APPENDIX 447 



Preparation of Fibers Impregnated with Litmus. A good 

 quality of raw silk is boiled in water containing a little soap, 

 rinsed thoroughly and placed for two hours at room temperature 

 in a sodium hydroxide solution containing 10 grams sodium 

 hydroxide in 100 c.c. of water. The silk is then thoroughly 

 washed with distilled water. Dyeing this treated silk 1 in a 

 10 per cent solution of purified litmus, acidified with 3 or 4 drops 

 of i : 4 sulphuric acid, produces a fiber of the proper color inten- 

 sity. In order to dye the silk properly, the acid litmus solution 

 containing it is evaporated to a thick syrup, the silk then removed 

 and washed in running water, neutralized carefully with very 

 dilute sodium hydroxide solution and again washed thoroughly. 

 If red and blue varieties of the silk are desired, these neutral 

 tinted fibers may be treated with dilute acetic acid for red or 

 with dilute sodium hydroxide for blue and then washed thor- 

 oughly in running water. 



The sensitiveness of the litmus silk depends upon the degree 

 of adsorption of the dye, the degree of purification of the raw 

 silk and the degree of purification of the litmus. 



If too little dye is adsorbed the color change is not distinct 

 enough. If too much dye is adsorbed the fiber becomes less 

 sensitive and the color is so deep that it renders the fiber opaque. 



The greater the degree of purification of the litmus the more 

 sensitive the dyed fiber, though this factor is not as important 

 as the two former ones. 



Preparation of Purified Litmus. The following procedure 

 (essentially Wartha's 2 method) is suggested for obtaining an 

 exceedingly pure litmus. Commercial litmus " cubes " are 

 extracted with 95 per cent alcohol until the alcoholic extract 

 no longer has a reddish tinge. They are then repeatedly 

 extracted with water until the greater part of the coloring 

 matter is removed, a current of air being blown through the 

 solution to prevent reduction. The filtered solution is carried 

 to a thick syrup in an evaporator on the water bath. The mass 

 is then evaporated several times with portions of absolute alcohol 



1 Chamot and Cole: J. Ind. Eng. Chem., IX (1917), 969. 

 2 Ber., 9 (1876), 217. 



