with a solution of 0.50M hydrochloric acid, p H 0.30, on the SA-2 paper in the hydrogen form. Both arsenic and tin were cationic in this sistem. Tin and arsenic had RP values '3f 0.40 and 0.91, respectively, and were separated by 20 cm. Arsenic was separated from the 25 other ions, and zinc was separated from all except vanadium ions by development with a solution of 0.50M sodium hydroxide, p H 14, on the SA-2 paper in
the sodium form. Both arsenic and zinc were anions in this system. Zinc and arsenic had RF values of 0.57 and 0.85, respectively, and were separated by about 10 cm. Except for the vanadium, the closest zone to the zinc is gold, which streaks from the origin to within 2 cm. LITERATURE CITED
(1) Sherma, J., Tahnta 9 , 775 (1962). (2) Sherma, J., Cline, C. W., Ibid., 10, 787 (1963). (3) Sherma, J., Evans, G. H., Frame,
H. D. Jr., Strain, H. H.. ANAL.CHEM. 224 ~~- flR6.7). (4) Strain, H. H., Binder, J. F., Evans, G. H., Frame, H. D., Jr., Hines, J. J., Ibid., 33. 527 (1961). JOSEPH SHERMA Lafayette College Easton, Pa. WORKsupported by a Summer Research Grant from Lafagette College. The electrochromatographic studies were performed at Argonne Sational Laboratory where the author was a Resident Research Associate during the summers of 1961 and 1962 in the laboratory of H. H. Strain. 35. I
\ - - - - ,
I
.
Indirect Volumetric Determination of Cobalt in Ferrous Alloys with EDTA as Indicated by Eriochrorne Black T SIR: The method of analysis for large amounts of cobalt in steel as reported by Lewis and Straul, ( 2 ) has been modified for use with Ekiochrome Black T to indicate the end point of the titration. Although Eriochrome Black T was reported by Relclier (3) as giving less satisfactory end points than other indicators, careful control of the titration variables resulted in accurate and precise recoveries of milligram amounts of cobalt from various 1 ypes of steel.
Table 1. Effect of Variables in Titrating Cobalt with EDTA (Eriochrome Black T)
Variable Time elapse during titration, seconds
Cobalt recoveredo,
75 .I
135 270 300 360 Temperature at start of titration, "C. 6 10 20 30
40
50
Total volume at start of titration, ml. 250 250 200 200 150 100 PH 6.6 7.4 8.1 9.0 10.0 a
20.40 mg. of cobalt taken.
mg.
20.24 20,39 20.09 20.09
20.24 20.54 19.94 20.39 20.09 20.09
19.97 20.24 20.39 20.39 20.24 20.09 19.97 19.82 20.12 20.35 20.39
The procedure used was similar to that of Lewis and Straub. However, some deviations are to be noted. Nitric acid was removed by baking the sample in perchloric acid rather than in hydrochloric acid. This approach lends itself readily to simple removal of chromium as chromyl chloride with gaseous hydrogen chloride. After chromium has been volatilized, the sample is fumed intensely in perchloric acid, cooled, and diluted with water. A filtering step then removes the hydrolyzable elements such as niobium, tantalum, and tungsten. This step has two advantages. Quantitative removal of these undesirable acid-insolubles simplifies cleaning of the resin so that it may be used for numerous analyses before discarding. Also, if these elements are not present in the resin, the chance of possible hydrolysis (1) during subsequent elution with weaker hydrochloric acid is avoided. That this hydrolysis does occur is evidenced by precipitates in the eluants and abnormally high recoveries. Among the indicators evaluated for possible use in the direct or indirect titration of cobalt with EDTA were murexide, pyrocatechol violet, pyrogallol red, and brompyrogallol red. Indefinite end points or low levels of cobalt tolerance ruled them out for use for milligram amounts of cobalt. Welcher
Table 11.
indicated that Eriochrome Black T had been used to titrate from 15 to 50 mg. of cobalt but with an unsatisfactory end point. Investigation of the titration conditions-pH, temperature, volume of solution, reducing agent, and time elapse from addition of indicator to finish-showed that the following conditions as chosen for this modified procedure result in accurate and precise cobalt recoveries (Table I). With proper care and no delays, titrations can be conducted in about two minutes; under most laboratory conditions, normal room temperature and addition of reagents a t the proper volume will provide a solution temperature of about 30' C. Best results with the reagents used are obtained in a total volume of about 200 ml.; the addition of 10 ml. of p H 10 buffer solution (60 grams of ammonium chloride and 5iO ml. of ammonium hydroxide diluted to 1 liter) will provide the optimum hydrogen ion concentration for this indicator. Xddition of about 0.5 gram of hydroxylamine hydrochloride to the solution to be titrated will retard atmospheric oyidation of the indicator. Table I1 is a compilation of some of the results obtained using the recommended procedure. Table I11 shows the types of alloys analyzed and their nominal compositions. As can be seen
Volumetric Cobalt Recoveries KO.
Sample NBS 153a NBS 167 NBS 168 NBS 349 NBS 1187 No. 524 No. 1 No. 2 No. 3
No. 4 No. 5
Certificate 8.47 42.90 41.20 13,95 20.80
...
... ...
...
...
...
Experimental av. 8.44 42.85 41.28 13.91 20.82 17.11 14.86 15.70 17.79 18.63 16.85
2u f0.07
10.20
of
determinations 5
f 0 .20 f 0 . 06
k0.14 & O . 07 f 0 .04
f O .10
f0.07
5
4
5 4 12 5 4 5
& O . 08 f 0 . 08
5
VOL. 36, NO. 3, MARCH 1964
a
5
691
Table 111.
Sample NBS 153a NBS 167 NBS 168 NBS 349 NBS 1187
Co 8.47 42.90 41.20 13.95 20.80
1-5, 524
Vary
Fe Bal.
Cr 3.72 20.00 20.33 19.50 21.62
2.13 3.43 0.13 27.4
Bal.
from Table 11, the average results on the five standard steels differ by only several hundredths of a per cent from the provisionally certified values. The precision as calculated by 2 c (reproducibility a t the 95% confidence limits) compares favorably with other reported procedures (4).
Ni 0,168 20.65 20.25 57.15 20.26 30.
Steels Analyzed by Proposed Procedure
Mo 8.85 3.90 3.95 4.04 3.41
Cu W 0.094 1.76 0.03 4.50 0.035 3.95 0.006
