Stable solutions containing thimerosal
The present invention relates to solutions containing thimerosal having greater stability than those of the prior art. Thimerosal, also called merthiolate, is a very effective antimicrobial. It may be used as a perservative for solutions intended for use in the eye or nose. However, when formulated with certain common buffers, such as phosphates and acetates, thimerosal degrades too quickly. 
     The present invention is based on the finding that thimerosal solutions containing specific buffer ingredients and having a pH in a specific range have much greater stability. 
     SUMMARY OF THE INVENTION 
     The present invention comprises a stable solution comprising: 
     (a) thimerosal, 
     (b) a compound selected from the group consisting of citric acid monohydrate, anhydrous citric acid, and combinations thereof, 
     (c) a compound selected from the group consisting of sodium citrate dihydrate, anhydrous sodium citrate and combinations thereof and 
     (d) water, 
     said solution having a pH between 3.5 and 7. 
     DETAILED DESCRIPTION OF THE INVENTION 
     The inventive solutions are preferably prepared by mixing two solutions. The first solution contains either citric acid monohydrate or anhydrous citric acid at a concentration of 0.002M. The second solution contains either sodium citrate dihydrate or anhydrous sodium citrate at a concentration of 0.002M. The two solutions are mixed together in selected volume ratios to yield a buffered solution having the desired pH as illustrated in the following table. 
     
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Parts by Volume of                                                        
               Parts by Volume of                                         
0.002 M citric acid                                                       
               0.002 M sodium citrate                                     
                              Resulting                                   
Solution       Solution       pH                                          
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1              0.21           3.5                                         
1              0.88           4.5                                         
1              2.23           5.5                                         
1              9.26           6.5                                         
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     Next, antimicrobial effective amounts of thimerosal and any active ingredients are added to the resulting mixture. 
     The resulting solution should have a pH in the range of 3.5 to 7, more preferably 3.5 to 5 and most preferably 4.5 to 5. 
    
    
     EXAMPLES 
     
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                  Example 1 Example 2                                     
Ingredient        mg/ml     mg/ml                                         
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Thimerosal        0.02      0.02                                          
Citric acid monohydrate                                                   
                  0.22      0.23                                          
Sodium citrate dihydrate                                                  
                  0.28      0.27                                          
Purified Distilled water q.s. ad:                                         
                  1 ml      1 ml                                          
pH                4         4.5                                           
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     Any active ingredient compatible with thimerosal and the citrate buffer system may be used. Examples of active ingredients are pilocarpine (present in a concentration of about 0.25 to 10 weight percent), beclomethasone dipropionate (present in a concentration of, for example, 0.044 weight precent) and prednisolone sodium phosphate (present in a concentration of, for example, 0.5 weight percent). 
     The solutions of Examples 1 and 2 with no additional ingredients are believed useful for disinfecting soft contact lenses. 
     The contact lenses are disinfected by immersing the contact lenses in the solution of Examples 1 and 2.