[Guitarmaniac86]

Hi,

Some insight required into the following problem.

The question is the following:

Determine the chain length n from the vaules of the end points. The general formula for a linear polymer for polyphosphoric acid is H[PO₃H]nOH where there are n strongly acidic hydrogens.

I have to determine the average chain length of a tripolyphosphate polymer using a derivative plot. From my results, the end points were at pH 6.5 at 21cm of NaOH added, pH 8 at 61cm NaOH added and pH 12 at 77.5cm NaOH added.

0.2g tripolyphosphate was dissolved in 250cm water.

Initial concentration: 5.44x10^-4 mol dm^-3

The sodium tripolyphosphate was converted into the triphosphoric acid and the initial pH was 2.

Im assuming that the polymer is going to be linear and that reaction is 2:1 acid to base since the polymer can react at both ends.

However, I cannot find the average chain length. Can anyone give me a starting point?

I have tried working out how much NaOH reacts with the acid at each end point using the following working:

pH=-log₁₀[H⁺

[H⁺ = 3.16x10^-7 moles.

Therefore 6.32x10^-7 NaOH reacts with the STPP

Im toying with Kw and Ka as well but Im actually stuck. Any help will be appreciated.

[Borek]

Can you use these numbers to determine molar mass of tripolyphosphate?

[Guitarmaniac86]

Can you use these numbers to determine molar mass of tripolyphosphate?

... No... Something has clicked though... I know the RMM of TPP is 367.86425 g/mol... If I can find the moles of TPP that has reacted with the NaOH at each point, I can then work out its mass. From there, I can determine n based on the difference in mass...?

Am I on the right track?

Thank you for your post.

Ah, Ive found another way.

If I plot [change in pH/change in volume] against volume, the formule n=2x/(y+z) gives me chain length.