Qualitative Determination of Nitrate with Triphenylbenzylphosphonium Chloride Donna A. Berry Shaker Heights H.S., Shaker Heights, OH 44120 Jerry J. Cole Ashland College, Ashland, OH 44805
The general procedure for the identification of nitrate is the anion group separation which is made with barium and silver solutions (I),or by division into three main groups: AgNOs group, BaCI2 group, and the soluble group (which are the anions that do not precipitate with AgN03 or BaClz), Nosis in this group (2). The separation procedures are then followed by the individual tests for the anions. T h e standard test for nitrate will he referred to as the "Brown Ring" test. Since the dependability of the"Brown Ring" test is questionable, a new qualitative scheme was written using the reagent, triphenylbenzylphosphonium chloride (TPBPC).
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01'
Triphenylhenzylphclsphonium chloride is a slightly crystalline solid that is soluhlc in alci~hol,but only slightly soluble in water and acetone. It is soluble in aliphatic and aromatic hvdro~arhms.It ha5 a molecular weiahr of'3SS.9. minimum G r i t y of 95% as supplied, and a meltYng point of 325-333"~ (3).When added to a solution containing nitrate, the triphenylbenzylphosphonium chloride forms triphenylbenzylphosphonium nitrate, a white crystalline solid. Standard Qualitative Test for Nitrate (4) The"Brou,n Ring" test for nitrate is made hy ndding ferrous iulfate to the unknown in a test tuheand thencarefullvadding concentrated sulfuric acid in such a manner that i t does not mix but forms a separate layer a t the bottom of the tube. If nitrate is present, i fine, hiown ring will be formed a t the junction of the acid and the solution.
In the first reaction the nitrate ion is reduced to nitric oxide hy thc ferrous suliate. The nitric oxide then comhines with excess ferrous ion to form a hmwn complex, [Fec N01I2'. Interferences in the Brown Ring- Test The reaction only occurs a t the junction of the solution and the HnSOa. where the acidity is high. - If the solution is mixed, no test is obtained. Bromides and iodides react with the concentrated HzS04 to liberate free iodine or bromine. Sulfides, sulfites, and other reducing agents interfere with the reaction or ferrous sulfate and nitric acid ( 5 ) . Chromates are reduced by ferrous sulfate to chromium sulfate, which will appear as a green layer a t the place where the brown ring should form.
60
Journal of Chemical Education
Permanganates interfere because of their color. T h e triphenylhenzylphosphonium chloride (TPBPC) reagent was tested to see if it precipitated any anions other than nitrate, or would interfere with the standard qualitative testa (61, (7),for anions other than nitrate. As a result of the tests, a new qualitative scheme had to be written, since TPBPC precipitated the anions: Br-, NOz-, I-, and MnOa-. New Qualitative Procedure (This Procedure follows the laboratory manual by Hered and Nebergall) (8): Procedure 6 Identification of NOS- in the absence of NOz-, Br-, and I-. To 1 ml of the prepared solution add 5-8 drops of the prepared reagent (TPBPC). A white crystalline precipitate confirms the presence of NOa-. Procedure 7 Identification of NO3- in the presence of NOz: If the NO2- is between0.3 and 0.1 M there is no interference with TPRPC~ ..-.-. Removal of NO2-. To 6 drops of the prepared solution add 4 M H2S04until acidic, and then add 4 drops of 1 M (NH&S04 solution. Place the mixture in a casserole and slowly evaporate to a moist residue (not dryness). Add 4 drops of H2S01 and evaporate t o a moist residue a second time. Dissolve the residue in 10 dram of HQOand transfer the R q w . 1 1 the test lor thewnfirmation mixture t
