In pH measurements, if the solution is acidic, the H+ ions diffuse out of the layer and a negative charge is established on the outer side of the membrane.

• A. True
• B. False
• C. The Outer Surface Becomes Positively Charged
• D. The Diffusion Is Independent Of pH

Answer: (B)

In pH measurements with a glass membrane, surface charges are set by proton exchange at the outer surface. When the surrounding solution is acidic, abundant H+ ions interact with the glass surface and protonate the surface groups, which reduces negative charge and can give the outer surface a positive character. In other words, H+ tends to move into and bind at the surface layer, not diffuse outward to leave a negative charge on the outer side. Therefore the outer surface does not establish a negative charge in an acidic solution; the statement is not correct.