Sentence:  One of the proteases has <WSD>pH</WSD> optimum of about 3.7 and another of about 5.7 ( McQuillan , Stanley and Trikojus , 1954 ; Alpers , Robbins and Rall , 1955 ) . 
Meaning of the word marked with <WSD> tags:  (from potential of Hydrogen) the logarithm of the reciprocal of hydrogen-ion concentration in gram atoms per liter; provides a measure on a scale from 0 to 14 of the acidity or alkalinity of a solution (where 7 is neutral and greater than 7 is more basic and less than 7 is more acidic);