0.4001
the cross-linked resin is useful as a chromatographic stationary phase for fractionation of mixtures of these compounds. A detailed report is being prepared.
ANTHRACENE
a300
0.100 0.1000 0
43
b,,
LITERATURE CITED
$020/]
I
45
,
47
Figure 1.
,
49
(1) De Ligny, C. L., Schmidt, H. M., De Vries, W., Rec. Trav. Chim. 82, 1061 11963). ( 2 ) James, A. T., Martin, A. J. P., Bzochem. J . 50, 679 (1952). ( 3 ) Smets, G., Balogh, V., Castrille, Y., J . Polymer Scz., Pt. C , S o . 4, 1467 (1964).
‘-%+
51
53 55 57 ml. SOLVENT
59
61
63
,
65 6 5
Elution curve for anthracene and pyrene
Bed depth, 4 8 cm.; bed diameter, 1.2 cm.; particle size, 2 5 0 - 2 7 0 mesh; solvent, 3 :1 by volume acetone-n-hexane (Spectrograde); flow rate, 3.03.5 ml./hr.; fraction size, 0.38-0.42 ml.; sample size, 3 . 0 6 mg. of anthrocene, 3 . 1 5 mg. of pyrene; R,, 0.72 far anthracene, 0.60 far pyrene; H E T P , 0.39 cm.; R, value, 2.45
linear T N B P with various donors obtained by us, and on the work of Smets et al. ( 3 ) related to the study of electron acceptor polymers. The charge-transfer spectra associated with T N B P exhibit absorption maxima in the ultraviolet
and visible between 350 and 500 mp. Our observations indicate that T N B P forms complexes with such molecules as naphthalene, anthracene, pyrene, carbazole, naphthylamine, aniline, transstilbene and dimethylphenol and that
Department of Chemistry Florida State University Tallahassee. Fla. 32306 RECEIVEDfor review July 6, 1964. Accepted Auguat 4, 1964. Work supported by Research Grant 536-A from the Petroleum Research Fund and by Research Grant Gbl 10064 from the United States Public Health Service.
Coulometric Diffusion Layer Titrations Using the Ring-Disk Electrode with Amperometric End Point Detection SIR: A rapid, simple, and sensitive electrochemical technique has been developed which is based upon the titration of a species present in the diffusion layer of a rotating ring-disk electrode. This method has been applied to the analysis of arsenious solutions over the Concentration range 1 x 10-~J1to 1 x l O - 3 M and utilizes the unique hydrodynamic features of a ring-disk electrode, the simplicity of a constant-current coulometric titration with internally generated reagent, and the sensitivity of an amperometric end point detection. Thiq technique is described below in terms of reaction between bromine and arsenious ion. Bromine can be generatpd a t a 100% current efficiency from an acid bromide supporting electrolyte by anodic oxidation a t a platinum disk electrode. If the supporting electrolyte also contains arsenious ion (which is not electroactive a t the potential bromide is oxidized to bromine), and the reaction Brz
+ As(III)+
.\s(V)
+ 2 Br-
is fast compared to convective diffusion, the surface concentration of bromine a t the disk electrode will remain zero until the anodic current flowing through the disk electrode (iajD exceeds the maximum flux of arsenious to the electrode, 2 186
ANALYTICAL CHEMISTRY
f4a(III))
by the amount shown in Equa-
tion l : cia)D
2 2 Ff4s(III) A
(1)
where d is the electrode area and F is Faraday’s constant. If the disk electrode is surrounded by a closely spaced, but electrically insulated, ring electrode, and the potential of the ring electrode is adjusted so as to reduce any bromine which reaches the ring electrode, the excess bromine generated a t the disk electrode will produce a cathodic ring current ( i J R . I t is not necessary to solve this boundary value problem analytically to arrive a t the form of the ( Q R us. (i.)D curve. Excess bromine generated at the disk electrode will diffuse into the solution normal to the disk electrode and will reach the ring by convective radial flow of supporting electrolyte and diffusion. Before any free bromine can reach the ring electrode, the bromine muqt react with additional As(II1) over and above that given by the minimum current in Equation 1. First, excess bromine will penetrate into the diffusion layer normal to the disk electrode, producing a zero As(II1) concentration for some distance away from the disk electrode and thereby consuming s o r e of the excess bromine. Second, the As(II1) present in the cylindrical shell normal to
the annulus between the ring and disk will react with excess bromine before any bromine can be reduced a t the ring electrode. Ring current will be observed initially when the bromine reaches is the inner edge of the ring; as (ia)D increased, the ring current will increase nonlinearly until the concentration in the region normal to the ring surface has been reduced to zero for some distance away from the ring electrode. Further increase in ( i J Dwill then produce a linear increase in (iJR. Extrapolation of this linear portion back to the ring electrode residual current will give a value of ( i , j D proportional to the concentration of As(II1) in the supporting electrolyte. EXPERIMENTAL
Apparatus and Reagents.
