Table VI. Evaluations of Formation Constants for All-Reversible Cadmium(I1)-Thiocyanate Systems by Three Different Calculation Methods Calcd System Ma method .f (BO’or BO) 81 BZ P3 P4 ~ ( ~ 1 0 3 ) 11 I 7 ( P o ’ = 0.9994 14.6f 1 . 8 46.0Ik 5.1 0 8 8 . 6 f 4.1 0.20694 A(1) f0.0340) (10Y 11 7 1 4 . 6 f 1 . 7 45.9 f 5 . 0 0 88.6Ik 2 . 6 0,20699 (10P 111 6 ( B o = -0.0278 21.4 f 8 . 3 36.7 Ik 11.9 0 90.9 f 3 . 5 ,0.14325 10.1240) B(2) 12 I 7 (~o’=l.O007 11.9f1.5 63.3izlO.O 2 0 . 5 k 1 6 . 7 4 9 . 1 I k 7 . 1 0.11173 f0.0250) (1lY I1 7 11.9iz1.4 63.3f10.1 20.6f16.7 4 9 . 1 & 7 . 0 0,11181 Ill 6 ( P o = 1.2769 8.1Ik6.4 76.4f24.0 5.9f29.2 5 3 . 9 + 1 0 . 4 0,09599 (1lY Ik0.4618) C(2) 37 I 32 ( B o ’ = 1.0058 18.0f 1 . 9 137+ 13 86.2iz27.3 1 7 7 f 14 0.26549 f0.0326) Wb I1 32 18.3 f 1 . 4 137 f 13 8 7 , 6 f 26.4 178 k 13 0.26658 (36Y 111 31 (Po 1.1597 14.8 & 3.8 155 f 21 63.0 iz 35.8 187 k 16 0.24414 =to.1642) A System Cd(Il)-KSCN-KN03. The experimental data are taken from Ref. 1. B System Cd(I1)-NaSCN-NaNOa. The experimental data are taken from Ref. 2. C System Cd(I1)-NaSCN-NaCIO,. The experimental data are taken from Ref. 2. 1 The systematic method described in this text. 11 The l/Fo*-weightmethod with slight changes in calculation processes from Ref. 1. 111 The method in which O-order term of C,in the equation for Foi is taken as a variable Po as in Refs. 26 and 27. The number of experimental data set ( I d and E,,*) used as the realizations of corresponding stochastic variables for the evaluation, The experimental data for the ligand-free state are treated as without errors. 0
The results with method I11 show the importance of plotting the point at C i = 0 for the AR curve-fitting. Although the method gives the least S/fand thus the best fits around the Fo-curves among the tested procedures, unreasonable intercepts and low precisions are obtained because the plotted points at C i = 0 are not given. The evaluated values for system C with method I1 show that the effect of the no-plot at C i = 0 on the curve-fittings is minimized naturally because the M is given as much ldrger than in A and B, as might be expected also from Figure 5 . The observation will serve as
another support for the above interpretation of these F-function methods systematically with Figure 5. Thus, the present study for PR systems has given considerable understanding of the polarographic F-function methods with introducing another systematic F-function procedure applicable to both AR and PR systems. RECEIVED for review December 15, 1970. Accepted June 16, 1971.
Extension of the Microcoulometric Determination of Total Bound Nitrogen in Hydrocarbons and Water L. A. Fabbro, L. A. Filachek, and R. L. Iannacone Cities Seroice Oil Company, Cranhury, N . J .
R. T. Moore, R. J . Joyce, Y. Takahashi, and M. E. Riddle Dohrmann Dioision, Enoirotech Corp., Mountain View, Gal$ The microcoulometric determination of total bound nitrogen in hydrocarbons add water has been extended in application, range, and precision through modification of system parameters and improvement in sampling techniques. For low ppm nitrogen samples, the precision has been improved to =t0.03 ppm or 3.00J0, whichever is larger, through increased sample size, system parameter optimization and by separating the needle/septum blank from the sample peak. The use of ,a Solid/Liquid Sample Inlet has permitted accurate nitrogen determination for high boiling petrochemical distillates and has facilitated the separation of volatile and nonvolatile nitrogen-bearing compounds in waste water analysis. Through extensive modification of the system parameters, the sampling capacity for solid samples has been extended to accomodate sample sizes up to 10 mg with a total nitrogen content of up to 100 pg in a single five-minute analysis.
