ph of mobile phase containg water:methanol:o-phosphoric acid

[chilam]

Hi,

what is pH of ph of mobile phase containg water:methanol:o-phosphoric acid (60: 20:20).

when i prepared mobile phase pH its going down to 2.6. But accourding to paper it should be 4.4.

Can you give me suggestions

[Markus Laeubli, Metrohm]

Dear chliam

For me that pH = 4.4 looks more like NaH2PO4.
pH = 4.4 is almost in the middle of pKa1 and pKa2 of phosphoric acid.

[tom jupille]

water:methanol:o-phosphoric acid (60: 20:20)

20% phosphoric acid ?????

[chilam]

i am using 0.1% of phosphoric acid

[bisnettrj2]

When are you measuring the pH? Before or after the addition of methanol?

[chilam]

i am measuring ph after preparing complete mobile phase ( water:methanol:0.1% phosphoric acid)

[bisnettrj2]

Measure the pH before the addition of the methanol, see if it's different.