Abstract
The direct potentiometric determination of thiocyanate ion can be carried out with a bromide ion selective electrode. The following parameters of the bromide ion electrode were studied in detail; Nernstain response to thiocyanate ion concentration, the effect of pH on thiocyanate response and the electrode response to other anions. The application of this membrane electrode as an indicator electrode for potentiometric titration was discussed.
The bromide membrane electrode was preconditioned by soaking in 10-2M potassium thiocyanate solution for over one hour. During the measurement, the test solution was stirred by magnetic-mixer to obtain stable potential valves because of large stirring effect.
The response time for thiocyanate ion was fairly short and reaches equilibrium within ten seconds after the electrode was immersed in the test solution. The potential of the bromide membrane electrode was dependent on the thiocyanate ion concentration to below 10-6M potassium thiocyanate solutions adjusted to constant ionic strength with potassium nitrate. The response is slightly less than Nernstain and a linear calibration curve is obtained and the value of the slope of potentials vs. logarithmic concentration was approximately 50mV for thiocyanate ion over the concentration range 10-1 to 10-5M thiocyanate.
The selectivity was measured in 10-14M solution of the interference, and measured selectivity valves for anions were listed. The electrode does not respond to most common anions such as F-, NO-3, SO42-, CO3-, OH- but responds partly metal complex cyanide ions such as [Fe(CN)6]3-, [Fe(CN)6]4-. This fact suggests that the bromide membrane electrode responds more unstable complex cyanide ion such as [Cd(CN)4]2-, [Zn(CN)4]2-, [Cu(CN)4]2-.
The interference encountered with S2O32- is serious. The extremely high selectivity for S2O32- should result from reduction of the membrane. This electrode was not influenced by pH in the range pH 2.012.0 at 10-3M thiocyanate ion.
Thiocyanate ion has been titrated potentiometrically with Ag+, Hg2+ as a reagent and using the bromide selective electrode as an indicator. The well defined reversed S shape of the titration curve could be obtained, and standard deviation of σ=±0.5 on both cases. The potentiometric titration of thiocyanate ion with Ag+, Hg2+ was also followed by means of the silver metal electrode. The difference between the bromide membrane electrode and silver metalel ectrode was compared.
The bromide electrode responds [Ag(SCN)2]-, [Ag(SCN)3]2-, [Hg(SCN)3]- and [Hg(SCN)4]2- formed during the titration, and only one end point observed.
In the case of silver metal electrode, two break point observed in the titration with Hg2+.
