September, 1925
IIL’DUSTRIAL AND ENGINEERING CHEMISTRY
965
Determination of Arsenic in Steel‘ By Alan E. C a m e r o n hlASSACHESETTS
M
INSTITUTE OF TECHNOLOGY. CAMBRIDGE,
A S Y methods for the determination of arsenic in
steel and iron have been published in the literature. With one or two exceptions they all depend upon the principle of the quantitative distillation of arsenic as arsenic trichloride in the presence of hydrochloric acid. The arsenic is first obtained as arsenic acid by the evaporation of a solution of the metal in dilute nitric acid, with or without the presence of sulfuric acid. The residue resulting from this evaporation, consisting of basic ferric salts, is then taken up with excess concentrated hydrochloric acid, cuprous chloride or ferrous sulfate added, and the whole quantitatively distilled. The resulting distillate contains all the arsenic as arsenic trichloride in a strong hydrochloric acid solution. The arsenic in this solution may be determined either volumetrically or gravimetrically, the latter being the more common method employed when appreciable quantities of arsenic are present. Gravimetric M e t h o d s
Two general methods for the gravimetric determination of the arsenic in this distillate, variously modified, are given. Both methods, as a preliminary, precipitate the arsenic mith hydrogen sulfide. The Bureau of Standards,’,* Lord and Demorest,* and others precipitate the arsenic by a rapid flow ( J f hydrogen sulfide through the cold distillate, allow the resulting precipitate to settle for a number of hours, filter, wash, dry, and weigh this precipitate as arsenious sulfide. The Research Laboratories of the American Rolling Mill Company3 pas:, hydrogen sulfide through the distillate from the beginning of the distillation while the distillate is being heated almost to the boiling point with the use of a small electric hot plate. This latter method has also been recommended by Lord and DenioresL4 Hall and Williams5 and the Research Laboratories of the American Rolling Mill Company3 prefer to weigh the arsenic as magnesium pyroarsenate. To do this Hall and Williams recommend that the sulfide precipitate, after filtering, bhould be dissolved in ammonia, the excess ammonia removed by boiling, and the solution evaporated to dryness with concentrated nitric acid, leaving a residue of arsenic acid. The Research Laboratories of the -4merican Rolling Mill Company dissolve the sulfide precipitate, after filtering, with concentrated nitric acid and potassium chlorate. The evaporation to dryness of this solution leaves a residue of arsenic acid as before. The resulting residue is then dissolved in a little mater or dilute hydrochloric acid and the arsenic precipitated as magnesium pyroarsenate with magnesia mixture. The precipitate is filtered on a Gooch crucible, washed, ignited, and weighed. Volumetric M e t h o d s
Different procedures have been suggested for the volumetric determination of the arsenic in the hydrochloric acid distillate. Stead6 neutralized the strongly acid distillate with sodium bicarbonate, added bicarbonate In excess, and titrated the arsenic with a standardized iodine solution. Mazzetti and Agnostini? obtained the arsenic as a precipitate from an aqua regia solution of the metal treated with Betten-
’
Received May 19, 1923 Work done in partial fulfilment of the requirements for the degree of doctor of science in metallurgy a t the Massachusetts Institute of Technology * Numbers in text refer to bibliography a t end of article
MASS.
dorf’s reagent, filtered off the arsenic, washed free from iron, dissolved the arsenic in 0.1 N iodine solution, and estimated excess iodine by titration with a standardized arsenite solution in the presence of sodium bicarbonate. The Bureau of Standards recommends a procedure whereby the arsenic is ultimately obtained as silver arsenate and the silver of this precipitate determined by titration with 0.01 N potassium thiocyanate solution using ferric alum as an indicator. Mellors quotes several methods, including Mohr’s iodine titration method and Bennett’s modification of Pearce’s silver arsenate method. Rambergg recommends titration with potassium bromate solut’ion. Present Method
The procedures outlined practically all have the disadyantages of multiplicity of operations, with their consequent consumption of time. For a more rapid and a t the same time reasonably accurate method for t’hisdetermination the writer has found the following modification of llohr’s met’hod satisfactory. Dissolve 5.00 grams of steel drillings in a 500-cc. round-bottomed flask by the gradual addition of nitric acid (1.2), cooling under the tap to prevent too violent action. When action ceases, place on a sand bath or hot plate, heat gradually until the solution clears, and then evaporate to dryness and bake to the removal of red fumes. Complete removal of nitrous fumes by the baking is essential. BangsAnrecommends the addition of a few crystals of potassium permanganate, and Rambergg removes nitrous acids by the addition of ammonium oxalate. A‘ofe-The use of sulfuric acid will give a closer temperature control a n d reduce the possibilities of loss of arsenic by volatilization, but the addition of this arid causes the formation of a heavy precipitate of ferric sulfate, which gives rise to diEculties in evaporating to dryness. The writer did not find material differences in the values obtained in determinations made with or without sulfuric acid.
