NEUTRAL LITMUS SOLUTION 179 



important that the twenty minutes should not be exceeded in 

 sterilisation, as neglect of this precaution may discolour the 

 litmus or lead to the production of yellowish tints when the 

 tubes are subsequently inoculated with acid-forming bacteria. 



Neutral Litmus Solution. 



The most satisfactory is the Kubel-Tiemann, pre- 

 pared by Kahlbaum. It can however be made in the 

 laboratory as follows : 



1. Weigh out 



Commercial litmus 50 grammes, 



and place in a well stoppered 500 c.c. bottle; measure 

 out and add 300 c.c. alcohol 95 per cent. 



2. Shake well at least once a day for seven days 

 the alcohol acquires a green colour. 



3. Decant off the green alcohol and fill a further 300 

 c.c. 95 per cent, alcohol into the bottle and repeat the 

 shaking. 



4. Repeat this process until on adding fresh alcohol 

 the fluid only becomes tinged with violet. 



5. Pour off the alcohol, leaving the litmus as dry as 

 possible. Connect up the bottle to an air pump and 

 evaporate off the last traces of alcohol. 



6. Transfer the dry litmus to a litre flask, measure in 

 600 c.c. distilled water and allow to remain in contact 

 24 hours with frequent shakings. 



7. Filter the solution into a clean flask and add one or 

 two drops of pure concentrated sulphuric acid until the 

 litmus solution is distinctly wine-red in colour. 



8. Add excess of pure solid baryta and allow to stand 

 until the reaction is again alkaline. 



9. Filter. 



10. Bubble CO 2 through the solution until reaction 

 is definitely acid. 



n. Sterilise in the steamer at 100 C. for thirty 

 minutes on each of three consecutive days. This 

 sterilises the solution and also drives off the carbon 

 dioxide, leaving the solution neutral. 



