Response of Cyanide Ion Selective Membrane Electrodes in the Presence of Metal Ions Marco Mascini and Aldo Napoli lnstituto di Chimica Analitica, Citta Universitaria, 00185 Roma, l t a l y
The cyanide membrane electrode has widespread interest because of the claimed possibility to determine directly the amount of cyanide in solution. The presence of a metal ion in such solutions is generally the rule. It, therefore, seems necessary to study the response of the cyanide membrane electrode to cyanide ion in solutions containing metal ions. Toth and Pungor ( I ) reported the qualitative effect of several metal ions and recently Fleet and Van Storp (2) suggested dividing the metal ions in two groups. In the first group are the cations forming stable soluble complexes, which do not respond at the electrode surface, while in the latter group are those which form complexes weaker than Ag(CIK)2- and which show a response due to the dissociation of the cyanide by a disproportionation reaction a t the electrode surface. Recently ( 3 ) it was shown how the response of the cyanide electrode is affected by pH and it was concluded that the following reaction:
SHCN
+ AgI
=+= Ag(CN)?-
THEORY At the silver iodide membrane surface, in contact with cyanide ions, the following reaction occurs:
+ 2CK
e Ag(CN)?-
+ I-
K
=
10'
(1)
The equilibrium constant is large enough and it is postulated that iodide concentration is proportional to the cyanide concentration and fixes the membrane potential through the Equation, a t 25 "C:
E
=
E" - 0.0.59 log[I-]
+
where X - is Br- or C1- and K = 108 and 1O1O, respectively. The limits into which the electrode potential will indicate either the total cyanide or the free cyanide concentration have been investigated, considering also the presence of a metal ion in the solution; both these values are, in fact, attractive from several points of view. If C C N and CM are the total concentrations of cyanide and of the metal ion a t the surface of the iodide membrane electrode, the following equations can be written:
K Toth and E Pungor, Anal Chim Acta, 51, 221 (1970) B Fleet and H Von Storp, Ana/ Chem , 43, 1575 (1971) M Mascini, Ana/ Chem 45, 614 (1973) D H Evans, Anal Chem , 44,875 (1972) (5) M S Frant, Plating, 58, 686 (1971)
where K1 is the protonation constant of the cyanide ion ( K 1 = lo9) and pL are the overall formation constants of the Me(CN), complexes. In Equation 5 , eventual side reactions of the metal ion must be considered, particularly the formation of hydroxo complexes. In this case, neglecting polynuclear species, the term 8,y,[Me][OH-]' should be added, in which y L is the cumulative formation constant of the Me(OH), species. Furthermore, from Equation 1:
[I-]
(2)
By means of this model, the behavior of the membrane electrode was explained either in solution of free cyanide (pH > 11) (2, 4 ) or in the presence of HCN (pH < 11)( 3 ) . The membrane containing solid silver iodide is separated from the sample solution by a porous barrier ( 5 ) (of AgzS in homogeneous membrane or by polythene in the heterogeneous membrane electrodes, used in this study). The electrode response should be interpreted in terms of the mass transfer species through the barrier and in terms of equilibrium considerations. However, in this theoretical treatment, the differences of the diffusion coefficients for the various complex species are neglected, because they are not very different and not available a t all. Thus, it is assumed that the mass transfer does not change the ratio of the concentration of the species in solution. (1) (2) (3) (4)
+
+ 21- + 2H'
that fits the behavior of the cyanide electrode as a function of pH, must be considered. In this paper, the presence of cations is studied and general equations are derived, taking in account the influence of any cyanide complexes on the response of selective membrane electrodes.
AgI
Moreover, the bromide and chloride ion selective membrane electrodes could be employed as well as the iodide membrane electrode in order to obtain a cyanide response ( I ) . The function mechanism of these electrodes is analogous and the diffusion barrier model ( 4 ) can also be invoked. Equilibrium 1 will be substituted by the following one: AgX 2CN- e Ag(Ch')?- Xi3 )
=
y T [CY-]
(7)
By introducing Equations 6 and 7 into 4 and 5 , the following is obtained:
Cc,
= 81-1
+ [CN-Jl + K,[H+l + C ia,[Me][CK-]'-
[I-](2
+ K-'" + [Ht]K,K-"' + C , l P K-l'-[&le][I-! C,
=
[Me](l
+ B,P,K-"'[I-I
)
=
)
(h)
(9)
)
Equations 8 and 9 can be solved to obtain [I-] and [Me]; furthermore, the iodide concentration is related to the membrane potential through Equation 2 . From Equation 8, the influence of pH on the electrode potential can be derived. In fact, neglecting the metal complexes, the following equation can be obtained:
r(~ \
'I-]
= 2
+
K-1 -
+
_
K,K-/?[Hf]
_ (10)
ANALYTICAL CHEMISTRY, VOL. 46, NO. 3, MARCH 1974
447
Zn (IIi
I
13
L
3
2
1
409
L
C"
2
-log C"
1
Figure 3. Theoretical curves and experimental points for the response of silver iodide and silver bromide electrodes to cyanide solution in the presence of zinc(l1) ions for a total cyanide concentration 1 0 - 3 M
For each total cyanide c'oncentration, the free cyanide concentration in the buik of solution and the bromide concentration on the electrode surface are shown. Values of the stability constants are reported in the text
3
2
-log CM
Figure 1. Theoretical curves and experimental points for the response of the silver iodide electrode to cyanide solutions in the presence of cadmium(l1) ions for two total cyanide concentrations
L
3
The curves are referred to free cyanide concentration in the buik of solution and, respectively, to iodide and bromide concentrations on the electrode surfaces. Values of the stability constants are reported in the text
1
Figure 2. Theoretical curves and experimental points for the response of the silver bromide electrode to cyanide solutions in the presence of cadmium(l1) ions for two total cyanide concentrations For each total cyanide concentration. the free cyanide concentration in the buik of solution and the bromide concentratton on the electrode surface are shown Values of the stability constants are reported in the text
This Equation, by considering that K - l I 2
