Problem Areas in the Use of SI Units

Some individuals have advocated reporting of acidity of body fluids in terms of hydrogen ion concentration instead of pH. Their rationale is that pH is a measure of the chemical potential of hydrogen ions rather than a measure of the concentration of the ions,. Because chemical potential is a function of the activity coefficient, which is not known with certainty, the antilog of pH cannot be assumed to equal the hydrogen ion concentration. pH values should be treated as primary variables and reported as measured. 

The base unit katal (symbol kat), mol/sec, is the catalytic amount of any catalyst, including enzymes, that catalyzes a reaction rate of 1 mol per second in an assay system. The kind of quantity measured is identified as catalytic amount. There is a constant relationship between the international unit (I pmol/min) and the katal (1 mol/sec) to convert, a y ue. in international units to nmol/sec, the value is multiplied by 16.67. Note, however, that dependence on reaction conditions applies to. SI units in the same way as to international units therefore data reported in the same units but obtained under different conditions may not be comparable. Replacement of the international unit for reporting enzyme activity is likely to be slow even units that antedated the international unit are sometimes used in clinical laboratory practice. (See Chapters 8 and 21 for further details on the expression of enzyme activity.) 

The correct unit for reporting pressure, whether it is the partial pressure of blood gases or blood pressure, is the pascal. Because the numerical values expressed in pascals are too small to be used for most chnically important pressure 

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