agree. Any disagreement between the two values provides an indication of the deviation from the theoretical diffusional model (2).This deviation can be of two forms: convection and diffusional shielding of the electrode by the support. Convection usually produces poor experimental precision and deviations from linearity in the i ( t )vs. l/fiplots. Shielding does not influence the linearity of the i(t) vs. 1/ &-plot, rather it distorts the diffusional model which is reflected as an increase in the effective radius of the electrode causing the observed currents to be smaller than theoretically predicted (assuming D is known). Also, and more significantly, the diffusion coefficient calculated from the intercept of the i ( t ) vs. l / f i p l o t . i s smaller than that obtained from the slope. These considerations form the basis for determining whether the micrometer fed capillary can serve as the electrode support for a HMDE without need to resort to an empirical correction for the distorted spherical diffusional model. Figure 1 contains data for the reduction of iodate ion a t three different electrode sizes supported by the FTC, GTC, and G P supports, respectively. The 14.5-mg electrode represents the largest mercury drop that could be supported on the micrometer-capillary assembly. In all cases, excellent linearity is observed. However, extensive diffusional shielding of the electrode is observed a t the FTC support, and only for the largest mercury drops supported on the GTC does the shielding decrease to an acceptable level. Diffusion coefficients for iodate ion calculated from the data contained in Figure 1 and for thallous ion obtained from an analogous study are summarized in Table I. The
data indicate that electrode support geometry has little, if any, effect upon the slope of i ( t ) vs. l / v T p l o t s and, therefore, cannot form the basis for detecting distortions in the diffusional model. In cases where shielding is present, increases in the electrode size result in the effective .radius approaching the true drop radius and, thus, as the drop size increases, the intercept of the i ( t ) vs. 1 / d p l o t increases toward the theoretical value. For the micrometer fed electrodes (FCT and GTC), data in Table I seems to indicate that this trend is present. The long extrapolation required to obtain the intercept and the resultant sensitivity to small changes in the slope may have obscured its magnitude. The important conclusion to be drawn from Table I is that the GTC is an acceptable HMDE support for long time experiments when large electrodes are used, while the FTC as delivered may be, a t best, a support of dubious value for accurate theoretical studies.
LITERATURE CITED (1) W. Kemula and 2. Kublik, "Applications of Hanging Mercury Drop Elec-
trodes in Analytical Chemistry" in "Advances in Analytical Chemistry and Instrumentation," Vol. 2, C. N. Reilley, Ed., Interscience. New York. N.Y.,
1963. (2) I. Shain and K. J. Martin, J. Phys. Chem.,65, 254 (1961). (3) R. W. Murray and D. J. Gross, J. Electroanal. Chem., 13, 1326 (1967). (4) P. E. Whitson, H. W. Vanderborn, and D. H. Evans, Anal. Chem., 45, 1298 (1973). (5) E. R. Brown, D. E. Smith, and G. L. Booman, Anal. Chem., 40, 1411 (1968).
RECEIVEDfor review October 2, 1974. Accepted December 23, 1974.
Determination of Thiocyanate in Water with a Cyanide Selective Electrode Giorgio Nota lstituto Chimico, Universita di Napoli, Via Mezzocannone 4, 80 134 Napoli, /ta/y
Potentiometric determination of traces as well as high concentrations of anions and cations in water by selective ion electrodes (many of which are commercially available), is a widely used technique nowadays. This method has been reviewed by Buck ( I ) . Many electrodes are highly specific, such as those for the determination of "3, CN-, S2and F-, while others are subjected to several and severe interferences. For instance, the commercially available solid state, thiocyanate electrode cannot be employed if Br-, C1-, "3, S20s2-, CN-, I-, and S2- are present in solution even though in small amounts (2); however the liquid membrane electrode does not appear to be so severely affected ( 3 ) . Because of interest in the analysis of waste water, in which the presence and the amount of cyanide and of thiocyanate are of particular significance, a gas-chromatographic method for the detection of CN- and CNS- in water was developed ( 4 ) . Furthermore, it was found that concentrations of thiocyanate and cyanide can be determined potentiometrically by using the CN- selective electrode only if the water has previously been subjected to a simple chemical treatment which consists in a quantitative transformation of CNS- into CN-.
EXPERIMENTAL Apparatus. A digital pH meter (Model 701, Orion Research, Cambridge, Mass.) equipped with a cyanide specific ion electrode (Model 04-06, Orion Research) and with a single junction reference electrode (Model 90-01, Orion Research) was used. Reagents. All reagents were of analytical grade. Bromine water was a saturated solution of bromine in distilled water. The SO2 saturated solution was obtained by bubbling gaseous SO2 into distilled water. To evaluate the accuracy of the method, a standard stock solution of KSCN was used. Procedure. To a 7-ml water sample containing up to 10-"M CNS- in a 10-ml flask, 0.5 ml of 20% was added. Bromine water was then added dropwise until a deep yellow color became persistent. The flask was occasionally shaken, and the excess of bromine was removed after 5 min by adding 0.2 ml of a 5% aqueous solution of phenol. The BrCN was then reduced by adding 0.1 ml of an SOz-saturated solution. The volume of sample was finally brought up to 10 ml with 4 M NaOH and transferred into the pHmeter cell. The potentials were measured a t room temperature according to the Orion Research instruction manual for the cyanide electrode. Metal cations, if they are present in the sample, are removed before the oxidation step by passage through a cationic exchange resin. Ten ml of the water sample are absorbed on the top of a 1.5X 5-cm Dowex 50 X8 column and eluted by 25 ml of 10-3N H2S04. Part of the eluate is then treated as described above. ANALYTICAL CHEMISTRY, VOL. 47, NO. 4, APRiL 1975
763
Table I. Analytical Data Obtained by Using a CN- Electrode on Water Samples Containing Known Amounts of CN- and SCNConcentrations Expressed in ppm
.
