Using the method of mixing monobasic and dibasic sodium or potassium phosphate solutions to get a phosphate buffer of any pH in the 6-8 range allows you to follow a simple recipe to get any desired pH in that range, without requiring a pH meter. At the end, the buffers will all have the same phosphate concentration no matter what the pH, although they will differ in their cation concentrations.

You could lower the pH of a phosphate solution using HCl, as long as you don't mind having chloride ion in your buffer, but you will require a pH meter to make the adjustment. Similarly, you will need a pH meter to adjust the pH upwards with NaOH. It isn't wrong to make the buffer this way, but it isn't the easiest or quickest way.

Caution, there's difference between phosphate buffer and PBS! Phosphate buffer is phosphate at certain pH (which may be supplemented by additives), while PBS contains salt as well.

So for phosphate buffer, it doesn't make sense to use HCl as you're introducing something new. It's the phosphate that is your buffer. Similarly for NaOH if you're using potassium phosphate.

However, if you used phosphoric acid or potassium hydroxide for K-phosphate buffer, I wouldn't see any problem in it. Although I encountered similar objection, but the answers were mostly that you can use the same acid/base

https://www.researchgate.net/post/Why_couldnt_I_titrate_KH2PO4_to_obtain_buffer_of_desired_pH

If you're using PBS, then even HCl and NaOH should not matter except you will have undefined concentration.

You should not use HCl to adjust the pH unless you want to add the extraneous chloride ion to the sodium phosphate buffer solution. Moreover, HCl is a strong acid, so that its mixture with NaOH (strong base) does not contribute to the pH buffering capacity.

You may consider to add H3PO4, NaOH, or any of the sodium phosphate salts; depending on the intended pH buffering range.

Sodium phosphate buffer solution is not to be confused with phosphate-buffered saline (PBS) solution.

Thanks all for your kind remarks:)

I just want to drop some extra questions after going through the answers:

What does excess chloride or hydroxide ions really do? How do they affect the buffering system? Is there any chance that Cl(-) ions react with Na(+) and form salts? I know it sounds ridiculous but I'm just curious.

Carlos Araújo Queiroz Tomáš Hluska Adam B Shapiro

By adding HCl to sodium phosphate to adjust the pH, you do of course create NaCl, although it will be mostly dissociated to Na+ and Cl- ions. The phosphate buffer will therefore be a mixture of sodium phosphate and sodium chloride, instead of just sodium phosphate. Depending on the situation, this may or may not matter to you.

As for adding NaOH to adjust the pH, this increases the Na+ concentration. Hydroxide ion (OH-) is of course already present anyway, since water is in equilibrium with H3O+ and OH-, with the position of the equilibrium depending on the pH. Again, depending on the situation, the extra Na+ may or may not matter to you.