Anal. Chem. 1980, 52, 143-145
143
Determination of the Ammonium Ion by Evolution of Ammonia and Ultraviolet Absorption Spectrometry in the Gas Phase Christine C. Muroski and Augusta Syty" Department of Chemistry, Indiana University of Pennsylvania, Indiana, Pennsylvania 15705
Ammonium ion in aqueous solution is determined by injection of 1-mL aliquots into strong base and measuring the transient absorbance signal of the evolved ammonia in the gas phase a1 194 nm. A glass reaction vessel and an atomic absorption spectrophotometer equipped with a flow-through absorption cell are used. The method is rapid, selective, and offers a detection limit of about 1 pg/mL NH4+. Reproducibility is 4 Yo relative standard deviation. The method is applied to the determination of total N in wheal flour. Concentrations of NH4+ found in Kjeldahl digests of flour by the proposed method agree very well with those obtained by the conventional method. The detection limit corresponds to 0.03 YO N in flour, based on 0.63 samples.
T h e ammonium ion has been determined by a variety of methods widely described in the literature. As far as users of atomic absorption and flame emission instrumentation are concerned, there have been reported several possible methods based on the spectra of CN, NH,NHz, and NO species excited in some flames and several indirect methods for a few Ncontaining species ( I ) . T h e present paper shows t h a t the ammonium ion in aqueous solution can be determined directly, rapidly, and precisely by evolution and measurement of ammonia, using an atomic absorption spectrophotometer which, after simple modification, is made to function as a nonflame molecular absorption spectrophotometer. The reaction vessel used has recently been described by one of the authors in a report on the determination of sulfide by evolution and measurement of HzS (2). Similar apparatus and approach have been used by one of the authors and co-workers in the determination of sulfite evolved as SO2 (3, 4 ) and of iodide and bromide evolved as Iz and Brz (5). A different apparatus and a more time-consuming procedure for the evolution and measurement of ammonia have been described by others (6). T h e proposed method for the determination of the ammonium ion is applied to the evaluation of the Kjeldahl digests of wheat flour. T h e Kjeldahl method, with its many modifications, is one of the most widely used analyses (7) and is the standard method for determining protein N in grains, meats, and other biological materials. Several evaluations of the Kjeldahl digests by means other than distillation and titration of ammonia have been reported, including the use of the ammonia-selective electrode (8), colorimetry (9),and coulometry ( I O ) . T h e use of the proposed method for NH4+ permits a particularly rapid evaluation of the digest, which is also direct, accurate, and free from interferences.
EXPERIMENTAL Apparatus. Absorption of UV radiation by evolved ammonia gas was measured using the Perkin-Elmer model 460 atomic absorption spectrophotometer, with readout presented on a 10-mV strip-chart recorder. The modification of the instrument consisted of removing the burner head from the nebulizer/burner and replacing it with a 15-cm long, quartz-windowed flow-through glass absorption cell. The deuterium arc lamp, whose normal function in the spectrophotometer is to provide background correction, 0003-2700/80/0352-0143$01 .OO/O
served as the source of exciting radiation. Absorbance measurements were made a t 194 nm with a slit setting of 2 nm. The simple and convenient reaction vessel used for the evolution of ammonia is illustrated in Figure 1of reference 2. The cylindrical glass vessel has a volume of about 60 mL. Measured aliquots of NaOH solution enter the vessel from a buret via a sidearm. Ammonium-containing sample solutions are introduced into the vessel through the rubber septum covering the injection port by means of a 1-mL Hamilton syringe. Removal ol'the spent reagent is made quick and convenient by the stopcock a t the bottom of the reaction vessel. Nitrogen, which serves as carrier gas, enters the reaction vessel through a plain glass tip submerged in the NaOH and sweeps the evolved ammonia to the absorption cell. The nitrogen pressure is set at 10 psi and the gas is never turned off while the apparatus is in use, allowing continuous flushing of the reaction vessel and of the absorption cell. Procedure. A 6-mL aliquot of 10 N NaOH is delivered into the reaction vessel from the buret. With the carrier gas flowing at a rate of 1.5 L/min, a base line is established on the recorder. A 1.00-mL aliquot of sample solution is injected into the vessel and absorbance due to the evolved ammonia begins to be