MANYDIFFERENT SITUATIONS exist where it is important t o measure the concentration of total bound nitrogen in hydrocarbons and water. In the petrochemical industry, the determination of nitrogen is important because of catalyst poisoning by the nitrogen. In the feed and grain industry, the nitrogen concentration is a measure of the protein content (or food value) of the product. Quality control of nitrogenous pharmaceutical products can often be effected through total nitrogen determinations. Eutrophication of water resources is critically dependent upon total nitrogen content since algae growth is dependent, in part, upon the total nitrogen content. Municipal sewage plants can continuously monitor internal processes by checking total nitrogen as well as ammoniacal nitrogen ( I ) . (1) D. K. Albert, ANAL.CHEM.,39, 1113 (1967).
ANALYTICAL CHEMISTRY, VOL. 43, NO. 12, OCTOBER 1971
1671
T - 400-H TITRATION CELL I
I
Figure 1. Low current system for total nitrogen in liquid samples
I I
llOoC
:’:
SAMPLE
INLET ( a )
eoo*c
3OOOC
400- BOO’C
N o OH O N 20/40 A L U N D U M
I
II
For purposes of reviewing current analytical techniques, the nitrogen concentration can be roughly divided into two broad ranges; low (1 .O N). In the case of high nitrogen concentrations (>1. O x N) the standard analytical techniques include micro and automatic Kjeldahl(2,3), automatic Dumas (4) and the carbon-hydrogennitrogen (CHN) analyzers (5, 6). Sample size and nitrogen bond type determine which technique is suitable. The Kjeldahl technique will not recover N-N linkages, requires special modifications for N - 0 linkages, takes two t o four hours for digestion and titration, but is readily adapted t o all types of samples. The C H N analyzers are limited to approximately 1 mg of sample. The automatic Dumas technique appears t o be a most promising technique with up to 500-mg sample size and only 15-minute analysis time. For nitrogen concentrations below 1.O N , the techniques of Kjeldahl and Ter Meulen are classically applied. Ter Meulen’s method (7-9) with colorimetric finish can achieve a precision of approximately 0.1 ppm but has been limited t o samples which boil below 450 “C. The Kjeldahl technique is capable of the same precision but required extraction of large samples and analysis time of two hours or more. In 1966, a microcoulometric cell was utilized as a detector
x
INLET ~
il_l ! I
i
CENTER
OUTLET ~
x
(2) G. M. Gustin, “Organic Analysis: Nitrogen,” Treatise on Anal, Chem., Part 11, Section B, Vol. 11, 404-499, John Wiley & Sons, New York, N. Y., 1965. (3) E. W. Catanzaro, G. R. Goldgraben, and R. M. Gasko, Automation in Analytical Chemistry, Technicon Symp., 240 (1966). (4) Coleman Instrument Co., Bulletin B-291A, Maywood, Ill., 1968. ( 5 ) Hewlett-Packard Corp., F & M Scientific Division, Bulletin 1850-A, Avondale, Pa., 1968. (6) H. J. Francis, ANAL,CHEM.,36 (7), 31A-45A (1964). (7) J. G. Holowchak, E. C. Wear, and E. L. Baldeschwieler, ibid., 24, 1754 (1952). (8) R. W. King and W. M. Faulconer, ibid., 28, 255 (1956). (9) E. C. Schluter, Jr., ibid.,31,1576 (1959).
