close agreement rn Ith classical analyses and certified values on Kational Bureau of Standards bteel samples (Table I). S o interference ha5 bpen encountered from as much ab 5% of niobium, lTc of tantalum, 3yo of titanium, and 4.5% of tungiten. or combinations of these alloying elements. The range investigated mas from 0.002 to 0.1 w.’o. phosphorus. The reproducibility at-
the 95Yo confidence limits or two sigma ranges from +0.0005 to xt0.006 w./o. a t the low and high extremes of the range or an average precision of 10.003 w.10. LITERATURE CITED
(1) Fogg,
D. N., Wilkinson, N. F., Analyst 83, 406-14 (1958).
(2) Morrison, G. H., Freiser, H., “Solvent Extraction in Analytical Chemistry,” p. 250, Wiley, New York, 1950. GERALD .4.BACER Research Laboratory The Carpenter Steel Co. Reading, Pa. Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Pittsburgh, Pa., 1964.
Spectrophotometric Determination of Molybdenum in Complex Ferrous Alloys SIR: Alan)- variations of the thiocyanate method for determining molybdenum have been reported ( S ) , some of uhich use extraction into immiscible organic solvents to separate the complexed molybdenum from interferences. Some observations made on a system of this type in which molybdenum thiocyanate is extracted into normal butyl alcohol are herein reported. When cobalt in amounts exceeding about 5 mg. was present in a 100-mg. portion of alloy samples such as Waspaloy and Type S-816, its influence was evident from high molybdenum recoveries. Because the sample solution is evaporated to fumes of perchloric acid to remove nitric and hydrochloric acids, cobalt is present as the perchlorate which is pink or red in color. I t is believed, therefore, that in this state this element will absorb some light at the same wave lengths as the amber to red molybdenum complex. However, in this system an extra addition of stannous chloride solution (40y0 by weight in concentrated hydrochloric acid), beyond the two used in the absence of cobalt, and extraction eliminated this effect with no further interference from cobalt noted. During the investigation of copper and its influence on molybdenum recovery, this element precipitated as the thiocyanate when a reducing agent such as stannous chloride was added to its solution. Particularly when larger amounts of copper (4 mg. or more) were present, this precipitate tended to collect a t the interface between the inorganic and organic phases and contaminate the extract. Filtration of Table I.
Effect of Iron on Molybdenum Absorbance ( 1 mg. of molybdenum)
Iron added, rng. 0
25
--
e50 10
100 150
Absorbance 0,300 0 494 0.520 0,520 0,540 0.550
Table It.
5
Molybdenum Recoveries on
N.B.S.Standards
Molybdenum, % Experimental S.B.S.So. Certificate Range 36a 0.92 .91-. 92 50c 0.082 .079-, 088 132 7.07 6.98-7.10 132a 4.51 4.45-4.50 152 0,013 ,013-,018 153a 8.85 8.80-9.00 160a 2.83 2.80-2.90 349 4.04 4.00-4.10 Reproducibility a t the 95y0 confidence limits.
the extract usually caused some loss of the element sought. Possible means of eliminating copper were tried, such as complexing and chemical separation. These approaches were eventually abandoned as either ineffective or too lengthy for routine work. Nevertheless, it was found in Remy (1) that cuprous thiocyanate is soluble in a n excess of a thiocyanate solution. Test samples of the precipitate were treated with excesses of thiocyanate until they did, indeed, redissolve. Up to 5 mg. of copper were held in solution by adding 5 grams of thiocyanate (sodium or ammonium) in water to complex molybdenum. This practice was adopted as standard procedure for all samples and was used during instrument calibration. Because iron plays an influential role in procedures where thiocyanate is used to develop a measurable colored complex of molybdenum ( 2 ) , its effect was examined more closely. Increasing quantities of an iron solution (5 w./v. yo ferric chloride in water) were made to constant molybdenum quantities. an illustration, Table I shows the effect on the absorbance obtained when 0 to 150 mg. of iron were added to 1 mg. of molybdenum. There is an increase in absorbance u p to about 50 mg. of iron, beyond which point the slight increase may be considered negligible. This observation held true a t other levels of molybdenum, indicating that in this system there is a critical minimum iron
Av . 0.92 0.083 7.04 4.47 0.015 8.92 2.86 4.05
Precisiona fO.O1
f0,008
10.12 10.05 f 0 .004 f 0 . 16 f0.06 f0.08
to molybdenum ratio of 50 to 1 by weight. Further evidence of the importance of maintaining this rat,io was obtained when low molybdenum recoveries were realized on standard steel samples with less than 50 to 1 iron to molybdenum ratios until enough iron was added to reach this minimum. Table I1 shows the type of results that may be expected using a general procedure of the addition of a large excess of thiocyanate to complex and color molybdenum in perchloric acid solut’ion, reduction with stannous chloride, and extraction into normal butyl alcohol. Determinations were made on a Beckman Model DU spectrophotometer at a wavelength of 525 mp a t which tungsten does not interfere. The reproducibility of the procedure at the 9501, confidence limits or two sigma varies from ~k0.004w./o. to +0.16 w./o. over the range of 0.01 to 9% molybdenum. LITERATURE CITED
(1) Remy, H., “Treatise on Inorganic Chemistry,” T’ol. 11, p. 382, Elsevier,
Amsterdam, Yetherlands, 1056. (2) Sandell, E. B., “Colorimetric Determination of Traces of Metals,” pp, 644-6 Interscience, Sew York, 1959. (3) Snell, F. TI., Snell, C. T., “Colorimetric Methods of Analysis,” T‘ol. IIA, pp. 370-4, Iran Sostrand, Princeton, P;. J., 1959.
GERALD A . B.\UER
Research Laboratory The Carpenter Steel Co. Reading, Pa. VOL. 37,
NO. 1, JANUARY 1965
155