Cool the residue from the evaporation, take up with 100 to 150 cc. of concentrated hydrochloric acid, warming gently to aid solution. Cool the clear solution, add 20 grams cuprous chloride, connect the flask to a spiral condenser, and distil. The separatory funnel for the distillation should have a stopcock. The outlet tube should be provided with a hole on the side about 1 cm. from the opening a t the end within the distillation flask, to allow for circulation in the outlet tube and thus prevent small particles in the flask from passing over into the distillate. The tip of the condenser should dip about 1 cm. into 100 cc. of water contained in a 400-cc. beaker, and the latter should be suitably cooled. The distillation must be carried o u t slowly. If brown fumes appear in the flask, nitrogen acids are still present and the experiment must be rejected. When about 60 to 7 i5 cc. have been distilled over, remove the flame, cool, and add 50 cc. of concentrated hydrochloric acid to the flask. Resume distillation until a further 50 cc. has distilled over. These two distillations should be sufficient to volatilize all the arsenic as arsenic trichloride. Make the distillate alkaline by means of a strong solution of sodium hydroxide. Make slightly acid with concentrated hydrochloric acid and neutralize with excess sodium bicarbonate. Cool to room temperature and titrate with a standardized iodine solution, using starch as an indicator. Prepare the iodine solution by dissolving 1.275 grams of pure iodine and 2 grams of potassium iodide in 1 liter of water. This solution must be standardized with a standard arsenic solution. To prepare the standard arsenic solution dissolve 0.66 gram of pure arsenious oxide in about 10 cc. of concentrated hydrochloric acid. This solution is obtained readily without boiling. Dilute the solution with a little water, neutralize the acid with excess sodium bicarbonate, and then dilute to 1 liter. It is obvious that arsenic-free reagents must be used. A blank test is made on the same quantities of acids and salts as are used in the determination, and the amount of iodine consumed is deducted from the results of the determination,
I N D U S T R I A L A N D ELVGINEERING CHEMISTRY
966
Test Analyses
As a test of the method, 0.0250 gram of pure arsenious oxide was dissolved in 90 cc. of concentrated hydrochloric acid, 20 grams cuprous chloride added, and the solution distilled. After about 70 cc. of the solution had distilled over a further 25 cc. of acid were added and the distillation continued until a further 25 cc. of distillate had collected. At the same time a blank was run on the reagents used. Titration gave: -Arsenic, 0.0198 0.0007
In solution In reagents
Gram0.0195 0.0007
--
-
Arsenic returned Arsenic added
0.0191 0.0260 X 0.7571
0,0188 0.0189
A Bureau of Standards standard steel sample (Fo. J l c ) , with arsenic given as 0.012 per cent, yielded on titration, 0.009 per cent and 0.010 per cent. Comparative determinations made on two arsenic bearing steels made by gravimetric methods (weighed as As&&) and the above volumetric method yielded the following: Samole 1
2
Gravimetric Gram 0.256 0.254 0.268 0.268 0.199 0 . 1 9 4 0.202
Volumetric Gram 0.262 0 . 2 5 8 0.263 0 . 2 6 2 0.202 0 . 1 9 8 0 . 2 0 5 0.211
Vol. 17, 30. 9
From the foregoing results it is evident that the volumetric method as outlined will give results that are quite as accurate gs those obtained by the more laborious and timeconsuming gravimetric method. With steels carrying up to 0.25 per cent arsenic an accuracy to within 0.01 per cent is possible. Acknowledgment
The author expresses his thanks to W. T. Hall, associate professor of analytical chemistry, Massachusetts Institute of Technology, for criticism and heip during the course of the investigations. Bibliography 1-Bureau of Standards Method for Arsenic, private communication 2-Lord and Demorest, “Metallurgical Analyses,” 1992. 3-American Rolling Mill Co., “Research and Methods of Analyses of Iron and Steel.” 4-Lord and Demorest, “MetaHurgical Analyses,’’ 1918. &-Hal and Williams, ”The Examination of Iron, Steel, and Brass.” I 7 0 f f Steel Insl. (London), 1896, p. 100. -tend, 7-Mazzetti and Agnostini, Ibid., 1923, 11, p. 485 (abs.). 8-Mellor, “Quantitative Inorganic Analyses.” &Ramberg and Sjostrom, Commission on Arsenical Poisoeing in Sweden; Cox, Analyst, PO, 3 (1925). 10-Bangs, see Reference 9.
