Discuss the relative buffering effectiveness of a 25 mM versus a 50 mM buffer as

Question

Discuss the relative buffering effectiveness of a 25 mM versus a 50 mM buffer as demonstrated by these titrations. Which buffer concentration better buffers against pH change? How do you know this? Why is this true?

The identities and concentrations of the buffers below are unknown. (buffer choices are phosphate, acetate, and glycine). To determine the concentrations, you need to determine the moles of OH- needed to reach endpoint. I started with 50mL of each buffer and added 25 microliter increments of 5 M NaOH until buffer was titrated (buffer A used 11 increments, buffer B used 42 increments, buffer C used 10 increments).

Explanation / Answer

The effectiveness of a buffer is given in an absolute sense using a parameter called maximum buffering capacity. This gives the precise number of moles of a strong acid/base required to increase the pH in a buffer solution by one unit (+1 in case of base and -1 in case of acid). But to find this value, numerous tedious calculations are required. In a scenario like the one given where the relative buffering effectiveness is called for between two or more buffers, from a plot of pH change with concentration of acid/base added, one can easily find out the effectiveness of a buffer.

A buffer as defined above loses its effectiveness as a buffer when it is subjected to a certain concentration of acid or base which results in protons or hydroxide ions in the medium that is too much for the buffer to quench and thus maintain the pH without significant change. In the given case, as the aliquots of base added is same throughout the titrations for all buffers, the increments of base added serve as a measure for buffer effectiveness as greater number of increments show greater quantity of base required to destabilize the buffer. This being established it can be easily observed that the most effective buffer is Buffer B as it buffered 42 25 microlitre aliquots of 5M NaOH before destabilizing while Buffer A and Buffer C required only 11 and 10 respectively showing they are both almost equally effective.

The destabilization of a buffer is characterised by the sudden spike or steep dip in the pH change despite constant increments of acid/base added as upon addition of that last aliquot, the buffer finds itself with too much ions in the medium for it to neutralize and hence tips off the balance.

Top of all, a buffer's effectiveness is not altered greatly by concentration of dilution thus, it can be noted that Buffer B is the most effective buffer among the three tested.

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