Volume 51, Number 10, 1997 APPLIED SPECTROSCOPY 15850003-7028 / 97 / 5110-1585$2.00 / 0q 1997 Society for Applied Spectroscopy

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Determination of Urea, Nitrate, andAmmonium in Aqueous SolutionUsing Nitrogen-14 Nuclear MagnaticResonance

LARRY S. SIMERALAlbemarle Corporation, Technical Center,P.O. Box 14799, Baton Rouge, Louisiana 70898

Index Headings: Nitrogen analysis; Aqueous urea, ammonium, andnitrate analysis; Nitrogen-14 NMR.

INTRODUCTION

The quantitative determination of nitrogen-containingspecies in aqueous solution is important in biological,chemical, and agricultural analysis. For example, the ni-trogen content of aqueous fertilizers is an important spec-i® cation, process control, and analytical parameter. Thestandard methods for determination of nitrogen contentare based on the Kjeldahl total nitrogen analysis and usu-ally involve three steps: (1) conversion of the nitrogen-containing species to ammonia using sulfuric acid and acatalyst; (2) neutralization of the solution from step 1 anddistillation of the ammonia; and (3) ® nish with a titra-tion.1± 5 Analysis of blanks is always included in thesemethods. These related classical wet chemical methodsare very labor intensive, require careful standardizationof multiple reagents, and do not provide unequivocalidenti® cation of the nitrogen-containing species in a sin-gle method. Further, wet chemical methods for nitrogenoften suffer interferences from common species presentin aqueous solution or in fertilizer, such as nitrite, phos-phate, or sulfate. Yet, Kjeldahl-related techniques arewidely used.

Nitrogen-14 nuclear magnatic resonance (NMR) rep-resents a new approach to quantitative analysis of nitro-gen species in solution. Nitrogen-14 NMR is often over-looked because broad resonances are expected to resultfrom the nitrogen-14 quadrupole moment. However, ni-trate and ammonium, important aqueous solution species,have narrow resonances, and the nitrogen-14 NMRchem-ical shift range is large.6 Several recent reports indicate

Received 31 October 1996; accepted 15 March 1997.

promise for nitrogen-14 NMR in studies of nitrogen me-tabolites in biological ¯ uids and tissues.7± 10 However,these reports did not explore the wider application of ni-trogen-14 NMR as a quantitative analytical tool for com-parison to classical wet chemical methods. This reportdemonstrates that quantitative nitrogen-14 NMR providesa convenient method for analysis of nitrogen content ofaqueous solutions. The nitrogen-14 NMR method is rap-id, does not require standardization of hazardous re-agents, has few interferences, and provides identi® cationand quantitation of the important nitrogen-containing spe-cies in a single analysis. Further, the method is automat-able on modern NMR instruments.

EXPERIMENTAL

ACS reagent-grade acetonitrile, urea, and ammoniumchloride and 99.995% sodium nitrate were purchasedfrom Aldrich Chemical. Reagent-grade cupric sulfatepentahydrate was purchased from J. T. Baker Chemicals.Deuterium oxide, 99.9 isotope % d, was purchased fromCambridge Isotopes. Accurately weighed standard solu-tions were prepared in distilled water from the urea, am-monium chloride, and sodium nitrate. Acetonitrile wasweighed into the NMR sample to provide a quantitativeinternal standard. The ratio of sample weight to aceto-nitrile was about 10:1 at 4% total nitrogen content, 15:1at 2% nitrogen, and 60:1 at 0.4% nitrogen. Five to tenpercent deuterium oxide was added to each NMR samplefor ® eld/frequency lock.