indicated by the recorder in a few seconds. The absorbance signal reaches a maximum in about 25 s. If the reagent mixture were left in the vessel, all of the ammonia would be flushed out of the train and the base line reestablished after about 115 additional seconds. Instead, immediately upon recording the signal maximum, the recorder is placed on standby, the reaction vessel is drained via the stopcock at the bottom and immediately refilled with a fresh aliquot of base. These operations take less than a minute and, when the recorder is turned on again, the base line is quickly reestablished and the apparatus is ready for the next sample injection. Signals are measured from the base line to the top of the recorded peak. Rinsing of the reaction vessel between runs is not necessary. The nitrogen flow is never interrupted. Obviously, by draining the reaction vessel as soon as the signal maximum is recorded, significant amounts of ammonia are discarded without evolution and passage through the absorption cell but this is of no consequence because, as the data below indicate, the recorded peak heights are reproducible arid proportional to the quantity of the ammonium ion injected over a substantial concentration range. Furthermore, simply measuring the peak heights is much more convenient and less time-consuming than measuring areas under the gradually tailing peaks. Since no drying agent is included in the gas train, a constant concentration of water vapor is continuously passing through the absorption cell. When first putting the apparatus into operation on a given day, it is necessary to condition it by letting the carrier gas bubble through the NaOH solution for about a minute before making injections in order to allow a flat base line to be established on the recorder. The first few injections are disregarded since they consistently yield low signals. For instance, on a given day the initial four injections of a 1000 pg/mL NH4+solution yielded absorbances increasing from 0.543 to 0.833, with subsequent injections leveling off around the latter value. The reason for the need to condition the apparatus in this manner prior to colleciing useful data is unclear. According to the manufacturer's literature, the D2arc lamp requires only 3-5 min for warmup. Kjeldahl digests were prepared by accurately weighing approximately 0.6-g samples of flour on filter paper, folding the paper up, and transferring it into the Kjeldahl flasks. Approximately 10 g of K2S04,several crystals of CuSO,, 4 glass beads, and 25 mL of concentrated H2S04were added. The solutions were heated at a gentle boil for 2-3 h until digestion was complete. A blank C 1979 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 52, NO. 1, JANUARY 1980
Table I. Effect of NaOH Concentration average time from concn peak height, injection to peak NaOH, N absorbance maximum, s 10 0.400 24 8 0.355 34 6 0.294 39 4 0.211 51 2 0.144 66
W A V E L E N G T H , nm
Figure 1. Absorption spectrum of evolved ammonia
was also prepared. After cooling, the digests were quantitatively diluted to 250.0 mL with deionized water. Aliquots of the diluted digests were removed for analysis. For evaluation by standard addition, 25.00-mL aliquots of the digests were mixed with 5.00 mL of 1000 pg/mL NH4+ and diluted to 50.00 mL. For the conventional Kjeldahl evaluation, 50.00-mL aliquots of the digests were treated with 25 mL of 10 N NaOH, distilled into 30.00 mL of 0.1456 N HCl and backtitrated potentiometrically with 0.1174 N NaOH. Standard solutions were prepared by weighing certified grade NH4C1and all dilutions were made with deionized water. NaOH, 10 N, was prepared by dissolving pellets.
RESULTS AND DISCUSSION Wavelength. T o determine the wavelength of maximum absorbance of the evolved ammonia, repeated injections of 339 pg/mL NH4+ solution were made while the wavelength was varied from 185 to 400 nm. The absorption spectrum shown in Figure 1 was confirmed by trapping some of the evolved gas and testing it on the Cary 14. All of the data for this report were collected a t 194 nm with a slit of 2 nm. Effects of NaOH Volume and Concentration. The volume of 10 N NaOH introduced into the reaction vessel was varied from 1.00 to 11.00 mL and repeated injections of 339 pg/mL NH4+solution into fresh aliquots of base were made. The optimum volume of reagent is expected to depend mainly on the shape of the reaction vessel and on the proximity of the tip of the carrier gas inlet tube to the bottom of the vessel, which affect efficient mixing of reagents and rapid evolution of gaseous ammonia. Signal intensity varied only slightly from 5 to 8 mL. At volumes smaller than 5 mL, the tip of the carrier gas inlet tube was not submerged in the NaOH and only