12 PUBLIC HEALTH CHEMISTRY 



3. Phenolphthalein is colourless in solution. Acids cause 

 no change in colour ; alkalies change it to purple-red. 



Used in the cold for titration of 

 Alkaline hydrates, except ammonia. 

 Mineral acids. 

 Organic acids (oxalic, tartaric, acetic, citric, and 



others). 

 Carbonates to bicarbonates. 



May be used in alcoholic solutions, and hence for 

 organic acids insoluble in water. Also for acids combined 

 with bases, like morphia, quinine, brucine, etc., the organic 

 base having no effect on it. 



4. Rosolic Acid is pale yellow in solution. With acids, 

 unchanged ; with alkalies, violet-red. 



Good for mineral acids and oxalic. 

 Not reliable for organic acids. 



5. Turmeric, yellow in colour. With acids, bright 

 yellow ; with alkalies, reddish-brown. 



6 Lacmoid, blue and red papers are best form for use. 

 These are an excellent substitute for methyl-orange when 

 latter cannot be used. It is a derivative of resorcin, and 

 is allied to litmus. 



The other indicators will be alluded to as their use is 

 required. 



Rules as to use of indicators commonly employed : 

 Methyl-orange for mineral acids. 

 Phenolphthalein for organic acids. 

 Litmus for organic acids in presence of free CO 2 (e.g. 

 in beer). 



ALKALIMETRY AND ACIDIMETRY. 



Perform the following exercises : 



1. Titrate 1 c.c. 50 per cent NaOH diluted with a little 

 water (distilled) with N/i H 2 S0 4 , using a few drops of 

 litmus as indicator. 



Take the NaOH in a porcelain basin add the water and 

 the litmus solution. Then take a burette and fill it with 

 the normal acid solution ; be careful that the nozzle is 



