The proposed modified Conway microdiffusion
method provides for consecutive determinations
of NH?- and NO?-N in a given
aliquot of soil extract. Analyses of primary
nitrate standards showed essentially complete
recovery in the range of 1 to 20 ppm NO?-N
(4 to 80 µg N/aliquot). Results for (NH? +
NO?)-N and NO?-N in soil extracts are comparable
to those obtained, respectively, by
macrodistillation with Devarda's alloy and by
the phenoldisulfonic acid colorimetric method.
The method is rapid and suitable for routine
analyses of soil extracts, the equipment is
inexpensive, and no interferences are apparent

Carter, J.N.

Schwaninger, Daniel E.

Simpson Jr, Elmer C.

Stanford, George

The proposed modified Conway microdiffusion
method provides for consecutive determinations
of NH₄- and NO₃-N in a given
aliquot of soil extract. Analyses of primary
nitrate standards showed essentially complete
recovery in the range of 1 to 20 ppm NO₃-N
(4 to 80 µg N/aliquot). Results for (NH₄ +
NO₃)-N and NO₃-N in soil extracts are comparable
to those obtained, respectively, by
macrodistillation with Devarda's alloy and by
the phenoldisulfonic acid colorimetric method.
The method is rapid and suitable for routine
analyses of soil extracts, the equipment is
inexpensive, and no interferences are apparent

