Answer to Question #110718 in Molecular Biology for FRIDAY

Question

The recipe for PBS says to dissolve compounds in 800 ml of water, adjust the pH to 7.4, then add water up to 1 liter. The final pH should still be 7.4, because the pH of buffer solutions remains stable when they are diluted as long as the concentration of its constitutive acid and base is not too low.

Question 10: Why do you think the protocol does not say to dissolve compounds directly in 1 liter of water?

Answer 1

There are two main reasons not to dissolve salts directly in 1 liter of water. At first, the dissolved compounds can increase the volume of the solution compared to pure water. As a result, the final volume of the solution will be higher than 1 liter. Second, if pH is needed to be adjusted, a certain volume of sodium hydroxide or hydrochloric acid must be added to the solution. This will also contribute to the final volume of the solution changing its molarity and buffer strength. As a result, it is required to dissolve components, adjust pH, and add water to 1 liter. In this case, the parameters of the buffer will not be changed.