Which substance, when stored with formalin, can cause the stock to become acidic?

• A. Calcium carbonate
• B. Sodium chloride
• C. Sodium bicarbonate
• D. Magnesium sulfate

Answer: (A)

Formalin can slowly formic acid as it oxidizes, which tends to lower the pH of the solution. When calcium carbonate is present, it readily reacts with that acid to form calcium formate and carbon dioxide. The CO2 that’s produced can dissolve in the watery medium and convert to carbonic acid, contributing to an acidic environment. So the stock becomes more acidic in this scenario because the carbonate reacts with the acid by producing CO2 and carbonic acid, which keep the solution on the acidic side. The other substances wouldn’t promote this acidification in the same way: sodium chloride is neutral, sodium bicarbonate would neutralize acid and raise pH, and magnesium sulfate has little impact on acidity in this context.