Treatment of sodium dithionite reaction mixture

A method is taught for increasing the yield of anhydrous sodium dithionite by adding an organic compound that is thiosulfate reactive to a batch reactor containing a puddle solution of methanol and fed with formic acid or an alkali formate as a first feed, an aqueous alkali compound as a second feed, an aqueous alkali formate solution as a third feed, and a methanolic SO.sub.2 solution as a fourth feed. This organic compound may be added prior to, combined with, or concurrently with one of the four feeds to the reactor. Preferably, it is added concurrently with the third feed and throughout the entire course of the reaction, ending with the beginning of the cooling period. A suitable addition rate is 0.4-0.6 wt. %/minute, preferably 0.5 wt. %/minute. All of the organic compound is consumed, and at least a portion of the thiosulfate ion is destroyed. The organic compound is selected from the group consisting of epoxy compounds having the formula ##STR1## or halogenated hydrocarbons having the general formula R.sub.2 X or XR.sub.2 X, R.sub.1 being hydrogen, an alkyl group containing from 1 to 8 carbon atoms, a halogenated alkyl group containing from 1 to 2 carbon atoms, a phenyl group, or a substituted phenyl group. The compound represented by this formula includes ethylene oxide, propylene oxide, butylene oxide, epichlorohydrin, epibromohydrin, and styrene oxide. R.sub.2 is a primary or secondary alkyl group containing from 1 to 8 carbon atoms, an allyl group or a 2-methylallyl or 2-ethylallyl group, and X is a halogen atom. Suitable compounds include methyl iodide and allyl chloride.

BACKGROUND OF THE INVENTION 
This invention relates to the manufacture of anhydrous alkali dithionites 
by reacting an alkaline formate, an alkali metal agent, and sulfur dioxide 
in an alcohol/water solvent. It particularly relates to improving this 
process by removing troublesome impurities from the reaction mixture while 
the reaction is occurring. 
In the process for the manufacture of alkali metal dithionites from an 
alkali metal salt of formic acid, an alkali metal hydroxide, carbonate, or 
bicarbonate, and sulfur dioxide, the pertinent chemistry is believed to be 
as shown in the following equations, using sodium formate and sodium 
hydroxide for illustration: 
EQU 2NaOH+2SO.sub.2 .fwdarw.Na.sub.2 S.sub.2 O.sub.5 +H.sub.2 O, (1) 
EQU 2HCOONa+2SO.sub.2 +H.sub.2 O .fwdarw.2HCOOH+Na.sub.2 S.sub.2 O.sub.5, (2) 
EQU 2HCOOH+2Na.sub.2 S.sub.2 O.sub.5 .fwdarw.2Na.sub.2 S.sub.2 O.sub.4 
+2CO.sub.2 +2H.sub.2 O. (3) 
According to these equations, and assuming that an excess of sodium 
metabisulfite is present, one mol of sodium dithionite should be produced 
via Equation (3) for each one mol of formic acid generated as shown in 
Equation (2). 
In practice it is found that substantially less than one mol of sodium 
dithionite is produced per mol of formic acid. Empirically it has been 
established that approximately 0.8 mol of sodium dithionite is produced 
per mol of formic acid. 
One reason for this yield deficiency is that formic acid in the alcohol 
reaction medium used for the process undergoes a certain degree of 
esterification: 
EQU HCOOH+CH.sub.3 OH.revreaction.HCOOCH.sub.3 +H.sub.2 O. (4) 
The alcohol used for the reaction medium is recovered for re-use via 
distillation. The methyl formate in the alcohol should be similarly 
recovered. If the methyl formate recovery were 100% efficient, no yield 
loss would be occasioned by the chemistry of Reaction (4). In practice, 
however, it has been found that methyl formate losses do occur, owing 
principally to its very high volatility. Some loss occurs directly from 
the reactor, as the methyl formate is carried out by the effluent carbon 
dioxide. Other losses occur in the act of filtering, washing, and blowing 
the product filter cake. Still more loss is occasioned by the distillation 
process. The severity of these various losses will be determined by the 
perfection of the design and operation of the equipment used to carry out 
each of these functions. 
