For steels low in carbon, the Volhard method has been modified by omitting the evaporation with H2SO4 and not filtering from the precipitate of ferric hydroxide produced by the ZnO, but instead simply decanting an aliquot part of the somewhat turbid liquid and titrating directly in the presence of the nitrates. In this case the permanganate should be standardized on a steel of similar kind in which the manganese has been determined gravimetrically. High carbon steel should be evaporated to dryness with HNO3 and baked. REFERENCES: STONE, J. Am. Chem. Soc. (1896), 228. ORTHEY, Z. anal. Chem., 47, 547-560. THE BISMUTHATE-ARSENITE METHOD FOR MANGANESE This method for manganese, when it is present in moderate amounts (up to 2.5 per cent), is, in the writer's opinion, the most accurate known. No elements interfere if the process is properly carried out. The delicacy of the reaction between manganese and bismuthate in HNO3 is remarkable, as little as 0.00001 gram in 50 c.c. can be easily detected. HCl must be absent, as it is ruinous to the accuracy of the results. The method is based on the fact that in a cold HNO3 solution of the proper strength manganese is oxidized to permanganic acid by the bismuthate. This is very stable in cold HNO3 (sp. gr. 1.135) but in hot solution the excess of bismuthate is decomposed and dissolved and then the permanganate is destroyed. In the cold, however, the excess of bismuthate may be filtered off and the permanganate titrated by a standard reducing agent. The reactions may be written: 5Bi20+2Mn(NO3)2 + 26HNO3 = 10Bi(NO3)3 + 2HMnO4 + 12H2O. 2HMnO4 + 5NaзASO3 + 4HNO3 = 5Na3ASO1 + 2Mn(NO3)2 + 3H2O. Process for Iron and Steel.-If the sample does not contain over 1 per cent Mn, use a 1-gram sample; if 1 to 2 per cent, use a 0.5-gram sample. Dissolve in 45 c.c. of water and 15 c.c. of HNO3 (sp. gr. 1.42) in a 150-c.c. flask. When dissolved boil until nitrous fumes are gone. Set the flask off the hot plate and cool a moment, then add 0.25 gram of bismuthate and shake and continue adding the bismuthate in 0.25-gram lots until a perman ganate color comes which persists after a few minutes' boiling. This indicates complete oxidation of the solution. On boiling, the permanganic acid is gradually decomposed to MnO2. Now add a few small crystals of KNO2 to dissolve the MnO2 and boil the solution several minutes to expel nitrous fumes. A little Na2CO3 added now will aid the expulsion of the fumes. Add water to bring the volume up to its original volume and cool to tap-water temperature. When cold, add 0.5 gram of bismuthate and shake the flask well. Add 20 c.c. of water and again shake, then filter through asbestos, preferably after settling, and wash several times with distilled water. Titrate the filtrate with standard arsenite solution. If the manganese is high, a little MnO2 may appear at the end but this does no harm, as the titration is continued until the brown MnO2 disappears. This only takes a drop or two after the pink disappears. Arsenite Solution.-Add to 0.908 gram pure As2O, in a beaker, hot Na2CO3 solution until the As2O; all dissolves, then dilute to a liter. One cubic centimeter should be equal to 0.0002 gram Mn. To standardize, treat a sample of steel with known Mn content as above described. An equally good method of standardization is as follows: Dissolve 6 grams of KMnO4 in 1 liter, heat and pass H2S through the solution until all the manganese is precipitated as MnS, filter and wash 20 times with H2S water. Burn the paper and precipitate at a low temperature in a platinum crucible, add H2SO4 until the residue is well moistened, and very gradually raise the temperature of the crucible in a muffle until the H2SO4 is all gone and the temperature remains at 500°C. for 10 minutes, cool the crucible and weigh the MnSO4. Dissolve the MnSO4 in a sufficient amount of 1 per cent H2SO4 so that 1 c.c. contains 0.00020 gram manganese. MnSO, contains 0.3638 manganese. Dissolve 0.5 gram of Armco iron or other pure iron in which the manganese has been determined, add 25 c.c. of the solution above prepared and determine the manganese as directed. It is not possible to make an accurate check against KMnO4 directly, owing to its variable composition. Notes on the Process.-It is necessary to add the bismuthate to the hot HNO3 solution until all carbon, sulfur, etc. are oxidized as well as the manganese. Otherwise results will run low. This complete oxidation is indicated when the MnO2, formed when the HMnO, is decomposed by boiling, remains after boiling a minute or so. Chromium is partly, and vanadium completely, oxidized to the higher forms H2CrO4 and HзVO, by the bismuthate in cold solution. These do not harm if the titration is carried out as above directed. If, however, titration is carried out by adding an excess of arsenite or other reducing agent and then oxidizing back to standard KMnO4, the results will be high if chromium or vanadium are present, as the chromic acid and vanadic acid will be reduced by the arsenite. The titration should be carried out by adding the arsenite until the permanganate color just disappears. The best material for filtering off the Bi2O4 is asbestos and glass wool made as follows: Shake together in a flask, with water, sufficient glass wool to fill a funnel one-fourth full and about the same amount of pure asbestos fiber. Pour into a funnel and wash out all HCl. This filtering medium does not easily stop up and yet filters perfectly. It may be used for a large number of filtrations without changing. When pig irons are being analyzed they should be filtered when dissolved in order to remove the graphite. In the analysis of white irons it will be necessary to treat the solution several times with bismuthate in order to oxidize the large amount of combined carbon present. The solution should be nearly colorless when cold. Process for Iron Ores.