The pH of a solution is 3. When its pH changes to 6, then H+ ion concentration:

When its pH changes to 6, then H+ ion concentration decreases 1000 times. pH tells the concentration of hydrogen ions; lower the pH, higher the hydrogen ion concentration. The formula for calculating pH is: pH = -log[H+]. pH of 3 = 10^-3 H+. pH of 6 = 10^-6 H+. From the above values, we can clearly see that H+ ion concentration decreases by 1000 times.