Principles and practice of agricultural analysis. Volume 2 (of 3), Fertilizers

by Harvey Washington Wiley

(6) Bulbed U tubes or Will’s bulbs, as described under the soda-lime method:

Manipulation.—(a) Clean the U tube and introduce ten cubic centimeters of standard acid.

(b) Fit the cork and glass connecting tube. Fill the tube as follows: (1) A loosely fitting plug of asbestos, previously ignited, and then two and five-tenths to three and five-tenths centimeters of the thiosulfate mixture: (2) The weighed portion of the substance to be analyzed is intimately mixed with from five to ten grams of the sugar and sulfur mixture: (3) Pour on a piece of glazed paper or in a porcelain mortar a sufficient quantity of thiosulfate mixture to fill about twenty-five centimeters of the tube; then add the substance to be analyzed, as previously prepared, mix carefully, and pour into the tube; shake down the contents of the tube; rinse off the paper or mortar with a small quantity of the thiosulfate mixture and pour into the tube; then fill up with soda-lime to within five centimeters of the end of the tube: (4) Place another plug of ignited asbestos at the end of the tube and close with a cork: (5) Hold the tube in a horizontal position and tap on the table until there is a gas-channel along the top of the tube: (6) Make connection with the U tube containing the acid; aspirate and see that the apparatus is tight.

The Combustion.—Place the prepared combustion tube in the furnace, letting the open end project a little, so as not to burn the cork. Commence by heating the soda-lime portion until it is brought to a full red heat. Then turn on slowly jet after jet toward the outer end of the tube, so that the bubbles come off two or three a second. When the whole tube is red hot and the evolution of the gas has ceased and the liquid in the U tube begins to recede toward the furnace, attach the aspirator to the other limb of the U tube, break off the end of the tube, and draw a current of air through for a few minutes. Detach the U tube and wash the contents into a beaker or porcelain dish; add a few drops of the cochineal solution, and titrate.

176. Observations.—In our experience we have found it much more satisfactory to adhere to the earlier directions for preparing the mixture of thiosulfate and alkali. We much prefer to make the mixture with soda-lime and without the previous drying of the sodium salt. Ruffle himself says that the sodium thiosulfate should be dry but not deprived of its water of crystallization.[146] The best method to dry the crystal powder without depriving it of its crystal water is to press it between blotting papers. The official method also contains a typographical error in prescribing that the combustion tube should have a length of thirty centimeters where evidently thirty inches were meant. Ruffle’s original tube was twenty-two inches in length.

As is seen from the above description the method is essentially a reduction process by the action of a powerful deoxidizer in the presence of an alkali. The crystals of the thiosulfate salt cannot be brought into direct contact with a pure alkali, like soda or potash, without forming at once a wet mass which would tend to cake and obstruct the tube. The soda-lime is therefore a mechanical device to prevent this fusion. Where many analyses are to be made an iron tube, for economical reasons, may be substituted for the glass; but the glass tube permits a more intelligent observation of the progress of the analysis.

Since charcoal has very high absorbent powers it will be found always to contain a little nitrogen which may be in a form to generate ammonia during the combustion. The charcoal used should therefore be previously boiled with caustic soda or potash solution, dried, powdered, and preserved in well-stoppered bottles. Although pure sugar is practically free of nitrogen, even when it is used, it is advisable to occasionally make a blank determination and thus ascertain the correction to be made for possible contamination.

177. Boyer’s Modification of Ruffle’s Method.—The principle of the method rests on the observation that if nitrates be heated in a combustion tube with calcium oxalate and soda-lime, not more than two-thirds of the total nitrogen appear as ammonia; but if a certain proportion of sulfur be added the whole of the nitrogen is recovered.[23] The process may be divided into two reactions; viz.:

(1) Action of the calcium oxalate upon the sodium nitrate in presence of soda-lime:

(2) The action of sulfurous acid and of calcium oxalate upon the sodium nitrate in presence of soda-lime.