Anal. Chem. 1002, 64, 1313-1315
1919
TECHNICAL NOTES
Determination of Nitrite and Nitrate by Differential Pulse Polarography with Simultaneous Nitrogen Purging Walter Holak' and John J. Specchio Food and Drug Administration, New York Regional Laboratory, Brooklyn, New York 11232
INTRODUCTION Nitrites and nitrates have historically been determined by conventional dc polarography in supporting electrolytes containing polyvalent cations such as lanthanum or uranium. In these media the nitrite and nitrate waves appear at sufficiently positive potentials suitable for practical applications. Anions, such aa sulfate and phosphate, however, interfere by precipitating the cations. In addition, the determination is complex when both species are present in solution. More recently, nitrite has been determined by differential pulse polarography (DDP) as nitrous acid in a citrate buffer' and after nitrosating diphenylamine.* Nitrate did not react under these conditions. This paper will describe a method whereby both nitrite and nitrate can be determined sequentially by DPP. This has been achieved by selective reduction of each species to nitric oxide followed by purging with nitrogen and nitrosating diphenylamine. A technique has been developed which allows the signal to be measured a t a voltage set to peak potential.
r i
(1) Application Note 156. EG & Princeton Applied Research, Princeton, NJ, 1979. (2) Chang, S.;Kozeniauskes, R.; Harrington, G. W. Anal. Chem. 1977, 49, 2272-2275. This artlcle not subject to U.S. Copyright.
D H
Flgure 1. Dlagram of lnstrumentatlon: (A) nitrogsn purge gas; (8) alkallne pyrogallol trap; (C) flow meter; (D) sample purge tube; (E) polarographlc cell; (F) dropping-mercury electrode capillary wlth Teflon sleeve; (0)polarographlc analyzer; (H) strip chart recorder.
EXPERIMENTAL SECTION Apparatus. Polarographicmeasurementswere obtained with an EG&G Princeton Applied ResearchModel 264A polarographic analyzer with a Model 303 static mercury-drop electrode, platinum-wirecounter electrode,and Ag-AgC1 referenceelectrode and a Model RE0089 X-Y recorder. A strip chart recorder was connectedfor monitoringthe signalat constant-voltageoperation. The polarographic conditions were as follows: initial potential, -0.50 V; final potential, -0.85 V; modulation amplitude, 50 mV; scan rate, 5 mV/s; drop time, 1 s; mode, differential pulse; range 10 pA. Constant-voltage operation: same as above except that the scan rate was set to 0 and the voltage was manually set to peak potential (-0.66 V vs Ag/AgCl) with NO2 standard in deaerated electrolyte-solution. A capillary sleeve was made by cutting a 47-mm-diameter Millipore LS Mitex (PTFE) type, 5pm filter into a 25- X 37-mm rectangle. It was rolled lengthwise onto a used capillary and fused into a cylinder by momentarily pressing on hot plate set at mediumheat. The sleevethus formed could be moved along the capillary and set at any position. In use, the sleeve was set extending below the capillary tip by about 2 mm. The purge-trap apparatus was connected to a polarographic cell with l/l,&-i.d. Teflon tubing through the hole of electrode base, adjacent to the capillary. The tubing was positioned so that it did not interfere with the operation of the capillary. Purging nitrogen was bubbled through an alkaline pyrogallol solution to remove traces of oxygen. The apparatus was assembled as shown in Figure 1.
* Corresponding author.
W
50 100 150 200 UL NABR SOLUTIOH (85%) ADDED
Figure 2. Reductlon of nitrite wlth NaBr. The solutlon contains 10 pg of NOn In 10 mL of 4.5 M H2S0,.
Reagents. All chemicalswere reagent grade. Standard nitrite and nitrate solutions were prepared from NaNOz and NaNOs, respectively, to contain lo00 pg/mL anion. Working Solutions of the desired NO2 and NO3 concentrations were prepared from the stock solutionsdaily. The supporting electrolybtrapsolution contained diphenylamine, perchloric acid, and NaSCN prepared aa described.2 Procedure. To the sample tube were added 5 mL of 9 M H2S04 and 150 pL of 85% NaBr solution, while to the polarographic cell was added 10.0 mL of electrolyte-trapping solution. The instrument was set to "run" mode while the nitrogen flow
Publldwd 1992 by the AmericBn Chemical Soclety
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ANALYTICAL CHEMISTRY, VOL. 64, NO. 11, JUNE 1, 1992
Table I. Analysis of Sampler and Spikes for Nitrite, p g / g found sample ref 2 method proposed method smoked chubs (Leuciecuscephalus) 0 0
American cheese imported Swiss cheese bacona
0
added
0 0.3 31.gb
0.3 30.2
total found 102.0
100.0 200.0 40.0 70.0
102 98 87 94 96
196.0 34.7
66.0 41.6 2.7 90.4
50.0 3.0 60.0
Imported Swiss cheese and bacon samples were also analyzed for nitrate by the pro& of four determinations, std dev i 2 . 1 .
