Urine pH was measured immediately after each sampling hour using a combination electrode (PHC2002-8; Hach Lange APS, Brønshøj,Denmark) and a pH meter calibrated at pH 4.005 and 7.000 (PHM 240; Hach Lange APS, Brønshøj, Denmark). Immediately after each pH measurement, a sub-sample of
5 mL of urine was collected and stored at 18 1C.