Question

how do you prepare 50mL of 25mM of NaPi (sodium phosphate) at pH 7?

Answer 1

Sodium Phosphate is buffer solution of the combination of Mono Sodium Phosphate (NaH₂PO₄ .H₂O) and Sodium Hydrogen Phosphate (Na₂HPO₄).

For buffers

P^H = P^Ka + log [Salt]/ [Acid]

For this P^H =7

P^ka = 7.21

7 = 7.21 + log [salt]/ [Acid]

log [salt]/ [Acid] = -0.21

[salt]/[acid] = 10^-0.21  = 0.61

[salt] = 0.61 [Acid] (1)

We can prepare 25mM solution

[salt] + [acid] =25mM (2)

From 1 & 2

[acid] 0.61 +[Acid] = 25 mM

[Acid] [1.61] =25

[Acid] = 15.527mM

[salt] =25 -15.527 = 9.473mM

For calculation of weight of Acid

Molarity = weight / Gram molecular weight * (1000/ v in ml)

15.527 mM converts moles = 0.0155M

0.0155 =w/138 * (1000/50) [138 is gram molecular weight of monosodium phosphate]

W = 0.106gm

For salt

M= w/ GMW * (1000/v in ml)

9.473mM converts moles

0.009473 = w/142 * (1000/50) [142gm is the GMW of sodium hydrogen phosphate]

W = 0.066gm

For 50ml 0f 25mM sodium phosphate requires 0.106gm of monos odium phosphate and 0.066gms of sodium hydrogen phosphate