Cobalt selenocyanates

Three papers have reported on the formation of soluble Co -SeCN complexes. Toropova [56TOR] concluded from spectrophotometric measurements that the interactions in water were too weak to be quantified by this technique. Golub and Skopenko [62GOL/SKO] studied the system in water, aqueous acetone, acetone, and methanol by means of spectrophotometric, potentiometric (Ag/AgSeCN-electrode), and conductometric methods. The potentiometric data in water n = 5) were collected at an undefined temperature with [Co ]tot = [SeCN ],ot = 0.02 to 0.04 M and in the absence of a supporting electrolyte. The calibration of the electrode system is not described. Data were ana- 

Also the work by Satyanarayana, Sahu and Das [75SAT/SAH] is based on po-tentiometric measurements with the Ag/AgSeCN-electrode and conducted in the absence of a supporting electrolyte. The Davies equation was applied to the calculation of the activity coefficient of Co, 0.03 to 0.08 M, and the activity coefficients of SeCN and CoSeCN were assumed to cancel in the calculation of the formation constant. On this basis, Ai(V.134) = (30.913.8) M was calculated from 6 experimental data at 298.15 K. 

Considering the inherent uncertainties associated with the evaluation of /f(V.I34) in both papers, the review can only accept the value by Satyanarayana et al. [75SAT/SAH] assigned with an uncertainty of 0.3 logarithmic units  

Satyanarayana et al. [75SAT/SAH] also determined the enthalpy change of Reaction (V.134) by direct calorimetry. The equilibrium constant evaluated from the potentiometric measurements was used to calculate concentration changes. This implies that a close connection exists between the two quantities evaluated and hence, that the reaction enthalpy reported is connected with a considerable uncertainty. The review therefore accepts the reaction enthalpy reported assigned with a considerably larger uncertainty than given in the paper  

The unsymmetrical uncertainty limits are suggested by the results obtained for the Ni PSeCN system in the same paper, cf. Section V. 11.1.4. 

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Cookeas EG, Efstathiou CE (1994) Flow injection amperometric determination of thiocyanate and selenocyanate at a cobalt phthalocyanine modified carbon paste electrode. Analyst 119 1607-1612... 

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