A 24-year-old man with type 1 diabetes presents with vomiting and abdominal pain. His blood glucose concentration is 28 mmol/L, blood ketones are 6.4 mmol/L and venous pH is 7.18; his serum potassium is 2.9 mmol/L (reference range 3.5–5.0). After an initial bolus of intravenous 0.9% sodium chloride, which one of the following is the most appropriate next step?