1198
ANALYTICAL CHEMISTRY
and from a control animal. Fat from the former steer, known to have ingested an excessive amount of toxaphene, was found to contain 700 p.p.m. more organic chlorine (calculated as toxaphene) than that from the control. These samples were treated by Carter et al. (1)to reduce the fat content and thereby concentrate the chlorinated compound. This treatment followed the method of Schechter et al. (2). The dashed curve in the upper half of Figure 1 is the infrared spectrum of the treated fat from the control steer, and the solid curve is the spectrum of the treated fat from the steer which had been fed toxaphene-sprayed alfalfa. The bands a t 7.67, 8.17, 10.54, 10.92, 11.11, 11.23, 11.52, 12.56, and 12.94 microns indicate the presence of toxaphene in the fat of the toxaphene-fed steer. It is evident that none of these bands appears in the fat from the control. The infrared absorption method of detection of toxaphene is
not necessarily applicable to all types of materials. In the present instance, no attempt a t quantitative estimation was made because of the uncertainty of the extraction and concentration procedure and the possible dissimilarity of the nontoxaphene portions of the fat from the two animals. Without the use of the concentration procedure referred to above, it would have been impossible to detect the bands attributable to toxaphene in the original untreated fat. LITERATURE CITED
(1) Carter, R. H., Nelson, R. H., and Gersdorff, W. A., Advances in Chem. Series, No. 1, 271-3 (1950). (2) Schechter, M. S., Pogorelskin, M. A., and Haller, H. L., . ~ N A L . CREM.,19, 51 (1947).
RECEIVED for review February 13, 1952. Accepted March 12, 1952.
Amperometric Titration of Zinc with Potassium Ferrocyanide A. L. WOODSONl, B. H. JOHNSON*, AND s. R. COOPER Howard University, Washington, D. C. PALENKA ( 4 ) and Riccoboni and Goldschmied (3) have perS formed amperometric titrations of zinc with potassium ferrocyanide. They recommended that, in the presence of free acid, the ferrocyanide solution be titrated with zinc solut,ion. Under this condition they noted that K,ZnS[Fe(CN)& was a product of the reaction. Nimer, Hamm, and Lee (2) have titrated zinc ions directly in ammonium acetate solution with potassium ferrocyanide. Their precipitate was calculated to be ZntFe(CN)e. This investigation was undertaken t o ascertain the possibility of obtaining reproducible and sufficiently accurate stoichiometric results in the direct titration of small amounts of zinc with potassium ferrocyanide under acid conditions. APPARATUS AND SOLUTIONS
The olarimeter, which was employed in this work, was constructei in accordance with the directions given by Neuberger (1). One exception was made, in that a Leeds & Northrup Type P galvanometer equipped with a lamp and scale was used for measuring the current. The galvanometer sensitivity was 0.008 pa. per mm. and its period was 8.8 seconds. The reference electrode was similar to that proposed by Keuberger. It consisted of a 250-ml. wide-mouthed bottle fitted with a two-hole rubber stopper, and containing a large pool of mercury overlaid with 0.1 Y potassium chloride solution. A platinum wire sealed in a glass tube served as the metal contact for this electrode. The liquid contact was made by means of a salt bridge, which contained 0.1 N potassium chloride and was plugged a t its open end with a 2% solution of agar agar in 0.1 N potassium chloride. The platinum wire contact and the salt bridge were passed through the rubber stopper that sealed the electrode. The titration vessel was a suitable wide-mouthed bottle, fitted with a rubber stopper bearing five holes. These were for the insertion of the dropping electrode, a microburet, the arm of the external electrode, and inlet and outlet tubes for nitrogen. The inlet tube for nitrogen extended to the bottom of the container, while the outlet tube was flush with the bottom of the stopper. C . P . potassium ferrocyanide, zinc sulfate, and potassium chloride were recrystallized. The zinc sulfate solution was standardized by evapordtion and weighing as ZnS04. The potassium ferrocyanide was standardized against standard potassium permanganate. The remaining solutions were approximate.
