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Anal. Chem. 1982, 5 4 , 1690-1692
Determination of Ammonia with a Stark Microwave Cavity Resonator Sachlo Hlrose, Mitsuhlro Hayashl, Norlyoshl Tamura, and Tamio Kamidate Central Research Laboratory, Mitsubishi Petrochemical Co., Ltd., Wakagurl, Ami-cho, Inashiki-gun, Ibaraki 300-03, Japan
Isao Karube" and Shuichi Surukl Research Laboratory of Resources Utillzation, Tokyo Institute of Technology, Nagatsuta-cho, Midor-ku, Yokohama 227, Japan
The system for the determination of liquid ammonia consists of a Stark microwave cavity resonator and a dlaiyzer in which ammonia in a sample solution Is Vaporized by a strong alkaline. When a sample solutlon (a strong alkaline) is pumped into the flow system continuously, the potential based on the concentration of ammonla increases until a steady state is reached. The steady-state potential is obtained within 5 min. The callbration plot of the potential difference vs. concentration of ammonia is linear up to 2.0 mg.L-'. No interfering substances are observed. The system is applied to three kinds of process waters in a petrochemical factory and agreement with the conventional method Is f2%. The system can be used continuously and shows long-term stability.
The determination of ammonia is very important in fields such as medical, environmental, and industrial process analyses. Continuous monitoring of ammonia is especially necessary for the urea manufacturing process in the petrochemical industry. Many methods for ammonia determination are known. A chemical and spectrophotometric method based on the reaction of ammonia with phenol to produce indophenol is usually used. Recently, an enzymatic method based on NADH-coupled reaction which consumes ammonia has been reported ( I ) .
NH4+ + NADH
+ a-ketoglutarate
glutamate dehydrogenase
(E.C.1.4.1.2)
glutamate
+ NAD+
These two methods involve relatively large amounts of reagents and complicated procedures. Furthermore, continuous determination of ammonia is difficult by these methods. An ammonia gas electrode consisting of a combined glass electrode and a gas-permeable membrane is available for the determination of ammonia. However, interfering substances such as volatile amines cause poor selectivity of the electrode. An ammonia gas sensor composed of immobilized nitrifying bacteria and an oxygen electrode has been developed (2). The selectivity of the ammonia gas sensor was excellent and it could be used for continuous determination of ammonia. However, long-term stability is insufficient because bacteria must be living in the sensor. On the other hand, a continuous monitor for gaseous ammonia using a Stark microwave cavity resonator has been developed (3). As previously reported, the concentration of ammonia gas can be determined with the monitor based on a rectangular Stark microwave cavity tightly coupled through an iris to a Gunn oscillator. This monitor showed excellent long-term stability. Therefore, liquid ammonia concentration may be determined by introducing a vaporized ammonia into a Stark microwave cavity resonator (SMCR). By use of a dialyzer, ammonia in a sample solution is vaporized under 0003-2700/82/0354-1690$01.25/0
strong alkaline conditions, and then gaseous ammonia is permeated through a Teflon membrane and sent to SMCR by nitrogen carrier gas. This paper describes characterization of an ammonia sensor consisting of SMCR and a dialyzer and its application to process waters in a petrochemical factory containing ammonium ion. EXPERIMENTAL SECTION Materials. SMCR (Model SMA-3) was purchased from Mitsubishi Petrochemical Engineering Co., Ltd. (Tokyo,Japan). Plastic tubes having different diameters were purchased from Eiken Kizai Co., LM. (Tokyo,Japan). A proportioning pump and a dialyzer were purchased from Technicon Co., Ltd. Other reagents were commercially available analytical reagents or laboratory grade materials. Ten milligrams per liter of ammonium sulfate solution was used as a standard. Process waters of ammonia and urea plants built in a petrochemical factory (Kashima, Japan) were used as sample solutions. Determination of Ammonia by a Conventional Method. Ammonia was determined by a conventional spectrophotometric method (4). The generation of indophenol from ammonia and phenol was measured from optical density at 630 nm. SMCR Conditions. The apparatus used in this study is schematically shown in Figure 1. The measurements were carried out under the following conditions: the resonant frequency of the cavity, 23 875 MHz; dc voltage 220 V; ac voltage, 150 V peak to peak (100 kHz); the time constant, 10 s; the sample pressure in the absorption cell, 4 torr; the temperature, 37.5 "C. The full range of recorder 10 mV corresponded to about 20 mgL-l (ammonia gas). System. Figure 2 shows a schematic diagram of the system which consists of a proportioning pump, a dialyzer with a Teflon membrane (Sumitomo Electric Industry, Ltd., pore size 1pm), a SMCR, and a recorder (Type 3066, Yokogawa Electric Works Ltd., Tokyo, Japan). Procedure. The temperature of the system was maintained at room temperature (21 f 1 "C) except the SMCR. A strong alkaline solution (6 N sodium hydroxide) was continuouslypassed through the dialyzer at 0.16 mL.min-', while a sample solution containing ammonia at 0.32 ml-min-l. Both solutions mixed at the mixing joint were transferred to the sensor system. The flow of the solution was interrupted by air periodically. Consequently, ammonia vaporized in the alkaline solution was permeated through the Teflon membrane and was transferred by nitrogen gas to the SMCR which was maintained at reduced pressure ( 4 torr). When the output of the SMCR reached a steady state, distilled water was passed through the system at 0.32 mL.min-' for the washing of the system. RESULTS AND DISCUSSION Response of the System. The initial output was constant when a strong alkaline solution passed through one stream of the dialyzer and nitrogen gas passed through the other. When a standard solution containing 10 mgL-l of ammonia was injected into the system, the output voltage increased as shown in Figure 3. The Stark dc bias voltage was so adjusted as to give a peak intensity of the derivative output of the 0 1982 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 54, NO. 11, SEPTEMBER 1982
8
1891
I
14
t-
i.I 2
4
6
Sodium Hydroxide
8
Conc.(N)
Flgure 4. Effect of concentration of sodium hydroxide on the output of the system. Other conditions than the NaOH concentration were the same as Figure 3.
