From the two formulas already given we have the relations between the MnO2, FeSO4 and KMnO4 as follows: One atom of Mn in the form of brown precipitate (MnO2) will oxidize two atoms of Fe as ferrous sulfate. Two molecules of permanganate will oxidize ten atoms of Fe as ferrous sulfate, that is to say, two molecules of permanganate will oxidize the same amount of iron as will five molecules of MnO2 containing five atoms of manganese. Knowing the iron value of the KMnO4 and FeSO4 solutions, the manganese values are easily calculated from the proportionMn:2Fe 54.93:2X55.84 = 0.4918. Determination of the MnO2.-To the asbestos and MnO2 in the beaker, add the solution of ferrous sulfate from a burette 5 c.c. at a time until, after stirring and warming, the MnO2 is completely dissolved. It is best to take the same burette used in standardizing. Break up all lumps of asbestos and precipitate with a glass rod, as they may enclose undissolved particles of MnO2. Now add a little water and run in the permanganate solution till a pink color is produced, not disappearing under two or three minutes. Read the burette and deduct the amount used from that to which the amount of ferrous sulfate taken would have been equivalent; the difference is equivalent to the Mn present in the precipitate. This, corrected by the factor for the permanganate solution, will give the amount of Mn in milligrams. As an example: Suppose that 5 c.c. of ferrous sulfate solution equaled 9.6 c.c. of permanganate solution, and 10.3 c.c. permanganate equaled 0.1425 gram of ammonium ferrous sulfate. If 15 c.c. of ferrous sulfate solution were added to dissolve the MnO2 and the permanganate required to oxidize the excess was 4.5 c.c., then the calculation is as follows: 3X9.6 28.8 the permanganate equivalent to the FeSO4 used. 4.5 the "titer back." = 24.3= the number of cubic centimeters of permanganate equivalent to the precipitate. 10.3:10 (x being the true amount of correct permanganate). x = 23.6 = Process for Ores.-Take 5 grams. Dissolve in 50 c.c. of con centrated HCl, evaporate to dryness, avoiding a temperature above 100°; add 20 c.c. HCl, and then water. When dissolved, filter into a No. 2 beaker. Add 50 c.c. concentrated HNO3, evaporate to a syrup, then add 100 c.c. of concentrated HNO3 and proceed as before. Process for Pig Iron.-Dissolve 5 grams of the metal in HN03 (sp. gr. 1.2) taking about 60 c.c. Then add 25 c.c. HCl, evaporate to dryness and bake. Dissolve in HCl, filter from the SiO2, and to the filtrate add 0.2 gram ammonium fluoride or a few drops of hydrochloric acid. Then add 50 c.c. of HNO3. Concentrate to a syrup add 100 c.c. HNO3 and proceed as before. The hydrofluoric acid expels traces of SiO2 from the solution and greatly accelerates the filtration from the MnO2 (E. F. WOOD). The above process, carefully conducted, is capable of giving very accurate results, but it requires practice and should be tried on metals in which the manganese has been carefully determined by another method until the two give concordant results. REFERENCES: Trans. Am. Inst. Min. Eng., 9, 397. Am. J. Sci., 5 (4), 260 J. Anal App. Chem., 2, 249. Process for Materials High in Manganese.-The writer prefers to use a gravimetric method on these materials, as follows: Transfer the washed MnO2 and asbestos mat to a beaker, dissolve the MnO2 with 10 c.c. of HCl in 100 c.c. of water and with the addition of enough H2O2 to completely dissolve the MnO2, boil until the H2O2 is completely destroyed, add a few drops of methyl red and then ammonia until the color turns yellow. Boil for two minutes, filter off the asbestos and small amount of iron. which has precipitated and wash with 1 per cent of ammonium chloride solution. Add bromine to the filtrate, heat to boiling, filter off the MnO2 and wash with hot water 10 times. Wash the precipitate into a weighed porcelain or silica dish, burn the filter paper and add the ash to the dish, add 10 c.c. of 1:3 H2SO4 and H2O2 until the MnO2 all dissolves, evaporate to dryness and finally ignite to constant weight in a muffle just below red temperature, cool and weigh. The increase in weight is manganese sulfate (MnSO4) containing 0.3638 manganese. It has been shown by Blum and also by Huber that the method of weighing manganese as manganese sulfate is a method of high precision when care is taken to ignite at a temperature of 500 to 550°C. REFERENCE: HUBER, Z. anal. Chem., 61 (1922), 103–107. The Knorre Method for Manganese. This method depends upon the precipitation of manganese as MnO2 from an H2SO4 solution by means of (NH4)2S2Os and avoids the unpleasant fumes produced by the chlorate method. As in all methods depending upon the precipitation of MnO2, the presence of cobalt interferes as the cobalt precipitates with the manganese. Process of Analysis.-Proceed exactly as directed in the Volhard process until the fumes of sulfuric acid are obtained. Dilute with water to 200 c.c., add H2SO4 if necessary to make a 5 per cent solution, add 10 grams of ammonium persulfate and boil 15 minutes. Cool, add 5 grams more and boil five minutes. Filter off the MnO2, wash and determine the manganese as previously directed either volumetrically or gravimetrically. REFERENCE: NICOLARDOT, Ann. Chem. Anal. Chem. Appl. 4 (1922), 102–110; c.f. C. A. 16, 1719. VOLHARD'S PROCESS FOR MANGANESE This is a volumetric process depending upon the reactions between potassium permanganate and manganous salts by which all the manganese is precipitated as MnO2. 