REACTANT
SUPPLY
GAS
H p ( 3 0 0 cc/minl
,
-
for a n improved Ter Meulen method (10). Here the nitrogen was converted to ammonia over a modified Ter Meulen catalyst and subsequently titrated automatically in a constant p H cell. A precision of k0.2 ppm and a typical analysis time of three minutes utilizing 5- to 10-pl samples was achieved. Further investigation led to several developments: a granular nickel catalyst for handling high boiling materials (11); a new scrubber material for water samples and oxygenated hydrocarbons (12) ; Solid/Liquid Sample Inlet for measuring solid and viscous materials which would otherwise plug the sample injection needle in a hot inlet (13); the use of a catalyst train for sequential cracking and hydrogenation (14); and the use of a palladium sensor electrode for lower cell noise and a precision of + 0.02 ppm N for nonoxygenated hydrocarbons
(15). This report describes methods for improving precision to
+0.03 ppm or 3.0x, whichever is greater, for the standard direct injection system-namely, the use of increased sample sizes and the separation of the needle/septum blank from the sample peak itself. Also described is the use of a Solid/Liquid Sample Inlet (13) which enables accurate analysis of high boiling petrochemical distillates and additives which may decompose in the hot needle of a standard direct injection system with subsequent low recovery. This same Solid/Liquid Sample Inlet also facilitates the separation of volatile and non(10) R. L. Martin, ANAL.CHEM.,38, 1209 (1966). (11) E. Fredericks, Shell Development Co., Emeryville, Calif., personal communication, 1964. (12) R. T. Moore and J. A. McNulty, Enuiron. Sci. Technol.,3, 741 (1969). (13) Dohrmann Division of Envirotech Corp., Tech Bulletin AI-27, Mountain View, Calif., 1968. (14) I. J. Oita, ANAL.CHEM., 40, 1753 (1968). (15) D. R. Rhodes, 115th National Meeting, American Chemical Society, Petroleum Section, Paper No. 37, San Francisco, Calif., 1968.
I.\
OUARTZ GUIDE R O D
S Y R I N G E INJECTION OF LIQUID SAMPLES INTO BOAT
Figure 2. Solid/liquid sample inlet
-y,SILICONE
R U B B E R SEAL /-CARROUSEL
T O P (REMOVABLE)
TO F U R N
STEEL
‘RAY ( R E M OVA B L E 1 VALVE
1672
ANALYTICAL CHEMISTRY, VOL. 43, NO. 12, OCTOBER 1971
TRAY GUIDE (ROTATABLE)
SYRlNGE lWJECT H 50ec/mln 2
SAMPLE BOAT
EXPANSION CATALYST
PUSHROD H2 l S O c c l m i n
(a)
-GENERATOR VENTS AT TO!? MISSES SENSOR
Figure 3. Schematic of high current nitrogen system 0. Pyrolysis tube b. Titration cell C. Circuit schematic
REFERENCE
G E NE R AT 0R
ACCESSORY
YICROCOULOMETER
rl RECORDER
(C)
volatile nitrogen compounds in waste water analysis. High correlation between the N H 3 content of waste water samples as measured by the Nestler technique and the volatile nitrogen content was observed. Efforts to increase the sampling capacity of the system for insoluble solids have resulted in extensive modification of the system components, with the result that the sampling capacity has been extended to accommodate sample sizes up to 10 mg with a total nitrogen content of Up to 100 Pg Per sample in a single five-minute analysis.
EXPERIMENTAL
Instrumentation. Figure 1 is a schematic diagram of the low current system (16) arranged for normal syringe injection of the samples into a hot inlet. A Solid/Liquid Sample Inlet suitable for solids, polymerizable liquids, or high boiling petrochemical distillates, none of which can be sampled accurately Using normal syringe injection into a hot inlet, is shown in Figure 2. All of the instrumentation in Figures
-
(16) Dohrmnn Division of Envirotech Corp., Tech Bulletin AI-1 1, Mountain View, Calif., 1968.
ANALYTICAL CHEMISTRY, VOL. 43, NO. 12, OCTOBER 1971
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Table I. Reduction Catalyst Evaluation Recovery, Benzene conversion Pyridine in Azobenzene in efficiency,a 5 pl/sec isooctane, 100 ppm isooctane, 100 ppm 96.0 100 43
Catalyst Granular nickel (10/20 mesh) Harshaw nickel catalyst 99.5 10% Pt on Alundum Failed at l/z (10/20 mesh) pl/sec 5 Ni on Alundum (10/20 mesh) ... Nickel wire a Methane/benzene ratio using a flame ionization detector.
100
. . I
Scrubber material KzC03 (20/40 mesh) CaO (20/60 mesh)
400
60
Ascarite granules (10/20 mesh)
180
50% NaOH on Alundum (20/ 40 mesh)
300
0
100
..
