TFP does not endorse any particular phone app but you may find members who like a particular one. Some apps are only available on one platform (iOS-only or Android-only), few have any kind of data logging and some are just not designed well. Many fail to take into account what is taught here in terms of recommended levels and proper dosing (be careful to make sure any app you use calculates dosages correctly...for example, calculating what is needed to increase/decrease pH is actually a complicated, iterative mathematical process that needs to take into account many different factors and so few apps will do that calculation correctly).
One thing you can do is save Pool Math as an offline web-page. That way, if you don't have an internet or cellular connection, you can still use Pool Math to do the necessary calculations.