Author Options:

If I have the pH and volume of 2 solutions, and the target pH of a combination of them, how do I calculate how much... Answered

 If I have the pH and volume of 2 solutions, and the target pH of a combination of them, how do I calculate how much I need?



Best Answer 8 years ago

Well...it can get pretty complicated.  It's fairly simple if the two solutions are either strong acid or strong base.  If not... the practical solution is to dip a pH electrode into one solution, with continuous stirring, then add the other solution til the target pH is obtained.  (That practical method, by the way, is from a recent textbook in analytical chemistry by Harris.  He points out that activity effects can make significant differences between calculated and actual values.)

If an approximate solution (no pun intended) is desired, the nature of the two solutions must be known; i.e., are they strong acids or weak acids?

Best of luck -- Prfesser

 Right, if you don't have any information about the dissociation constants or concentrations of the two solutions or anything, If you have some means of measuring pH, the best thing to do would be to simply vary the amounts of each until you get the right pH, as Prfesser suggests.  Even easier, most likely, is simply to find some place like a health food store, and buy powdered citric acid.

 Is it really that simple? There's no molar mass or logarithms involved? Even so, I still can't entirely figure it out...

Substance A has a pH of 2.5. Substance B has a pH of 6.6 and has 1.892L. How much of Substance A should I add to reach 5.5?

Think ratios and differences (and go back and read the chapter of your textbook with this homework problem).  Since pH is logarithmic, the differences in pHs correspond to ratios of volumes.  That statement, of course, is wrong, since there are buffering and reaction effects which alter the lab-bench result.  Prfesser is correct below that you need to titrate by hand to actual reach your target pH.

 Sadly, I'm not actually a chemistry student. I'm doing this for fun...kinda. The idea is cheesemaking. I have a kit that uses pure citric acid, I want to try to substitute lemon juice. Lemon juice is substance A, the milk is Substance B. If I had a chemistry textbook handy, I'd look it up. But I don't.

Are either of them buffers?


Think ratios.