Determination of Nitrate in Water with an Ammonia Probe John Mertens, Pierre Van den Winkei, and D. L. Massart Laboratorium Analytische Chemie, Farmaceutisch lnstituut, Vroe Universiteit Brussel, Paardenstraat, 67, B- 1640 Sint Genesius Rode, Belgium
This article describes manual and automatic procedures for the determination of nitrates in water containing ammonia by means of an ammonia probe. The nitrates are determined by measuring the ammonia produced during a heterogeneous reduction by means of Devarda alloy powder. The yield of the nitrate reduction with the Devarda alloy was quantitative. The method shows good accuracy and reproducibility and can be applied in waters ranging from mineral water to sewage. Procedures for the elimination of excess ammonium and nitrite are proposed. Other reduction methods, which did not yield good results, are also discussed.
Most of the existing methods for nitrate determination are based on manual or automatic colorimetric procedures. Direct spectrophotometric determination in the UV region was used by several authors (1-7). Since the nitrate ion selective electrode became commercially available, several papers were published about its a p plications. The nitrate selective electrode has been used, not always with the best results, for the analysis of usually large concentrations ( > l o ppm) of nitrate in plants, soil extracts, food, a n d water (8-12). Mertens et al. (13) described the determination of nitrates in drinking waters of low nitrate content using a fluoride electrode as reference. However, the nitrate electrode has a rather low selectivity, so that many interferences were found and acceptable results were obtained only for waters with low mineral content. Colorimetric methods for the determination of nitrate are often based on reduction to nitrite or to ammonium. A summary of methods based on the reduction to ammonium can be found in the introduction of a n article by Lingane and Pecsok (14). Ammonia probes, recently introduced by Orion and E.I.L., permit the direct determination of ammonia or organic nitrogen after Kjeldahl digestion in a variety of samples (15-20). Since the ammonia probe is free from ionic interferences, its use for the determination of nitrates after reduction to ammonia seemed quite attractive. Studies on interferences ( 1 8 ) have shown t h a t t h e volatile amines a r e the only possibly interfering substances. A method for the nitrate determination with such a probe after reduction with aluminum dust was proposed by Orion Research (21 ). In the concentration range of interest for the analysis of most waters ( < 5 ppm), this method did not yield good results in our hands. This is not surprising because, according to Ross ( 2 2 ) , only 90% conversion is obtained for 6.2 p p m NOS- and, in addition, the manual recommends that t h e recovery should be studied in each type of sample measured. A number of other reducing agents were studied by us. It was found t h a t Devarda alloy permits t h e determination of nitrate concentrations as low as 0.1 p p m in samples ranging from mineral waters t o sewage. I n addition, the suitability of this method for automatic determination was investigated. EXPERIMENTAL Manual Procedure. Apparatus. The reduction is carried out in 100-ml polythene bottles. Small holes are bored into the outer 522
screwcap and the inner stopper and between both a piece of hydrophobic paper (Whatman PS) is fitted. The latter contains a few perforations. Hydrogen gas can escape and no solution will be lost if the holes in the paper and in the stopper are not directly in contact with each other. The measurements are carried out in a thermostated (25 f 0.1 "C) polythene vessel with an ammonia probe (Orion 95-10), mounted at a 45' angle to prevent hydrogen bubble entrapment at the membrane. The solution is stirred moderately by means of a Teflon coated magnetic stirring bar. The potential is measured with an Orion 801 digital pH meter, the analog output of which is connected to a Kipp en Zonen BD 9 recorder via a home-made base-line adjustment unit. The upper part of the electrode body and the top cap are wrapped up in aluminium paper, which is grounded to avoid static electricity effects. The elimination of the NH4+ background is carried out in Pyrex tubes provided with a 75-ml mark, heated in a Technicon heating block for discrete sample destruction. Reagents. All the reagents were reagent grade. Stock solutions of 1000 ppm NH4+ and NOa- were prepared from NH4C1 and KN03. Both were stored in polyethylene bottles in a cool dark place after addition of a few drops of CHC13 as a preservative. Dilutions were made with ammonium free water made by bidistillation of deionized water in a silica apparatus. New stock solutions were prepared every three weeks. The following reductors were used: Devarda alloy (p.a. U.C.B.), nickel aluminium alloy 50/50 (catalytic BDH), aluminum powder (p.a. Merck), defatted prior to use by washing with diethyl ether and ethyl alcohol. Raney nickel was prepared as described in reference ( 7 ) with the nickel-aluminum alloy. Recommended Procedure. a ) No NH4+ or NOz- Present. Add 5 ml of 1 M NaOH and 1 g Devarda alloy t o a 50-ml sample in the reduction bottle. Close the bottle and shake it for 30 minutes. Transfer the contents of the bottle to the measuring cell. Insert the probe, stir the solution moderately, and read the potential after 3 to 8 minutes. Take care that no bubbles become trapped at the membrane. Prepare a calibration line by treating Nos- standards of appropriate concentration in the same manner. 