[evagirlrules]

Hi everyone! I've been trying to figure this out for the longest time and am completely stuck, can someone please help? I am asked to explain what is happening in this test:

Transfer 40.0 mL of 1 N hydrochloric acid to a 250-mL beaker, add 50 mL of water, and titrate potentiometrically with 1 N sodium hydroxide VS to the endpoint. Record the volume of 1 N sodium hydroxide VS consumed as the blank. Transfer a portion of Dibasic Sodium Phosphate, accurately weighed, equivalent to about 2.5 g of Na2HPO4, to a 250-mL beaker, add 40.0 mL of the 1 N hydrochloric acid and 50 mL of water, and stir until dissolved. Titrate the excess acid potentiometrically with 1 N sodium hydroxide VS to the inflection point at about pH 4, and record the buret reading. Subtract this buret reading from that of the blank, and designate the volume of 1 N sodium hydroxide VS resulting from this subtraction as A. Continue the titration with 1 N sodium hydroxide VS to the inflection point at about pH 8.8, record the buret reading, and calculate the volume (B) of 1 N sodium hydroxide required in the titration between the two inflection points (pH 4 to pH 8.. Where A is equal to or less than B, each mL of the volume A of 1 N sodium hydroxide is equivalent to 142.0 mg of Na2HPO4. Where A is greater than B, each mL of the volume 2B-A of 1 N sodium hydroxide is equivalent to 142.0 mg of Na2HPO4.


This is from the most recent USP, and I understand the chemistry of it, but I dont know how to explain volumes A and B and why a certain calculation is necessary if one is larger than the other.

What I have:
The amount of blank (HCl) is added in 2 eq's of the Na2HPO4, so to convert all species to H3PO4, which is a low pH. Phosphoric acid is polyprotic and has three pka's, at around 2, 6, and 12. The titration to pH 4 is the amount of NaOH requred to convert H3PO4 to H2PO4 and neutralize excess H+ ins. Volume A, which is the blank volume - this, is then equal to the amount of H3PO4 ions. The titration to pH8 is the amount required to convert H2PO4 to HPO4, which corresponds to the amount of H2PO4 ions. Thus, volume A and B should be theoritically the same, so I am confused as to why the calculation of the assay is different. Can anyone help please? Sorry for the long post!

[sjb]

Can any side reactions occur?

[evagirlrules]

^do you mean reaction of HCl + NaOH? or the reverse reaction of titration? x___ x I'm still completely confused...

[sjb]

Neither. (I could be leading you astray here, so don't take this as gospel).

How fresh are your acid and base solutions?

[evagirlrules]

they are standardized every month.... techanically they should be very accurate...

[sjb]

they are standardized every month.... techanically they should be very accurate...

Are you aware of terms like primary and secondary standards?

[evagirlrules]

Yes... the one used is a secondary standard...


my main question is why is there two equations for calculation? The two volumes should theoritically be the same right? (according to balanced chemical equation), so why are there two different ways to calculate?

sorry for being so lost x.x

[sjb]

Why is sodium hydroxide a secondary standard, rather than a primary one?

[evagirlrules]

because it was made from a primary one .

i figured out the calculations though, it wasn't really based on the titrant since the exact normality is known (from standardization), it is to account for any experimental differences in weighing out the sample . thanks for the help though!

[sjb]

Glad you managed to sort it, despite my ramblings.

For what it's worth, I was considering the fact that sodium hydroxide solutions can absorb CO₂ from the atmosphere to form the bicarbonate salt (does anyone have any kinetic data on this?), and so if you were solely relying on pH values to let you know when a change was occurring this may be misleading.