Precision is hard to obtain when you're getting very high resolution. Especially the last digits become non-saying. You could calibrate your pH sensor, but that costs time and you need reference measurements to do that (i.e. get liquids you know are exactly a pH value and make a reference measurement with an already calibrated device).
The only reason I could see a high bit ADC is remotely useful in if you want to see small very drifts in pH without losing your Dynamic Range. This is only useful if the noise floor of your system (from sensor to ADC input) is not exceeding that of your ADC. I have no idea what the noise level of your pH sensor is. Maybe you can refer to the datasheet (if it's mentioned at all).
You could use oversampling to reduce the noisiness of your signal, but then your output sampling rate goes down (or you need to get a faster converter/processing unit!). This only makes life more complicated.
Furthermore, temperature drifts may be a huge factor as stevenvh pointed out, not only from your sensor but also from the electronics. But as I said, if you know temperature is a constant factor in your measurements you can see relative changes more accurately.
About actual ADCs: There are differential ADCs with PGA. PGA stands for Programmble Gain Amplifier. You can amplify the signal by certain factors, like 1x, 8x, 16x or even more. It depends on your pH sensor whether you can easily amplify the region of 6-9pH.