to its entrance into the vessel the nitrogen had been passed through 0.1 N potassium chloride solution. After the stirring had been completed, a microburet containing the ferrocyanide solution was pushed through the remaining hole in the stopper. The drop time and voltage were set. Then the sensitivity of the galvanometer was adjusted so as to give a deflection of 100 mm. After the expiration of a few minutes, the current became steady and the initial mean of the galvanometer deflections was read. A small quantity of the titrant was added, the solution was stirred with nitrogen for 1 minute, and the current was read 2 minutes thereafter. This latter procedure was continued throughout the titration. Other titrations were performed in which the effect of acid was studied. In all cases the ratio of the volume of zina solution to the potassium chloride solution was 1 to 15. The extrapolation method was employed in ascertaining the volume of ferrocyanide used in each titration. , Figure 1 is a typical titration curve. Table I gives the data for these titrations. DISCUSSION
Experiments 1 to 6 showed that the titration gave fairly reproducible results when free acid was absent.
let
TITRATIONS
Two milliliters of the zinc solution were placed in the titration vessel, 30 ml. of 0.1 N potassium chloride solution were added, and the stopper bearing the contact arm of the reference electrode was inserted. The dropping electrode was put in place, and the solution stirred for 10 minutes with oxygen-free nitrogen. Prior 1 2
Present address, Suburban Chemical Co., Chicago, Ill. Present address, Central State College, Wilberforce, Ohio.
1 2 4 6 8 10 12
0
ML. OF TITRANT ADDED Figure 1. Titration of 10 iM1. of Zinc Sulfate Solution with Potassium Ferrocyanide End point 9.70 ml. of potaeeium ierrocyanids
V O L U M E 24, NO. 7, J U L Y 1 9 5 2 Table I.
Titration of Zinc with Potassium Ferrocyanide
(Drop time 1.8 seconds. Room temp. E = -1.30 volts. 0.0495 M KIFe(CiY)s. 0.073 .If ZnSOd) Zn++ 6 0.1 KCI. s KdFe(CN)o, Soh., HzS04, NO. hI1. MI. nr1. 111. 1 2.00 0.0 30 0 1.82 2 2.00 0 0 30 0 1.78 3 2.00 0.0 30.0 1.825 4 2.00 0.0 30.0 1.81 5 2.00 0.0 30.0 1.81 6 30.0 2.00 0.0 1.825 7 30.0 2.00 1.92 1.0 8 30.0 2.00 2.0 1.95 9 30.0 1,925 2.00 3.0 10 30.0 2.00 4.0 1.94 11 2.00 5.0 30.0 1.93 12 2.00 1.0 30.0 1.92 2.00 1.0 30.0 1.93 , 13 14 2.00 1. 0 30.0 1.94 15 1.0 30.0 1.95 2.00 16 2.00 1.0 30.0 1.94 17 2.00 2.0 30.0 1.95 18 2.00 2.0 30.0 1.93 19 2.00 2.0 30.0 1.93 20 2.00 2.0 30.0 1.95 21 1.94 22 9.70 23 9.72 24 9.72 25 9.70 26 4.85 27 4.85 28 4.84 29 4.84 1.939 m1./2 ml. of Zn A\7
Experiments 7 to 11 were performed to study the effect of free acid on the end point of the titration. Graphs of these and all other titrations showed, before the end point had been reached, the same initial curvature f o l l o ~ e dby a straight line. This curvature became more extensive as the free acid content was increased. Thus those titrations in which larger amounts of acid were employed gave a smaller number of points on the straight-line portion of the curve. Several preliminary titrations RThich were performed a t a temperature of 25" f 0.1" C. gave similar graphs.