100
42
Very slight negative Rate limited negative Rate limited negative Very slight oscillation
1 and 2 is available from Dohrmann Division, Envirotech Corp., Mountain View, Calif. (13, 16). Figure 3 is a schematic diagram illustrating the modifications t o the system of Figure 1 which are necessary in order to measure solid samples of up to 10 mg with a nitrogen content of up to 100 p g in a single five minute analysis. The inlet is the Solid/Liquid Sample Inlet of Figure 2. The modified combustion tube of Figure 3a has an expansion volume for allowing the sample to expand and mix with hydrogen, and a dilution chamber where further hydrogen dilution reduces the possibility of overloading the catalyst with excessive hydrocarbon. The catalyst temperature is lowered to 700 "C here to favor N H 3 in the equilibrium Nz 3H2 e 2NH3. The titration cell cap of Figure 36 is heavily plated with Pt black to virtually eliminate sensor degradation due to the oxygen generated at the generator anode when H+ ions are being regenerated. To further eliminate sensor degradation, the generator anode is raised to allow the generated oxygen to escape from solution rather than poison the classical platinum/hydrogen sensor electrode. The cell body is modified to allow the Hzgenerated at the cathode to escape rather than to force the very basic electrolyte in the generator sidearm back into the cell body with subsequent high recovery. The electronic interface (high current accessory) of Figure 3c, consisting of a follower amplifier (Fairchild 714) with its associated power supply further protects the platinum contacts of the output chopper of the microcoulometer from current overload. Chemicals. Reagent grade chemicals and ultra pure hydrogen (99.9998%) are required. Water for the electrolyte must be distilled or double deionized. Catalyst Selection. Several catalysts were evaluated for hydrogenation capacity, ammonia conversion capability, and peak shape (Table I). A ionization detector following a 6-ft X '/*-in. Porapak Q column programmed at 6 "C/min from 50 "C to 200 "C was used to measure the methane/benzene ratio in the catalyst effluent during continuous benzene injection into the inlet of the pyrolysis tube. 1674
, .
Table 11. Acid Gas Scrubber Evaluation Temperature, System response to Halide and "C 10 pl acetone injection sulfur response 450 Very negative None
Ascarite granules (10/20 mesh)
+
48
78
Comments Sinters Shrinks, cokes Not quantitative capacity of 10/20 mesh Ni Sinters, tails l/3
...
Comments Excellent for nonoxygenated hydrocarbons only Suitable for all sample types, but difficult to condition Rate limited; absorbs HzO
...
Negative response to acetone
None
Best performer; suitable for all sample types
None
The 10/20 granular nickel catalyst gave the best overall performance. Catalyst activation is necessary and is detailed elsewhere (17). The Harshaw catalyst (No. 0707-T-1/8 ground to 10/20 mesh) performed very well for a short time but persisted (3 batches) in losing its nitrogen to ammonia conversion capability after it had been used a while, particularly after it had seen large hydrocarbon samples. Oxidation at 900 "C did not regenerate this catalyst once it had failed. Because it does not sinter and can be removed without breaking the pyrolysis tube a desirable form of the nickel catalyst was 5.0% Ni on 10/20 mesh Alundum. Although this catalyst quantitated on standards under conditions of slow injection (0.2 pl/sec), coke and unpyrolyzed hydrocarbons resulted from injection rates above 0.5 pl/sec, indicating that the nickel concentration was not sufficient. An attempt to obtain higher concentrations of nickel on the Alundum surface by increasing the amount of N i N 0 3 in the plating solution failed due to the rather low surface area of the Alundum. U p to 20% Ni was attempted, but most of the Ni remained on the sides of the plating crucible and no improvement in catalytic performance was observed. Scrubber Material. Several scrubber materials were investigated to find the "universal" scrubber material and the appropriate scrubbing conditions so that all types of samples (water, oxygenated and nonoxygenated hydrocarbons, high sulfur and chlorine content, etc.) could be run interchangeably. The results of this study are shown in Table 11. This investigation revealed that a double application of 5 t o 8 drops of 50% NaOH on 20/40 mesh Alundum at 300 "C proved to be the best such material. We were led to this after trying ascarite, a form of NaOH on asbestos, for our water samples (18). The ascarite was used at room temperature to trap the water completely, which quickly saturated (17) Zbid.,Tech. Bulletin IM-IOA (1970). (18) D. R. Rhodes, Chevron Research, Richmond, Calif., personal
communication, (1970).
