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Ref. No. [UMCES] CBL 2015-010
ACT VS15-03
APPENDIX I
Laboratory test results for rapid pH transitions during acid/base additions
Laboratory tests involved week long testing at a set combination of three temperatures
(10, 20 and 30
o
C) and three salinities (0.3, 22 and 35 psu). After 4 to 6 days of testing at a stable
T-S condition and ambient pH, pH was cycled over a reasonable range using acid-base additions
to the water of the test tank (7.5 to 8.5 for seawater and 6.5 to 8.8 for freshwater). One or two,
raised - lowered pH cycles were conducted at each T-S condition over the course of three to six
hours. Acid/base additions were done by first mixing known quantities of acid/base into several
liters of the current test solution and then adding this solution into the test tank to facilitate
mixing and rapid equilibration. Shifts in pH were typically complete in 10 – 15 minutes, so
usually within 1 measurement cycle of the instrument. There was no attempt to calculate a
specific response time because the shifts were not implemented on a specific schedule relative to
the sampling rate.
Results are presented for each of the three temperatures for a given salinity on the same
page. Results for the Metrohm electrode measuring at 15 min intervals are presented in the solid
magenta line along with the discrete dye reference pH measurement in yellow circles. The
Metrohm electrode was directly calibrated to the dye pH results, hence the direct agreement.
Results for the Idronaut 305 Plus, also measuring at 15 min intervals are shown with blue
triangles. The 305 Plus closely tracked the pH shifts within one measurement cycle and
appeared to be in full equilibrium to the new level and corresponding dye reference pH
measurements over each of the cycled intervals. The response was similar when going between
raised and lowered pH shifts.
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