Rapid Determination of Ammonia and Total Nitrogen in Municipal Wastewater by Microcoulometry D. K. Albert, R. L. Stoffer, and I. J. Oita Research and Development Department, American Oil Company, Whiting, Ind. 46394
R. H. Wise Robert A . Taft Water Research Center Federal Water Pollution Control Administration, U. S . Department of the Interior, 4676 Columbia Parkway, Cincinnati, Ohio 45226 NITROGENDETERMINATIONS are very important in water pollution control because ammonia, nitrates, nitrites, and other nitrogenous compounds can act as nutrients for algae and other troublesome aquatic plants. Nitrogen in wastewaters may range fron less than 0.1 to several ppm and may occur as refractory materials a t various stages of waste treatment and purification. Whereas standard analytical methods ( I ) , such as titrations or spectrophotometry for ammonia, Kjeldahls for organic nitrogen, and colorimetry or spectrophotometry for inorganic nitrogen, give satisfactory results in routine analyses of water samples, they are tedious and time-consuming. For the rapid determination of both ammonia and total nitrogen in municipal wastewater, we have therefore adapted a microcoulometric technique that was originally developed for determining organic nitrogen in petroleum products (2, 3) and later modified for determining total nitrogen in either petroleum products or aqueous samples (4,5). Inorganic and organic nitrogen compounds are pyrolyzed and reduced to ammonia in a stream of hydrogen over a suitable catalyst, either granular nickel (4) or a combination of a hydrocracking catalyst (rhodium on a support such as silica-alumina) and nickel-on-magnesia (ter Meulen) catalyst (5). Acidic pyrolysis products are removed in a suitable alkaline scrubber, either calcium oxide (4) or magnesium oxide (5), and the ammonia is titrated automatically with coulometrically generated hydrogen ion. In our adaptation, we have added a specially designed sample inlet that includes a calcium oxide scrubber so that the catalyst can be bypassed when only ammonia need be determined. Analyses of a variety of municipal wastewaters have shown that the precision and accuracy of both the ammonia and total nitrogen determinations, with either catalyst system, is about &6% relative. Results agree closely with those obtained by conventional methods. EXPERIMENTAL
Materials. All gases had a minimum purity of 99.995%. Grangular nickel (20-50 mesh) was obtained from Dohrmann Instruments Company, Mountain View, Calif. The hydrocracking catalyst and ter Meulen catalyst were prepared as described (5). Calcium oxide (20-50 mesh) was screened from lump lime (National Formulary). Standard solutions (1) American Public Health Association, et al., “Standard Methods for the Examination of Water and Wastewater,” 12th ed.,American Public Health Association, Inc., New York, 1965. (2) R. L. Martin, ANAL.CHEM., 38, 1209 (1966). (3) Dohrmann Instruments Co., Mountain View Calif., Technical Bulletins 508 and 522. (4) R. Moore and J. A. McNulty, “Determination of Total Nitrogen in Water by Microcoulometric Titration,” Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Cleveland, Ohio, March 4, 1968. (5) I. J. Oita, ANAL.CFIEM., 40, 1753 (1968). 1500
ANALYTICAL CHEMISTRY
SAMPLE INLEtr TOTAL NITROGEN
S A M P L E INLETS
AMMONIA
PYROLYSIS
TITRATION
FURNACE
L
SCRUBBER
l
i
i
__f
AUXILIARY F L O W 120 r c / m l n .
