Process for the preparation of aqueous solutions of tetraalkylammonium hydroxides

There is provided a simple method of producing in a good yield a tetraalkylammonium hydroxide aqueous solution in which the content of metal ions such as alkali metal ions is reduced to an extremely low level. High-purity trialkylamine and high-purity alkyl chloride are reacted with each other in ultrapure water to obtain an extremely high-purity tetraalkylammonium chloride aqueous solution substantially free from metal ions such as alkali metal ions, and then, the tetraalkylammonium chloride aqueous solution is subjected to electrolysis-dialysis in a solution state without drying it into a solid to produce a tetraalkylammonium hydroxide aqueous solution. Thereby, an extremely high-purity tetraalkylammonium hydroxide aqueous solution with little reduction in purity is obtained in a high yield.

FIELD OF TECHNOLOGY 
This invention relates to a novel method of producing efficiently 
tetraalkylammonium hydroxide (may be abbreviated as TAAH hereinafter) 
aqueous solution having extremely high-purity. 
BACKGROUND OF THE INVENTION 
A TAAH aqueous solution is a useful chemical as a strong base in a chemical 
reaction and has been recently used also as a processing chemical in a 
semiconductor-related field, such as the cleaning and etching of a 
semiconductor substrate and the development of a resist in the production 
of ICs and LSIs. 
For use as a processing chemical in the semiconductor-related field, a 
high-purity tetraalkylammonium hydroxide aqueous solution containing no 
impurities such as metal ions and organic substances has been desired, 
along with progress made in the integration of semiconductor devices. 
Heretofore, the above TAAH aqueous solution has been produced by dissolving 
solid tetraalkylammonium chloride (may be abbreviated as TAAC hereinafter) 
in water to prepare a TAAC aqueous solution and electrolyzing and 
electrodialyzing the TAAC aqueous solution to effect ion-exchange between 
chlorine ions and hydroxyl ions. 
In the above production method of the TAAH aqueous solution, reasons why 
TAAC is used in a solid state are as follows. That is, TAAC has been 
heretofore produced by reacting a trialkylamine with an alkyl chloride in 
a polar solvent such as water, isopropanol or the like. However, in the 
industrial production, water as a solvent has been rarely used actually 
and an organic solvent such as isopropyl alcohol has been generally used. 
Therefore, to prepare a TAAC aqueous solution, there is employed a method 
in which solid TAAC obtained by drying a TAAC aqueous solution to remove 
the solvent is dissolved in water. This is one of the reasons for handling 
TAAC in a solid state. 
Another reason is that TAAC is generally used as an additive such as a 
phase-transfer catalyst or reagent and it must be solid when it is 
directly added to a reaction system. 
Still another reason is that solid TAAC is advantageous in transportation 
between factories, handling and the like. 
However, the above-mentioned application involves problems. That is, the 
method of producing a TAAH aqueous solution from the above solid TAAC 
requires a process of drying TAAC into a solid. Therefore, concentrated 
metal impurities such as metal ions derived from a solvent and raw 
materials are contained in TAAC, and further, when TAAC is dissolved in 
water, impurities contained in water are added thereto. Consequently, the 
purity of the obtained TAAH aqueous solution is greatly lowered. 
Further, at the time of drying, TAAC is partially decomposed, whereby the 
yield thereof is lowered and at the same time, the purity of the obtained 
TAAH aqueous solution is lowered by the decomposition product, like in the 
above case. 
Meanwhile, use of purified TAAC as a raw material of the TAAH aqueous 
solution to improve the purity of the TAAH aqueous solution is known as 
disclosed in Japanese Laid-open Patent Application 60-131985 
(131985/1985), for example. The above publication teaches that the 
impurities of alkali metals and alkaline earth metals out of impurities 
derived from raw materials can be reduced to such an extent that a 
required purity of a quaternary hydroxide can be obtained by selecting a 
quaternary ammonium salt as a raw material with care and purifying it. 
