Discussion 61 Determination of total nitrogen in presence of nitrate and organic N

Nitrogen may be present in several forms: ammonium, cyanamide (NH2CN), nitrate, urea (carbamide, CO(NH2)2), and slow-release ureaformaldehyde condensates. Any nitrate must first be reduced to ammonia/ammonium by the use of an appropriate reducing agent. Some reducing agents are:

• Arnd's alloy (60% Cu, 40% Mg) in 20% (m/v) MgCl2.6H2O solution containing 1.5% (m/v) MgSO4.7H2O and 0.2% (m/v) MgO.

• Devarda's alloy (45% Al, 5% Zn, 50% Cu) in 30% (m/v) NaOH solution.

• Raney catalyst powder (50% Ni, 50% Al) in 20% v/v H2SO4 containing 10.67% (m/v) K2SO4.

• Ulsch method, powdered iron, reduced in hydrogen, in 1.6% (m/v) H2SO4.

• Zinc dust and salicylic acid (5% m/v) in H2SO4 (93-98% m/v).

Calcium cyanamide and urea must be subjected to a Kjeldahl digest to convert —NH2 to NH4+. Various modifications to procedures are necessary when several different compounds are present in the same fertilizer sample, and these may be consulted in the above references. It is important to avoid generating heat in the sample grinding process, which could affect any heat-sensitive organic constituents or cause moisture loss; a pestle and mortar is therefore recommended. Conversely, the absorption of atmospheric moisture by hygroscopic constituents, such as ammonium nitrate, must be avoided; therefore the sample preparation stage should be accomplished as speedily as possible before storing in an inert airtight container. The sample is finely ground for two reasons: the fertilizer may contain various compounds with different crystal properties which could lead them to segregate, causing a loss in homogeneity; second, only a semi-micro amount of sample is taken, therefore it is vital that it should be homogeneous and truly representative of the bulk sample. The following procedure uses salicylic acid and zinc dust as reducing agents, and the reactions involved are described in Method 5.6a.i, Reduction of nitrate prior to digestion and colorimetric autoanalysis. The digestion is carried out using micro-Kjeldahl digestion units to benefit from the various economies of scale. Potassium sulphate is omitted from the digestion mixture so that the same digest solution may be analysed for potassium; however, because of the resulting lower boiling point, the digestion may take 4-5 h.

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