VARIATION OF pH IN REACTION OF SODIUM SULFIDE WITH CARBONYL COMPOUNDS 2009
dependent of the nature of an HA acid (confirmed for
hydrochloric, hydrobromic, formic, acetic, chloro-
acetic, trichloroacetic, sulfuric, oxalic, malonic, ortho-
phosphoric, and citric acids; the differences in pKa
values between the first and subsequent stages of dis-
sociation of polybasic acids are of no fundamental
importance). Also unimportant is the presence in so-
lution in amounts of up to 20 wt % of water-soluble
organic substances (e.g., propanol-2, ethyl cellosolve).
However, these characteristics strongly depend on
the ratio of the initial concentrations of the carbonyl
compound and Na2SO3. This is readily illustrated by
the data in Fig. 3, in which the ratio between the num-
ber of mol-equiv of the acid introduced and the initial
concentration of the reagent taken in deficiency is
plotted along the abscissa axis. It was confirmed ex-
perimentally that, with the data presented in this form,
it is the ratio of the initial concentrations of Na2SO3
and the carbonyl compound that is important. At dif-
ferent initial concentrations of these reagents, but
equal ratios of these concentrations, the dependence in
the coordinates of Fig. 3 will be represented by actual-
ly a single curve.
Fig. 3. Variation of the pH value in the course of
(1 3) process (IV) and (4 6) its stage (VI) under model
conditions with the relative amount of the acid introduced,
n
/n (n , amount of the reagent taken in deficiency).
HA
0 0
Temperature 23 C, agitation with a magnetic stirrer. Dosage
(mole): Na SO : (1, 2) 0.05 and (3, 6) 0.10; acetone:
2
3
(1, 3) 0.05 and (2) 0.10. Acid: (1 4) hydrochloric, (5) mo-
nochloroacetic, and (6) para-toluenesulfonic acid.
stant (denoted by the horizontal line) when the re-
sidual amount of the reacting carbonyl compound will
approach to zero (see table).
Curves describing how the pH value varies upon
introduction of the acid in the case of reaction (VI)
occurring under model conditions with the use of HCl
and a number of other acids are plotted in Fig. 3 as aux-
iliary lines. For these curves, the quantity chosen to be
As can be seen in Fig. 3 (curves 4 6), the frac-
tion of Na2SO3 consumed in reaction (VI) by the in-
stant of time when this pH value is reached depends
on the nature of an HA acid used. It is about 50%
for HCl, 40% for HNO3, 33 35% for NaHSO4 and
ClCH2COOH, 17% for para-CH3C6H4SO3H, etc. If
the fractions corresponding to the above percentages
are denoted by , the following formula can be sug-
gested for calculating the number of moles (mol-equiv)
of the acid to be introduced for reaching pH 6.3 6.5
in the case of an excess amount of Na2SO3:
plotted along the abscissa axis is also nHA /nN0 a SO
.
2
3
This is done to facilitate the interpretation of how
the pH value varies in the case when the carbonyl
compound and Na2SO3 are taken in excess in carrying
out the main process.
As can be seen from a comparison of curves 1
and 2 in Fig. 3, the excess of acetone leads to higher
pH values over virtually the entire range of varia-
tion of the molar ratio of the amounts of acid and
a reagent taken in deficiency. The only exception
are the values close or equal to unity, when the cur-
ves under consideration converge to a point. This
circumstance is quite understandable, because such
an excess will favor a decrease in the running concen-
trations of Na2SO3 and will shift the equilibrium of
reaction (I) to the right; and the higher the reactivity
of a carbonyl compound and its concentration, in
the more alkaline medium will the process develop
(Fig. 1).
nHA = n0
+ (n0Na SO
n0C=O).
(4)
C=O
2
3
Equation (4) is highly important for several rea-
sons. First, the quantity nHA is one of the most easily,
rapidly, and accurately determinable in a kinetic ex-
periment. At the same time, simultaneous monitoring
of pH and nHA can furnish a sufficient information
about the course of the process in such an exper-
iment. Second, the lower the reactivity of a carbonyl
compound, the larger excess of Na2SO3 is necessary
to faster obtaining a hydrosulfite derivative in a
quantitative yield. In this case, not only the value of
nHA /n0Na SO is important, but also that of . Naturally,
In the case of an excess of Na2SO3, the amount of
the acid introduced will be constituted by the amount
consumed for a quantitative conversion of the car-
bonyl compound by reaction (IV) and for partial con-
version of the excess amount of Na2SO3 into NaHSO3
on reaching pH 6.3 6.5, which corresponds to the in-
2
3
the choice should ensure the highest value of . Third,
Eq. (4) shows how the instant of completion of the
target process can be determined if there are no other
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 77 No. 12 2004