PURPOSE: To simultaneously measure the pH and carbon dioxide and oxygen differential pressures of blood by using a specific calibration liquid by preparing the calibration liquid by mixing a buffer having a specific pH and alkali salt carbonate solution at a specific temperature.

CONSTITUTION: A buffer selected from among Good's buffer having a 6.3-7.5pH at 37°C, phosphate buffer, tris buffer having a 6.8-7.4pH at 37°C, and the aqueous solution of a water soluble Good's buffer reagent having a 6.5-7.6pH at 37°C is used as a first liquid and an alkali salt carbonate solution is used as a second liquid and the first and second liquids are balanced with the oxygen concentration in the atmosphere. Then the calibration liquid having a 6.5-7.6pH and 10-150mmHg carbon dioxide differential pressure is prepared by mixing the first and second liquids with each other. When the calibration liquid is used, the pH and carbon dioxide and oxygen differential pressures of blood can be measured simultaneously.