Method of preparing storage-stable aqueous sodium peroxymonsulfate solutions
The present invention relates to a method of preparing storage-stable, aqueous sodium peroxymonosulfate solutions which contain 5% to 50% by weight sodium peroxymonosulfate (NaHSO 5 ). These solutions are prepared by the partial neutralization of an aqueous solution containing Caro&#39;s acid (H 2  SO 5 ) and sulfuric acid with sodium hydroxide or sodium carbonate. 
     BACKGROUND OF THE INVENTION 
     Peroxymonosulfuric acid (Caro&#39;s acid) and its salts (peroxymonosulfates) are becoming increasingly important as oxidation agents in the oxidative removal of pollutants from waste waters. Caro&#39;s acid is effective in such methods; however, it must be produced in situ or directly prior to being used because of its limited storage stability. The etching solutions containing Caro&#39;s acid for the production of printed circuits such as those described e.g. in European Patent Application EP-A 0 256 284 have limited stability as regards the available oxygen; under the indicated test conditions -- 5 hours at 70° C. -- the loss is approximately 15% if the solution used contains approximately 4.6% by weight peroxymonosulfuric acid. Thus, such solutions can not be marketed. 
     Even though peroxymonosulfates of the alkaline metals and alkaline-earth metals as well as of a few other metals and of ammonia are known -- cf. German patent DE 10 80 083 --, only potassium peroxymonosulfate in the form of the storage-stable triple salt 2KHSO 5  KHSO 4  K 2  SO 4  and ammonium peroxymonosulfate in the form of a solution, the so-called &#34;ammonium-stabilized Caro&#39;s acid&#34;, have become accepted in practice. However, since the above-mentioned potassium triple salt must generally be dissolved prior to use and the ammonium ion of the Caro&#39;s acid stabilized therewith must be considered itself as a critical component of waste water, there is a need for readily manageable and ecotoxicologically harmless, storage-stable peroxymonosulfate solutions. Absolute eco-toxicological safety could be expected from solutions containing sodium peroxymonosulfate. 
     According to the methods described in DE patent 10 80 083, metal monopersulfates are prepared by means of reacting Caro&#39;s acid or mixtures of Caro&#39;s acid and sulfuric acid with the hydroxides or carbonates of the metals concerned up to a pH of the reaction mixture of not more than 3. The reaction mixture which is obtained at first is transferred immediately after its preparation by means of vacuum drying or spray [flash]drying into a salt mixture containing the corresponding peroxymonosulfate. In order to obtain sufficiently storage-stable products, diluting agents with a stabilizing action are added preferably before or during the drying. 
     Solutions containing Caro&#39;s acid and sulfuric acid can be used in the method of DE 10 80 083 which were obtained either by means of electrolysis of a sulfuric acid with a content of preferably 35% to 50% by weight, with subsequent hydrolysis of the peroxodisulfuric acid formed or by means of the reaction of 50% to 88% by weight aqueous hydrogen peroxide with concentrated or fuming sulfuric acid (oleum). In order to prepare sodium peroxymonosulfate, such H 2  SO 5  /H 2  SO 4  solutions are neutralized with a soda solution up to pH 2.9, freed of the precipitated Glauber salt and subsequently dried. 
     The method evaluated above for the preparation of solutions containing NaHSO 5  proved to be unsatisfactory because a considerable loss of available oxygen was observed during the neutralization -- even in the relative example of DE 10 80 083, the yield of NaHSO 5  in the solution is indicated at only 85% of theory. In addition, the storage stability of the solution was unsatisfactory: The available oxygen content decreased to an intolerable degree and at the same time the solution was opacified again and again by precipitating Glauber salt. Obviously, the problems detected during the reworking of the method were also the cause for the fact that the usefulness of the solution was not pointed out anywhere in DE 10 80 083, but rather it was always concentrated by evaporation. However, the drying of the solution containing NaHSO 5  in order to obtain a storage-stable, solid product is associated with a further loss of available oxygen and is also expensive, which reduces the economic value of the process. 
     SUMMARY OF THE INVENTION 
     The object of the present invention is to provide a method of preparing storage-stable, aqueous sodium peroxymonosulfate solutions which does not have the disadvantages of the method disclosed in DE 10 80 083. The present invention seeks to provide a method which yields storage-stable solutions and minimizes the loss of available oxygen during the neutralization of the H 2  SO 5  /H 2  SO 4  solutions. 