I
7
8
Flgure 1. Block diagram of the Stark cavity ammonia detector: (1) micrometer; (2) movable short; (3) crystal detector: (4) Stark cavity cell; (5) Stark electrode; (6) coupling irk; (7) tuning screw; (8) Gunn oscillator; (9) Gunn power supply; (10) Stark voltage supply; (1 1) lock-in amp; (12) recorder; (13) inlet; (14) outlet.
O
0.2
0.4
F l o w rate
0.6
0.8
(mt.min-')
Figure 5. Effect of flow rate of 6 N NaOH on the output of the system. Conditions other than the flow rate of NaOH were the same as Figure 3.
Nz gas
e-7
2
I 6l
Flgure 2. Schematic diagram of a liquid ammonia monitor: (1) sample turntable; (2) proportioning pump; (3) sample mixer; (4) dialyzer; (5) Stark cavity ammonia detector; (6) recorder; (7) vacuum pump.
.
!; 0 4.0
tr?
0
6
12
Time
(min)
18
Flgure 3. Typical response curve of the system. The experiments were carried out under standard conditions: (1) ammonia sulfate solution (ammonb 10.0 mg-L-'; flow rate 0.32 mL.min-'); (2) 6 N NaOH (0.16 mLamin-'); (3) air (0.42 mL-min-I).
electric-resonance i3ignal. The output of the system increased markedly and a steady state was obtained within 5 min. The output of the system was closely related with the gaseous ammonia concentration. The experiments were carried out under the standard conditions as described in the Experimental Section. Figure 4 shows the effect of concentrations of sodium hydroxide on the output of the system. The output increased with increasing concentraion of sodium hydroxide. Six normal sodium hydroxide was uwd for further works.
Table I. Effect of Interference Substances to the Output of the System ammonia N concn, content, interference mg.L-' mg.L-' substances 10.00
refa 10.00
0 noneb 9.72 100 urea 9.90 10 methyl alcohol 9.90 10 see-butyl alcohol 9.83 10 isobutyl alcohol 10 10.10 n-butylamine 9.88 10 see-butylamine 9.88 10 isobutylamine 9.82 100 tert-butylamine 9.90 10 hydroxylamine HCl 10.25 100 dimethylamine 9.75 100 diethylamine 9.90 100 triethylamine 9.96 100 triethanolamine 10.03 100 aniline 10.09 100 hydrazine hydrate 12.86 10.07 100 acetamide 10.15 100 benzamide A a The conventional method (indophenol method). standard solution containing ammonia N (10 mg.L-').
Figure 5 shows the effect of the flow rate of 6 N sodium hydroxide solution on the output of the system. The highest output was obtained at a flow rate of 0.16 mlsrnin-l. At lower flow rates, sufficient sensitivity was not attained because ammonia in the solution was not thoroughly vaporized. While at higher flow rate, dilution of the sample solution occurred and the output of the system based on gaseous ammonia decreased. The flow rate of 0.16 ml-min-l of the alkaline solution was used for further works. Figure 6 shows the effect of the flow rate of the sample solution on the output of the system. The output increased with increasing the flow rate of the sample solution, because ammonia concentration in the mixed solution (6 N sodium hydroxide and the standard solution) increased with increasing
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ANALYTICAL CHEMISTRY, VOL. 54, NO. 11, SEPTEMBER 1982 ~~
Table 11. Comparison of the Ammonia N Concentration Determined by the Conventional Method and by the Proposed Method ammonia N concn remarks sample conventional proposed re1 total N so lut iona method,b method (a), difference, concn (b), BOD: (a)/(b)X % mg.L-' 100, % PH mg,L-' (dilution) mg.L-' mg.L-'
COD,d mg.L-'
s-1 (1/100) s-1 (1/200)
1000 1000
1020 1010
2.0 1.0
460 460
s - 2 (1/20) s-3 (1/200)
107 1250
109
1.8
1230
1.6
1000 1000 120 2000
102
7.2 7.2
101
420 4 20
90.8
9.7
18
61.5
8.3
170
9.8 110
S-1, recycle water in an ammonia plant; 5-2, service water in a urea plant; S-3, cooling water in a urea plant. Indophenol method. Biological oxygen demand. Chemical oxygen demand.
