2918
THE NEW ZEALAND GAZETTE
[No. 72

(ii) The quantity of ammonia shall be determined by distillation into standard acid after liberation with alkali, and where mercury has been used with the addition also of sodium or potassium sulphide solution.

(b) Nitrogen when nitrates are present:
(i) A weighed portion of the sample shall be transferred to a Kjeldahl digestion-flask; 30 cubic centimetres of concentrated sulphuric acid, containing 1 gram of salicylic acid, shall be added, and the flask shall be shaken so as to mix its contents without delay. The shaking shall be continued at intervals during ten minutes, the flask being kept cool, and then 5 grams of sodium thiosulphate and 10 grams of potassium sulphate shall be added. The flask shall be heated until the contents are colourless or nearly so. Copper sulphate or mercury may be used as above described in paragraph (3) (a) (i).

(ii) The quantity of ammonia shall be determined as above prescribed in paragraph (3) (a) (ii).

(c) Nitrogen in form of ammonium salts:
A weighed portion of the sample shall be transferred to a distillation-flask, and the quantity of ammonia shall be determined as above prescribed in paragraph (3) (a) (ii).

(d) Nitrogen in nitrates in the absence of ammonium salts and organic nitrogen:
One gram of the sample shall be placed in a half-litre Erlenmeyer flask with 50 cubic centimetres of water. 10 grams of reduced iron and 20 cubic centimetres of sulphuric acid of 1·35 specific gravity shall be added. The flask shall be closed with a rubber stopper provided with a thistle tube, the head of which shall be half filled with glass beads. The liquid shall be boiled for five minutes, and the flask shall then be removed from the flame, any liquid that may have accumulated among the beads being rinsed back with water into the flask. The solution shall be boiled for three minutes more, and the beads again washed with a little water. The quantity of ammonia shall then be determined as above prescribed in paragraph (3) (a) (ii).

In cases in which the proportion of nitrates is small a larger quantity of the sample shall be taken.

(e) Control experiment in determination of nitrogen:
The materials used in any of the methods described under this paragraph (3) shall be examined as to their freedom from nitrogen by means of a control experiment carried out under similar conditions with the same quantities of the reagents which have been employed in the actual analysis, in the case of (a) one gram of pure sugar being used in place of the weighed portion of the sample. The quantity of standard acid used in the control experiment shall be deducted from the total quantity of acid found to have been neutralized in the distillation of the sample.

(4) Determination of phosphates.
(a) Phosphates soluble in water:
In the case of superphosphates, dissolved bones, and similar substances, 20 grams of the sample shall be continuously agitated for thirty minutes in a litre flask with 800 cubic centimetres of water. The flask shall then be filled to the mark and again shaken, and the contents shall be filtered. Twenty-five cubic centimetres of the filtrate shall be boiled with 10 cubic centimetres of concentrated nitric acid and the phosphoric acid shall be determined by the molybdate method described below in paragraph (4) (d).

In the case of fertilizers in which the proportion of phosphate soluble in water is small, a larger quantity of the filtrate prepared as above shall be taken.

(b) Phosphates soluble in the prescribed citric acid solution:
Five grams of the sample shall be transferred to a stoppered bottle of about 1 litre capacity. Ten grams of pure crystallized citric acid shall be dissolved in water, the volume shall be made up to 500 cubic centimetres, and the solution shall be added to the weighed portion of the sample in the bottle. To lessen the possibility of caking, the portion of the sample in the bottle may be moistened with 5 cubic centimetres of alcohol or methylated spirit before the citric acid solution is added; and in that case the volume of the citric acid solution shall be 495 cubic centimetres instead of 500 cubic centimetres. The bottle shall be at once fitted into a mechanical shaking apparatus

and shall be continuously agitated during 30 minutes at a temperature of approximately 65° F. The solution shall then be filtered through a large "folded" filter, the whole of the liquid being poured on the paper at once. If not clear, the filtrate shall be again poured through the same paper. Fifty cubic centimetres of the filtrate shall be taken and the phosphoric acid shall be determined by the molybdate method prescribed below in paragraph (4) (d). The precipitation as phospho-molybdate shall be carried out immediately after filtration.

(c) Total phosphoric acid:
A weighed portion of the sample, in which portion, if necessary, the organic matter has been destroyed by ignition and the silica removed by appropriate means, shall be dissolved in nitric acid and boiled, the solution being made up to a definite bulk. The phosphoric acid shall be determined in an aliquot part of the solution by the molybdate method prescribed below in paragraph (4) (d).

(d) Molybdate method:
To the solution, which should preferably contain from 0·1 to 0·2 gram of phosphoric oxide (P₂O₅), obtained as above described in paragraphs (4) (a), (b), or (c), 100 to 150 cubic centimetres of molybdic acid solution prepared as described below, or an excess of such solution—i.e., more than is sufficient to precipitate all the phosphoric oxide present in the solution, shall be added, and the vessel containing the solution shall be placed in a water bath maintained at 70° C., for 15 minutes, or until the solution has reached 70° C. It shall then be taken out of the bath and allowed to cool, and the solution shall be filtered, the phospho-molybdate precipitate being washed several times by decantation and finally on the paper with 1 per cent. nitric acid solution. The filtrate and washings shall be mixed with more molybdic acid solution and allowed to stand for some time in a warm place in order to ascertain that the whole of the phosphoric oxide has been precipitated.

The phospho-molybdate precipitate shall be dissolved in cold 2 per cent. ammonia solution, prepared as described below, and about 100 cubic centimetres of the ammonia solution shall be used for the solution and washings. Fifteen to 20 cubic centimetres of magnesia mixture prepared as described below, or an excess of such mixture, i.e., more than sufficient to precipitate all the phosphoric oxide present, shall then be added drop by drop, with constant stirring. After standing at least 2 hours with occasional stirring, the precipitate shall be filtered off, washed with 2 per cent. ammonia solution, dried, and finally weighed as magnesium pyrophosphate. The filtrate and washings shall be tested by the addition of more magnesia mixture.

(e) Preparation of molybdic acid solution:
The molybdic acid solution shall be prepared as follows: 125 grams of molybdic acid and 100 cubic centimetres of water shall be placed in a litre flask, and the molybdic acid shall be dissolved by the addition, while the flask is shaken, of 300 cubic centimetres of 8 per cent. ammonia solution, prepared as described below. Four hundred grams of ammonium nitrate shall be added, the solution shall be made up to the mark with water, and the whole added to 1 litre of nitric acid. (Sp. Gr. 1·19.) The solution shall be maintained at about 35° C. for twenty-four hours and then filtered.

(f) Preparation of magnesia mixture:
The magnesia mixture shall be prepared as follows: 110 grams of crystallized magnesium chloride and 140 grams of ammonium chloride shall be dissolved in 1,300 cubic centimetres of water. This solution shall be mixed with 700 cubic centimetres of 8 per cent. ammonia solution, and the whole shall be allowed to stand for not less than three days and shall be then filtered.

(g) Preparation of ammonia solutions:
The 8 per cent. ammonia solution shall be prepared as follows: One volume of ammonia solution of Sp. Gr. 0·880 shall be mixed with three volumes of water. This solution shall then be adjusted by the addition thereto of more strong ammonia solution or water as required until the specific gravity of the solution is 0·967.

The 2 per cent. ammonia solution shall be prepared as follows: One volume of 8 per cent. ammonia solution shall be mixed with three volumes of water.