However, TAAC cannot be purified by distillation due to its properties, and 
the industrial-scale purification thereof is limited to a separation 
method in an ion state such as electrodialysis. For example, when are 
purification is conducted by letting quaternary ammonium ions passing 
through a cation exchange membrane, most of alkali metal ions pass through 
a separating membrane together with tetraalkylammonium ions in the 
purification because alkali metal ions have a smaller ion diameter than 
tetraalkylammonium ions. Meanwhile, for removing alkali metal ions by a 
univalent perm-selective membrane completely, it takes extremely long time 
and hence, this is not practical. 
Therefore, it has been difficult to purify TAAC to such an extent that the 
concentration of alkali metal ions is several ppb, which is the target of 
the present invention, with the conventional method. 
The purification by electrodialysis of the obtained TAAH aqueous solution 
is disclosed also in Japanese Laid-open Patent Application 60-131985. 
According to this publication, a means to purify the TAAH aqueous solution 
containing the above impurities comprises supplying the TAAH aqueous 
solution to be purified to an anode chamber formed by installing a cation 
exchange membrane between an anode and a cathode, supplying water to a 
cathode chamber and applying electricity between the both electrodes to 
obtain purified TAAH from the cathode chamber. 
However, in this case, too, the removal of univalent ions such as sodium 
and potassium is not sufficient like the purification of the aforesaid 
TAAC aqueous solution and hence, an extremely high-purity TAAH aqueous 
solution does not yet come to be obtained. 
As described above, in any of the above conventional methods for obtaining 
a high-purity TAAH aqueous solution, purification is carried out in a 
condition that separation of metal ions such as sodium and potassium is 
difficult to conduct, and the level of high purity thereof is required to 
be further improved. 
DISCLOSURE OF THE INVENTION 
It is therefore an object of the present invention to provide a simple 
method of producing in a high yield a TAAH aqueous solution in which the 
content of metal ions such as alkali metal ions is reduced to an extremely 
low level. 
The inventors of the present invention have conducted diligent studies to 
attain the above object, and found that it is possible to remove metal 
ions on a high level from an trialkylamine and alkyl chloride, which are 
raw materials of TAAC, by distillation, and an extremely high-purity TAAC 
aqueous solution substantially free from metal ions such as alkali metal 
ions can be obtained by causing them to react with each other in ultrapure 
water, and that an extremely high-purity TAAH aqueous solution with little 
reduction in purity can be obtained in a high yield by producing a TAAH 
aqueous solution by electrodialysis in an aqueous solution state without 
drying it into a solid. The present invention has been accomplished based 
on these findings. 
That is, the present invention is a method of producing a high-purity 
tetraalkylammonium hydroxide aqueous solution comprising the steps of: 
reacting trialkylamine and alkyl chloride, both having a metal ion impurity 
content of 500 ppb or less, in ultrapure water to form an aqueous solution 
of tetraalkylammonium chloride, and 
subjecting the resulting aqueous solution of tetraalkylammonium chloride to 
electrolysis and electrodialysis to produce a high-purity 
tetraalkylammonium hydroxide aqueous solution.

In the drawing, reference numeral 1 denotes a cathode, 2 anode, 3-a and 3-b 
cation exchange membranes, 4 anion exchange membrane, 5 cation exchange 
membrane, 101 a cathode chamber, 102 to 104 intermediate chambers, 105 an 
anode chamber, 201 and 202 demisters, 203 and 204 distillation columns and 
205 a reactor. 
DETAILED DESCRIPTION 
In the present invention, any trialkylamine that is soluble in water at a 
reaction temperature can be used without particular restriction. The 
method of the present invention is suitable for trialkylamines having an 
alkyl group with 1 to 4 carbon atoms, particularly suitable for 
trimethylamine. 
Any alkyl chloride that is soluble in water at a reaction temperature can 
be used also without particular restriction. The method of the present 
invention is suitable for alkyl chlorides having an alkyl group with 1 to 
4 carbon atoms, particularly suitable for methyl chloride. 
