[Analytical2015]

Hi everyone, I have a bit of a problem with a UPLC method I am currently developing and was wondering could anyone help me.
I am using a Sodium Dihydrogen Orthophosphate Buffer, 3.12 g in a litre. Its MW is 156.01, therefore 156.01g/mole or 0.15601g/mM, hence the 3.12 g comes from it being a 20mM solution (20 X 0.15601).

Its resting or start pH is 4.65-4.70. I need to bring it down to 2.90, +-0.02 units. I am trying to eliminate the use of the pH meter for this step as it is not very reliable and Im looking for a more accurate way to change the pH. Is there a calculation or formula that can be used to calculate the exact amount of orthophosphoric acid required to bring it down to a fixed pH value? I need an amount that will bring it from the starting pH to 2.90 and 2.50. The acid I use typically is orthophosphoric acid MW 98. Even if i had to make up a 1-10% acid solution from this, that would be fine. I typically make either 3.12g in a litre or 6.24 g in 2 litres as the buffer.Thanks in advance!

[Borek]

I don't think it is realistic to get ±0.02 pH unit accuracy without a reliable pH meter.

I am using a Sodium Dihydrogen Orthophosphate Buffer

Just sodium dihydrogen phosphate is not a buffer.

Judging from the molar mass what you use is sodium dihydrogen phosphate dihydrate (which is not a buffer either).

[Analytical2015]

I use the 3.12 g of this chemical in a litre of Water for its buffering capabilities, just to confirm.
Im almost sure there was an equation out there to calculate the amount of acid needed to bring the pH of a known molar quantity of a solution down to a specific figure, does anyone out there know of such an equation?

[Borek]

I use the 3.12 g of this chemical in a litre of Water for its buffering capabilities, just to confirm.

I believe that's the procedure you follow, but buffering properties of such solution are quite low, so I would not call it a buffer.

To be more precise - buffering capacity of this solution is around 6×10^-5, that's over two orders of magnitude less than the buffering capacity of the 0.02 M (same concentration) acetate buffer at pH 5.0. Compare discussion at

http://www.chembuddy.com/?left=buffers&right=buffer-capacity

Im almost sure there was an equation out there to calculate the amount of acid needed to bring the pH of a known molar quantity of a solution down to a specific figure, does anyone out there know of such an equation?

I guess you mean Henderson-Hasselbalch equation. Problem is not with calculations, these can be done relatively easily (see these examples or just use Buffer Maker, whichis much more precise than just HH equation). Problem is with preparation - there are too many factors that have to be taken into account to be sure what you have prepared has the requested pH. The only way of knowing if the pH of your solution is the one you need is to check it with a good, properly calibrated pH meter, and adjusting it if needed. If you believe preparative approach is easier and more reliable than the pH meter route, you are wrong.