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Recently, I've been told that similar pH shift (of about 0.5 units) was observed with 20 times NPS solution as well.

In case of NPS the authors provide the following recipe for the 20 times solution:
H2O 900 ml,
(NH4)2SO4 66 g,
KH2PO4 136 g,
Na2HPO4 142 g. with expected pH to be about 6.75.

Why is that?

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    $\begingroup$ Asking for answers and not providing relevant details, purpose or background of questions are contradictory decisions. Also, questions should be self containing, making sense even without link target availablity. $\endgroup$
    – Poutnik
    Commented Oct 19, 2022 at 6:38
  • $\begingroup$ What is the meaning of the words "10xPBS stock" ? Is it ten times something ? Ten times what ? What is PBS ? What is 1xsolution ? $\endgroup$
    – Maurice
    Commented Oct 19, 2022 at 8:15
  • $\begingroup$ Thanks! Edited for better understanding $\endgroup$
    – abc
    Commented Oct 19, 2022 at 10:41
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    $\begingroup$ Activity coefficients of ions with bigger charge ($\ce{HPO4^2-}$ versus $\ce{H2PO4-}$) drop with growing ionic strength faster, therefore the phosphate buffer gets more acidic with higher concentrations, and vice versa. $\endgroup$
    – Poutnik
    Commented Oct 19, 2022 at 12:56
  • $\begingroup$ @Poutnik, thanks. Maybe you can post your comment as an answer? $\endgroup$
    – abc
    Commented Oct 19, 2022 at 18:44

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Activity coefficients of ions with bigger charge ($\ce{HPO4^2−}$ versus $\ce{H2PO4−}$) drop with growing ionic strength faster, therefore the phosphate buffer gets more acidic with higher concentrations, and vice versa.

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