I have an enzyme - inorganic pyrophosphatase from E. coli, and I am trying to measure its kinetic properties. I have some doubts in what I am doing, because the data I am getting doesn’t make a lot of sense. 

I am using a reaction buffer with Tris-HCl pH 8.0, KCl, 10 mM Mg2+ and 0.1 mg/mL BSA, diluting the enzyme in it, adding the sodium pyrophosphate  in different concentrations, incubating, and adding a mixture of H2SO4, acetone and ammonium molybdate to develop a yellow color, corresponding to an amount of liberated phosphate. Last few tries my procedure was: dilute the enzyme, add a reaction mix with the enzyme and pyrophosphate, take an aliquot every 30 seconds up to 8-10 times, developing each one immediately. This process was repeated for 0.8, 0.4, 0.2, 0.1, 0.05 mM of final pyrophosphate concentration in the same conditions, and as I understand, the slope of a linear region near the start of reaction (which I am pretty sure I am at) should depict lower speeds at lower substrate concentrations (if substrate concentration is near or lower than Km of an enzyme), but my experimental data shows exactly she same slope for each substrate concentration. Approximate enzyme Km in these conditions should be \~0.2-0.1 mM of PPi, and my detection method is not very sensitive at detecting lower than 0.02mM of phosphate. Am I doing something wrong? What can I try next?

Trouble measuring enzyme kinetics