The green-colored di-sodium hydrogen phosphate/potassium dihydrogen phosphate buffer solution is gravimetrically prepared using high-quality reagents at a registered calibration laboratory. Its pH value of 7.00 at 20°C is measured following a 5-point calibration method, using a combined glass electrode. The measurement is made following DIN 19268 using reference buffer solutions compliant to DIN 19266, NIST, IUPAC, Ph. Eur, and USP. Its pH value is strongly dependent on the temperature and therefore a constant temperature must be maintained during its use.