A second source of yield deficiency is chemical decomposition of sodium 
dithionite after it is made. While several modes of sodium dithionite 
decomposition are possible, the predominating loss results in the 
formation of sodium thiosulfate and other unidentified sulfur compounds 
which are found in the reactor throughout the course of the reaction: 
EQU 2Na.sub.2 S.sub.2 O.sub.4 .fwdarw.Na.sub.2 S.sub.2 O.sub.3 +Na.sub.2 
S.sub.2 O.sub.5. (5) 
It has also been found that the sodium thiosulfate forming reaction is 
auto-catalytic with respect to sodium thiosulfate. That is, as the 
concentration of sodium thiosulfate increases in the reactor, its rate of 
formation similarly increases. This increasing rate of formation may be 
owing to thiosulfate itself or to the accompanying sulfur compounds. A 
number of factors are known to influence the rate of sodium thiosulfate 
formation, principally the reaction temperature, the pH, and the 
water-to-alcohol ratio in the reaction medium. Despite all efforts to 
optimize these various conditions and minimize the loss of sodium 
dithionite, this loss continues to be a principal cost in the manufacture 
of sodium dithionite. 
Japanese Patent Publication No. 28,397/75 teaches a process for 
manufacturing anhydrous sodium dithionite in an alcohol/water solvent from 
sodium formate, an alkali compound, and sulfur dioxide, followed by 
filtering the sodium dithionite crystals from the mother liquor. The 
publication discloses a method for recycling the reaction filtrate with 
reduced distillation of the filtrate by treating the filtrate with 
1-to-4-fold excess on a molar basis of ethylene oxide, propylene oxide, or 
a mixture thereof over the amount of sodium thiosulfate contained in the 
reaction filtrate and by allowing the reaction mixture to stand for 
several hours at room temperature. The reaction filtrate is combined with 
the methanol used in washing the separated crystals of sodium dithionite 
to form a mixture of which a part is distilled to recover the methanol and 
isolate the addition product, which is discarded, of sodium thiosulfate 
and ethylene oxide or propylene oxide. This is hereinafter referred to as 
the filtrate purification/recycle method. 
Japanese Patent Disclosure No. 110,407/83 teaches a method for producing 
dithionites by reacting a formic acid compound, an alkali compound, and 
sulfur dioxide in a water-organic solvent mixture and by adding an epoxy 
compound, a halogenated hydrocarbon of the general formula R-X, or a 
mixture of two or more compounds of these types to the reaction mixture in 
the final stage of the reaction. Suitable epoxy compounds include ethylene 
oxide, propylene oxide, butylene oxide, isobutylene oxide, styrene oxide, 
cyclohexene oxide, epichlorohydrin, and epibromohydrin. In the halogenated 
hydrocarbon, R is a primary or secondary alkyl group having 1-8 carbons, 
an allyl group, a 2-methylallyl group, or a 2-ethylallyl group, and X is a 
halogen. The filtrate obtained by isolating the dithionite crystals, the 
organic solvent used for washing the crystals, or a mixture thereof is 
recycled and reused in the reaction. 
The reaction is conducted according to Patent Disclosure No. 110,407/83 by 
dissolving sodium formate in hot water, adding methanol, and heating at 
82.degree. C. under an applied pressure of 1.0 kg/cm.sup.2 gauge while 
stirring in a reactor equipped with a reflux condenser and a deep-cooling 
condenser. Subsequently, 50% sodium hydroxide solution and a methanolic 
solution containing methyl formate and sulfur dioxide are added 
simultaneously and dropwise over a 90-minute period. Stirring is continued 
for an additional 150 minutes at the same temperature and pressure. 
Cooling to 73.degree. over a period of 20 minutes is then started, and 
simultaneously the epoxy compound, the halogenated hydrocarbon, or a 
mixture thereof is added within less than five minutes. The dithionite 
crystals are separated out by filtration under applied pressure with 
carbon dioxide and subsequently washed with methanol and then dried under 
reduced pressure. Both the filtrate and the washing liquid were 
demonstrated to be equivalent to distilled methanol as the organic solvent 
for producing sodium dithionite. This is referred to hereinafter as the 
mother liquor cooling/purification method. 
