Thermo Fisher Scientific Ion Selective Electrodes Cyanide User Manual
Page 10
Cyanide Electrode
Instruction Manual
10
Using Table 3, the maximum ratio is:
[Br
-1
]
———— = 5X10
3
[CN
-1
]
[Br
-1
] = 5X10
3
[CN-1]
= (5X10
3
) (1X10
-5
)
[Br
-1
] = 5X10
-2
M = maximum bromide concentration for no interference
Complexation
Total concentration, Ct, consists of free ions, Cf, and complexed or bound ions, Cc, in solutions:
Ct = Cf + Cc
Since the electrode only responds to free ions, any complexing agent in the solution reduces the
measured concentration of ions.
Hydrogen ions and many metal ions form complexes with cyanide ions. The presence of any
complexing agent lowers the measured concentration. Since the electrode measures only free
cyanide ions, use of 10M NaOH (ISA) is essential, since it eliminates complexation by hydrogen.
EDTA can be used to break up cyanide complexes formed with many metal ions, including
cadmium, copper, nickel, and zinc. To a sample whose cyanide concentration is not more than 10
ppm, or about 1X10
-3
M, add acetic acid to make the sample solution's pH 4. Add EDTA (disodium)
to about 0.02M (or about 0.76 grams Na
2
EDTA per 100 ml sample). In a hood, heat the mixture to
about 50
o
C for about five minutes to speed up the decomplexation. After cooling the solution, add
10M NaOH (ISA) until the pH 13. The cyanide remains free long enough for concentration
measurements to be made, since EDTA complexes of the metals break up very slowly.
This method will not work for silver, mercury, gold, or cobalt, since they will bind the cyanide too
strongly.
Temperature Influences
Samples and standards should be within ±1
o
C of each other, since electrode potentials are
influenced by changes in temperature. A 1
o
C difference in temperature results in a 2% error at the
1.0X10
-3
M concentration level. Because of the solubility equilibrium on which the electrode
depends, the absolute potential of the reference electrode (or reference portion of the combination
electrode) changes slowly with temperature. The slope of the electrode, as indicated by the factor
"S" in the Nernst equation, also varies with temperature. Table 4 gives values for the "S" factor in
the Nernst equation for the cyanide ion.