small broad absorbance peaks were recorded. At volumes greater than 9 mL, excessive splashing took place in the reaction vessel, increasing the possibility of NaOH solution droplets reaching the absorption cell and often giving nonreproducible absorption signals with several maxima. Most of the data in this report were collected using 6.00-mL aliquots of base. By varying the concentration of NaOH from 2 to 10 N, while making repeated injections of 339 pg/mL NH4" into fresh aliquots of base, the data given in Table I were collected. Although signal reproducibility was approximately three times better when the 4 and 6 N NaOH concentrations were used, the 10 N NaOH was selected because it afforded the greatest speed (24 s between injection and peak maximum) and good sensitivity. The advantage of being able to drain the reagents from the reaction vessel through the bottom stopcock immediately after recording the signal maximum is clearly evident when one compares the 24-s interval required for recording just the signal maximum with the 2-3 min required for the residual ammonia to become completely flushed out of the train and for the entire absorbance signal to be recorded if the reaction vessel is not drained. I t is useful to note that not only the optimum volume of the NaOH aliquot but also the speed with which the recorded
absorbance rises to a maximum following injection of ",+-containing sample is dependent upon the shape of the reaction vessel and the proximity of the tip of the carrier gas inlet tube to the bottom of the vessel. Thus, when, during repairs, the bottom portion of the vessel was constricted from a diameter of 2.7 cm to that of 1.7 cm, with the tip of the N2 inlet tube 1 cm from the bottom, the recorded absorbance rose to a maximum only 13 instead of 24 s after injection. Effect of Nitrogen Flow Rate. Variation of the carrier gas flow rate from 0.4 to 2.0 L/min had little effect on the intensity of the recorded absorbance signal, but it did affect the shape of the recorded signals and the overall speed of analysis. Flow rates below 0.8 L/min are undesirable because precision tends to decrease as peak maxima become very broad and less well defined. Also, the analysis time is extended because the maximum absorbance is reached slowly and an additional waiting interval after refilling the reaction vessel with fresh NaOH becomes necessary before the residual NH3 becomes flushed out of the train and the base line is reestablished. On the other hand, nitrogen flow rates of about 1.9 L/min and above give rise to very sharp signals and rapid return to the base line but excessive bubbling in the reaction vessel increases the possibility of solution droplets reaching the absorption cell. Most of the data were collected a t a convenient intermediate flow rate of 1.5 L/min. When working with NH4+concentrations approaching the detection limit, however, it is best to work with higher flow rates in order to assure well-defined easily measurable peaks. Calibration Curve, Reproducibility, and Detection Limit. The relationship between absorbance of the evolved ammonia and the concentration of the injected ammonium ion is linear to approximately 400 pg/mL NH4+ but the calibration curve remains usable up to a t least 1000 pg/mL
NH4+. Reproducibility was demonstrated by making 15 repeated injections at several concentrations of the ammonium ion. The standard deviation was 3.3% with 336 pg/mL NH4+,3.8% with 101 pg/mL NH4+,and 3.9% with 24.0 pg/mL NH4+. At the 24.0 pg/mL NH4+level, the average absorbance was 0.0269 and the average deviation from the mean signal for a series of 15 repeated injections was 0.0008. Defining the detection limit as that concentration which yields a signal twice the size of the average deviation from the mean yielded a detection limit of about 1 pg/mL NH4+. Interferences. Ten concomitant anions were tested for interference with the evolution of ammonia or the measurement of its absorbance a t 194 nm. A variety of cations was not tested because of the limited solubility of metal hydroxides. Solutions containing 3000 pg/mL of the potential interferents (as Na or K salts) and solutions containing 3000 pg/mL of the potential interferents plus 100 pg/mL NH4+ (as chloride) were prepared in distilled water and tested under constant conditions. No absorbance was detected for the or C03'solutions of SO$-, I-, Br-, NOz-, NOB-, SCN-, alone, and the absorbance of 100 pg/mL NH4+was unaffected by the presence of these concomitants. A S*--containing solution was prepared by dissolving (NH&S to yield a solution 100 pg/mL in NH4+ and consequently 88.9 pg/mL in S2-.The observed absorbance was
ANALYTICAL CHEMISTRY, VOL. 52, NO. 1, JANUARY 1980