     These and other objects are achieved by a method of preparing storage-stable, aqueous sodium peroxymonosulfate solutions with a content of 5% to 50% by weight sodium peroxymonosulfate by means of the partial neutralization of a solution which contains Caro&#39;s acid (H 2  SO 5 ) and sulfuric acid and which was obtained by means of reacting sulfuric acid or oleum with at least 50% by weight aqueous hydrogen peroxide or by means of the electrolysis of an at least 30% by weight aqueous sulfuric acid. The partial neutralization is accomplished by hydrolysis of the peroxydisulfuric acid formed, with sodium hydroxide or sodium carbonate at a temperature below 50° C. which is characterized in that such a solution containing Caro&#39;s acid and sulfuric acid is used which contains less than 3 parts by weight hydrogen peroxide per 100 parts by weight Caro&#39;s acid (H 2  SO 5 ) and that the partial neutralization is carried out up to an equivalent proportion of total SO 3  to sodium ions in the range of 1 to 1 up to 1 to 1.3 and that, to the extent desired, the NaHSO 5  content is adjusted by means of diluting with water or by means of distilling off water under reduced pressure during or after the partial neutralization. 
     It has been found that both the yield of available oxygen during the partial neutralization of a sulfuric Caro&#39;s acid as well as the storage stability of the solution obtained containing sodium peroxymonosulfate as regards the available oxygen content depend to a significant degree on the content of hydrogen peroxide in the sulfuric Caro&#39;s acid solution used and in the sodium peroxymonosulfate solution. The lower the H 2  O 2  content in the H 2  SO 5  /H 2  SO 4  solution and in the NaHSO %  /NaHSO 4  is, the lower the losses of available oxygen in the partial neutralization and during storage. This behavior was not to have been expected previously. It is assumed based on analytic experiments regarding the method of the invention that hydrogen peroxide reacts during the neutralization stage, in part immediately, and somewhat more slowly during storage, with decomposition of H 2  SO 5  and NaHSO 5 , that one must start with an equimolar decomposition reaction. 
     If the equivalent proportion of the invention of total SO 3  to sodium ions is 1 to 1 up to 1 to 1.3, NaHSO 5 , solutions with a pH of approximately 1 or below 1 are obtained from which practically no solid matter is separated during a storage of several weeks. The solutions prepared in accordance with the invention are therefore economical, that is, they can be prepared with a high yield of available oxygen and are marketable because of their high storage stability. The user of aqueous solutions containing peroxymonosulfate, e.g. for treating waste water for the oxidative removal of toxic waste-water contents, no longer needs to prepare such solutions himself immediately before use or to use solutions containing ammonia, but rather he can make use of the solutions of the invention, which are sufficiently storage-stable. In order to further increase the storage stability, known stabilizers for peroxymonosulfates can be added in an effective amount to the sodium peroxymonosulfate solutions prepared in accordance with the invention, during or after the preparation of the solutions, e.g. dipicolinic acid, stannates and chelate complexing agents, especially from the series of phosphonates such as hydroxyalkylidene diphosphonates, ethylene diamine tetra(methylene phosphonic acid) and diethylene triamine penta(methylene phosphonic acid). However, the solutions prepared in accordance with the invention are even much stable in the absence of such stabilizers than peroxymonosulfuric acid solutions with a comparable content of available oxygen. 
     Solutions with an equivalent proportion of total SO 3  to sodium ions of 1 to greater than 1.3 have a pH above 1 and are no longer sufficiently storage stable since solid matter precipitates --see Examples 12 to 14. The term &#34;total SO 3  &#34; denotes that molar amount which is bound in the sulfuric Caro&#39;s acid used in the form of H 2  SO 5 , H 2  SO 4  and H 2  S 2  O 8 . The stability as regards the available-oxygen content decreases at an equivalent proportion of 1 to less than 1 since the low stability of the Caro&#39;s acid becomes increasingly apparent in this case --see Example 1. 
     The solutions containing Caro&#39;s acid and sulfuric acid and to be used in the method of the invention can be obtained in a known manner - see e.g. DE 10 80 083. There are essentially two potential methods, namely: 1) The reaction of concentrated sulfuric acid or oleum 
     (solutions of SO 3  in sulfuric acid) with aqueous hydrogen peroxide and 2) The hydrolysis of an electrolytically produced, aqueous sulfuric solution containing peroxydisulfuric acid. 