J'
605 0 3
401
C K - content from the elecmode response
30'
Sample
$1
A B
1.0
C
/'
D
/' 0.5
4
D"
I/"'
Potential
CN-
-130
-120
Response curve of the cyanide electrode for standard
4Br2
+
4H,O
SO2
+
H,O
-
-
CNBr CNBr
t
HCN HCN
+
OH-
+
SO,'-
-
7Br-
- B r - + SO,'-
--L
C Y
t
HLO
8H'
(1)
- 2H'
(2)
T
(3)
The steps do not require any further comment except that the first one has been suggested in 1923 by Schulek as a volumetric method for determination of thiocyanate ( 5 ) . The procedure is rapid and very simple. The reaction conditions, i.e., temperature and time, as reported in the Experimental section do not seem to play a critical role. The reaction products S042- and Br-, the latter up to 5 X 10" [CN-1, do not interfere with the cyanide selective ion electrode (2). The response is quantitative within the working SCN-. i.e., from 10-5 to range of the electrode (6), Figure 1 shows a plot of the potential of the cyanide electrode us. the log of the thiocyanate concentration in samples of a standard KCNS solution previously treated as described in the experimental section. The plot is linear and shows a slope of -62 mV per logarithmic unit which is in good agreement with the theoretical value of -59.1 a t 25 "C (6). Thiocyanate can be determined according to the procedure suggested above, even though cyanide is present in the sample. It was observed that standard thiocyanate solutions containing comparable amounts of thiocyanate and cyanide, gave a 1 E response related to the sum of thiocyanate and cyanide concentrations. Therefore, provided that the cyanide determination has been carried out before the oxidation procedure, the amount of thiocyanate can be ob-
764
1 1 1
1 1
20 20
1 0.9
1 20
...
After oxidation
6 100 2
20 0.9
E l e c t r o d e , MV
RESULTS AND DISCUSSION Thiocyanate can be quantitatively transformed into cyanide by a three-step procedure according to the following equations:
+
5
100 1
Before oxidation
-150
-140
KSCN solution treated as described in the experimental section
SCN-
CN-
After 5-min boiling at pH 8 -110
-100
-90
Figure 1.
SCN
ANALYTICAL CHEMISTRY, VOL. 47, NO. 4, APRIL 1975
tained by difference. Table I reports some analytical results obtained from samples containing known amount of SCN- and CN-. Since the electrode experimental error is quoted to be f 10% (see Ref. 5 ) , cyanide eventually present in strong excess, Le., [CN-] higher than 10 [CNS-1, must be removed. This can be easily performed by boiling the sample adjusted a t p H 8 for 5 min. The use of formaldehyde for removing cyanide excess (7), found to be very satisfactory in the GLC method ( 4 ) , is not suggested here since a poisoning effect on the electrode has been observed. If metal cations, such a Fez+, Fe3+, Co2+, Co3+, Cd2+, Cu", etc., are present in the water sample, they must be removed since they could form complexes with the cyanide obtained from thiocyanate. This can easily be performed by passing the sample solution through a cationic exchange column. This treatment allows the metal-thiocyanate comlexes to be destroyed too, whose formation constants for metals such as those mentioned above are not higher than l o 3 (8). The recovery of CNS-, tested with the Schulek method ( 5 ) ,after the Dowex 50 column step, was quantitative for samples containing Cu2+,Cd2+,Fe", Cr3+ up to 10 x [CNS-1, also after several-hours standing of the samples. As far as the instrumental requirelhents are concerned, the method suggested in this paper is much simpler than the previously reported GLC method ( 4 ) ; thus, it can be advantageously employed in routine analyses.
LITERATURE CITED (1)R. P. Buck, Anal. Cbem: 44,270 R (1972). (2)Orion Research Inc., Analytical Methods Guide," Cambridge, Mass., 1972. (3)C. J. Coetzee and H. Freiser, Anal. Chem., 41, 1128 (1969). (4)G. Nota and R. Palombari. J. Chromatogr.,84,37 (1973). (5)E. Schulek, 2.Anal. Cbem., 62, 337 (1923). (6)Orion Research Inc.. Instruction manual for cyanide ion activity electrode Mod. 94-06,"Cambridge, Mass., 1972. (7)R. Lang. 2.Anal. Chem., 67, l(1925). (8) "Stability Constants of Metal-Ion Complexes," compiled by L. G. Sillen and A. E. Martell, The Chemical Society, London, 1964.
RECEIVEDfor review July 9, 1974. Accepted November 25, 1974.