CHEMAnalytical Reagent Grade chemicals and conductivity water were used. Sulfuric acid should be freshly fumed for best results. The supporting electrolyte was 0.2M KBr and 1, O M
ICALS.
HzSOI. RING-DISKELECTRODE. An electrode with a disk diameter of 0.7772 cm., inner ring diameter of 0.7973 cm., and outer ring diameter of 0.8886 cm. was used. The disk and ring were constructed of platinum. A detailed description of the construction of such electrodes will be published in the near future.
CURRENT SOURCE. A circuit diagram of this electronically regulated device is given elsewhere ( I ) . .4 linear current scan was obtained by coupling a synchronous motor to a 10-turn helical potentiometer furnishing the reference voltage to the control amplifier. RECORDERS.For As(II1) concentrations less than 10-6.M a Sargent 1-mv. recorder was used to record ring currents as a function of time, while scanning the anodic disk current linearly with time. Above 10-6-V A k ( I I I ) , a Jfoseley Model 2.1 X-Y recorder was used to record ring a n d disk currents simultaneously. Procedure. T h e ring-disk electrode was introduced into the cell and rotated a t 21.1 radians per second; ( i J R was recorded as a function of (ia)D.Using the automatic current scanning technique, the rate of scan must be chosen sufficiently slow to e n w r e t h a t the rate of establishment of steady state ring currents is rapid compared t o the rate of current scan. Disk currents were scanned in both increasing and decreasing directions. If the current scan rate was not too fast, no hysteresis was observed. A typical titration curve can be obtained in 5 minutes.
i
Figure 2.
Relationship between ( i e , p , ) D and C5/.~,cm)
0 - iD X A - iD X 0 - iD X A - iD X
1 02,C h X 1 0 4 M , slope = 5.42 X 1 Os p a . / M 10, C” X 1 OKM,slope = 5.2 1 X 1 OK p a . / M 1 , C bX 1 06M,slope = 5.2 1 X 1 Os pa./M 1 O-’, C5 X 1 O’M, slope = 5 . 2 8 X 1 O5 pa./M Ring potential = +0.3 volt vs. S.C.E. w = 21.1 rodians/second
determination of (ie ) D is &2% and is not affected by the current level. The supporting electrolyte gives a blank equivalent to 4 X 1 0 - ~ 5 1 As(II1) in RESULTS one experiment (A,Figure 2) and 2.4 X 10-6M As(II1) in another experiment Typical ( i J R us. (iJ0 curves are ( 0 , Figure 2 ) and represents varying shown in Figure 1. The rising linear amounts of reducing impurities present portion was extrapolated to the ring in the supporting electrolyte. Supportresidual current to obtain the end point ing electrolyte prepared from freshly disk current (ie ) D . Plots of (ie )D US. bulk arsenious concentration [ C 5 ~ a ( ~ ~ ~fumed ) ] , sulfuric acid yields the lowest blank. The average slope of the lines are shown in Figure 2 and dem0.07 X lo5 pa. in Figure 2 is 5.28 onstrate that (i, ) D is a linear funcper Ji As(II1) over the concentration tion of [CbA,crrr)]from 1 X lO-’M range 1 x lO-’M to 1 x 10-3v. On to 1 X 10-3JL. Reproducibility of the
+
10.0
7.5
0
1.17
2.34
4.70
7.05
-
the basis of our preliminary experiments it appears to be possible to extend the concentration range to a t least a minimum of 10-8V and a maximum of lO-*X. The loner concentration-limit of this technique is determined by the ratio of the residual current a t the ring electrode to the bromine reduction current a t the ring electrode. Increasing the speed of rotation will decrease this ratio and permit the application t o more dilute ,Is(III) solutions. The ultimate limitation of this method u-ill occur at high rotational speeds with the onset of turbulence and or chemical kinetic, rather than convective, diffusion control. I t is obvious that the method described above may be applied to any rapid titration reaction for which it is possible to electrically generate the reactant. Further applications of this technique are currently being investigated and will be published in the near future. LITERATURE CITED
(1) Rouse, T. O., Ph.1). thesis, Cniversity of Minnesota, 1960.
20
40
60
(id0 pa.
Figure 1 ,
Diffusion layer titration curves
Bulk concn. of As(III) X 1 05Mgiven in flgure
80
RECEIVEDfor review July 31, 1964. Accepted August 11, 1964. Work carried out under a grant from the Xational Science Foundation. ST~NLE BY R~CKENSTEIN I ~ E Z I XC.I SJOHKSOK Department of Chemistry I’niverslty of Minnesota Minneapolis, Minn. 55455
VOL. 36, NO. 1 1 , OCTOBER 1964
2187
