V O L U M E 27, NO. 6, J U N E 1 9 5 5
977
that concentrates of normil CIS,CIS, C?o, C", and CZZwere obtaintid. %. slight decrease in melting point was noted near the end of FeveraI plateaus. This is attributed to isoparaffin and cyclop;traffin hydrorarhons present, in the xas. The still has heen used t o prepare narrow boiling fractions from lubricating oil stocks as the first step in a separations program. The opcration of the unit has been satisfactory for the heaviest fraction distilled so far. In this rase a maximum temperat,ure of 600" F.~ w olxerved. s LITERATURE CITED (1) 131,ewer.
4.K., and Madorsky, S. L.. J . Renearch S a t l . Bur.
Staridads, 38, 129 (19.17).
( 2 ) Bpron. E. 8.. Bowman, J. R., and Coull, J., Ind. Eno. Chem., 43, 1002 (1951).
Fawvcett, E . W.. and McCoxen, J . L., U. 9. Patent 2,073,202 (llarch 9, 193i). (4) Fenske, 31. R . , I n d . Eng. Chern.. 2 4 , 4 8 2 (1932). (5) liadorsky. S.L., J . Research S a t l . Bur. Standards, 44, 136 (1960). (6) lIair, B. J.,Pignocco, A. J . , and Rossini, F. D., ASAL.CHEY.,2 7 , (3)
190 (1955).
(7) Llulliken. R. S.. and Harkins, W. D . , J . A m . C h e m . SOC.,44, 37 (1922). (8) T r e r o y . D. J.,Ind. Eng. C h e m . , 44, 1888 (1952). RECEIVED for review Noremher 26, 1934. Accepted February 5 , 1955. Presented hefore tlie Division of Petrolellin Chemistry at the 125th Meeting of the A M E R I C ~ CHEMICAL S SOCIETY, Kansas City, N o . , 19.54,
Determination of Traces of Nickel and Zinc in Copper and Its Salts LOUIS MEITES Sterling Chemistry Laboratory, Y a l e University, N e w Haven, Conn.
A solution of the sample in an ammoniacal ammonium chloride medium containing excess hydrazine is electrol>zedwith a mercury cathode, the potential of which is automatically maintained constant at -0.85 volt Z'S. S.C.E. for the removal of the copper. Nickel and zinc are determined polarographically in the residual solution. With certain modifications in rare instances, results accurate and precise to about &t;% or better can easily be attained over the range of nickel and zinc contents from lo-' to 10-6%. An entire analjsis can be completed within 7.5 minutes.
T""'
, ( cxterminat,ion of traces of nickel a n d zinc in metallic copper and its salts presents a rather difficult problem. Since copper in large amounts interferes in most procedures for the determinat,ion of nickel and zinc, i t must be removed practirallj. complct~ely.without. contaminating the sample with significant ctnionnts of nickel and zinc which may he present, in the reagents. This pxper presents a method by n-hich as little as 10-q?O of eithci, nickel or zinc may be determined in cupriferous materials. The copper is removed by electrolyzing an ammoniacal solution of t h r sample with a mercury cathode the potent'ial of x-ihich is licipt roi~.qt:inta t -0.85 volt Z'R.S.C.E. .is is shown by Figure 1. this potrntial suffice$ for the complete reduction of copper to the metal hut is without effect on nickel and zinc. T h e peculiar virtue of the electrolytic sixparation lies in the fact that, i t practically eliminates the possihility of adventitious contamination of the s:tnipk. Indeed, this can be entirely eliminated by a simple prel)iir~ifir:ttionof the supporting electrolyte solution. PROCEDURE
T h e followiiig procedurc is recommended for the determination of nickel and zinc in sample? containing not more t h a n ahout 20 times as much nickel as zinc, and not less than :$bout 5 X 10-477, of either. Dissolve a weighed sample containing about 1 gram of copper iri a minimal volume of water or dilute nitric arid and transfer to a 250-nil. volumetric flask. Add 60 ml. of a stock solution rontaining 1.V ammonia, 4M ammonium chloride, and 1M hydrazine hydrochloride. This solution should he made up fresh every week or two hecause of the slow air oxidation of the hydrazine, and should he stored in a thoroughly cleaned polyethylene bottle. Allow the mixture t o stand in the umtoppered flask for a minute or two. During this period the cupric ammonia complex is practically completely reduced t o the cuprous state according t o the NzHa = 4Cu(NHJ)2+ 4SH4+ equation 4Cu(NH3],++ N2. When vigorous evolution of nitrogen has ceased and the solution is nearly colorless, dilute i t to volume and transfer a portion of i t to a clean dry cell suitable for controlled potential electrolysis.
+
+
+
T h r cell used in this work was that, shown in Figure 3 of a previous paper (3), Jvhere its advantages over previously described cells were discussed in detail. I n this procedure the working anode is a helix of platinum wire wrapped around the stirrer shaft, and it is to depolarize this electrode that the hydrazine is included in the supporting electrolyte. As a referener electrode the author used a saturated calomel electrode of the type supplied for use with Beckman pH meters (Type B-I 1i0, Sational Techniral Laboratories, South Pasadena, Calif,). Unlike some instruments previously described, the .inalytical Inst,ruments, Inc., Bristol, Conn., potentiostat used in this work does not appear to object to the presence of tlie resistance of this type of electrode in its control circuit. Adjust the potentiostat t o maintain the potential of the mercury cathode constant a t -0.85 volt v s . S.C.E. and an efficient stirrer immersed in the mercury-solution interface t o keep the surface of the mercury in rapid motion, and alloiv the electrolysip to proreed unattended for 45 to 60 minutes. exclude air, from the solution during the electr oxidation of the cuprous ammonia complex at, the surface of the solution. ivill reduce t h e concentration of copper remainto about 0.01% of its initial value. Though the height of the copper wave in such a solution will be too small to interfere with the measurement of the nickel and zinc wave height. for most samples, it may very rarely be desirable t o continue the electrolysis for anot,her 15 minutes or FO to reduce the copper concentration still further. After the desired length of time, IT-ithdrair-about 50 ml. of the solution into a dry polarographic cell without interrupting t,he electrolysis current. Add one drop (0.05 nil.) of a 20/, aqueous solution of Triton S-100 (Rohm and Haas Co., Philadelphia, Pa.) to serve as a maximum suppressor, deaerate nitrogen or hydrogen, and record the polarogra from -0.8 to -1.5 volt,s vs. S.C.I
