How do I prepare neutral buffered formalin with disodium hydrogen phosphate dihydrate and sodium dihydrogen phosphate dihydrate instead of sodium dihydrogen phosphate monohydrate and disodium hydrogen phosphate anhydrous?
Say you want to prepare 0.1 M phosphate buffer with pH 7. Prepare 0.5 L of 0.1 M disodium hydrogen phosphate dihydrate and 0.5 L of 0.1 M sodium dihydrogen phosphate dihydrate solutions. Take the first solution and measure pH. Add the second solution by small portion until pH reaches the desired value.
The hydration of the salts is only relevant when you ned to use the molecular weight to calculate how much of the solid to add to water to make a solution of the desired concentration. Once the salts are dissolved, the hydration no longer matters.
For example, A 1 M solution of sodium dihydrogen phosphate dihydrate is exactly the same as a 1 M solution of sodium dihydrogen phosphate monohydrate, although they require different weights of the solids to prepare.
Use sodium hydrogen phosphate dissolve it in water and adjust the pH by titrating with x% Phosphoric acid. The more dilute the phosphoric acid, the more precisely it can be adjusted.
A small amount of the buffer is enough for you to test the concentration at your discretion.
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