What happens to pCO2 and pH when air bubbles are present in the sample?

• A. pCO2 decreases and pH increases
• B. pCO2 increases and pH decreases
• C. pCO2 decreases and pH decreases
• D. pCO2 increases and pH increases

Answer: (A)

Air bubbles provide a gas–liquid interface that lets CO2 diffuse out of the sample into the bubble, because the partial pressure of CO2 in the bubble is lower than in the sample. As CO2 leaves, less carbonic acid forms, which lowers hydrogen ion concentration and raises pH. In the bicarbonate system, pH increases when pCO2 decreases, since less CO2 shifts the equilibrium toward fewer H+ ions. So CO2 falls and pH rises.