Sentence:  The fraction of <WSD>exchange</WSD> was determined as the ratio of the counts / minute observed in the carbon tetrachloride to the counts / minute calculated for the carbon tetrachloride fractions for equilibrium distribution of the activity between the chlorine and carbon tetrachloride , empirically determined correction being made for the difference in counting efficiency of * * f in * * f and * * f . 
Meaning of the word marked with <WSD> tags:  chemical process in which one atom or ion or group changes places with another