6 ) No NO*- and an Excess of NH4+. If less than a 2:1 excess of ammonia is present, the ammonia concentration is measured before and after reduction. The difference is used to calculate the NOS- content. If more than a 2:l excess is present, add 5 ml of 1 M NaOH to a 50-ml sample in a graduated Pyrex tube. Transfer the tubes to the heating block and heat at 80-90 "C. Nitrogen, which has been purified by passing successively through 1 M NaOH and 1M HzS04 is bubbled through the solution for at least one hour. Cool the tubes and make the solution up to the 75-ml mark with 0.1 M NaOH. Proceed as described under a). c ) NOz- und NH4+ Present. Add 1 g of sulfamic acid to a 50ml sample in a Pyrex tube. Wait 3 minutes and proceed as described under b). Automatic Procedure. The flow diagram is given in Figure 1. The AutoAnalyzer modules consisted of a Sampler I1 and a proportioning Pump I. The sampling speed amounts to 2O/hour with a sample to wash ratio %. The air (channel 2) is bubbled through a 1M HzS04 solution to prevent ammonia uptake. The samples are protected by covering the sample dish. The debubbler is mounted on the end block just before resampling and all connections are made with thin Teflon tubes to minimize carry-over. NaOH is added before the reduction column and the degassing system. The solution is then pumped to the electrode cell by means of a Minipulspump (Gilson) with adjustable speed. Instead of a conventional flow-through cell an open flow system is used. The latter, represented in Figure 2, eliminates pulse oscillations
ANALYTICAL CHEMISTRY, VOL. 47, NO. 3, MARCH 1975
Flgure 1. Flow diagram of the automatic system B.A.U.. home made baseline adjustment unit: P, Minipulspump with adjustable
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20
10
.
T~*Ci*INl
Figure 3. Potential observed with the ammonia probe during the reduction of 20 ppm NOs- as a function of time From t = 0 to A: aluminum dust. At point A, Devarda alloy was added
When trying to use this method, difficulties were encountered with the aluminum powder which adhered to the membrane and caused potential fluctuations. To avoid this, the unreacted powder was filtered away before the measurement. Reduction yields of less than 1%were then obtained. This is due to the fact that the reduction of nitrate with aluminum powder does not take place in acid mediLm, as the procedure proposed by Orion leads one to suppose, but in alkaline medium. Indeed, one arrives a t this conclusion because much better results are obtained if the a1c;minum powder is removed by filtration after alkalinization of the sample. Even then, however, we were not able to attain yields higher than 90% and, in any case, the method proposed was not sensitive enough for the analysis of nitrates in most waters. The use of two other metallic reductors, Raney nickel and Devarda alloy was therefore investigated. With Raney nickel, good results were achieved in weakly acid medium: yields of 100%were obtained with 2 ppm NO3- after reduction during 30 minutes a t 50 "C. However, the reproducibility was not very high and the time needed for reduction was judged too long. This was not the case when Devarda alloy was used. The superiority of Devarda alloy compared to aluminum dust as a reductant in alkaline medium is illustrated best with the following experiment (see Figure 3). Twenty ppm Nos- is subjected to reduction, first with aluminum dust and the potential of the ammonia probe is recorded continuously. One observes a slowly rising potential. On addition of Devarda alloy to this reaction mixture, a potential jump is obtained and the potential stabilizes rapidly (after 5 minutes). This shows clearly that the kinetics of the reduction with Devarda alloy are much more rapid than with aluminum dust. These observations caused us to investigate systematically the determination of nitrate in water with the ammonia probe using Devarda alloy as the reductant. It should be mentioned here, that to avoid incomplete reduction of low concentrations, the reduction time was 30 minutes in the manual procedure. In the automatic procedure, it can be estimated at 1minute. Choice of pH. As shown in Figure 4,the reaction is faster a t pH 13 than at p H 11. This may be due to the fact that at the higher pH, all the ionic constituents originating from the reductor which is composed of Cu, Zn, and Al, are present as soluble oxygen-containing anions, while at pH 11 they are partly present as a precipitate of the hydroxides,
\F= 1 f r o m minipuis
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a
t
b
Figure 2. (a) The reduction column with the degassing system. (b) The open flow-system I = the metallic inlet tube, C = metallic screw-clamp
caused by the proportioning pump and minimizes cell hold-up. The reduction column (Figure 2) (4.5-mm i.d. X 80 mm) is packed with Devarda alloy beads of 0.18 to 0.25-mm diameter. The beads are prepared from a mixture of polystyrene or poly(viny1chloride) and Devarda alloy. The polymers are dissolved in chloroform in the case of polystyrene and in tetrahydrofuran in the case of poly(vinylch1oride) and mixed with the Devarda alloy. After evaporation of the solvent, the mixture is ground in a water cooled mixer and the resulting powder is sieved to obtain beads of appropriate dimensions. The degassing system consists of a syringe mounted on the reduction column (Figure 2). The volume in this syringe is adjusted to 0.1-0.2 ml by means of the Minipulspump. By placing a plastic netting a t the syringe inlet, finely divided gas bubbles are obtained. This avoids gas-uptake a t the needle inlet. In freshly prepared columns, abundant hydrogen gas evolution takes place so that frequent adjustment of the pump speed is necessary. After a few hours, only sporadic adjustments are required.
