I'm from Colombia and I joined this site to ask a question that I could not solve yet. I am calculating the amounts to prepare a phosphate buffer with two salts. I want to prepare a buffer system with pH = 7.0 and a total concentration of Ct = 0.1 M. All my calculations are in the image attached. When I mix the calculated amounts, the obtained pH = 6.5 in the lab. I thought it was my mistake in calculations or an error when I weighed the amounts, but I repeated everything several times and still not get the pH = 7, and the value always was 6.4 or 6.5.
I don't know the reason but I would like to know this. Please help me. I think that If the concentration of the monobasic salt is greather than the dibasic, the pH tends to decrease, but in the calculations the relation [dibasic]/[monobasic] is always less than 1 and this again causes that [monobasic] > [dibasic]. Thanks, I really need a help.