In European Patent Publication No. 68,248 and in U.S. Pat. No. 4,388,291, a 
process is disclosed for producing anhydrous dithionites in which the 
washing liquid discharged from the washing step is sequentially divided 
into two portions, a first discharge liquid and a second discharge liquid, 
the former being treated to convert undesirable substances inhibiting the 
production of dithionites into substances which do not exert an adverse 
influence on the production of dithionites by adding an organic compound 
selected from the group consisting of compounds represented by Formulas I 
and II and cyclohexene oxide. Formula I is as follows: 
##STR2## 
wherein R.sub.l is hydrogen, an alkyl group containing from 1 to 8 carbon 
atoms, a halogenated alkyl group containing from 1 to 2 carbon atoms, a 
phenyl group, or a substituted phenyl group. The compound represented by 
this formula includes ethylene oxide, propylene oxide, butylene oxide, 
epichlorohydrin, epibromohydrin, and styrene oxide. Formula II is as 
follows: 
EQU R.sub.2 --X, 
wherein R.sub.2 is a primary or secondary alkyl group containing from 1 to 
8 carbon atoms, an allyl group or a 2-methylallyl or 2-ethylallyl group, 
and X is a halogen atom. Suitable compounds include methyl iodide and 
allyl chloride. 
In the examples, the reaction of the first discharge liquid (48 parts) with 
the treatment compound (0.07-0.13 parts) occurred at 25.degree.-45.degree. 
C. for 1-24 hours after filtration at 73.degree. C. A portion of the 
treated first discharge liquid was mixed with nearly twice as much of the 
untreated second discharge liquid and used to prepare sodium dithionite 
after adjusting for the amount of water in the discharge liquid mixture. 
The resulting purities and yields for the sodium dithionite product were 
substantially identical to those obtained with pure methanol. This is 
hereinafter referred to as the washing liquid purification/recycle method. 
The use of a thiosulfate-reactive compound for destroying thiosulfate ions 
in the mother liquor, the reactor filtrate, or the washing liquid before 
re-use of the methanol therein, as respectively taught in Japanese No. 
110,407/83, Japanese No. 28,397/75, and European No. 68,248, enables the 
initial reaction mixture of the next run to be substantially free of 
thiosulfate ions, but it does nothing to diminish the formation of 
thiosulfate ions during the dithionite-forming reaction. The use of such a 
thiosulfate-reactive compound in the final (i.e., cooling) stage of the 
formate/SO.sub.2 reaction, as taught in Japanese No. 110,407, similarly 
enables the reaction filtrate or the washing liquid to be utilized for 
making sodium dithionite at yields and purities equal to results from 
manufacture with distilled methanol. Again, however, such protection for 
the next batch provides no help for the current batch. 
Accordingly, there is clearly a need for destroying thiosulfate ions as 
they are being formed within the reaction vessel in order to minimize 
destruction of the sodium dithionite product. Moreover, if the thiosulfate 
ions could be at least partially destroyed in the current batch, the 
treatment needed for the mother liquor during the cooling period, for the 
filtrate, or for the wash liquid according to prior art methods could also 
be decreased before each recycle of filtrate and/or wash liquid to the 
next batch. Similarly, an in situ treatment to diminish the harmful 
effects of thiosulfate and other deleterious sulfur compounds would allow 
the use of raw materials containing these contaminants in the manufacture 
of sodium dithionite. 
SUMMARY OF THE INVENTION 
It has surprisingly been discovered that the auto-catalytic action of 
sodium thiosulfate and accompanying sulfur compounds can be negated or at 
least minimized by adding an organic compound to the reactor prior to 
starting the sodium dithionite producing process so that the sodium 
thiosulfate is consumed as soon as it is produced. 
The organic compound can also be pumped into the reactor throughout the 
course of the sodium dithionite producing process to obtain a similar 
effect. Preferably, the organic compound is pumped into the pressurized 
reactor throughout the course of the entire reaction, beginning when the 
reactor contents have been heated to about 50.degree. C. and ending wtih 
the start of the cooling period. The pumping rate is suitably 0.4-0.6 wt. 
%/minute and preferably about 0.5 wt. %/minute. 
