Method 56b Determination of organic plus ammoniumN by digestion and distillationa

Nitrogen in the sample is converted to ammonium-N by digestion with sulphuric acid and sodium sulphate with a copper-selenium catalyst. The ammonia liberated with sodium hydroxide is removed by steam distillation and determined titrimetrically.

a(MAFF/ADAS, 1986, pp. 150-151 with Crown Copyright Permission).

Digestion stage Apparatus.

• Macro-Kjeldahl digestion unit - with adjustable heating. Reagents.

• Copper-selenium catalyst tablets - each tablet contains 0.5 g of cupric sulphate pentahydrate, and 0.02 g of selenium.

• Sodium sulphate tablets - each tablet contains 2.5 g of anhydrous sodium sulphate.

• Sulphuric acid, approximately 98% m/m H2SO4.

Procedure. Transfer 5 g of air-dried soil, ground to pass a 2-mm mesh sieve, into a Kjeldahl flask and add four sodium sulphate tablets and one copper-selenium tablet. Add 25 ml sulphuric acid. Swirl the acid until no particles of the sample adhere to the bottom of the flask.

Heat the flask gently until frothing ceases, more strongly until the solution clears, and then for a further 1 h with the sulphuric acid condensing in the lower part of the neck of the flask. Allow to cool, carefully add approximately 100 ml of water and warm to dissolve the soluble material. When cool, transfer quantitatively into a 250-ml volumetric flask and dilute to 250 ml. Retain the diluted digest for the determination of ammonium nitrogen.

Carry out a blank determination using 2 g of sucrose in place of the sample.

Distillation stage Apparatus.

• Distillation unit - the Markham semi-micro distillation unit is suitable.


• Ammonium-N standard solution, 0.28 mg ml-1 of nitrogen - dry ammonium sulphate at 102°C for 1 h and cool in a desiccator. Dissolve 1.321 g of the dried salt in water and dilute to 1 l.

• Boric acid solution, approximately 1% m/v.

• Methyl red-methylene blue solution - dissolve 1.25 g of methyl red and 0.825 g of methylene blue in 1 l of ethanol.

Procedure. Steam out the distillation unit for 20 min. Pipette 5 ml of ammo-nium-N standard solution into the unit. Add 7 ml of sodium hydroxide solution and steam distil the liberated ammonia into 5 ml of boric acid solution. Collect 20 ml of distillate. Add 2-3 drops of methyl red-methylene blue solution and titrate with 0.01 M sulphuric acid until the green colour changes to purple. Carry out a blank determination using 5 ml of water in place of the ammonium-N standard. The titre of the ammonium-N standard minus the distillation blank should be 5.00 ml.

Pipette 10 ml of the sample digest (see under 'Digestion stage') into the distillation unit, and proceed with addition of sodium hydroxide solution, distillation and titration as above.

Calculation. Subtract the titre given by the blank from that given by the sample digest. Multiply the difference by 1.4. The result gives the g kg-1 of nitrogen in the sample. (Multiply the difference by 0.14 to get the % N in the sample.) If required, express results for oven-dry soil as in Method 5.2, Calculation (2).

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