V O L U M E 2 3 , NO. 11, N O V E M B E R 1 9 5 1
1683
chloroform, making the extraction difficult. Chromium, for example, if present in large amounts may partially precipitate as chromium(II1) fluoride and cause trouble. The extraction may usually be carried out successfully, however, if more water is added to increase the volume of the aqueous phase. Residues of molybdic or tungstic acid, which appeared during solution of some steel samples, were allowed to settle in the volumetric flask when the sample solutions were made up and were ignored in \ubsequent operations. In making the interference studies, the results of which are shown in Table 11, the procedure was as follows:
h weighed quantity of the substance or substances to be investigated was dissolved in 25 ml. of a cobalt nitrate solution containing 198 micrograms of cobalt. The solution was then extracted with thiocyanate tetraphenylarsonium chloride and chloroform. The extract was diluted with chloroform to 25 ml. and the transmittancy was measured at 620 mp.
Table 111. Test of Method with Bureau of Standards Samoles Sample and X.B.S. Certificate Values
Per Cent Found ~______~ 1
Ni steel 126A Co 0.30, Xi 35.89, C 0.056, M n 0.414, Si 0 . 3 0 0.194, Cu0.092, Cr0.054 Co, Mo, W steel 153 Co8.45. C 0 . 8 6 4 , M n 0.219, P 0.025, S 8 . 4 5 0.008, Si 0.187, Cu 0.099, Xi 0.107, Cr
2. 3.
4.14,V2.04,hlo8.38,W1.58 Ni. Cr. alloy 161 Co 0.47, C 0.34, RIn 1.29. P 0.012, S 0 . 4 8 0.005, Si 1.56, Cu 0 . 0 4 , SI 64.3, Cr 16.9, V0.03, hI00.005, Fe 1.5.0 SI,Cu alloy 162 Co 0.54. Xi 66.38, Cu 28.93. .\In 2.34, 0 53 Si 0.67, Fe 0.34, Cr 0.24. AI 0.23, Ti 0 20, c o . 1 1 , s o . 0 0 2
4.
3.
2
3
4
0.30
0.29
0 30
8.42
8.42
8.44
0.48
0.48
...
0 52
0 62
0 32
DETERMINATION O F COBALT
Procedure in Absence of Copper and Vanadium. Introduce a suitable aliquot of the solution to be analyzed into a small separatory funnel. Add water if necessary to increase the volume to about 25 ml. Add 2 to 5 ml. of 50% ammonium thiocyanate solution. Add solid ammonium fluoride until the red color of the ferrithiocyanate complex is completely removed and then add 200 to 300 mg. in excess. Add 15 drops of 0.05 N aqueous tetraphenylarsonium chloride solution and shake the resulting precipitate and solution with 8 to 10 ml. of chloroform. Tranvfer the chloroform layer to a 25-ml. volumetric flask. Repeat the extraction two times, adding each time 5 drops of tetraphenylarsonium chloride and 5 ml. of chloroform. Dilute t,he m1nt)iried extracts to volume with chloroform and measure the tranamittancy against a chloroform extract of the reagents or :igainst pure chloroform if the reagent blank has been shown to kw riegligible. Procedure in Presence of Copper. Take a suitable aliquot of the solution in a small beaker, neutralize, and then make just, widic with hydrochloric acid. Add solid potassium iodide until t,he precipitate of copper( I) iodide redissolves, and then add 500 mg. in excess. Add 10Co sodium thiosulfate solution until ihe color of the triiodide ion is completely removed. Add 3 to 5 drops in excess. Add 500 mg. of solid ammonium fluoride. Transfer the solution t,o a small separatory funnel. Add 10 ml. of 50% ammonium thiocyanate and 15 drops of 0.05 M tetraphenylarsonium chloride solution. Proceed with the extraction as outlined above. Procedure in Presence of Vanadium. Introduce a suitable aliquot of the solution to he analyzed into a small separatory funnel. Add solid ferrous ammonium sulfate until all the ranadium has been reduced to the plus four oxidation state. iidd solid ammonium fluoride unt,il the solution has a greenishyellow color; do not stop at the blue color of t,he vanadyl ion. Add thiocyanate, tetraphen\‘larsorliulll chloride, and chloroform xnd proceed as above.
ACCURACY
The accuracy of the procedures has been r.stablished by the analysis of five Bureau of Standards alloy samples (Table 111). LITERATURE CITED
(1) Aflsprung, H. E., M.*L thesis, Washington L niversity, 1950. (2) Xyres, G. H . , .\SAL. CHEM., 21, 652 (1949). (3) Rabko, A . K., and D r a k o . 0. F., J . Geri. Chem. ( U . S . S . R . ) .19, 1809 (1949). (4) Barnes, K.A , , h1.S. thesis. Washington University, 194i. (5) Bayliss, N. S., a n d Pickering, R. W.,IND.Eso. CHEM..A s . 4 ~ . ED.,18, 446 (1946). (6) Bobtelsky, hl.. and Spiegler. K. S.. ,J. Cheni. SOC..1949, 143. ii) Dwver. F. P.. Gibson. S . .I..and Kyholm. R. S.,J . Pro? Rou. ,