IMICIO.1
E C 0 U L 0 METE R-
HYD R‘O G EN
RECORDER
Figure 1. Modified nitrogen analyzer were prepared from reagent grade diethanolamine, potassium nitrate, and ammonium sulfate. Water for the solutions and the instrument calibrations was deionized (Amberlite MB-3 resin) and then distilled from sulfuric acid (pH 1-2) in an all-glass apparatus. Apparatus. Figure 1 is a diagram of the apparatus. The pyrolysis furnace (nickel catalyst), titration cell, microcoulometer, and recorder (1-mV, with Disc integrator) are as employed in the Dohrmann Nitrogen Analyzer (3). The calcium oxide scrubber, ammonia inlet, and hydrogen humidifier are laboratory-constructed. The ammonia inlet is located so that the calcium oxide scrubber is common to both the total nitrogen and ammonia determinations. There is also an auxiliary hydrogen flow to stabilize the partial pressure of the gas at the platinum-hydrogen electrode during sample injection (4). Each sample inlet has a siliconerubber septum through which samples are injected with a hypodermic syringe. The gas supply system and the humidifier are illustrated in Figure 2. The humidifier, which is located downstream from the regulating needle valves and flow meters on the pyrolysis furnace, reduces the rate of coking of organic compounds on the catalyst and maintains a CaO-Ca(OH)2 equilibrium in the scrubber for effective removal of acidic compounds (4); the auxiliary flow is not humidified. The flow pattern is such that the humidifier need not be disconnected for recharging, nor need the flow through the catalyst be interrupted. In addition to the conventional two-stage pressure regulators on the cylinders, another pressure regulator (Model R06122 NNKA, C. A. Norgren Co.) is placed near the pyrolysis furnace for more precise control. Details of the ammonia inlet, the scrubber, and the vaporization block are shown in Figure 3. The ball joints are secured with clamps equipped with locking screws. The lS/9 joint is lubricated with a thin film of silicone grease, but the 12/5 joint is not lubricated so as to avoid contamination of the titration cell. The ball joints, ammonia-inlet and scrubber, and the cell inlet are insulated with asbestos cloth and glass wool to eliminate cold zones. The top of the vaporization block is removable so that the scrubber tube can be
BY-PASS-----+ THROUGH NEEDLE VALVE A N D FLOW METER
T O TOTAL-N
INLET
LOK U N I O N W I T H E-RUBBER O - R I N G S PRESSURE REGULATOR
TOGGLE
VALVES
- 2 5 0 - C . C . GAS-WASHING
BOTTLE
W I T H COARSE FRIT
CHECK VALVES T W O - S T A G E PRESSURE REGULATORS
(V1.5 ARE REGULATING V A L V E S )
-CYLINDERS
Figure 2. Gas supply system replaced easily. Each section of the block is insulated with asbestos and, in operation, the block is placed on an asbestos base which is supported on a small adjustable jack. For analyses with the hydrocracking-ter Meulen catalyst, the apparatus and method of Oita (5) were used without modification, except for ammonia analyses which were obtained by inserting the "inlet-scrubber" between the outlet of the catalyst tube and the inlet of the titration cell. Procedures. The apparatus is operated at the hydrogen flow rates shown in Figure 1, with the pyrolysis furnace and scrubber temperatures at 800 and 430 OC., respectively. The Norgren pressure regulator, Figure 2, is generally adjusted to an output pressure of 30 psi with an input pressure of 60 to 80 psi. The titration cell is operated with a bias potential in the range of 100 to 112 mV, a stirring rate of about one third the maximum stirring rate, and with the sensor electrode positioned toward the reference electrode. The cell electrolyte, 0 . 0 4 z sodium sulfate, is maintained at a level of about ' 1 4 inch above the platinum electrodes. The microcoulometer is generally operated at a constant sensitivity of 30 ohms with the recorder base line arbitrarily adjusted to a position of 0.03 mV. The latter provides a constant background count for the integrator, and this count is then subtracted from the sample peak-area count. For maximum instrument stability, operating temperatures are maintained when the apparatus is not in use, but with hydrogen flow reduced to about 20 to 30 cc/min. To remove volatile impurities and fine particles of calcium oxide, a freshly packed scrubber is preconditioned as follows: With the apparatus in place, but without the cell attached, the temperature is increased t o 500 "C,left there for about 30 minutes, and then decreased to the operating temperature of 430 "C. During this time, eight to ten 10-111portions of water are injected. Finally, the outlet of the scrubber tube is cleaned with a cotton swab, and the cell is connected. A base line noise no greater than 0.005 mV and the absence of a negative cell response to 5 p1 of distilled water are criteria for determining whether the calcium oxide is sufficiently conditioned. [The life of the scrubber packing is not known precisely, but it can be used for at least a week in daily operation. No attempts were made in this work to regenerate the spent calcium oxide ( 4 ) because it was easy to replace.] The apparatus is calibrated with replicate runs of reference solutions of diethanolamine (10.0 pprn N) for total nitrogen determinations and of ammonium sulfate (5.0 ppm N) for
FOR THERMOC (1RON.CONSTA I B R A S S VAPORIZATION BLOCK, BOTTOM H A L F ]
I T LOUIS. MO
, Dimensions in inches except as n o t e d
Figure 3. Ammonia inlet and scrubber ammonia determinations. Generally, 5.O-pl samples are adequate. Sample injections in the range of 1-5 pl., with 30 ohms sensitivity, are generally adequate to cover the concentration range of about 2-50 ppm N. Samples for total nitrogen are injected at a rate of only 1-2 pl/sec. to allow sample equilibrium to be established in the combustion tube. For ammonia, the rate can be increased to about 3-4 pl/sec. In routine operation, the instrument is calibrated periodically (e.g., after every tenth sample) to detect any changes in response. Wastewater samples are injected in the same manner as the reference solutions, and are run in duplicate. Results are calculated by multiplying the sample peak area (integrator counts) per microliter of sample by the calibration factor which is expressed as ppm N per unit area (integrator count) per microliter of reference solution. VOL. 41, NO. 11, SEPTEMBER 1969
1501
related to micrograms of nitrogen by Faraday's laws (Q, which reduce to: (Peak area, square inches) 1.742 PgN = (sensitivity, ohms)
Table I. Nitrogen Determinations on Aqueous Reference Solutions
z
Run No."
