Say you were adding a strong base to a strong acid I know the PH curve will start ataround 2 and rise slowly but stay low as long as long as there is still some H+ present from the acid. Just before the vertical stretch however the PH curve starts to increase more quickly and could not think of why??
I know the acid is in less excess at this point but surely the concentration of H+ will be decreasing at the same rate. Or is it linked to the fact that while the rate of decrease of H+ ions will be the same, the percentage decrease in the concentration will be more thus having a greater effect on PH.
Also does an indicator such as methyl orange have a red colour all the way to its end point, change and then retain that colour no matter how high the ph goes or is the indicator "activated within a certain PH range.
Indicators as weak acids and can be expressed as follows: HI <-> H+ + I-
HI is one colour, I- is another colour.