76 recovered
80 98
method. Found 97.7 and 60.0 cg/g, respectively.
b Average
e -
%
-t i
E-3
1 2 6 a
T
N -
4
10
mL H,SO. ADDED
--
Ftgm 3. Reduction of nitrate with Nak after addltbn of H2SO4. The solutlon contains 10 pg of NO3 In 10 mL of 4.5 M H2S04 150 pL of Naar (85%). was set at 0.5 L/min, scan rate at 0, and purge time at 2 min.
+
Voltage was previously manually set at peak potential. At the end of the purge cycle, the instrument waa advanced to “scan” mode. At this time, 5 mL of HzO containing the required quantities of standard, usually 5-10 pg of NO2 and NOS, was added to the sample tube. Within a short time, NO signal generated from NO2 appeared and reached a maximum value in 5-6 min. Then, 6 mL of concentrated HsSO4 was added to the sample tube. The NO signal generated from NO3 appeared as above, and when the maximum value was reached, the run was terminated. Aqueous samples prepared as below were run similarly. Food samples (10 g) were blended with 70 mL of water and 12 mL of 2 7’% NaOH solution. The slurry was transferred to a 200-mL volumetric flask and heated on a steam bath for 60 min. Then, 10 mL of 0.42 M ZnSO4 solution was added, and the mixture was mixed and heated for additional 10 min. The contentswere cooled to room temperature and diluted to volume with water. The precipitate was allowed to settle, and an aliquot was removed, mixed with 200 mg of charcoal, and filtered. An aliquot of this solution (5 mL), or further diluted if necessary, was used for the determination of nitrite and nitrate.
A 3
2
I
4
5
6
TIME, MINUTES
I3
I
RESULTS AND DISCUSSION The measurement technique is unique in that unlike conventionalpolarography,the solution is being purged during the analysis. This is facilitated by the Teflon sleeve placed onto the dropping-mercury capillary which shields the mercury drops from the turbulence produced by the purging gas. The technique allows the monitoring of the signal as the analyta is being delivered to the cell, resulting in an efficient analysis. After all the nitric oxide has been purged from the sample, the voltage may be scanned in the usual manner to obtain a differential pulse polarogram. This may be useful in providing additional specificity or to check for interferences from other evolving species. T h e precision a t 10-pg levels for nitrite and nitrate in terms of relative standard deviation was 2.8 and 4.9 % , respectively. Nitrite is readily reduced by sodium bromide at relatively low acidity. The optimum amount of sodium bromide was
1
-0.5
I
-0.6
POTENTIAL
-0.7
-0.8
( E vs. A g f A g C l )
Flguro 4. (a, Top) typical signal profiks of solutions containing (A) nlbtte aqd (B) nttrtte and nitrate. The arrow indicates the pokrt at whlch concentrated H2SO4 was added. (b, Bottom) dmerentlal pul8e1 POlarograma obtained after g”ing dlphenylnitn”lne (A) from NOn and (e) from NO2 NO3.
+
determined by experiment, as shown in Figure 2. The reduction of nitrate required the addition of concentrated sulfuric acid which also results in a generation of heat. The optimum amount of sulfuric acid was found by experiment, as illustrated in Figure 3. Other reductante for nitrite and
ANALYTICAL CHEMISTRY, VOL. 84, NO. 11, JUNE 1, lQQ2 1815
nitrate reported in the literature3 were tried but were unsuitable for the present technique;no signal was obtained. Recently vanadium(II1) was reported to reduce NO2 and NO3 in less acid solutions.4 Figure 4a shows signal profiles for nitrile (A) and a mixture of nitrite and nitrate (B)obtained as described in the procedure. Concentrated sulfuric acid was added to solution A containing only nitrite to demonstrate that no additional signal is produced after the nitrite had reached ita maximum signal in dilute sulfuric acid. In Figure 4b are shown the corresponding differential pulse polarograms; no change in the peak potential vs time was observed. The electrode reaction, which accounts for the measured current& results from the electroreduction of diphenylnitrosamineat a dropping-mercury electrode. In acidic solutions, nitrosamines undergo an irreversible four-electrode reduction to the cor(3) Cox, R.D.A d . Chem. 1980,62, 332-335. (4)Braman, R. S.; Hendrix, S. A. A n d . Chem. 1989,61, 2715-2718.
responding asymmetrical hydrazine.6 I t should be pointed out that although many N-nitrosamines are potent carcinogens, diphenylnitrosamine, generated in the present procedure, is not.2 The method described above was applied to the analysis of several samples of food, and the resulta are presented in Table I.
RECEIVED for review October 8, 1991. Accepted February 21, 1992. Registry No. Nitrate, 14797-55-8;nitrite, 14797-66-0. ~~
(5)Hwebe, K.;Osteryoung, J. A n d . Chem. 1976,47,2142.
