Question

if the ph of incoming and outgoing waters are 7.2 and 8.4 average ph value of water

Answer 1

I understand the question this way.  I hope it is correct.
Into and from a container, there are incoming and outgoing water flows at the same rate V litres per hour.   Let us say there are always V2 litres in the container.

concentration of [ H+] in incoming water = 10^{-7.2} moles / litre
concentration of [ H+] ions in outgoing water = 10^{-8.4} moles / litre

So number of ions which came in t hours  = 10^{-7.2} V t   moles
Number of ions which went out = 10^{-8.4} V t   moles

So concentration of [ H+ ]ions in the container at time t hours = 
      { 10^{-7.2} - 10^{-8.4} ) V t  = V t * [ (10^{1.2} - 1) ] * 10^{-8.4}   

Average number of [ H+ ] in moles per litre at t hours : 
        =  V t * [ (10^{1.2} - 1) ] * 10^{-8.4} / V2
 
      [ H+ ]   = [ (10^{1.2} - 1) ] * 10^{-8.4} * (V t / V2)  moles

Taking  logs :     pH  = -7.228 + Log_10 (V t / V2)

The average pH value in the container increases continuously as the time progresses.  We are letting in more number of H+ ions and draining only a fraction of that.