USDA - ARS - NWISRL

FifiST& 1121.29aNitrate Determination by a Modified Conway MicrodiffusionMethodGEORGE STANFORD, JOHN N. CARTER,' ELMER C. SIMPSON, JR., andDANIEL E. SCHWANINGERPlant Physiology Institute, Agricultural Research Service, U.S. Department of Agriculture,Beltsville, Md. 20705The proposed modified Conway microdiffu-sion method provides for consecutive deter-minations of NH 4- and NO3-N in a givenaliquot of soil extract. Analyses of primarynitrate standards showed essentially completerecovery in the range of 1 to 20 ppm NO3-N(4 to 80 lAg N/aliquot). Results for (N114+NO 3)-N and NO 3-N in soil extracts are com-parable to those obtained, respectively, bymacrodistillation with Devarda's alloy and bythe phenoldisulfonic acid colorimetric method.The method is rapid and suitable for routineanalyses of soil extracts, the equipment isinexpensive, and no interferences are apparent.Nitrates in soils are derived from mineraliza-tion of soil organic nitrogen and from applicationof nitrogen fertilizers (1). A knowledge of amountsof nitrate present in soils and in drainage watersemanating therefrom may have important im-plications in guiding judicious management and,hence, minimizing excessive use of nitrogenfertilizers.Various interferences are encountered in deter-mining nitrate in soil extracts, using colorimetricmethods (2) or the nitrate-selective electrode (3).Aqueous extracts of soils and water samples fre-quently are colored and/or contain suspendedclay or organic matter that interfere in color-imetry. Although extraction of soils with aqueoussalt solutions (sulfates or chlorides of potassiumor calcium are commonly used) flocculates thecolloids, extracts often are colored. Chloride inter-feres with nitrate determinations by the phenol-disulfonic acid colorimetric method and by thenitrate-selective electrode, thus necessitating pre-liminary removal of chloride or use of compen-sating chloride solutions in nitrate standards forelectrode calibration. These interferences can beI Present address: Snake River Conservation Research Cen-ter, Route 1, Box 188, Kimberly, Idaho 83341.avoided by using a modified Conway micro-diffusion method.Despite the foregoing advantages, the micro-diffusion method has not tended to replace exist-ing methods for determining nitrates in water andsoil extracts. Moreover, the microdiffusion meth-ods most recently reported (2, 4) have not beenwidely adopted because (1) procedures are labo-rious and time consuming; (2) NOa-N is estimatedindirectly as the difference between separate de-terminations of (N11 4 -1-NO3)-N and NH4-N; and(3) exacting techniques are required to insurecomplete reduction of NO3-N to NH4-N. TheObrink modification (5) of the Conway micro-diffusion unit, in providing a simple and rapidmeans of sealing the dish from the atmosphere,eliminates an objectionable feature of other units(2, 4). The Obrink modification has been usedsuccessfully to measure amounts of NH 4-N insoil extracts (5, 6), and its use for the directdetermination of NO 3-N as well as (NH4+NO3)-N is described in the present paper.METHODApparatus(a) Microdiffusion dish.—Obrink modification (5)of Conway microdiffusion dish is obtainable asmolded plastic unit consisting of dish and cover (Bel-Art Products, Pequannock, N.J. 07440). Units usedin present study (Fig. 1) were ca 83 mm in diameterwith sample (No. 2) and acid chamber (No. 1) ca-pacities of ca 6 and 3 ml, respectively. Cover fitsperipheral groove or moat (No. 3) which seals unitfrom atmosphere.(b) Buret.—Micrometer 0.2 ml buret with divi-sions of 0.2 Al.Reagents(a) Boric acid solution.-1 %. Add 10 g H3B03 ,200 ml 95 % ethanol, 700 ml distilled water, 10 mlmixed indicator (b), and 0.25 ml NPX Tergitol (non-ionic wetting agent, 1+10.5 nonylphenol-ethylene1366	JOURNAL OF THE AOAC (Vol. 56, No. 6, 1973)FIG. 1—The Obrink modification of the Conway microdiffusion dish, showing the central boric acid chamber (No.1),the peripheral sample chamber (No. 2), and the annular moat (No. 3). The clear polypropylene cover, when seatedin the moat, seals off the acid and sample chambers.oxide condensate) to 1 L volumetric flask and letsolid dissolve. Cool solution, adjust to faint pinkwith 0.005N NaOH, and dilute to volume withdistilled water.(b) Mixed indicator.—Dissolve 0.066 g methyl redand 0.033 g bromocresol green in 100 ml 95%ethanol.(c) Devarda's alloy.-20 mesh or finer (Baker,reagent grade).(d) Potassium carbonate solution.-45% (w/v) inwater. Add 4-5 drops of NPX Tergitol/L.Nitrate Determination(a) Microdiffusion.