RESULTS AND DISCUSSION Manual Method. Orion (21) proposes a method in which aluminum dust is added together with HC1 and NaF to the sample. After 5 to 7 minutes, NaOH is added to achieve a pH >11 and the ammonia concentration is measured.
ANALYTICAL CHEMISTRY, VOL. 47, NO. 3, MARCH 1975
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I
Table I. Yield of Reduction as a Function of the Quantity of Devarda Alloy ( C N O ~=- 500 ppm)
m4
Amount of Devarda 9/50 m l nitTate soludon
Yield of reduction, A
0.1 0.2 0.4 0.6 0.8 1 2
35) 0 66, 0 89, 0 98, 0 99,7 100 100
~~
Table 11. Elimination of NH4 + Background Concn after bubbling through of NZ gas Sample
Synthetic Sewage (from a slaughterhouse)
Figure 4.
influence of pH on the reduction rate
Curve 1: pH 13; Curve 2: pH 11
covering the surface of the reductor and thereby passivating it. Quantity of Devarda Alloy. In Table I, the % reduction obtained for a solution containing 500 ppm NO3- as a function of the quantity of Devarda alloy added, is given. One observes that 1gram of Devarda alloy is amply sufficient for the reduction of 500 ppm NO3-. Choice of t h e Reaction Cell. Because the reduction is carried out at pH >11 all the ammonium formed is present as ammonia. It is possible that part of the ammonia gas escapes before the measurement of the potential. This possibility was investigated and it was found that such losses do indeed occur for open stirred solutions. For an initial concentration of 5 ppm, losses of 6.0, 9.1, 15.5, and 22.8% were found after, respectively, 10,20,30, and 40 minutes. However such losses do not occur in the reaction cell described in the Experimental Section. The use of a completely closed recipient would offer, of course, still more guarantees that no NHs is lost. However, during the reduction H2 gas is formed and a pressure is built up. This has an unfavorable effect on the reduction yield. In the 0.5- to 4ppm range, in which the half closed reaction cell described in the Experimental Section allows reduction yields of loo%, only about 90% reduction is obtained using a completely closed vessel. Elimination of NH4+ Background. In natural waters, NOs- must usually be measured in the presence of NH4+. When the differences in concentrations are not too large, one can measure the NH4+ content before and after the reduction step. The difference is then equal to the NO3- concentration. When NO3- has to be measured in the presence of a large excess of NH4+ (in raw sewage ratios of NO3-: NH4+ < 1:lOO are not uncommon), this is no longer possible. It seemed to us that it would be possible to overcome this difficulty by eliminating the NH4+ present before the reduction step as NH3 gas. To do this, the sample is made alkaline and nitrogen gas is bubbled through the solution (see exact procedure under Experimental). In Table 11, the residual quantities of NH4+ found after treating synthetic 524
Original concn, PPm
1 10 100 940 920 945
30 min
60 min
0.15 0.54
0.08 0.16 0.31 0.03 0.04 0.03
...
... ...
...
NH4+ samples and sewage in this way are summarized. One observes that the NH4+ is completely eliminated after 1 hour. The same experiment was carried out on NO3- solutions. It was found as could be expected, that no NO3- is lost since for original concentrations of 1, 2, 3, and 5 ppm, concentrations of, respectively, 1.05, 2.1, 3.0, and 4.95 ppm were recovered after 1 hour of treatment. Elimination of Nitrites. Nitrites are also reduced to NH4+ by Devarda alloy. In many instances, this does not matter because it is the custom to report the sum of nitrite and nitrate concentrations as the result. When one does need the NO3- concentrations without NOz-, the latter must also be removed. The elimination of NOz- by the addition of sulfamic acid is described in the literature ( 2 3 ) . A difficulty which arises is that, on alkalinization, sulfamic acid is partly converted into NH4+ (1 g causes a background concentration of 4 ppm NH4+). This can be avoided by the procedure discussed earlier. In fact, by adding sulfamic acid, alkalinization and bubbling through N2 gas, a procedure is obtained for the combined elimination of NOz- and NH4+ (present in the original sample or originating from the sulfamic acid). As an example, results obtained on a sewage sample from a slaughterhouse can be cited. The sample contained 940 ppm NH4+, 12 ppm NOz-, and 309 ppm NO3- (determined by colorimetry). After carrying out successively the elimination and the reduction procedures, concentrations of 0.35 ppm NH4+,