a
6
I
L 0
0.8 1.2 1.6 Flow r a t d L . min-1)
0.4
Figure 6. Effect of flow rate of the sample solution on the output of the system. Conditions other than the sample flow rate were the same as Figure 3.
the flow rate of the sample solution when the flow rate of 6 N sodium hydroxide solution was constant. The output reached a plateau above 0.32 mlnmin-l, where the amount of ammonia vaporized by alkali became constant. Thus the flow rate was adjusted a t 0.32 mlamin-l throughout the following experiments. Figure 7 shows the effect of the flow rate of the air for separating the sample solution on the output of the system. The maximum output was obtained at 0.42 mlemin-l. However the effect of the air flow rate on the output was small for the determination of gaseous ammonia. The flow rate of the air was adjusted at 0.42 ml-min-' hereafter. In order to obtain the highest sensitivity of the system, we examined the effect of a temperature of the dialyzer on the output of the system. The output remarkably increased with increasing temperature. When the temperature of the dialyzer was higher than that of the SMCR the reproducibility was not so good. This was caused by the condensation of vapor water in the SMCR and the absorption of ammonia gas into the condensed water. Therefore, the dialyzer of the system was maintained at room temperature. The output was reproducible within 3.0% of the relative error when a standard solution containing 10.0 mg.L-l of ammonia was employed at room temperature. The standard deviation was 0.17 rng-l-' in 20 experiments. Calibration. A linear relationship was obtained between the output and concentration of ammonia below 20 mgL-l. The minimum detectable concentration was 1.0 rngL-l. Selectivity. The selectivity of the system for ammonia is shown in Table I. Alcohols, amines, and various amino compounds were applied to the system. However none of them interfered with the measurement of ammonia when applied at the same or ten times higher concentration of ammonium sulfate solution. Acetamide slightly interfered,
0
0.4
0.8
Flow rate
1.2
1.6
(mL.min-')
Flgure 7. Effect of flow rate on air on the output of the system. Conditions other than the air flow rate were the same as Figure 3.
when ammonia N concentration was 10.07 mg.L-l (determined by the conventional method). Application. The system was applied to the determination of ammonia in industrial waters of ammonia and urea plants in a petrochemical factory. The concentrations of ammonia and several other indicators of the sample solutions were determined by the indophenol method and the JIS (Japanese Industrial Standard) method ( 5 ) ,respectively. As shown in Table 11, a good agreement was obtained between the ammonia concentration determined by the conventional method and that obtained by the system proposed here. The relative difference between the two methods was less than 2%. Wastewaters in an ammonia plant (S-1) usually contain impurities such as methyl alcohol. Urea is contained in service water (S-2) and cooling water (S-3) in a urea plant. However these compounds did not affect the results as shown in Table I. pH, biological oxygen demand, and chemical oxygen demand also had no effect on the results of ammonia determination. Total N concentrations (b) were also measured by the Kjeldahl method (6). The total N concentration of the industrial waters in the urea plant is mainly due to the presence of ammonia and urea. The effect of total N was also negligible in the present system.
LITERATURE CITED (1) Albert, B. L.; Terry, F. D.; Mllton, A. J. Appl. Environ. Microbiol. 1979, 38, 212-215. (2) Karube, I.; Okada, T.; Suzuki, S.Anal. Chem. 1981, 5 3 , 1852-1854. (3) Uehara, H.; Ijuuin, Y.; Morlno, Y.; Kaidate, T.; Nakamura, A,; Imai. H. Rev. Sci. Instrum. 1980, 51, 334-337. (4) Weatherburn, M. W. Anal. Chem. 1987, 39, 971-974. (5) Japanese Industrial Standard Committee "Testing Methods for Industrial Waste Water", JIS K 0102, 1974, P36. (6) Steyermark, A. I . Anal. Chem. 1951, 2 3 , 523-528.
RECEIVED for review December 29, 1981. Accepted June 1, 1982.
