A 31-year-old woman with type 1 diabetes is being treated for diabetic ketoacidosis with intravenous insulin and 0.9% sodium chloride. Four hours later, her blood glucose concentration is 11.8 mmol/L, blood ketones are 2.1 mmol/L and venous pH is 7.29. She is alert and her serum potassium is 4.1 mmol/L (reference range 3.5–5.0). Which one of the following is the most appropriate adjustment to treatment?