ANALYTICAL CHEMISTRY, VOL. 43, NO. 12, OCTOBER 1971
Table IV. Low Range Nitrogen, ppm Average of Duplicates
Table 111. Results on Standards: Low Current System ( Recovery at 20 ppm N and 200 ppm N) Organic Nitrogen in Isooctane 100 Analine Octylamine 99 Pyridine 98 Indole 101 Quinoline 102 Azobenzene 404 Organic Nitrogen in HPO Pyridine Diethanolamine Aminotriazol
a
Inorganic Nitrogen in HPO Ca(N0h KNOI Ni(NOd2 400 “C inlet temperature: 9 5 z .
Microcoulometric method Direct injection
Kjeldahl extraction Product (4 (b) (C) Hydro feed 0.177 f 0 . 1 0 0.309 0.315 f 0.01 Benzene unit feed 0.190 i 0.05 . . . 0.069 f 0.01 Liquid product 0.334i0.04 ... 0.079fO.02 Reformer feed 0.447 i 0 . 1 0 0.161 0.131 f 0.03 Reformer feed (hydro 0.720 i 0.01 . . , 0.215 f 0.01 Treated) Topper unit gasoline 0.745 =k 0.04 . . . 0.278 f 0.01 Raw kerosene 1 . 5 0 0 3 ~0 . 2 0 , . , 0 . 9 2 6 f 0.02 Premium gasoline 3.980 i 0.03 . , . 2.850 f 0.05 Premium gasoline 14.000 f 0.04 . . . 10,100 i 0.01 a 100 grams of sample taken. * 500 grams of sample taken; duplicates not available. 10-32 gl of sample taken.
98 100
90
100 100 99
the scrubber material with water and necessitated frequent changing of the scrubber. The need for very large water samples (30- to 50-PI) when determining 1.0 ppm nitrogen or less led to the idea of using NaOH on a support such as 20/40 mesh Alundum at a n elevated temperature. Titration Cell Parameter Selection. For the standard T-400-H low current titration cell (26), a n investigation of cell parameters indicated that an electrolyte concentration of 1.0% Na2S04, a microcoulometer gain of 300 and a bias of 100, a platinum black coating of 150 mA for 3 seconds yielded the lowest noise plus the optimum response eke. The reduced microcoulometer gain required at ?he 1.O electrolyte concenrration is probably the major factor in reducing the noise level. The high current cell utilized 2.0% Na2S04 electrolyte, a bias of -30, a microcoulometer gain of 4000, and a platinum black coating of 60 mA for 3 minutes. The platinum black plating procedure is detailed elsewhere (19).
::
RESULTS AND DISCUSSION
Low Current System. RESULTSON STANDARDS. The results of various organic and inorganic nitrogen standards which were run on the system described in Figure 1 are shown in Table 111. These results are consistent with results reported previously (IO, 14, 20) for the “standard” nitrogen system, and are representative of recoveries in nitrogen compounds of similar bonding type. One precaution is evident from the low recovery o n azobenzene (40%:) a t the 650 “C inlet temperature. Since azo compounds tend to decompose to N P instead of being reduced to NHs, lower inlet temperatures and higher Hz flow rates significantly increase recovery by reducing high temperature exposure a t the inlet. This reduces the tendency for the molecules t o “crack” a t the N-N bond with subsequent formation of N?. RESULTS OF PETROLEUM SAMPLES.Comparative analysis of petroleum samples is shown in Tables IV-VI. All higher concentration samples were diluted to approximately 50 ppm in cyclohexane. The following tabulated results illustrate the precision and accuracy of the nitrogen analyzer for samples in the low to medium ppm range. ~~~~
(19) Dohrmann Division of Envirotech Corp., “Sub-Micro Ele-
mental Analysis by Microcoulometry,” Mountain View, Calif., 1969. (20) P. Gouverneur and F. Van de Croats, Analyst, 93, 782-787 (1968).
