[dpx90]

I am preparing buffered hypochlorite reagent for an experiment. The paper says to "Dissolve 1.48g of NaOH in 70mL water, add 4.98g Na₂HPO₄ and 20mL of sodium hypochlorite (5% NaOCl). Use less or more hypochlorite solution if the NaOCl concentration is higher or lower than indicated. Check pH to ensure value between 11.4-12. Add additional NaOH if required to raise pH. Dilute to 100mL."

I need to calculate the pH of the solution. I can measure it after with a pH meter but I have to be able to show the calculation. I am using H-H to calculate buffer pH.

The relevant equations are
   HPO₄^2-  +  H₂O     PO₄^3-  +  H₃O⁺   Ka = 2.2x10^-13
   HClO  +  H₂O     ClO^-  +  H₃O⁺   Ka = 3.0x10^-8

The first reaction is NaOH and dibasic sodium phosphate
   Na₂HPO₄  +  NaOH     Na₃PO₄  +  H₂O

How do I proceed with H-H to get pH of the solution? What are the species partaking in buffering the hypochlorite?

[Babcock_Hall]

Near pH 12, how much of the hypochlorite is in the form HClO versus ClO^-?

[dpx90]

For every 1 mol of ClO^- that reacts with water, 1 mol of HClO and OH^- are formed.
At equilibrium pH was calculated to be 10.32. That is before NaOH or Na₂HPO₄ is added. I guess my confusion lies when you add the other two species. Hypochlorite can be a buffer system on its own and so can dibasic sodium phosphate.
Does NaOH prefer to react with one versus the other?

[Babcock_Hall]

Other people here have more expertise than I do, but I can tell you how I would approach the problem.  NaOH is a strong base, and it will react with any weak acid until all of it is consumed.  But it should react with the stronger acid first.  Which is the stronger acid HClO or HPO₄^2-?  That was the point of my previous question.