To obtain a TAAC aqueous solution at a high-purity, it is important to 
remove metal impurities such as metal ions contained in the trialkylamine 
and alkyl chloride, which are the raw materials of the above reaction, 
before subjecting them to the reaction. That is, a process for removing 
impurities from the TAAC aqueous solution containing impurities such as 
metal ions is extremely complicated, and invites an inclusion of 
impurities produced by the decomposition of TAAC in the removing process 
step and as a result, a reduction in the yield of TAAC even if the 
impurities can be removed. 
Therefore, it is preferred that the above removal operation for 
purification is carried out to such an extent that the concentration of 
metal ion impurities contained in the raw materials is preferably 500 ppb 
or less, more preferably 100 ppb or less. 
As the method of removing the above metal impurities for purification, a 
method by distillation is suitable. In the case where the trialkylamine 
and alkyl chloride are gaseous at normal temperature, that is, in the case 
of trimethylamine and methyl chloride, liquefied products thereof are 
subjected to distillation. In this case, to prevent a purified material 
from being mixed with metal impurities by entrainment, demisters 201 and 
202 are desirably installed in top portions of distillation columns 203 
and 204, respectively, as shown in FIG. 1. 
In the present invention, a reaction between trialkylamine and alkyl 
chloride is carried out using ultra-pure water as a solvent. As far as the 
reaction is carried out in ultrapure water, other conditions are not 
particularly limited. In this specification, there is preferably used 
ultrapure water having a resistivity of 18.0 M.OMEGA..multidot.cm or more 
and containing particles having a particle diameter of 0.2 .mu.m or more 
at a density of one particle per 10 cc or less. 
The reaction is carried out in a reactor filled with ultrapure water. The 
trialkylamine and alkyl chloride, which are raw materials, may be supplied 
in a liquid or gaseous state. For example, when the above trimethylamine 
and methyl chloride are used, they are supplied preferably in a gaseous 
state. In this case, part of the gases is dissolved in ultrapure water and 
the reaction proceeds in the ultrapure water. As a matter of course, it is 
possible to supply these raw materials in a liquid state above atmospheric 
pressure and partially dissolve them in the ultrapure water to carry out a 
reaction. As the case may be, it is also possible that one or both of the 
raw materials may be dissolved in the ultrapure water solvent and then 
supplied to a reaction system. 
Any reaction conditions are acceptable without particular limitation if the 
raw materials can be dissolved in the ultrapure water. The reaction 
temperature is preferably 5 to 80.degree. C., more preferably 20 to 
60.degree. C. The reaction pressure is preferably 0 to 9.9 kg/cm.sup.2, 
particularly preferably 0 to 4 kg/cm.sup.2. As for the amounts of 
trialkylamine and alkyl chloride to be supplied for the reaction, the 
molar ratio of trialkylamine to alkyl chloride is generally selected from 
the range of 1:1 to 1:1.5. Particularly, the ratio is preferred to be 
1:1.2 to 1:1.5 such that the alkyl chloride which can be easily removed 
from the solution after the reaction by evaporation to be described later 
becomes excessive in quantity. Further, the reaction time which slightly 
differs according to the kinds of the raw materials cannot be limited 
definitely but is generally 1 to 24 hours, particularly suitably 1 to 5 
hours. 
After the reaction, operation (aging) for elevating temperature by 50 to 
60.degree. C. from the reaction temperature and maintaining that 
temperature for 1 to 5 hours is preferably carried out to further increase 
the yield of the reaction. 
