260 CHAPTER XIII 



in 50 per cent, alcohol, allowed to dry and cut into strips. One end ot a 

 strip is cut off, leaving a ragged edge ; 100 c.c. or other convenient quantity 

 of the juice is placed in a suitable vessel to which (if acid) decinormal alkali 

 is allowed to flow from a burette. As the end point is approached, the ragged 

 edge of the paper is dipped into the juice, and, after immersion, is examined 

 by transmitted light ; the end point or exact neutrality is taken as being 

 when a delicate orange-red colour can be detected on the transparent torn 

 edge of the paper. Determinations sensitive to o.i c.c. decinormal acid 

 or alkali can be made by this method, and, when a juice is said to have an 

 acidity of 3 c.c. normal acid, nothing more than the result of the execution 

 of this or a similar test is intended. 



If litmus be used as an indicator, results different from those found with 

 phenolphthalein obtain, the acidity being less and the alkalinity being 

 greater. That is to say, on titrating an acid juice with alkali, the end 

 point appears with litmus before it is seen with phenolphthalein. This 

 difference is of especial importance in the control of sulphitation. Normal 

 sulphites of the formula M 2 SO 3 are alkaline towards litmus and neutral 

 towards phenolphthalein ; accordingly, if a juice containing free sulphurous 

 acid be gradually neutralized with an alkali, a neutral reaction will be given 

 to litmus when both normal sulphite and acid sulphite are present. The 

 complete neutralization and disappearance of acid sulphite and presence of 

 free alkali is shown by the appearance of a red colour with phenolphthalein ; 

 when this body is colourless, free acid or acid sulphite may equally be present. 



The natural colouring matters of cane juice also to some extent serve as 

 indicators, three colour phases being observed. At the point where phenol- 

 phthalein becomes pink, cane juice changes to a golden yellow ; with the 

 addition of acid the colour changes to an olive brown, which persists over 

 0-5 c.c. of normal acid per looc.c. of juice, counting from the appearance of 

 the golden yellow colour ; the addition of more acid gives an almost colourless 

 juice ; the change from olive brown to colourless takes place very nearly 

 at the point where litmus becomes distinctly red. These changes are prob- 

 ably due to the presence of several colouring matters in juice. 



The relative advantages of litmus and of phenolphthalein in technical 

 sugar-house control have at times led to controversy. Without doubt 

 litmus papers are superior for routine inspection and for supervision, and 

 generally in defecation processes juices which afford a barely perceptible 

 bluish tint settle well ; when tested with phenolphthalein papers such 

 juices give no change of colour, and hence afford no indication of a critical 

 point as is given by litmus. For the definite expression of analytical results, 

 however, the end point as afforded by phenolphthalein is much sharper and 

 more distinct. In the carbonation process, moreover, the appearance of a 

 very faint pink with phenolphthalein forms one of the critical points. 



The Action of Acids on Cane Sugar. Cane sugar in acid solution is con- 

 verted into equal parts of glucose and of fructose. This process is vulgarly 

 called inversion, and is actually an hydrolysis, the acid acting as a catalyst. 

 Symbolically, the process follows the equation : 



QaHfflOu + H 2 = C 6 H 12 O 6 + C 6 H 12 6 



The rate of inversion is dependent on the concentration of the hydrogen 

 ions, or on the strength of the acid used, and actually the study of the hydrolysis 

 of cane sugar is one of the classic methods by which the strengths of acids 



