I have always made borate buffer using sodium borate octahydrate (Na2[B4O5(OH)4]·8H2O), which has a pH ~9-10, is there a reason you're using boric acid?

Also, if you're having trouble getting it to dissolve you can heat the mixture up while stirring, just make sure you use a lower volume than your final to account for additional pH adjustment volume additions. If you're using it for HPLC then also make sure you filter the solution before running it through your instrumentation. Adjustments should be made via dropwise(ish) addition of concentrated NaOH.

Dear Dr Krasel and Dr Schmitz,

Thank you for the replies. I've read the discussion about boric acid in the link you shared before posting here. However, given my limited understanding in chemistry, I'm not sure if I fully understood what the discussion attempt to explain.

And to answer Dr Schmitz, the link shared above mentioned that using either both boric acid-NaOH and borax-HCl will yield sodium borate as well. I chosen boric acid as one post on researchgate mentioned that sodium tetraborate decahydrate required larger quantities and may have solubility issues.

(P/S: I actually have both chemicals in the lab)

Now, back to the link of the discussion of borate buffer, it seems that the presence of ionic species in borate buffer isn't entirely understood yet, and thus I found it difficult to determine which recipe online is the right one and whether if my calculations is valid in the real world in anyway.

More advices from experts are needed.

Your calculations are accurate but, in practice, you need to look at the solubility of your analyte in order to determine whether or not you can dissolve that amount of solid material at a reasonable temperature and pressure. Generally speaking, hydrates and salts have a much higher water solubility, and in the case of borax the water solubility increases with near-linearity at increasing temperatures (starting at 30 g/L at STP to over 500 g/L at the boiling point).

If you use the octahydrate, be sure to include the full formula mass (including the 8 complexed water molecules) in your calculations.

Thanks Dr Schmitz, I ended up preparing 0.4 N of sodium tetraborate octahydrate. Yes, I did include the full formula mass with 8 complexed in water molecules in my calculations.

Thank you all so much.