In order to prevent metal ions from being eluted out from the devices into 
a TAAC aqueous solution and a TAAH aqueous solution that is the end 
product, it is preferred that at least the inside surfaces of a reactor 
for obtaining the TAAC aqueous solution and an electrolysis and 
electrodialysis cell for electrolyzing and electrodialyzing the TAAC 
aqueous solution should be made from a material which does not elute metal 
ions substantially, and these devices should be connected by an airtight 
pipe of which at least inside surface is made from a material which does 
not elute metal ions substantially. The material which does not elute 
metal ions substantially is selected from fluororesins such as 
polytetrafluoroethylene (PTFE) and PFA and other resins such as 
polypropylene (PP), polyethylene (PE), polyvinyl chloride (PVC) or the 
like. Of these, the treatment by fluororesins such as PTFE and PAF is the 
most preferred. 
Means for constituting these devices using the material that does not elute 
metal ions substantially is to form all the parts of the device from such 
material or line the surfaces of all the parts of the device with the 
material. 
In the present invention, unreacted raw materials are removed from the TAAC 
aqueous solution obtained by the above reaction, and evaporation operation 
is carried out as required until an appropriate concentration is obtained. 
When the unreacted raw materials are to be removed, the conditions of a 
temperature of 50 to 60.degree. C. and a operation time of 1 to 5 hours 
are generally employed for evaporation operation. Topping using nitrogen 
or inert gas may be used as required. 
To use the obtained TAAC aqueous solution as a raw material of the TAAH 
aqueous solution, the TAAC aqueous solution preferably has a concentration 
of 40 to 50 wt % and, as required, evaporation operation is carried out 
until such concentration is obtained. 
The removal and/or concentration of the unreacted materials is preferably 
carried out by heating the TAAC aqueous solution at a reduced pressure of 
10.sup.5 to 10 Pa. 
The unreacted materials removed by the above operation and distilled out in 
a gaseous state together with part of water are preferably collected, and 
recycled as required. For example, when methyl chloride is a main 
unreacted material, it is recommended that its gaseous distillate be 
brought into contact with the above ultrapure water to be dissolved 
therein and collected, because it has a low boiling point and is difficult 
to be condensed. The ultrapure water which has absorbed the unreacted 
material may be recycled to the above reaction system. 
Further, in the removal of the unreacted materials, the solution is 
desirably maintained at an alkaline level, for example, pH of 8 or more, 
preferably 8 to 12. The high-purity TAAH aqueous solution obtained by the 
present invention is advantageously used to adjust the pH. 
Since the TAAC aqueous solution obtained by the method of the present 
invention does not need to be dried and re-dissolved in water unlike TAAC 
obtained by the prior art method, it can be used in the production of a 
TAAH aqueous solution while retaining high purity. 
In this connection, the content of metal ion impurities in the TAAC aqueous 
solution is 500 ppb or less, preferably 100 ppb or less, based on TAAC. 
Particularly, the content of alkali metal ions is 20 ppb or less, 
preferably 10 ppb or less, based on TAAC. 
The production of the above TAAH aqueous solution is carried out by 
subjecting the above high-purity TAAC aqueous solution to electrolysis and 
electrodialysis, that is, a so-called electrolysis-dialysis. The chlorine 
ions of TAAC are exchanged with hydroxyl ions by the electrolysis-dialysis 
to form TAAH. 
As a method for the above electrolysis-dialysis, conventionally known 
methods are employed without particular restriction. Generally speaking, a 
TAAC aqueous solution is supplied to an electrolytic/electrodialytic cell 
having an intermediate chamber(s) formed by arranging only a cation 
exchange membrane or a combination of a cation exchange membrane and an 
anion exchange membrane between a cathode and an anode, an acid is 
supplied to an anode chamber formed on one side of the cell, and the TAAH 
aqueous solution is obtained from the cathode chamber formed on the other 
side of the cell. 
Electrolysis-dialysis is carried out by supplying an aqueous solution of an 
acid such as hydrochloric acid to the anode chamber, the TAAC aqueous 
solution to the intermediate chambers formed by cation exchange membranes 
and an anion exchange membrane and high-purity water, particularly 
ultrapure water or a high-purity TAAH aqueous solution to the cathode 
chamber. 
