No abstract

The combination of two low-cost classical procedures based on titrimetric techniques is presented for the determination of pyridoxine hydrochloride in pharmaceuticals samples. Initially some experiments were carried out aiming to determine both pKa1 and pKa2 values, being those compared to values of literature and theoretical procedures. Commercial samples containing pyridoxine hydrochloride were electrochemically analysed by exploiting their acid-base and precipitation reactions. Potentiometric titrations accomplished the reaction between the ionizable hydrogens present in pyridoxine hydrochloride, being NaOH used as titrant; while the conductimetric method was based on the chemical precipitation between the chloride of pyridoxine hydrochloride molecule and Ag+ ions from de silver nitrate, changing the conductivity of the solution. Both methods were applied to the same commercial samples leading to concordant results when compared by statistical tests (95 and 98% confidence levels). Recoveries ranging from 99.0 to 108.1% were observed, showing no significant interference on the results.

<p>The climate system experienced several periodic oscillations over the last ca. 800 ka known as glacial-interglacial (G-IG) cycles. Disruptions of the global carbon cycle were evident on this time scale, promoting fluctuations in the atmospheric CO<sub>2</sub> concentration leading to global climate variability. In the more recent interglacials, both Antarctic temperatures and atmospheric CO<sub>2</sub> concentrations are significantly higher than in the previous “lukewarm interglacials” (ca. 800 – 430 ka) before the Mid-Brunhes Transition (MBT). Changes in the Atlantic Meridional Overturning Circulation (AMOC) and deepwater formation rate around Antarctica have been invoked to explain a 30 ppm increase in the atmospheric CO<sub>2 </sub>­during post-MBT interglacial periods. Deepwater variability is tightly coupled to the ventilation of CO<sub>2 </sub>in the Southern Ocean by atmospheric and oceanic connections, contributing to carbon storage in the deep ocean and the atmospheric CO<sub>2</sub>. Here, we present a new 770 ka benthic foraminifera δ<sup>13</sup>C record from sediment core GL-854 retrieved from the western South Atlantic (WSA) at 2200 m water depth. We compare our record with published δ<sup>13</sup>C data from the eastern margin to investigate the zonal gradient variability of the North Atlantic Deep Water (NADW) in the deep South Atlantic basin. WSA δ<sup>13</sup>C variability and absolute values strongly mimic the North Atlantic mid-depth record at the NADW formation region. This similarity is interpreted as NADW preferentially carrying a modified signal through the deep western boundary current towards the WSA (rather than towards the eastern margin) after the MBT. The δ<sup>13</sup>C gradient based on the difference between benthic foraminifera <em>C. wuellerstorfi </em>from both margins (Δδ<sup>13</sup>C<sub>w-e</sub>) gradually increases after a transitional period between ca. 400 ka to 300 ka towards the Holocene. We suggest that the mechanism behind this long-term increasing trend on the Δδ<sup>13</sup>C<sub>w-e</sub> record post-MBT is the result of enhanced production of North Component Water due to Agulhas Leakage intensification driven by reduced sea-ice extent after the MBT. Furthermore, reduced sea-ice extent decreases the Antarctic Bottom Water density and formation in the Southern Ocean, contributing to the deepening of the AMOC during post-MBT interglacial periods. Our interpretation proposes a framework connecting sea-ice and ocean-atmosphere dynamics to deepwater geometry within the South Atlantic basin, which ultimately contributed to the climate change observed across the MBT.</p>