These organic compounds that are capable of reacting with or complexing 
sodium thiosulfate include the epoxy compounds such as ethylene oxide, 
propylene oxide, butyl and isobutyl oxide, epichlorohydrin, and 
epibromohydrin. These organic compounds also include halogenated 
hydrocarbons of the general formula R.sub.2 X, or XR.sub.2 X, where 
R.sub.2 is an alkyl group of carbon number 1 to 8, or an allyl, methallyl, 
or ethallyl group, and X is a halogen. In the case of ethylene oxide, 
propylene oxide, and the like, the result of the reaction is a Bunte salt: 
##STR3## 
In the case of the alkyl halides, the reaction is: 
EQU R.sub.2 X+Na.sub.2 S.sub.2 O.sub.3 .fwdarw.R.sub.2 -S-SO.sub.3 Na+NaX. (7) 
When carrying out repetitive batches on a large scale, it becomes 
critically important that every detail of the operational procedure be 
carried out exactly as scheduled. Such an operational procedure is 
typically developed, after numerous technical studies and operational 
trials, in unending efforts to maximize product purity and yield. Even 
small fractional improvements are greatly valued. Accordingly, any 
destruction of thiosulfate ions and other harmful sulfur compounds during 
the course of the dithionite-producing reaction that could minimize 
product losses and increase yield would be an important improvement over 
known batch production methods.

DESCRIPTION OF PREFERRED EMBODIMENTS 
A series of pilot plant experiments was conducted to demonstrate this 
process. The organic compound was chosen to be propylene oxide owing to 
its lesser hazard and ease of handling. Ethylene oxide would be the 
commercial epoxy compound of choice, however, owing to its lower molecular 
weight and lower cost. The data and calculated results are shown in the 
Table accompanying Example 9. 
EXAMPLE I 
A series of thirteen base line runs was made while adding no propylene 
oxide to the reactor. These runs were begun by adding to a 100-gallon 
reactor, as a first feed, 150 lbs of distilled recovered methanol 
containing 3.67% methyl formate and 0.96% sulfur dioxide. Next, as a 
second feed, a solution of 7 lbs of 96% sodium formate dissolved in 5 lbs 
of water was added. The reactor contents were heated to 50.degree. C. with 
agitation, at which time the third and fourth feeds were started 
simultaneously. The third feed consisted of 69 lbs of 99% sodium 
hydroxide, 135 lbs of 96% sodium formate, and 109 lbs of water. Its feed 
rate was controlled so that it was fed in its entirety in 65 minutes. The 
fourth feed consisted of 310 lbs of distilled recovered methanol of the 
same composition as the first feed and containing additional sulfur 
dioxide of a quantity such that between the first and fourth feeds a total 
of 201 lbs of sulfur dioxide would enter the reactor. The feed rate of the 
fourth feed was controlled so that 80% of its total amount was fed to the 
reactor in 65 minutes. 
Owing to the exothermic nature of the reaction between the third and fourth 
feeds, the contents of the reactor self-heated to 84.degree. C. over a 
fifteen-minute time period. Temperature control was then initiated to 
maintain 84.degree. C. throughout the entire remaining course of the 
reaction. Owing to the evolution of carbon dioxide, the reactor pressure 
increased to 40 psig in this same 15-minute period. Pressure control was 
then initiated to maintain 40 psig throughout the entire remaining course 
of the reaction. The vented carbon dioxide exited the reactor through 
condensers and a scrubber that was fed with essentially pure recovered 
methanol at a rate of 0.364 lb per minute. When the third feed was 
terminated at 65 minutes, the rate of feed of the fourth feed was reduced 
so that the remaining 20% was fed over an additional 65 minutes. At the 
conclusion of this feed, an additional 65-minute period at 84.degree. C. 
and 40 psig was allowed for the reaction to go to completion. The sodium 
thiosulfate concentration was monitored via sampling the reactor contents 
at the end of each of the above 65 minute periods. The concentration is 
expressed as the sodium thiosulfate titer of a standard iodine solution. 
At the conclusion of this third 65-minute period, the reactor contents 
were cooled to 73.degree. C. and discharged to the filtering apparatus. 
After methanol washing, the filter cake was vacuum dried to produce the 
sodium dithionite product. The thirteen runs made in this manner averaged 
a titer of 3.9 at the end of the first 65 minute period, 4.3 at the end of 
the second 65 minute period, and 7.0 at the end of the third 65 minute 
period. The product averaged 237 lbs in weight at an assay of 91.57%, or a 
yield of 1.246 mols of sodium dithionite. 
EXAMPLE 2 
A series of five runs was made by adding 4 lbs of propylene oxide to the 
first feed. All other quantities and conditions were identical to Example 
1 except that the distilled recovered methanol used for feeds one and four 
contained 3.56% methyl formate. The five runs made in this manner averaged 
a titer of 1.9 at the end of the first 65 minute period, 2.5 at the end of 
the second 65 minute period, and 4.6 at the end of the third 65 minute 
period. The product averaged 245 lbs in weight at an assay of 91.64%, or a 
yield of 1.289 mols of sodium dithionite. 