—Distribute 40-50 mg De-varda's alloy in peripheral sample chamber (No. 2,Fig. 1) of Obrink-Conway dish. Pipet 4 ml solutionto be analyzed for nitrate into same chamber. Pipet1.5 ml boric acid solution into center well (No. 1) ofdish. Place 1.5-2 ml 45% K2CO3 solution in outermoat (No. 3) and 1 ml in sample chamber. Seat lid inmoat. After 16 hr at room temperature, remove lidand titrate boric acid to end point with 0.02N H2SO4,using micrometer buret (1 ml 0.02 N H2SO4 titrates280 Aig NH 4-N). Stir solution with buret tip duringtitration. This procedure measures (NH 4 + NO3)-N.To perform consecutive determinations of NH4-and NO3-N using single set of sample solutions andmicrodiffusion units, proceed as follows: First deter-mine NH4-N by procedure outlined above, omittingDevarda's alloy. After > 16 hr, titrate diffusedNH4-N. Then distribute Devarda's alloy in samplechamber and replace cover. After > 16 hr, titrateagain to measure NO 3-N. (Because significantamounts of nitrite are rarely found in soil extracts,the present study was limited to NH4- and NO3-N.Bremner (2) has described a procedure for estimat-ing NO 2-N in microdiffusion analyses, involving itspreliminary destruction with sulfamic acid.)(b) Extraction of soil nitrate.—Several methodscommonly are used to extract nitrates from soils (1,7). The methods used in the present study are brieflydescribed under Results in connection with specificexperiments.ResultsNitrate Recovery from Primary Standards.—Nitrate standard solutions prepared from driedpotassium nitrate were analyzed by the modifiedConway method and results are given in Table 1.Within the range of 1-20 ppm NO 3-N, averagerecoveries from primary standards were essen-tially complete. Although coefficients of varia-tion are relatively high for determinations in the1-2 ppm range, estimates are sufficiently accurateto meet normal requirements in soil and wateranalyses.STANFORD ET AL.: NITRATE BY MODIFIED CONWAY MICRODIFFUSION METHOD 	 1367Table 1. Determination of nitrogen in potassiumnitrate primary standards by the modifiedConway methodNO 3 -N	 NO 3 -N byconcn	 modifiedin std	Conway(calcd),	 method,'ppm	 ppm1 1.1 0.15 13.52 2.1 0.17 8.24 4.0 0.10 2.58 8.0 0.07 0.810 9.9 0.27 2.720 19.3 0.45 2.3Average of 8 determinations.6 Coefficient of variation =(standard deviation/mean)X 100; for statistical methods, see Snedecor (8).Microdiffusion vs. Macrodistillation Method.-Nine soils containing a wide range in mineral Ncontents were extracted with IN KC1 (100 g air-dried soil in 250 ml extractant with 4 replicates).After 2-4 intermittent shakings over a half-hourperiod, the suspension was allowed to settle.Duplicate 4 ml aliquots from the supernatantliquid were analyzed for (NH 4 +NO3)-N by themodified Conway method described earlier. Forcomparison with a macrodistillation method, ad-ditional extract was recovered by filtering. Theextract (150 ml), placed in an 800 ml Kjeldahlflask, was analyzed for (NH4 +NO3)-N by aDevarda's reduction and distillation procedure(5).Results in Table 2 show reasonably good agree-ment between amounts of (NH 4+NO3)-N deter-mined by the microdiffusion and macrodistillationmethods. Coefficients of variation were relativelylow by both methods. The averages for the micro-and macro-methods, respectively, are based on 8and 4 determinations.Microdiffusion vs. Phenoldisulfonic Acid Meth-od.-Soils were sampled from a number of rate-of-N field experiments on sugar beets in Idaho.The samples were ground and mixed, and a por-tion of each was sent to the Beltsville laboratoryfor various chemical determinations, includingNH4- and NO3-N. Soil samples were extractedby heating with 0.01M CaCl2 in a steam chest(100°C) for 16 hr as described by Stanford (7).The NH4-N fraction in the extract was deter-mined by microdiffusion after which Devarda'smetal was added and NO3-N was determined asdescribed under Method.Independent determinations of NO3-N wereTable 2. Comparison of a macrodistillation and amodified Conway method for determining(NH 4 +NO 3 )-N in soil extracts, using Devarda'salloy as the nitrate reducing agentMacrodistillation b Modified Conway'	Coeff. of	 Coeff. of	ppm var.; %	 ppm var., %Kranzburg sil 24.3 2.5 22.3 4.0Barnes I 16.7 5.3 15.1 3.2Aastad cl 34.5 2.9 32.3 3.0Cecil sl 6.9 3.6 6.6 4.8Minidoka sil 18.1 5.2 17.5 3.6Parshall fsl 5.9 6.6 6.2 2.0Hagerstown sil 16.5 3.9 16.1 4.4Pullman sicl 41.4 3.6 40.9 2.3Amarillo fsl 23.0 4.3 22.9 1.2si =silt; I =loam ; c =clay; s =sandy; fs =fine sandy.6 Average of 4 separate extractions.See footnote b, Table 1.a Average of 8 determinations (duplicate of 4 extracts).made at the Snake River Conservation ResearchCenter (Kimberly, Idaho), using a different meth-od of extraction and the phenoldisulfonic acidcolorimetric procedure for determination ofNO3-N. The extractant was a dilute aqueous solu-tion of Ag2SO4 (0.17 g/L) and CuSO4 . 