Table V. Medium Range Nitrogen, ppm Average of Duplicates Microcoulometric Product Kjeldahl method 21.1 i 0 . 3 Commercial motor oil 1 8 . 0 f 1.0 Gas engine oil 24.5 f 4 . 0 20.9 f 0 . 4 Virgin light gas oil 2 8 . 9 i 0.3 2 1 . 2 1 G.I* No. 2 side stream 36.2& 4 . 0 3 2 . 6 4 ~0.1 Commercial No. 2 fuel oil 47.8 i 0 . 1 51.01- 1 . 5 Hydro treated blend oil 6 2 . 0 3 ~3 . 0 S0.2& 0.3 Commercial additive 63.5 i 6 . 0 63.9 -L 1 . 5 No. 2 fuel oil 87.5 i 0 . 2 9 2 . 8 =t0 . 5 Experimental oil No. 2 1 5 5 . 0 3 ~0 . 1 160.0 f 4 . 0 181.0 & 1 . 5 Experimental oil No. 1 176.0 i 14.0 ‘ Solid/liquid sample inlet: 24.0 f 0.3 ppm. Table VI.=’ High Range Nitrogen, ppm Average of Duplicates Microcoulometric methoda Direct Solidiliquid Kjeldahl injection sample inlet Product 202 f 10 190 f 5 Light cycle oil 218 i 17 313 i 11 305 i 4 295 f 1 Gear oil Commercial motor oil 423 f 25 Varies f 80 430 i 6 515 i 15 4 6 6 f 10 582 i 6 Gas engine oil Commercial motor 405 i 5 582 i 10 579 f 3 oil 460 f 2 730 f 20 735 f 15 Transmission fluid 634 i 4 758 i 18 769 =k 17 Industrial oil 2600zk 100 2300 f 100 2 5 0 0 f 100 A1 drawing oil Outboard motor oil 5600 =k 100 Varies i 2000 5400 f 100 a Diluted to -50 ppm N in cyclohexane.
In the case of trace analysis (Table IV), there is generally good agreement (to within 0.5 pprn) between the usual 100gram macro Kjeldahl extraction procedure utilizing 95 % H2SO4 and the microcoulometer results. On two of the samples there was enough material remaining t o analyze one 50-gram extraction. The agreement with the microcoulometric data improved considerably (better than 0.1 ppm). Unfortunately this unexpected result could not be verified for the remaining samples, so that the results can only indicate that a more detailed study on the very trace samples should be undertaken to check whether the agreement is in fact better than 0.1 ppm N.
ANALYTICAL CHEMISTRY, VOL. 43, NO. 12, OCTOBER 1971
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I
I Figure 4. Separation of needle/ septum blank from thesample peak: direct injection system for trace nitrogen Deionized water b. Pyridine in water, 0.21 ppm nitrogen a.
SAMPLE
I
.3
1
PEAK
.22 PPM
I
I
I
I
I m
I
.2
I ,I
I
4-45
ut SAMPLE
SAMPLE INJECT
0
INJECT
TIME-
The precision attained in the trace analysis in Table IV was generally better than +0.03 ppm N and was achieved through large sample sizes (up to 30 pl), slow injection rate ( 0
E
‘
\
29.5
PPM
37.0 P P M
4 2 MIN
S A M P L E INJECT BOAT I N I M M E D I A T E L Y
1676
1I
ANALYTICAL CHEMISTRY, VOL. 43, NO. 12, OCTOBER 1971
SAMPLE INJECT E+
BOAT I N
100
Table VII. Waste Water Analyses: Solid/Liquid Sample Inlet Applications Microcoulometric method Solidiliauid samde inlet NonNessler Kjeldahl Volatile volatile Sample NHB Total N Total N N N Raw sewage before 27.8 13.0 39.0 40.0 processing 22.0 Nonchlorinated 32.6 13.0 48.0 47.0 planteffluent 29.0 Final plant effluent 29.0 17.2 47.0 46.5 No. 1 25.0 Final plant effluent 37.0 67.0 29.5 65.0 No. 2 26.0