     It is essential that only such solutions are to be supplied to the partial neutralization of the invention whose hydrogen peroxide content is less than 3 parts by weight, preferably less than 2.5 and especially less than 2 parts by weight per 100 parts by weight Caro&#39;s acid. It can be advantageous, depending on the intended use of the solution containing NaHSO 5 , to use such H 2  SO 5  /H 2  SO 4  solutions which exhibit not only as low an H 2  O 2  content as possible but at the same time as low a peroxydisulfate content as possible, preferably below 5 parts by weight H 2  S 2  O 8  per 100 parts by weight Caro&#39;s acid. The selection of the suitable solution requires a knowledge of the available oxygen bound as H 2  O 2 , H 2  SO 5  and H 2  S 2  O 8 . The individual components can be readily determined potentiometrically using a cerium(IV)sulfate solution and sodium arsenite solution in the same specimen in succession: At first, the H 2  O.sub. 2 content is determined by means of titration of a specimen with a Ce(SO 4 ) 2  solution; the value for the H 2  SO 5  content is obtained after the addition of an excess of NaAsO 2  solution to the specimen titrated as described above and back titration with the Ce(SO 4 ) 2  solution; the H 2  S 2  O 8  content is obtained after a new addition of an NaAsO 2  solution in excess, 15 minutes of boiling and back titration with the Ce(SO 4 ) 2  solution. The same method of determination is also suitable for determining the content of the partially neutralized solutions. 
     It is possible to obtain H 2  SO 5  solutions with a low H 2  O 2  content by means of the hydrolysis of an electrolytically prepared solution containing H 2  S 2  O 8  however, the H 2  S 2  O 8  content is as a rule distinctly above 5, usually above 10 parts by weight per 100 parts by weight H 2  SO 5 . However, as the hydrolysis proceeds further, there is the danger of an increase of the H 2  O 2  content and, in addition, a reduction of the available-oxygen content. 
     The reaction of hydrogen peroxide with sulfuric acid or oleum is an equilibrium reaction. It is possible, depending on the concentration of the reactants and the molar ratio selected, to obtain solutions with differing H 2  SO 5  content and H 2  O 2  residual content. Aqueous H 2  O 2  solutions with an H 2  O 2  content of at least 50 % by weight, especially 70 to approximately 85% by weight, as well as fuming sulfuric acid, also called oleum, with an S03 content of at least 20% by weight, preferably over 50% by weight, especially approximately 65% by weight are used in order to prepare solutions containing H 2  SO 5  and H 2  SO 4  with an H 2  O 2  content below 3 parts by weight per 100 parts by weight H 2  SO 5 . The molar ratio of hydrogen peroxide to total SO 3  - sum of SO 3  and SO 3  bound as H 2  SO 4  - is 1 to 1.1 up to 1 to 1.6. The concentration of H 2  O 2  in the aqueous hydrogen peroxide, the concentration of SO 3  in the oleum as well as the molar ratio are to be coordinated in such a manner with each other that the molar ratio of the water bought with the aqueous hydrogen peroxide into the reaction system to the sulfur trioxide brought in with the oleum is less than 1; a molar ratio in a range around 0.8 is well suited. It is possible under the cited conditions to obtain in a high yield the solutions with an H 2  SO 5  content of 50 - 65% by weight, an H 2  SO 4  content of 20% to 40% by weight and an H 2  O 2  content of less than 1.5% by weight, especially less than 1% by weight, which solutions are to be used with preference and which can be supplied directly or after partial dilution with water to the partial neutralization. 
     Sodium hydroxide solution or a soda solution are suitable for the partial neutralization. A sodium hydroxide solution with a content of 10%-50% by weight is preferred; insofar as highly concentrated NaHSO 5  solutions are desired, a sodium hydroxide solution in the upper concentration range is used. The neutralization heat is generally removed by cooling, so that a reaction temperature below 50° C., preferably below 35° C. is maintained. The cooling can be achieved by means of a customary cooling of the reactor, e.g. by means of heat exchangers or by means of evaporation cooling. 
