The Journal of Industrial and Engineering Chemistry I9
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Jan., 1918 THE JOURNAL OF INDUSTRIAL AND ENGINEERING CHEMISTRY I9 THE DETERMINATlON OF MANGANESE IN STEEL IN the weight of the sample has been added as chromate THE PRESENCE OF CHROMIUM AND VANADIUM immediately before titrating. Under the usual BY ELECTROMETRIC TITRATION conditions of analysis most of the chromium is present By G. L. RELLBY, M. G. SPBNCBR,C. B ILLINGWORTHAND T. GRAY as chromic salt and such amounts of chromium in Received July 14, 1917 this state make the end-point obscure. In the pres- The determination of manganese in steel in the ence of 5 per cent or more of chromium all of these presence of chromium and vanadium has long offered methods, except those which involve the separation difficulties to the analyst in steel works laboratories. of manganese, are extremely uncertain. Numerous methods have been developed with the object of alleviating or overcoming these difficulties. In this paper we describe a method which has suffi- cient :accuracy for all technical purposes and which, Cain' and \Tatters2 recommend devices for the separa- without the separation of chromium and vanadium, tion of the manganese. Cornelius3 and Dedericks,* is not interfered with by these elements under the con- although not particularly concerned with the analysis of+'steel, have recommended procedures available for ditions of analysis. The oxidation of the manganese the quantitative separation of manganese from chro- in this method may be accomplished either by the bis- muthate or persulfate procedures, and titration is mium and vanadium. Talminger6 has proposed a method of the latter type, using von Knorre's procedure, made electrometrically, using mercurous nitrate as the viz., the precipitation of manganese by ammonium reducing agent. In the course of the examination of a long list of reducing agents this was the only reagent persulfate. KoesterO has investigated Engel's method found which would reduce permanganate quantita- for the electrodeposition of manganese in the pres- ence of chromium, and finds chromium occluded. tively and rapidly at ordinary temperatures without Cain' finds that the Ford-Williams method gives reducing chromates or vanadates. high results due to the occlusion of chromium. Even THE STANDARDIZATION OF THE MERCUROUS NITRATE in the Volhard-WoX8 method chromium and vanadium SOLUTION interfere. The two methods most in use for the determination IO. 5 g. of mercurous nitrate are dissolved in I jo cc. of manganese in steel are known as the persulfate and of water to which 2 cc. of nitric acid have been added. bismuthate methods. In the first of these the man- Any undissolved salt is removed by decantation and ganese in a nitric acid solution of the steel is oxidized the solution made up to a volume of one liter. This with ammonium persulfate and silver nitrate, after is compared electrometrically on the apparatus made which it is titrated with sodium arsenite. Wdow- for this laboratory by the Leeds & Northrup Com- iszewskig reports good results with this method, even pany,1 with a solution of potassium permanganate in the presence of I per cent chromium. In this which has been standardized against sodium oxalate. laboratory, however, it has been noted that the method The permanganate solution contains 0. 5 g. of Mn rapidly becomes less useful as the percentage of chro- per liter, each cc. being equivalent to o.oj per cent mium rises, owing to the obscurity of the end-point. of Mn in a I-g. sample of steel. The medium in In the bismuthate method the manganese in a solu- which the titration is made is a solution containing tion of the sample is oxidized with sodium bismuthate, 50 cc. of sulfuric acid (sp. gr. 1.58) and zoo cc. of the excess filtered out and an added excess of ferrous water. The:temperature should not be above 40'. sulfate titrated with permanganate. Here even a For purposes of this titration, permanganate is trace of chromium may cause trouble and this diffi- added to the solution in any convenient amount and culty rapidly increases with the larger percentage of titrated with mercurous nitrate. The details of the chromium often met in commercial steels. By cooling titration will be given at a later point in this article. the solution thoroughly with ice before oxidizing the manganese the tendency of the chromium to oxidize THE REACTION is depressed, and if this procedure is followed by rapid When titration is complete, the solutions have a filtration fairly satisfactory determinations of man- brown color suggesting dissolved manganese dioxide. ganese may be made even in the presence of 5 per The solutions, however, appeared to be quite stable. cent of chromium in the sample. The consistently When- solutions containing IO cc. of permanganate successful analysis of such material by this method, had,been titrated and were allowed to stand in a however, requires a high degree of skill. Demorestlo stoppered Erlenmeyer flask at room temperature, no suggests titrating the solution with sodium arsenite precipitates appeared after some weeks. Filtration until the color of the permanganate disappears. He removed only a small amount of the solids. With has tested this method by titrating solutions of steel solutions containing as much as 40 cc. permanganate to which chromium corresponding to 3 per cent of solution, a faint turbidity appeared after 24 hrs. On 1 THISJOURNAL, 3 (1911), 630. warming, even to 40°, manganese dioxide was pre- 2 Met. b Chem. Eng., 9 (1911), 244. 