PROCEDURE
Preparation of the solution for analysis
7.—7.1 See Method 2.
Determination
7.2 According to the variant chosen, place in the receiving flask an exactly measured quantity of standard sulphuric acid as indicated in the Table. Add the appropriate quantity of the chosen indicator solution (4.10.1 or 4.10.2) and finally, sufficient water to give a volume of 50 ml. The end of the extension tube of the condenser must be below the surface of the solution. Fill the bubble trap with distilled water.
Using a pipette, take an aliquot part of the clear solution as indicated in the Table and place in the distillation flask. Add sufficient water to the distillation flask to obtain a volume of 250-300 ml, 5 ml ethanol (4.11) and 4 g Devarda’s alloy (4.8).
(Note: In the presence of calcium salts such as calcium nitrate and calcium ammonium nitrate, it is necessary to add, before distillation for each gram of sample present in the aliquot, 0.700 g disodium hydrogen phosphate (Na2HPO4.2H2O) to prevent the formation of calcium hydroxide.)
Taking the necessary precautions to avoid loss of ammonia, add to the flask about 30 ml 30% sodium hydroxide solution (4.9) and finally, in the case of acid soluble samples, an additional quantity sufficient to neutralise the quantity of hydrochloric acid (4.1) present in the aliquot part taken for the analysis. Connect the distillation flask to the apparatus, ensuring the tightness of connections. Carefully shake the flask to mix the contents.
Warm gently, so that the release of hydrogen decreases appreciably over about half an hour and then liquid will boil. Continue the distillation, increasing the heat so that at least 200 ml liquid distils in about 30 minutes. (Do not prolong the distillation beyond 45 minutes.)
When the distillation is complete, disconnect the receiving flask from the apparatus, carefully wash the extension tube and bubble trap, collecting the rinsings in the titration flask. Titrate the excess acid according to the procedure in Method 2.
Blank test
7.3 Carry out a blank test under the same conditions omitting only the sample and allow for this in the calculation of the final results.
Control test
7.4 Before carrying out the analysis, check that the apparatus is working properly and that the correct application of the method is used, using an aliquot of a freshly prepared solutionj of sodium nitrate (4.12) containing, according to the variant chosen, 0.050 g to 0.150 g nitrate nitrogen.