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Table 11. Determination of Total Nitrogen in Flour
peak ht. before wt. of flour, g
0.5371 0.5270 0.6447 0.7466 0.5206 0.5347 0.5839
peak ht. after
spiking,
spiking,
absorbance 0.077 0.069 0.087 0.104 0.070 0.076 0.079
absorbance 0.160 0.161 0.164 0.175 0.158 0.161 0.160
unaffected by the presence of sulfide. In the absence of an alternate source of soluble sulfide, a higher concentration of sulfide alone was not tested. Solutions of CN-, freshly prepared from five different lots of certified grade NaCN and KCN, gave no absorbance when tested alone and did not affect the absorbance caused by 100 kg/mL NH4+. However it was noted t h a t upon standing tightly stoppered, either in glass or plastic containers, injection of cyanide solutions into 10 N NaOH gave rise to an absorbance signal the magnitude of which reflected the age of the solution. For example, a 3000 pg/mL CN- solution yielded an absorbance of 0.000 on the day of preparation, 0.003 after 2 days, 0.010 after 5 days, 0.020 after 12 days, and 0.024 after 16 days. In solutions containing 100 pg/mL NH4+plus 3000 kg/mL CN-, the absorbance signal increased in the same manner and remained additive. T h e nature of the CN- reaction is being studied. Determination of N in Kjeldahl Digests of Flour. Seven samples of flour were prepared for analysis by Kjeldahl digestion as described under Procedure. The digests were diluted to 250.0 mL and portions thereof were evaluated by the proposed and by the conventional distillation methods. The results are given in Table 11. The detection limit of 1 kg/mL NH4+for the proposed method corresponds to 0.03% N in flour based on 0.6-g samples. Consequently, there is no difference between the 2.18% N average obtained by the proposed method using standard addition and the 2.20% N obtained by the conventional method. The use of the calibration curve however resulted in an average % N value of 2.26 which represents a relative error of 3%. Whereas the blank consisting of filter paper, K2S04, catalyst, and H 2 S 0 4 gave a barely measurable signal, it was noted that the presence of the H2S04-K2S04matrix significantly enhanced the absorbance signal to ammonia evolved from NH4+ standards. For example, the absorbance of a 50.0 pg/mL NH4+standard was enhanced 3%, that of the 400 pg/mL NH4+was enhanced 7%, and that of the 1000 pg/mL NH4+ was enhanced 12 %. Consequently, in preparing the calibration curve, all standards
calibration curve, proposed method 2.36 2.15 2.22 2.29 2.21 2.34 2.23 av. = 2.26
N , 7% standard addition, proposed
method 2.28 2.02 2.19 2.20 2.13 2.25 2.19 __ av. = 2.18
disLiilation/ titration 21. 24
2.21 2.18 2.18
21.18 __ av. = 2.20
were made to contain the same quantities of H 2 S 0 4and of K2S04as the flour digests. The working range corresponding to the linear part of the calibration curve extends to 13% N in flour. The authors prefer evaluation by standard addition to t h a t by use of the calibration curve. Not counting the minutes required initially to warm up the spectrophotometer, set the carrier gas flow rate, and condition the apparatus by making several practice injections, an operator can make 30 runs per hour, which represents 10 samples if triplicate injections are made of all solutions. Inasmuch as it takes only about 24 s after injecting the sample for the signal maximum to be attained, the remainder of the 2-min interval required per injection is spent in draining the reaction vessel, refilling it with a fresh aliquot of base, and rinsing and filling the syringe. T h e results of evaluation by the proposed method of evolving ammonia and measuring UV absorbance in the gas phase agree well with those obtained by the conventional evaluation of the Kjeldahl digest. T h e proposed method is fast and quite selective for the ammonium ion.
LITERATURE CITED (1) Syty, A. in "Flame Emission and Atomic Absorption Spectrometry", Dean, J. A., Rains, T. C. Eds., Marcel Dekker: New York, 1975; Chapter 14. (2) Syty, A. Anal. Chem. 1979, 51, 911. (3) Syty, A. Anal. Chem. 1973, 45, 1744. (4) Winkler, H. E.; Syty, A. Environ. Sci. Techno/. 1976, IO, 913. ( 5 ) Nicholson. . ... , G.: . Svtv. . Chem. 1976. 48. 1481. .,~,. A. Anal. Cresser, M. S. Anal. Chim. Acta 1976, 85, 253. (7) Ogg, C. L. I n "Treatise on Analytical Chemistry", Kolthoff. I. M., Elving, P. J., Eds., Interscience Publishers: New York, 1965: Part 11, Volume 11. (8) Eastin, E. F. Commun. Soil Sci. Plant Anal. 1976, 7, 477; Anal. Absrr. 1976, 37, 5G2. (9) Felker. P. Anal Chem. 1977, 49, 1080. (IO) Bostroem, C. A.; Cedergren. A.; Johansson, G.; Pettersson, I. Talanta 1974, 21, 1123.
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RECEIVED for review June 11,1979. Accepted September 27, 1979.