tile nitrogen" content was observed (Table VII). The technique is also appealing since needle fouling is prevented, allowing the possibility of handling samples with particulate matter or high inorganic salt concentrations (sea water, brines, etc.). For trace nitrogen in water analysis where the needle is inserted through the septum inlet into the hot zone, the needle/ septum blank must be subtracted from the total peak area. This fact is emphasized in Figure 4, where the needle/septum blank is separated from the sample peak, and is as much as twenty times as large. For the Solid/Liquid Sample Inlet there is no needle/septum blank because the sample is injected into the boat in the cool zone. However, care must be taken to measure the quantity of sample which may be volatilized out of the needle by radiation from the hot inlet, just as in the case of injecting into the hot inlet, the sample volume in the syringe should be checked both before and after injection. For trace nitrogen in water analysis, the cell bias also plays a role since absolutely pure water at p H 7.0 entering the cell requires the generation of some hydrogen ions to bring the pH to the 6.0 level maintained in the cell by the microcoulometer. This result is an apparent positive blank of approximately 0.03 ppm (12). By adjusting the cell bias (and hence the p H of the cell electrolyte) the blank on water of known purity can be adjusted to within 0.01 ppm of the actual nitrogen content. Changing the bias voltage from 120 mV t o 110 mV on the coulometer changed the apparent blank on a sample of double deionized water from -0.02 ppm to +0.01 ppm. High Current System. Figure 6 illustrates the results achieved on the high current system of Figure 3 when 10 p1 of standards of pyridine in isooctane and ammonia water at the
90
Es P W
> 0
0 P W
z u
0
t z
70
600
650
NICKEL
700
750
800
CATALYST TEMPERATURE ( * C )
Figure 6. High current system: Per cent recovery on standards at 1% nitrogen us. catalyst temperature 1 C,
0
Nitrogen from ammonia in water 2 4 sample 10-pl sample Nitrogen from pyridine in isooctane 2-pl sample 10-pl sample
z
1
A A
1.0% nitrogen level were injected at various catalyst temperatures. As an additional check, the ammonia in water standard yielded 100% recovery when injected directly into the cell electrolyte with a 6-in. needle. The decrease in recovery of the ammonia standard when injected through the furnace at catalyst temperatures above 700 "C indicated a loss in catalytic activity a t the higher temperatures: only below 700 "C was NH, strongly favored in the equilibrium N2 3H2 e 2" 3. When the sample size was decreased below the 10-pl level, the recovery increased, reaching 100% at the 20,000-nanogram level for both standards at catalyst temperatures below 750 "C. Table VlII illustrates the results achieved on IO-mg solid samples of varying composition at the 1.0% nitrogen level which were weighed directly into the sample boats on a semimicrobalance. Recovery increased and peak tailing reduced
+
Table VIII.
Results on Solid Samples in the High Current System % N, microcoulometric data high current system inlet temperature ( "C) N KjeldahP 700 850 lo00 1120 1.290 f 0.010 1.17 =k 0.01 ... 1.25 f 0.020 1.270 f 0.010 1.390 i. 0.010 1.09 i 0.01 ... 1.28 i 0.030 1.280 i 0.030 0.950 zk 0.015 0.88 f 0.01 , . . ... 0.900 f 0.010
z
Sample Sample consistency Food additive Fine powder Prune leaf Dry flakes Polymeric oil Viscous tar additive Catsup No. 1 Paste 0.211 i 0.005 ... 0.150 i 0.007 Catsup No. 2 Paste 0.222 i 0.005 ... 0.161 f 0.003 Standardb Liquid 1.ooO f 0.005 0.96 f 0.01 ... Method modified to include nitrate nitrogen. Prepared standard of pyridine in isooctane injected at port A of Solid/Liquid Sample Inlet.
