Some alkaloids and other nitrogen-containing organic compounds fail to yield all of their nitrogen upon digestion with sulfuric acid;therefore these methods cannot be used for the determination of nitrogen in all organic compounds.

METHOD I

Nitrates and Nitrites Absent— Place about 1g of the substance,accurately weighed,in a 500-mL Kjeldahl flask of hard borosilicate glass.The material to be tested,if solid or semisolid,may be wrapped in a sheet of nitrogen-free filter paper for convenience in transferring it to the flask.Add 10g of powdered potassium sulfate or anhydrous sodium sulfate,500mg of powdered cupric sulfate,and 20mLof sulfuric acid.Incline the flask at an angle of about 45,and gently heat the mixture,keeping the temperature below the boiling point until frothing has ceased.Increase the heat until the acid boils briskly,and continue the heating until the solution has been clear green in color or almost colorless for 30minutes.Allow to cool,add 150mLof water,mix the contents of the flask,and again cool.Add cautiously 100mLof sodium hydroxide solution (2in 5),in such manner as to cause the solution to flow down the inner side of the flask to form a layer under the acid solution.Immediately add a few pieces of granulated zinc,and without delay connect the flask to a Kjeldahl connecting bulb (trap),previously attached to a condenser,the delivery tube from which dips beneath the surface of 100mLof boric acid solution (1in 25)contained in a conical flask or a wide-mouth bottle of about 500-mLcapacity.Mix the contents of the Kjeldahl flask by gentle rotation,and distill until about four-fifths of the contents of the flask has distilled over.Titrate with 0.5Nsulfuric acid VS,determining the endpoint potentiometrically.Perform a blank determination,and make any necessary correction.Each mLof 0.5Nsulfuric acid VSis equivalent to 7.003mg of nitrogen.

Nitrates and Nitrites Present— Place a quantity of the substance,accurately weighed,corresponding to about 150mg of nitrogen,in a 500-mL Kjeldahl flask of hard borosilicate glass,and add 25mLof sulfuric acid in which 1g of salicylic acid previously has been dissolved.Mix the contents of the flask,and allow the mixture to stand for 30minutes with frequent shaking.To the mixture add 5g of powdered sodium thiosulfate,again mix,then add 500mg of powdered cupric sulfate,and proceed as directed under Nitrates and Nitrites Absent,beginning with “Incline the flask at an angle of about 45.”

METHOD II

Apparatus— Select an appropriate 300-mL Kjeldahl flask,from which the nitrogen is first liberated by acid digestion and then transferred quantitatively to the titration vessel by steam distillation.

Procedure— Place an accurately weighed or measured quantity of the material,equivalent to 2to 3mg of nitrogen,in the digestion flask of the apparatus.Add 1g of a powdered mixture of potassium sulfate and cupric sulfate (10:1),and wash down any adhering material from the neck of the flask with a fine jet of water.Add 7mLof sulfuric acid,allowing it to rinse down the wall of the flask,then,while swirling the flask,add 1mLof 30percent hydrogen peroxide cautiously down the side of the flask.(Do not add hydrogen peroxide during the digestion.)