The pH value is essentially a continuous variable. Therefor in a statistical study it can be treated as a continuous as well as a discrete variable. Which of the two method is appropriate will depend on the purpose for which the data is used.
For example, if the data pertains to the pH value of shampoo manufactured by a company as collected by the process control department, it will be best to treat it as a continuous variable. However if the data was being collected in a market research project to find the relationship between shampoo pH value and the user perception about the quality of shampoo, it may be desirable to classify shampoos in a fixed number of classes based on their pH values. In this case, the pH value will be treated by the study as a discrete variable.
However, if I have to choose one of the two types for all applications, it is best to treat pH value as a continuous variable.