Chemistry Reference
In-Depth Information
First of all the stability of rongalite and sodium dithionite in solution was
investigated. From the literature, it is clear that sodium dithionite is rela-
tively unstable over the entire pH range
15-19
.This experiment, conducted at
a pH of 5.5, showed that sodium dithionite indeed decomposes relatively
fast, while rongalite remained stable at room temperature. At elevated tem-
peratures (>330 K), rongalite decomposed very slowly, with a rate of 0.005
mol l
-1
h
-1
, which was independent of the rongalite concentration, indicating
that this rate is only an apparent value. It is expected that the oxygen uptake
rate in the solution determines this apparent rate, because oxygen is a com-
pound taking part in the reaction and its concentration in solution is low
owing to limited solubility.
11.3.3 Reduction rate of Ni(II) to metallic Ni in
presence of rongalite
In this section of the research, the aim is to investigate the optimal com-
position of the Ni(II)-containing solution as a function of the Ni(II) reduc-
tion rate, the total amount of Ni reduced, the fraction of NiS formed and
PAN-fibre properties, such as specific electrical resistance. In Fig. 11.2, data
are shown for the variation of Ni(II) and rongalite concentration as a func-
tion of time and temperature starting from a constant initial Ni(II) con-
centration, while the initial rongalite concentration was increased. It must
be pointed out that in this section of the research, no fibre was immersed
in the solution, so the pure kinetic parameters of the reduction reaction of
Ni(II) by rongalite is studied. Similar experiments were performed with dif-
ferent initial Ni(II) concentrations.
First, the initial reduction rate (slope of the decreasing Ni(II) curves
during the first 15-30 min) is high. In all cases, the reaction finished after
about 60 min, independently of the ratio of rongalite/Ni(II). This is
explained by the formation of formaldehyde in solution, owing to the
decomposition of rongalite into formaldehyde and sulphite. Formaldehyde
is an inhibitor of the Ni(II) reduction reaction by rongalite. As formalde-
hyde is formed by the compound that reduces Ni(II), this type of inhibition
is also called 'auto-inhibition'. Secondly, the reaction rate of the Ni(II)
reduction increases with temperature. In addition, it can be seen that at
room temperature the rate of reduction is somewhat limited and too low
for practical use. Higher temperatures should be used in order to obtain
higher reduction rates and relatively shorter production times. The fact that
the linear section in the initial decreasing Ni(II) reduction curves shortens
with increasing temperature confirms the fact that formaldehyde inhibits
further reduction. At elevated temperatures, a higher decomposition rate
for rongalite is observed, resulting in higher formaldehyde concentrations