-Treat 1 gram in a beaker with 20 c.c. HCl until all iron is in solution. Add 4 c.c. H2SO4 and evaporate until the H2SO4 fumes freely, taking particular care that all the HCl is removed. Cool and dissolve in 45 c.c. of water and 15 c.c. of HNO3, filter and proceed as in the case of steels. It may be necessary to examine the residue for manganese. For manganese ores proceed as above except to dilute to the mark in a calibrated flask, shake well and take an aliquot part equal to 1 per cent of manganese on a 1-gram sample. However, the Volhard process is more especially adapted to high manganese ores. REFERENCES: BLAIR, "Analysis of Iron and Steel," 7th ed., p. 121. DEMOREST, "The Bismuthate Method for Manganese," J. Ind. Eng. BLUM, J. Am. Chem. Soc., 26, 793. COLOR METHOD USING AMMONIUM PERSULFATE This method is due to Marshal and Walters. It depends upon the act that if ammonium persulfate (NH4)2S2O, is added to a solution of manganese in dilute nitric or sulfuric acid, it will, on warming, promptly and completely oxidize the manganese to permanganic acid, provided a small amount of silver nitrate is present. The silver salt is essential, for, if not present, the manganese will be precipitated as MnO2. If too much silver salt is present, silver peroxide will precipitate and make the solution muddy. The solution should not be boiled, but merely warmed until the color develops. It is said that the persulfate should be slightly damp, but we have used this salt dried in a desiccator with satisfactory results. Walter's Process for Steel.-Dissolve 0.1 or 0.2 gram of the steel, according to the percentage of manganese, in 10 c.c. of HNO3 (sp. gr. 1.2). Heat until all nitrous fumes are driven off. Now add 15 c.c. of a solution of silver nitrate containing 1.33 gram of the salt to 1 liter of water. This will cool the solution considerably. Now immediately add about 1 gram of ammonium persulfate, and warm until the color commences to develop, and then for about a half minute longer. Remove from the heat and set it in cold water while the evolution of oxygen continues. As soon as the solution is cool, compare it with a solution of standard steel treated in the same manner. The solution of the standard steel may be prepared in quantity, as noted before, by dissolving several grams of the metal in a sufficient amount of nitric acid (sp. gr. 1.2) and diluting with the same strength acid until 10 c.c. of the solution contain 0.2 gram of steel. Ten cubic centimeters of this solution are then used with each set of determinations. REFERENCE: Chemical News, Feb. 15 and Nov. 15, 1911. Persulfate-arsenite Method. This method has the advantage of requiring no filtrations as the bismuthate method does. Proceed as above directed for the persulfate method until the ammonium persulfate has oxidized the manganese. Cool, add 10 c.c. of a 2.5 per cent NaCl solution, stir and titrate with sodium arsenite as usual. The sodium chloride is added to precipitate the silver, otherwise the excess persulfate will make results for manganese run high. CHAPTER VII THE DETERMINATION OF SULFUR Sulfur occurs in iron ores as sulfides, such as pyrite (FeS2) and sphalerite (ZnS) and also as sulfates, such as gypsum (CaSO4, 2H2O), barite (BaSO4) and celestite (SrSO1). In iron and steel it occurs as sulfide only. In the gravimetric methods for determining sulfur it is first converted into sulfuric acid and then the sulfuric acid precipitated by barium chloride and weighed as barium sulfate. DIRECT OXIDATION METHODS Conversion of the Sulfides to Sulfates. All sulfides are completely oxidized to sulfates when fused with a mixture of dry Na2CO3 and NaNO3 or with Na2O2. As free sulfur and certain disulfides give off sulfur vapor at a comparatively low temperature (below the fusing point of Na2CO3), when these are present care must be taken to prevent loss by the escape of this vapor. The mixture of ore and flux must be covered with a layer of the pure "fusion mixture" and heated carefully. After fusion all the sulfur, whether originally present as sulfide or sulfate (even in BaSO4), will be found as Na2SO4, the bases present remaining as oxides or carbonates. When the fused mass is boiled with water till thoroughly disintegrated and then filtered off and washed, the sulfate all passes into the filtrate. Sulfides can be more or less completely oxidized to sulfates in the "wet way" by treating them with hot concentrated HNO3 or aqua regia. Wet methods are not very satisfactory, as free sulfur is liable to separate and fuse to globules, its melting point being below the boiling point of HNO3. Once in this form it is very slowly oxidized by boiling with ordinary oxidizing agents. Iron sulfide can be completely oxidized, however, by heating with a large excess of concentrated HNO3 and adding a little powdered KCIO3. When iron sulfide, or even iron containing but little sulfur, is dissolved in dilute HNO3 (1.2 sp. gr.) a considerable proportion of the sulfur separates as such and escapes oxidation. Solutions containing ferric sulfate, on evaporation to dryness and "baking," as is common in iron analysis, may lose SO3 unless enough |