     The method of the invention can be carried out either discontinuously or continuously. In the case of a continuous embodiment, the reaction is preferably carried out in a reaction loop. The loop contains a reactor, preferably a tubular reactor which is preferably equipped with mixing devices, e.g. one or several static mixers, at least one metering position for the sodium hydroxide solution or soda solution and the solution containing Caro&#39;s acid and an effective cooler; other components of the loop are a conducting main, equipped with a transport member, and a removal position for the partially neutralized sodium peroxymonosulfate solution. The sulfuric Caro&#39;s acid is metered into the partially neutralized solution of the desired concentration of NaHSO 5  and NaHSO 4  circulating in the loop in the first part of the reactor and is mixed with the circulating solution; sodium hydroxide solution is metered in at a position removed from the first metering position to obtain a sufficiently homogeneous mixture and is mixed. The reactor contents are maintained at a temperature of below 35° C. by means of external cooling or by means of an integrated vacuum evaporative cooler. The volume of the reaction loop is maintained constant by removing the partially neutralized solution from the reactor. 
     An important advantage of the method of the invention is the fact that it is possible to make available in an economic fashion storage-stable and thus marketable solutions with a content of 5% to 50% by weight, preferably 10% to 40% by weight and especially 15% to 35% by weight sodium peroxymonosulfate. Whereas during the previously known preparation of solutions containing NaHSO 5  for the purpose of drying the same, the yield of available oxygen decreased sharply even during the partial neutralization of the sulfuric Caro&#39;s acid, it is possible, according to the method of the invention, to obtain the desired solutions with a yield of available oxygen of in general over 90%. It is possible in the especially preferred embodiment in which an H 2  SO 5  /H 2  SO 4  solution obtainable from 70% to 85% by weight hydrogen peroxide and 50% to 70% by weight oleum is partially neutralized, especially in the continuous method of operation presented, to maintain the available oxygen yield over 90% relative to hydrogen peroxide used. The satisfactory storage stability as regards the available oxygen content as well as the freedom from opacifying substances in the solutions prepared in accordance with the invention permits the elimination of the expensive and yield-reducing drying of the solutions for the purpose of obtaining a storage-stable NaHSO 5  product; in addition, no dissolving station is needed at the site of use. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The invention and its advantages are disclosed further by means of the following examples and reference examples. 
     EXAMPLES 1 to 7 
     The aqueous hydrogen peroxide solution is placed in a flask provided with an agitator, a thermometer and external cooling. Sulfuric acid or oleum is added dropwise with strong cooling -- reaction temperature 0° to 10° C. The molar ratio of H 2  O 2  to the total SO 3 , the concentrations of the reactants, the yield as well as the analyses of the H 2  SO 5  /H 2  SO 4  solution obtained are given in Table la. 
     For partial neutralization, sodium hydroxide solution (37% by weight) and the H 2  SO 5  /H 2  SO 4  solution prepared above are added in the indicated molar ratio via two dropping funnels into a flask equipped as above. A small amount of NaHSO 5  /NaHSO 4  solution from a comparable preceding batch or water is placed in a receiver for the purpose of better temperature control and mixing. The reaction temperature is maintained at 25° to 35° C. The molar ratio of total SO 3  to NaOH, the analysis of the solution and the yield are given in Table lb. 
     Examples 3 and 7 are in accordance with the invention; Examples 1, 2, 5 and 6 are partially outside of the molar ratios of the invention and/or contain too much H 2  O 2  in the H 2  SO 5  /H 2  SO 4  solution, which results in low yields during the partial neutralization and/or in insufficient storage stability. 
     Example 4 is based on the molar ratios of the invention; however, a precipitation of essentially sodium sulfate occurred on account of too strong a concentration by evaporation: Example 4 thus shows that there are limits to a concentration by evaporation with increasing Na excess in relation to the total SO 3 . 
