Question

At iso-electric point:
A: conc. of cation is equal to the conc. of anion
B: Net charge is zero
C: Maximum conc. of di-polar ion (zwitter ion) will be present
D: All of the above

Answer 1

Iso-electric point is the $pH$ at which a molecule which is electrically neutral. $pH$ of a substance is defined as the concentration of hydrogen ions in that molecule. An iso-electric point molecule or the surface carries no net electrical charge.

Complete step by step answer:
The Iso-electric point is that $pH$ at which the molecules carry no net electrical charge. $pH$ of a substance is defined as the concentration of hydrogen ions in that molecule. Cations are the positively charged ions. Cations are formed when an atom or molecule loses its electrons. Anions are the negatively charged species. Anions are formed when an atom or molecule gains electrons. Zwitter ion is a molecule which is composed of positively and negatively charged functional groups. The overall charge on zwitter ion is zero that is zwitter ion is electrically neutral. Every zwitter ion has an iso-electric point. At iso-electric point the concentration of zwitter ion is maximum (as zwitter ion is electrically neutral it will not conduct any electric charge). This means option C is correct.
Option A states that conc. of cation is equal to the conc. of anion is equal at iso-electric point. Since the concentration of positive charge is equal to the concentration of negative charge, the overall charge will be zero and hence there will be no flow of electric charge. This means option A is also correct.
Option B states that Net charge is zero at iso-electric point, which is also true (explained above). This means option B is also correct.

So, the correct answer is Option D.

Note:
Amino acids are zwitter ions. Positively charged functional group in these acids is $N{H^{3 + }}$ and the negatively charged functional group is $CO{O^ - }$. The average iso-electric point ($pH$ at which net charge is zero) of an amino acid is $5.5$.