Buffer solutions contain citric acid 1 H₂O (11,768 g); sodium hydroxide 50% (6 ml); hydrochlorid acid 35% (3,96 ml); preservative 1% (5 ml) and water (1 l).

• pH (25 to 40 °C): 4,00 +0,01
• pH (20 °C): 4,00 ±0,02
• pH (15 °C): 4,00 +0,01
• pH (10 °C): 4,00 +0,02
• pH (5 °C): 4,00 +0,04
• pH (0 °C): 4,00 +0,05