Manganese and Chromium in Steel Modified Persulfate-Arsenite Method R. P. FORSYTH AND W. F. BARFOOT, Chamber of Mines Steel Products Ltd., Germiston, South Africa
T
were obtained on Ridsdale steels “J” and “V”, using the procedure described below; chromium was introduced where necessary by means of bichromate:
HE persulfate-arsenite method for manganese is widely used in routine analysis of steel, since it is rapid, economical, and clean as compared with the alternative bismuthate method. I n solutions as normally used, arsenite reduces septivalent manganese to more or less midway between the trivalent and quadrivalent state. Consequently the method suffers in reliability, since standardization is empirical and varies with the operator, and the end point is indefinite, particularly in the presence of sexivalent chromium. Lang (3) found that the use of catalysts such as potassium iodide and iodate causes the reduction of permanganic acid by arsenite to proceed quantitatively to the bivalent condition in sulfuric or hydrochloric acid solutions. Bright (1) confirmed this and showed that arsenious oxide could be used as a direct primary standard in permanganimetry. Gleu (2) proposed the use of a 0.01 M solution of osmium tetroxide as catalyst, and Park (4) investigated and found satisfactory the use of this catalyst for evaluating manganese in steel by the bismuthate procedure, using arsenite as reducing agent. In the present work it has been shown that osmium tetroxide is a suitable catalyst for the persulfate-arsenite method for manganese in steel. Potassium iodide and iodate were ineffective in the same solutions, because of precipitation by silver ions; but they were successful after precipitation of silver as chloride, provided excess hydrochloric acid was present in concentrations above 1 ml. of hydrochloric acid (1to 2) per 150 ml. of solution. However, in the presence of these catalysts chromium is reduced from the sexivalent to the trivalent state. Chromium, if present, must be evaluated separately and manganese found by deduction. Using the routine persulfate method on Ridsdale standard steels, arsenite with osmium tetroxide as catalyst gave high results for manganese. This was found to be due mainly to a direct oxidation of arsenite by undecomposed persulfate, and not to the reoxidation of manganese by persulfate. This direct oxidizing effect in equivalents of manganese was 0.17, 0.14, and 0.07 mg. for persulfate concentrations of 2.0, 1.0, and 0.20 gram per 150 ml., respectively. For accurate work by this method, almost complete decomposition of persulfate is necessary, and this means longer boiling periods. To stabilize permanganic acid throughout this period, the following reagent concentrations (per 100 ml. of solution) were found necessary and sufficient within the usual range of manganese concentrations : Sulfurio soid (SP.gr. 1.84) Phoaphorio mid (sp. gr. 1.75) Persulfate Silver nitrate
5 2 2 50
Standard Steel
Constituents Present Cr V
Mn
Mg. J
Mg.
7.68 Nil Nil 7.68 3.54 Nil V 5.42 8.61 2.73 Average of three determinations.
J
0
Mg.
Constituents Founds Cr V Mg. Mg. 7.69 Nil Nil 7.71 3.51 Nil 5.32 8.50 2.74
Mn
Mg.
Manganese and Chromium on One Sample SOLUTIONS REQUIRED.Silver nitrate, 10 grams dissolved in 1 liter of distilled water. Ammonium persulfate, 250 grams dissolved in water, filtered if necessary, and made up to 500 ml. Sulfuric-phosphoric acid. Pour 1 liter of sulfuric acid into 6 liters of distilled water, and add 300 ml. of phosphoric acid (sp. gr. 1.75). Sodium arsenite solution, 0.0316 N . Dissolve 7.8 grams of pure arsenious oxide in 500 ml. of distilled water and 50 ml. of sodium hydroxide (1 pound per liter). When completely in solution dilute to 5 liters with distilled water. Standardize against sodium oxalate through permanganate. PROCEDURE. Transfer 1 gram of the steel drillings t o a 500ml. Erlenmeyer flask. Take up in 50 ml. of sulfuric-phosphoric acid mixture. Heat gently until in solution and then oxidize ferrous iron t o ferric by adding 5 ml. of nitric acid (sp. gr. 1.20). Boil to expel oxides of nitrogen and add 50 ml. of distilled water and 5 ml. of silver nitrate solution. Boil and add 5 ml. of ammonium persulfate solution. Boil for a further period of not less than 4 minutes and not greater than 6 minutes. Cool rapidly in a water bath to 25’ C. Add 3 drops of osmium tetroxide (0.01 M ) and then run in sodium arsenite in slight excess (5-ml. excess). Titrate excess arsenite by adding potassium permanganate dropwise to the first detectable pink tinge. The volume of arsenite so obtained represents manganese plus chromium (a). Return solution to hot plate. Boil and add 5 ml. of persulfate solution. Boil the solution for 8 to 10 minutes, and run in 4 to 5 ml. of hydrochloric acid (1 to 3). Boil for 5 minutes after the pink color of the solution has been dischar ed. Cool to 25” C., add 3 drops of potassium iodate (0.0025 MY, and run in a small excess of arsenite. Titrate to a faint pink tint with potassium permanganate. The volume of arsenite so used represents chromium ( b ) . By deducting (b) from (a) the manganese equivalent is obtained. For routine work the theoretical equivalent of arsenite and permanganate may be used for obtaining the permanganate equivalent of (a) and (b). However, for accurate work the volumes obtained for (a) and (b) must be corrected for dilution effect and color interference. This usually amounts to 0.2 to 0.3 ml. for chromium contents up to 1 per cent and should be added to both (a) and (b). This may be determined by boiling the solution after the final titration for 10 minutes. After cooling, the volume of potassium permanganate required to give a perceptible pink tint is noted and this volume added to both (a) and (a). Vanadium, if present, is best estimated on the solution remaining by the ferrous sulfate-persulfate method, followed by titration with potassium permanganate. In this the volume correction for dilution and color interference must be deducted from the potassium permanganate buret reading.
mt ml. grama mg.
In such solutions. the minimum period for effective decomposition of persulfate by boiling is 4 minutes. Permanganic acid is stable after an %minute boiling period. Chromium, when present, was satisfactorily determined on the same solution by the usual persulfate method. For the titration of sexivalent chromium arsenite was used with a 0.0025 M solution of potassium iodide as catalyst. Vanadium does not interfere and this is an advantage, since the permanganate end point is not subject to the fading that must be guarded against in the presence of vanadium, when ferrous sulfate is used for reduction. Owing to its stability, it is suggested that arsenite replace ferrous sulfate for volumetric determinations of chromium and manganese. The following tabulated results
Literature Cited (1) Bright, H. A., IND.ENG.CHEM.,Anal. Ed.,9, 577 (1937). (2) Gleu, K.,2. anal. Chem., 95, 303 (1933). (3) Lang, Rudolph, 2. anolrg. allgem. Chem., 152, 197 (1926). (4) Park, B.,IND.ENG.CHEM., Anal. Ed., 7, 427 (1935).
625
