The phosphoric acid is determined in one-half of the filtrate and in the remaining half the iron is reduced with zinc and determined with potassium permanganate in the usual way. The phosphoric acid and iron having been thus determined the alumina is estimated by difference. The chief objection to this process is in the excessive quantity of acetic acid used and the danger of solution of the precipitated phosphates caused thereby.
43. Estimation of the Lime and Magnesia.—The filtrate and washings from the first precipitation, ([paragraph 41],) of iron and alumina in the method of Krug and McElroy, above described, are collected and sufficient ammonium oxalate is added to precipitate the calcium. The precipitated calcium is very fine and should be collected on a gooch, under pressure. The filtrate and washings from the calcium precipitate are again collected, and a solution of sodium phosphate added to precipitate the magnesia. The solution must be kept cool and slightly alkaline with ammonia during the above operations in order to prevent the separation of molybdic anhydrid.
44. Estimation of Sulfuric Acid.—As a rule, sulfates are not abundant in mineral phosphates. In case the samples are pyritiferous, however, considerable quantities of sulfuric acid may be found after treatment with aqua regia.
The acid is precipitated with barium chlorid, in the usual way, in an aliquot portion of the filtrate first obtained. The precipitate of barium sulfate is washed with hot water until clean, dried, ignited, and weighed. If the portion of the filtrate taken represent half a gram of the original material then the weight of barium sulfate obtained multiplied by 0.6858 will give the quantity of sulfur trioxid in one gram.
45. Estimation of Fluorin by the Method of Berzelius as Modified by Chatard.—The method of estimating fluorin as proposed by Berzelius, has been found quite satisfactory in the laboratory of the Geological Survey, with the modifications given below.[34]
Two grams of the phosphate are intimately mixed in a large platinum crucible with three grams of precipitated silica and twelve grams of pure sodium carbonate, and the mixture is gradually brought to clear fusion over the blast-lamp. When the fusion is complete the melt is spread over the walls of the crucible, which is then rapidly cooled (preferably by a blast of air). If this have been properly done, the mass separates easily from the crucible, and the subsequent leaching is hastened. The mass, detached from the crucible, is put into a platinum dish into which whatever remains adhering to the crucible, or its lid, is also washed with hot water. A reasonable amount of hot water is now put into the dish, which is covered and digested on the water-bath until the mass is thoroughly disintegrated. To hasten this, the supernatant liquid may, after awhile, be poured off, the residue being washed into a small porcelain mortar, ground up, returned to the dish and boiled with fresh water until no hard grains are left. The total liquid is then filtered, and the residue is washed with hot water. The filtrate (which should amount to about half a liter) is nearly neutralized with nitric acid (methyl orange being used as indicator), some pure sodium bicarbonate is at once added, and the solution (in a platinum dish, if one large enough is at disposal, otherwise in a beaker) is placed on the water-bath, when it speedily becomes turbid through separation of silica. As soon as the solution is warm it is removed from the bath, stirred, allowed to stand for two or three hours, and then filtered by means of the filter-pump and washed with cold water.
The filtrate is concentrated to about a quarter of a liter and nearly neutralized, as before, some sodium carbonate is added, and the phosphoric acid is precipitated with silver nitrate in excess. The precipitate is separated by filtration and washed with hot water, and the excess of silver in the filtrate is removed with sodium chlorid.
The filtrate from the silver chlorid (after addition of some sodium bicarbonate) is evaporated to its crystallizing point, then cooled and diluted with cold water; still more sodium bicarbonate is added, and the whole is allowed to stand, when additional silica will separate, and this is to be removed by filtration.
This final solution is nearly neutralized, as before; a little sodium carbonate solution is added; it is heated to boiling and an excess of solution of calcium chlorid is added. The precipitate of calcium fluorid and carbonate must be boiled for a few minutes, when it can be easily filtered and washed with hot water. The precipitate is then washed from the filter into a small platinum dish and evaporated to dryness, while the filter, after being partially dried and used to wipe off any particles of the precipitate adhering to the dish in which it was formed, is burned, and the ash is added to the main precipitate. This, when dry, is ignited, and allowed to cool; dilute acetic acid is added in excess, and the whole is evaporated to dryness, being kept on the water-bath until all odor of acetic acid has disappeared. The residue is then treated with hot water, digested, filtered on a small filter, washed with hot water, partially dried, put into a crucible, carefully ignited, and weighed as calcium fluorid. The calcium fluorid is then dissolved in sulfuric acid by gentle heating and agitation, evaporated to dryness on a radiator, ignited at full red heat, and weighed as calcium sulfate. From this weight the equivalent weight of calcium fluorid should be calculated, and this should be very close to that actually found as above, but should never exceed it. The difference, which is generally about a milligram (sometimes more), is due to silica precipitated with the fluorid. The percentage of fluorin is, therefore, always calculated from the weight of the sulfate, and not from that of the fluorid obtained.
The main improvements in this method are the use of sodium bicarbonate to separate the silica, and the keeping of the earlier solutions as dilute as possible, which can not be done, if ammonium carbonate be used for the separation of the silica. These changes make the fluorin estimation, although still tedious, far more rapid than before, and the results are very satisfactory.