Wagner proposes to neutralize the basicity of a slag in the following manner:[66] Five grams of the slag are placed in a half liter flask which is then filled up to the mark with a one per cent solution of citric acid and shaken for half an hour. After filtering, fifty cubic centimeters are titrated with a standard soda solution using phenolphthalein as indicator. This gives the quantity of citric acid necessary to neutralize the slag. To a second portion of five grams of the sample in a half liter flask are added 200 cubic centimeters of water and enough five per cent citric acid solution to neutralize the lime and then 200 cubic centimeters of acid ammonium citrate made as indicated below. After filling to the mark with water it is shaken for half an hour and filtered. To fifty cubic centimeters of the filtrate are added 100 cubic centimeters of molybdic solution and heated to 80°. After cooling, the precipitate is filtered and the phosphoric acid estimated in the usual way.

The acid ammonium citrate solution used is made as follows: Dissolve 160 grams of citric acid with enough ammonia to represent about twenty-eight grams of nitrogen and make up with water to one liter. The exact method is given in 82.

The molybdic solution is made by dissolving 125 grams of molybdic acid in a slight excess of two and a half per cent of ammonia, adding 400 grams of ammonium nitrate, diluting to one liter and pouring the solution into one liter of nitric acid having a specific gravity of 1.19. After allowing to stand at room temperature for one day the mixture is filtered and is then ready for use.

81. Wagner’s Shaking Apparatus.—The latest directions given by Wagner for determining the phosphoric acid in slags and raw phosphates soluble in citrate solutions, are the following:[67] Five grams of the material as it is sent into commerce without grinding or sifting, are placed in a half liter flask and covered with nearly a quarter liter of water, and then 200 cubic centimeters of citrate solution added, prepared as described below. The flask is filled to the mark with water. The flasks, which are of the shape shown in the figure, are closed with rubber stoppers, and without delay placed for half an hour in a rotating apparatus, ([Fig. 6]) which is turned on its axis from thirty to forty times a minute. If a shaking apparatus be used instead of the one mentioned, 200 cubic centimeters of the citrate solution should be placed in a half liter flask, filled to the mark with water, and the contents poured into a liter flask containing the phosphate. This flask should be placed in a nearly horizontal position in the apparatus and the agitation be continued for half an hour. On removal from the apparatus the mixture is filtered and fifty cubic centimeters thereof treated with double the quantity of molybdic solution at 80° and the precipitate separated after cooling. The precipitate is carefully washed with one per cent nitric acid mixture, after which the filter is broken and the precipitate washed into a beaker with two per cent ammonia and the filter washed therewith until about 100 cubic centimeters have been used. If the solution is turbid from the presence of silicic acid it should be precipitated a second time by addition of molybdic solution until the acid reaction is restored. The ammoniacal solution of the yellow precipitate is treated, drop by drop, with constant stirring, with fifteen cubic centimeters of magnesia mixture, and set aside for two hours. The precipitate is collected, washed, ignited, and weighed in the usual manner. The direct precipitation of the phosphoric acid by the magnesia solution in presence of citrate is not advisable because of the almost general presence of silicic acid which would cause the results to be too high.

Figure. 6.

Wagner’s Digestion Apparatus for Slags.

The chief objection to this method of Wagner lies in the failure to control the temperature at which the digestion with citrate solution is made. Huston has shown, as will be described further on, that the temperature exercises a great influence in digestion with citrate. Since the laboratory temperature, especially in this country, may vary between 10° and 35°, it is evident that on the same sample the Wagner method would give very discordant results at different seasons of the year.

82. Solutions Employed in the Wagner Method.—1. Ammonium Citrate.—In one liter there should be exactly 150 grams of citric acid and 27.93 grams of ammonia, equivalent to twenty-three grams of nitrogen. The following example illustrates the preparation of ten liters of the solution: In two liters of water and three and a half liters of eight per cent ammonia, 1,500 grams of citric acid are dissolved and the cooled solution made up exactly to eight liters. Dilute twenty-five cubic centimeters of this solution to 250 cubic centimeters and treat twenty-five cubic centimeters of this with three grams of calcined magnesia and distill into forty cubic centimeters of half normal sulfuric acid. Suppose the ammonia nitrogen found correspond to twenty cubic centimeters of fourth normal soda-lye. Then in the eight liters are contained