     
                                           TABLE 1a                                
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Preparation and analysis of the H.sub.2 SO.sub.5 /H.sub.2 SO.sub.4        
solution                                                                  
                                             Yield                        
                  Batch       Analysis       % H.sub.2 SO.sub.5           
Example                                                                   
     H.sub.2 O.sub.2                                                      
           Oleum  Molar Ratio                                             
                         H.sub.2 SO.sub.5                                 
                              H.sub.2 SO.sub.4                            
                                   H.sub.2 S.sub.2 O.sub.8                
                                        H.sub.2 O.sub.2                   
                                             based on                     
No.  (% by wt.)                                                           
           (% by wt.)                                                     
                  H.sub.2 O.sub.2 :                                       
                         (wt. %)                                          
                              (wt. %)                                     
                                   (wt. %)                                
                                        (wt. %)                           
                                             H.sub.2 O.sub.2              
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1    70    65     1:1    65   16   1.2  6    77                           
2    As in Example 1                                                      
3    70    65     1:1.5  58   32   2.8  1.2  92                           
4    As in Example 3                                                      
5    70    20     1:1.3  49   30   1    5    74                           
6    85    O (═H.sub.2 SO.sub.4)                                      
                  1:1.3  49   30   1    5.5  74                           
7    70    65      1:1.54                                                 
                         56   33   2.8  1.0  92                           
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                       TABLE 1b                                                    
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Partial neutralization of the solutions of Table 1a                       
                                   Yield                                  
                                   % NaHSO.sub.5                          
       Molar ratio                 relative                               
Example                                                                   
       total SO.sub.3 :                                                   
                 Analysis (% by wt.)                                      
                                   to H.sub.2 SO.sub.5                    
No.    NaOH      NaHSO.sub.5                                              
                          NaHSO.sub.4                                     
                                 H.sub.2 O.sub.2                          
                                       added                              
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1      1:0.5     22       21     3.2   96                                 
2      1:1.0     33       16     2.2   86                                 
3      1:1.0     30       21     0.5   98                                 
4       1:1.27   44       not    0.3   .sup.  96**)                       
                          measured                                        
5      1:1       25       20     2.0   85                                 
6      1:1       28       20     2.5   87                                 
7      1:1         30.5   22     0     95                                 
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 *)The solution contains in addition 18% by weight H.sub.2 SO.sub.5 and 9%
 by weight H.sub.2 SO.sub.4 ; in spite of the high H.sub.2 O.sub.2 content
 the yield was high; however, the solution was insufficiently storagestabl
  loss of HSO.sub.5.sup.-  within one week 20° C. 41.5% of the     
 initial value.                                                           
 **)The partial neutralization took place under reduced pressure and      
 evaporation of water, at which time some solid precipitated (the analysis
 refers to the filtrate).                                                 
 
    
     EXAMPLES 8 and 9 
     In order to illustrate the influence of the hydrogen peroxide content on the yield during the partial neutralization and to illustrate the storage stability, 4.5% by weight hydrogen peroxide was added to the batch of Example 7 immediately before the partial neutralization (Example 8). 
     3% by weight hydrogen peroxide was added to the solution of Example 7 after the end of the partial neutralization (Example 9). The results, NaHSO 5  content and H 2  O 2  content immediately after the preparation of the solutions including the additional addition of H 2  O 2  and after 1 week of storage at room temperature, as well as yield and residual value of NaHSO 5  relative to H 2  SO 5  added in the partial neutralization are given in Table 2. 
     
                       TABLE 2                                                     
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                                Yield of NaHSO.sub.5                      
            NaHSO.sub.5                                                   
                      H.sub.2 O.sub.2                                     
                                (% relative to                            
Example No. (% by wt.)                                                    
                      (% by wt.)                                          
                                H.sub.2 SO.sub.5 added)                   
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7 after preparation                                                       
            30.5      0         95                                        
  after 1 week                                                            
            30.0      0         94                                        
8 after preparation                                                       
            24.5      2.2       81                                        
  after 1 week                                                            
            16.0      0         53                                        
9 after preparation                                                       
            28.6      3.1       94                                        
  after 1 week                                                            
            21.7      1.5       71                                        
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     EXAMPLES 10 and 11 
     The test apparatus for the partial neutralization consisted of a multi-neck flask with inner thermometer, agitator, distillation bridge with receiver, vacuum pump as well as two pumps for very precise metering of the reaction components into the multi-neck flask. 