Solution Diethanolamine
1 2 3 1 2 1 2 1 2 3
Ammoniumsulfate Potassium nitrate Diethanolamine
Nitrogen, ppm Added Foundb
TOTAL NITROGENC 9.9 9.9 10.1 9.3 10.2 10.5 9.9 9.9 9.9 9.9 0.99 0.9 1.1 1.0
Recovery, av
f 0.2 It 0 . 3 It 0.1 f 0.1 f 0.5 f 0.7 f 0.4 f O . l f 0.1 f 0.1
100 102 94 103 97 100 100 91 102 101
AMMONIUM NITROGEN Ammonium sulfate 0.
b
5.0 1 .o
5.1 f 0.1 0.91 z!= 0.03
102 90
Indicates determinations on successive days, Average of 3 or 4 determinations. Nickel catalyst.
As illustrated in Figure 2, whenever the water in the humidifier must be replenished during an analysis, the following procedure is used: Open valve Vl and close V s but leave Vz open temporarily. Open V 3 which controls the water flow from a reservoir, and allow the hydrogen flowing through V, and V3 t o purge the water in the reservoir of dissolved gases. Close V,, open V 4 ,and allow water to drain from the reservoir into the humidifier. If necessary, apply slight suction through V , t o start water flow. Close V l , Va, V4 and open V2, V5to restore the normal flow pattern. To avoid loss of ammonia, all wastewater samples are stabilized by the addition of 0.8 ml of concentrated sulfuric acid per liter (I). Turbid solutions, especially those with large amounts of particulate matter, are filtered (glass-fiber filters) and/or homogenized to facilitate sampling with a hypodermic syringe. In general, homogenization is recommended because nitrogen losses are possible with filtration. RESULTS AND DISCUSSION
A base line noise no greater than 0.005 mV, with a catalytic conversion efficiency (as tested with a reference solution of diethanolamine) of at least 9 5 x of theory, are criteria for judging whether the catalyst is satisfactory. Peak area is
In analyses of reference solutions and several different types of municipal waste waters, the microcoulometric technique has given satisfactory results with either the granular nickel or the combined hydrocracking-ter Meulen catalyst. Moreover the (6) Dohrmann Instruments Co., Mountain View, Calif., operation
manual for C-200-A microcoulometrer.
.7
SEN S ITlVlTY=30 O H M S (NICKEL CATALYST) .6
A M M O N I A INLET TOTAL-N INLET .5
POTASSIUM N I1 ;ATE
( 10.1 ppm N )
THANOLAMINE .4
'
9 . 9 ppm N
AMMONIUM SULFATE
AMMONIUM SULFATE
(
)
i . 2 ppm N
10.0 ppm N
)
v)
5
0
>
-5
2.3
.2
-
x
.I
9 m
0
I
I
I
I
I
I
I
MINUTES
Figure 4. Response of system to aqueous reference solutions 1502
ANALYTICAL CHEMISTRY
I
I
Table 11. Ammonia Nitrogen in Various Municipal Effluents Ammonia nitrogen, ppm Sample No. 1 2 3 4
a
Sample type Raw sewage Primary emuent Secondary effluent Secondary effluent
Microcoulometry
Nessler
25 i- 1 49 i 2 36 f 0 4.4 i0 . 2
21 37 35 4.4
Distillationtitration 21 46 35
Not run
Table 111. Total Nitrogen in Various Municipal Effluents Total nitrogen, ppm Microcoulometry Hydrocracking Sample No. Sample type Nickel catalyst ter Meulen catalyst 25 =k 2 26.5 + 0 . 5 1 Raw sewage 51 f 1 50 i 1 2 Primary effluent 11.4 =t0.5 12.8 i 0 . 6 3 Primary effluent 6.4 + 0 . 7 5 . 6 i- 0.1 4 Secondary effluent Not run 4.2 i0.3 5 Secondary effluent Not run 6.4 i0.7 6 Secondary effluent 6.8 i0.2 Not run 7 Secondary effluent Low because nitrates not included.
