# Potassium phosphate buffer + EDTA

Given 10 mM potassium phosphate buffer (pH = 7.0), 50 mM potassium propionate and 20 mM EDTA (acid). After addition of all 3 components pH becomes 4 since EDTA has 4 $\ce{OH}$ groups.

How to calculate the ionic strength if I am pHing it with $\ce{KOH}$ up to 7.1? I can calculate ionic strength for binary mixtures, but 4 different components makes me stuck.

• Are you sure you are using EDTA (the acid) and not any of its salts? The water solubility of EDTA is quite limited and I think it would be difficult to prepare a 0.020 M solution. Please check, because it might affect your calculations. – Bive May 30 '17 at 18:48
• yep, it was acid and it dissolved completely upon bringing pH to 7.1 – physavage Jun 9 '17 at 16:51
• Ok. I suggest you state the charge balance for all protolytes that are present in significant amounts at pH 7.1. Then you calculate the concentration of each protolyte by using the Henderson-Hasselbalch's equation. Finally, you multiply each protolyte concentration with the charge of the protolyte squared. Sum up all products and divide by two and you will get the ionic strength of the solution. Show me! – Bive Jun 10 '17 at 14:24