3MnSO4 +2KMnO4 + 2H2O = 5MnO2 + 2KHSO4 + H2SO4. The method is especially applicable to high manganese ores. For low manganese ores and for iron and steels, the bismuthate-arsenite method is best. The solution must be neutral or very nearly so. The titration must be made with the solution nearly at the boiling point and very dilute or the precipitate will not settle quickly and it will be difficult to see the end reaction. For the same reason the solution must be vigorously shaken or stirred and the permanganate added rather slowly, especially at first. Too rapid addition of the permanganate will cause the MnO2 to precipitate on the glass, forming a firmly adherent yellow-brown stain which makes it very difficult to see the final pink color produced by the excess of permanganate when the titration is finished. The permanganate solution used in the iron assay will serve for manganese. If 1 c.c. equals 0.01 Fe, then 1 c.c. will equal 0.002951 Mn. In this process the iron is separated from the manganese by means of zinc oxide. ZnO precipitates the iron completely as hydroxide from a dilute solution of ferric sulfate, while manganese sulfate is not affected and remains in the solution. Sufficient ZnO must be added to neutralize any free sulfuric acid present as well as to precipitate the iron. The separation of the iron is really due to hydrolysis, the ZnO serving to keep the solution neutral. The reaction is: Fe2(SO4)3 + 3ZnO + 6HOH = 2Fe(OH)3 + 3ZnSƆ1 + 3H2O. The ZnO must not contain any alkali, as Na2CO3, as this would precipitate manganese. Process for Ores.-Weigh 1.00 gram of the sample; ignite it to redness if carbonaceous matter is present; then transfer it to a casserole and digest with 15 c.c. of concentrated HCl. If any ferrous iron is present, add a few small crystals of KClO3 to oxidize it to the ferric state. Finally, boil until all free chlorine is expelled. Now add 10 c.c. of dilute H2SO4 (1:1) and evaporate till fumes of H2SO4 begin to come off, then cool the casserole and add 75 c.c. of water. Warm till all of the ferric sulfate goes into solution and then transfer to a 500-c.c. graduated flask. Add a saturated solution of Na2CO3 to the contents of the flask until a slight precipitate appears which is dissolved in a few drops of dilute H2SO4. Add 10 grams of ZnSO4, heat to boiling and add 1 gram of ZnO. Cool, dilute to the 500-c.c. mark, mix and decant off 200 c.c. Heat to boiling and titrate with standard KMnO4, with vigorous shaking and with the liquid kept at boiling. When a permanganate color appears, cool somewhat and add 1 to 2 c.c. of glacial acetic acid when the color will disappear. Now finish the titration in the hot but not boiling solution. The permanganate color should persist after the solution has been shaken several times. The color will then persist for one to two hours. The potassium permanganate used should be carefully standardized against an ore of known manganese content, or as follows: Dissolve 0.5 gram of Armco iron, the manganese in which has been determined by the bismuthate method, and treat the sample as directed above. After the sulfuric acid solution has been evaporated to fumes and diluted, add 0.400 gram of MnSO4 obtained as directed on page 85, then proceed as directed in the Volhard method. If the iron value of the permanganate is used to calculate the manganese value, the following calculations must be used: The permanganate when used to titrate iron in acid solution undergoes a valence change of 5, but when used to titrate manganese in a neutral or slightly acid solution, undergoes a valence change of 3, hence its oxidizing strength is only three-fifth as great as when used for titrating iron. The manganese that is being titrated. undergoes a valence change of 2, hence Mn = 2Fe, and the final proportion used will be Mn: 2Fe::x:3% iron value of permanganate, or the manganese value equals 0.2951 times the iron. value of the permanganate. Notes on the Process. Where the amount of manganese in the sample is less than 0.7 or 0.8 per cent, the process is not satisfactory, unless more of the sample is taken, since the precipitate will not clot and settle properly if there are less than 5 or 6 mg. of manganese in the liquid. Therefore, in applying the process to ores or metals quite low in manganese, enough sample must be weighed out that there shall be at least this amount of manganese in the liquid titrated. The bismuthate method is much better for low manganese samples. As more than a trace of free mineral acid interferes with the titration, the amount of HNO3 added must not exceed that indicated or the precipitate will settle badly and the end point be indistinct. Evaporation with H2SO4 is necessary to destroy the carbonaceous matter as well as to expel the HNO3, both of which may affect the titration. Instead of taking 1 gram for the analysis, it is usually more convenient to take such an amount of the sample as will make 1 c.c. of the permanganate equivalent to 1 per cent of manganese. This amount can be calculated from the iron standard of the permanganate. Thus, if 1 c.c. of the KMnO4 equals 0.01 Fe, take 0.5902 gram of the sample for the process, then the half of the solution taken for the titration will contain 0.2951 gram of the sample and each cubic centimeter of permanganate used will obviously represent 1 per cent of Mn. |