ECT around 1,000 pC/ml are detected with pressboard use
while less than 1,000 pC/ml of ECT are detected without
pressboard. When 34 ppm of BTA is added, on the other hand,
the ECT stay less than 500 pC/ml regardless of pressboard use.
Thus high concentration of BTA suppresses the increase in
ECT and pressboard enhances the increase in ECT of the oil
with the addition of either octyl sulfide or octyl sulfoxide,
which reflects the adsorption of BTA on pressboard. The
effect of pressboard is obvious when the concentration of
BTA is low. Thus 34 ppm of BTA is appropriate for
suppressing the increase in ECT compared to 10 ppm of BTA.