The nitric acid (Merck, Darmstadt, Germany) used to prepare
the standard solutions was further purified by sub-boiling distillation
in a quartz sub-boiling still (Kürner Analysentechnik, Rosenheim,
Germany). All containers and glassware were soaked in 1.4 mol L− 1
nitric acid for 24 h and rinsed with deionised water before use.