5H20 (2.5g/L). A 50 g soil sample was shaken 10 min with200 ml extractant, the extract was filtered, andan aliquot was taken for determination of NO 3-Nby the phenoldisulfonic acid method, essentiallyas described by Bremner (2).Average NO 3-N contents of 122 soils as deter-mined by the colorimetric and microdiffusionmethods, respectively, were 12.8 and 12.5 ppm.The relation between NO 3-N (ppm) by the colori-metric method, Y, and NO3-N (ppm) by themodified Conway method, X, is depicted by thefollowing regression equation (8): Y = 0.2+1.005X. The coefficient of determination, r 2 , was0.976. Although the overall range in NO 3-N con-tents of 122 soils extended from approximately1 to 45 ppm, about 90% of the soils contained 25ppm or less and 70% contained 15 ppm or less,as shown in Table 3.DiscussionThe determination of NH 4-N and/or NO3-Nby the modified Conway microdiffusion methodas outlined in this report has certain advantagesover the methods with which it has been com-pared, from the standpoint of interferences, sim-plicity, and cost of equipment. The actual timerequired per NO3-N determination by an ex-perienced analyst (excluding the diffusion period)Soil seriesStd dev.,	 Coeff. of	 andppm	 var.,b %	texture°1368	 JOURNAL OF THE AOAC (VOl. 56, No. 6, 1973)Table 3. Percentage distribution among ranges ofNO 3-N contents in 122 soils (ppm NO 3 -N based onair-dried soil) as determined by the phenoldisulfonicacid and modified Conway microdiffusion methodsRange in	 Phenoldisulfonic	 Modifiedsoil NO 3 •N	 acid	Conwaycontents, ppm	 method, %	 method, %0-5 17.2 18.05.1-15 54.1 55.715.1-25 18.9 18.025.1-35 7.4 6.635.1-45 2.4 1.7is about 3 min, or 4-5 min for consecutivedeterminations of NO3- and NH 4-N. On theaverage, the initial sample and reagent transfersconsume about 1.5 min, and the titration from 1to 1.5 min. Since these operations are done onconsecutive days, an analyst can average 100 ormore determinations per day, in addition to per-forming the soil extractions and cleaning of Con-way dishes and glassware.The importance of thorough cleaning of theConway dishes in order to remove the last tracesof alkali deserves special emphasis. Routinely,the dishes are flushed under a tap water faucetwhile the Devarda alloy residue is removed bybrushing. Dishes then are soaked in a vessel oftap water containing detergent, followed by soak-ing in a dilute acid bath. Finally, the dishes arerinsed thoroughly in tap water, given a final rinsewith distilled water, and drained. The lids arerinsed thoroughly in tap water and finally withdistilled water.A unique feature of the modified microdiffusionReceived March 13, 1973.Mention of trade names or company names is for the benefitof the reader and does not imply endorsement by the U.S.Department of Agriculture.method as proposed herein is that it provides forthe direct determinations of NH 4- and NO3-N.In the method described by Bremner (2), involv-ing solutions containing both mineral forms of N,separate determinations are made for NH 4-Nand for (NR4 +NO3)-N, and NO3-N is obtainedby difference. The proposed direct method in-volves an initial determination of diffusibleNH4-N, followed by addition of Devarda's alloyand determination of NO3-N. Bremner and Shaw(4) concluded that Devarda's alloy used withstrong alkalies (potassium carbonate and potas-sium hydroxide) was unsatisfactory for deter-mining nitrates in soil extracts. - They proposedthe use of titanous sulfate with MgO-suspensionfor nitrate reduction in microdiffusion analyses,a method which entails mixing to minimize gelformation and facilitate diffusion of NH 3 (2).Results of the present study, however, show satis-factory recovery of NO 3-N from standards andgood agreement with values obtained by themacrodistillation and phenoldisulfonic acid meth-ods.REFERENCES(1) Smith, S. J., & Stanford, G. (1971) Soil Sci. 111228-232(2) Bremner, J. M. (1965) Methods of Soil Analys,,,Pt. 2, American Society of Agronomy, 1110 .Madison, Wis., pp. 1179-1237(3) Bremner, J. M., Bundy, L. G., & Agerwal, A. S(1968) Anal. Lett. 1, 837-844(4) Bremner, J. M., & Shaw, K. (1955) J. Agr. ,ti,46, 320-328(5) Obrink, K. J. (1955) Biochem. J. 59, 134-131;(6) Stanford, G., & DeMar, W. H. (1970) Soil ,'.'109, 190-196(7) Stanford, G. (1968) Soil Sci. 106, 345-351(8) Snedecor, G. W. (1956) Statistical Methods, 5111Ed., Iowa State University Press, AmesReprinted from the Journal of the Association of Official Analytical Chemists, Vol. :0,November 1973.

Nitrate Determination by a Modified Conway Microdiffusion Method

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Nitrate Determination by a Modified Conway Microdiffusion Method

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