By carrying out the above electrolysis-dialysis method, a TAAH aqueous 
solution containing an extremely small amount of metal ions can be 
obtained from the cathode chamber. But, in this case, chlorine ions 
contained in the TAAC aqueous solution may move to the cathode chamber by 
diffusion, so that the obtained TAAH aqueous solution is contaminated with 
chlorine ions. 
To obtain a TAAH aqueous solution of higher-purity by preventing the 
diffusion of chlorine ions effectively, an electrolysis-dialysis cell is 
preferably used which is, for example, constituted by arranging two or 
more cation exchange membranes 3-a and 3-b, anion exchange membrane 4 and 
cation exchange membrane 5 from the cathode side between the cathode 1 and 
the anode 2 sequentially to form chambers defined by these ion exchange 
membranes, as shown in FIG. 1. And, electrolysis is carried out by 
supplying water or a high-purity TAAH aqueous solution to the cathode 
chamber 101 comprising a cathode, an aqueous solution containing an 
electrolyte essentially composed of TAAH to the intermediate chamber 102 
formed by two cation exchange membranes, the high-purity TAAC aqueous 
solution obtained by the above method to the intermediate chamber 103 
formed by the cation exchange membrane 3-b on the cathode side and the 
anion exchange membrane 4, and an aqueous solution of an acid to the 
intermediate chamber 104 formed by the anion exchange membrane 4 and the 
cation exchange membrane 5 that is present on the anode side and the anode 
chamber 105 comprising the anode 2, and applying a direct current between 
the electrodes. 
In the above method, an aqueous solution of hydrochloric acid is 
advantageously used as the aqueous solution of an acid to be supplied to 
the intermediate chamber 104, and an aqueous solution of sulfuric acid is 
advantageously used as the aqueous solution of an acid to be supplied to 
the anode chamber 105. The concentration of chlorine ions contained in the 
electrolyte aqueous solution to be supplied to the intermediate chamber 
102 is preferably adjusted to 30 ppb or less, more preferably 20 ppb or 
less to reduce the concentration of chlorine ions contained in the 
obtained TAAH aqueous solution to a target value. 
According to the above method, it is possible to reduce the concentration 
of chlorine ions contained in the obtained TAAH aqueous solution to 5 ppb 
or less. 
Known cation exchange membranes and anion exchange membranes which have 
been conventionally used in the production of a TAAH aqueous solution by 
electrolysis-dialysis can be used without particular restriction as the 
cation exchange membrane and anion exchange membrane used for the above 
electrolysis-dialysis. For example, the cation exchange membrane is a 
membrane having a sulfonic acid group, carboxylic acid group, phosphoric 
acid group or the like; and the anion exchange membrane is a membrane 
having at least one of strong basic ion exchange groups such as a 
quaternary ammonium salt group, sulfonium salt group, phosphonium salt 
group or the like and primary, secondary and tertiary amines bonded. The 
substrate of the ion exchange membrane is selected from hydrocarbon-, 
fluorocarbon- and perfluorocarbon-based resins. Particularly, the cation 
exchange membrane that constitutes the cathode chamber is preferably made 
from a perfluorocarbon resin which is stable in a basic atmosphere and has 
excellent durability, and a perfluorocarbon-based ion exchange membrane 
having oxidation resistance is preferably used for the ion exchange 
membrane in contact with the anode chamber because an oxidative gas(es) 
such as halogen gas, oxygen gas or the like generate(s) in the anode 
chamber. 
A so-called insoluble electrode of carbon, platinum-coated titanium, 
ruthenium, iridium-coated titanium or the like is advantageously used as 
the anode. A material which is stable in a strong basic atmosphere and has 
a low overpotential, such as SUS316, platinum, Raney nickel or the like, 
is advantageously used as the cathode. 
In the above electrolysis-dialysis, the current density is suitably 1 to 50 
amp/dm.sup.2 and the temperature is preferably controlled to a temperature 
not higher than 90.degree. C., more preferably 30 to 50.degree. C. 
