Test aimed at determining the content of nitrous nitrogen (nitrite) in a sample of wastewater (pursuant to Legislative Decree No. 152 03/04/2006 Table 3 All.5 Part III), underground (according to D .Lgs. N ° 152 03/04/2006 Table 2 All.5 Part IV), land delivery (pursuant to Legislative Decree N ° 152 03/04/2006 Table 4 All.5 Part III) or for the human consumption (pursuant to Legislative Decree No. 31 02/02/2001)