Abstract
Succcssive amperometric titrations of thiocyanateand cyanate with silver nitrate solution and a rotatingplatinum electrode (1000rpm) have been carried out at zero applied potential with a saturated calomel electrode.
Suspended silver thiocyanate does not cathodically depolarize the rotating platinum electrode due to its low solubility, while silver cyanate readily gives rise to a cathodic current because of its much greater solubility in aqueous solution. They scarcely coprecipitate in an argentometric titration; therefore, thiocyanate in a neutral supporting electrolyte such as potassium nitrate, or barium nitrate can be titrated in the presence of cyanate at room temperature (about 20°C). Then cyanate can be titrated below 5°C by adding 25vol% methanol to decrease the solubility of silver cyanate.
The recommended procedure is as follows: Add 10ml of 1F potassium nitrate solution (or barium nitrate if carbonate is present) and 4ml of 0.5% gelatin solution to a neutral sample solution containing thiocyanate and cyanate, and dilute to about 100ml with re-distilled water. Titrate with silver nitrate solution at about 20°C to the thiocyanate end point. Then titrate the cyanate at about 2°C after adding 40ml methanol.
The possible concentration ranges were 0.00010.01F for thiocyanate, and 0.0050.01F for cyanate, respectively. The relative percentage errors were±0.2% for 0.01F thiocyanate, ±1% for 0.01F cyanate, ±13% for 0.001F thiocyanate, and O.01F cyanate.
The time required is about 30minutes.
