The salt concentration at the isospheric point c iso scales linearly with the polymer's charge fraction ξ, provided the charge fraction is low (ξ 0.05), see Figure . As the polymer's charge fraction increases, the isospheric point weakly deviates from the original linear trend and it is also gradually smeared out to a concentration range, as discussed in detail in the SI. Very weakly hydrated ions, e.g., perchlorate ClO 4 -, do not exhibit the isospheric point in experiments. Our theoretical model predicts such behavior for salts having ε anion 1.1 k B T.