Question

Prepare an aqueous 0.08M potassium phosphate buffer, pH 7.5 from the appropriate solid salt (conjugate acid form), water, and approximately 1M solution of the appropriate strong base (potassium hydroxide), and a pH meter. Dissolve the desired mass of salt in deionized water to about 80% of the final volume (1 L). Measure and record the pH of the solution and add the base in portions equal to 0.5% the final volume of buffer, recording the pH after each addition. When you reach pH ≥ 7.3, decrease the volume of base added to 0.1% of the final volume . Continue adding volumes of base until the pH reaches 7.5. Quantitatively transfer the buffer to a volumetric flask, dilute to volume with deionized water, mix thoroughly, and return the buffer to the beaker.

I am very confused, can someone please help write a procedure how to do this with calculations? Thanks.

Answer 1

There's actually only one calculation which has to be done here, which involves the concentration of phosphate species in the buffer. We are told that we want to prepare a 0.08 M solution, adding initially NaH₂PO₄ (a salt of the conjugate acid of the acid-base pair that makes up the buffer). In order to calculate the mass to add, we can use:

In this case, the molar mass of the salt is 120 g/mol and we want 1 L of buffer, so we calculate:

This mass has to be added to the water and then the pH is adjusted using the base and a pH-meter, which is a 100% experimental approach, which doesn't need any further calculations.

It says to add it to 80% of the final volume (800 mL) and then add portions of base that represent 0.5% of the final volume, that is: 5 mL each time. Once the pH is over 7.3, the volume of base added has to be lower: 0.1% of the final volume, which is equivalent to 1 mL. After each addition, you have to stir the mixture and measure the pH, until you reach the desired value of 7.5.