1182
Anal. Chem. 1983, 5 5 , 1182-1184
be designed even in field. I t is for example possible to study photosynthetic activity on the carbonate system in lake waters, which may increase p H by -2 units by COz(aq) depletion (6, 7). For this purpose the probe is well suited for two reasons. First, the titrations are performed in a vessel without any gas volume and, second, the titrations are performed in a constant ionic medium which allows a concentration scale to be used for both pH and the dissociation constants of COz(aq) (8). The latter feature is very important for the accuracy of speciation calculations in the carbonate system because the equilibrium constants used can be applied to the same media in which they were determined. If the probe is to be used for complete titration experiments the time required will be considerably longer than 5 min since the equilibration times may not be as fast as in a solution buffered by free protons. In the future a portable field computer will be constructed by low-power CMOS circuits together with a printing unit and a digital panel meter all powered by NiCd batteries.
ACKNOWLEDGMENT Thanks are due to Michael Sharp for revising the English text. Registry No. H,O, 7732-18-5.
LITERATURE CITED (1) Gran6l1, A.; Anfalt, T. Anal. Chlm. Acta 1977, 91,175-180. (2) Anderson, L.; GranBII, A.; Strandberg, M. Anal. Chim. Acta 1978, 103,489-494. (3) Lid&, J.; Ginstrup, 0.; Ingrl, N. Chem. Scr. 1960, 15, 203-205. (4) Hansson, I.; Jagner, D. Anal. Chim. Acta 1973, 6 5 , 363-373. (5) Arnek, S.;SiIIBn, L. G.; Wahlberg, 0 Ark. Keml 1969, 353-363. (6) Talllng, J. F. J . Ecol. 1976, 6 4 , 79-121. Samuelsson, G.; Sloberg, S.; Ingri, N. Plant Cell. En(7) Hofslagare, 0.; vlron., in press. (8) Hansson, I. Acta Chem. Scand. 1973, 2 7 , 931-944.
RECEIVED for review December 13, 1982. Accepted January 31, 1983. The method described in this paper is the subject of a patent application.
Determination of Nitrogen-ISINitrogen-I4 in Nitrate Samples by Conversion to Benzamide Nigel J. Bunce" and Jon LaMarre Department of Chemistry, University of Guelph, Guelph, Ontario, Canada N 1G 2 W 1
Robert L. Merrlck and Charles T. Corke Department ofEnvironmental Biology, University of Guelph, Guelph, Ontario, Canada N 10 2W1
In connection with a problem in soil microbiology, we needed an analytical method for determining lsN:14N ratios in nitrate samples. Literature methods for this determination depend on converting the nitrate ion to a gas, e.g., NO (1)or N2 ( 2 , 3 ) which , is then analyzed by mass spectrometry. We wished t o devise a method by which the nitrate could be converted to a solid derivative. This would eliminate the need for gas handling facilities and would make it possible for a laboratory lacking a mass spectrometer to send the sample to a mass spectrometry laboratory for analysis. In this paper, we describe a method for converting nitrate to the solid derivative benzamide; the procedure is simple, rapid, and uses minor modifications of known chemistry. The new method lends itself conveniently to tracer work involving the use of 16N compounds but is not sufficiently precise for estimation of 15N:14Nratios a t near natural abundance.
EXPERIMENTAL SECTION I4Ncompounds were reagent grade laboratory chemicals; I5N compounds (99% nominal isotopic purity) were obtained from Stohler Isotope Chemicals Ltd. All were dried before use. Nessler's reagent was prepared by the standard method ( 4 ) ; quantitative analyses were done by using absorbance measurements at 415 nm, calibrated against ammonium salts of known concentration. Devarda's alloy, benzoyl chloride, aniline, and 2-naphthol were reagent grade chemicals supplied by Fisher Scientific. The 2-naphthol was purified by crystallization from toluene/cyclohexane. Standard Procedure. The nitrate sample (sufficient t o contain 2-3 mg of N) is placed in a 500-cm3boiling flask equipped for simple distillation, together with enough water to make the total volume 100-200 cm3. One gram of Devarda's alloy and 10 cm3of 20% NaOH solution are added (cf. ref 5 ) , and solution is boiled gently for 1 h. The mixture is then distilled slowly into 2 cm3of 0.1 M HC1 until a total of 5-10 cm3of distillate has been
collected (see (a) below). The distillate is then basified with 20% NaOH solution and 1 cm3 of benzoyl chloride is added. The mixture is shaken (standard Schotten-Baumann conditions, ref 6), and when the benzoyl chloride has reacted completely, the alkaline solution is extracted with five successive 10-cm3portions of dichloromethane(see (b) below). The combined organic extracts are dried and fiitered and the solvent evaporated,to leave a residue of benzamide which is recrystallized from warm toluene (see (c) below) and analyzed by mass spectrometry (see (d) below). Notes: (a) The progress of the distillation is conveniently monitored by using Nessler's reagent to check for NH3 in the distillate. (b) The solution must be alkaline before extracting with dichloromethane or else benzoic acid M , = 122 will also be extracted. (c) An alkane, e.g., hexane, having molar mass