Nitrogen-14 NMR spectra were obtained onBruker/GE Omega 400WB and 500-MHz instruments.On the Omega 500 a 10-mm `̀ low band’ ’ N-15 (50-MHz)to Ge-73 (17-MHz) probe tuned to 36.1 MHz for N-14was used. On the Omega 400WB instrument a 10-mmP-31 (161-MHz) to N-15 (40.5-MHz) probe tuned to 28.9MHz for N-14 (28.9 MHz) was used, but the impedancematch was very poor (about 50% re¯ ected power). Ni-trogen-14 NMR spectra were obtained by using a 30-m spulse (45 8 at 500 MHz and 308 at 400 MHz), 7-m s pre-acquisition delay, 5-s post-acquisition delay, 32-kHzsweep width, 32K real data points, 32± 256 acquisitions,and 50-Hz exponential line broadening for data process-ing. Data acquisition for samples with added relaxationagent (cupric sulfate) employed 8K real data points,0.03-s post-acquisition delay, and 500 acquisitions. Pro-ton decoupling was not employed. Relaxation times weremeasured by using the inversion-recovery method. Thenitrate resonance was assigned to 0 ppm.

1586 Volume 51, Number 10, 1997

FIG. 1. Nitrogen-14 NMR spectrum of an aqueous 4 wt % total nitro-gen solution of urea, sodium nitrate, and ammonium chloride. Aceto-nitrile is present as a quantitative internal standard. The spectrum rep-resents 256 acquisitions obtained at 28.9 MHz as described in the Ex-perimental section. This spectrum was processed with 10-Hz exponen-tial line broadening. The nitrate resonance is assigned to 0 ppm.

TABLE I. Nitrogen-14 NMR relaxation times for aqueous nitrogenanalysis.

Species T1 in seconds

UreaNitrateNH4

1

Acetonitrile

0.030.061.1a

0.004

a This time is reduced to 0.04 s with the addition of 0.5 mg of cupricsulfate pentahydrate per gram of sample. The other values remain un-

changed.

TABLE II. Comparison of prepared and nitrogen-14 NMR determined wt % nitrogen.

Prepared Found Prepared Found Prepared Founda

As UreaAs NitrateAs NH4

1

Total

2.120.751.204.07

2.00, 2.090.79, 0.791.22, 1.224.01, 4.10

0.970.350.551.86

1.00, 0.950.35, 0.330.57, 0.571.93, 1.85

0.220.080.120.42

0.23, 0.270.09, 0.080.12, 0.140.44, 0.49

(0.19, 0.20)(0.09, 0.08)(0.13, 0.12)(0.41, 0.40)

a Values in parentheses were determined by using cupric sulfate added as a relaxation agent.

Parameters for data acquisition and processing werechosen with quantitation of the nitrogen-14 resonances inmind. The 30-m s pulse width was chosen, rather than themuch longer 908 pulse widths, to ensure uniform irradi-ation of the entire nitrogen-14 spectral region of interest.The 50-Hz line broadening provided the best overallcompromise for the widely different linewidths in thespectrum and the best quantitation on integration. Thesweep width chosen was 3± 4 3 the chemical shift rangeof the resonances of interest here, and a Butterworth ® lterwas set for 3-db roll-off at the edges of the total spectralwidth. Thus, the integration of the spectrum was not com-promised by ® lter or pulse power roll-off.

The regions of integration were as follows: 2 10 to 10ppm for the nitrate resonance; 2 100 to 2 190 ppm forthe acetonitrile resonance; 2 250 to 2 345 ppm for urea;and 2 345 to 2 370 ppm for ammonium. These choices

were reasonable and provided consistent quantitation ofthe peaks.

RESULTS AND DISCUSSION

Figure 1 shows the nitrogen-14 NMR spectrum of anaqueous solution of sodium nitrate, urea, ammoniumchloride, and acetonitrile. The dispersion of nitrogen-14chemical shifts for these species allows easy identi® ca-tion and integration. Interferences from phosphate, sul-fate, or other species not containing nitrogen are, ofcourse, absent. Nitrite (N-14 NMR chemical shift at1 240 ppm) does not interfere and could easily be in-cluded in the analysis. Acetonitrile was chosen as theinternal standard for quantitation on the basis of its chem-ical shift in an open area of the spectrum, its solubilityin water, and its inertness to the species of interest here.