EXAMPLE 3 
A series of five runs was made by adding 6 lbs of propylene oxide to the 
first feed. All other quantities and conditions were identical to Example 
1 except that the distilled recovered methanol used for feeds one and four 
contained 3.00% methyl formate. The five runs made in this manner averaged 
a titer of 1.6 at the end of the first 65 minute period, 2.4 at the end of 
the second 65 minute period, and 4.5 at the end of the third 65 minute 
period. The product averaged 240 lbs in weight at an assay of 93.35%, or a 
yield of 1.287 mols of sodium dithionite. 
EXAMPLE 4 
A series of five runs was made by adding 8 lbs of propylene oxide to the 
first feed. All other quantities and conditions were identical to Example 
1 except that the distilled recovered methanol used for feeds one and four 
contained 2.71% methyl formate. The five runs made in this manner averaged 
a titer of 1.5 at the end of the first 65 minute period, 2.3 at the end of 
the second 65 minute period, and 4.7 at the end of the third 65 minute 
period. The product averaged 246 lbs in weight at an assay of 91.17%, or a 
yield of 1.288 mols of sodium dithionite. 
EXAMPLE 5 
A series of four runs was made by pumping 6 lbs of propylene oxide into the 
reactor at a rate of 0.031 lb per minute over the 195 minute reaction 
duration. All other quantities and conditions were identical to Example 1 
except that the distilled recovered methanol used for feeds one and four 
contained 2.58% methyl formate. The four runs made in this manner averaged 
a titer of 1.5 at the end of the first 65 minte period, 1.3 at the end of 
the second 65 minute period, and 1.2 at the end of the third 65 minute 
period. The product averaged 247 lbs in weight at an assay of 90.03%, or a 
yield of 1.277 mols of sodium dithionite. 
It will be noted that according to Equation (6), sodium hydroxide is 
produced along with the Bunte salt when propylene oxide reacts with sodium 
thiosulfate. In that sodium hydroxide is a raw material in the production 
of sodium dithionite, it should be possible to remove a quantity of sodium 
hydroxide from the third feed. 
EXAMPLE 6 
A series of six runs was made by pumping 6 lbs of propylene oxide into the 
reactor at a rate of 0.031 lb/minute over the 195-minute reaction 
duration. All other quantities and conditions were identical to Example 1 
with two exceptions; the distilled recovered methanol used for feeds one 
and four contained 2.63% methyl formate, and the sodium hydroxide content 
of the third feed was 66 lbs. The six runs made in this manner averaged a 
titer of 1.7 at the end of the first 65 minute period, 1.9 at the end of 
the second 65 minute period, and 1.8 at the end of the third 65 minute 
period. The product averaged 242 lbs in weight at an assay of 92.63%, or a 
yield of 1.287 mols of sodium dithionite. 
EXAMPLE 7 
A series of two runs was made by pumping 10 lbs of propylene oxide into the 
reactor at a rate of 0.051 lb/minute over the 195-minute reaction 
duration. All other quantities and conditions were identical to Example 1 
with two exceptions; the distilled recovered methanol used for feeds one 
and four contained 3.00% methyl formate, and the sodium hydroxide content 
of the third feed was 65 lbs. The two runs made in this manner averaged a 
titer of 1.5 at the end of the first 65 minute period, 1.9 at the end of 
the second 65 minute period, and 1.2 at the end of the third 65 minute 
period. The product averaged 244 lbs in weight at an assay of 92.15%, or a 
yield of 1.291 mols of sodium dithionite. 
EXAMPLE 8 
A series of three runs was made by pumping 8 lbs of allyl chloride into the 
reactor at a rate of 0.041 lb/minute over the 195-minute reaction 
duration. All other quantities and conditions were identical to Example 1 
except that the distilled recovered methanol used for feeds one and four 
contained 3.00% methyl formate. The three runs made in this manner 
averaged a titer of 2.0 at the end of the first 65 minute period, 1.3 at 
the end of the second 65 minute period, and 1.2 at the end of the third 65 
minute period. The product averaged 243 lbs in weight at an assay of 
92.83%, or a yield of 1.295 mols of sodium dithionite. 
