<Dl> <Dd> (H) = (H) + (HSO </Dd> </Dl> <Dd> (H) = (H) + (HSO </Dd> <P> An alternative scale, the' free scale', often denoted' pH', omits this consideration and focuses solely on (H), in principle making it a simpler representation of hydrogen ion concentration . Only (H) can be determined, therefore (H) must be estimated using the (SO) and the stability constant of HSO, K: </P> <Dl> <Dd> (H) = (H) − (HSO) = (H) (1 + (SO) / K) </Dd> </Dl>

Ph is the negative logarithm of the hydrogen ion concentration