The resonances are all `̀ simple’ ’ singlets. At the pH ofthe samples investigated here, the exchange of protonsbetween ammonium ion and water is fast. Therefore, noscalar coupling between nitrogen-14 and protons is ob-served. The electric ® eld gradients at the nitrate and am-monium nitrogens are small; hence, there are narrow res-onances. The resonances for urea and acetonitrile, as ex-pected,6 are much broader than those of nitrate and am-monium. Urea nitrogen-14 NMR relaxation is rapid andalso removes scalar coupling to the protons.

In contrast to previous studies,7 severe baseline rollfrom acoustic ringing was not a problem. This bene® twas accomplished through a combination of factors:higher nitrogen concentrations, higher NMR observationfrequency, probe tuning slightly off-frequency, and opti-mization of the pre-acquisition delay. For quantitativeanalysis at 0.1% nitrogen or higher, acoustic ringingshould not present a signi® cant obstacle. Curve-® ttingprocedures to reduce or eliminate any baseline roll wouldimprove analysis where baseline roll was signi® cant.

Table I shows the nitrogen-14 NMR relaxation timesfor nitrate, urea, ammonium ion, and acetonitrile in theaqueous solutions studied here. The ammonium nitrogenclearly has the longest T1, 1.1 s. A recycle time of 6 swas chosen to ensure complete relaxation of all species.In the absence of the ammonium, much faster pulse rep-etition rates could be used. As the sensitivity of nitro-gen-14 to NMR detection is relatively high, it wasn’t nec-essary in the present study to optimize the signal-to-noiseratio per unit time.

Rapid analysis of large numbers of aqueous samplescontaining nitrate, ammonium, and urea may be an im-portant application of the N-14 NMR method demonstrat-ed here. Addition of paramagnetic relaxation agents willshorten the relaxation time of the ammonium nitrogen tospeed the data acquisition without compromising the re-sults. Addition of 0.5 mg of cupric sulfate pentahydrate

APPLIED SPECTROSCOPY 1587

per gram of sample (2 3 102 3 M Cu1 2) reduces the N-14relaxation time of ammonium to 0.04 s. The total dataacquisition time for the analysis of samples containing0.4% total nitrogen is easily reduced from about 30 minwithout cupric sulfate to under 3 min with cupric sulfate.The determination of individual species and total nitrogenwas not compromised. The determination of urea, am-monium, and nitrate in water at 0.4% total nitrogen for20± 30 samples per hour is possible with the use of stan-dard instrumentation with a 10-mm NMR probe tuned toN-14 and an autosample changer. Higher nitrogen con-tents will, of course, allow higher sample throughputs.

Table II compares the nitrogen content of the individ-ual species and the total nitrogen content determined byusing nitrogen-14 NMR with the theoretical (weighed)amounts of the species. The nitrogen-14 NMR determi-nations were performed in duplicate. The data show goodagreement with the actual amounts of nitrogen presentfor the individual species and the total nitrogen content.The presence of signi® cant amounts of alkyl or arylamines may interfere with the determination of urea as aseparate species. However, total nitrogen content couldstill be determined, as well as separate nitrate and am-monium contents. As noted above, nitrite, phosphorus,and sulfur species present no interferences.

The ultimate detection limit for nitrogen species inaqueous solution is much lower than shown here. Thenitrogen-14 NMR spectrum for 0.4 wt % nitrogen wasobtained with excellent signal-to-noise in about 30 minwithout added relaxation agent and in about 3 min withrelaxation agent and the use of a poorly tuned probe (seeExperimental section). Clearly a detection limit for nitro-gen an order of magnitude lower is obtainable in a rea-sonable amount of time. One could considerably extendthe limit of detection with a larger diameter probe opti-mized for nitrogen-14 detection to accommodate largervolume NMR samples.7 Forty ppm nitrogen would notbe out of the question as an `̀ ultimate’ ’ limit of detection.

