524 SIMPLE CHEMICAL MANIPULATIONS 



use by the addition of alkali. Litmus is the most commonly used 

 indicator for various acids and alkalies. 



Rosolic Acid, or Corallin. This is an aniline stain. It is dissolved 

 in the proportion of 0.5 gr. to 50 c.c. of 95 per cent, alcohol. After 

 solution 50 c.c. of distilled water is added. The indicator is yellow in 

 neutral and acid solutions and turns rose red in alkaline solutions. 

 It is particularly useful in the titration of organic acids, and is, there- 

 fore, used in the determination of lactic, butyric, formic, succinic, and 

 acetic acids. 



Phenolphthalein. This is a coal-tar derivative, and is very much 

 used in work in bacteriology. It is generally employed in the exact 

 titration of culture media, as previously explained in detail. The 

 indicator is prepared as a 1 per cent, alcoholic solution. It is colorless 

 in neutral and acid solutions and turns red in alkaline solutions. 

 The change of color in this solution can also be well seen by artificial 

 illumination. It also takes place promptly in hot fluids, and the 

 phenolphthalein indicator has, therefore, a wide range of application. 

 It can, however, not be depended upon for the determination of 

 ammonia and the weak alkalies, but it is excellent for the hydrates of 

 sodium, potassium, calcium, and barium. 



Empirical Standard Solution. These are based upon a different 

 principle. They are not prepared according to the molecular formula 

 of the chemical compound employed, but in such a manner that 1 c.e. 

 of the standard solution will be equivalent to 10 milligrams of the 

 substance which is to be determined quantitatively by the volumetric 

 analysis. 



FEELING'S STANDARD SOLUTION. A fluid of this type is Fehling's 

 standard solution for the quantitative determination of sugars (glucose, 

 dextrose, maltose, lactose). As the quantitative determination of 

 sugar, either to ascertain how much of it has been fermented by 

 certain bacteria or has been formed from starch, frequently must be 

 made in bacteriological work, the formula for Fehling's copper solution 

 and the steps in the analysis will be given here. 



Fehling's Standard Solution: 



SOLUTION 1. 



Sulphate of copper, chemically pure crystals 34.638 grams 



Sulphuric acid, chemically pure 1 c.c. 



Dissolved in enough distilled water to make 500 c.c. 



SOLUTION 2. 



Tartrate of sodium and potassium, chemically pure (Rochelle salt) 175 grams 



Sodium hydrate, chemically pure .... 125 grams 



Distilled water enough to make . . . . , .- . .\ . . . 500 c.c. 



Two cubic centimeters of the copper and alkaline solution mixed 

 is equivalent to 10 milligrams of glucose or dextrose, to 16 milligrams 

 of maltose, and to 13.5 milligrams of lactose. Solutions No. 1 and 

 No. 2 must be kept separate. They are mixed only in exactly equal 