     An aqueous H 2  SO 5  /H 2  SO 4  solution, prepared according to the data in Example 1, and a 37% by weight sodium hydroxide solution are fed, in the desired amount ratio, in a continuous manner, into 100 ml water in the flask. A reaction temperature of 25° to 35° C. is maintained via the vacuum system at a pressure in the range of approximately 10 to 30 mbars; at the same time, the water supplied with the raw materials and produced during the neutralization can be removed in order to obtain the desired concentration of NaHSO 5 . A partial current can be removed from the flask in order to maintain the volume of reaction mixture constant; this was not done, however, in Examples 10 and 11. The batch amounts and analyses as well as the yields are given in Table 3. 
     The loss of available oxygen during storage at room temperature of the solution of Example 10 containing sodium peroxymonosulfate and prepared in accordance with the invention is approximately 1.7% each 10 days during the 40 days of testing. 
     
                       TABLE 3                                                     
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                 Example 10                                               
                           Example 11                                     
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Batch for H.sub.2 SO.sub.5 /                                              
H.sub.2 SO.sub.4 solution                                                 
Conc. H.sub.2 O.sub.2 (% by weight)                                       
                   69          69                                         
Conc. oleum (% by weight)                                                 
                   65          65                                         
Molar ratio                                                               
H.sub.2 O.sub.2 :total SO.sub.3                                           
                   1:1.54      1:1.54                                     
Analysis of the H.sub.2 SO.sub.5 /H.sub.2 SO.sub.4                        
solution                                                                  
H.sub.2 SO.sub.5 (% by weight)                                            
                   55.5        56.3                                       
H.sub.2 O.sub.2 (% by weight)                                             
                   0.95        0.96                                       
H.sub.2 S.sub.2 O.sub.8 (% by weight)                                     
                   3.2         3.4                                        
Partial neutralization                                                    
Molar ratio        1:1.23      1:1                                        
Total SO.sub.3 :NaOH                                                      
Amount H.sub.2 SO.sub.5 /H.sub.2 SO.sub.4                                 
                   100         100                                        
solution (g/h)                                                            
Sodium hydroxide solution                                                 
                   110.5       89.6                                       
(g/h)                                                                     
Analysis of the H.sub.2 SO.sub.5 /                                        
H.sub.2 SO.sub.4 solution                                                 
                   26.97       30.45                                      
NaHSO.sub.5 (% by weight)                                                 
NaHSO.sub.4 (% by weight)                                                 
                   10.11       22.18                                      
H.sub.2 O.sub.2 (% by weight)                                             
                   0.02        0.02                                       
Yield of NaHSO.sub.5 relative                                             
                   94.3        95.0                                       
to H.sub.2 SO.sub.5 added                                                 
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     EXAMPLES 12 to 14 
     An aqueous H 2  SO 5  /H 2  SO 4  solution was partially neutralized according to the method of Examples 1 to 7. 70% by weight H 2  O 2  and 65% by weight oleum were added; the molar ratio of H 2  O 2  to the total SO 3  was 1 to 1.16. The partial neutralization with 37% by weight sodium hydroxide solution took place up to the pH indicated in the Table (measured with a glass electrode and pH meter). The resulting molar ratios of total SO 3  to NaOH and the yields of NaHSO 5  relative to added H 2  SO 5  are given in Table 4. 
     
                       TABLE 4                                                     
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Example  Molar ratio          Yield % NaHSO.sub.5                         
No.      total SO.sub.3 :NaOH                                             
                        pH    based on H.sub.2 SO.sub.5                   
______________________________________                                    
12       1:1.30         1     96.80                                       
13       1:1.36         2     93.40                                       
14       1:1.39         3     85.68                                       
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     Example 12 is within and Examples 13 and 14 outside of the limits of the invention. In Examples 13 and 14, solid matter precipitates during the partial neutralization as well as during storage. 
     EXAMPLE 15 
     An NaHSO 5  solution prepared in a manner analogous to Example 11 but with a higher concentration was diluted by means of the addition of water to various concentrations of NaHSO 5 . The storage stability was determined by storing 5  hours at 70° C. Table 5 shows the concentrations of the solutions of NaHSO 5  as well as the loss of available oxygen (AO loss) in % from the initial value. 
     
                       TABLE 5                                                     
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Concentration of   AO loss                                                
NaHSO.sub.5 (% by weight)                                                 
                   (%)                                                    
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5                  1.5                                                    
10                 4.2                                                    
15                 5.0                                                    
20                 7.7                                                    
25                 10.8                                                   
30                 14.0                                                   
35                 21.5                                                   
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