1199
As the ratio of the volume of the zinc solution to the potassium chloride was constant, it is probable that the solubility of the precipitate in the free acid was responsible for this initial curvature. The end points in these experiments do not show much variation. As the end points of titrations in which smaller amounts of free acid were present were easy t o ascertain, all the remaining experiments were performed with 1 to 2 ml. of free acid per 30 ml. of supporting electrolyte. The mean of experiments 12 to 29 is 1.94 ml. of potassium ferrocyanide solution per 2 ml. of zinc solution, with a standard deviation of 0.015 ml. The ratio of the moles of titrant to zinc is 1 to 1.52 or 2 to 3.04. This indicates that the reaction was probably taking place according to the following equation: 3Zn++
+ 2Fe(CS)---- + 2 R f
+ IinZna[Fe(CN)6]2
The range of acidity over which this equation appears t o hold and ab the same time to give easily interpreted graphs is approximately 0.18 to 0.35 S in sulfuric acid. Finally, the errors are compatible with this type of procedure. SUMMARY
Small amounts of zinc have been titrated with potassium ferrocyanide in the presence of free sulfuric acid with a fair degree of accuracy. The reaction appeared to follow the accepted equation of these reactants, in that KtZna[Fe(C?;)6]2was a product of the titration. LITERATURE CITED
(1) (2)
Neuberger, A , , Z . anal. Cheni., 116, 1 (1939). Nimer, E. L., Hamm, R. E., and Lee, G. L., ANAL.CHEM.,22,790
(3)
Riccoboni, L., and Goldschmied, P., Proc. X l t h Intern. Congr.
(4)
Spalenka, hI., Collection Czechosloa. Chem. Communs., 11, 146
(1950).
Pure and Applied Chem. ( L o n d o n ) , 1, 199 (1947). (1939).
RECEIVED for review January 18, 1952. Accepted April
1 1 , 1952.
Analysis of Mixtures of Chloride and Bromide by Ion-Exchange Chromatography WILLIAM RIEMAN I11 AND SIEGFRIED LINDENBAUM School of Chemistry, Rutgers University, Neu, Brunswick, N. J .
HE determination of chloride and bromide in a mixture of the two has long been a difficult analytical problem. -4tteberry and Boyd ( 1 ) have demonstrated that the halides can be separated by ion-exchange chromatography, but their data are expressed only as a graph which leaves some doubt concerning the completeness of the separation. This note reports an investigation, started before the publication of the paper by Atteberry and Boyd, on the quantitative analysis of chloride-bromide mixtures. APPARATUS AND REAGENTS
Amberlite XE-67, a strong-base anion-exchange resin, was used in a 16.8 em. X 3.83 sq. cm. column. It required no preliminary treatment except the removal of the fine particles by decantation and treatment with 0.60 M sodium nitrate in a column until the eluate contained no chloride. The eluant was 0.60 M sodium nitrate containing 0.40 ml. of Cutscum per liter. This is a nonionic wetting agent purchased from Fisher Scientific Co., N. Y . Cutscum was added a t first to ensure thorough wetting of the siphon pipet and hence a constant volume of delivery. It was found later that the addition of Cutscum permitted an increase in flow rate without the undesirable effects (flattening and tailing of the elution graphs) that usually accompany such an increase. Silver nitrate, approximately 0.1 M , was standardized by the titration of pure sodium chloride or potassium bromide dissolved in the eluant solution, with potassium chromate as indicator. Thus the conditions of standardization closely resembled those of the analysis.
D
-1 siphon pipet, used to deliver equa volumes of eluate, is illustrated in Figure 1 The drainage tube, D,must be large enough (4-to 6-mm. internal diameter) so that the entire sample is delivered before a drop forms a t T . The overflow tube, 8, must have a small diameter (2- to 3-mm. internal diameter) to prevent the solution from draining instead of siphoning. The overflow tube should be close to the neck, N , of the receiver, R, to minimize the effect of small changes in the position of the siphon pipet. The end of the delivery tube, E, should be ground open on one side for about 2 cm. to give complete drainage. PROCEDURE
F i g u r e 1. Siphon Pipet
Weighed quantities of sodium chloride and potassium bromide (not over 2.5 millimoles of either) were mixed and dissolved in 2 ml. of water in a small beaker. The solution was poured on the top of the column, and the beaker was rinsed with a small volume of eluant. Then the eluant was passed through the column at a rate of 1.0 cm. per minute. Fractions of 7.85 ml. of the eluate were collected with the siphon pipet. Each fraction (sometimes a mixture of two con-
