=112. Variation of Dudley and Noyes.=—The method of Emmerton to determine small quantities of phosphoric acid, or of phosphorus in presence of a large excess of iron, has been modified by Dudley and Pease,[93] and by Noyes and Royse.[94] As modified, the method is not intended for fertilizer analysis, but the principle on which it rests may some time, with proper modifications, find application in fertilizer work. The reduction is accomplished in a Jones’ tube,[95] much simplified, so as to render it suitable for common use. The molybdic acid is reduced to a form, or series of forms, corresponding to molybdenum sesquioxid, as in the Emmerton method, and subsequently as in that method, titrated by a set solution of potassium permanganate. The iron or steel filings, containing phosphorus, are brought into solution by means of nitric acid. For this purpose two grams of them are placed in a half liter flask together with fifty cubic centimeters of nitric acid of 1.18 specific gravity. The mixture is boiled for one minute, and ten cubic centimeters of permanganate solution of one and a quarter per cent added. Boil again until the pink color disappears. Ferrous sulfate solution is next to be carefully added, shaking meanwhile, until the solution clears. Cool to 50° and add eight cubic centimeters of ammonia of 0.90 specific gravity, stopper the flask, and shake until any precipitate which may form is redissolved. Cool or warm, as the case may be, until the solution is as many degrees above or below 60° as the molybdic solution is above or below 27°. Add sixty cubic centimeters of molybdic solution, stopper, and shake on a machine or by hand for five minutes. After remaining at rest for five minutes pour into a nine centimeter filter of fine texture and wash with the acid ammonium sulfate solution in quantities of from five to ten cubic centimeters each time. The filtrate and washings must be perfectly bright. Continue the washings until the filtrate gives no color with hydrogen sulfid. Dissolve the yellow precipitate with twelve cubic centimeters of 0.96 ammonia diluted with an equal volume of water, and wash the filter with 100 cubic centimeters of water. Finally add to the filtrate and wash-water eighty cubic centimeters of water and ten of strong sulfuric acid. Pass the mixture through the Jones’ reducing tube and follow it with 200 cubic centimeters of water, taking care that no air enter the tube during the operations. The solution collected in the flask should be at once titrated with potassium permanganate. _Solutions used_: (1) _Nitric acid._—One part of nitric acid of 1.42 specific gravity and two parts of water by volume. The specific gravity of the mixture is about 1.18. (2) _Permanganate solution for oxidizing._—Dissolve 12.5 grams of potassium permanganate in one liter of water. (3) _Ferrous sulfate._—Fresh crystals not effervesced and free from phosphorus.