I was preparing a phosphate buffer and adjusting its pH to 7.2 from 9.0 by adding monobasic phosphate. In the beginning, it started to decrease fast but then, it went slowly (it was about pH=7.6). Even though I added too much monobasic solution, its pH was not increased easily. Then, it went to pH=7.3 easily but it stopped at pH=7.3 again. I wonder why it happened. I know it is related to its titration and the titration graph is fine to understand but I need a comprehensive explanation for it.
Buffer mean which maintain PH, monobasic potassium phosphate it self pH 5 to 8, you need to Dilute HCl to decrease its pH. If you increase its pH you add NaOH or KOH solution.
Hi Devrim Yağmur Durur ,
Adding to the good comment of Saeed Ayaz Khan indeed to keep the buffer concentration constant you need to add 'pure' acid like HCl or NaOH and not add more phosphate. Next, why do you see such large consumption of acid? The titration curve tells you why. pH= 7.2 is the second equivalence point of phosphoric acid. Around the equivalence point, the buffer capacity is the largest, meaning that quite some acid or base is required to change pH. You can see this going through the titration curve, note that the shallow increase of pH with titrant added is always around the equivalence point (see attached image)
There is an excellent internet page on the topic of buffer capacity that explains this concept in far more detail: https://www.chembuddy.com/?left=pH-calculation&right=pH-buffer-capacity
Kind regards, Leo
Dear all, one point should be considered, buffers in the alkaline part of the pH are unstable because of the ambiant CO2 (having acid character), so better to work under an inert atmosphere. My Regards
- Badges
- Science topic
- Similar topics
- Chemistry