Explorative Study to Investigate the Acid-base Response to Sodium and Potassium Salts in Patients With Chronic Kidney Disease.

Inclusion Criteria: * Adult patients (≥ 18 years) * CKD G3b or G4 (44 - 15 ml/min/1.73 m2) * Use of RAAS-I (ACE-inhibitor or angiotensin receptor blocker, ARB) Exclusion Criteria: * Use of any of the following drugs or supplements: mineralocorticoid receptor antagonists, potassium-sparing diuretics, oral potassium binders, immunosuppressive medication, tolvaptan, acetazolamide, topiramate, sodium bicarbonate. * Patients using double RAAS blockade (i.e., ACE-inhibitor + ARB). * Metabolic alkalosis (plasma bicarbonate \>27 mmol/L) at last outpatient visit * Kidney transplant recipients * Patients with an active gastro-intestinal ulcer * Patients with previous history of ventricular cardiac arrhythmia * Patients with a life expectancy \< 6 months * Incapacitated subjects or subjects who are deemed unfit to adequately adhere to instructions from the research team * Women who are pregnant, breastfeeding or consider pregnancy in the coming 7 weeks * Patients with chronic respiratory acidosis in previous medical history * Hyperkalemia (plasma potassium \>5.5 mmol/L) at V0