The high-purity TAAH aqueous solution which is the object of the present 
invention cannot be obtained unless the high-purity TAAC obtained by the 
above method is subjected to the above electrolysis-dialysis. In other 
words, even when electrolysis-dialysis is carried out under the above 
conditions, if TAAC is contaminated by impurities, the impurities, 
particularly univalent ions such as sodium ions and potassium ions, are 
directly introduced to the cathode chamber in which TAAH is obtained, 
without being inhibited by the above ion exchange membranes. 
Further, ions having a valence of 2 or higher inhibited in the electrolytic 
cell may cause, in some case, fouling in the ion exchange membranes during 
long-term operation, thereby hampering stable operation or inviting 
re-inclusion into a product by diffusion. 
In the present invention, it is preferred that the raw materials and 
purified product present in a section extending from the distillation 
columns 203 and 204 up to the electrolysis-dialysis cell via the reactor 
205 shown in FIG. 1 should not come substantially in contact with the open 
air in order to obtain a higher-purity TAAH aqueous solution. Stated more 
specifically, such methods can be employed that the gas-phase portion of 
the reactor is sealed with an inert gas such as nitrogen gas or the like; 
the electrolysis-dialysis cell is constructed in a tight-closed type like 
a filter press type cell; the lines connecting the devices are made 
air-tight; and the like. 
As is understood from the above description, according to the method of the 
present invention, an extremely high-purity TAAH aqueous solution can be 
obtained efficiently, and the method of the present invention is extremely 
useful for the industrial production of a high-purity TAAH aqueous 
solution. 
The amount of metal ion impurities contained in the TAAH aqueous solution 
is 500 ppb or less, preferably 100 ppb or less, based on the TAAH. 
Particularly, the amount of alkali metal ions is 20 ppb or less, 
preferably 10 ppb or less, based on the TAAH. In the TAAH aqueous solution 
obtained from the preferred electrolysis-dialysis device shown in FIG. 1, 
it is possible to reduce the concentration of chlorine ions to 5 ppb or 
less. 
The following examples are provided for the purpose of further illustrating 
the present invention but are in no way to be taken as limiting. The metal 
ion impurities were measured by inductively coupled plasma mass 
spectrometry (IPC-MS). 
EXAMPLE 1 
Liquefied trimethylamine containing more than 1,000 ppb of metal ions was 
supplied to a distillation column 203 having a demister 201 at the column 
top and gasified to form trimethylamine having a reduced metal ion 
concentration of 50 ppb. Then, the obtained trimethylamine was supplied to 
a reactor 205 equipped with an external jacket. The above distillation 
column, reactor and pipes connecting these were lined with PTFE. 
Meanwhile, liquefied methyl chloride containing more than 1,000 ppb of 
metal ions was also supplied to a distillation column 204 having a 
demister 202 at the column top and gasified to form methyl chloride having 
a reduced metal ion concentration of 50 ppb. The obtained methyl chloride 
was supplied to the above reactor 205. The molar ratio of the supplied raw 
materials is 1.1 mols of methyl chloride per 1 mol of trimethylamine. 
In the above operation, ultrapure water was in advance contained in the 
reactor and the temperature of the reactor was controlled so as to 
maintain the reaction temperature at 40.degree. C. The amount of supplied 
gas was adjusted such that the pressure inside the reactor is to be about 
3 kg/cm.sup.2. 
The above reaction was carried out until the concentration of a tetramethyl 
ammonium chloride aqueous solution produced in the reactor became 50 wt %, 
and then, the pressure inside the reactor was reduced to 500 Torr by an 
ejector, and methyl chloride as a raw material was mainly removed. Waste 
gas going into the ejector was brought into contact with ultrapure water 
to absorb raw materials contained therein, and supplied to the reaction 
system upon being counted as part of the amount of methyl chloride to be 
supplied. 
The content of impurities contained in the obtained tetramethyl ammonium 
chloride aqueous solution, based on the solute (tetramethyl ammonium 
chloride) and the aqueous solution is shown in Table 1. 