1.92 -I 0.003 0.97 ~ ' 0 . 0 2 0
0.209 f 0.006 0.215 f 0.002 0.890 f 0.020
ANALYTICAL CHEMISTRY, VOL. 43, NO. 12, OCTOBER 1971
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1677
as the inlet temperature was raised from 700 "C to 1120 "C, indicating that the nitrogen which was fixing o n the carbon residue remaining in the sample boat was released only at the very high inlet temperatures. The decrease in recovery for the prepared standard of pyridine in isooctane at the 1120 "C inlet temperature indicates that decomposition of the bound nitrogen to N2 can occur. Thus a tradeoff may exist between increased recovery due to release of the nitrogen from the sample residue at higher inlet temperatures cs. decrease in recovery due to the formation of NPin the very hot inlet. The most significant aspect of these results is the average precision of 0.02z N obtained at the 1.0% nitrogen level. Thus by comparison with the more laborious classical methods of analysis, excellent precision and accuracy can be combined in a single five-minute analysis at the 1.0% nitrogen level. By an exchange of titration cells and bypassing the high current interface, this same system can be ready for very trace analysis (to 0.1 ppm) within 10 minutes. CONCLUSIONS
The microcoulometric method for trace nitrogen determinations has been extended in sensitivity and capacity from 0.1 f 0.03 ppm N to 1.0 i 0.02 N. This represents a sixfold improvement in sensitivity and a twentyfold improvement in capacity. The increased sensitivity was obtained through
increased sample sizes, isolation of the septum/needle blank contributions, and optimization of cell parameters. Increased capacity has been achieved through cell redesign, pyrolysis tube modification, the addition of a high current interface, and the use of high capacity choppers. The system can be converted from high capacity to high sensitivity or back again in less than 10 minutes through a sample cell exchange and a minimal electrical reconnection. The chief application of the high capacity system is for those solid samples in the 500 ppm to 1.0% nitrogen range which cannot be readily dissolved or diluted and where a 5-minute or less titration time with a precision of +2.0% relative is required. Comparison of results with the more laborious classical techniques will enable 2.0% accuracy as well for similar sample types. Very recent experience has indicated that for trace analysis in the low current system, better precision and accuracy may be expected at a catalyst temperature of 700 "C rather than the 800 "C temperature utilized in this study. Further work is required at this temperature to verify complete cracking and conversion to NH3 for a wide range of high temperature petroleum products, however. RECEIVED for review September 3, 1970. Accepted June 23, 1971.
NOTES
Influence of the Setchenow Salt Effect upon Analytical Results Involving Concentration by Solvent Extraction Techniques The0 Groenewald Chamber of Mines of South Africa, Research Organisation, Johannesburg, South Africa
SOLVENTEXTRACTION has become a n integral part of analytical chemistry, and is most often used for the preliminary purification and/or concentration of materials to be determined. Much attention has been paid to the effect of many of the fundamental aspects of solvent extraction upon the results obtained by an analytical technique incorporating a solvent extraction step. One aspect which has not attracted much attention is the effect upon the results of the mutual solubility of the aqueous phase and of the organic phase. If effectively quantitative transfer of the relevant material occurs during the solvent extraction, then the resultant concentration of this material in the organic phase will be affected by a change in volume of the organic phase in contact with the aqueous phase. Often no correction is made for this change in volume and, indeed, no correction may be necessary. A further complication is introduced by the influence of the Setchenow ( I ) salt effect, which also has a bearing upon the stoichiometry of the solvent extraction procedure as employed in analysis. Salting-out which is caused by the common-ion effect applicable to the extraction of a disso-
ciable species must be distinguished from the salt effect first described by Setchenow. The latter effect applies to the influence of salts upon the activity coefficients of non-electrolytes in aqueous solution. The Setchenow equation may be written log
SO,
--
sos
=
log f
=
K,,C,T
(1)
where So, is the solubility of the non-electrolyte in pure water, SO,is the solubility of the non-electrolyte in the aqueous salt solution, f is the activity coefficient of the non-electrolyte, K , is the Setchenow salting coefficient, and C , is the molar concentration of the salt in the aqueous phase. Salting-out usually results in a decrease in the solubility of the non-electrolyte in the aqueous phase, when K , has a positive value. Salting-in refers to the reversal of this process, when K , assumes a negative value. The relative merits of attempts to explain these phenomena will not be discussed here. The most recent theoretical treatment is by Masterson (2) and the latest reviews by Long (3) and by Sergeeva ( 4 ) (2) W. L. Masterson and T. P. Lee, J . Phys. Chem.. 74, 1776 (1970).
(1) J. Setchenow, Z . Physik. Chem., 4, 117 (1889).
1678
(3) F. A. Long and W. F. McDevit, Chem. Ret>.,51, 119 ( 1 9 5 2 ) . (4) V. F. Sergeeva, R u n . Chem. Rec., 34, 309 (1965).
ANALYTICAL CHEMISTRY, VOL. 43, NO. 12, OCTOBER 1971