EXAMPLE 9 
A series of two runs was made by pumping 4 lbs of 1-2 dichloroethane to the 
reactor at a rate of 0.021 lb/minute over the 195 minute reaction 
duration. All other quantities and conditions were identical to Example 1 
except that the distilled recovered methanol used for feeds one and four 
contained 3.00% methyl formate. The three runs made in this manner 
averaged a titer of 3.4 at the end of the first 65 minute period, 5.3 at 
the end of the second 65 minute period, and 7.8 at the end of the third 65 
minute period. The product averaged 237 lbs in weight at an assay of 
92.19%, or a yield of 1.255 mols of sodium dithionite. 
All of these examples demonstrate an enhanced yield of sodium dithionite 
achieved by the addition of the named chemical. The results become even 
more coherent if correction is made to the yield for the variable methyl 
formate content of the alcohol entering the reactor. According to Equation 
(4), any deficiency in methyl formate will be made up at the expense of 
the formic acid needed to produce sodium dithionite via Equation (3). It 
was established in earlier work that the approach to an equilibrium 
concentration of methyl formate in one pass through the reactor was only 
70%. As was noted earlier, the net efficiency of formic acid utilization 
in producing sodium dithionite is 80%. Therefore, each one mol deficiency 
in methyl formate in the methanol entering the reactor will result in a 
sodium dithionite yield deficiency of 0.56 mol. When this correction (to 
3.00% methyl formate) was applied to the reported yields of Examples 1-9, 
corrected yields were obtained as shown in the Table. 
__________________________________________________________________________ 
ADDITIONS AND YIELDS FOR REACTION EXAMPLES 
Organic Compound Methyl Yield 
Example 
Addition Formate 
Titer Period 
Yield 
Assay 
Reported 
Corrected 
Increased 
No. lbs 
wt. % @ 
Rate* 
% 1st 
2nd 
3rd 
lb % mol mol % 
__________________________________________________________________________ 
1 0 0.0 0.0 3.67 3.9 
4.3 
7.0 
237 91.57 
1.246 
1.217 0.00 
2 4 1.8 # 3.56 1.9 
2.5 
4.6 
245 91.64 
1.289 
1.265 3.94 
3 6 2.7 # 3.00 1.6 
2.4 
4.5 
240 93.35 
1.287 
1.287 5.75 
4 8 3.6 # 2.71 1.5 
2.3 
4.7 
246 91.17 
1.288 
1.300 6.82 
5 6 2.7 0.031 
2.58 1.5 
1.3 
1.2 
247 90.03 
1.277 
1.295 6.41 
6 6 2.7 0.031 
2.63 1.7 
1.9 
1.8 
242 92.63 
1.287 
1.303 7.07 
7 10 4.5 0.051 
3.00 1.5 
1.9 
1.2 
244 92.15 
1.291 
1.291 6.08 
8 8 3.6 0.041 
3.00 2.0 
1.3 
1.2 
243 92.83 
1.295 
1.295 6.41 
9 4 1.8 0.021 
3.00 3.4 
5.3 
7.8 
237 92.19 
1.255 
1.255 3.12 
__________________________________________________________________________ 
#All added to puddle (first feed). 
*Rate in lbs/min of continuous addition during all three 65minute periods 
@Of sodium dithionite product. 
These corrected yields clearly indicate that the yield increase was 
proportional to the quantity of propylene oxide added to the reactor up to 
8 lbs. At the 10 lbs level, a yield decrease was noted as compared to the 
identical procedure utilizing 6 lbs of propylene oxide. They also show an 
improved result when the propylene oxide is pumped in continuously, as 
opposed to adding it all at the beginning of the reaction, and they show a 
further improvement when the quantity of sodium hydroxide in the third 
feed is adjusted to compensate for the sodium hydroxide generated in situ 
owing to the use of propylene oxide. 
"Titer" is a measure of sodium thiosulfate content of the solution at the 
indicated time and is therefore an indication of the extent of 
decomposition of dithionite. It is obtained by mixing a 10 ml. sample of 
reactor filtrate with a neutral formaldehyde solution (to tie up 
bisulfite), adjusting the pH to 4.0, and then titrating the sample with 
0.1 N standard iodine solution. 
In the base case Example 1, no organic compound added, the titer affords a 
measure of the total thiosulfate content of the reaction filtrate but not 
including the thiosulfate existing as a solid within the product. In 
Examples 2-9, the titer measures only the thiosulfate in the filtrate that 
has not reacted or complexed with the organic compound added. No 
analytical methods are known to determine the quantity of thiosulfate 
present in the solid product within the reactor or present as a reaction 
product resulting from the organic compound addition. The titer may or may 
not indicate the quantity of the various unidentified sulfur compounds 
present in the filtrate depending on the structure of those compounds. 
