Potassium may be precipitated with excess of sodium tetraphenylborate solution as potassium tetraphenylborate. The excess of reagent is determined by titration with mercury (II) nitrate solution. The indicator consists of a mixture of iron(III) nitrate and dilute sodium thiocyanate solution. The end-point is revealed by the decolorisation of the iron(III) thiocyanate complex due to the formation of the colourless mercury(II) thiocyanate. The reaction between mercury (II) nitrate and sodium tetraphenylborate under the experimental conditions used is not quite stoichiometric; hence it is necessary to determine the volume of Hg(NO3)2 solution equivalent to a NaB(C6H5)4 solution. Halides must be absent.
Prepare the sodium tetraphenylborate solution by dissolving 6.0 g of the solid in about 200 mL of distilled water in a glass-stoppered bottle. Add about 1 g of moist aluminium hydroxide gel, and shake well at five-minute intervals for about 20 minutes. Filter through a Whatman No. 40 filter paper, pouring the first runnings back through the filter if necessary, to ensure a clear filtrate. Add 15 mL of 0.1M sodium hydroxide to the solution to give a pH of about 9, then make up to 1 L and store the solution in a polythene bottle.
Prepare a mercury(II) nitrate solution (0.03M) by dissolving 10.3 g recrystallised mercury(II) nitrate, Hg(NO3)2,H2O, in 800mL distilled water containing 20 mL 2M nitric acid. Dilute to 1 L in a graduated flask and then standardise by titrating with a standard thiocyanate solution using iron (III) indicator solution. Prepare the indicator solutions for the main titration by dissolving separately 5g hydrated iron(III) nitrate in 100 mL of distilled water and filtering, and 0.08 g sodium thiocyanate in 100 mL of distilled water.
Pipette 10.0 mL of the sodium tetraphenylborate solution into a 250 mL beaker and add 90 mL water, 2.5 mL 0.1 M nitric acid, 1.0 mL iron(III) nitrate solution, and 10.0 mL sodium thiocyanate solution. Immediately stir the solution mechanically, then slowly add from a burette 10 drops of mercury(II) nitrate solution. Continue the titration by adding the mercury(II) nitrate solution, slowly until the colour of the indicator is temporarily discharged. Continue the titration more slowly, but maintain the rapid state of stirring. The end point is arbitrarily defined as the point when the indicator colour is discharged and fails to reappear for 1 minute. Perform at least three titrations, and calculate the mean volume of mercury(II) nitrate solution equivalent to 10.0 mL of the sodium tetraphenylborate solution.
Pipette 25.0 mL of the potassium ion solution (about 10 mg K +) into a 50 mL graduated flask, add 0.5 mL 1M nitric acid and mix. Introduce 20.0 mL of the sodium tetraphenylborate solution, dilute to the mark, mix, then pour the mixture into a 150mL flask provided with a ground stopper. Shake the stoppered flask for 5 minutes to coagulate the precipitate, then filter most of the solution through a dry Whatman No. 40 filter paper into a dry beaker. Transfer 25.0 mL of the filtrate into a 250 mL conical flask and add 75 mL of water, 1.0 mL of iron(III) nitrate solution, and 1.0 mL of sodium thiocyanate solution. Titrate with the mercury (II) nitrate solution as described above.