Which method of buffer preparation one should choose?

Which method do you think is better for preparing a buffer. Method 1 might appear better than method 2 because we are required to titrate with NaOH; this implies that we are not going to add any phosphate species that could change the strength of buffer. There is, however, a possibility of overshooting the required pH while adding NaOH. What do we do if we actually overshoot the required pH? We are left with two options:

1. Discard all the contents and prepare fresh buffer.
2. Add H₃PO₄ to bring the pH down to 7.4; this would, however, increase the strength of the buffer.

Increase in buffer strength, however, is not that big a problem. The prepared can be diluted with water so as to obtain the buffer of desired strength. But, is there a way out that we don't change the buffer strength and still achieve the correct pH no matter how casually we perform the titrations? The answer is “yes”. This is how we go about it:

We know that Na₂HPO_4^-7H₂O and NaH₂PO₄ are required in 1.5488: 1 molar ratio. We can prepare the 200 mM solutions of both Na₂HPO_4^-7H₂O and NaH₂PO₄ in a 1.5488:1 volume ratio. As we need to prepare 500 ml  buffer, we would perform the calculation for a slightly higher volume, say 550 ml .

Although we have discussed the preparation of phosphate buffer, the concept and the protocol for making the buffer are general.