results compare favorably with those obtained by conventional techniques. As summarized in Table I, analyses of aqueous reference solutions of diethanolarnine, potassium nitrate, and ammonium sulfate show very good accuracy and precision, even for only 1 ppm nitrogen. The determined values were calculated from the respective peak areas. Illustrative peaks for total nitrogen and ammonia are shown in Figure 4. The ammonia peak from ammonium sulfate is sharp with only a minor amount of tailing, but the total nitrogen peak tails over a period of 4 to 5 minutes. Generally, about 3 minutes and 5 minutes, respectively, are required between successive ammonia and total nitrogen peaks for baseline stabilization. As shown in Tables I1 and 111, the microcoulometric methods compare favorably with other methods for the determination of both ammonia and total nitrogen in various municipal effluents. The fact that the microcoulometric value for the ammonia in raw sewage (Table 11, sampleNo. 1)is higher than the values obtained by either of the other methods indicates a possible interference (e.g., volatile amines or other basic compounds that enter the cell and are titrated with the ammonia). On the other hand. the Nessler value for the primary effluent (Table 11, sample No. 2) is significantly lower which indicates a possible interference with that method. Total nitrogen analyses (Table 111) with either the nickel catalyst or the hydrocracking-ter Meulen catalyst are, in general, in good agreement with the Kjeldahl analyses. The Kjeldahl value of 2 ppm nitrogen for sample No. 7 is low because of the inability of that method to determine nitrates. Nitrate analysis of this sample showed about 4 ppm of nitrate-nitrogen; the nitrate contents of the other samples were 0.5 ppm, or less. This further demonstrates that the microcoulometric technique includes inorganic, as well as organic, nitrogen. To assess the precision and accuracy of the microcoulometric methods, six samples consisting of a primary and a secondary (final) effluent from each of three municipal sources were analyzed for ammonia and total nitrogen, and the results were compared with those from routine methods a t American Oil
Kjeldahl 28 48 12.7 5.2 3.9 5.2 2a
Table IV. Ammonia Nitrogen: Comparison of Microcoulometric, Nessler,a and Phenol-HypochloritebMethods Ammonia nitrogen found, ppm sample Microcoulometry PhenolN0.c Av.d Std dev Nessler hypochlorite 1 2 3 4 5
6
7.6 7.9 19.4 23.1 8.2 7.9
0.3 0.3 0.9 2.0 0.5 0.2
7.7 8.2 14.0 24.3 7.9 7.1
4.5 7.8 12.6 23.0 4.8 6.0
Routine spectrophotometric method, American Oil Company. b Routine spectrophotometric (AutoAnalyzer) method, Robert A. Taft Water Research Center. c The samples are: No. 1 and 2 primaiy and secondary effluents from Source A; No. 3 and 4 primary and secondary effluents from Source B; No. 5 and 6 primary and secondary effluents from Source C . d Based on 5 to 6 determinations. a
and a t the Robert A. Taft Water Research Center. These evaluations are summarized in Tables IV and V. The precision of the microcoulometric method for ammonia (Table IV) is within =k6zrelative and, in general, the accuracy is within the same range. Although the microcoulometric method is only in fair agreement with the phenol-hypochlorite method, it is in generally good agreement with the Nessler method (Tables IT and IV). Table V further shows that the granular nickel catalyst and the combined hydrocracking-ter Meulen catalyst are equally satisfactory for the determination of total nitrogen in wastewater. The precision and accuracy with either catalyst is generally within =k6 relative. One shortcoming of the microcoulometric technique is that its general applicability for the direct determination of ammonia is not fully known. F o r example, samples that contain volatile amines or other basic and volatile titratable materials, all of which interfere with the microcoulometric VOL. 41, NO. 11, SEPTEMBER 1969
1503
Table V. Total Nitrogen : Comparison of Microcoulometric and Routine Methods Total nitrogen found, ppm Microcoulometry Sample Nickel catalyst Hydrocracking-ter Meulen catalyst Routine methods" No. Avb Std dev AV.~ Std dev American Oil TWRC~ 1 9.2 0.8 9.4 0.7 9.6 8.0 2 9.4 0.3 9.6 0.5 9.7 8.2 3 21 .o 1.6 23.1 0.8 22.9 18.2 4 25 1.1 28 1.3 28.4 28.8 5 12.4 0.7 9.6 0.6 10.2 9.0 6 11.2 0.7 10.1 0.8 9.6 8.2 Organic nitrogen and ammonia determined by Kjeldahl method; nitrates and nitrites determined spectrophotometrically. o Based on 4 to 6 determinations. d Robert A. Taft Water Research Center.
