Colorimetric Determination of Molybdenum I,. H. JAMES, Reo Motor Car Company, Lansing, Mich.
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HE most generally used method for the colorimetric determination of molybdenum in iron or
parison tubes, and dilute with butyl acetate saturated with sodium thiocyanate and stannous chloride until the color of the tubes is identical. The weight of molybdenum in the sample and standard is then directly proportional to the volumes. This proportionality will not hold, however, unless the solution used in matching the colors is treated as in the following directions for the preparation of the butyl acetate solution, a procedure which is necessary in order to prevent any iron present from oxidizing and the color produced by the molybdenum from fading.
steel was developed by Maag and McCollam (3) who combined methods previously described by the U. 8. Steel Corporation (4) and by King ( 2 ) . This method depends upon the extraction by ethyl ether of the brown coloration produced by the formation of molybdenum thiocyanate in a solution containing hydrochloric and sulfuric acid. The results obtained by this procedure are satisfactory, although considerable experience is required to evaporate and fume the solution successfully as directed, in order to remove the nitric acid entirely without spattering and a t the same time to prevent the formation of insoluble salts by overheating. In the following modified procedure perchloric acid is substituted for sulfuric acid, thereby eliminating entirely any tendency to spatter while removing the nitric acid, and there is moreover never any difficulty in redissolving the insoluble salts formed by boiling with this acid, Ethyl ether is also replaced as an extraction reagent by butyl acetate, an advantage in that there is no heat or pressure generated in the separatory funnel during the extraction of the molybdenum thiocyanate, a somewhat troublesome feature if ether is used for this purpose. Nickel, chromium, vanadium, silicon, and copper do not interfere in the percentages ordinarily encountered, although it is necessary to separate the insoluble copper thiocyanate formed before estimating the molybdenum, if the copper content is much more than 0.13 per cent. It is further determined that the time designated in the following procedure for fuming with perchloric acid in order to remove the nitric acid is twice the minimum time required to complete this operation satisfactorily, and that the accuracy of the results obtained are not increased by extracting more than once with butyl acetate. The color formed does not fade and is not affected by any reasonable variation in the amount of the various reagents used.
SOLUTIONS SODIUMTHIOCYANATE. Dissolve 50 grams in 1000 cc. of water. STANNOUS CHLORIDE.Heat 250 grams of stannous chloride with 200 cc. of hydrochloric acid (sp. gr. 1.19) until dissolved, and dilute without cooling with 1200 cc. of water. BUTYLACETATESOLUTION.Shake 250 cc. of butyl acetate (b. p. 118' t o 127" C.) in a separatory funnel with 5 cc. of sodium thiocyanate and 25 cc. of stannous chloride solution, and discard the aqueous solution. This solution must be freshly prepared each day, as it deteriorates in about 24 hours unless again saturated with the two salts. STANDARD MOLYBDENUM SOLUTION.Heat 0.71 gram of molybdic acid (85 per cent) with 100 cc. of nitric acid (sp. gr. 1.42) and 30 cc. of hydrochloric acid (sp. gr. 1.19)until dissolved. Add 100 cc. of water, boil, cool, and dilute to 2000 cc. Determine the molybdenum content of 50 cc. 0 : this solution as lead molybdate and calculate the concentration per cc. which should be approximately 0.0002 gram. DETERMINATIONS The accuracy with which molybdenum can be estimated by this method was determined in all cases by comparison with the same standard, which was prepared by adding 3 cc. of standard molybdenum solution to 0.1 gram of molybdenumfree steel. A standard prepared in this manner contained 0.000636 gram of molybdenum and was compared in all but the first two series of results with molybdenum-free steel, cast iron, or stainless steel samples prepared as the standard sample, in which, however, the molybdenum content was varied. The standard molybdenum solution was added from a IO-cc. precision buret graduated in 0.05 cc., and the colors matched against a white background.
