The Fertilisers (Sampling and Analysis) Regulations 1996

Determination

7.2  According to the variant chosen, place in the collecting flask a measured quantity of standard sulfuric acid as indicated in the Table on page16. Add the appropriate quantity of the chosen indicator solution (4.9.1 to 4.9.2) and, if necessary, water to obtain a volume of at least 50 ml. The condenser outlet must be below the surface of the standard acid in the collecting flask.

Transfer by pipette, according to the details given in the Table, an aliquot portion of the clear solution into the distillation flask of the apparatus. Add water to obtain a volume of about 350 ml and several grains of pumice to control the boiling.

Assemble the distillation apparatus and, taking care to avoid any loss of ammonia, add to the contents of the distillation flask 10 ml of concentrated sodium hydroxide solution (4.8) or 20 ml of the reagent in the cases where 20 ml hydrochloric acid (4.1) have been used in order to dissolve the sample. Warm the flask gently and when boiling commences distil at such a rate that about 200 ml are obtained in 30 minutes.

When no more ammonia is likely to be evolved, lower the receiving flask so that the tip of the condenser is above the surface of the liquid.

Test the subsequent distillate by means of an appropriate reagent to ensure that all the ammonia has been completely distilled. Wash the condenser with a little water and titrate the excess acid with the standard solution of sodium or potassium hydroxide prescribed for the variant adopted (see Note).

Note: Standard solutions of different strengths may be used for the titration provided that the volumes used do not, as far as possible, exceed 40 to 45 ml.