TABLE 1 
______________________________________ 
Analytical results 
Analyzed item 
Unit Based on solute 
Based on solution 
______________________________________ 
silver ppb &lt;0.2 &lt;0.1 
aluminum ppb &lt;0.4 &lt;0.2 
gold ppb &lt;2 &lt;1 
barium ppb &lt;0.6 &lt;0.3 
calcium ppb &lt;2 &lt;1 
cadmium ppb &lt;0.4 &lt;0.2 
cobalt ppb &lt;0.4 &lt;0.2 
chromium ppb &lt;1.2 &lt;0.6 
copper ppb &lt;1.0 &lt;0.5 
iron ppb &lt;1.0 &lt;0.5 
potassium ppb &lt;1.0 &lt;0.5 
lithium ppb &lt;0.1 &lt;0.05 
magnesium ppb &lt;0.2 &lt;0.1 
manganese ppb &lt;0.2 &lt;0.1 
sodium ppb &lt;2 &lt;1 
nickel ppb &lt;2 &lt;1 
lead ppb &lt;0.4 &lt;0.2 
strontium ppb &lt;0.6 &lt;0.3 
zinc ppb &lt;2 &lt;1 
______________________________________ 
The tetramethyl ammonium chloride aqueous solution obtained by the above 
method was subjected to an electrolysis-dialysis using devices shown in 
FIG. 1, in which intermediate chambers are formed by arranging cation 
exchange membranes and an anion exchange membrane between a cathode and an 
anode. 
SUS316 was used as the cathode and a titanium plate plated with platinum 
was used as the anode. Naphion 901 (a product of E. I. Du Pont) was used 
as the cation exchange membrane and AM-1 (a product of Tokuyama 
Corporation) was used as the anion exchange membrane. 
The electrolysis-dialysis were carried out by supplying a 0.5 N 
hydrochloric acid aqueous solution to the anode chamber 105 and the 
intermediate chamber 104, the tetramethyl ammonium hydroxide aqueous 
solution obtained in Example 1 to the intermediate chamber 103 formed by a 
cation exchange membrane and an anion exchange membrane, a 2.5 N TAAH 
aqueous solution having a chlorine ion concentration of 20 ppb or less to 
the intermediate chamber 102, and ultrapure water to the cathode chamber. 
A pipe connecting the electrolysis-dialysis cell and the reactor, and pipes 
and a tank provided for the electrolysis-dialysis cell were lined with 
PTFE. A filter press type cell was used as the electrolysis-dialysis cell 
and the frame of each chamber was made from PP. A gas phase portion of the 
reactor was supplied with a nitrogen gas and sealed. 
The current density was adjusted to 15 amp/dm.sup.2 and the temperature was 
maintained at 40.degree. C. 
A 2.5 N tetramethyl ammonium hydroxide aqueous solution was continuously 
obtained from the cathode chamber 101. The properties of the obtained 
tetramethyl ammonium hydroxide aqueous solution and the content of 
impurities contained therein are shown in Table 2. 
TABLE 2 
______________________________________ 
Analytical results 
Analyzed item 
Unit Based on solute 
Based on solution 
______________________________________ 
TMAH concentration 
Normal 2.5 
silver Normal &lt;0.4 &lt;0.1 
aluminum Normal &lt;0.9 &lt;0.2 
gold Normal &lt;4.4 &lt;1 
barium Normal &lt;1.3 &lt;0.3 
calcium Normal &lt;4.4 &lt;1 
cadmium Normal &lt;0.9 &lt;0.2 
cobalt Normal &lt;0.9 &lt;0.2 
chromium Normal &lt;2.6 &lt;0.6 
copper Normal &lt;2.2 &lt;0.5 
iron Normal &lt;2.2 &lt;0.5 
potassium Normal &lt;2.2 &lt;0.5 
lithium Normal &lt;0.22 &lt;0.05 
magnesium Normal &lt;0.4 &lt;0.1 
manganese Normal &lt;0.4 &lt;0.1 
sodium Normal &lt;4.4 &lt;1 
nickel Normal &lt;4.4 &lt;1 
lead Normal &lt;0.9 &lt;0.2 
strontium Normal &lt;1.3 &lt;0.3 
zinc Normal &lt;4.4 &lt;1 
chlorine Normal &lt;13.1 &lt;3 
______________________________________ 
COMATIVE EXAMPLE 1 
The reactor, the pipe connecting the electrolysis-dialysis cell and the 
reactor and the pipes and tank provided for the electrolysis-dialysis cell 
were made from stainless steel and not lined with PTFE. A reaction was 
carried out without supplying nitrogen gas to the gas-phase portion of the 
reactor. 