Nitrogen-14 NMR analysis of solids containing water-soluble nitrogen species, such as urea, nitrate, nitrite, andammonium, is straightforward. For example, solid fertil-izer containing these chemicals could be dissolved in wa-ter, or soil and intractable samples extracted with waterprior to analysis. Nitrogen-14 NMR could easily be usedto study the decomposition of urea or the conversion ofnitrite to nitrate in water or in fertilizer. Further, the N-14NMR analysis could be combined with classical total ni-trogen determinations where signi® cant amounts of nitro-gen-containing large molecules are present whose N-14NMR linewidths are too broad for detection to show howmuch of the total nitrogen is due to ammonium, nitrate,nitrite, and/or urea.

ACKNOWLEDGMENTS

The technical assistance of Catherine Wu and the Tulane UniversityCoordinated Instrument Facility, for use of the Omega 500 NMR, isgratefully acknowledged.

1. Annual Book of ASTM Standards, D3590-89 (American Society ofTesting Materials, Philadelphia, 1996), Vol. 11.01, pp. 316± 341.

2. Of® cial Methods of Analysis, K. Helrich, Ed. (Association of Of-® cial Analytical Chemists, Arlington, Virginia, 1990).

3. F. J. Rexroad and G. F. Krause, J. Assoc. Off. Anal. Chem. 61, 299(1978).

4. F. J. Johnson and D. L. Miller, J. Assoc. Off. Anal. Chem. 57, 8(1974).

5. P. R. Rexroad and G. F. Krause, J. Assoc. Off. Anal. Chem. 53,450 (1970).

6. J. M. Lehn and J. P. Kintzinger, Nitrogen NMR (Plenum, London,1973).

7. S. I. Tyukhtenko and S. V. Pavlova, in Spectroscopy of BiologicalMolecules, J. C. Merlin, S. Turrell, and J. P. Huvenne, Eds. (KluwerAcademic Publishers, Netherlands, 1995), pp. 23± 24.

8. S. I. Tyukhtenko and Z. Z. Rozhdkova, in Spectroscopy of Biolog-ical Molecules, J. C., Merlin, S. Turrell, and J. P. Huvenne, Eds.(Kluwer Academic Publishers, Netherlands, 1995), pp. 505± 506.

9. R. B. Lee and R. G. Ratcliffe, Planta 183, 359 (1991).10. S. Wray and D. R. Wilkie, NMR Biomed. 5, 137 (1992).

Spectroelectrochemical andPhotoelectrochemical Study ofTungsten Trioxide Particulate Films

LIANYONG SU* and ZUHONG LUNational Laboratory of Molecular and BiomolecularElectronics, Southeast University, Nanjing 210018,P. R. China

Index Headings: Spectroelectrochemis try; Electrochromic effect;Photochromic effect; WO3 ® lm.

INTRODUCTION

Photoelectrochemical conversion and storage of so-lar energy using semiconductor colloids have attractedconsiderable interest in recent years.1± 5 The approachof using semiconductor colloids for the design of op-tically transparent thin ® lms6± 9 has also been fascinat-ing in the sense that this technique is relatively simpleand inexpensive compared with other commonly em-ployed techniques such as molecular beam epitaxy orchemical vapor deposition (CVD). The transparent na-ture of these ® lms allows for direct monitoring of elec-tron transfer processes by spectroscopic means. Re-cently, for the ® rst time we have employed quantizedWO3 colloids for preparing optically transparent thin® lms on conducting quartz glass plates, and have in-vestigated their photochromic, electrochromic, andphotoelectrochemical properties.

EXPERIMENTAL

Preparation of WO3 Film. A transparent colloidalsuspension of WO3 was prepared by a method describedearlier.10 WO3´2H2O was dissolved in hot oxalic solution.WO3 particulate ® lm was obtained by dipping a con-ducting quartz glass substrate in the WO3 sol and sub-sequently pulling it up at a constant speed (0.1 mm/s).The resulting WO3 ® lm was subjected to heat treatment

Received 28 October 1996; accepted 6 March 1997.* Author to whom correspondence should be sent.