PH of neutrality is not exactly 7 (25 °C), although this is a good approximation in most cases. Neutrality is defined as the condition where [H] = [OH] (or the activities are equal). Since self-ionization of water holds the product of these concentration [H]×[OH] = K, it can be seen that at neutrality [H] = [OH] = , or pH = pK/2. pK is approximately 14 but depends on ionic strength and temperature, and so the pH of neutrality does also. Pure water and a solution of NaCl in pure water are both neutral, since dissociation of water produces equal numbers
PH-sensitive polymers include carboxylic acids (-COOH), sulfonic acids (-SOH), phosphonic acids, and boronic acids. Polyacids accept protons at low pH values. At higher pH values, they deprotonate and become negatively charged. The negative charges create a repulsion that causes the polymer to swell. This swelling behavior is observed when the pH is greater than the pKa of the polymer. Polybases are the basic equivalent of polyacids and are also known as cationic polymers. They accept protons at low pH like polyacids do, but they then become positively charged. In contrast, at higher pH values they are neutral. Swelling behavior is seen when