MODIFIEDMETHOD Dissolve a 0.1-gram sample of steel or iron and a standard sample of the same weight in 5 cc. of nitric acid (sp. gr. 1.13) and 5 cc. of perchloric acid (60 per cent), in 150-cc. beakers covered with raised watch glasses. The standard used may be either a steel of known molybdenum content or a standard prepared by adding to 0.1 gram of molybdenum-free steel a definite amount of standard molybdenum solution. In order to dissolve a sample of stainless steel, it is usually necessary to add to the acids used for steel 5 cc. of hydrochloric acid (sp. gr. 1.19) and 5 cc. of water. Continue to boil for 10 minutes after copious white fumes start to be given off, cool, and dissolve in 15 cc. of water. Add 5 cc. of sodium thiocyanate, agitate, and transfer to 100-cc. separatory funnels, rinsing the beakers first with 10 cc. of stannous chloride and then with 20 cc. of butyl acetate. If the sample contains less than 0.3 per cent of molybdenum, it is advisable, although not absolutely necessary, to extract with 10 rather than 20 cc. of butyl acetate, as the coloration thus produced is more readily compared with the standard. Shake the funnels for 1 minute and discard the aqueous solutions. Add 5 CC. of sodium thiocyanate and 15 cc. of stannous chloride to the butyl acetate solutions, and again shake for 1 minute. Draw off the wash solutions and filter if an appreciable amount of insoluble copper thiocyanate is present. Transfer the butyl acetate solutions t o color-com-
TABLEI MOLYBDEXUM DETERWINATIONS XOLYBDENUM
SAMPLE ADDIOD 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
C -
AYERAQ~
MOLYBDE~NUM FOUND--
%
%
%
%
%
%
0.014 0.15 0.12 0.21 0.32 0.63 0.74 0.84 1.05 1.68 2.10 0.11 0.63 1.05 0.32 0.42 1.05
0.015 0.16 0.13 0.23 0.33 0.60 0.72 0.88 0.98 1.62 2.16 0.11 0.63 1.08 0.34 0.43 1.02
0.013 0.15 0.12 0.19 0.32 0.63 0.74 0.84 1.04 1.68 1.98 0.12 0.66 1.14 0.32 0.43 1.08
0.015 0.16 0.12 0.22 0.33 0.65 0.72 0.87 0.01 1.62 2.16 0.12 0.64 1.05 0.34 0.42 1.08
0.014 0.15 0.12 0.23 0.33 0.60 0.71 0.81 1 .OB 1.64 2.34 0.11 0.63 0.98 0.33 0.45 1.14
0.014 0.15 0.12 0.22 0.33 0.62 0.72 0.83 1.02 1.64 2.16 0.11 0.64 1.06 0.33 0.43 1.08
The series of results numbered 1 to 11 in Table I were obtained on steel samples, the first two of which were Bureau of Standards samples Nos. 30B and 72, rather than samples prepared as described. The series numbered 12to 14 are castiron, and the series numbered 15 to 17, stainless steel samples. 89
ANALYTICAL EDITION
90
Molybdenum can be precipitated as the sulfide and weighed as lead molybdate so that the percentages thus obtained are accurate to * [0.02 (0.02 x the percentage)], or in other words, if molybdenum is determined in this manner, the results obtained should not deviate from the theoretical result for any definite molybdenum content by more than the value obtained by means of this equation. This value is designated as the allowable error in the following discussion, and if the results obtained deviate from the theoretical by more than this calculated error, it must be assumed that either the analyst or the method is a t fault (1). If the proposed method is to be considered as accurate as the method to which the above equation is applicable, the maximum deviations of the percentages of molybdenum determined from the theoretical percentages in Table I must be equal to or less than the allowable errors as calculated by this equation. It is evident, however, if the results are ex-
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amined in this manner, that the maximum deviations for the higher percentages in this table are greater than the calculated allowable errors. Because of this fact, it must obviously be concluded that the proposed method cannot be used for a molybdenum content of more than 0.8 per cent, if it is considered necessary to determine this element as accurately as it can be determined by precipitating as the sulfide and weighing as lead molybdate. LITERATURE CITED (1) (2) (3) (4)
Am. SOC. Testing Materials, "Standards," Part 1, p. 377 (1930). King, IND.ENG.CHEM.,15, 350 (1923). Maag and MoCollam, Ibid., 17, 524 (1925).