Trimethylamine and methyl chloride each containing more than 1,000 ppb of 
metal ions were first supplied from the respective bombs to isopropanol in 
the same ratio as in Example 1 and reacted with each other to obtain a 
tetramethyl ammonium chloride solution. Thereafter, the solution was 
filtrated and dried to obtain solid tetramethyl ammonium chloride. The 
obtained solid tetramethyl ammonium chloride was then dissolved in 
ultrapure water to prepare an aqueous solution having the same 
concentration as in Example 1. 
The content of impurities contained in the thus obtained tetramethyl 
ammonium chloride aqueous solution is shown in Table 3. 
TABLE 3 
______________________________________ 
Analytical results 
Analyzed item 
Unit Based on solute 
Based on solution 
______________________________________ 
silver ppb &lt;0.2 &lt;0.1 
aluminum ppb 2.0 1.0 
gold ppb &lt;2 &lt;1 
barium ppb 6.0 3.0 
calcium ppb 100 50 
cadmium ppb &lt;0.4 &lt;0.2 
cobalt ppb &lt;0.4 &lt;0.2 
chromium ppb 29.2 14.6 
copper ppb 13.6 6.8 
iron ppb 99.0 49.5 
potassium ppb 46.6 23.3 
lithium ppb 0.64 0.32 
magnesium ppb 3.6 1.8 
manganese ppb 2.0 1.0 
sodium ppb 230 115 
nickel ppb 2 1 
lead ppb 4.8 2.4 
strontium ppb 6.2 3.1 
zinc ppb 8 4 
______________________________________ 
The tetramethyl ammonium chloride aqueous solution obtained by the above 
method was subjected to an electrolysis-dialysis using membranes in the 
same manner as in Example 1. 
The properties of the obtained tetramethyl ammonium hydroxide aqueous 
solution and the content of impurities contained therein are shown in 
Table 4. 
TABLE 4 
______________________________________ 
Analytical results 
Analyzed item 
Unit Based on solute 
Based on Solution 
______________________________________ 
TMAH concentration 
Normal 2.5 
silver ppb &lt;0.4 &lt;0.1 
aluminum ppb 3.9 0.9 
gold ppb &lt;4.4 &lt;1 
barium ppb 6.6 1.5 
calcium ppb 382 87 
cadmium ppb &lt;0.9 &lt;0.2 
cobalt ppb &lt;0.9 &lt;0.2 
chromium ppb 39.1 8.9 
copper ppb 15.4 3.5 
iron ppb 108 24.7 
potassium ppb 112 25.6 
lithium ppb 0.79 0.18 
magnesium ppb 4.8 1.1 
manganese ppb 3.9 0.9 
sodium ppb 632 144 
nickel ppb &lt;4.4 &lt;1 
lead ppb 4.8 1.1 
strontium ppb 7.9 1.8 
zinc ppb 13 3 
chlorine ppb &lt;13 &lt;3 
______________________________________ 
Although the accumulation of ions having a valence of 2 or higher in the 
cell was seen, it was observed that most of metal impurities contained in 
the raw material (tetramethyl ammonium chloride) passed through the cation 
exchange membranes and were carried to the tetramethyl ammonium hydroxide 
aqueous solution.