THE ACTION OF HEAT, ALKALIES AND ACIDS 259 



is a body which, on solution in water, splits up into free hydrogen ions, 

 carrying a positive charge of electricity and into ions carrying a negative 

 charge. Thus hydrochloric acid represented by the formula HC1 on solution 

 in water consists of H-|- and Cl together with undissociated HC1. 



The strength of an acid is believed under this conception to be due to 

 the degree of dissociation or to the number of free hydrogen ions present. 

 Conversely, an alkali is a body which splits up into hydroxyl ions, OH , 

 and into a base, caustic soda in solution being under this conception believed 

 to consist of sodium ions, Na-f and hydroxyl ions, OH , together with 

 undissociated NaOH. 



The routine analytical process for the determination of acidity depends 

 on the use of indicators, or of bodies which change their colour, depending 

 on whether free hydrogen or free hydroxyl ions are present. Such a body 

 very widely used in analysis is phenolphthalein which is colourless in acid 

 and deep crimson in alkaline solution. 



A normal solution of an acid is one that contains in 1,000 c.c. the hydrogen 

 equivalent of the acid expressed in grams. Thus a normal solution of hydro- 

 chloric acid of the formula HC1 contains in 1,000 c.c. 36 5 grams of acid ; 

 a normal solution of sulphuric acid, H 2 S0 4> contains 49 grams of sulphuric 

 acid, and a normal solution of caustic soda, NaOH, contains 40 grams of 

 caustic soda ; and equal quantities of normal solutions of acids and of alkalies 

 will exactly neutralize each other. This statement does not imply that the 

 strength of all acids and alkalies is the same, for, as an acid is gradually 

 neutralized by an alkali, dissociation of the undissociated portion con- 

 tinually takes place until all is dissociated and the end point must in every 

 case be the same.* If, then, a material is said to have an acidity of 3 c.c. 

 normal acid per 100 c.c., all that is meant is that 3 c.c. of normal alkali are 

 required to induce the colour change in the presence of some suitable indicator. 

 In the case of different acids, the number of free hydrogen ions present 

 originally before the addition of alkali and the effects due to acidity are very 

 different, although the test shows the same acidity in the different cases. 



Determination of Acidity and Alkalinity. Where the expression " an acidity 

 of 3 c.c. normal" occurs in this chapter it is to be understood that 100 c.c. 

 of the material required the addition of 3 c.c. of normal alkali solution to 

 induce the colour change with the selected indicator. Alkalinity is ex- 

 pressed in a similar way. Elsewhere in the sugar industty it is often usual 

 to express acidity in terms of milligrams of lime per 1,000 c.c. of juice, and an 

 alkalinity of 280 milligrams of lime per 1,000 c.c. is the same as I c.c. normal 

 alkalinity per 100 c.c. Similarly, an acidity of 410 milligrams of sul- 

 phurous acid per 1,000 c.c. is the same as I c.c. normal acidity per 100 c.c. 



In the determination of acidity and of alkalinity, the end point is the 

 term used to denote the colour change of the indicator when the point of 

 exact neutrality is just passed. All indicators do not show the same end 

 point, and it is also affected by the presence of neutral salts. For technical 

 control in the sugar industry this difference has some importance, as is ex- 

 plained later. 



The indicators most commonly used in the sugar industry are litmus and 

 phenolphthalein. The analytical routine followed by the writer is as 

 follows : White filter paper is soakedin a neutral solution of phenolphthalein 



* Except in so far as regards some finer points which do not affect the technical correctness of this statement. 



