ASTM D1157 Standard Test Method for Total Inhibitor Content (TBC) of Light Hydrocarbons
7. Sampling
7.1 Supply samples to the laboratory in high-pressure sample cylinders. Use the procedures described in Practice D1265 or similar methods.
8. Calibration and Standardization
8.1 Preparation of Standard Solutions - Prepare a standardization curve showing the relation between the absorbance and the catechol content as follows: Make up solutions of known catechol content by pipetting 0, 1, 2, 3, 4, and 5-mL portions of the standard catechol solution (1.00 mL = 100 mg/kg) into separate 100-mL volumetric flasks, or stoppered, graduated mixing cylinders. Then add enough water to each flask or cylinder to make a total volume of approximately 90 mL. Add 5.0 +/- 0.1 mL of standard FeCl3 solution, bring the total volume to 100 mL and mix well.
8.2 Measurement of Standards - Five to fifteen minutes after the addition of the FeCl3 reagent, measure the absorbance of the solution by means of a photoelectric photometer, using water as a reference standard and using light that is predominantly blue (425 nm).
8.3 Preparation of Calibration Curve - Subtract the reading obtained for the zero catechol standard from each of the above readings, using the same volumes of FeCl3 solution and water and observing the same time limits. Record the difference as the respective "net" absorbance. Assuming 1.0 mL of the standard catechol solution to be equal to 100 mg/kg of catechol in the butadiene, plot the net absorbance against the amount of added catechol in mg/kg.
NOTE 1 - While this test method of plotting a curve is the recognized method, it has been found that the blank or zero reading using the FeCl3 reagent described in 6.3 does not change, thus enabling, for control work, the plotting of a curve that can be read directly.
