically valid pH scales. Thus the protonation constants reported are of equal validity to those normally utilized in aqueous solutions. The choice of methanol as the nonaqueous solvent component for this study was based on both experimental and theoretical considerations. Alcohols closely resemble water in the nature of their protolytic behavior and are readily amenable to study without specialized techniques. Yet these (2) D. B. Rorabacher and F. R. Shu, unpublished data; W. J. MacKellar and D. B. Rorabacher, J . Amer. C/zem. Soc., in press. (3) D. B. Rorabacher, F. R. Shu, W. J. MacKellar, and R. W. Taylor, Abstract 127, Proceedings of the XIIIth International Conference
on Coordination Chemistry, Cracow-Zakopane, Poland, September 1970. ANALYTICAL CHEMISTRY, VOL. 43, NO, 4, APRIL 1971
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