The potassium tetraoxalate certified buffer solution is gravimetrically prepared using high-quality reagents and water at a registered calibration laboratory. Its pH value of 1.68 at 25°C is measured using a combined glass electrode following a 5-point calibration method. The measurement is made following DIN 19268 using reference buffer solutions compliant to DIN 19266, NIST, IUPAC, Ph. Eur, and USP. The buffer′s pH value depends on the temperature and therefore a constant temperature must be maintained during its use.