U. S. Steel Corp., "Methods of Chemists of the lJ; S. Steel Corp. for Sampling and Analysis of Alloy Steels, 2nd ed., p. 72 (1921).
RECEIVEDJuly 16, 1931.
Determination of Total Sulfur in Gases E. LIEBERAND R. ROSEN,Standard Oil Detelopment Company, Linden, N. J . A N IMPROVED method for the determinaa boiling point of 92" to 93" C., HE use of gases containof gases is and thiophene (synthetic), Easting a minimum amount tion of the total sulfur Of organic is presented. A study of its reliability based upon Analysis man Kodak in this Co.,laboratory 83" to 85"ofC. a tial for the successful operation anab'sis of a series O f of a number of industrial procmixfures con- sample of sodiu'm c a r b o n a t e esses. In the development of taz'ning a number of types of sulfur compounds from Eimer and Amend labeled in concentrations found in refinery gases, and "special purified, sulfur free," methods for the removal of oralso covering a wide range of B. t. u. values, showed zero sulfur. ganic sulfur from gases it is preThe gases ethane, propane, requisite that a method be showed a mean error of 0.03 per cent, correspondand butane were available in able for the determination of pering t~ a mean centages of organic sulfur of the Of 2-4 per cent On a basis 3-pound cylinders. Samples of of the actual sulfur content of the gas. Anmagnitude of one grain per 100 these gases when burned by the alysis by two operators on similar equipment8 combustion apparatus described cubic f e e t , 0.008 per cent by volume. analyzing samples of the same gas agree within in this Paper showed zero Sulfur. Oxygen, n i t r o g e n , and The methods wed 1 per cent of the actual sulfur content. hydrogen were obtained from for official gas testing are those in which the gas is burned with An analysisy by the procedure described rethe Air R e d u c t i o n Company. quired from 1 to 100 liters of gas. Combustion oxygen or air, the resulting sulfur Samples of these gases burned may be completed in 1 to 4 hours, and the sulfur with hydrogen in the combusdioxide being converted to sulin the absorbent medium is determined granition apparatus also showed zero furic acid and the final product determined by gravimetric Or metrically as barium sulfate, sulfur. volumetric means. Dennis (2) The various sulfur compounds were diluted by means of a gives a bibliography including many of these and describes the three methods recommended specially purified benzene which showed 0.015 per cent sulfur by the Bureau of Standards (1)-i. e., the Referees, the Hin- by the lamp method (correction was applied for this sulfur man-Jenkins, and their modification of the Drehschmidt (5). in all determinations in which it was used). The Bureau of Standards (1) states that each has its parAPPARATUS ticular advantage and that it is not possible to combine all of these in any composite apparatus, nor is any of the The complete set-up for the proposed modified Drehthree superior to the other two for all purposes. Further, Schmidt apparatus is shown in Figure 1. It consists of the it is noteworthy that none of the authors has demonstrated flowmeters, etc., which are used for measuring the volumes the reliability of his method on the basis of the analysis of of the gases used in the combustion process; the combusgases of known sulfur content. tion apparatus, E , shown in detail in Figure 2, in which the The purpose of this paper is to present a method which, gases are burned; and the Milligan absorption bottles, L, in addition to its demonstrated accuracy, possesses a number which contain the absorption solution through which the of advantages over the standard methods in use in the in- products of combustion are bubbled. A is a 5 0 0 - ~ round~. dustry. bottom flask with stopper as shown, and B is an ordinary wash bottle whose inlet tube has been severed midway; MATERIALS USED these are expansion bulbs provided to even out fluctuations Carbon bisulfide, high grade c. P., showed no residue in pressure. The combustion apparatus (Figure 2) consists of the upon distillation and boiled within a narrow range (b. p. 46.0' to 46.2" C.). Ethyl sulfide, Eastman Kodak Co., had chamber in which the combustion takes place and the con-
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