Logarithms pH scale

Hydrogen ion concentration is more conveniently expressed using the logarithmic pH scale, in which pH = — logio[H ]. 

For the same reasons that it is convenient to express hydrogen ion concentrations on the logarithmic pH scale, it is common practice to express acid strengths as pKa = — log Aa. Strong acids have pKaS of zero or less, while those of weak acids are positive. 

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