Blue-Violet for Nitrate Ion The most common test that is employed in general chemistry for the qualitative analysis of the nitrate ion is the brawn ring test. A far superior test, however, is the diphenylamine test for the nitrate ion. The diphenylamine test takes advantage of the oxidizing ability of the nitrate ion in highly acidic solutions tooxidize colorless diphenylamine to the violet-colored compound called diphenylbenzidine violet. The diphenylamine test has the following advantages over the brawn ring test: i t is instantaneous, easier to perform and observe, more sensitive, and the reagents and test eolor are stable. In additian, all interfering ians are easily removed without a deleterious effect on the test, and the test can be used as a screening test for oxidizing and reducingions in an anion analysis scheme. The diphenylamine reagent solution can be prepared by taking advantage of the heat of solution of sulfuric acid t o dissolve the diphenylamine. To prepare 100 ml of the test reagent, 0.4 gof diphenylamine is added to80 ml of 3 M H z S 0 4 . To this mixture is added 20 ml of 18 MHISOI and the mixture is stirred. The heat evolved in the solution of the H ~ S O I is sufficient to bring about the rapid dissolution of the diphenylamine. Using this reagent, the test for the nitrate ion is performed as follows. Three drops of the test solution are placed in a 10 X 75 mm test tube. This is followed by the addition of 3 drops of the diphenylamine solution and then 10 drops of 18 M H 2 S 0 4 .The tuhe is shaken to mix the reactants. If the nitrate ion is present in the test solution, the mixture immediately turns deep hlue in color and then slowly changes
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The diphrnylnmine test was tound to he quite acnsitive. Hcndily ohservahlr cestr were obtainrd wbth concentrations of nitrate :on as low ar OW1 M. The eolor, however, depends upon the concentration of rhe S O n ion that LS emplwrd in thr t m At all concentrations ot nitrate ion an inirinl blue cdor is ubtarned. At low ronrentratinns "ithe NO1 ton, concentrations of the order of 0.04 M and less, the blue eolor remains stable. At higher concentrations the color slowly changes to violet and if the concentration of N O 3 ion is of the order of 0.1 M or greater, the violet slowly turns into a deep wine red color. Using one drop of 0.2 M NOs- and 2 drops of water in the above test procedure for the nitrate ion gives a very stable violet eolor. Because of the intensity of the colors, the colors are best characterized by shaking the tube and viewing the calor through the film of the solution that momentarily adheres to the side of the tube. The diphenylamine test is not, of course, a specific test for the nitrate ion, but is rather a test that depends upon the oxidation potential of the system. The presence of other strong oxidizing agents will give a positive test and the presence of reducing agents will prevent the test from occurring or will cause a rapid fading of the eolor. In spite of this limitation, however, none of the common anions will interfere with its use as a confirmatory test far NO3 - in single anion unknowns. The chromate ion gives a positive test with the diphenylamine, but the yellow color of the Cr0,2- ion should preclude its identification as the colorless NO3- ion. The nitrite ion gives a positive test that slowly fades to brown on standing. The NOz- ion is readily distinguished from the N O 1 ion by adding 3 more drops of the test solution after the additian of the 10 drops of 18 M H1S04. If the ion in the test solution is the N O 2 ion, the eolor turns to yellow as a result of the reducine of the N O 1 ion. If the ion in the test solution is the NO1- ion. the blue-violet color remains. The iodide . " nhilitv ~~~~, ~. ion g1ve.i frrrt a red color when used in the diphenylamine test and thli changes to a \,:olet-hlark color as more elrvnrntal it,dine forms. These rotor changes are readih disringuished from the blue and then hlue-violet colors that arr given hg the nitrate ion. Advantage can be taken of the fact that the color of the diphenylamine dependsupon the oxidation potential of the system t o classify a single anion unknown as a n oxidizing anion, areducing anion, or a nori-redox anion. The development of a blue-violet color when the test is performed indicates that the anion is a n oxidizing anion: Nos-, CrO&, or NO%-. To test for a reducing anion, a positive diphenylamine test is prepared with a known 0.2 M NOs- solution. To this positive test is added 5 drops of the unknown solution. The common reducing anions, NOz-, I - , Br-, S2-, S032-, and SCN-, all rapidly change the blue-violet calor to yellow. Common ians that will neither give a positive test nor destroy a positive test are: C1-, Sol2-, C O i - , P0a3-, CZHJOZ-, F-, B0+, C2042-, and As0+3-. This test procedure classifies these anions as non-redoa anions. The diphenylamine test can be successfully used as the confirmatory test for the N O a ion for unknowns that contain mixtures of anions. The test can be performed in the presence of the non-redox ions listed above; they do not interfere with the test. Test solutions that are knawn to contain, or which may contain, CrOez-, NOS-, I-, B r r , S2-, SOzZ-, and SCN- ions must be treated to eliminate these interfering ians before the test for nitrate is performed. The Br-, I-, and SCN- ions can he removed by precipitation with a saturated solution of silver sulfate; the system is centrifuged and 3 drops of the eentrifugate are used for the nitrate test. The CrO&, SZ-, and SO& ions can be removed by precipitation with a 0.20 M solution of leadacetate; again, the system is centrifuged and 3 drops of the centrifugate are used for the nitrate test. The nitrite ion is removed prior to the nitrate test by reduction with aqueous NaHSOs. This procedure is accomplished by adding 5 drops of 0.2 M NaHS08 to 3 drops of the test solution in a 10 x 75 mm test tuhe. This mixture is shaken and 2 drops of 18 MHISOl are added. The tube is heated in a boiling water bath for 5 min to bring about the reduction of the nitrite ion and to destroy the excess H&Oa. The hydrogen sulfite ion is known to reduce the NO*- ion to NHI through a series of intermediate reduction products. ~
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Leonard C. Grotz University of Wisconsin Center Waukesha, 53186
Volume 50, Number 1, January 1973 / 63
