I have the following salts:
Na2HPO4.12H2O
NaH2PO4.2H2O
please how do I go about it.
From a search, it seems that the desired pH is 6.8.
If you have a pH meter and magnetic stirplate available, you can simply add a 67 mM solution of one of the salts to a 67 mM solution of the other until the pH of 6.8 is reached.
The molecular weight of Na2HPO4.12H2O is 358.14. One liter of 67 mM contains 24.0 grams. It is a mildly basic solution.
The molecular weight of NaH2PO4.2H2O is 156.01. One liter of 67 mM contains 10.45 grams. It is a mildly acidic solution.
You will need approximately 46% by volume of the Na2HPO4 solution and approximately 54% by volume of the NaH2PO4 solution (at 25oC). Use the pH meter to get precisely the right pH, however, since these proportions are for a 100 mM solution.
Thank you Adam B. Shapiro. But if I make the solution up to 1000ml wouldn't the pH change?
The pH is determined mainly by the relative proportion of the monobasic and dibasic salts and, to a lesser extent, their actual concentrations (This chemical activity effect is mainly observed at high concentrations such as 1 Molar, and has very little effect on the dilute solutions you will be working with.)
It is not necessary to bring the volume up to 1 liter if your are mixing the two 67 mM solutions. The solution will be 67 mM no matter how much of the two solutions you mix together. If you want 1 liter, then mix about 460 ml of dibasic and about 540 ml of monobasic 67 mM solutions (use a pH meter to check the actual pH).
- Badges
- Science method