Device for Maintaining a Slow, Constant Flow of Liquid‘ By Charles Van Brunt GENERALELECTRIC Co.. SCHENECTADY. N. Y.
E Y E R Y chemist has experienced the difficulty of adjusting and maintaining a slow but steady flow of liquid when the head is such that a cock, valve, or clamp must be “cracked” for the purpose. Sooner or later the flow slackens, and ultimately may stop altogether. Presumably, this is due to solid impurities in the liquid, which gradually accumulate behind the minute opening of the valve. Capillary adsorption may also play a part12but it is believed that the first-mentioned cause is the principal one, since an effect due to adsorption would reach an equilibrium. Further, the flow can always be reestablished by opening the valve slightly for a moment only. The desired throttling of flow can better be attained by using an elongated capillary of such bore as readily to pass any solid matter in the liquid. Ostwald has recommended two or more screw clamps in series on rubber tubing, which is an application of the same principle. But the length of capillary required is often inconvenient, and the nature of the suspended matter frequently makes such a remedy quite impracticable. The writer has adopted a modification of the long capillary idea with such success as to justify the belief that others will find it of service. A reference to the device was made in a recent article,3 but its efficiency in the trying circumstances under which it was there used justifies more specific mention. The problem was that of continuously draining off, from the bottom of a vessel, a t a rate as low as 5 cc. per minute, and under a 30-cm. head, a suspension of gelatinous or gummy carbonaceous and soapy matter more or less emulsified with 1 2
8
Received June 8, 1925. Wilson and Barnard, THIS JOURNAL, 14, 683 (1922). Van Brunt and Miller, Ibid., 17, 421 (1925), see Figure 4.
oil in a dilute solution of silicate of soda and sodium soaps. This mixture contains the various solid impurities removed from used automobile crank-case oil in the process described. It is inhomogeneous and “lumpy’’ to a degree which causes immediate clogging of any cracked valve or any tube of less than 3 or 4 mm. bore. Such a tube would not only have to be of inordinate length to produce the desired rate of flow under the head used, but it must have a sharp downward slope throughout to avoid stoppage by sedimentation. The problem was solved by cutting a helical groove about 0.5 mm. deep around an 8-mm. steel rod. When this “screw” is inserted into a close-fitting housing it forms a comparatively fine capillary of any desired length in a very compact form. Clogging is prevented by rotating the rod in its housing, which serves as a bearing. The r. p. m. may be as low as desired. A small windmill run by a jet of compressed air is a convenient source of power. The direction of rotation should be such that any solid particles are carried by the screwing action in the direction of flow. So far as the writer’s experience has gone this device is absolutely nonclogging. Not a single stoppage, or anything more than a momentary slowing up, has occurred in several months of use. Evidently, the only thing that could cause stoppage would be a mass too large to enter the capillary and too tough to be disintegrated by the movement, and this would have to be caught and held in the continually moving opening of the groove. Any particle entering the capillary must eventually work through unless it sticks in the groove so tightly that friction against the walls of the housing cannot move it. This has so far never occurred. It should be understood that the helical pump or screw conveyor action of this device does not appreciably affect the
