Using these corrections the estimation of phosphoric acid becomes one of the most accurate of known analytical methods. =61. The Color of the Magnesium Pyrophosphate.=—After the final ignition of the magnesium pyrophosphate, whether secured by the citrate or the molybdic method, a black or grayish tint is often noticed. This may be due to traces of organic matter brought down by the precipitate and especially to a lack of care in the initial ignition. Many devices have been proposed for the purpose of avoiding this coloration, although general experiments have shown that there is no appreciable increase in the weight of the precipitate when colored in this way. When the precipitation is carried on according to the citrate method, Neubauer[47] proposes to eliminate this coloration by the use of ammonium sulfate. About seven cubic centimeters of a saturated solution of ammonium sulfate should be added to the solution before the precipitation by the magnesium mixture. With this precaution it is possible to obtain a perfectly white precipitate after five minutes of ignition. The lively glowing of the precipitate throughout the whole mass at the time of changing into pyrophosphate, is much more easily observed by this treatment than when the mass is gray or black. Even should the addition of the ammonium sulfate solution to one containing a large amount of lime produce a precipitate of crystalline calcium sulfate, it is of no importance inasmuch as the ammonium citrate immediately dissolves large quantities of the calcium salt. In this laboratory a white pyrophosphate is easily obtained by treating the precipitate on the gooch after washing free of chlorids with a drop or two of ammonium nitrate. The ignition is commenced very gently at first and afterwards when the mass is white the blast is used. If the ignited residue be gray it may sometimes be whitened by moistening with a drop or two of nitric acid, burning at a very low temperature, followed by the blast. There is no appreciable difference in weight between a gray and white pyrophosphate. =62. Determination of Phosphoric Acid and Nitrogen in the Same Solution by Treatment with Sulfuric Acid and Mercury.=—Fertilizing materials which contain organic nitrogen and phosphoric acid, such as bones, are of such a nature that it is often difficult to obtain a fair sample of them in quantities suited to the direct determination; _viz._, about one gram. Thus it often becomes important to take a much larger quantity of the material, to bring it into solution and to take an aliquot part thereof. It may also often happen that it is important to determine the phosphoric acid in the same sample which has been used for the determination of the nitrogen by moist combustion with sulfuric acid and mercury. In this connection, however, it is somewhat difficult to avoid the precipitation of some of the mercury with the phosphoric acid.