Division 1.3 Determination of K2O

by precipitation and weighing of potassium tetraphenylborate.

in a platinum dish on a water-bath then in a sand bath. After dense fumes of perchloric acid have been given off (continue until dry), dissolve with 20 millilitres of hot water and 2-3 millilitres concentrated HCl.

Transfer to a 200 millilitres graduated flask and adjust to volume with distilled water.

Reagents: 6% solution of sodium tetraphenylborate: dissolve 1·5 grammes of the reagent in 250 millilitres distilled water. Remove the light cloudiness which remains by adding 1 gramme of hydrated aluminia. Shake for five minutes and filter, taking care to re-filter the first 20 millilitres obtained.

Washing solution for the precipitate: prepare a little of the potassium salt by precipitation in a solution of about 0·1 grammes KCl to 50 millilitres N/10 HCl into which the solution of tetraphenylborate is poured while stirring, until precipitation ceases. Filter through a sinter. Wash with distilled water. Dry in a desiccator at room temperature. Then pour 20-30 milligrammes of that salt into 250 millilitres of distilled water. Stir from time to time. After thirty minutes, add 0·5-1 gramme of hydrated alumina. Stir for a few minutes. Filter.

Method of operation: Take an aliquot of the acid digest corresponding to about 10 milligrammes of K2O. Dilute to about 100 millilitres. Slowly add the reagent solution, about 10 millilitres per assumed 5 milligrammes of K2O, while gently stirring. Allow to stand for a maximum of fifteen minutes then filter through a tared sintered crucible of porosity 3 or 4. Wash with washing solution. Dry for thirty minutes at 120 °C. Conversion factor 0·13143 for K2O.