Because of the shortcomings noted above, a rigorous correlation between the 
dithionite yield increase achieved and the measured titer would not be 
expected. However, it can be said that an increased dithionite yield was 
accompanied by a decrease in the titer measurement, with the magnitude of 
the decrease being a function of the way in which the organic compound was 
added to the reactor. 
In the previously discussed Japanese patents describing the use of the 
various organic chemicals to eliminate the sodium thiosulfate in the 
filtrate, no examples are given for the production of sodium dithionite in 
the absence of the various organic chemicals. It is not possible, 
therefore, to demonstrate a yield change owing to the use of these 
organics. It is assumed, however, that since no yield increase was noted 
or claimed, that none was observed. To demonstrate this point, the 
experiments described in Example 10 were performed. 
EXAMPLE 10 
As a comparative example, a series of four runs was made by adding 6 lbs of 
propylene oxide to the reactor at the end of the 195-minute reaction 
period and simultaneously with cooling the reactor contents to 73.degree. 
C., a process requiring approximately 15 minutes. All other quantities and 
conditions were identical to Example 1 except that the distilled recovered 
methanol used for feeds one and four contained 3.00% methyl formate. The 
four runs made in this manner averaged a titer of 2.9 at the end of the 
first 65 minute period, 6.8 at the end of the second 65 minute period, and 
12.6 at the end of the third 65 minute period. The product averaged 241 
lbs in weight at an assay of 88.57% or a yield of 1.226 mols of sodium 
dithionite. 
This yield, 1.226 mols, is a slight improvement (0.74%) as compared to the 
base line (0 lb propylene oxide) runs of Example 1. The small yield 
increase is probably owing to a decreased sodium dithionite decomposition 
during both the slurry cooling period and the filtration and cake wash 
periods. A filtrate sample taken after the propylene oxide addition 
resulted in a titer of 2.7. 
Example 10 uses the process taught in Japan No. 110,407/83. In both this 
procedure and in the procedure of Examples 2-9, analysis of the filtrate 
shows no residual unreacted propylene oxide. That which does not react 
with the sodium thiosulfate reacts with either water or methanol as shown 
in the following equations: 
##STR4## 
The products of both of these reactions were found in the filtrate. 
EXAMPLE 11 
In the process taught in Japan No. 28397/75, the organic chemical was added 
to the cooled filtrate after the product sodium dithionite was removed. 
When this procedure was followed, immediate analysis confirmed the 
presence of the added propylene oxide, 1.1%. After 5 hours, the 
concentration had decreased to 0.71%, and after 16 hours to 0.62%. Still 
following the instructions of Japan No. 28397/75, appropriate amounts of 
both sodium hydroxide and sodium formate were added to the filtrate. A 
sample then showed 0.45% propylene oxide. When heated to 70.degree. C., 
again as in Japan No. 28397/75, a sample showed the propylene oxide to be 
totally consumed, with the same reaction products noted previously in 
Equations (8) and (9). 
The mother liquor purification/cooling method, the filtrate 
purification/recycle method, and the washing liquid purification/recycle 
method of the prior art may be classified as the cooling period treatment 
and filtrate/wash liquid treatment methods of the prior art for destroying 
thiosulfate ions for recycle purposes. These methods and the reaction 
mixture treatment method of this invention totally consume the added 
organic chemical compound prior to commencing the subsequent synthesis of 
the next batch of sodium dithionite. These methods also appear to destroy 
at least a portion of the thiosulfate ions. It consequently follows that 
the filtrate and/or the wash liquid produced by following the method of 
this invention can be employed in the cooling period or filtrate/wash 
liquid treatment methods of the prior art, by recycling these liquids as 
the methanol source for the next batch reaction or as the feed methanol 
for a continuous reaction, with reduced usage of an epoxy compound or a 
halogenated hydrocarbon for treating the filtrate and/or the wash liquid. 
What is regarded as the invention and is desired to be protected is defined 
in the accompanying claims.