pH is a logarithmic expression of concentration of $[H]$

$-\log ([H^{+}])=pH$ Instrument: Most accurate is pH indicator. The $Kw$ is called ionic product of water. $[H^{+}][O{H}^{-}]=Kw=10^{-14}$ Take log of both sides to get $pH+pOH=14$ Both are same in water, so the neutral for water would be 7.

The pH is temperature dependent because of Le Chatelier’s Principle on how changes in temperature modify the $k$ values. So pH should always be listed with temperature beside it. Also note that the units inside to calculate pH should be $mold{m}^{-3}$