Sentence:  A microsomal de-iodinase with a <WSD>pH</WSD> optimum of around 8 , and requiring reduced triphosphopyridine nucleotide for activity , has been identified in the thyroid ( Stanbury , 1957 ) . 
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);