8 Phnrm. Ztg., 68, 427. cipitated. 4 Ibid., p. 446. After titrating 40 cc. of the KMn04 solution with 6 Chem -Ztg., 34 (19101,1877. a 2. Elektvochem., 17 (1911), 57. mercurous nitrate, the titration was continued with 7 LOG. cit ferrous sulfate of equivalent concentration on the 8 Stahl u. Eisen, 88 (1913). 633. same apparatus. This was found to require 12 CC. 9 Ibid., 28 (1908),1067. bo THISJOURNAL, 4 (1912), 19. 1 THIS JOURNAL,^ (19171, 780. 20 THE JOURNAL OF INDUSTRIAL AND ENGINEERING CHEMISTRY 1701. IO, No. I of ferrous sulfate. It is believed that the latter re- in its support. We give below a brief outline of some action was the reduction of quadrivalent manganese of our observations with their implications as under- to the bivalent state. Our efforts to prove this point stood by us, omitting, however, experimental details were not entirely successful. When the solutions after because of the inconclusive character of the work. titration were warmed, both in the original volume When manganous sulfate' is added to the solution of 2jo cc., and diluted to larger volumes up to one of permanganic acid before titration with mercurous liter, the amount of MnOz precipitated varied in an nitrate, it has the effect of diminishing the amount of uncertain manner. Changing the temperature from mercurous nitrate necessary. The effect, however, to boiling did not serve to indicate a procedure 40' is not a regular one, for the addition of small amounts which would give consistent results. The MnOz so of manganous salt produces -a proportionately larger precipitated was collected on asbestos and, after washing, mixed with dilute sulfuric acid. An excess effect upon the titration than large additions. Such additions noticeably alter the color of the solution, of ferrous sulfate was then added and the solution changing it from brownish yellow to brownish red, titrated with permanganate electrometrically. From suggesting the formation of manganic salts. Barne- 8 to IO cc. of ferrous sulfate were required for titra- bey1 reviews the explanations which have been offered tion, an amount always less than that required for a to account for the effect of manganous salts upon similar titration before precipitation. One possible the titration of ferrous iron with permanganate in explanation of this is that the manganese dioxide as the presence of hydrochloric acid. quotes Vol- formed is in a hydrated condition and while in this He hard as suggesting that the action of manganous condition it may be that it very readily undergoes salts on the permanganate results in the formation of decomposition in part into manganous sulfate and quadrivalent manganese, while Birch suggests the oxygen. That there is a diminution in oxidizing formation of trivalent manganese. When the solu- power by precipitation is shown by the fact that the tion after titration with mercurous nitrate is treated oxidizing power of the precipitate and filtrate com- with manganous salt, the same reddish color appears bined is insufficient to oxidize 12 cc. of ferrous sulfate as when the manganous salt is added before titra- as is done when the titration with ferrous sulfate is tion. If we are correct in our belief that the product of made without precipitation. the reaction is manganic sulfate in both instances, In the foregoing discussion 40 cc. of the solution this would constitute additional evidence of the pres- were taken as a unit in the study of the quantitative ence of quadrivalent manganese in the titrated solu- precipitation of manganese dioxide. In the discussion tion. which follows we shall adhere to this volume. From the fact that we are unable to remove MnOz To determine the amount of mercurous salt present from the solution by filtration after titration, this sub- in the mercurous nitrate solution 40 cc. were diluted stance, if present, must be evident either as a sulfate to 200 cc., and 200 cc. of a solution containing j g. or in the colloidal condition. Witzemann,2 discussing of sodium chloride gradually added with stirring. the conditions under which colloidal solutions of man- This was followed by the addition of IO g. of sodium ganese dioxide are stable, points out that small amounts acetate in the form of a filtered solution. The pre- of either salts or acids cause immediate precipitation.