method (as well as with the conventional titration method), cannot now be reliably analyzed. With such samples, a n investigation of auxiliary methods (e.g., distillation or gas chromatography) to separate ammonia from the interferences would be worthwhile. Partial differentiation between total organic nitrogen and toral inorganic nitrogen might be possible by selective fractionation of the sample a t the total-nitrogen inlet. For example, volatile organic compounds in water might be completely volatilized a t an inlet temperature of about 300 "C, or less; inorganic nitrogen (including nonvolatile organic nitrogen) could then be decomposed and volatilized a t 500-600 "C. Also, a more compact apparatus for the determination of ammonia and total nitrogen could be made by installing a n inlet in the Dohrmann pyrolysis furnace for the direct determination of ammonia. The inlet, which would bypass the nickel catalyst, could then be used in conjunction with the
"internal' scrubber that is normally used in the furnace for total nitrogen determinations. ACKNOWLEDGMENT
The authors are particularly grateful to A. L. Conn, of American Oil Company, for helpful advice, and to R. T. Williams, Robert A. Taft Water Research Center, FWPCA, for technical assistance. RECEIVED for review March 24, 1969. Accepted June 16, 1969. This work was performed for the Federal Water Pollution Control Administration (FWPCA), U. S. Department of the Interior, under Contract No. 14-12-109. Mention of products is for identification only and does not constitute endorsement by the Federal Water Pollution Control Administration or the U. S. Department of the Interior.
Precise Peak Area Determination for Ge(Li) Detectors Paul Quittner' Texas A 6 M University, Activation Analysis Research Laboratory, College Station, Tex. 77843
THE INTENSITIES of y-rays measured with Ge(Li) detector3 are usuaily determined by calculating the counts in the full energy peaks produced by the radiations in question. For this a base line estimation and a base area computation are necessary because the peaks are superimposed on the background and often on a Compton continuum as well. The net counts in the peak are given by N = T - B, where T is the total measured count under the peak and B is the area, measured in counts, under the base line in the peak region. Previous methods have used a straight line approximation for the construction of the base line, even if this line has been calculated by fitting different functions to the measured data (1-3). If B is comparable to T, as is the case when measuring low intensity radiations or y rays in the presence of intense higher energy y rays, small deviations from the linearity can cause large errors in the intensity determination, N . To 1 Present address, Central Research Institute for Physics, Budapest, Hungary
overcome this, a method is described here which uses higher order polynomials for the base line construction. Because of the excellent energy resolution of Ge(Li) detectors, there are several channels on each side of the peak of interest which do not contain any other peaks, except for very unfortunate cases. In these regions a polynomial of a second or a third degree is fitted to the measured values with least squares techniques using 2 k ~f 1 and 2kR 1 points around the centers X L = X P - I L and X R = X P IR, respectively, where x p is the location of the peak (Figure 1). Using the tables of Savitzky and Golay (4) for the constants c, and d,, the values ( p ~JJR) , and the slopes ( q L , q R ) of these polynomials can be expressed as functions of the measured counts Y(j) in channel j by the following:
+ +
kw
PM =
i = -k.u
c,(k.dY(xM
+ i) and q.w
=
hw
--x.
dXkM)Y(X.w M
+ i);
(1)
( M = L, R ) (1) D. F. Covell, ANAL.CHEM., 31, 1785 (1959). (2) H. P. Yule, ibid., 40, 1430 (1968). (3) W. Schulze, 2. Anal. Chern., 234,401 (1968). 1504
ANALYTICAL CHEMISTRY
(4) A. Savitzky and M. J. E. Golay, ANAL.CHEM., 36, 1627 (1964).
